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Metabolic trapping refers to a localization mechanism of synthesized radiocompounds in the human body. It can be defined as the intracellular accumulation of a radioactive tracer based on the relative metabolic activity of the body's tissues . [ 1 ] It is a basic principle of the design of radiopharmaceuticals as metabolic probes [ 2 ] for functional studies or tumor location. [ 3 ]
Metabolic trapping is the mechanism underlying the ( PET ) scan, [ 4 ] an effective tool for detecting tumors, as there is a greater uptake of the target molecule by tumor tissue than by normal tissue.
In order to use it as a diagnostic tool in medicine, scientists have studied the trapping of radioactive molecules within different tissues throughout the body. In 1978, Gallagher et al. studied glucose tagged with Fluorine-18 (F-18) to see how it metabolized in the tissues of different organs. This group studied how long it took the lungs, liver, kidneys, heart, and brain to metabolize radioactive glucose. They found the molecule distributed uniformly, and then, after two hours, only the heart and the brain had significant levels of radioactivity from the F-18 due to metabolic trapping. This trapping occurred because once the glucose was pulled into the cells, the glucose was phosphorylated to cause the concentration of glucose in the cell to appear lower than it is, which then promotes the transport of more glucose. This phosphorylation of the radioactive glucose caused the metabolic trapping in the heart and the brain. The lungs, liver, and kidneys did not experience metabolic trapping, and the radioactive glucose that was not trapped was excreted in the urine. F-18 radiolabeled glucose did not get collected by the kidneys and cycled back into the system, as it would do for normal glucose. This suggests that the active transporter requires the hydroxyl (-OH) group found on the C-2 position of the sugar, where the F-18 atom was placed. Without the active transport, the radiolabeled glucose that was not trapped was then excreted as waste instead of being phosphorylated in the cell. [ 5 ]
A 2001 study of metabolic trapping used choline derivatives, which were synthesized using F-18, to label prostate cancer. The experiments were conducted first in mice and then in human patients. Choline (CH) and choline radiolabeled with F-18 (FCH) were both found to primarily migrate to the kidneys and liver in their experiment. This is different from the earlier experiment with glucose due to the difference in mechanism and metabolic need of glucose versus choline in the body. Phosphorylation was again found to be responsible for the trapping of the tracer in the tissues. [ 6 ] | https://en.wikipedia.org/wiki/Metabolic_trapping |
Metabolic wastes or excrements are substances left over from metabolic processes (such as cellular respiration ) which cannot be used by the organism (they are surplus or toxic ), and must therefore be excreted . This includes nitrogen compounds, water , CO 2 , phosphates , sulphates , etc. Animals treat these compounds as excretes. Plants have metabolic pathways which transforms some of them (primarily the oxygen compounds) into useful substances.
All the metabolic wastes are excreted in a form of water solutes through the excretory organs ( nephridia , Malpighian tubules , kidneys ), with the exception of CO 2 , which is excreted together with the water vapor throughout the lungs . The elimination of these compounds enables the chemical homeostasis of the organism.
The nitrogen compounds through which excess nitrogen is eliminated from organisms are called nitrogenous wastes ( / n aɪ ˈ t r ɒ dʒ ɪ n ə s / ) or nitrogen wastes . They are ammonia , urea , uric acid , and creatinine . All of these substances are produced from protein metabolism . In many animals, the urine is the main route of excretion for such wastes; in some, it is the feces .
Ammonotelism is the excretion of ammonia and ammonium ions. Ammonia (NH 3 ) forms with the oxidation of amino groups.(-NH 2 ), which are removed from the proteins when they convert into carbohydrates. It is a very toxic substance to tissues and extremely soluble in water. Only one nitrogen atom is removed with it. A lot of water is needed for the excretion of ammonia, about 0.5 L of water is needed per 1 g of nitrogen to maintain ammonia levels in the excretory fluid below the level in body fluids to prevent toxicity. [ citation needed ] Thus, the marine organisms excrete ammonia directly into the water and are called ammonotelic . [ 2 ] Ammonotelic animals include crustaceans , platyhelminths , cnidarians , poriferans , echinoderms , and other aquatic invertebrates. [ 3 ]
The excretion of urea is called ureotelism. Land animals, mainly amphibians and mammals , convert ammonia into urea, a process which occurs in the liver and kidney. These animals are called ureotelic . [ 3 ] Urea is a less toxic compound than ammonia; two nitrogen atoms are eliminated through it and less water is needed for its excretion. It requires 0.05 L of water to excrete 1 g of nitrogen, approximately only 10% of that required in ammonotelic organisms. [ citation needed ]
Uricotelism is the excretion of excess nitrogen in the form of uric acid . [ 4 ] Uricotelic animals include insects , birds and most reptiles . Though requiring more metabolic energy to make than urea, uric acid's low toxicity and low solubility in water allow it to be concentrated into a small volume of pasty white suspension in feces, compared to the liquid urine of mammals. [ 3 ] Notably however, great apes and humans, while ureotelic, are also uricotelic to a small extent, with uric acid potentially causing problems such as kidney stones and gout , but also functioning as a blood antioxidant.
These compounds form during the catabolism of carbohydrates and lipids in condensation reactions, and in some other metabolic reactions of the amino acids. Oxygen is produced by plants and some bacteria in photosynthesis, while CO 2 is a waste product of all animals and plants. Nitrogen gases are produced by denitrifying bacteria and as a waste product, and bacteria for decaying yield ammonia, as do most invertebrates and vertebrates. Water is the only liquid waste from animals and photosynthesizing plants. [ 5 ]
Nitrates and nitrites are wastes produced by nitrifying bacteria , just as sulfur and sulfates are produced by the sulfur-reducing bacteria and sulfate-reducing bacteria . Insoluble iron waste can be made by iron bacteria by using soluble forms. In plants, resins, fats, waxes, and complex organic chemicals are exuded from plants, e.g., the latex from rubber trees and milkweeds. Solid waste products may be manufactured as organic pigments derived from breakdown of pigments like hemoglobin, and inorganic salts like carbonates, bicarbonates, and phosphate, whether in ionic or in molecular form, are excreted as solids. [ 5 ]
Animals dispose of solid waste as feces . | https://en.wikipedia.org/wiki/Metabolic_waste |
Metabolic water refers to water created inside a living organism through metabolism , by oxidizing energy-containing substances in food and adipose tissue. Animal metabolism produces about 107–110 grams of water per 100 grams of fat , [ 1 ] 41–42 grams of water per 100 g of protein , and 60 grams of water per 100 g of carbohydrate . [ 2 ] [ 1 ] [ 3 ]
Some organisms, especially xerocoles — animals living in the desert — rely exclusively on metabolic water. Migratory birds must rely exclusively on metabolic water production while making non-stop flights, facilitated by the high metabolic rate during such flights. [ 4 ] [ 5 ] Humans, by contrast, obtain only about 8–10% of their water needs through metabolic water production. [ 6 ]
In mammals , the water produced from metabolism of protein roughly equals the amount needed to excrete the urea which is a byproduct of the metabolism of protein. [ 6 ] Birds, however, excrete uric acid and can have a net gain of water from the metabolism of protein. | https://en.wikipedia.org/wiki/Metabolic_water |
Metabolism: Clinical and Experimental is a monthly peer-reviewed medical journal covering all aspects of human metabolism . It was established in 1952 and is published by Elsevier . [ 1 ] The editor-in-chief is Christos Socrates Mantzoros ( Harvard Medical School ) who has reinvigorated the journal during his tenure.
The journal is abstracted and indexed in
According to the Journal Citation Reports , the journal has a 2021 impact factor of 13.93 and a current Cite Score (the equivalent of a 4-year impact factor) of 16.5 placing the journal in the top 3% of Endocrinology, Diabetes and Metabolism Journals. [ 5 ] | https://en.wikipedia.org/wiki/Metabolism:_Clinical_and_Experimental |
Metabolite damage can occur through enzyme promiscuity or spontaneous chemical reactions. Many metabolites are chemically reactive and unstable and can react with other cell components or undergo unwanted modifications. Enzymatically or chemically damaged metabolites are always useless and often toxic. To prevent toxicity that can occur from the accumulation of damaged metabolites, organisms have damage-control systems that:
Damage-control systems can involve one or more specific enzymes . [ 1 ] [ 2 ]
Similarly to DNA and proteins , metabolites are prone to damage, which can occur chemically or through enzyme promiscuity. Much less is known about metabolite damage than about DNA and protein damage, in part due to the huge variety and number of damage-prone metabolites.
Many metabolites are chemically reactive and unstable, and thus prone to chemical damage. In general, any reaction that occurs in vitro under physiological conditions can also occur in vivo . [ 3 ] [ 4 ] Some metabolites are so reactive that their half-life in a cell is measured in minutes. [ 2 ] For example, the glycolytic intermediate 1,3-bisphosphoglyceric acid has a half-life of 27 minutes in vivo . [ 5 ] Typical types of chemical damage reactions that can occur to metabolites are racemization , rearrangement , elimination , photodissociation , addition , and condensation .
Although enzymes are generally specific towards their substrate, enzymatic side activities ( enzyme promiscuity ) can lead to toxic or useless products. These side reactions proceed at much lower rates than their normal physiological reactions, but build-up of damaged metabolites can still be significant over time. For example, the mitochondrial malate dehydrogenase reduces alpha-ketoglutarate to L-2-hydroxyglutarate 10 7 times less efficiently than its regular substrate oxaloacetate , but L-2-hydroxyglutarate can still accumulate to several grams per day in a human adult. [ 6 ]
Metabolite damage-control systems fall into three different categories:
Damage repair is the conversion of a damaged metabolite back to its original state via one or more enzymatic reactions; the concept is similar to DNA repair and protein repair. For example, the promiscuous activity of malate dehydrogenase causes reduction of alpha-ketoglutarate to L-2-hydroxyglutarate. This compound is a dead-end metabolite and is not a substrate for any other enzyme in central metabolism, and its accumulation in humans causes L-2-Hydroxyglutaric aciduria . The repair enzyme L-2-hydroxyglutarate dehydrogenase oxidizes L-2-hydroxyglutarate back to alpha-ketoglutarate , thus repairing this metabolite. In humans, L-2-hydroxyglutarate dehydrogenase uses FAD as the cofactor, while the E. coli enzyme reduces molecular oxygen. [ 7 ]
Pre-emption prevents damage from happening. This is done either by converting reactive metabolites to less harmful ones, or by speeding up an insufficiently fast chemical reaction. The reactive metabolite can be either a side product, or a normal, but highly reactive intermediate .
For example, a side activity of Rubisco yields small amounts of xylulose-1,5-bisphosphate, which can inhibit Rubisco activity. The CbbY enzyme dephosphorylates xylulose-1,5-bisphosphate to the natural metabolite xylulose-5-phosphate, thereby preventing inhibition of Rubisco. [ 8 ]
Directed overflow is a special case of damage pre-emption, where excess of a normal, but reactive metabolite could lead to toxic products. Preventing this excess is thus pre-emption of potential damage.
The first two intermediates in riboflavin biosynthesis are highly reactive and can spontaneously break down to 5-phosphoribosylamine and Maillard reaction products, which are highly reactive and harmful. The enzyme COG3236 hydrolyzes these two first intermediates into two less harmful products, thus preventing the harm they would otherwise cause. [ 9 ]
In humans, L-2-Hydroxyglutaric aciduria was the first disease linked to a missing metabolite repair enzyme. [ 7 ] Mutations in the L2HGDH gene cause accumulation of L-2-hydroxyglutarate, which is a structural analog to glutamate and alpha-ketoglutarate and presumably inhibits other enzymes or transporters. [ 7 ]
Metabolic network modelling aims at reproducing cellular metabolism in silico . Metabolite damage and repair create cellular energy costs, and consequently need to be incorporated into genome-scale metabolic models so that these models can more effectively guide metabolic engineering design. [ 1 ]
In addition, genes encoding so-far unrecognized metabolite damage-control systems may constitute a significant fraction of the many conserved genes of unknown function found in the genomes of all organisms. [ 1 ] [ 2 ]
When an alien pathway is installed in a host ('chassis') organism, and even when a native pathway is massively upregulated, reactive intermediates may accumulate to levels that negatively impact viability, growth, and flux through the pathway because a matching damage-control system is absent or has been overwhelmed. [ 10 ] Engineering damage-control systems may thus be needed to support synthetic biology and metabolic engineering projects. [ 11 ] | https://en.wikipedia.org/wiki/Metabolite_damage |
Metabolite pool is a collective term for all of the substances involved in the metabolic process in a biological system .
Metabolic pools are within cells (or organelles such as chloroplasts ) and refer to the reservoir of molecules upon which enzymes can operate. The size of the reservoir is referred to as its "metabolic pool." The metabolic pool concept is important to cellular biology . [ 1 ]
In certain ways, a metabolic pathway is similar to a factory assembly line. Products are assembled from parts by workers who each perform a specific step in the manufacturing process. Enzymes of a cell are like workers on an assembly line; each is only responsible for a particular step in the assembly process. A lag period also occurs when a new factory is constructed, a time period before finished products begin to roll off the assembly line at a steady rate. This lag period partially results from the time needed to fill supply bins with the necessary parts. As you might imagine, when parts are not readily available, production slows or stops. Metabolite pools are somewhat analogous to the parts bins of a factory. The Calvin-Benson cycle will only operate at full speed when the cellular 'bins' are full of the molecular building blocks that lie between PGA and RUBP .
This biology article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metabolite_pool |
The metabolome refers to the complete set of small-molecule chemicals found within a biological sample. [ 1 ] The biological sample can be a cell , a cellular organelle , an organ , a tissue , a tissue extract, a biofluid or an entire organism . The small molecule chemicals found in a given metabolome may include both endogenous metabolites that are naturally produced by an organism (such as amino acids , organic acids , nucleic acids , fatty acids , amines , sugars , vitamins , co-factors , pigments , antibiotics , etc.) as well as exogenous chemicals (such as drugs, environmental contaminants , food additives , toxins and other xenobiotics ) that are not naturally produced by an organism. [ 2 ] [ 3 ]
In other words, there is both an endogenous metabolome and an exogenous metabolome. The endogenous metabolome can be further subdivided to include a "primary" and a "secondary" metabolome (particularly when referring to plant or microbial metabolomes). A primary metabolite is directly involved in the normal growth, development, and reproduction. A secondary metabolite is not directly involved in those processes, but usually has important ecological function. Secondary metabolites may include pigments, antibiotics or waste products derived from partially metabolized xenobiotics. The study of the metabolome is called metabolomics .
The word metabolome appears to be a blending of the words "metabolite" and " chromosome ". It was constructed to imply that metabolites are indirectly encoded by genes or act on genes and gene products. The term "metabolome" was first used in 1998 [ 1 ] [ 4 ] and was likely coined to match with existing biological terms referring to the complete set of genes (the genome ), the complete set of proteins (the proteome ) and the complete set of transcripts (the transcriptome ). The first book on metabolomics was published in 2003. [ 5 ] The first journal dedicated to metabolomics (titled simply "Metabolomics") was launched in 2005 and is currently edited by Prof. Roy Goodacre . Some of the more significant early papers on metabolome analysis are listed in the references below. [ 6 ] [ 7 ] [ 8 ] [ 9 ]
The metabolome reflects the interaction between an organism's genome and its environment. As a result, an organism's metabolome can serve as an excellent probe of its phenotype (i.e. the product of its genotype and its environment). Metabolites can be measured (identified, quantified or classified) using a number of different technologies including NMR spectroscopy and mass spectrometry . [ 10 ] Most mass spectrometry (MS) methods must be coupled to various forms of liquid chromatography (LC), gas chromatography (GC) or capillary electrophoresis (CE) to facilitate compound separation. Each method is typically able to identify or characterize 50-5,000 different metabolites or metabolite "features" at a time, depending on the instrument or protocol being used. Currently it is not possible to analyze the entire range of metabolites by a single analytical method.
Nuclear magnetic resonance (NMR) spectroscopy is an analytical chemistry technique that measures the absorption of radiofrequency radiation of specific nuclei when molecules containing those nuclei are placed in strong magnetic fields . The frequency (i.e. the chemical shift ) at which a given atom or nucleus absorbs is highly dependent on the chemical environment (bonding, chemical structure nearest neighbours, solvent) of that atom in a given molecule. The NMR absorption patterns produce "resonance" peaks at different frequencies or different chemical shifts – this collection of peaks is called an NMR spectrum . Because each chemical compound has a different chemical structure, each compound will have a unique (or almost unique) NMR spectrum. As a result, NMR is particularly useful for the characterization, identification and quantification of small molecules, such as metabolites. The widespread use of NMR for "classical" metabolic studies, along with its exceptional capacity to handle complex metabolite mixtures is likely the reason why NMR was one of the first technologies to be widely adopted for routine metabolome measurements. As an analytical technique, NMR is non-destructive, non-biased, easily quantifiable, requires little or no separation, permits the identification of novel compounds and it needs no chemical derivatization. NMR is particularly amenable to detecting compounds that are less tractable to LC-MS analysis, such as sugars, amines or volatile liquids or GC-MS analysis, such as large molecules (>500 Da) or relatively non-reactive compounds. NMR is not a very sensitive technique with a lower limit of detection of about 5 μM. Typically 50-150 compounds can be identified by NMR-based metabolomic studies.
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of molecules. Molecules or molecular fragments are typically charged or ionized by spraying them through a charged field ( electrospray ionization ), bombarding them with electrons from a hot filament ( electron ionization ) or blasting them with a laser when they are placed on specially coated plates (matrix assisted laser desorption ionization). The charged molecules are then propelled through space using electrodes or magnets and their speed, rate of curvature, or other physical characteristics are measured to determine their mass-to-charge ratio. From these data the mass of the parent molecule can be determined. Further fragmentation of the molecule through controlled collisions with gas molecules or with electrons can help determine the structure of molecules. Very accurate mass measurements can also be used to determine the elemental formulas or elemental composition of compounds. Most forms of mass spectrometry require some form of separation using liquid chromatography or gas chromatography . This separation step is required to simplify the resulting mass spectra and to permit more accurate compound identification. Some mass spectrometry methods also require that the molecules be derivatized or chemically modified so that they are more amenable for chromatographic separation (this is particularly true for GC-MS). As an analytical technique, MS is a very sensitive method that requires very little sample (<1 ng of material or <10 μL of a biofluid) and can generate signals for thousands of metabolites from a single sample. MS instruments can also be configured for very high throughput metabolome analyses (hundreds to thousands of samples a day). Quantification of metabolites and the characterization of novel compound structures is more difficult by MS than by NMR. LC-MS is particularly amenable to detecting hydrophobic molecules ( lipids , fatty acids) and peptides while GC-MS is best for detecting small molecules (<500 Da) and highly volatile compounds ( esters , amines, ketones , alkanes , thiols ).
Unlike the genome or even the proteome , the metabolome is a highly dynamic entity that can change dramatically, over a period of just seconds or minutes. As a result, there is growing interest in measuring metabolites over multiple time periods or over short time intervals using modified versions of NMR or MS-based metabolomics.
Because an organism's metabolome is largely defined by its genome, different species will have different metabolomes. Indeed, the fact that the metabolome of a tomato is different from the metabolome of an apple is the reason why these two fruits taste so different. Furthermore, different tissues, different organs and biofluids associated with those organs and tissues can also have distinctly different metabolomes. The fact that different organisms and different tissues/biofluids have such different metabolomes has led to the development of a number of organism-specific and biofluid-specific metabolome databases. Some of the better known metabolome databases include the Human Metabolome Database or HMDB, [ 11 ] the Yeast Metabolome Database or YMDB, [ 12 ] the E. coli Metabolome Database or ECMDB, [ 13 ] the Arabidopsis metabolome database or AraCyc [ 14 ] as well as the Urine Metabolome Database, [ 15 ] the Cerebrospinal Fluid (CSF) Metabolome Database [ 16 ] and the Serum Metabolome Database . [ 17 ] The latter three databases are specific to human biofluids. A number of very popular general metabolite databases also exist including KEGG , [ 18 ] MetaboLights, [ 19 ] the Golm Metabolome Database , [ 20 ] MetaCyc , [ 21 ] LipidMaps [ 22 ] and Metlin . [ 23 ] Metabolome databases can be distinguished from metabolite databases in that metabolite databases contain lightly annotated or synoptic metabolite data from multiple organisms while metabolome databases contain richly detailed and heavily referenced chemical, pathway, spectral and metabolite concentration data for specific organisms.
The Human Metabolome Database (HMDB) is a freely available, open-access database containing detailed data on more than 40,000 metabolites that have already been identified or are likely to be found in the human body. The HMDB contains three kinds of information:
The chemical data includes >40,000 metabolite structures with detailed descriptions, extensive chemical classifications, synthesis information and observed/calculated chemical properties. It also contains nearly 10,000 experimentally measured NMR , GC-MS and LC/MS spectra from more than 1,100 different metabolites. The clinical information includes data on >10,000 metabolite-biofluid concentrations, metabolite concentration information on more than 600 different human diseases and pathway data for more than 200 different inborn errors of metabolism. The biochemical information includes nearly 6,000 protein (and DNA) sequences and more than 5,000 biochemical reactions that are linked to these metabolite entries. The HMDB supports a wide variety of online queries including text searches, chemical structure searches, sequence similarity searches and spectral similarity searches. This makes it particularly useful for metabolomic researchers who are attempting to identify or understand metabolites in clinical metabolomic studies. The first version of the HMDB was released in Jan. 1 2007 and was compiled by scientists at the University of Alberta and the University of Calgary . At that time, they reported data on 2,500 metabolites, 1,200 drugs and 3,500 food components. Since then these scientists have greatly expanded the collection. The version 3.5 of the HMDB contains >16,000 endogenous metabolites, >1,500 drugs and >22,000 food constituents or food metabolites. [ 24 ]
Scientists at the University of Alberta have been systematically characterizing specific biofluid metabolomes including the serum metabolome, [ 17 ] the urine metabolome, [ 15 ] the cerebrospinal fluid (CSF) metabolome [ 16 ] and the saliva metabolome. These efforts have involved both experimental metabolomic analysis (involving NMR, GC-MS, ICP-MS , LC-MS and HPLC assays) as well as extensive literature mining. According to their data, the human serum metabolome contains at least 4,200 different compounds (including many lipids), the human urine metabolome contains at least 3,000 different compounds (including hundreds of volatiles and gut microbial metabolites), the human CSF metabolome contains nearly 500 different compounds while the human saliva metabolome contains approximately 400 different metabolites, including many bacterial products.
The Yeast Metabolome Database is a freely accessible, online database of >2,000 small molecule metabolites found in or produced by Saccharomyces cerevisiae ( Baker's yeast ). The YMDB contains two kinds of information:
The chemical information in YMDB includes 2,027 metabolite structures with detailed metabolite descriptions, extensive chemical classifications, synthesis information and observed/calculated chemical properties. It also contains nearly 4,000 NMR, GC-MS and LC/MS spectra obtained from more than 500 different metabolites. The biochemical information in YMDB includes >1,100 protein (and DNA) sequences and >900 biochemical reactions. The YMDB supports a wide variety of queries including text searches, chemical structure searches, sequence similarity searches and spectral similarity searches. This makes it particularly useful for metabolomic researchers who are studying yeast as a model organism or who are looking into optimizing the production of fermented beverages (wine, beer).
Secondary electrospray ionization - high resolution mass spectrometry SESI-HRMS is a non-invasive analytical technique that allows us to monitor the yeast metabolic activities. SESI-HRMS has found around 300 metabolites in the yeast fermentation process, this suggests that a large number of glucose metabolites are not reported in the literature. [ 25 ]
The E. Coli Metabolome Database is a freely accessible, online database of >2,700 small molecule metabolites found in or produced by Escherichia coli (E. coli strain K12, MG1655). The ECMDB contains two kinds of information:
The chemical information includes more than 2,700 metabolite structures with detailed metabolite descriptions, extensive chemical classifications, synthesis information and observed/calculated chemical properties. It also contains nearly 5,000 NMR, GC-MS and LC-MS spectra from more than 600 different metabolites. The biochemical information includes >1,600 protein (and DNA) sequences and >3,100 biochemical reactions that are linked to these metabolite entries. The ECMDB supports many different types of online queries including text searches, chemical structure searches, sequence similarity searches and spectral similarity searches. This makes it particularly useful for metabolomic researchers who are studying E. coli as a model organism.
Secondary electrospray ionization (SESI-MS) can discriminate between eleven E. Coli strains thanks to the volatile organic compound profiling. [ 26 ]
In 2021, the first brain metabolome atlas of the mouse brain – and of an animal (a mammal) across different life stages – was released online. The data differentiates by brain regions and the metabolic changes could be "mapped to existing gene and protein brain atlases". [ 27 ] [ 28 ]
Human intestinal microbiota contribute to the etiology of colorectal cancer via their metabolome. [ 29 ] In particular, the conversion of primary bile acids to secondary bile acids as a consequence of bacterial metabolism in the colon promotes carcinogenesis . [ 29 ] | https://en.wikipedia.org/wiki/Metabolome |
Metabolomics is the scientific study of chemical processes involving metabolites , the small molecule substrates, intermediates, and products of cell metabolism . Specifically, metabolomics is the "systematic study of the unique chemical fingerprints that specific cellular processes leave behind", the study of their small-molecule metabolite profiles. [ 1 ] The metabolome represents the complete set of metabolites in a biological cell, tissue, organ, or organism, which are the end products of cellular processes. [ 2 ] Messenger RNA (mRNA), gene expression data, and proteomic analyses reveal the set of gene products being produced in the cell, data that represents one aspect of cellular function. Conversely, metabolic profiling can give an instantaneous snapshot of the physiology of that cell, [ 3 ] and thus, metabolomics provides a direct "functional readout of the physiological state" of an organism. [ 4 ] There are indeed quantifiable correlations between the metabolome and the other cellular ensembles ( genome , transcriptome , proteome , and lipidome ), which can be used to predict metabolite abundances in biological samples from, for example mRNA abundances. [ 5 ] One of the ultimate challenges of systems biology is to integrate metabolomics with all other -omics information to provide a better understanding of cellular biology.
The concept that individuals might have a "metabolic profile" that could be reflected in the makeup of their biological fluids was introduced by Roger Williams in the late 1940s, [ 6 ] who used paper chromatography to suggest characteristic metabolic patterns in urine and saliva were associated with diseases such as schizophrenia . However, it was only through technological advancements in the 1960s and 1970s that it became feasible to quantitatively (as opposed to qualitatively) measure metabolic profiles. [ 7 ] The term "metabolic profile" was introduced by Horning, et al. in 1971 after they demonstrated that gas chromatography-mass spectrometry (GC-MS) could be used to measure compounds present in human urine and tissue extracts. [ 8 ] [ 9 ] The Horning group, along with that of Linus Pauling and Arthur B. Robinson led the development of GC-MS methods to monitor the metabolites present in urine through the 1970s. [ 10 ]
Concurrently, NMR spectroscopy , which was discovered in the 1940s, was also undergoing rapid advances. In 1974, Seeley et al. demonstrated the utility of using NMR to detect metabolites in unmodified biological samples. [ 11 ] This first study on muscle highlighted the value of NMR in that it was determined that 90% of cellular ATP is complexed with magnesium. As sensitivity has improved with the evolution of higher magnetic field strengths and magic angle spinning , NMR continues to be a leading analytical tool to investigate metabolism. [ 8 ] [ 12 ] Recent efforts to utilize NMR for metabolomics have been largely driven by the laboratory of Jeremy K. Nicholson at Birkbeck College, University of London and later at Imperial College London . In 1984, Nicholson showed 1 H NMR spectroscopy could potentially be used to diagnose diabetes mellitus, and later pioneered the application of pattern recognition methods to NMR spectroscopic data. [ 13 ] [ 14 ]
In 1994 and 1996, liquid chromatography mass spectrometry metabolomics experiments [ 15 ] [ 16 ] were performed by Gary Siuzdak while working with Richard Lerner (then president of the Scripps Research Institute ) and Benjamin Cravatt , to analyze the cerebral spinal fluid from sleep deprived animals. One molecule of particular interest, oleamide , was observed and later shown to have sleep inducing properties. This work is one of the earliest such experiments combining liquid chromatography and mass spectrometry in metabolomics.
In 2005, the first metabolomics tandem mass spectrometry database, METLIN , [ 17 ] [ 18 ] for characterizing human metabolites was developed in the Siuzdak laboratory at the Scripps Research Institute . METLIN has since grown and as of December, 2023, METLIN contains MS/MS experimental data on over 930,000 molecular standards and other chemical entities, [ 19 ] each compound having experimental tandem mass spectrometry data generated from molecular standards at multiple collision energies and in positive and negative ionization modes. METLIN is the largest repository of tandem mass spectrometry data of its kind. The dedicated academic journal Metabolomics first appeared in 2005, founded by its current editor-in-chief Roy Goodacre .
In 2005, the Siuzdak lab was engaged in identifying metabolites associated with sepsis and in an effort to address the issue of statistically identifying the most relevant dysregulated metabolites across hundreds of LC/MS datasets, the first algorithm was developed to allow for the nonlinear alignment of mass spectrometry metabolomics data. Called XCMS, [ 20 ] it has since (2012) [ 21 ] been developed as an online tool and as of 2019 (with METLIN) has over 30,000 registered users.
On 23 January 2007, the Human Metabolome Project , led by David S. Wishart , completed the first draft of the human metabolome, consisting of a database of approximately 2,500 metabolites, 1,200 drugs and 3,500 food components. [ 22 ] [ 23 ] Similar projects have been underway in several plant species, most notably Medicago truncatula [ 24 ] and Arabidopsis thaliana [ 25 ] for several years.
As late as mid-2010, metabolomics was still considered an "emerging field". [ 26 ] Further, it was noted that further progress in the field depended in large part, through addressing otherwise "irresolvable technical challenges", by technical evolution of mass spectrometry instrumentation. [ 26 ]
In 2015, real-time metabolome profiling was demonstrated for the first time. [ 27 ]
The metabolome refers to the complete set of small-molecule (<1.5 kDa) [ 22 ] metabolites (such as metabolic intermediates, hormones and other signaling molecules, and secondary metabolites) to be found within a biological sample, such as a single organism. [ 28 ] [ 29 ] The word was coined in analogy with transcriptomics and proteomics ; like the transcriptome and the proteome, the metabolome is dynamic, changing from second to second. Although the metabolome can be defined readily enough, it is not currently possible to analyse the entire range of metabolites by a single analytical method.
In January 2007, scientists at the University of Alberta and the University of Calgary completed the first draft of the human metabolome. The Human Metabolome Database (HMDB) is perhaps the most extensive public metabolomic spectral database to date [ 30 ] and is a freely available electronic database (www.hmdb.ca) containing detailed information about small molecule metabolites found in the human body. It is intended to be used for applications in metabolomics, clinical chemistry, biomarker discovery and general education. The database is designed to contain or link three kinds of data:
The database contains 220,945 metabolite entries including both water-soluble and lipid soluble metabolites. Additionally, 8,610 protein sequences (enzymes and transporters) are linked to these metabolite entries. Each MetaboCard entry contains 130 data fields with 2/3 of the information being devoted to chemical/clinical data and the other 1/3 devoted to enzymatic or biochemical data. [ 31 ] The version 3.5 of the HMDB contains >16,000 endogenous metabolites, >1,500 drugs and >22,000 food constituents or food metabolites. [ 32 ] This information, available at the Human Metabolome Database and based on analysis of information available in the current scientific literature, is far from complete. [ 33 ] In contrast, much more is known about the metabolomes of other organisms. For example, over 50,000 metabolites have been characterized from the plant kingdom, and many thousands of metabolites have been identified and/or characterized from single plants. [ 34 ] [ 35 ]
Each type of cell and tissue has a unique metabolic ‘fingerprint’ that can elucidate organ or tissue-specific information. Bio-specimens used for metabolomics analysis include but not limit to plasma, serum, urine, saliva, feces, muscle, sweat, exhaled breath and gastrointestinal fluid. [ 36 ] The ease of collection facilitates high temporal resolution, and because they are always at dynamic equilibrium with the body, they can describe the host as a whole. [ 37 ] Genome can tell what could happen, transcriptome can tell what appears to be happening, proteome can tell what makes it happen and metabolome can tell what has happened and what is happening. [ 38 ]
Metabolites are the substrates, intermediates and products of metabolism . Within the context of metabolomics, a metabolite is usually defined as any molecule less than 1.5 kDa in size. [ 22 ] However, there are exceptions to this depending on the sample and detection method. For example, macromolecules such as lipoproteins and albumin are reliably detected in NMR -based metabolomics studies of blood plasma. [ 39 ] In plant-based metabolomics, it is common to refer to "primary" and "secondary" metabolites. [ 3 ] A primary metabolite is directly involved in the normal growth, development, and reproduction. A secondary metabolite is not directly involved in those processes, but usually has important ecological function. Examples include antibiotics and pigments . [ 40 ] By contrast, in human-based metabolomics, it is more common to describe metabolites as being either endogenous (produced by the host organism) or exogenous . [ 41 ] [ 42 ] Metabolites of foreign substances such as drugs are termed xenometabolites. [ 43 ]
The metabolome derives from a large network of metabolic reactions, where outputs from one enzymatic chemical reaction are inputs to other chemical reactions. Such systems have been described as hypercycles . [ citation needed ]
Metabonomics is defined as "the quantitative measurement of the dynamic multiparametric metabolic response of living systems to pathophysiological stimuli or genetic modification". The word origin is from the Greek μεταβολή meaning change and nomos meaning a rule set or set of laws. [ 44 ] This approach was pioneered by Jeremy Nicholson at Murdoch University and has been used in toxicology, disease diagnosis and a number of other fields. Historically, the metabonomics approach was one of the first methods to apply the scope of systems biology to studies of metabolism. [ 45 ] [ 46 ] [ 47 ]
There has been some disagreement over the exact differences between 'metabolomics' and 'metabonomics'. The difference between the two terms is not related to choice of analytical platform: although metabonomics is more associated with NMR spectroscopy and metabolomics with mass spectrometry -based techniques, this is simply because of usages amongst different groups that have popularized the different terms. While there is still no absolute agreement, there is a growing consensus that 'metabolomics' places a greater emphasis on metabolic profiling at a cellular or organ level and is primarily concerned with normal endogenous metabolism. 'Metabonomics' extends metabolic profiling to include information about perturbations of metabolism caused by environmental factors (including diet and toxins), disease processes, and the involvement of extragenomic influences, such as gut microflora . This is not a trivial difference; metabolomic studies should, by definition, exclude metabolic contributions from extragenomic sources, because these are external to the system being studied. However, in practice, within the field of human disease research there is still a large degree of overlap in the way both terms are used, and they are often in effect synonymous. [ 48 ]
Exometabolomics, or "metabolic footprinting", is the study of extracellular metabolites. It uses many techniques from other subfields of metabolomics, and has applications in biofuel development, bioprocessing , determining drugs' mechanism of action , and studying intercellular interactions. [ 49 ]
The typical workflow of metabolomics studies is shown in the figure. First, samples are collected from tissue, plasma, urine, saliva, cells, etc. Next, metabolites extracted often with the addition of internal standards and derivatization. [ 38 ] During sample analysis, metabolites are quantified ( liquid chromatography or gas chromatography coupled with MS and/or NMR spectroscopy). [ 50 ] The raw output data can be used for metabolite feature extraction and further processed before statistical analysis (such as principal component analysis , PCA). Many bioinformatic tools and software are available to identify associations with disease states and outcomes, determine significant correlations, and characterize metabolic signatures with existing biological knowledge. [ 51 ]
Initially, analytes in a metabolomic sample comprise a highly complex mixture. This complex mixture can be simplified prior to detection by separating some analytes from others. Separation achieves various goals: analytes which cannot be resolved by the detector may be separated in this step; in MS analysis, ion suppression is reduced; the retention time of the analyte serves as information regarding its identity. This separation step is not mandatory and is often omitted in NMR and "shotgun" based approaches such as shotgun lipidomics .
Gas chromatography (GC), especially when interfaced with mass spectrometry ( GC-MS ), is a widely used separation technique for metabolomic analysis. GC offers very high chromatographic resolution, and can be used in conjunction with a flame ionization detector (GC/FID) or a mass spectrometer (GC-MS). The method is especially useful for identification and quantification of small and volatile molecules. [ 52 ] However, a practical limitation of GC is the requirement of chemical derivatization for many biomolecules as only volatile chemicals can be analysed without derivatization. In cases where greater resolving power is required, two-dimensional chromatography ( GCxGC ) can be applied.
High performance liquid chromatography (HPLC) has emerged as the most common separation technique for metabolomic analysis. With the advent of electrospray ionization , HPLC was coupled to MS. In contrast with GC , HPLC has lower chromatographic resolution, but requires no derivatization for polar molecules, and separates molecules in the liquid phase. Additionally HPLC has the advantage that a much wider range of analytes can be measured with a higher sensitivity than GC methods. [ 53 ]
Capillary electrophoresis (CE) has a higher theoretical separation efficiency than HPLC (although requiring much more time per separation), and is suitable for use with a wider range of metabolite classes than is GC. As for all electrophoretic techniques, it is most appropriate for charged analytes. [ 54 ]
In direct-infusion mass spectrometry (DI-MS), sample is directly introduced into the spectrometer and separation steps are skipped. DI-MS can be employed to perform single cell metabolic analysis of human cells. [ 55 ]
Mass spectrometry (MS) is used to identify and quantify metabolites after optional separation by GC , HPLC , or CE . GC-MS was the first hyphenated technique to be developed. Identification leverages the distinct patterns in which analytes fragment. These patterns can be thought of as a mass spectral fingerprint. Libraries exist that allow identification of a metabolite according to this fragmentation pattern [ example needed ] . MS is both sensitive and can be very specific. There are also a number of techniques which use MS as a stand-alone technology: the sample is infused directly into the mass spectrometer with no prior separation, and the MS provides sufficient selectivity to both separate and to detect metabolites.
For analysis by mass spectrometry, the analytes must be imparted with a charge and transferred to the gas phase. Electron ionization (EI) is the most common ionization technique applied to GC separations as it is amenable to low pressures. EI also produces fragmentation of the analyte, both providing structural information while increasing the complexity of the data and possibly obscuring the molecular ion. Atmospheric-pressure chemical ionization (APCI) is an atmospheric pressure technique that can be applied to all the above separation techniques. APCI is a gas phase ionization method, which provides slightly more aggressive ionization than ESI which is suitable for less polar compounds. Electrospray ionization (ESI) is the most common ionization technique applied in LC/MS. This soft ionization is most successful for polar molecules with ionizable functional groups. Another commonly used soft ionization technique is secondary electrospray ionization (SESI) .
In the 2000s, surface-based mass analysis has seen a resurgence, with new MS technologies focused on increasing sensitivity, minimizing background, and reducing sample preparation. The ability to analyze metabolites directly from biofluids and tissues continues to challenge current MS technology, largely because of the limits imposed by the complexity of these samples, which contain thousands to tens of thousands of metabolites. Among the technologies being developed to address this challenge is Nanostructure-Initiator MS (NIMS), [ 56 ] [ 57 ] a desorption/ ionization approach that does not require the application of matrix and thereby facilitates small-molecule (i.e., metabolite) identification. MALDI is also used; however, the application of a MALDI matrix can add significant background at < 1000 Da that complicates analysis of the low-mass range (i.e., metabolites). In addition, the size of the resulting matrix crystals limits the spatial resolution that can be achieved in tissue imaging. Because of these limitations, several other matrix-free desorption/ionization approaches have been applied to the analysis of biofluids and tissues.
Secondary ion mass spectrometry (SIMS) was one of the first matrix-free desorption/ionization approaches used to analyze metabolites from biological samples. [ citation needed ] SIMS uses a high-energy primary ion beam to desorb and generate secondary ions from a surface. The primary advantage of SIMS is its high spatial resolution (as small as 50 nm), a powerful characteristic for tissue imaging with MS. However, SIMS has yet to be readily applied to the analysis of biofluids and tissues because of its limited sensitivity at >500 Da and analyte fragmentation generated by the high-energy primary ion beam. Desorption electrospray ionization (DESI) is a matrix-free technique for analyzing biological samples that uses a charged solvent spray to desorb ions from a surface. Advantages of DESI are that no special surface is required and the analysis is performed at ambient pressure with full access to the sample during acquisition. A limitation of DESI is spatial resolution because "focusing" the charged solvent spray is difficult. However, a recent development termed laser ablation ESI (LAESI) is a promising approach to circumvent this limitation. [ citation needed ] Most recently, ion trap techniques such as orbitrap mass spectrometry are also applied to metabolomics research. [ 58 ]
Nuclear magnetic resonance (NMR) spectroscopy is the only detection technique which does not rely on separation of the analytes, and the sample can thus be recovered for further analyses. All kinds of small molecule metabolites can be measured simultaneously - in this sense, NMR is close to being a universal detector. The main advantages of NMR are high analytical reproducibility and simplicity of sample preparation. Practically, however, it is relatively insensitive compared to mass spectrometry-based techniques. [ 59 ] [ 60 ]
Although NMR and MS are the most widely used modern-day techniques for detection, there are other methods in use. These include Fourier-transform ion cyclotron resonance , [ 61 ] ion-mobility spectrometry , [ 62 ] electrochemical detection (coupled to HPLC), Raman spectroscopy and radiolabel (when combined with thin-layer chromatography). [ citation needed ]
The data generated in metabolomics usually consist of measurements performed on subjects under various conditions. These measurements may be digitized spectra, or a list of metabolite features. In its simplest form, this generates a matrix with rows corresponding to subjects and columns corresponding with metabolite features (or vice versa). [ 8 ] Several statistical programs are currently available for analysis of both NMR and mass spectrometry data. A great number of free software are already available for the analysis of metabolomics data shown in the table. Some statistical tools listed in the table were designed for NMR data analyses were also useful for MS data. [ 63 ] For mass spectrometry data, software is available that identifies molecules that vary in subject groups on the basis of mass-over-charge value and sometimes retention time depending on the experimental design. [ 64 ]
Once metabolite data matrix is determined, unsupervised data reduction techniques (e.g. PCA) can be used to elucidate patterns and connections. In many studies, including those evaluating drug-toxicity and some disease models, the metabolites of interest are not known a priori . This makes unsupervised methods, those with no prior assumptions of class membership, a popular first choice. The most common of these methods includes principal component analysis (PCA) which can efficiently reduce the dimensions of a dataset to a few which explain the greatest variation. [ 37 ] When analyzed in the lower-dimensional PCA space, clustering of samples with similar metabolic fingerprints can be detected. PCA algorithms aim to replace all correlated variables with a much smaller number of uncorrelated variables (referred to as principal components (PCs)) and retain most of the information in the original dataset. [ 65 ] This clustering can elucidate patterns and assist in the determination of disease biomarkers – metabolites that correlate most with class membership.
Linear models are commonly used for metabolomics data, but are affected by multicollinearity . On the other hand, multivariate statistics are thriving methods for high-dimensional correlated metabolomics data, of which the most popular one is Projection to Latent Structures (PLS) regression and its classification version PLS-DA. Other data mining methods, such as random forest , support-vector machines , etc. are received increasing attention for untargeted metabolomics data analysis. [ 66 ] In the case of univariate methods, variables are analyzed one by one using classical statistics tools (such as Student's t-test , ANOVA or mixed models) and only these with sufficient small p-values are considered relevant. [ 36 ] However, correction strategies should be used to reduce false discoveries when multiple comparisons are conducted since there is no standard method for measuring the total amount of metabolites directly in untargeted metabolomics. [ 67 ] For multivariate analysis , models should always be validated to ensure that the results can be generalized.
Machine learning is a powerful tool that can be used in metabolomics analysis. Recently, scientists have developed retention time prediction software. These tools allow researchers to apply artificial intelligence to the retention time prediction of small molecules in complex mixture, such as human plasma, plant extracts, foods, or microbial cultures. Retention time prediction increases the identification rate in liquid chromatography and can lead to an improved biological interpretation of metabolomics data. [ 68 ]
Toxicity assessment/ toxicology by metabolic profiling (especially of urine or blood plasma samples) detects the physiological changes caused by toxic insult of a chemical (or mixture of chemicals). In many cases, the observed changes can be related to specific syndromes, e.g. a specific lesion in liver or kidney. This is of particular relevance to pharmaceutical companies wanting to test the toxicity of potential drug candidates: if a compound can be eliminated before it reaches clinical trials on the grounds of adverse toxicity, it saves the enormous expense of the trials. [ 48 ]
For functional genomics , metabolomics can be an excellent tool for determining the phenotype caused by a genetic manipulation, such as gene deletion or insertion. Sometimes this can be a sufficient goal in itself—for instance, to detect any phenotypic changes in a genetically modified plant intended for human or animal consumption. More exciting is the prospect of predicting the function of unknown genes by comparison with the metabolic perturbations caused by deletion/insertion of known genes. Such advances are most likely to come from model organisms such as Saccharomyces cerevisiae and Arabidopsis thaliana . The Cravatt laboratory at the Scripps Research Institute has recently applied this technology to mammalian systems, identifying the N -acyltaurines as previously uncharacterized endogenous substrates for the enzyme fatty acid amide hydrolase (FAAH) and the monoalkylglycerol ethers (MAGEs) as endogenous substrates for the uncharacterized hydrolase KIAA1363 . [ 69 ] [ 70 ]
Metabologenomics is a novel approach to integrate metabolomics and genomics data by correlating microbial-exported metabolites with predicted biosynthetic genes. [ 71 ] This bioinformatics-based pairing method enables natural product discovery at a larger-scale by refining non-targeted metabolomic analyses to identify small molecules with related biosynthesis and to focus on those that may not have previously well known structures.
Fluxomics is a further development of metabolomics. The disadvantage of metabolomics is that it only provides the user with abundances or concentrations of metabolites, while fluxomics determines the reaction rates of metabolic reactions and can trace metabolites in a biological system over time.
Nutrigenomics is a generalised term which links genomics, transcriptomics, proteomics and metabolomics to human nutrition. In general, in a given body fluid, a metabolome is influenced by endogenous factors such as age, sex, body composition and genetics as well as underlying pathologies. The large bowel microflora are also a very significant potential confounder of metabolic profiles and could be classified as either an endogenous or exogenous factor. The main exogenous factors are diet and drugs. Diet can then be broken down to nutrients and non-nutrients. Metabolomics is one means to determine a biological endpoint, or metabolic fingerprint, which reflects the balance of all these forces on an individual's metabolism. [ 72 ] [ 73 ] Thanks to recent cost reductions, metabolomics has now become accessible for companion animals, such as pregnant dogs. [ 74 ] [ 75 ]
Volatolomics is a development of metabolomics that studies volatile organic compounds (VOCs) emitted by a biological systems .
Plant metabolomics is designed to study the overall changes in metabolites of plant samples and then conduct deep data mining and chemometric analysis. Specialized metabolites are considered components of plant defense systems biosynthesized in response to biotic and abiotic stresses. [ 76 ] Metabolomics approaches have recently been used to assess the natural variance in metabolite content between individual plants, an approach with great potential for the improvement of the compositional quality of crops. [ 77 ] | https://en.wikipedia.org/wiki/Metabolomics |
In biochemistry , a metabolon is a temporary structural-functional complex formed between sequential enzymes of a metabolic pathway , held together both by non-covalent interactions and by structural elements of the cell, such as integral membrane proteins and proteins of the cytoskeleton .
The formation of metabolons allows the intermediate product from one enzyme to be passed (channelling) directly into the active site of the next consecutive enzyme of the metabolic pathway. The citric acid cycle is an example of a metabolon that facilitates substrate channeling. [ 1 ] [ 2 ] Another example is the dhurrin synthesis pathway in sorghum, in which the enzymes assemble as a metabolon in lipid membranes. [ 3 ] During the functioning of metabolons, the amount of water needed to hydrate the enzymes is reduced and enzyme activity is increased [ citation needed ] .
The concept of structural-metabolic cellular complexes was first conceived in 1970 by A. M. Kuzin of the USSR Academy of Sciences, [ 4 ] and adopted in 1972 by Paul A. Srere of the University of Texas for the enzymes of the citric acid cycle . [ 5 ] This hypothesis was well accepted in the former USSR and further developed for the complex of glycolytic enzymes (Embden-Meyerhof-Parnas pathway) by B.I. Kurganov and A.E. Lyubarev. [ 6 ] [ 7 ] [ 8 ] [ 9 ] In the mid-1970s, the group of F.M. Clarke at the University of Queensland , Australia also worked on the concept. [ 10 ] [ 11 ] The name "metabolon" was first proposed in 1985 by Paul Srere [ 12 ] during a lecture in Debrecen, Hungary. [ 13 ]
In Chaetomium thermophilum , a complex of a metabolon exists between fatty acid synthase and a MDa carboxylase, [ 14 ] and was observed using chemical cross-linking coupled to mass spectrometry and visualized by cryo-electron microscopy . The Fatty acid synthesis metabolon in C. thermophilum is highly flexible, and although a high-resolution structure of Fatty acid synthase was possible, the metabolon was highly flexible, hindering high-resolution structure determination. [ citation needed ] | https://en.wikipedia.org/wiki/Metabolon |
A metaborate is a borate anion consisting of boron and oxygen , with empirical formula BO − 2 . Metaborate also refers to any salt or ester of such anion (e.g. salts such as sodium metaborate NaBO 2 or calcium metaborate Ca(BO 2 ) 2 , and esters such as methyl metaborate CH 3 BO 2 ). Metaborate is one of the boron's oxyanions . Metaborates can be monomeric , oligomeric or polymeric .
In aqueous solutions metaborate anion hydrolyzes to tetrahydroxyborate [B(OH) 4 ] − . For this reason, solutions or hydrated salts of the latter are often improperly named "metaborates".
In the solid state of their salts , metaborate ions are often oligomeric or polymeric , conceptually resulting from the fusion of two or more BO − 2 through shared oxygen atoms. In these anions, the boron atom forms covalent bonds with either three or four oxygen atoms. Some of the structures are:
The cyclic trimer anions dissociate almost completely in aqueous solution giving mainly tetrahydroxyborate anions: [ 13 ] [ 14 ]
Other molecules and anions, such as B(OH) 3 , [B 3 O 3 (OH) 4 ] − , [B 3 O 3 (OH) 5 ] 2− , and B 4 O 5 (OH) 2− 4 are less than 5% at 26 °C. [ 15 ] [ 16 ]
In 1937, Nielsen and Ward claimed that the metaborate anion in solution has a linear symmetric structure − O−B + −O − with negative charges on the oxygens and a positive charge on the boron, or O=B − =O with negative charge on the boron. [ 17 ] However, this claim has been disproved.
The vapor of cesium metaborate has neutral monomers CsBO 2 and dimers Cs 2 [B 2 O 4 ] as well as ionized versions thereof. [ 18 ] The same situation holds for thallium metaborate TlBO 2 . [ 19 ]
In 1964 Hisatsune and Surez investigated the infrared spectrum of metaborate anions in dilute solid solutions of potassium salt in alkali halides such as potassium chloride KCl. [ 20 ] | https://en.wikipedia.org/wiki/Metaborate |
A metabotropic receptor , also referred to by the broader term G-protein-coupled receptor , [ 1 ] is a type of membrane receptor that initiates a number of metabolic steps to modulate cell activity. The nervous system utilizes two types of receptors : metabotropic and ionotropic receptors. While ionotropic receptors form an ion channel pore , metabotropic receptors are indirectly linked with ion channels through signal transduction mechanisms, such as G proteins .
Both receptor types are activated by specific chemical ligands . When an ionotropic receptor is activated, it opens a channel that allows ions such as Na + , K + , or Cl − to flow. In contrast, when a metabotropic receptor is activated, a series of intracellular events are triggered that can also result in ion channels opening or other intracellular events, but involve a range of second messenger chemicals. [ 2 ]
Chemical messengers bind to metabotropic receptors to initiate a diversity of effects caused by biochemical signaling cascades . G protein-coupled receptors are all metabotropic receptors. When a ligand binds to a G protein-coupled receptor, a guanine nucleotide-binding protein , or G protein , activates a second messenger cascade which can alter gene transcription , regulate other proteins in the cell, release intracellular Ca 2+ , or directly affect ion channels on the membrane. [ 3 ] [ 4 ] These receptors can remain open from seconds to minutes and are associated with long-lasting effects, such as modifying synaptic strength and modulating short- and long-term synaptic plasticity. [ 5 ]
Metabotropic receptors have a diversity of ligands, including but not limited to: small molecule transmitters, monoamines , peptides , hormones , and even gases. [ 5 ] [ 6 ] [ 7 ] In comparison to fast-acting neurotransmitters , these ligands are not taken up again or degraded quickly. They can also enter the circulatory system to globalize a signal. [ 3 ] Most metabotropic ligands have unique receptors. Some examples include: metabotropic glutamate receptors , muscarinic acetylcholine receptors , GABA B receptors. [ 2 ] [ 8 ]
The G protein-coupled receptors have seven hydrophobic transmembrane domains. Most of them are monomeric proteins, although GABA B receptors require heterodimerization to function properly. The protein's N terminus is located on the extracellular side of the membrane and its C terminus is on the intracellular side. [ 2 ]
The 7 transmembrane spanning domains, with an external amino terminus, are often claimed as being alpha helix shaped, and the polypeptide chain is said to be composed of around 450–550 amino acids. | https://en.wikipedia.org/wiki/Metabotropic_receptor |
Metachirality is a stronger form of chirality .
It applies to objects or systems that are chiral (not identical to their mirror image ) and where, in addition, their mirror image has a symmetry group that differs from the symmetry group of the original object or system. [ 1 ]
Many familiar chiral objects, like the capital letter 'Z' embedded in the plane, are not metachiral.
The symmetry group of the capital letter 'Z' embedded in the plane consists of the identity transformation and a rotation over 180˚ (a half turn).
In this case, the mirror image has the same symmetry group.
In particular, asymmetric objects (that only have the identity transformation as symmetry, like a human hand) are not metachiral,
since the mirror image is also asymmetric.
In general, two-dimensional objects and bounded three-dimensional objects are not metachiral.
An example of a metachiral object is an infinite helical staircase .
A helix in 3D has a handedness (either left or right, like screw thread ), whereby it differs from its mirror image.
An infinite helical staircase, however, does have symmetries: screw operations , that is, a combination of a translation and a rotation .
The symmetry group of the mirror image of an infinite helical staircase also contains screw operations.
But they are of the opposite handedness and, hence,
the symmetry groups differ.
Note, however, that these symmetry groups are isomorphic .
Of the 219 space groups , 11 are metachiral.
A nice example of a metachiral spatial structure is the K 4 crystal , [ 2 ] also known as Triamond,
and featured in the Bamboozle mathematical artwork. [ 3 ]
This stereochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metachirality |
An ecological metacommunity is a set of interacting communities which are linked by the dispersal of multiple, potentially interacting species. [ 1 ] [ 2 ] [ 3 ] The term is derived from the field of community ecology , which is primarily concerned with patterns of species distribution, abundance and interactions. Metacommunity ecology combines the importance of local factors (environmental conditions, competition, predation) and regional factors (dispersal of individuals, immigration, emigration) to explain patterns of species distributions that happen in different spatial scales.
There are four theoretical frameworks, or unifying themes, that each detail specific mechanistic processes useful for predicting empirical community patterns. These are the patch dynamics , species sorting , source–sink dynamics (or mass effect) and neutral model frameworks. Patch dynamics models describe species composition among multiple, identical patches, such as islands. In this framework, species are able to persist on patches through tradeoffs in colonization ability and competitive ability, where less competitive species can disperse to unoccupied patches faster than they go extinct in others. [ 4 ] Species sorting models describe variation in abundance and composition within the metacommunity due to individual species responses to environmental heterogeneity , such that certain local conditions may favor certain species and not others. Under this perspective, species are able to persist in patches with suitable environmental conditions resulting in a strong correlation between local species composition and the environment. This model represents the classical theories of the niche-centric era of G. Evelyn Hutchinson and Robert MacArthur . Source-sink models describe a framework in which dispersal and environmental heterogeneity interact to determine local and regional abundance and composition. This framework is derived from the metapopulation ecology term describing source–sink dynamics at the population level. High levels of dispersal among habitat patches allow populations to be maintained in environments that are normally outside the species environmental range. Finally, the neutral perspective describes a framework where species are essentially equivalent in their competitive and dispersal abilities, and local and regional composition and abundance are determined primarily by stochastic demographic processes and dispersal limitation. [ 5 ] The neutral perspective was recently popularized by Stephen P. Hubbell following his groundbreaking work on the unified neutral theory of biodiversity. | https://en.wikipedia.org/wiki/Metacommunity |
Metacresol purple or m -cresol purple , also called m -cresolsulfonphthalein , is a triarylmethane dye and a pH indicator . It is used as a capnographic indicator for detecting detect end-tidal carbon dioxide to ensure successful tracheal intubation in an emergency. [ 1 ] [ 2 ] It can be used to measure the pH in subzero temperatures of saline or hypersaline media. [ 3 ]
In colorimetric capnography , the indicator is incorporated in an aqueous matrix that provides a pH just above the indicator's colour change. [ 4 ] When exposed to carbon dioxide (CO 2 ), it undergoes a colour change from purple to yellow, because when CO 2 dissolves in the matrix, it forms carbonic acid . [ 5 ]
In chemistry, it has two useful indicator ranges: [ 2 ]
This article about an organic compound is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metacresol_purple |
Metadata controller (or MDC) is a storage area network (SAN) technology for managing file locking , space allocation and data access authorization.
This is needed when several clients are given block level access to the same disk volume, data storage sharing.
MDCs are only used on high-end servers. These are never found on user computers. In the absence of MDC over a SAN there is no possible way of ensuring privacy of the stored data. This controller can also play its role as a sharing device in case the administrators allow other servers to access certain blocks in a particular SAN. The access granted to the servers is of different levels. Some times it may happen that the server is not able to see a block or make changes in it in case of a locked file. This is caused by grant of low level access. If different clients on SAN happen to know each other, access may be granted to shift a certain block from one server to another. This allows the recipient server to use the block and make changes in it.
MDCs work as enzymes . They require certain types of SANs and networks to work properly. If a controller is connected to the right network it will boost its output. In case of wrong connection i.e. with the incorrect network, it will decrease its performance.
This computer-storage -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metadata_controller |
Metadynamics (MTD; also abbreviated as METAD or MetaD) is a computer simulation method in computational physics , chemistry and biology . It is used to estimate the free energy and other state functions of a system , where ergodicity is hindered by the form of the system's energy landscape . It was first suggested by Alessandro Laio and Michele Parrinello in 2002 [ 1 ] and is usually applied within molecular dynamics simulations. MTD closely resembles a number of newer methods such as adaptively biased molecular dynamics, [ 2 ] adaptive reaction coordinate forces [ 3 ] and local elevation umbrella sampling. [ 4 ] More recently, both the original and well-tempered metadynamics [ 5 ] were derived in the context of importance sampling and shown to be a special case of the adaptive biasing potential setting. [ 6 ] MTD is related to the Wang–Landau sampling. [ 7 ]
The technique builds on a large number of related methods including (in a chronological order) the
deflation, [ 8 ] tunneling, [ 9 ] tabu search , [ 10 ] local elevation , [ 11 ] conformational flooding, [ 12 ] Engkvist-Karlström [ 13 ] and Adaptive Biasing Force methods. [ 14 ]
Metadynamics has been informally described as "filling the free energy wells with computational sand". [ 15 ] The algorithm assumes that the system can be described by a few collective variables (CV). During the simulation, the location of the system in the space determined by the collective variables is calculated and a positive Gaussian potential is added to the real energy landscape of the system. In this way the system is discouraged to come back to the previous point. During the evolution of the simulation, more and more Gaussians sum up, thus discouraging more and more the system to go back to its previous steps, until the system explores the full energy landscape—at this point the modified free energy becomes a constant as a function of the collective variables which is the reason for the collective variables to start fluctuating heavily. At this point the energy landscape can be recovered as the opposite of the sum of all Gaussians.
The time interval between the addition of two Gaussian functions, as well as the Gaussian height and Gaussian width, are tuned to optimize the ratio between accuracy and computational cost. By simply changing the size of the Gaussian, metadynamics can be fitted to yield very quickly a rough map of the energy landscape by using large Gaussians, or can be used for a finer grained description by using smaller Gaussians. [ 1 ] Usually, the well-tempered metadynamics [ 5 ] is used to change the Gaussian size adaptively. Also, the Gaussian width can be adapted with the adaptive Gaussian metadynamics. [ 16 ]
Metadynamics has the advantage, upon methods like adaptive umbrella sampling , of not requiring an initial estimate of the energy landscape to explore. [ 1 ] However, it is not trivial to choose proper collective variables for a complex simulation. Typically, it requires several trials to find a good set of collective variables, but there are several automatic procedures proposed: essential coordinates , [ 17 ] Sketch-Map , [ 18 ] and non-linear data-driven collective variables. [ 19 ]
Independent metadynamics simulations (replicas) can be coupled together to improve usability and parallel performance. There are several such methods proposed: the multiple walker MTD, [ 20 ] the parallel tempering MTD, [ 21 ] the bias-exchange MTD, [ 22 ] and the collective-variable tempering MTD. [ 23 ] The last three are similar to the parallel tempering method and use replica exchanges to improve sampling. Typically, the Metropolis–Hastings algorithm is used for replica exchanges, but the infinite swapping [ 24 ] and Suwa-Todo [ 25 ] algorithms give better replica exchange rates. [ 26 ]
Typical (single-replica) MTD simulations can include up to 3 CVs, even using the multi-replica approach, it is hard to exceed 8 CVs in practice. This limitation comes from the bias potential, constructed by adding Gaussian functions (kernels). It is a special case of the kernel density estimator (KDE). The number of required kernels, for a constant KDE accuracy, increases exponentially with the number of dimensions. So MTD simulation length has to increase exponentially with the number of CVs to maintain the same accuracy of the bias potential. Also, the bias potential, for fast evaluation, is typically approximated with a regular grid . [ 27 ] The required memory to store the grid increases exponentially with the number of dimensions (CVs) too.
A high-dimensional generalization of metadynamics is NN2B. [ 28 ] It is based on two machine learning algorithms: the nearest-neighbor density estimator (NNDE) and the artificial neural network (ANN). NNDE replaces KDE to estimate the updates of bias potential from short biased simulations, while ANN is used to approximate the resulting bias potential. ANN is a memory-efficient representation of high-dimensional functions, where derivatives (biasing forces) are effectively computed with the backpropagation algorithm. [ 28 ] [ 29 ]
An alternative method, exploiting ANN for the adaptive bias potential, uses mean potential forces for the estimation. [ 30 ] This method is also a high-dimensional generalization of the Adaptive Biasing Force (ABF) method. [ 31 ] Additionally, the training of ANN is improved using Bayesian regularization, [ 32 ] and the error of approximation can be inferred by training an ensemble of ANNs. [ 30 ]
In 2015, White, Dama, and Voth introduced experiment-directed metadynamics, a method that allows for shaping molecular dynamics simulations to match a desired free energy surface . This technique guides the simulation towards conformations that align with experimental data, enhancing our understanding of complex molecular systems and their behavior. [ 33 ]
In 2020, an evolution of metadynamics was proposed, the on-the-fly probability enhanced sampling method (OPES), [ 34 ] [ 35 ] [ 36 ] which is now the method of choice of Michele Parrinello 's research group. [ 37 ] The OPES method has only a few robust parameters, converges faster than metadynamics, and has a straightforward reweighting scheme. [ 38 ] In 2024, a replica-exchange variant of OPES was developed, named OneOPES, [ 39 ] designed to exploit a thermal gradient and multiple CVs to sample large biochemical systems with several degrees of freedom. This variant aims to address the challenge of describing such systems, where the numerous degrees of freedom are often difficult to capture with only a few CVs. OPES has been implemented in the PLUMED library since version 2.7. [ 40 ]
Assume we have a classical N {\textstyle N} -particle system with positions at { r → i } {\textstyle \{{\vec {r}}_{i}\}} ( i ∈ 1... N ) {\textstyle (i\in 1...N)} in the Cartesian coordinates ( r → i ∈ R 3 ) {\textstyle ({\vec {r}}_{i}\in \mathbb {R} ^{3})} . The particle interaction are described with a potential function V ≡ V ( { r → i } ) {\textstyle V\equiv V(\{{\vec {r}}_{i}\})} . The potential function form (e.g. two local minima separated by a high-energy barrier) prevents an ergodic sampling with molecular dynamics or Monte Carlo methods.
A general idea of MTD is to enhance the system sampling by discouraging revisiting of sampled states. It is achieved by augmenting the system Hamiltonian H {\textstyle H} with a bias potential V bias {\displaystyle V_{\text{bias}}} :
The bias potential is a function of collective variables ( V bias ≡ V bias ( s → ) ) {\textstyle (V_{\text{bias}}\equiv V_{\text{bias}}({\vec {s}}\,))} . A collective variable is a function of the particle positions ( s → ≡ s → ( { r → i } ) ) {\displaystyle ({\vec {s}}\equiv {\vec {s}}(\{{\vec {r}}_{i}\}))} . The bias potential is continuously updated by adding bias at rate ω {\displaystyle \omega } , where s → t {\displaystyle {\vec {s}}_{t}} is an instantaneous collective variable value at time t {\displaystyle t} :
At infinitely long simulation time t sim {\displaystyle t_{\text{sim}}} , the accumulated bias potential converges to free energy with opposite sign (and irrelevant constant C {\displaystyle C} ):
For a computationally efficient implementation, the update process is discretised into τ {\displaystyle \tau } time intervals ( ⌊ ⌋ {\displaystyle \lfloor \;\rfloor } denotes the floor function ) and δ {\displaystyle \delta } -function is replaced with a localized positive kernel function K {\displaystyle K} . The bias potential becomes a sum of the kernel functions centred at the instantaneous collective variable values s → j {\displaystyle {\vec {s}}_{j}} at time τ j {\displaystyle \tau j} :
Typically, the kernel is a multi-dimensional Gaussian function , whose covariance matrix has diagonal non-zero elements only:
The parameter τ {\displaystyle \tau } , ω {\displaystyle \omega } , and σ → {\displaystyle {\vec {\sigma }}} are determined a priori and kept constant during the simulation.
Below there is a pseudocode of MTD base on molecular dynamics (MD), where { r → } {\displaystyle \{{\vec {r}}\}} and { v → } {\displaystyle \{{\vec {v}}\}} are the N {\displaystyle N} -particle system positions and velocities, respectively. The bias V bias {\displaystyle V_{\text{bias}}} is updated every n = τ / Δ t {\displaystyle n=\tau /\Delta t} MD steps, and its contribution to the system forces { F → } {\displaystyle \{{\vec {F}}\,\}} is { F → bias } {\displaystyle \{{\vec {F}}_{\text{bias}}\}} .
The finite size of the kernel makes the bias potential to fluctuate around a mean value. A converged free energy can be obtained by averaging the bias potential. The averaging is started from t diff {\displaystyle t_{\text{diff}}} , when the motion along the collective variable becomes diffusive:
Metadynamics has been used to study:
PLUMED [ 47 ] is an open-source library implementing many MTD algorithms and collective variables . It has a flexible object-oriented design [ 48 ] [ 49 ] and can be interfaced with several MD programs ( AMBER , GROMACS , LAMMPS , NAMD , Quantum ESPRESSO , DL_POLY_4, CP2K , and OpenMM). [ 50 ] [ 51 ]
Other MTD implementations exist in the Collective Variables Module [ 52 ] (for LAMMPS , NAMD , and GROMACS ), ORAC , CP2K , [ 53 ] EDM, [ 54 ] and Desmond . | https://en.wikipedia.org/wiki/Metadynamics |
A metagame , broadly defined as "a game beyond the game", typically refers to either of two concepts: a game which revolves around a core game; or the strategies and approaches to playing a game. [ 1 ] A metagame can serve a broad range of purposes, and may be tied to the way a game relates to various aspects of life. [ 2 ] : 2,14 [ 3 ]
In competitive games, the metagame can refer to the most popular strategy, often called a game's meta, or preparation for a match in general. [ 4 ]
In tabletop role-playing games , metagaming has been used to describe players discussing the game, sometimes simply rules discussions and other times causing the characters they control to act in ways they normally would not within the story. [ 5 ]
The word metagame is composed of the Greek -derived prefix meta – (from μετά, meta, meaning "after") and the noun game . [ 4 ] Metagame was used in the context of playing zero-sum games in a publication by the Mental Health Research Institute in 1956. [ 6 ] It is alternately claimed that the first known use of the term was in Nigel Howard's book Paradoxes of Rationality: Theory of Metagames and Political Behavior published in 1971, where Howard used the term in his analysis of the Cold War political landscape using a variation of the Prisoner's Dilemma ., [ 2 ] : 10 however Howard used the term in Metagame Analysis in Political Problems published in 1966. [ 7 ] In 1967, the word appeared in a study by Russell Lincoln Ackoff [ 8 ] and in the Bulletin of the Operations Research Society of America. [ 9 ]
In casual gaming , the metagame generally refers to any meaningful interaction between players and elements not directly part of the game. [ 2 ] [ 4 ] The concept gained traction in game design in a column written in 1995 by Richard Garfield , the creator of Magic: The Gathering , for The Duelist . In a 2000 talk at the Game Developers Conference , Garfield expanded on this, defining metagame as "how a game interfaces beyond itself", and asserted that this can include "what you bring to a game, what you take away from a game, what happens between games, [and] what happens during a game". [ 2 ] : 14 [ 3 ] Stephanie Boluk and Patrick Lemieux extend and refine Garfield's term to apply to potentially all forms of play and gaming, arguing that metagames are often more important than video games themselves. [ 2 ] : 8 They go on to describe that metagaming "results from the entanglement of philosophical concepts, the craft of game design, and the cultures of play that surrounds videogames." [ 2 ] : 21
In the world of competitive games , rule imprecisions and non-goal oriented play are not commonplace. As a result, the extent of metagaming narrows down mostly to studying strategies of top players and exploiting commonly-used strategies for an advantage. [ 4 ] Those may evolve as updates are released or new, better, strategies are discovered by top players. [ 10 ] The opposite metagame of playing a relatively unknown strategy for surprisal is often called off-meta . [ 4 ] [ better source needed ]
This usage is particularly common in games that have large, organized play systems or tournament circuits. Some examples of this kind of environment are tournament scenes for tabletop or computer collectible card games like Magic: The Gathering , Gwent: The Witcher Card Game or Hearthstone , tabletop war-gaming such as Warhammer 40,000 and Flames of War , or team-based multiplayer online games such as Star Conflict , Dota 2 , League of Legends , and Team Fortress 2 . In some games, such as Heroes of the Storm , varied level design makes the battleground a significant factor in the metagame. [ 10 ]
The meta in these environments is often affected by new elements added by the game's developers and publishers, such as new card expansions in card games, or adjustments to character abilities in online games. [ 11 ] The metagame may also come within player communities as reactions to win over currently-popular strategies, creating ebbs and flows of strategy types over time.
In competitive games, more pervasive forms of metagaming like teaming in free-for-all multiplayer games can be interpreted as cheating or as bad sportsmanship. [ 12 ] [ 2 ] [ 13 ] Writer Richard Garfield's book, Lost in the Shuffle: Games Within Games , considers instead teaming as just a form of metagaming. [ 14 ] The practice of losing individual games to dodge stronger opponents in tournaments has also been interpreted as a form of metagaming, [ 4 ] sometimes considered as unfair. [ 12 ] [ 2 ]
Exploiting the meta is commonplace in esports . [ 4 ] In StarCraft , a player's previous matches with the same opponent have given them insight into that player's play style and may cause them to make certain decisions which would otherwise seem inferior. Another instance of using the meta in esports was in 2012 at The International , a Dota 2 competition, when one team was able to exploit "predictable, economical strategies and that summer's metagame, the in-game decisions and team configurations that were fashionable" to counter a play by the other team. [ 2 ] : 215
In fighting games , the meta is also played through character selection. The opposing character has various strengths that can be avoided and weaknesses that can be exploited more easily depending on the character you choose provided you are aware of those strengths and weaknesses (called a "match up"). For a basic example, a character with a projectile attack has the advantage over a grappler who must be close to the opponent to be effective. Match up metagaming is very important in tournament settings. In recent fighting games, blind select has been implemented for online modes. This makes it so that neither player can see what character the other player chose. In tournaments, players have the option to opt for a blind select where they tell a judge in confidence the character they intend to select in the match, making their character choice mandatory. A newer trend in more recently released titles is to allow the selection of multiple characters at once which the player can then switch between on the fly, rendering match-up picking excessively hard and virtually impractical.
In popular trading card games , such as Magic: The Gathering , Pokémon Trading Card Game , or Yu-Gi-Oh! Trading Card Game players compete with decks they have created in advance and the meta consists of the deck types that are currently popular and expected to show up in large numbers in a tournament. The knowledge of metagame trends can give players an edge against other participants, both while they are playing by quickly recognizing what kind of deck opponents have and guessing their likely cards or moves, and during the deck building process, by selecting cards that do well against current popular deck types at the possible expense of performance against rarer ones. Another example of metagaming would be bluffing opponents into expecting cards that you do not have, or surprising the competition with novel decks that they may not be prepared for. The secondary market of cards is heavily influenced by metagame trends: cards become more valuable when they are popular, often to the point of scarcity. [ 15 ]
Rogue-likes often gate content behind completion of basic runs, usually to convey a sense of progression and/or not to inundate less experienced players with too many choices. This is usually referred to as the "metaprogression" of the game. [ 16 ] Most roguelikes present this in one form or another, such as in FTL: Faster Than Light , Inscryption , Slay the Spire , and Rogue Legacy .
The chess metagame has developed over time to include particularly effective opening moves and reactions to them. However, since the game rules themselves are static, the metagame does not evolve in the same ways it does for games where the rules are regularly updated by developers. [ 17 ]
As in other games, chess can also involve a metagame in which players use their knowledge of how the game is being played in a larger competitive context, beyond the rules of the game itself. Researchers identified a set of games in which players may have colluded in early rounds of chess tournaments to obfuscate player strength for a matchmaking advantage in later rounds. [ 18 ]
In tabletop role-playing games , metagaming can refer to aspects of play that occur outside of a given game's fictional universe . In particular, metagaming often refers to having an in-game character act on knowledge that the player has access to, but the character should not. For example, having a character bring a mirror to defeat Medusa when they are unaware her gaze can petrify them, or being more cautious when the game is run by a merciless gamemaster .
Some consider metagaming to benefit oneself bad sportsmanship. [ 5 ] [ 19 ] It is frowned upon in many role-playing communities, as it upsets suspension of disbelief , and affects game balance . [ 20 ] [ 21 ] However, some narrativist indie role-playing games deliberately support metagaming and encourage shared storytelling among players. [ 20 ] [ 21 ] [ 22 ]
The metagame for game developers refers to the extra set of rules and logic that are independent of the core gameplay. This can involve extra progressions or an economic market appended to the core gameplay that add mid- and long-term goals for players. Some researchers argue that having a metagame for players can increase user engagement with those games. [ 23 ] | https://en.wikipedia.org/wiki/Metagame |
Metagame analysis involves framing a problem situation as a strategic game in which participants try to realise their objectives by means of the options available to them. The subsequent meta-analysis of this game gives insight in possible strategies and their outcome.
Metagame theory was developed by Nigel Howard in the 1960s as a reconstruction of mathematical game theory on a non-quantitative basis, hoping that it would thereby make more practical and intuitive sense ( Howard 1971 , pp. xi). Metagame analysis reflects on a problem in terms of decision issues, and stakeholders who may exert different options to gain control over these issues. The analysis reveals what likely scenarios exist, and who has the power to control the course of events. The practical application of metagame theory is based on the analysis of options method, first applied to study problems like the strategic arms race and nuclear proliferation .
Metagame analysis proceeds in three phases: analysis of options, scenario development, and scenario analysis.
The first phase of analysis of options consists of the following four steps:
The dependencies between options should typically be formulated as "option X can only be implemented if option Y is also implemented", or "options Y and Z are mutually exclusive". The result is a metagame model, which can then be analysed in different ways.
The possible outcomes of the game, based on the combination of options, are called scenarios . In theory, a game with N stakeholders s 1 , ..., s N who have Oi options (i = 1, ..., N), there are O 1 ×...×O N possible outcomes. As the number of stakeholders and the number of the options they have increase, the number of scenarios will increase steeply due to a combinatorial explosion . Conversely, the dependencies between options will reduce the number of scenarios, because they rule out those containing logically or physically impossible combinations of options.
If the set of feasible scenarios is too large to be analysed in full, some combinations may be eliminated because the analyst judges them to be not worth considering. When doing so, the analyst should take care to preserve these particular types of scenarios ( Howard 1989 , pp. 243 ff):
The next step in the metagame analysis consists of the actual analysis of the scenarios generated so far. This analysis centres around stability and is broken down in the following four steps ( Howard 1989 , pp. 248–255):
This analysis procedure shows that the credibility of threats and promises (sanctions and improvements) is of importance in metagame analysis. A threat or promise, one that the stakeholder prefers to carry out for its own sake, is inherently credible. Sometimes a stakeholder may want to make credible an 'involuntary' threat or promise, to use this to move the situation in the desired direction. Such threats and promises can be made credible in three basic ways: preference change, irrationality, and deceit ( Howard 1989 , pp. 257).
Metagame analysis is still used as a technique in its own right. However it has been further developed in distinct ways as the basis of more recent approaches: | https://en.wikipedia.org/wiki/Metagame_analysis |
The word metagenics uses the prefix meta and the suffix gen . Literally, it means "the creation of something which creates". In the context of biotechnology, metagenics is the practice of engineering organisms to create a specific enzyme, protein, or other biochemicals from simpler starting materials. The genetic engineering of E. coli with the specific task of producing human insulin from starting amino acids is an example. E. coli has also been engineered to digest plant biomass and use it to produce hydrocarbons in order to synthesize biofuels. The applications of metagenics on E. coli also include higher alcohols, fatty-acid based chemicals and terpenes. [ 1 ]
The depletion of petroleum sources and increase in greenhouse gas emissions in the twenty and twenty-first centuries has been the driving factor behind the development of biofuels from microorganisms. E. coli is currently regarded as the best option for biofuel production because of the amount of knowledge available about its genome. The process converts biomass into fuels, and has proven successful on an industrial scale, with the United States having produced 6.4 billion gallons of bioethanol in 2007. Bioethenol is currently the front-runner for alternative fuel production and uses S.cerevisiae and Zymomonas mobilis to create ethanol through fermentation. However, maximum productivity is limited due to the fact that these organisms cannot use pentose sugars, leading to consideration of E.coli and Clostridia. E.coli is capable of producing ethanol under anaerobic conditions through metabolizing glucose into two moles of formate, two moles of acetate, and one mole of ethanol. While bioethanol has proved to be a successful alternative fuel source on an industrial scale, it also has its shortcomings, namely, its low energy density, high vapor pressure, and hygroscopicity . Current alternatives to bioethanol include biobutanol, biodiesel, propanol, and synthetic hydrocarbons. [ 2 ] The most common form of biodiesels is fatty acid methyl esters and current synthesis strategies involve transesterification of triacylglycerols from plant oils. However, plant oils have a major limitation in availability of oil-seed supplies at competitive prices, leading to an interest in direct synthesis of fatty acid methyl esters in bacteria. This process bypasses transesterification, leading to higher energy yields and lower production cost. [ 3 ] One of the principal obstacles in production of viable biofuels is that the maximum blend ratio of biofuel to petroleum is between 10% and 20%, Current biofuels are not compatible with high-performance, low-emission engines and costly changes in infrastructure and engine remodeling would be required. A University of Exeter study sought to overcome this obstacle through production of biofuels that can replace current fossil fuels through sustainable means, namely, the production of n -alkanes, iso -alkanes, and n -alkenes, as these are the hydrocarbons that compose current retail transport fuels. The study found suitable substrates for production of the aforementioned hydrocarbons by means of the P. luminescens fatty acid reductase (FAR) complex. [ 4 ] A study published in Biotechnology for Biofuels used S. cerevisiae to produce short- and branched-chain alkyl esters biodiesel through metabolic engineering. Negative regulators for the INO1 gene, Rpd3 and Opi1 were deleted to boost S. cerevisiae's ability to produce fatty acid esters. To increase the production of alcohol precursors, five isobutanol pathway enzymes were overexpressed. [ 5 ]
Increase in the demand for recombinant insulin can be explained by an increase in the number of diabetic patients globally, as well as alternative delivery methods such as inhalation and oral routes, which require higher doses. [ 6 ] Through the use of recombinant DNA technology, E. coli can be used for the production of human insulin. The biosynthesis of insulin within the human body confers a significant advantage over bovine or porcine synthesis, which are often immunogenic in diabetic patients. [ 7 ] To accomplish this, synthetic genes for human insulin are fused with the β-galactosidase gene of E.coli , where they undergo transcription and ultimately translation into proteins. [ 8 ] The limiting factor for the use of microorganisms like E. coli in biosynthesis of gene products like insulin is time, yet due to advancements in the synthesis of oligonucleotides and liquid chromatography, the production time needed for DNA fragments has greatly decreased. [ 9 ] Recombinant human insulin was first approved for clinical trials in 1980. At this time the A and B chains of insulin were produced separately and then chemically joined. [ 10 ] Joining of the two chains was often carried out through air oxidation with low efficiency. A 1978 study by Goedell et al. successfully accomplished correct joining of the A and B chains through S-sulfonated derivatives and an excess of the A chain, resulting in 50-80% correct joining. [ 8 ] Recent advances have allowed the chains to be synthesized together by inserting the human proinsulin gene into E. coli cells, which produce proinsulin through fermentation. [ 10 ] | https://en.wikipedia.org/wiki/Metagenics |
In chemistry, metal-catalysed hydroboration is a reaction used in organic synthesis . It is one of several examples of homogeneous catalysis .
In 1975, Kono and Ito reported that Wilkinson's catalyst (Rh(PPh 3 ) 3 Cl) can undergo oxidative addition with catecholborane (HBcat) or 4,4,6-trimethyl-1,3,2-dioxaborinane. [ 1 ] These two borane compounds are otherwise slow to participate in hydroboration. [ 2 ] [ 3 ] In 1985, Männig and Nöth demonstrated for the first time that Wilkinson's catalyst indeed catalyzes hydroboration of alkenes with HBcat.
Whereas uncatalyzed hydroboration using HBcat leads to reduction of the carbonyl group, the catalyzed version is selective for the alkene. [ 4 ]
As indicated by subsequent research, transition metal-catalyzed hydroboration proceeds with attractive functional group-, regio-, stereo-, and chemo- selectivity.
The rhodium-catalyzed hydroboration reaction is thought to be initiated with the dissociation of a triphenylphosphine from the Rh(I) centre. Oxidative addition of the B-H bond of the borane reagent to this 14 e − species is then followed by coordination of the alkene to the 16e − Rh(III) hydride complex. Subsequent migratory insertion of the alkene into the rhodium-hydride bond can give two regioisomeric alkyl rhodium(III) boride complexes. Reductive elimination of the boronate ester regenerates the catalyst. Catalyst prepared and handled under anaerobic condition reverses the selectivity to favor the secondary boronate ester. What has been debated is the coordination of the alkene. In the dissociative mechanism, proposed by Männig and Nöth, [ 4 ] and supported by Evans and Fu [ 5 ] the coordination is accompanied by the loss of one triphenylphosphine ligand.
In the associative mechanism (see below), proposed by Burgess et al., [ 6 ] the alkene binds trans to the chloride without dissociation of a triphenylphosphine ligand. The mechanism has been studied by computational methods. [ 7 ] [ 8 ] Dorigo and Schleyer excluded the associative mechanism by an ab initio study on the dissociative mechanism, [ 9 ] whereas Musaev and co-workers support the associative mechanism. [ 10 ]
Apart from the original evidence provided by Männig and Nöth, the total synthesis of (+)-ptilocaulin also demonstrates selective hydroboration of a terminal alkene in the presence of a ketone. [ 11 ]
In terms of regioselectivity, the catalyzed hydroboration differs from the uncatalyzed parallel. Depending on the ligands and the alkene, either Markovnikov or anti-Markovnikov product result. The difference in regioselectivity is more pronounced in the hydroboration of vinylarenes with HBcat. Wilkinson's catalyst or the cation Rh(COD) 2 (in the presence of PPh 3 ) produces the Markovnikov product. [ 12 ] [ 13 ] The anti-Markovnikov product is produced in the absence of a catalyst. [ 14 ] It is worth noticing that the use of RhCl 3 ·nH2O produces selectively the anti-Markovnikov product. [ 15 ] To account for the high regioselectivity of catalyzed hydroboration, Hayashi proposed a mechanism involving a η 3 -benzylrhodium complex. [ 12 ]
Catalyzed hydroboration-oxidation of substituted alkenes can be rendered enantioselective. In 1990, Brown and co-workers achieved asymmetric hydroboration using an achiral catalyst and chiral borane sources derived from ephedrine and pseudoephedrine. In most cases, the regioselectivity was poor although the ee values can be close to 90%. [ 16 ]
Use of a chiral catalyst and an achiral borane source is more common, e.g. chiral diphosphines such as BINAP . [ 17 ] [ 18 ] [ 19 ]
Styrene or its simple derivatives are usually the prochiral substrate. [ 20 ] [ 21 ] [ 22 ]
Enantioselectivity tends to be lowered with ortho -substituents on the aromatic ring, as well as further substitution on the olefin. Successful results have also been obtained on other reactants. [ 23 ] [ 24 ] The second class of ligands is phosphinamine ligands. In 1993, Brown first reported the successful use of QUINAP in asymmetric alkene hydroboration. [ 25 ] QUINAP improve upon the intolerance of substitution on the aromatic ring as observed for diphosphine ligands. Reactions using styrene and derivatives with electron-donating groups on the para position still gave high ee values. Similar results were also obtained on cyclic vinyl arenes. Such results expand the scope of asymmetric hydroboration to more sterically demanding alkenes. Several new ligands of this class have also been developed. Some recent results are summarized below. [ 26 ] [ 27 ] [ 28 ]
The studies above have all utilized oxidation of the boronate esters to produce alcohols, which is a severe limitation to the synthetic scope of such species, especially when they can be made enantioselectively. Another important class of compounds that can be derived from boronate esters is α-substituted benzylamines , some of which are commercially useful. The synthesis of such chiral amines via catalytic hydroboration involves conversion of the catecholboronate ester to trialkylborane by diethyl zinc or methylmagnesium chloride. Reaction of the trialkylborane with hydroxylamine-O-sulfonic acid produces primary benzylamines. [ 29 ] Secondary amines can also be prepared by in situ formation of N- chloramines . [ 30 ] | https://en.wikipedia.org/wiki/Metal-catalysed_hydroboration |
Metal-catalyzed cyclopropanations are chemical reactions that result in the formation of a cyclopropane ring from a metal carbenoid species and an alkene . [ 1 ] In the Simmons–Smith reaction the metal involved is zinc. Metal carbenoid species can be generated through the reaction of a diazo compound with a transition metal). The intramolecular variant of this reaction was first reported in 1961. [ 2 ] Rhodium carboxylate complexes, such as dirhodium tetraacetate , are common catalysts. Enantioselective cyclopropanations have been developed. [ 3 ]
Definitive mechanistic studies of rhodium-catalyzed cyclopropanation are lacking. However, the mechanism has been rationalized based on product distribution and stereoselectivity. [ 4 ] Attack of the diazo compound on the metal center generates a zwitterionic metal alkyl complex, which expels nitrogen gas to afford a metal carbene intermediate. Concerted addition of the metal carbene to the olefin (without direct coordination of the olefin to the metal) generates the observed cyclopropane product. [ 5 ] The configuration of the olefin is retained throughout the process; [ 6 ] however, metal carbenes with heterotopic faces may generate a mixture of diastereomers, as shown at the right of Eq. (2).
(2)
The configuration of the product is determined by the trajectory of approach of the olefin to the metal carbene. In reactions of monosubstituted metal carbenes with terminal olefins, the olefin likely approaches "end-on" (with the carbon-carbon double bond of the olefin nearly parallel to the metal-carbon double bond of the carbene) with the olefin R group pointed away from the substituent of the carbene. [ 7 ] A second transition state model has been proposed for reactions of vinyl-substituted carbenes. In this model, the olefin approaches "side-on" (with the carbon-carbon double bond of the olefin perpendicular to the metal-carbon double bond of the carbene) with the olefin R group far from the vinyl group. [ 8 ]
Methods for the stereoselective synthesis of cyclopropanes from diazocarbonyl compounds and olefins have relied on either the use of pre-formed chiral rhodium catalysts or chiral auxiliaries on the diazocarbonyl compound. For example, Rh 2 [ S -DOSP] 4 is a highly effective catalyst for the enantioselective cyclopropanation of alkenes. [ 9 ]
(3)
Chiral auxiliaries derived from readily available chiral alcohols (such as pantolactone) may be used for diastereoselective cyclopropanations with diazo esters. [ 10 ]
(4)
Cyclopropanation of olefins with diazocarbonyl compounds is commonly accomplished using rhodium carboxylate complexes, although copper was originally used. [ 11 ] The scope of the olefin is generally quite broad—electron-rich, [ 12 ] neutral, [ 13 ] and electron-poor [ 14 ] olefins have all been cyclopropanated efficiently using rhodium-based catalyst systems. This section describes the various classes of diazocarbonyl compounds that react with olefins under rhodium catalysis to afford cyclopropanes.
Diazoacetates that possess a single carbonyl substituent attached to the diazo carbon, have been used for the cyclopropanation of a wide array of olefins. Diastereoselectivity for the ( E ) cyclopropane increases as the size of the ester group increases. In addition, adding electron density to the catalyst (for instance by replacing acetate ligands with acetamide, acam) increases the diastereoselectivity of the reaction. [ 15 ]
(5)
Diazocarbonyl compounds substituted with two electron-withdrawing groups, such as diazomalonates, are prone to experience side reactions under cyclopropanation conditions. [3+2] Cycloaddition [ 16 ] and C-H insertion [ 17 ] side products have been observed.
(6)
Diazoacetates substituted with a vinyl or aryl group on the diazo carbon are unreactive towards trans -alkenes. This result has been explained by invoking the transition state model in Eq. (2). Reactions of these substrates are highly selective for the ( E ) cyclopropane isomer. [ 18 ]
(7)
Vinyl diazoacetates react with dienes to afford divinyl cyclopropanes, which undergo Cope rearrangement to afford cycloheptadienes. [ 19 ] The more substituted double bond of the diene reacts preferentially. [ 20 ]
(8)
(9)
Furans react similarly with vinyl diazoacetates, although the intermediate cyclopropane may transform either into the Cope rearrangement product or an opened unsaturated carbonyl compound. The distribution of these products is highly dependent on the substitution pattern of the furan. [ 21 ]
(10)
Pyrroles react with vinyl diazoacetates to form nitrogen-bridged cycloheptadienes. The use of methyl lactate as a chiral auxiliary on the vinyl diazoacetate led to moderate diastereoselectivity in the tandem cyclopropanation/Cope rearrangement of Boc-protected pyrrole. [ 22 ]
(11)
The enantioselectivity of asymmetric cyclopropanations may depend profoundly on the solvent. [ 23 ]
Enantioselective intermolecular cyclopropanation has been applied to the synthesis of the chiral cyclopropane antibiotics cilastatin. [ 24 ] (12) Tandem cyclopropanation/fragmentation is a key step in the synthesis of 12-hydroxyeicosatetraenoic acid. [ 25 ]
(12)
Simmons-Smith cyclopropanation , which employs carbenes derived from diiodomethane, is a popular alternative to rhodium-catalyzed cyclopropanation. In the presence of a chiral diamine, Simmons-Smith cyclopropanation is enantioselective; however, selectivities are not as high as the corresponding rhodium-catalyzed reactions. [ 26 ]
(13)
Substituted zinc carbenoids can be prepared from the corresponding ketones or aldehydes through a sequence analogous to the mechanism of the Clemmensen reduction . Cyclopropanation of olefins with these intermediates occurs with moderate diastereoselectivity and yield. [ 27 ]
(14)
Other diazo compounds besides diazocarbonyl compounds have been used for rhodium-catalyzed cyclopropanations; [ 28 ] however, these substrates are much more difficult to handle and unstable than diazocarbonyl compounds. Thus, they have not been extensively adopted for organic synthesis.
(15) | https://en.wikipedia.org/wiki/Metal-catalyzed_cyclopropanations |
A metal-centered cycloaddition is a subtype of the more general class of cycloaddition reactions . In such reactions "two or more unsaturated molecules unite directly to form a ring", [ 1 ] incorporating a metal bonded to one or more of the molecules. Cycloadditions involving metal centers are a staple of organic and organometallic chemistry, and are involved in many industrially-valuable synthetic processes.
There are two general types of metal-centered cycloaddition reactions: those in which the metal is incorporated into the cycle (a metallocycle ), and those in which the metal is external to the cycle. These can be further divided into "true" cycloadditions (those that take place in a concerted fashion), and formal cycloadditions (those that take place in a stepwise fashion). Beyond that, they are classified by the number of atoms contributed to the cycle by each of the participants.
For example, olefin metathesis using a Grubbs catalyst typically involves a reversible [2+2] cycloaddition . A Ruthenium alkylidene and an alkene (or alkyne ) react to form a metallocycle .
A common role for a metal centre in cycloaddition reactions is to exert control over the conformation of the reactants. Metal ions are frequently a component of 1,3-dipolar cycloadditions , and Diels-Alder reactions. A Lewis acidic can coerce a Diene into the reactive cisoid conformation, thereby catalyzing the reaction the Diels-Alder reaction. [ 2 ] [ 3 ]
A crucial role of the metal in many cycloadditions reactions is to bind simultaneously to the reactants. This brings them into close proximity and encourages them to cyclize. The ligands associated with the metal can direct the approach of the reactants, providing control over regiochemistry and stereochemistry .
Cycloadditions that require unstable synthons such as carbanions or carbenes are often possible using organometallic compounds. Several synthetic routes to cyclopropyl and cyclopropenyl compounds involve the cycloaddition of a metal carbene to an alkene or alkyne. [ 4 ] [ 5 ] [ 6 ] Metal-stabilized allyl and pentadienyl complexes are used in [4+3] and [5+2] cycloadditions for preparing seven-membered rings. [ 7 ]
Alkylidenes and other carbene analogs participate readily in cycloaddition reactions. Cycloaddition reactions of Ruthenium phosphinidenes with alkenes and alkynes is an active area research and has promise as catalytic cycle for hydrophosphination . [ 8 ] [ 9 ]
Underlying any attempt to explain cycloaddition reactions is Frontier Molecular Orbital Theory , which describes the interaction between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) of the reactants. A cycloaddition will only proceed if the HOMO and LUMO have an allowed symmetry and are similar in energy. Metals play a crucial role in cycloaddition reactions because they can bind to unsaturated molecules, changing the symmetries and energy levels of the HOMO and/or LUMO. The Woodward-Hoffmann rules and Green-Davies-Mingos rules can provide some indication of the effects of metal-bonding on cycloaddition reactions.
As an example, free Benzene is extremely unreactive in cycloadditions due to its aromaticity . Coordination of Benzene to a highly reduced Tricarbonylmanganese centre allows the Benzene to undergo cycloaddition with Diphenylketene . [ 10 ]
Although cyclobutadienes can only exist briefly in the free state, they can exist indefinitely as metal ligands. They can be formed as ligands in-situ by the [2+2] cycloaddition of sterically bulky alkynes bound to a metal. [ 11 ]
The Dötz reaction is a formal [3+2+1] cycloaddition of two alkynes, a carbene, and a carbonyl ligand to form a benzene ring. [ 12 ]
An unusual formal [5+4] cycloaddition was reported by Kreiter et al. [ 13 ] Nine-membered rings are unusual and only a handful of synthetic routes to rings of this size are known. | https://en.wikipedia.org/wiki/Metal-centered_cycloaddition_reactions |
Metal-coded affinity tag is a method used for quantitative proteomics by mass spectrometry that uses a metal chelate complex 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA) coupled to different lanthanide ions. [ 1 ] [ 2 ] The metal complexes attach to the cysteine residues of proteins in a sample.
For bottom-up proteomics , the proteins can be separated by two-dimensional gel electrophoresis and analyzed by matrix-assisted laser desorption/ionization (MALDI) or electrospray ionization mass spectrometry for relative quantification or by inductively coupled plasma mass spectrometry for absolute quantification. For top-down proteomics , the undigested labeled proteins are analyzed.
This article about analytical chemistry is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metal-coded_affinity_tag |
Combined with certain metallic species, amorphous films can crystallize in a process known as metal-induced crystallization (MIC). The effect was discovered in 1969, when amorphous germanium (a-Ge) films crystallized at surprisingly low temperatures when in contact with Al, Ag, Cu, or Sn. [ 1 ] The effect was also verified in amorphous silicon (a-Si) films, [ 2 ] as well as in amorphous carbon [ 3 ] and various metal-oxide films. [ 4 ]
Likewise, the MIC evolved from simple temperature-driven annealing approaches to others involving laser [ 5 ] [ 6 ] or microwave radiation, [ 7 ] [ 8 ] for example.
A very common variant of the MIC procedure is the metal-induced lateral crystallization (MILC). [ 9 ] In this case, the metal is deposited (onto the top or at the bottom) of some selected areas of the desired amorphous film. Upon annealing, crystallization starts from the portion of the amorphous film that is in contact with the metal species, and the MIC proceeds laterally.
So far, lots of studies have been carried out to investigate the MIC phenomenon -- invariably by applying different sample production methods and characterization tools. According to them, the MIC process is highly susceptible to the type and amount of the metallic species, the sample history (production method, geometry and annealing details), as well as to the methodology to determine crystallization. Besides, the MIC process is well beyond the mere diffusion of species (as it is usually discussed in studies involving layered sample structures) and involves many complex atomic-thermodynamic processes at the microscopic level. [ 10 ] | https://en.wikipedia.org/wiki/Metal-induced_crystallization |
Metal-induced embrittlement (MIE) is the embrittlement caused by diffusion of metal, either solid or liquid, into the base material. Metal induced embrittlement occurs when metals are in contact with low-melting point metals while under tensile stress. The embrittler can be either solid ( SMIE ) or liquid ( liquid metal embrittlement ). Under sufficient tensile stress , MIE failure occurs instantaneously at temperatures just above melting point. For temperatures below the melting temperature of the embrittler, solid-state diffusion is the main transport mechanism. [ 1 ] This occurs in the following ways:
The main mechanism of transport for SMIE is surface self-diffusion of the embrittler over a layer of embrittler that’s thick enough to be characterized as self-diffusion at the crack tip. [ 1 ] In comparison, LMIE dominant mechanism is bulk liquid flow that penetrates at the tips of cracks.
Studies have shown that Zn, Pb, Cd, Sn and In can embrittle steel at temperature below each embrittler’s melting point.
Similar to liquid metal embrittlement ( LME ), solid metal-induced embrittlement results in a decrease in fracture strength of a material. In addition, a decrease in tensile ductility over a temperature range is indicative of metal-induced embrittlement. Although SMIE is greatest just below the embrittler’s melting temperature, the range over which SMIE occurs ranges from 0.75 × T m to T m , where T m is the melting temperature of the embrittler. [ 4 ] The reduction in ductility is caused by formation and propagation of stable, subcritical intergranular cracks. SMIE produces both intergranular and transgranular fracture surfaces in otherwise ductile materials. [ 4 ]
Crack extension, as opposed to crack onset, is the rate determining step for solid induced-metal embrittlement. The main mechanism leading to solid metal induced embrittlement is multilayer surface self-diffusion of the embrittler at the crack tip. [ 1 ] [ 4 ] [ 5 ] Propagation rate of a crack undergoing metal-induced embrittlement is a function of the supply of embrittler present at the crack tip. Crack velocities in SMIE are much slower than LMIE velocities. [ 5 ] Catastrophic failure of a material via SMIE occurs as a result of the propagation of cracks to a critical point. To this end, the propagation of the crack is controlled by the transport rate and mechanisms of the embrittler at the tip of nucleated cracks. SMIE can be mitigated by increasing the tortuosity of crack paths such that resistance to intergranular cracking increases.
SMIE is less common that LMIE and much less common that other failure mechanisms such as hydrogen embrittlement , fatigue , and stress-corrosion cracking . Still, embrittlement mechanisms can be introduced during fabrication, coatings , testing or during service of the material components. Susceptibility for SMIE increases with the following material characteristics: | https://en.wikipedia.org/wiki/Metal-induced_embrittlement |
Metal-ligand cooperativity (MLC) is a mode of reactivity in which a metal and ligand of a complex are both involved in the bond breaking or bond formation of a substrate during the course of a reaction. This ligand is an actor ligand rather than a spectator , and the reaction is generally only deemed to contain MLC if the actor ligand is doing more than leaving to provide an open coordination site. MLC is also referred to as "metal-ligand bifunctional catalysis." Note that MLC is not to be confused with cooperative binding .
The earliest reported metal-ligand cooperativity was from the Fujiwara group in the 1950s, in which they reported formation of stilbene from styrene and arenes using a palladium chloride catalyst. [ 1 ] Shvo's catalyst was developed for one of the earliest uses of ketone hydrogenation by an outer-sphere mechanism . [ 2 ] Noyori has developed many chiral catalysts for asymmetric hydrogenation . [ 3 ] Transfer hydrogenation , one of the most commonly used applications of MLC, is employed broadly in industry for large scale Noyori-type reductions. [ 4 ] [ 5 ] [ 6 ] [ 7 ]
There are a variety of modes in which this cooperativity has been demonstrated. Four primary modes are generally accepted under MLC: the ligand can (1) act with Lewis acidity, (2) act with Lewis basicity, (3) play a role in aromatization and dearomatization, or (4) be redox non-innocent. [ 8 ]
The ligand can act as a Lewis acid and accept electrons from an incoming substrate as it binds to the metal, as in employed in dehydrogenation catalysis. Conversely, the ligand can be Lewis basic and bind the substrate; this Lewis basicity is most frequently seen in hydrogenation catalysis.
The aromatization and dearomatization of a ligand can serve to facilitate a reaction. As shown in the figure, a ligand can be dearomatized by a base and thus activated toward cleaving a C-H or H-H bond and be subsequently rearomatized during substrate bond cleavage. NHC ligands and other pincer ligands are frequently employed in this mode of MLC. [ 9 ] In some reports, with bidentate ligands, ligand dearomatization is not observed when the complex is treated with base but rather a complex with a formal metal-carbon bond is observed (that then acts as a Lewis basic ligand). [ 10 ] [ 11 ]
The ligand can also be redox non-innocent to facilitate reactions that the metal would otherwise be unable to activate. [ 8 ] The ligand can act as an electron reservoir, which is enabled when ligands contain frontier orbitals of suitable energy to participate in the redox event themselves, and can accept or donate electrons during the course of the reaction, allowing the metal to modulate its oxidation state. This allows metals which normally only participate in one electron regimes to be used in two electron regimes with a redox non-innocent ligand to store electrons during the reaction. Dithiolate ligands have been used extensively as one electron redox active ligands in metal complexes. [ 12 ] For example, dithiolates have been demonstrated to allow for the selective and reversible reduction of ethylene in the presence H 2 , CO, and H 2 S. This has applications in the purification of ethylene gas streams, in which ethylene can be reduced electrochemically by a dithiolate, selectively removed from the impurities in the stream, and then reversibly desaturated. [ 13 ]
Electrochemical metal-ligand cooperativity in redox reactions allows for ease of tuning the potential of the ligands to avoid off-target reactivity. [ 14 ]
There are a number of other ligand modes of reactivity which are sometimes classified under MLC. This includes reactions in which the ligand accepts or loses a proton, though not directly from or to the substrate. [ 15 ] Ligands can also be used to form stabilizing H-bonds , which can be applied in molecular recognition catalysis. [ 15 ] Ligands can also be designed to be photoresponsive, with applications in molecular switches . [ 15 ] Ligands may also be considered to be involved in MLC while acting only in the second coordination sphere (not directly bound to the metal) but acting as a proton shuttle. [ 16 ] Frustrated Lewis pairs , in which an ion pair of the type [R 3 B-H] − [H-Ar 3 ] + transfer a hydride and proton are also sometimes classified under MLC. [ 16 ]
MLC is most frequently used in hydrogenations, with many applications in asymmetric catalysis and in process scale production of chemicals. In a hydrogenation, there is a transfer of a hydride and a hydrogen to a substrate. Typical substrates include aldehydes, ketones, and imines. As this is a common use for MLC, it is instructive in understanding the mechanism of metal-ligand cooperativity. MLC occurs through an outer sphere mechanism. An outer sphere mechanism does not necessitate that the metal undergo oxidative addition or reductive elimination. Thus, H 2 is not added across the metal, but rather across the metal and a ligand; alternatively, the metal complexes are preformed to contain a hydride ligand as well as a ligand with a hydrogen alpha to the metal. Thus, the hydride and hydrogen are adjacent to one another, facilitating the transfer to the substrate; this transfer occurs without the substrate ever binding to the metal itself. [ 17 ] Though amine is by far the most used ligand in cooperativity, other actor ligands include alkoxides and thiols.
In contrast, in an inner sphere mechanism, the substrate will be inserted into the metal and reaction with hydrogen will then afford the hydrogenated product. This mechanism does not employ MLC. The differentiation between an outer sphere mechanism relying on MLC and an inner sphere mechanism is exemplified by cobalt hydrogenation with an amine pincer ligand. [ 8 ] In the outer sphere mechanism, the hydrogen on the pincer ligand is added into the ketone along with a hydride ligand on the metal. [ 18 ] It is worth noting that there is debate over the concertedness of the transition state of this outer sphere hydrogenation step, and different reactions and catalysts may be either concerted or stepwise, and in some scenarios there may be multiple pathways at play. [ 5 ] In comparison to the ketone hydrogenation, an olefin undergoes an inner sphere mechanism under the same reaction conditions, in which the olefin inserts directly into the metal. These mechanistic differences between the ketone and olefin are corroborated by the observation that the ketone hydrogenation will not occur with an N-Me pincer ligand, and the olefin hydrogenation will proceed with the N-Me ligand, suggesting the ketone requires the presence of the N-H bond while the olefin does not. [ 18 ]
MLC is most broadly used with M-NH systems. Nyori and others have developed an extensive library of diamine ligands which serve in hydrogenation reactions, following the general outer sphere mechanism illustrated above. These systems are typically ruthenium complexes containing phosphine ligands as the spectator ligands. [ 19 ] [ 20 ] Many of these diphosphine ligands, such as BINAP , contain arene rings and impart chirality from atropisomerism ; the rigidity of the phosphene ligands can impart chirality on prochiral substrates with high fidelity, allowing for asymmetric hydrogenation. Reactivity of metal complexes used in MLC can be tuned greatly by the use of different diphosphine spectator ligands.
M-OH metal ligand systems have application in MLC. Shvo's catalyst was one of the earliest complexes developed for ketone and aldehyde reductions to alcohols. The ruthenium complex ( 1 ), upon heating, dissociates into a 18 electron complex ( 2 ) and a 16 electron complex ( 3 ), the former of which is catalytically active. [ 21 ] The hydroxy group on the cyclopentadienyl is the actor ligand, donating a hydrogen in an outer sphere mechanism. Bäckvall has developed use for Shvo's catalyst in the dynamic kinetic resolution of alcohols with lipases. [ 22 ]
Bergman and coworkers developed a sulfur ligand for activation of H-H as well as Si-H bonds. [ 23 ] [ 24 ] A titanium sulfide complex binds H 2 across the titanium and sulfur, yielding a hydride and thiol ligand. A similar mode of reactivity is seen with H-Si bonds, in which the sulfide forms a bond with the silicon, and the titanium accepts the hydride. The use of sulfur ligands in MHC has continued to expand since Bergman's early work in the field. Iridium and rhenium complexes with bridging sulfides have been demonstrated to heterolytically cleave H 2 . [ 25 ] [ 26 ]
Metal boron complexes have also been demonstrated to be useful in activating H 2 . [ 27 ] [ 28 ] [ 29 ] [ 30 ] These ligands are less developed for the purpose of MLC, and commonly suffer from off target alkyl and aryl migration from the boron ligand to other ligands or substrates which disrupts the catalytic cycle [ 9 ] | https://en.wikipedia.org/wiki/Metal-ligand_cooperativity |
A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R 3 P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis . [ 1 ] [ 2 ] Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh 3 ) 3 Cl), Grubbs' catalyst , and tetrakis(triphenylphosphine)palladium(0) . [ 3 ]
Many metal phosphine complexes are prepared by reactions of metal halides with preformed phosphines. For example, treatment of a suspension of palladium chloride in ethanol with triphenylphosphine yields monomeric bis(triphenylphosphine)palladium(II) chloride units. [ 4 ]
The first reported phosphine complexes were cis - and trans -PtCl 2 (PEt 3 ) 2 reported by Cahours and Gal in 1870. [ 5 ]
Often the phosphine serves both as a ligand and as a reductant. This property is illustrated by the synthesis of many platinum-metal complexes of triphenylphosphine : [ 6 ]
Phosphines are L-type ligands . Unlike most metal ammine complexes , metal phosphine complexes tend to be lipophilic , displaying good solubility in organic solvents .
Phosphine ligands are also π-acceptors. Their π-acidity arises from overlap of P-C σ* anti-bonding orbitals with filled metal orbitals. Aryl- and fluorophosphines are stronger π-acceptors than alkylphosphines. Trifluorophosphine (PF 3 ) is a strong π-acid with bonding properties akin to those of the carbonyl ligand . [ 8 ] In early work, phosphine ligands were thought to utilize 3 d orbitals to form M-P pi-bonding, but it is now accepted that d-orbitals on phosphorus are not involved in bonding. [ 9 ] The energy of the σ* orbitals is lower for phosphines with electronegative substituents , and for this reason phosphorus trifluoride is a particularly good π-acceptor. [ 10 ]
In contrast to tertiary phosphines, tertiary amines , especially arylamine derivatives, are reluctant to bind to metals. The difference between the coordinating power of PR 3 and NR 3 reflects the greater steric crowding around the nitrogen atom, which is smaller.
By changes in one or more of the three organic substituents, the steric and electronic properties of phosphine ligands can be manipulated. [ 11 ] The steric properties of phosphine ligands can be ranked by their Tolman cone angle [ 7 ] or percent buried volume. [ 12 ]
An important technique for the characterization of metal-PR 3 complexes is 31 P NMR spectroscopy . Substantial shifts occur upon complexation. 31 P- 31 P spin-spin coupling can provide insight into the structure of complexes containing multiple phosphine ligands. [ 13 ] [ 14 ]
Phosphine ligands are usually "spectator" rather than "actor" ligands. They generally do not participate in reactions, except to dissociate from the metal center. In certain high temperature hydroformylation reactions, the scission of P-C bonds is observed however. [ 15 ] The thermal stability of phosphines ligands is enhanced when they are incorporated into pincer complexes .
One of the first applications of phosphine ligands in catalysis was the use of triphenylphosphine in " Reppe " chemistry (1948), which included reactions of alkynes , carbon monoxide , and alcohols . [ 16 ] In his studies, Reppe discovered that this reaction more efficiently produced acrylic esters using NiBr 2 ( PPh 3 ) 2 as a catalyst instead of NiBr 2 . Shell developed cobalt-based catalysts modified with trialkylphosphine ligands for hydroformylation (now a rhodium catalyst is more commonly used for this process). [ 17 ] The success achieved by Reppe and his contemporaries led to many industrial applications. [ 18 ]
The popularity and usefulness of phosphine complexes has led to the popularization of complexes of many related organophosphorus ligands. [ 5 ] Complexes of arsines have also been widely investigated, but are avoided in practical applications because of concerns about toxicity.
Most work focuses on complexes of triorganophosphines, but primary and secondary phosphines, respectively RPH 2 and R 2 PH, also function as ligands. Such ligands are less basic and have small cone angles. These complexes are susceptible to deprotonation leading to phosphido-bridged dimers and oligomers :
Nickel(0) complexes of phosphites, e.g., Ni[P(OEt) 3 ] 4 are useful catalysts for hydrocyanation of alkenes. Related complexes are known for phosphinites (R 2 P(OR')) and phosphonites (RP(OR') 2 ).
Due to the chelate effect , ligands with two phosphine groups bind more tightly to metal centers than do two monodentate phosphines. The conformational properties of diphosphines makes them especially useful in asymmetric catalysis , e.g. Noyori asymmetric hydrogenation . Several diphosphines have been developed, prominent examples include 1,2-bis(diphenylphosphino)ethane (dppe) and 1,1'-Bis(diphenylphosphino)ferrocene , the trans spanning xantphos and spanphos . The complex dichloro(1,3-bis(diphenylphosphino)propane)nickel is useful in Kumada coupling . | https://en.wikipedia.org/wiki/Metal-phosphine_complex |
Metal L-edge spectroscopy is a spectroscopic technique used to study the electronic structures of transition metal atoms and complexes . This method measures X-ray absorption caused by the excitation of a metal 2p electron to unfilled d orbitals (e.g. 3d for first-row transition metals), which creates a characteristic absorption peak called the L-edge . Similar features can also be studied by Electron Energy Loss Spectroscopy . According to the selection rules , the transition is formally electric-dipole allowed, which not only makes it more intense than an electric-dipole forbidden metal K pre-edge (1s → 3d) transition, [ 1 ] but also makes it more feature-rich as the lower required energy (~400-1000 eV from scandium to copper) results in a higher-resolution experiment. [ 2 ]
In the simplest case, that of a cupric (Cu II ) complex, the 2p → 3d transition produces a 2p 5 3d 10 final state. The 2p 5 core hole created in the transition has an orbital angular momentum L=1 which then couples to the spin angular momentum S=1/2 to produce J=3/2 and J=1/2 final states. These states are directly observable in the L-edge spectrum as the two main peaks (Figure 1). The peak at lower energy (~930 eV) has the greatest intensity and is called the L 3 -edge, while the peak at higher energy (~950 eV) has less intensity and is called the L 2 -edge.
As we move left across the periodic table (e.g. from copper to iron ), we create additional holes in the metal 3d orbitals. For example, a low-spin ferric (Fe III ) system in an octahedral environment has a ground state of ( t 2g ) 5 ( e g ) 0 resulting in transitions to the t 2g (dπ) and e g (dσ) sets. Therefore, there are two possible final states: t 2g 6 e g 0 or t 2g 5 e g 1 (Figure 2a). Since the ground-state metal configuration has four holes in the e g orbital set and one hole in the t 2g orbital set, an intensity ratio of 4:1 might be expected (Figure 2b). However, this model does not take into account covalent bonding and, indeed, an intensity ratio of 4:1 is not observed in the spectrum.
In the case of iron, the d 6 excited state will further split in energy due to d-d electron repulsion (Figure 2c). This splitting is given by the right-hand (high-field) side of the d 6 Tanabe–Sugano diagram and can be mapped onto a theoretical simulation of a L-edge spectrum (Figure 2d). Other factors such as p-d electron repulsion and spin-orbit coupling of the 2p and 3d electrons must also be considered to fully simulate the data.
For a ferric system, all of these effects result in 252 initial states and 1260 possible final states that together will comprise the final L-edge spectrum (Figure 2e). Despite all of these possible states, it has been established that in a low-spin ferric system, the lowest energy peak is due to a transition to the t 2g hole and the more intense and higher energy (~3.5 eV) peak is to that of the unoccupied e g orbitals. [ 3 ]
In most systems, bonding between a ligand and a metal atom can be thought of in terms of metal-ligand covalent bonds, where the occupied ligand orbitals donate some electron density to the metal. This is commonly known as ligand-to-metal charge transfer or LMCT . In some cases, low-lying unoccupied ligand orbitals (π*) can receive back-donation (or backbonding ) from the occupied metal orbitals. This has the opposite effect on the system, resulting in metal-to-ligand charge transfer, MLCT , and commonly appears as an additional L-edge spectral feature.
An example of this feature occurs in low-spin ferric [Fe(CN) 6 ] 3− , since CN − is a ligand that can have backbonding. While backbonding is important in the initial state, it would only warrant a small feature in the L-edge spectrum. In fact, it is in the final state where the backbonding π* orbitals are allowed to mix with the very intense e g transition, thus borrowing intensity and resulting in the final dramatic three peak spectrum (Figure 3 and Figure 4). [ 4 ]
X-ray absorption spectroscopy (XAS), like other spectroscopies, looks at the excited state to infer information about the ground state. To make a quantitative assignment, L-edge data is fitted using a valence bond configuration interaction (VBCI) model where LMCT and MLCT are applied as needed to successfully simulate the observed spectral features. [ 3 ] These simulations are then further compared to density functional theory (DFT) calculations to arrive at a final interpretation of the data and an accurate description of the electronic structure of the complex (Figure 4).
In the case of iron L-edge, the excited state mixing of the metal e g orbitals into the ligand π* make this method a direct and very sensitive probe of backbonding. [ 4 ] | https://en.wikipedia.org/wiki/Metal_L-edge |
The report Metal Stocks in Society: Scientific Synthesis [ 1 ] was the first of six scientific assessments on global metals to be published by the International Resource Panel (IRP) of the United Nations Environment Programme . The IRP provides independent scientific assessments and expert advice on a variety of areas, including:
Metals were an early priority for the International Resource Panel since little was known about them, their impacts, their economic importance or their scarcity. The report aimed to calculate the amount of metals present in society and assess the potential for utilising in-use stock to offset demand for virgin metal. Knowing how much metal stock there is in use, and how long the lifespan of the metal is, can help planners know when these metal stocks will enter recycling or waste streams. It suggested that these 'mines above ground' had the growing potential for future metals supply; the authors found that it is about 50 kg of above-ground copper for every person on earth, and more than two tons of iron per capita. [ 2 ] However, they noted that enormous disparities in global metals stocks existed between developed and developing nations including Brazil, China and India. [ 3 ]
Calculating ‘anthropogenic stocks’ of metals already in use in society is a precursor to stimulating efforts to increase recycling . The authors reported that very little information is presently known about different metals, making it difficult for policymakers to develop and plan recycling systems. However, what is known is that recycling can not only reduce negative impacts on the environment but also save energy. For example, 95% of the energy used to make aluminium from bauxite ore is saved by using recycled material. [ 4 ]
The authors quantified per capita stocks of the following metals, for some countries (stocks were mostly quantified for developed countries but data was also available for some developing nations).
Extant in-use metal-stock estimations for the major engineering metals:
Some primary stocks of rare but useful metals are already running low. For example, rhenium only occurs at seven parts per billion in the Earth’s crust, making it one of the rarest elements on the planet. However, its high melting point of 3,186 °C makes it valuable in the manufacture of jet engines. Demand for the metal is rising, with increasing air travel, but its rarity means increasing extraction is not simple. This is where recycling comes in; rhenium is one of the few metals that has witnessed a rise in recycling rates. It is likely that recycling will become a more viable option than extraction for other metals in future, saving energy, cutting greenhouse gases emitted to the atmosphere and reducing negative impacts on the environment. [ 5 ] | https://en.wikipedia.org/wiki/Metal_Stocks_in_Society_report |
Metal allergies inflame the skin after it has been in contact with metal. They are a form of allergic contact dermatitis . They are becoming more common, as of 2021 [update] , except in areas with regulatory countermeasures.
People may become sensitized to certain metals by skin contact, usually by wearing or holding consumer products (including non-metal products, like textiles and leather treated with metals), or sometimes after exposure at work. Contact with damaged skin makes sensitization more likely. Medical implants may also cause allergic reactions. Diagnosis is by patch test , a method which does not work as well for metals as it does for some other allergens.
Prevention and treatment consists of avoiding the metal allergen; there is no other treatment, as of 2021 [update] . It can be difficult to identify and avoid the allergen, because many metals are common in the environment, and some are biologically necessary to humans. Regulations have successfully reduced the rates of some metal allergies in Europe, but are not widespread. The social and economic costs of metal allergies are high.
Metal allergies are type IV allergies ; the metals are haptens . The toxicity of some allergenic metals may contribute to the development of allergies. [ 1 ]
Nickel allergy and allergies to mercury and chromium have long been recognised; gold , palladium , and cobalt have gotten attention more recently. [ 2 ] [ 3 ] There is often cross-sensitization, where a person allergic to one metal may become allergic to another, but monosensitization, reacting to just one metal, is also possible. [ 4 ] For instance, many people allergic to nickel are often also allergic to cobalt (a similar element often found in the same places as nickel) and palladium. But it is also possible to only be allergic to one of these metals. [ 1 ]
Nickel is one of the most common contact allergens. [ 5 ]
Most cases of metal allergy are caused by consumer products containing metal; exposure at work can also cause metal allergies. [ 3 ] The largest human exposure to metals is ingestion; while food or drink containing metals can cause an allergic reaction in people who already have an allergy, it's not clear if it can cause a new allergy, as of 2021 [update] . Some metal allergens are nutritionally necessary to humans. Airborne metals have been linked to higher rates of sensitization. [ 1 ]
It can be difficult to figure out what allergen a person with contact dermatitis is reacting to, especially if the allergic reaction is systemic, rather than just occurring where the allergen entered the body. [ 7 ]
Consumer products that have induced allergies include jewellery (both cheap and expensive, brand-name jewellery may release metal allergens [ 8 ] ), buttons, clothing fasteners (such as zippers, [ 1 ] buckles, and hooks), dental restorations, mobile phones, and leather [ 3 ] (from the tanning process). Metal hair fasteners may also leach allergens. [ 8 ] The increase in consumer products, including consumer electronics, that use metal nanomaterials , mainly silicon, titanium, zinc and aluminum, increases exposure. [ 1 ]
Tattoo inks contaminated with metal allergens have been known to cause severe reactions, sometimes years later, when the original ink is not available for testing. [ 1 ]
Implants and prosthetics, including dental repairs, are also an exposure; dental work is the main way in which the general population is sensitized to palladium, and dental workers may get occupational palladium allergies, though cross-sensitization may also be a common way in which people develop an allergy to this fairly rare metal. Medications containing metals could also potentially cause sensitization. [ 1 ]
Exposure on damaged skin, such as chapped hands or a piercing, increases the risk of sensitization from a low-level exposure to the allergen. [ 1 ]
Diagnosis is by patch testing , a method first used in 1895. Patches containing potential allergens are stuck on the skin, and the skin is monitored for inflammation. For metal allergens, patch test reproducibility is low, and the extent to which they predict implant failures is debated. If the person being tested has a rash already, it may be difficult to do a patch test. Patch testing may also worsen the allergy. it is also difficult to distinguish co-sensitivity from cross-sensitivity using a patch test. [ clarification needed ] [ 1 ]
In-vitro tests, where a blood sample is examined for metal-sensitive T cells, are in development, but not widely used, partly due to cost. Many non-allergic people also have metal-specific T cells, and in some cases they seem to have more than some allergic individuals, which makes the test less useful. [ 1 ]
Metal allergies are rapidly becoming more common. Nickel is the most common contact allergen worldwide (of people with contact dermatitis, 11.4% in Europe, 8.8–25.7% in China, and 17.5% in North America are allergic to nickel). [ 1 ]
Nickel allergy, and contact allergies more generally, can develop when people are any age, but they are most likely to develop in early adulthood. This may be due to patterns in exposure or changes in the immune system with age, or both. [ 1 ]
Preventing and treating contact allergies largely involves avoiding the allergen, which may be difficult when it is a common metal. There are no other treatments for metal allergies, as of 2021 [update] . [ 1 ]
In the Netherlands , regulations that limit the release of nickel from consumer products, introduced in the 1990s, [ 1 ] worked. Dutch women are now significantly less likely to develop nickel allergies. [ clarification needed ] [ 3 ] Sweden followed in 1994, and later [ when? ] regulations were made Europe-wide. These limits cover objects inserted into piercings (0.2 μg/cm²/week) and those in direct or prolonged contact with the skin (0.5 μg/cm 2 ). They also set target values for nickel in ambient air; (20 ng/mg 3 ) increases in nickel concentrations in ambient air, even when absolute levels are quite low, have been linked to increased rates of sensitization in human populations. Nickel allergy rates in Europe have decreased, though it is still the most common contact allergy. [ 1 ] Regulation is generally inadequate, given the amount of the social and economic harm caused by metal allergies. [ 1 ]
Regulation encouraged use of metals other than nickel, and that caused more cases of allergies to other metals. Nickel remains the most common, but cobalt is the second most common allergy, and in 2020 the EU introduced a temporary generic concentration limit (GCL) of 0.1% on cobalt. Limits on nickel and cobalt in textiles (130mg/kg nickel, 110 mg/kg cobalt) and leather (70mg/kg nickel, 60 mg/kg cobalt), were proposed in 2020 by France and Sweden. There is no allergen regulation of pallidium in Europe as of October 2021 [update] . [ 1 ] | https://en.wikipedia.org/wiki/Metal_allergy |
Metal amides (systematic name metal azanides ) are a class of coordination compounds composed of a metal center with amide ligands of the form NR 2 − . Amido complexes of the parent amido ligand NH 2 − are rare compared to complexes with diorganylamido ligand, such as dimethylamido. Amide ligands have two electron pairs available for bonding.
In principle, the M-NX 2 group could be pyramidal or planar. The pyramidal geometry is not observed.
In many complexes, the amido is a bridging ligand . Some examples have both bridging and terminal amido ligands. Bulky amide ligands have a lesser tendency to bridge. Amide ligands may participate in metal-ligand π-bonding giving a complex with the metal center being co-planar with the nitrogen and substituents. Metal bis(trimethylsilyl)amides form a significant subcategory of metal amide compounds. These compounds tend to be discrete and soluble in organic solvents.
Lithium amides are the most important amides. They are prepared from n-butyllithium and the appropriate amine
The lithium amides are more common and more soluble than the other alkali metal analogs. Potassium amides are prepared by transmetallation of lithium amides with potassium t-butoxide (see also Schlosser base ) or by reaction of the amine with potassium , potassium hydride , n-butylpotassium , or benzylpotassium . [ 2 ]
The alkali metal amides, MNH 2 (M = Li, Na, K) are commercially available. Sodium amide (also known as sodamide) is synthesized from sodium metal and ammonia with ferric nitrate catalyst. [ 3 ] [ 4 ] The sodium compound is white, but the presence of metallic iron turns the commercial material gray.
Lithium diisopropylamide is a popular non-nucleophilic base used in organic synthesis . Unlike many other bases, the steric bulk prevents this base from acting as a nucleophile . It is commercially available, usually as a solution in hexane. It may be readily prepared from n-butyllithium and diisopropylamine .
Amido derivatives of main group elements are well developed. [ 5 ]
Early transition metal amides may be prepared by treating anhydrous metal chloride with alkali amide reagents. In some cases, two equivalents of a secondary amine can be used, one equivalent serving as a base: [ 6 ]
Transition metal amide complexes have been prepared by these methods: [ 6 ]
Highly cationic metal ammine complexes such as [Pt(NH3)6]4+ spontaneously convert to the amido derivative:
Transition metal amides are intermediates in the base-induced substitution of transition metal ammine complexes . Thus, the Sn1CB mechanism for the displacement of chloride from chloropentamminecobalt chloride by hydroxide proceeds via an amido intermediate: [ 8 ] | https://en.wikipedia.org/wiki/Metal_amides |
In chemistry , metal aquo complexes are coordination compounds containing metal ions with only water as a ligand . These complexes are the predominant species in aqueous solutions of many metal salts , such as metal nitrates , sulfates , and perchlorates . They have the general stoichiometry [M(H 2 O) n ] z + . Their behavior underpins many aspects of environmental , biological , and industrial chemistry . This article focuses on complexes where water is the only ligand (" homoleptic aquo complexes"), but of course many complexes are known to consist of a mix of aquo and other ligands. [ 1 ] [ 2 ]
Most aquo complexes are mono-nuclear, with the general formula [M(H 2 O) 6 ] n + , with n = 2 or 3; they have an octahedral structure . The water molecules function as Lewis bases , donating a pair of electrons to the metal ion and forming a dative covalent bond with it. Typical examples are listed in the following table.
Tutton's salts are crystalline compounds with the generic formula (NH 4 ) 2 M (SO 4 ) 2 ·(H 2 O) 6 (where M = V 2+ , Cr 2+ , Mn 2+ , Co 2+ , Ni 2+ , or Cu 2+ ). Alums , MM′(SO 4 ) 2 (H 2 O) 12 , are also double salts. Both sets of salts contain hexa-aquo metal cations.
Silver(I) forms [Ag(H 2 O) 4 ] + , a rare example of a tetrahedral aquo complex. [ 8 ] Palladium(II) and platinum(II) were once thought to form square planar aquo complexes. [ 9 ]
Aquo complexes of lanthanide(III) ions are eight- and nine-coordinate, reflecting the large size of the metal centres.
In the binuclear ion [Co 2 (OH 2 ) 10 ] 4+ each bridging water molecule donates one pair of electrons to one cobalt ion and another pair to the other cobalt ion. The Co-O (bridging) bond lengths are 213 picometers, and the Co-O (terminal) bond lengths are 10 pm shorter. [ 10 ]
The complexes [Mo 2 (H 2 O) 8 ] 4+ and [Rh 2 (H 2 O) 10 ] 4+ contain metal-metal bonds. [ 8 ]
Monomeric aquo complexes of Nb, Ta, Mo, W, Mn, Tc, Re, and Os in oxidation states +4 to +7 have not been reported. [ 9 ] For example, [Ti(H 2 O) 6 ] 4+ is unknown: the hydrolyzed species [Ti(OH) 2 (H 2 O) n ] 2+ is the principal species in dilute solutions. [ 11 ] With the higher oxidation states the effective electrical charge on the cation is further reduced by the formation of oxo-complexes.
Lanthanide salts often or perhaps characteristically form aquo complexes. The homoleptic tricationic aquo complexes have nine water ligands. [ 12 ]
Some reactions considered fundamental to the behavior of metal aquo ions are ligand exchange, electron-transfer, and acid-base reactions.
Ligand exchange involves replacement of a water ligand ("coordinated water") with water in solution ("bulk water"). Often the process is represented using labeled water H 2 O· :
In the absence of isotopic labeling , the reaction is degenerate, meaning that the free energy change is zero.
Rates vary over many orders of magnitude. The main factor affecting rates is charge: highly charged metal aquo cations exchange their water more slowly than singly charged cations. Thus, the exchange rates for [Na(H 2 O) 6 ] + and [Al(H 2 O) 6 ] 3+ differ by a factor of 10 9 . Electron configuration is also a major factor, illustrated by the fact that the rates of water exchange for [Al(H 2 O) 6 ] 3+ and [Ir(H 2 O) 6 ] 3+ differ by a factor of 10 9 also. [ 4 ] Water exchange usually follows a dissociative substitution pathway, so the rate constants indicate first order reactions.
This reaction usually applies to the interconversion of di- and trivalent metal ions, which involves the exchange of only one electron. The process is called self-exchange, meaning that the ion appears to exchange electrons with itself. The standard electrode potential for the following equilibrium:
shows the increasing stability of the lower oxidation state as atomic number increases. The very large value for the manganese couple is a consequence of the fact that octahedral manganese(II) has zero crystal field stabilization energy (CFSE) but manganese(III) has 3 units of CFSE. [ 13 ]
Using labels to keep track of the metals, the self-exchange process is written as:
The rates of electron exchange vary widely, the variations being attributable to differing reorganization energies: when the 2+ and 3+ ions differ widely in structure, the rates tend to be slow. [ 14 ] The electron transfer reaction proceeds via an outer sphere electron transfer . Most often large reorganizational energies are associated with changes in the population of the e g level, at least for octahedral complexes.
Solutions of metal aquo complexes are acidic owing to the ionization of protons from the water ligands. In dilute solution chromium(III) aquo complex has a p K a of about 4.3, affording a metal hydroxo complex :
Thus, the aquo ion is a weak acid , of comparable strength to acetic acid (p K a of about 4.8). This pK a is typical of the trivalent ions. The influence of the electronic configuration on acidity is shown by the fact that [Ru(H 2 O) 6 ] 3+ ( p K a = 2.7 ) is more acidic than [Rh(H 2 O) 6 ] 3+ ( p K a = 4 ), despite the fact that Rh(III) is expected to be more electronegative. This effect is related to the stabilization of the pi-donor hydroxide ligand by the ( t 2g ) 5 Ru(III) centre. [ 8 ]
In concentrated solutions, some metal hydroxo complexes undergo condensation reactions, known as olation , to form polymeric species. Many minerals are assumed to form via olation. Aquo ions of divalent metal ions are less acidic than those of trivalent cations.
The hydrolyzed species often exhibit very different properties from the precursor hexaaquo complex. For example, water exchange in [Al(H 2 O) 5 OH] 2+ is 20000 times faster than in [Al(H 2 O) 6 ] 3+ . | https://en.wikipedia.org/wiki/Metal_aquo_complex |
Metal aromaticity or metalloaromaticity is the concept of aromaticity , found in many organic compounds , extended to metals and metal-containing compounds. [ 1 ] The first experimental evidence for the existence of aromaticity in metals was found in aluminium cluster compounds of the type MAl − 4 where M stands for lithium , sodium or copper. [ 2 ] These anions can be generated in a helium gas by laser vaporization of an aluminium / lithium carbonate composite or a copper or sodium / aluminium alloy , separated and selected by mass spectrometry and analyzed by photoelectron spectroscopy . The evidence for aromaticity in these compounds is based on several considerations. Computational chemistry shows that these aluminium clusters consist of a tetranuclear Al 2− 4 plane and a counterion at the apex of a square pyramid . The Al 2− 4 unit is perfectly planar and is not perturbed by the presence of the counterion or even the presence of two counterions in the neutral compound M 2 Al 4 . In addition its HOMO is calculated to be a doubly occupied delocalized pi system making it obey Hückel's rule . Finally a match exists between the calculated values and the experimental photoelectron values for the energy required to remove the first 4 valence electrons. The first fully metal aromatic compound was a cyclogallane with a Ga 3 2- core discovered by Gregory Robinson in 1995. [ 3 ]
D-orbital aromaticity is found in trinuclear tungsten W 3 O − 9 and molybdenum Mo 3 O − 9 metal clusters generated by laser vaporization of the pure metals in the presence of oxygen in a helium stream. [ 4 ] In these clusters the three metal centers are bridged by oxygen and each metal has two terminal oxygen atoms. The first signal in the photoelectron spectrum corresponds to the removal of the valence electron with the lowest energy in the anion to the neutral M 3 O 9 compound. This energy turns out to be comparable to that of bulk tungsten trioxide and molybdenum trioxide . The photoelectric signal is also broad which suggests a large difference in conformation between the anion and the neutral species. Computational chemistry shows that the M 3 O − 9 anions and M 3 O 2− 9 dianions are ideal hexagons with identical metal-to-metal bond lengths . Tritantalum oxide clusters (Ta 3 O 3 − ) also are observed to exhibit possible D-orbital aromaticity. [ 3 ]
The molecules discussed thus far only exist diluted in the gas phase. A study exploring the properties of a compound formed in water from sodium molybdate ( Na 2 MoO 4 ·2H 2 O ) and iminodiacetic acid also revealed evidence of aromaticity, but this compound has actually been isolated. X-ray crystallography showed that the sodium atoms are arranged in layers of hexagonal clusters akin to pentacenes . The sodium-to-sodium bond lengths are unusually short (327 pm versus 380 pm in elemental sodium) and, like benzene, the ring is planar. In this compound each sodium atom has a distorted octahedral molecular geometry with coordination to molybdenum atoms and water molecules. [ 5 ] The experimental evidence is supported by computed NICS aromaticity values. | https://en.wikipedia.org/wiki/Metal_aromaticity |
Metal bellows are elastic vessels that can be compressed when pressure is applied to the outside of the vessel, or extended under vacuum. When the pressure or vacuum is released, the bellows will return to its original shape, provided the material has not been stressed past its yield strength . They are used both for their ability to deform under pressure and to provide a hermetic seal that allows movement.
Precision bellows technology of the 20th and 21st century is centered on metal bellows with less demanding applications using ones made of rubber and plastic. These products bear little resemblance to the original leather bellows used traditionally in fireplaces and forges .
There are three main types of metal bellows: formed, welded and electroformed.
Formed bellows are produced by reworking tubes, normally produced by deep drawing , with a variety of processes, including cold forming (rolling), and hydroforming . They are also called convoluted bellows or sylphons .
Welded bellows (also called edge-welded, or diaphragm bellows) are manufactured by welding a number of individually formed diaphragms to each other. The comparison between the two bellows types generally centers on cost and performance. Hydroformed bellows generally have a high tooling cost, but, when mass-produced, may have a lower piece price. However, hydroformed bellows have lower performance characteristics due to relatively thick walls and high stiffness. Welded metal bellows are produced with a lower initial tooling cost and maintain higher performance characteristics. The drawback of welded bellows is the reduced metal strength at weld joints, caused by the high temperature of welding. [ 1 ]
Electroformed bellows are produced by plating ( electroforming ) a metal layer onto a model (mandrel), and subsequently removing the mandrel. They can be produced with modest tooling costs and with thin walls (25 micrometres or less), providing such bellows with high sensitivity and precision in many exacting applications, and may also be produced in shapes that would be exceptionally difficult to produce by other means with little additional difficulty. [ 2 ] [ 3 ]
Another area of comparison is in metals of construction. Hydroformed and rolled bellows are limited to metals with high plastic elongation characteristics, whereas welded bellows may be fabricated from a wider variety of standard and exotic alloys, such as stainless steel and titanium , as well as other high-strength, corrosion-resistant materials. Electroformed bellows can be produced of nickel , its high-strength alloys , and copper .
Metal bellows are used in a large number of industrial applications. Below you will find a few;
Welded bellows can be fabricated from a variety of exotic metals and alloys, whereas formed bellows are limited to alloys with good elongation – brass being a prime example. Welded bellows are not fabricated from brass because of its fundamentally poor weldability. Other advantages to welded bellows include compactness (higher performance in a smaller package), ability to be compressed to solid height with no damage, resistance to nicks and dents, and dramatically greater flexibility.
The welding of metal bellows is a microscopic welding process, typically performed under laboratory conditions at high magnification.
Hydroformed bellows are produced by forcing a metal tube to expand under hydraulic pressure inside a bellows-shaped mold, and assume the convoluted shape of the mold.
Electroformed bellows are produced by plating metal onto a bellows-shaped model (mandrel), and the subsequent mandrel removal by chemical or physical means. Due to the low tooling cost and short manufacturing cycle, electroforming of bellows is not only an inexpensive manufacturing method, but also a perfect prototyping tool.
There are a variety of expansion joints and not each one can accept the same types of deflection. The various types of deflections are axial, lateral, angular, torsional, cyclic, or any combination that can occur at the same time. [ 4 ] | https://en.wikipedia.org/wiki/Metal_bellows |
A metal carbido complex is a coordination complex that contains a carbon atom as a ligand . They are analogous to metal nitrido complexes . Carbido complexes are a molecular subclass of carbides , which are prevalent in organometallic and inorganic chemistry. Carbido complexes represent models for intermediates in Fischer–Tropsch synthesis, olefin metathesis , and related catalytic industrial processes. [ 1 ] Ruthenium-based carbido complexes are by far the most synthesized and characterized to date. Although, complexes containing chromium, gold, iron, nickel, molybdenum, osmium, rhenium, and tungsten cores are also known. Mixed-metal carbides are also known. [ 2 ] [ 3 ]
Most molecular carbido complexes are clusters, usually featuring carbide as a six-fold bridging ligand . Examples include [ Rh 6 C(CO) 15 ] 2− , and [ Ru 6 C(CO) 16 ] 2− . [ 5 ] Though exceptions exist, such as the nonanuclear Ruthenium cluster (μ-C)Ru 9 (CO) 14 (μ 3 -η 5 : η 2 :η 2 -C 9 H 7 ) 2, containing a tripped trigonal prism geometry around the carbide. [ 6 ]
The iron carbonyl carbides exist not only in the encapsulated carbon ([ Fe 6 C(CO) 16 ] 2− ) but also with exposed carbon centres as in Fe 5 C(CO) 15 and Fe 4 C(CO) 13. [ 7 ]
Bimetallic and exotic clusters such as metal carbide clusterfullerenes (MCCF's) have also been able to be prepared. [ 8 ] [ 9 ]
Bridging carbido ligands can be subdivided into three classes:
Cumulenic compounds generally bridge two metal atoms of the same element and are symmetrical. [ 11 ] However, there are exceptions to this. [ 12 ]
In contrast, metallocarbyne compounds are generally constitutionally heterobimetallic, with complexes containing varying coordination geometries being common. These moieties have been able to serve as precursors to elaborate molecular scaffolds such as porphyrin derivatives. [ 13 ]
The polar covalent class is distinguished from metallocarbynes by a very fine line. This carbide-metal interaction is considered labile in nature. Carbon here can be understood fundamentally as being similar to CO ligands, that is, dative (L-type). Although, this class has also been described to some extent being analogous to the behavior of Lewis acid adduct-forming terminal nitrido and oxo complexes e.g. (PMe 2 Ph) 2 Cl-Re≡N-BCl 3 and tBu(CH 2 ) 3 (Br)W=O-AlBr 3 . [ 14 ]
In rare cases, carbido ligands are terminal. One example is RuC(PCy 3 ) 2 Cl 2 with a Ru-C distance of 163 pm , typical for a triple bond. [ 15 ] The complex can be obtained by metathesis of vinyl acetate to give [Ru(CH- p -C 6 H 4 Me)(PCy 3 ) 2 Cl 2 ] results in a metastable Ru(Cl 2 )(PCy 3 ) 2 C 2 HOAc complex, which eliminates acetic acid. [ 16 ]
Such transition metal, one coordinate-carbon bonded complexes are comparable to carbon monoxide, cyanide, and isonitrile analogues. These carbides can be used as synthons to access a wide range of carbyne complexes, the most notable being Fischer carbynes. [ 17 ] American chemist Christopher C. Cummins is one of the pioneers of this area.
Synthesis of carbido clusters can be accomplished by hydrolysis, thermolysis of labile ligands, thermal rearrangements, and photolysis. Their synthesis has historically been crudely achieved by serendipitous chance following apparent random molecular organization. One example is the following reaction:
Synthetic routes to cumulenic carbido complexes can be efficient and lead to rapid, near quantitative product formation with simple purifications. [ 18 ] This dimerization involves the formation of a vinylidene from an alkyne. Mechanistically, there are various proposed pathways, starting with oxidative addition of the alkyne to the metal core, followed by either intramolecular 1,2-H shifts or intermolecular 1,3-H shifts. [ 19 ] For Ruthenium coordination complexes, bridging Ru-Cl bond lengths have been observed to lie in the range of 1.76-1.8 Å. Ru-C bonds can vary significantly as a result of trans effect phenomena which is caused by the respective ethylene and vinylidene ligands.
The appropriate halocarbyne precursors of choice can be reacted with organolithium reagents to afford the respective lithiocarbyne derivate by virtue of lithium/halogen exchange . [ 20 ] This species can serve as a lynchpin for subsequent carbide linkage with an additional metal complex. Phosphine-based analogues were first introduced by Templeton and co. [ 21 ] These types of complexes can be characterized crystallographically and are distinguishable by their C s symmetry.
Addition of tricyclohexylphosphine to the carbene complex (PPh 3 ) 2 (Cl) 2 Ru=C(CHCO 2 Me) 2 results in olefin extrusion and yields an air stable anionic carbido complex. This species displaces a dimethyl sulfide ligand from PdCl 2 (SMe) 2 to give the μ-carbido bimetallic complex (PCy 3 ) 2 Cl 2 Ru≡C-PdCl 2 (SMe 2 ). Spark towards a novel type of bonding was proposed following empirical observations wherein the carbido-palladium interaction could be readily disturbed. Reversible coordination ensues upon exposure of the bimetallic complex to carbon monoxide. Additionally, no coordination occurs if the anionic carbido complex contains bulky ligands such as H 2 IMes . This indicates that the thermodynamic sink towards making the C-M bond is not very favorable, suggesting a weak interaction. Although not intuitive, characterization of this type of bonding can be inferred if 13 C NMR shifts are observed to be far downfield, and C-M bond lengths are similar to those of complexes proven to contain carbon-based σ-donor ligands such as [(Et 2 H 2 Im)PdCl(μ-Cl)] 2. [ 22 ]
Metathesis using Grubbs-type alkylidene complexes can be used to synthesize terminal carbido-containing complexes. One example is RuC(PCy 3 ) 2 Cl 2 with a Ru-C distance of 163 pm , typical for a triple bond. [ 23 ] The complex can be obtained by metathesis of vinyl acetate to give [Ru(CH- p -C 6 H 4 Me)(PCy 3 ) 2 Cl 2 ] results in a metastable Ru(Cl 2 )(PCy 3 ) 2 C 2 HOAc complex, which eliminates acetic acid. [ 24 ]
The "naked" carbido ligand is weakly basic, forming complexes with other metal centers. The C-M bond is typically found to be around 1.65 Å . The 13 C NMR resonance values for the carbido carbons vary widely, but range from δ211-406. [ 25 ] Another example of a terminal carbido complex is Li[MoC(NR2)3] (Mo-C distance of 172 pm), which forms upon deprotonation of the respective methylidyne precursor. [ 26 ] | https://en.wikipedia.org/wiki/Metal_carbido_complex |
Metal carbon dioxide complexes are coordination complexes that contain carbon dioxide ligands . Aside from the fundamental interest in the coordination chemistry of simple molecules, studies in this field are motivated by the possibility that transition metals might catalyze useful transformations of CO 2 . This research is relevant both to organic synthesis and to the production of "solar fuels" that would avoid the use of petroleum-based fuels. [ 1 ]
Carbon dioxide binds to metals in only a few ways. The bonding mode depends on the electrophilicity and basicity of the metal centre. [ 2 ] Most common is the η 2 -CO 2 coordination mode as illustrated by Aresta's complex, Ni(CO 2 )( PCy 3 ) 2 , which was the first reported complex of CO 2. [ 3 ] [ 4 ] This square-planar compound is a derivative of Ni(II) with a reduced CO 2 ligand. In rare cases, CO 2 binds to metals as a Lewis base through its oxygen centres, but such adducts are weak and mainly of theoretical interest. A variety of multinuclear complexes are also known often involving Lewis basic and Lewis acidic metals, e.g. metallacarboxylate salts (C 5 H 5 )Fe(CO) 2 CO 2 − K + . In multinuclear cases (compounds containing more than one metal), more complicated and more varied coordination geometries are observed. One example is the unsymmetrical compound containing four rhenium centres, [(CO) 5 ReCO 2 Re(CO) 4 ] 2 . [ citation needed ] Carbon dioxide can also bind to ligands on a metal complex (vs just the metal), e.g. by converting hydroxy ligands to carbonato ligands. [ citation needed ]
Transition metal carbon dioxide complexes undergo a variety of reactions. Metallacarboxylic acids protonate at oxygen and eventually convert to metal carbonyl complexes:
This reaction is relevant to the potential catalytic conversion of CO 2 to fuels. [ 5 ]
N-heterocyclic carbene (NHC) supported Cu I complexes catalyze carboxylation of organoboronic esters. [ 6 ] The catalyst forms in situ from CuCl, an NHC ligand, and KO t Bu. Copper tert -butoxide can transmetallate with the organoboronic ester to generate the Cu I -C bond, which intermediate can insert into CO 2 smoothly to get the respective carboxylate. Salt metathesis with KO t Bu releases product and regenerates catalyst (Scheme 2).
Apart from transmetallation, there are other approaches forming Cu-C bond. C-H functionalization is a straightforward and atom economic method. Base can help deprotonate acidic C-H protons and form Cu-C bond. [( Phenanthroline )Cu(PR 3 )] catalyst effect C-H carboxylation on terminal alkynes together with Cs 2 CO 3 . [ 7 ] NHC-Cu-H species to deprotonate acidic proton to effect carboxylation of terminal alkynes. [ 8 ] Cu-H species were generated from Cu-F and organosilanes . The carboxylate product was trapped by silyl fluoride to get silyl ether. For non-acidic C-H bonds, directed metalation with i Bu 3 Al(TMP)Li is adopted followed by transmetallation with copper to get Cu-C bond. Allylic C-H bonds and phenyl C-H bonds got carboxylated with this approach by Hou and co-workers: [ 9 ] [ 10 ]
Carbometallation to alkynes and allenes using organozinc and organoaluminum reagents followed by transmetallation to copper is also a strategy to initiate carboxylation. Trimethylaluminium is able to insert into unbiased aliphatic internal alkynes with syn fashion directed by ether directing group. Vinyl copper complexes are formed by transmetallation and carboxylation is realized with a similar pathway giving tetrasubstituted aliphatic vinyl carboxylic acids. [ 11 ] In this case, regioslectivity is controlled by the favor of six-membered aluminum ring formation. Furthermore, carboxylation can be achieved on ynamides and allenamides using less reactive dimethyl zinc via similar approach. [ 12 ] [ 13 ]
In the presence of palladium acetate under 1-30 bar of CO 2 , simple aromatic compounds convert to aromatic carboxylic acids. [ 14 ] [ 15 ] [ 16 ] [ 17 ] [ 18 ] A PSiP-pincer ligand ( 5 ) promotes carboxylation of allene without using pre-functionalized substrates. [ 19 ] Catalyst regeneration, Et 3 Al was added to do transmetallation with palladium. Catalyst is regenerated by the following β-H elimination . Apart from terminal allenes, some of internal allenes are also tolerated in this reaction, generating allyl carboxylic acid with the yield between 54% and 95%. This system was also applied to 1,3-diene, generating carboxylic acid in 1,2 addition fashion. [ 20 ] In 2015, Iwasawa et al. reported the germanium analogue ( 6 ) and combined CO 2 source together with hydride source to formate salts. [ 21 ]
Palladium has shown huge power to catalyze C-H functionalization . If the Pd-C intermediate in carboxylation reaction comes from C-H activation, such methodology must promote metal catalyzed carboxylation to a much higher level in utility. Iwasawa and co-workers reported direct carboxylation by styrenyl C-H activation generating coumarin derivatives. [ 22 ] Benzene rings with different electronic properties and some heteroaromatic rings are tolerated in this reaction with yield from 50% to 90%. C-H activation was demonstrated by crystallography study.
Similar to Cu(I) chemistry mentioned above, Rh(I) complexes can also transmetallate with arylboronic esters to get aryl rhodium intermediates, to which CO 2 is inserted giving carboxylic acids. [ 23 ] Later, Iwasawa et al . described C-H carboxylation strategy. Rh(I) undergoes oxidative addition to aryl C-H bond followed by transmetallation with alkyl aluminum species. Ar-Rh(I) regenerates by reductive elimination releasing methane. Ar-Rh(I) attacks CO 2 then transmetallates with aryl boronic acid to release the boronic acid of product, giving final carboxylic acid by hydrolysis. Directed and non-directed versions are both achieved. [ 24 ] [ 25 ] [ 26 ]
Iwasawa and co-workers developed Rh(I) catalyzed carbonation reaction initiated by Rh-H insertion to vinylarenes. In order to regenerate reactive Rh-H after nucleophilic addition to CO 2 , photocatalytic proton-coupled electron transfer approach was adopted. [ 27 ] In this system, excess amount of diethylpropylethylamine works as sacrificial electron donor (Scheme 5).
Carboxylation of benzyl halides has been reported. [ 28 ] The reaction mechanism is proposed to involve oxidative addition of benzyl chloride to Ni(0). The Ni(II) benzyl complex is reduced to Ni(I), e.g., by zinc, which inserts CO 2 delivering the nickel carboxylate. Reduction of the Ni(I) carboxylate to Ni(0) releases the zinc carboxylate (Scheme 6). Similarly, such carboxylation has been achieved on aryl and benzyl pivalate, [ 29 ] alkyl halides, [ 30 ] [ 31 ] and allyl esters. [ 32 ] | https://en.wikipedia.org/wiki/Metal_carbon_dioxide_complex |
Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands . Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis , such as hydroformylation and Reppe chemistry . In the Mond process , nickel tetracarbonyl is used to produce pure nickel . In organometallic chemistry , metal carbonyls serve as precursors for the preparation of other organometallic complexes.
Metal carbonyls are toxic by skin contact, inhalation or ingestion, in part because of their ability to carbonylate hemoglobin to give carboxyhemoglobin , which prevents the binding of oxygen . [ 1 ]
The nomenclature of the metal carbonyls depends on the charge of the complex, the number and type of central atoms, and the number and type of ligands and their binding modes. They occur as neutral complexes, as positively-charged metal carbonyl cations or as negatively charged metal carbonylates . The carbon monoxide ligand may be bound terminally to a single metal atom or bridging to two or more metal atoms. These complexes may be homoleptic , containing only CO ligands, such as nickel tetracarbonyl (Ni(CO) 4 ), but more commonly metal carbonyls are heteroleptic and contain a mixture of ligands. [ citation needed ]
Mononuclear metal carbonyls contain only one metal atom as the central atom. Except vanadium hexacarbonyl , only metals with even atomic number, such as chromium , iron , nickel , and their homologs, build neutral mononuclear complexes. Polynuclear metal carbonyls are formed from metals with odd atomic numbers and contain a metal–metal bond . [ 2 ] Complexes with different metals but only one type of ligand are called isoleptic. [ 2 ]
Carbon monoxide has distinct binding modes in metal carbonyls. They differ in terms of their hapticity , denoted η , and their bridging mode. In η 2 -CO complexes, both the carbon and oxygen are bonded to the metal. More commonly only carbon is bonded, in which case the hapticity is not mentioned. [ 3 ]
The carbonyl ligand engages in a wide range of bonding modes in metal carbonyl dimers and clusters. In the most common bridging mode, denoted μ 2 or simply μ , the CO ligand bridges a pair of metals. This bonding mode is observed in the commonly available metal carbonyls: Co 2 (CO) 8 , Fe 2 (CO) 9 , Fe 3 (CO) 12 , and Co 4 (CO) 12 . [ 1 ] [ 4 ] In certain higher nuclearity clusters, CO bridges between three or even four metals. These ligands are denoted μ 3 -CO and μ 4 -CO. Less common are bonding modes in which both C and O bond to the metal, such as μ 3 η 2 . [ citation needed ]
Carbon monoxide bonds to transition metals using "synergistic pi* back-bonding ". The M–C bonding has three components, giving rise to a partial triple bond. A sigma (σ) bond arises from overlap of the nonbonding (or weakly anti-bonding) sp-hybridized electron pair on carbon with a blend of d- , s- , and p-orbitals on the metal. A pair of pi (π) bonds arises from overlap of filled d-orbitals on the metal with a pair of π*- antibonding orbitals projecting from the carbon atom of the CO. The latter kind of binding requires that the metal have d-electrons, and that the metal be in a relatively low oxidation state (0 or +1) which makes the back-donation of electron density favorable. As electrons from the metal fill the π-antibonding orbital of CO, they weaken the carbon–oxygen bond compared with free carbon monoxide, while the metal–carbon bond is strengthened. Because of the multiple bond character of the M–CO linkage, the distance between the metal and carbon atom is relatively short, often less than 1.8 Å, about 0.2 Å shorter than a metal– alkyl bond. The M-CO and MC-O distance are sensitive to other ligands on the metal. Illustrative of these effects are the following data for Mo-C and C-O distances in Mo(CO) 6 and Mo(CO) 3 (4-methylpyridine) 3 : 2.06 vs 1.90 and 1.11 vs 1.18 Å. [ 5 ]
Infrared spectroscopy is a sensitive probe for the presence of bridging carbonyl ligands. For compounds with doubly bridging CO ligands, denoted μ 2 -CO or often just μ -CO, the bond stretching frequency ν CO is usually shifted by 100–200 cm −1 to lower energy compared to the signatures of terminal CO, which are in the region 1800 cm −1 . Bands for face-capping ( μ 3 ) CO ligands appear at even lower energies. In addition to symmetrical bridging modes, CO can be found to bridge asymmetrically or through donation from a metal d orbital to the π* orbital of CO. [ 6 ] [ 7 ] [ 8 ] The increased π-bonding due to back-donation from multiple metal centers results in further weakening of the C–O bond. [ citation needed ]
Most mononuclear carbonyl complexes are colorless or pale yellow, volatile liquids or solids that are flammable and toxic. [ 9 ] Vanadium hexacarbonyl , a uniquely stable 17-electron metal carbonyl, is a blue-black solid. [ 1 ] Dimetallic and polymetallic carbonyls tend to be more deeply colored. Triiron dodecacarbonyl (Fe 3 (CO) 12 ) forms deep green crystals. The crystalline metal carbonyls often are sublimable in vacuum, although this process is often accompanied by degradation. Metal carbonyls are soluble in nonpolar and polar organic solvents such as benzene , diethyl ether , acetone , glacial acetic acid , and carbon tetrachloride . Some salts of cationic and anionic metal carbonyls are soluble in water or lower alcohols. [ 10 ]
Apart from X-ray crystallography , important analytical techniques for the characterization of metal carbonyls are infrared spectroscopy and 13 C NMR spectroscopy . These two techniques provide structural information on two very different time scales. Infrared-active vibrational modes , such as CO-stretching vibrations, are often fast compared to intramolecular processes, whereas NMR transitions occur at lower frequencies and thus sample structures on a time scale that, it turns out, is comparable to the rate of intramolecular ligand exchange processes. NMR data provide information on "time-averaged structures", whereas IR is an instant "snapshot". [ 11 ] Illustrative of the differing time scales, investigation of dicobalt octacarbonyl (Co 2 (CO) 8 ) by means of infrared spectroscopy provides 13 ν CO bands, far more than expected for a single compound. This complexity reflects the presence of isomers with and without bridging CO ligands. The 13 C NMR spectrum of the same substance exhibits only a single signal at a chemical shift of 204 ppm. This simplicity indicates that the isomers quickly (on the NMR timescale) interconvert. [ citation needed ]
Iron pentacarbonyl exhibits only a single 13 C NMR signal owing to rapid exchange of the axial and equatorial CO ligands by Berry pseudorotation . [ citation needed ]
An important technique for characterizing metal carbonyls is infrared spectroscopy . [ 13 ] The C–O vibration, typically denoted ν CO , occurs at 2143 cm −1 for carbon monoxide gas. The energies of the ν CO band for the metal carbonyls correlates with the strength of the carbon–oxygen bond, and inversely correlated with the strength of the π-backbonding between the metal and the carbon. The π-basicity of the metal center depends on a lot of factors; in the isoelectronic series ( titanium to iron ) at the bottom of this section, the hexacarbonyls show decreasing π-backbonding as one increases (makes more positive) the charge on the metal. π-Basic ligands increase π-electron density at the metal, and improved backbonding reduces ν CO . The Tolman electronic parameter uses the Ni(CO) 3 fragment to order ligands by their π-donating abilities. [ 14 ] [ 15 ]
The number of vibrational modes of a metal carbonyl complex can be determined by group theory . Only vibrational modes that transform as the electric dipole operator will have nonzero direct products and are observed. The number of observable IR transitions (but not their energies) can thus be predicted. [ 16 ] [ 17 ] [ 18 ] For example, the CO ligands of octahedral complexes, such as Cr(CO) 6 , transform as a 1g , e g , and t 1u , but only the t 1u mode (antisymmetric stretch of the apical carbonyl ligands) is IR-allowed. Thus, only a single ν CO band is observed in the IR spectra of the octahedral metal hexacarbonyls. Spectra for complexes of lower symmetry are more complex. For example, the IR spectrum of Fe 2 (CO) 9 displays CO bands at 2082, 2019 and 1829 cm −1 . The number of IR-observable vibrational modes for some metal carbonyls are shown in the table. Exhaustive tabulations are available. [ 13 ] These rules apply to metal carbonyls in solution or the gas phase. Low- polarity solvents are ideal for high resolution. For measurements on solid samples of metal carbonyls, the number of bands can increase owing in part to site symmetry. [ 19 ]
Metal carbonyls are often characterized by 13 C NMR spectroscopy . To improve the sensitivity of this technique, complexes are often enriched with 13 CO. Typical chemical shift range for terminally bound ligands is 150 to 220 ppm. Bridging ligands resonate between 230 and 280 ppm. [ 1 ] The 13 C signals shift toward higher fields with an increasing atomic number of the central metal.
NMR spectroscopy can be used for experimental determination of the fluxionality . [ 26 ]
The activation energy of ligand exchange processes can be determined by the temperature dependence of the line broadening. [ 27 ]
Mass spectrometry provides information about the structure and composition of the complexes. Spectra for metal polycarbonyls are often easily interpretable, because the dominant fragmentation process is the loss of carbonyl ligands ( m / z = 28).
Electron ionization is the most common technique for characterizing the neutral metal carbonyls. Neutral metal carbonyls can be converted to charged species by derivatization , which enables the use of electrospray ionization (ESI), instrumentation for which is often widely available. For example, treatment of a metal carbonyl with alkoxide generates an anionic metallaformate that is amenable to analysis by ESI-MS:
Some metal carbonyls react with azide to give isocyanato complexes with release of nitrogen . [ 28 ] By adjusting the cone voltage or temperature, the degree of fragmentation can be controlled. The molar mass of the parent complex can be determined, as well as information about structural rearrangements involving loss of carbonyl ligands under ESI-MS conditions. [ 29 ]
Mass spectrometry combined with infrared photodissociation spectroscopy can provide vibrational informations for ionic carbonyl complexes in gas phase. [ 30 ]
In the investigation of the infrared spectrum of the Galactic Center of the Milky Way , monoxide vibrations of iron carbonyls in interstellar dust clouds were detected. [ 32 ] Iron carbonyl clusters were also observed in Jiange H5 chondrites identified by infrared spectroscopy. Four infrared stretching frequencies were found for the terminal and bridging carbon monoxide ligands. [ 33 ]
In the oxygen-rich atmosphere of the Earth, metal carbonyls are subject to oxidation to the metal oxides. It is discussed whether in the reducing hydrothermal environments of the prebiotic prehistory such complexes were formed and could have been available as catalysts for the synthesis of critical biochemical compounds such as pyruvic acid . [ 34 ] Traces of the carbonyls of iron, nickel, and tungsten were found in the gaseous emanations from the sewage sludge of municipal treatment plants . [ 35 ]
The hydrogenase enzymes contain CO bound to iron. It is thought that the CO stabilizes low oxidation states, which facilitates the binding of hydrogen . The enzymes carbon monoxide dehydrogenase and acetyl-CoA synthase also are involved in bioprocessing of CO. [ 36 ] Carbon monoxide containing complexes are invoked for the toxicity of CO and signaling. [ 37 ]
The synthesis of metal carbonyls is a widely studied subject of organometallic research. Since the work of Mond and then Hieber, many procedures have been developed for the preparation of mononuclear metal carbonyls as well as homo- and heterometallic carbonyl clusters. [ 38 ]
Nickel tetracarbonyl and iron pentacarbonyl can be prepared according to the following equations by reaction of finely divided metal with carbon monoxide : [ 39 ]
Nickel tetracarbonyl is formed with carbon monoxide already at 80 °C and atmospheric pressure, finely divided iron reacts at temperatures between 150 and 200 °C and a carbon monoxide pressure of 50–200 bar. [ 40 ] Other metal carbonyls are prepared by less direct methods. [ 41 ]
Some metal carbonyls are prepared by the reduction of metal halides in the presence of high pressure of carbon monoxide. A variety of reducing agents are employed, including copper , aluminum , hydrogen , as well as metal alkyls such as triethylaluminium . Illustrative is the formation of chromium hexacarbonyl from anhydrous chromium(III) chloride in benzene with aluminum as a reducing agent, and aluminum chloride as the catalyst: [ 39 ]
The use of metal alkyls, such as triethylaluminium and diethylzinc , as the reducing agent leads to the oxidative coupling of the alkyl radical to form the dimer alkane :
Tungsten , molybdenum , manganese , and rhodium salts may be reduced with lithium aluminium hydride . Vanadium hexacarbonyl is prepared with sodium as a reducing agent in chelating solvents such as diglyme . [ 9 ]
In the aqueous phase, nickel or cobalt salts can be reduced, for example by sodium dithionite . In the presence of carbon monoxide, cobalt salts are quantitatively converted to the tetracarbonylcobalt(−1) anion: [ 9 ]
Some metal carbonyls are prepared using CO directly as the reducing agent . In this way, Hieber and Fuchs first prepared dirhenium decacarbonyl from the oxide: [ 42 ]
If metal oxides are used carbon dioxide is formed as a reaction product. In the reduction of metal chlorides with carbon monoxide phosgene is formed, as in the preparation of osmium carbonyl chloride from the chloride salts. [ 38 ] Carbon monoxide is also suitable for the reduction of sulfides , where carbonyl sulfide is the byproduct.
Photolysis or thermolysis of mononuclear carbonyls generates di- and polymetallic carbonyls such as diiron nonacarbonyl (Fe 2 (CO) 9 ). [ 43 ] [ 44 ] On further heating, the products decompose eventually into the metal and carbon monoxide. [ citation needed ]
The thermal decomposition of triosmium dodecacarbonyl (Os 3 (CO) 12 ) provides higher-nuclear osmium carbonyl clusters such as Os 4 (CO) 13 , Os 6 (CO) 18 up to Os 8 (CO) 23 . [ 9 ]
Mixed ligand carbonyls of ruthenium , osmium , rhodium , and iridium are often generated by abstraction of CO from solvents such as dimethylformamide (DMF) and 2-methoxyethanol . Typical is the synthesis of IrCl(CO)(PPh 3 ) 2 from the reaction of iridium(III) chloride and triphenylphosphine in boiling DMF solution. [ 45 ]
Salt metathesis reaction of salts such as KCo(CO) 4 with [Ru(CO) 3 Cl 2 ] 2 leads selectively to mixed-metal carbonyls such as RuCo 2 (CO) 11 . [ 46 ]
The synthesis of ionic carbonyl complexes is possible by oxidation or reduction of the neutral complexes. Anionic metal carbonylates can be obtained for example by reduction of dinuclear complexes with sodium. A familiar example is the sodium salt of iron tetracarbonylate (Na 2 Fe(CO) 4 , Collman's reagent ), which is used in organic synthesis. [ 47 ]
The cationic hexacarbonyl salts of manganese , technetium and rhenium can be prepared from the carbonyl halides under carbon monoxide pressure by reaction with a Lewis acid . [ citation needed ]
The use of strong acids succeeded in preparing gold carbonyl cations such as [Au(CO) 2 ] + , which is used as a catalyst for the carbonylation of alkenes . [ 48 ] The cationic platinum carbonyl complex [Pt(CO) 4 ] 2+ can be prepared by working in so-called superacids such as antimony pentafluoride . [ 49 ] Although CO is considered generally as a ligand for low-valent metal ions, the tetravalent iron complex [Cp* 2 Fe] 2+ (16-valence electron complex) quantitatively binds CO to give the diamagnetic Fe(IV)-carbonyl [Cp* 2 FeCO] 2+ (18-valence electron complex). [ 50 ]
Metal carbonyls are important precursors for the synthesis of other organometallic complexes. Common reactions are the substitution of carbon monoxide by other ligands, the oxidation or reduction reactions of the metal center, and reactions at the carbon monoxide ligand. [ 1 ]
The substitution of CO ligands can be induced thermally or photochemically by donor ligands. The range of ligands is large, and includes phosphines , cyanide (CN − ), nitrogen donors, and even ethers, especially chelating ones. Alkenes , especially dienes , are effective ligands that afford synthetically useful derivatives. Substitution of 18-electron complexes generally follows a dissociative mechanism , involving 16-electron intermediates. [ 51 ]
Substitution proceeds via a dissociative mechanism :
The dissociation energy is 105 kJ/mol (25 kcal/mol) for nickel tetracarbonyl and 155 kJ/mol (37 kcal/mol) for chromium hexacarbonyl . [ 1 ]
Substitution in 17-electron complexes, which are rare, proceeds via associative mechanisms with a 19-electron intermediates.
The rate of substitution in 18-electron complexes is sometimes catalysed by catalytic amounts of oxidants, via electron transfer . [ 52 ]
Metal carbonyls react with reducing agents such as metallic sodium or sodium amalgam to give carbonylmetalate (or carbonylate) anions. Polynuclear or electron-imprecise clusters add the reductant to give mononuclear anions: [ 53 ]
Conversely, electron-precise mononuclear compounds lose CO and may form clusters: [ 53 ]
Mercury can insert into the metal–metal bonds of some polynuclear metal carbonyls:
The CO ligand is often susceptible to attack by nucleophiles . For example, trimethylamine oxide and potassium bis(trimethylsilyl)amide convert CO ligands to CO 2 and CN − , respectively. In the " Hieber base reaction", hydroxide ion attacks the CO ligand to give a metallacarboxylic acid , followed by the release of carbon dioxide and the formation of metal hydrides or carbonylmetalates. A well-studied example of this nucleophilic addition is the conversion of iron pentacarbonyl to hydridoiron tetracarbonyl anion :
Hydride reagents also attack CO ligands, especially in cationic metal complexes, to give the formyl derivative :
Organolithium reagents add with metal carbonyls to acylmetal carbonyl anions. O - Alkylation of these anions, such as with Meerwein salts , affords Fischer carbenes .
Despite being in low formal oxidation states , metal carbonyls are relatively unreactive toward many electrophiles . For example, they resist attack by alkylating agents, mild acids, and mild oxidizing agents . Most metal carbonyls do undergo halogenation . Iron pentacarbonyl , for example, forms ferrous carbonyl halides:
Metal–metal bonds are cleaved by halogens. Depending on the electron-counting scheme used, this can be regarded as an oxidation of the metal atoms:
Most metal carbonyl complexes contain a mixture of ligands. Examples include the historically important IrCl(CO)(P(C 6 H 5 ) 3 ) 2 and the antiknock agent (CH 3 C 5 H 4 )Mn(CO) 3 . The parent compounds for many of these mixed ligand complexes are the binary carbonyls, those species of the formula [M x (CO) n ] z , many of which are commercially available. The formulae of many metal carbonyls can be inferred from the 18-electron rule .
Large anionic clusters of nickel , palladium , and platinum are also well known. Many metal carbonyl anions can be protonated to give metal carbonyl hydrides .
Nonclassical describes those carbonyl complexes where ν CO is higher than that for free carbon monoxide. In nonclassical CO complexes, the C-O distance is shorter than free CO (113.7 pm). The structure of [Fe(CO) 6 ] 2+ , with d C-O = 112.9 pm, illustrates this effect. These complexes are usually cationic, sometimes dicationic. [ 59 ]
Metal carbonyls are used in several industrial processes. Perhaps the earliest application was the extraction and purification of nickel via nickel tetracarbonyl by the Mond process (see also carbonyl metallurgy ). [ citation needed ]
By a similar process carbonyl iron , a highly pure metal powder, is prepared by thermal decomposition of iron pentacarbonyl. Carbonyl iron is used inter alia for the preparation of inductors , pigments , as dietary supplements , [ 60 ] in the production of radar -absorbing materials in the stealth technology , [ 61 ] and in thermal spraying . [ citation needed ]
Metal carbonyls are used in a number of industrially important carbonylation reactions. In the oxo process , an alkene , hydrogen gas, and carbon monoxide react together with a catalyst (such as dicobalt octacarbonyl ) to give aldehydes . Illustrative is the production of butyraldehyde from propylene :
Butyraldehyde is converted on an industrial scale to 2-ethylhexanol , a precursor to PVC plasticizers , by aldol condensation , followed by hydrogenation of the resulting hydroxyaldehyde. The "oxo aldehydes" resulting from hydroformylation are used for large-scale synthesis of fatty alcohols, which are precursors to detergents . The hydroformylation is a reaction with high atom economy , especially if the reaction proceeds with high regioselectivity . [ citation needed ]
Another important reaction catalyzed by metal carbonyls is the hydrocarboxylation . The example below is for the synthesis of acrylic acid and acrylic acid esters:
Also the cyclization of acetylene to cyclooctatetraene uses metal carbonyl catalysts: [ 62 ]
In the Monsanto and Cativa processes , acetic acid is produced from methanol, carbon monoxide, and water using hydrogen iodide as well as rhodium and iridium carbonyl catalysts, respectively. Related carbonylation reactions afford acetic anhydride . [ 63 ]
Carbon monoxide-releasing molecules are metal carbonyl complexes that are being developed as potential drugs to release CO. At low concentrations, CO functions as a vasodilatory and an anti-inflammatory agent. CO-RMs have been conceived as a pharmacological strategic approach to carry and deliver controlled amounts of CO to tissues and organs. [ 64 ]
Many ligands are known to form homoleptic and mixed ligand complexes that are analogous to the metal carbonyls. [ citation needed ]
Metal nitrosyls, compounds featuring NO ligands , are numerous. In contrast to metal carbonyls, however, homoleptic metal nitrosyls are rare. NO is a stronger π-acceptor than CO. Well known nitrosyl carbonyls include CoNO(CO) 3 and Fe(NO) 2 (CO) 2 , which are analogues of Ni(CO) 4 . [ 65 ]
Complexes containing CS are known but uncommon. [ 66 ] [ 67 ] The rarity of such complexes is partly attributable to the fact that the obvious source material, carbon monosulfide , is unstable. Thus, the synthesis of thiocarbonyl complexes requires indirect routes, such as the reaction of disodium tetracarbonylferrate with thiophosgene :
Complexes of CSe and CTe have been characterized. [ 68 ]
Isocyanides also form extensive families of complexes that are related to the metal carbonyls. Typical isocyanide ligands are methyl isocyanide and t -butyl isocyanide (Me 3 CNC). A special case is CF 3 NC , an unstable molecule that forms stable complexes whose behavior closely parallels that of the metal carbonyls. [ 69 ]
The toxicity of metal carbonyls is due to toxicity of carbon monoxide , the metal, and because of the volatility and instability of the complexes, any inherent toxicity of the metal is generally made much more severe due to ease of exposure. Exposure occurs by inhalation, or for liquid metal carbonyls by ingestion or due to the good fat solubility by skin resorption. Most clinical experience were gained from toxicological poisoning with nickel tetracarbonyl and iron pentacarbonyl due to their use in industry. Nickel tetracarbonyl is considered as one of the strongest inhalation poisons. [ 70 ]
Inhalation of nickel tetracarbonyl causes acute non-specific symptoms similar to a carbon monoxide poisoning , such as nausea , cough , headache , fever , and dizziness . After some time, severe pulmonary symptoms such as cough, tachycardia , and cyanosis , or problems in the gastrointestinal tract occur. In addition to pathological alterations of the lung, such as by metalation of the alveoli, damages are observed in the brain, liver, kidneys, adrenal glands, and spleen. A metal carbonyl poisoning often necessitates a lengthy recovery. [ 71 ]
Chronic exposure by inhalation of low concentrations of nickel tetracarbonyl can cause neurological symptoms such as insomnia, headaches, dizziness and memory loss. [ 71 ] Nickel tetracarbonyl is considered carcinogenic, but it can take 20 to 30 years from the start of exposure to the clinical manifestation of cancer. [ 72 ]
Initial experiments on the reaction of carbon monoxide with metals were carried out by Justus von Liebig in 1834. By passing carbon monoxide over molten potassium he prepared a substance having the empirical formula KCO, which he called Kohlenoxidkalium . [ 73 ] As demonstrated later, the compound was not a carbonyl, but the potassium salt of benzenehexol (K 6 C 6 O 6 ) and the potassium salt of acetylenediol (K 2 C 2 O 2 ). [ 38 ]
The synthesis of the first true heteroleptic metal carbonyl complex was performed by Paul Schützenberger in 1868 by passing chlorine and carbon monoxide over platinum black , where dicarbonyldichloroplatinum (Pt(CO) 2 Cl 2 ) was formed. [ 74 ]
Ludwig Mond , one of the founders of Imperial Chemical Industries , investigated in the 1890s with Carl Langer and Friedrich Quincke various processes for the recovery of chlorine which was lost in the Solvay process by nickel metals, oxides, and salts. [ 38 ] As part of their experiments the group treated nickel with carbon monoxide. They found that the resulting gas colored the gas flame of a burner in a greenish-yellowish color; when heated in a glass tube it formed a nickel mirror. The gas could be condensed to a colorless, water-clear liquid with a boiling point of 43 °C. Thus, Mond and his coworker had discovered the first pure, homoleptic metal carbonyl, nickel tetracarbonyl (Ni(CO) 4 ). [ 75 ] The unusual high volatility of the metal compound nickel tetracarbonyl led Kelvin to the statement that Mond had "given wings to the heavy metals". [ 76 ]
The following year, Mond and Marcellin Berthelot independently discovered iron pentacarbonyl , which is produced by a similar procedure as nickel tetracarbonyl. Mond recognized the economic potential of this class of compounds, which he commercially used in the Mond process and financed more research on related compounds. Heinrich Hirtz and his colleague M. Dalton Cowap synthesized metal carbonyls of cobalt , molybdenum , ruthenium , and diiron nonacarbonyl . [ 77 ] [ 78 ] In 1906 James Dewar and H. O. Jones were able to determine the structure of diiron nonacarbonyl, which is produced from iron pentacarbonyl by the action of sunlight. [ 79 ] After Mond, who died in 1909, the chemistry of metal carbonyls fell for several years in oblivion. BASF started in 1924 the industrial production of iron pentacarbonyl by a process which was developed by Alwin Mittasch . The iron pentacarbonyl was used for the production of high-purity iron, so-called carbonyl iron , and iron oxide pigment . [ 40 ] Not until 1927 did A. Job and A. Cassal succeed in the preparation of chromium hexacarbonyl and tungsten hexacarbonyl , the first synthesis of other homoleptic metal carbonyls. [ citation needed ]
Walter Hieber played in the years following 1928 a decisive role in the development of metal carbonyl chemistry. He systematically investigated and discovered, among other things, the Hieber base reaction , the first known route to metal carbonyl hydrides and synthetic pathways leading to metal carbonyls such as dirhenium decacarbonyl . [ 80 ] Hieber, who was since 1934 the Director of the Institute of Inorganic Chemistry at the Technical University Munich published in four decades 249 papers on metal carbonyl chemistry. [ 38 ]
Also in the 1930s Walter Reppe , an industrial chemist and later board member of BASF, discovered a number of homogeneous catalytic processes, such as the hydrocarboxylation , in which olefins or alkynes react with carbon monoxide and water to form products such as unsaturated acids and their derivatives. [ 38 ] In these reactions, for example, nickel tetracarbonyl or cobalt carbonyls act as catalysts. [ 81 ] Reppe also discovered the cyclotrimerization and tetramerization of acetylene and its derivatives to benzene and benzene derivatives with metal carbonyls as catalysts. BASF built in the 1960s a production facility for acrylic acid by the Reppe process, which was only superseded in 1996 by more modern methods based on the catalytic propylene oxidation. [ citation needed ]
For the rational design of new complexes the concept of the isolobal analogy has been found useful. Roald Hoffmann was awarded the Nobel Prize in chemistry for the development of the concept. This describes metal carbonyl fragments of M(CO) n as parts of octahedral building blocks in analogy to the tetrahedral CH 3 –, CH 2 – or CH– fragments in organic chemistry. In example dimanganese decacarbonyl is formed in terms of the isolobal analogy of two d 7 Mn(CO) 5 fragments, that are isolobal to the methyl radical CH • 3 . In analogy to how methyl radicals combine to form ethane , these can combine to dimanganese decacarbonyl . The presence of isolobal analog fragments does not mean that the desired structures can be synthesized. In his Nobel Prize lecture Hoffmann emphasized that the isolobal analogy is a useful but simple model, and in some cases does not lead to success. [ 82 ]
The economic benefits of metal-catalysed carbonylations , such as Reppe chemistry and hydroformylation , led to growth of the area. Metal carbonyl compounds were discovered in the active sites of three naturally occurring enzymes. [ 83 ] | https://en.wikipedia.org/wiki/Metal_carbonyl |
In chemistry , a metal carbonyl cluster is a compound that contains two or more metal atoms linked in part by metal–metal bonds and containing carbon monoxide (CO) as the exclusive or predominant ligand. The area is a subfield of metal carbonyl chemistry, and many metal carbonyl clusters are in fact prepared from simple metal carbonyls. Simple examples include Fe 2 (CO) 9 , Fe 3 (CO) 12 , and Mn 2 (CO) 10 . [ 1 ] High nuclearity clusters include [Rh 13 (CO) 24 H 3 ] 2− and the stacked Pt 3 triangules [Pt 3n (CO) 6n ] 2− (n = 2–6). [ 2 ]
The first metal carbonyl clusters, Fe 3 (CO) 12 , Ir 4 (CO) 12 , and Rh 6 (CO) 16 , were reported starting in the 1930s, often by Walter Hieber. [ 3 ] [ 4 ] The structures were subsequently established by X-ray crystallography . [ 5 ]
Paolo Chini (1928–1980) was a pioneer for the synthesis and characterization of high-nuclearity metal carbonyl clusters. His first studies started in 1958, in the attempt to repeat a patent that claimed an improved selectivity in hydroformylation. From a mixture of iron and cobalt carbonyls the first bimetallic carbonyl cluster HFeCo 3 (CO) 12 was obtained. [ 6 ]
Binary carbonyl clusters consist only of metal and CO. They are the most widely studied and used metal carbonyl clusters. They arise in general by the condensation of unsaturated metal carbonyls. Dissociation of CO from Ru(CO) 5 would give Ru(CO) 4 , which could trimerize to Ru 3 (CO) 12 . The reaction mechanisms are more complicated than this simple scenario. Condensation of low-molecular-weight metal carbonyls requires decarbonylation, which can be induced thermally, photochemically, or using various reagents. The nuclearity (number of metal centers) of binary metal carbonyl clusters is usually no greater than six.
The synthesis and characterization of the platinum carbonyl dianions [Pt 3n (CO) 6n ] 2- (n = 1–10), also known as Chini clusters or more correctly Chini-Longoni clusters, are recognized by the scientific community as the most spectacular result of Chini's work. [ 6 ]
Chini clusters follow the general formula of [Pt 3 (CO) 6 ] n 2− , 1 < n < 10. [ 7 ] These clusters are prepared by reduction of hexachloroplatinate with strongly basic methanol under an atmosphere of CO. [ 8 ] These clusters consist of stacks of triangularly shaped Pt 3 subunits. Although these clusters were first reported in 1969 by Chatt and Booth, their structure were not established until Chini and Longoni's work in 1976. [ 7 ] [ 8 ]
Chini clusters are based on a planar triangular building block that can be condensed as multiple units forming chains usually anywhere from two to ten units long. The chains are formed by stacking of the planar units, extending through platinum to platinum bonds forming trigonal prismatic clusters. Within a triangular unit, the platinum–platinum bond lengths are 2.65 Å and between units the Pt–Pt bond lengths are 3.05 Å. Cluster structure is easily disrupted by deposition onto surfaces such as carbon or silicon, where the chains are broken, but the triangular subunits remain intact. [ 9 ] The tetramer [Pt 3 (CO) 6 ] 4 2− is the most common member of this series of clusters. [ 10 ] These clusters undergo reversible redox. They catalyze the hydrogenation of alkenes, ketones, and aldehydes.
Chini clusters can also be converted heterometal clusters and catalyze pH driven redox reactions and transport. First, the Chini clusters are the source of platinum atoms for the mixed metal cluster synthesis. [ 7 ] For instance, the reaction [Pt 12 (CO) 24 ] 2− with [Ag(PPh 3 ) 4 ] + produces heterometal cluster [Pt 3 Ag(CO) 3 (PPh 3 ) 5 ] + . Second, the Chini clusters with redox properties act as a catalyst that helps transport sodium ions and electrons in the same direction across a liquid membrane, driven by pH-gradient. The [Pt 3 (CO) 6 ] n-1 2− platinum clusters, where n=4 – 6, are reduced by OH − :
Although the nuclearity of binary metal carbonyl clusters is usually six or fewer, carbido clusters often have higher nuclearities. Metal carbonyls of the iron and cobalt triads are well known to form carbido derivatives . Examples include [Rh 6 C(CO) 15 ] 2− [ 12 ] and [Ru 6 C(CO) 16 ] 2− . [ 13 ] Carbonyl carbides exist not only with fully encapsulated carbon (e.g., [Fe 6 C(CO) 16 ] 2− ) but also with exposed carbon centres as in Fe 5 C(CO) 15 and Fe 4 C(CO) 13 . [ 14 ]
For low nuclearity clusters, bonding is often described as if it is localized. For this purpose, the 18-electron rule is used. Thus, 34 electrons in an organometallic complex predicts a dimetallic complex with a metal-metal bond. For higher nuclearity clusters, more elaborate rules are invoked including Jemmis mno rules and polyhedral skeletal electron pair theory .
Although clusters are often written with discrete M-M bonds, the nature of this bonding is unclear, especially when there are bridging ligands . [ 15 ] | https://en.wikipedia.org/wiki/Metal_carbonyl_cluster |
Metal carbonyl hydrides are complexes of transition metals with carbon monoxide and hydride as ligands . These complexes are useful in organic synthesis as catalysts in homogeneous catalysis , such as hydroformylation . [ 1 ]
Walter Hieber et al. prepared the first metal carbonyl hydride in 1931 by the so-called Hieber base reaction of metal carbonyls. In this reaction a hydroxide ion reacts with the carbon monoxide ligand of a metal carbonyl such as iron pentacarbonyl in a nucleophilic attack to form a metallacarboxylic acid . This intermedia releases of carbon dioxide in a second step, giving the iron tetracarbonyl hydride anion. The synthesis of cobalt tetracarbonyl hydride (HCo(CO) 4 ) proceeds in the same way. [ 2 ]
A further synthetic route is the reaction of the metal carbonyl with hydrogen. [ 3 ] The protonation of metal carbonyl anions, e.g. [Co(CO) 4 ] − , leads also to the formation of metal carbonyl hydrides.
The neutral metal carbonyl hydrides are often volatile and can be quite acidic. [ 5 ] The hydrogen atom is directly bounded to the metal. The metal-hydrogen bond length is for cobalt 114 pm, the metal-carbon bond length is for axial ligands 176 and 182 for the equatorial ligands. [ 6 ]
A direct metal-hydrogen bond was suspected by Hieber for H 2 Fe(CO) 4 . A number of metal carbonyl hydrides have been characterized by X-ray crystallography [ 7 ] and neutron diffraction . [ 6 ] [ 8 ] Nuclear magnetic resonance spectroscopy has also proved to be a useful characterization tool.
Metal carbonyl hydrides are used as catalysts in the hydroformylation of olefins . The catalyst is usually formed in situ in a reaction of a metal salt precursor with the syngas . The hydroformylation starts with the generation of a coordinatively unsaturated 16-electron metal carbonyl hydride complex like HCo(CO) 3 or HRh(CO)(PPh 3 ) 2 by dissociation of a ligand . Such complexes bind olefins in a first step via π-complexation, thus beginning the transformation of the alkene to the aldehyde.
Iron carbonyl hydrides occur in nature at the active sites of hydrogenase enzymes. [ 9 ] | https://en.wikipedia.org/wiki/Metal_carbonyl_hydride |
In metalworking and jewelry making, casting is a process in which a liquid metal is delivered into a mold (usually by a crucible ) that contains a negative impression (i.e., a three-dimensional negative image) of the intended shape. The metal is poured into the mold through a hollow channel called a sprue . The metal and mold are then cooled, and the metal part (the casting ) is extracted. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods. [ 1 ]
Casting processes have been known for thousands of years, and have been widely used for sculpture (especially in bronze ), jewelry in precious metals , and weapons and tools. Highly engineered castings are found in 90 percent of durable goods, including cars, trucks, aerospace, trains, mining and construction equipment, oil wells, appliances, pipes, hydrants, wind turbines, nuclear plants , medical devices, defense products, toys, and more. [ 2 ]
Traditional techniques include lost-wax casting (which may be further divided into centrifugal casting , and vacuum assist direct pour casting), plaster mold casting and sand casting .
The modern casting process is subdivided into two main categories: expendable and non-expendable casting. It is further broken down by the mold material, such as sand or metal, and pouring method, such as gravity, vacuum, or low pressure. [ 3 ]
Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) moldings. This method of mold casting involves the use of temporary, non-reusable molds.
Sand casting is one of the most popular and simplest types of casting, and has been used for centuries. Sand casting allows for smaller batches than permanent mold casting and at a very reasonable cost. Not only does this method allow manufacturers to create products at a low cost, but there are other benefits to sand casting, such as very small-size operations. The process allows for castings small enough fit in the palm of one's hand to those large enough for a train car bed (one casting can create the entire bed for one rail car). Sand casting also allows most metals to be cast depending on the type of sand used for the molds. [ 4 ]
Sand casting requires a lead time of days, or even weeks sometimes, for production at high output rates (1–20 pieces/hr-mold) and is unsurpassed for large-part production. Green (moist) sand, which is black in color, has almost no part weight limit, whereas dry sand has a practical part mass limit of 2,300–2,700 kg (5,100–6,000 lb). Minimum part weight ranges from 0.075–0.1 kg (0.17–0.22 lb). The sand is bonded using clays, chemical binders, or polymerized oils (such as motor oil). Sand can be recycled many times in most operations and requires little maintenance.
Loam molding has been used to produce large symmetrical objects such as cannon and church bells. Loam is a mixture of clay and sand with straw or dung. A model of the produced is formed in a friable material (the chemise). The mold is formed around this chemise by covering it with loam. This is then baked (fired) and the chemise removed. The mold is then stood upright in a pit in front of the furnace for the molten metal to be poured. Afterwards the mold is broken off. Molds can thus only be used once, so that other methods are preferred for most purposes.
Plaster casting is similar to sand casting except that plaster of paris is used instead of sand as a mold material. Generally, the form takes less than a week to prepare, after which a production rate of 1–10 units/hr-mold is achieved, with items as massive as 45 kg (99 lb) and as small as 30 g (1 oz) with very good surface finish and close tolerances . [ 5 ] Plaster casting is an inexpensive alternative to other molding processes for complex parts due to the low cost of the plaster and its ability to produce near net shape castings. The biggest disadvantage is that it can only be used with low melting point non-ferrous materials, such as aluminium , copper , magnesium , and zinc . [ 6 ]
Shell molding is similar to sand casting, but the molding cavity is formed by a hardened "shell" of sand instead of a flask filled with sand. The sand used is finer than sand casting sand and is mixed with a resin so that it can be heated by the pattern and hardened into a shell around the pattern. Because of the resin and finer sand, it gives a much finer surface finish. The process is easily automated and more precise than sand casting. Common metals that are cast include cast iron , aluminium, magnesium, and copper alloys. This process is ideal for complex items that are small to medium-sized.
Investment casting (known as lost-wax casting in art) is a process that has been practiced for thousands of years, with the lost-wax process being one of the oldest known metal forming techniques. From 5000 years ago, when beeswax formed the pattern, to today's high technology waxes, refractory materials, and specialist alloys, the castings ensure high-quality components are produced with the key benefits of accuracy, repeatability, versatility, and integrity.
Investment casting derives its name from the fact that the pattern is invested, or surrounded, with a refractory material. The wax patterns require extreme care for they are not strong enough to withstand forces encountered during the mold making. One advantage of investment casting is that the wax can be reused. [ 5 ]
The process is suitable for repeatable production of net shape components from a variety of different metals and high performance alloys. Although generally used for small castings, this process has been used to produce complete aircraft door frames, with steel castings of up to 300 kg and aluminium castings of up to 30 kg. Compared to other casting processes such as die casting or sand casting , it can be an expensive process. However, the components that can be produced using investment casting can incorporate intricate contours, and in most cases the components are cast near net shape, so require little or no rework once cast.
A durable plaster intermediate is often used as a stage toward the production of a bronze sculpture or as a pointing guide for the creation of a carved stone. With the completion of a plaster, the work is more durable (if stored indoors) than a clay original which must be kept moist to avoid cracking. With the low cost plaster at hand, the expensive work of bronze casting or stone carving may be deferred until a patron is found, and as such work is considered to be a technical, rather than artistic process, it may even be deferred beyond the lifetime of the artist.
In waste molding a simple and thin plaster mold, reinforced by sisal or burlap, is cast over the original clay mixture. When cured, it is then removed from the damp clay, incidentally destroying the fine details in undercuts present in the clay, but which are now captured in the mold. The mold may then at any later time (but only once) be used to cast a plaster positive image, identical to the original clay. The surface of this plaster may be further refined and may be painted and waxed to resemble a finished bronze casting.
This is a class of casting processes that use pattern materials that evaporate during the pour, which means there is no need to remove the pattern material from the mold before casting. The two main processes are lost-foam casting and full-mold casting.
Lost-foam casting is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of foam to simplify the investment casting process by removing the need to melt the wax out of the mold.
Full-mold casting is an evaporative-pattern casting process which is a combination of sand casting and lost-foam casting . It uses an expanded polystyrene foam pattern which is then surrounded by sand, much like sand casting. The metal is then poured directly into the mold, which vaporizes the foam upon contact.
Non-expendable mold casting differs from expendable processes in that the mold need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting. This form of casting also results in improved repeatability in parts produced and delivers near net shape results.
Permanent mold casting is a metal casting process that employs reusable molds ("permanent molds"), usually made from metal . The most common process uses gravity to fill the mold. However, gas pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush casting , produces hollow castings. Common casting metals are aluminum , magnesium , and copper alloys. Other materials include tin , zinc , and lead alloys and iron and steel are also cast in graphite molds. Permanent molds, while lasting more than one casting still have a limited life before wearing out.
The die casting process forces molten metal under high pressure into mold cavities (which are machined into dies). Most die castings are made from nonferrous metals , specifically zinc , copper, and aluminium-based alloys, but ferrous metal die castings are possible. The die casting method is especially suited for applications where many small to medium-sized parts are needed with good detail, a fine surface quality and dimensional consistency.
Semi-solid metal (SSM) casting is a modified die casting process that reduces or eliminates the residual porosity present in most die castings. Rather than using liquid metal as the feed material, SSM casting uses a higher viscosity feed material that is partially solid and partially liquid. A modified die casting machine is used to inject the semi-solid slurry into reusable hardened steel dies. The high viscosity of the semi-solid metal, along with the use of controlled die filling conditions, ensures that the semi-solid metal fills the die in a non-turbulent manner so that harmful porosity can be essentially eliminated.
Used commercially mainly for aluminium and magnesium alloys, SSM castings can be heat treated to the T4, T5 or T6 tempers. The combination of heat treatment, fast cooling rates (from using uncoated steel dies) and minimal porosity provides excellent combinations of strength and ductility. Other advantages of SSM casting include the ability to produce complex shaped parts net shape, pressure tightness, tight dimensional tolerances and the ability to cast thin walls. [ 7 ]
In this process molten metal is poured in the mold and allowed to solidify while the mold is rotating. Metal is poured into the center of the mold at its axis of rotation. Due to inertial force, the liquid metal is thrown out toward the periphery.
Centrifugal casting is both gravity and pressure independent since it creates its own force feed using a temporary sand mold held in a spinning chamber. Lead time varies with the application. Semi- and true-centrifugal processing permit 30–50 pieces/hr-mold to be produced, with a practical limit for batch processing of approximately 9000 kg total mass with a typical per-item limit of 2.3–4.5 kg.
Industrially, the centrifugal casting [ 8 ] of railway wheels was an early application of the method developed by the German industrial company Krupp and this capability enabled the rapid growth of the enterprise.
Small art pieces such as jewelry are often cast by this method using the lost wax process, as the forces enable the rather viscous liquid metals to flow through very small passages and into fine details such as leaves and petals. This effect is similar to the benefits from vacuum casting, also applied to jewelry casting.
Continuous casting is a refinement of the casting process for the continuous, high-volume production of metal sections with a constant cross-section. It's primarily used to produce a semi-finished products for further processing. [ 9 ] : 339 Molten metal is poured into an open-ended, water-cooled mold, which allows a 'skin' of solid metal to form over the still-liquid center, gradually solidifying the metal from the outside in. After solidification, the strand, as it is sometimes called, is continuously withdrawn from the mold. Predetermined lengths of the strand can be cut off by either mechanical shears or traveling oxyacetylene torches and transferred to further forming processes, or to a stockpile. Cast sizes can range from strip (a few millimeters thick by about five meters wide) to billets (90 to 160 mm square) to slabs (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an initial hot rolling process before being cut.
Continuous casting is used due to the lower costs associated with continuous production of a standard product, and also increased quality of the final product. Metals such as steel, copper, aluminum and lead are continuously cast, with steel being the metal with the greatest tonnages cast using this method.
The upcasting (up-casting, upstream, or upward casting) is a method of either vertical or horizontal continuous casting of rods and pipes of various profiles (cylindrical, square, hexagonal, slabs etc.) of 8-30mm in diameter. [ 10 ] Copper (Cu), bronze (Cu· Sn alloy), nickel alloys are usually used because of greater casting speed (in case of vertical upcasting) and because of better physical features obtained. The advantage of this method is that metals are almost oxygen-free and that the rate of product crystallization (solidification) may be adjusted in a crystallizer - a high-temperature resistant device that cools a growing metal rod or pipe by using water. [ 10 ]
The method is comparable to Czochralski method of growing silicon (Si) crystals, which is a metalloid .
Metal casting processes uses the following terminology: [ 11 ]
Some specialized processes, such as die casting, use additional terminology.
Casting is a solidification process, which means the solidification phenomenon controls most of the properties of the casting. Moreover, most of the casting defects occur during solidification, such as gas porosity and solidification shrinkage . [ 12 ]
Solidification occurs in two steps: nucleation and crystal growth . In the nucleation stage, solid particles form within the liquid. When these particles form, their internal energy is lower than the surrounded liquid, which creates an energy interface between the two. The formation of the surface at this interface requires energy, so as nucleation occurs, the material actually undercools (i.e. cools below its solidification temperature) because of the extra energy required to form the interface surfaces. It then recalescences, or heats back up to its solidification temperature, for the crystal growth stage. Nucleation occurs on a pre-existing solid surface because not as much energy is required for a partial interface surface as for a complete spherical interface surface. This can be advantageous because fine-grained castings possess better properties than coarse-grained castings. A fine grain structure can be induced by grain refinement or inoculation , which is the process of adding impurities to induce nucleation. [ 13 ]
All of the nucleations represent a crystal, which grows as the heat of fusion is extracted from the liquid until there is no liquid left. The direction, rate, and type of growth can be controlled to maximize the properties of the casting. Directional solidification is when the material solidifies at one end and proceeds to solidify to the other end; this is the most ideal type of grain growth because it allows liquid material to compensate for shrinkage. [ 13 ]
Cooling curves are important in controlling the quality of a casting. The most important part of the cooling curve is the cooling rate which affects the microstructure and properties. Generally speaking, an area of the casting which is cooled quickly will have a fine grain structure and an area which cools slowly will have a coarse grain structure. Below is an example cooling curve of a pure metal or eutectic alloy, with defining terminology. [ 14 ]
Note that before the thermal arrest the material is a liquid and after it the material is a solid; during the thermal arrest the material is converting from a liquid to a solid. Also, note that the greater the superheat the more time there is for the liquid material to flow into intricate details. [ 15 ]
The above cooling curve depicts a basic situation with a pure metal, however, most castings are of alloys, which have a cooling curve shaped as shown below.
Note that there is no longer a thermal arrest, instead there is a freezing range. The freezing range corresponds directly to the liquidus and solidus found on the phase diagram for the specific alloy.
The local solidification time can be calculated using Chvorinov's rule, which is:
Where t is the solidification time, V is the volume of the casting, A is the surface area of the casting that contacts the mold , n is a constant, and B is the mold constant. It is most useful in determining if a riser will solidify before the casting, because if the riser does solidify first then it is worthless. [ 16 ]
The gating system serves many purposes, the most important being conveying the liquid material to the mold, but also controlling shrinkage, the speed of the liquid, turbulence, and trapping dross . The gates are usually attached to the thickest part of the casting to assist in controlling shrinkage. In especially large castings multiple gates or runners may be required to introduce metal to more than one point in the mold cavity. The speed of the material is important because if the material is traveling too slowly it can cool before completely filling, leading to misruns and cold shuts. If the material is moving too fast then the liquid material can erode the mold and contaminate the final casting. The shape and length of the gating system can also control how quickly the material cools; short round or square channels minimize heat loss. [ 17 ]
The gating system may be designed to minimize turbulence, depending on the material being cast. For example, steel, cast iron, and most copper alloys are turbulent insensitive, but aluminium and magnesium alloys are turbulent sensitive. The turbulent insensitive materials usually have a short and open gating system to fill the mold as quickly as possible. However, for turbulent sensitive materials short sprues are used to minimize the distance the material must fall when entering the mold. Rectangular pouring cups and tapered sprues are used to prevent the formation of a vortex as the material flows into the mold; these vortices tend to suck gas and oxides into the mold. A large sprue well is used to dissipate the kinetic energy of the liquid material as it falls down the sprue, decreasing turbulence. The choke , which is the smallest cross-sectional area in the gating system used to control flow, can be placed near the sprue well to slow down and smooth out the flow. Note that on some molds the choke is still placed on the gates to make separation of the part easier, but induces extreme turbulence. [ 18 ] The gates are usually attached to the bottom of the casting to minimize turbulence and splashing. [ 17 ]
The gating system may also be designed to trap dross. One method is to take advantage of the fact that some dross has a lower density than the base material so it floats to the top of the gating system. Therefore, long flat runners with gates that exit from the bottom of the runners can trap dross in the runners; note that long flat runners will cool the material more rapidly than round or square runners. For materials where the dross is a similar density to the base material, such as aluminium, runner extensions and runner wells can be advantageous. These take advantage of the fact that the dross is usually located at the beginning of the pour, therefore the runner is extended past the last gate(s) and the contaminates are contained in the wells. Screens or filters may also be used to trap contaminates. [ 18 ]
It is important to keep the size of the gating system small, because it all must be cut from the casting and remelted to be reused. The efficiency, or yield , of a casting system can be calculated by dividing the weight of the casting by the weight of the metal poured. Therefore, the higher the number the more efficient the gating system/risers. [ 19 ]
There are three types of shrinkage: shrinkage of the liquid , solidification shrinkage and patternmaker's shrinkage . The shrinkage of the liquid is rarely a problem because more material is flowing into the mold behind it. Solidification shrinkage occurs because metals are less dense as a liquid than a solid, so during solidification the metal density dramatically increases. Patternmaker's shrinkage refers to the shrinkage that occurs when the material is cooled from the solidification temperature to room temperature, which occurs due to thermal contraction . [ 20 ]
Most materials shrink as they solidify, but, as the adjacent table shows, a few materials do not, such as gray cast iron . For the materials that do shrink upon solidification the type of shrinkage depends on how wide the freezing range is for the material. For materials with a narrow freezing range, less than 50 °C (122 °F), [ 23 ] a cavity, known as a pipe , forms in the center of the casting, because the outer shell freezes first and progressively solidifies to the center. Pure and eutectic metals usually have narrow solidification ranges. These materials tend to form a skin in open air molds, therefore they are known as skin forming alloys . [ 23 ] For materials with a wide freezing range, greater than 110 °C (230 °F), [ 23 ] much more of the casting occupies the mushy or slushy zone (the temperature range between the solidus and the liquidus), which leads to small pockets of liquid trapped throughout and ultimately porosity. These castings tend to have poor ductility , toughness , and fatigue resistance. Moreover, for these types of materials to be fluid-tight, a secondary operation is required to impregnate the casting with a lower melting point metal or resin. [ 21 ] [ 24 ]
For the materials that have narrow solidification ranges, pipes can be overcome by designing the casting to promote directional solidification, which means the casting freezes first at the point farthest from the gate, then progressively solidifies toward the gate. This allows a continuous feed of liquid material to be present at the point of solidification to compensate for the shrinkage. Note that there is still a shrinkage void where the final material solidifies, but if designed properly, this will be in the gating system or riser. [ 21 ]
Risers, also known as feeders , are the most common way of providing directional solidification. It supplies liquid metal to the solidifying casting to compensate for solidification shrinkage. For a riser to work properly the riser must solidify after the casting, otherwise it cannot supply liquid metal to shrinkage within the casting. Risers add cost to the casting because it lowers the yield of each casting; i.e. more metal is lost as scrap for each casting. Another way to promote directional solidification is by adding chills to the mold. A chill is any material which will conduct heat away from the casting more rapidly than the material used for molding. [ 25 ]
Risers are classified by three criteria. The first is if the riser is open to the atmosphere, if it is then it is called an open riser, otherwise it is known as a blind type. The second criterion is where the riser is located; if it is located on the casting then it is known as a top riser and if it is located next to the casting it is known as a side riser . Finally, if the riser is located on the gating system so that it fills after the molding cavity, it is known as a live riser or hot riser , but if the riser fills with materials that have already flowed through the molding cavity it is known as a dead riser or cold riser . [ 19 ]
Riser aids are items used to assist risers in creating directional solidification or reducing the number of risers required. One of these items are chills which accelerate cooling in a certain part of the mold. There are two types: external and internal chills. External chills are masses of high-heat-capacity and high-thermal-conductivity material that are placed on an edge of the molding cavity. Internal chills are pieces of the same metal that is being poured, which are placed inside the mold cavity and become part of the casting. Insulating sleeves and toppings may also be installed around the riser cavity to slow the solidification of the riser. Heater coils may also be installed around or above the riser cavity to slow solidification. [ 26 ]
Shrinkage after solidification can be dealt with by using an oversized pattern designed specifically for the alloy used. Contraction rule s , or shrink rule s , are used to make the patterns oversized to compensate for this type of shrinkage. [ 27 ] These rulers are up to 2.5% oversize, depending on the material being cast. [ 26 ] These rulers are mainly referred to by their percentage change. A pattern made to match an existing part would be made as follows: First, the existing part would be measured using a standard ruler, then when constructing the pattern, the pattern maker would use a contraction rule, ensuring that the casting would contract to the correct size.
Note that patternmaker's shrinkage does not take phase change transformations into account. For example, eutectic reactions, martensitic reactions, and graphitization can cause expansions or contractions. [ 27 ]
The mold cavity of a casting does not reflect the exact dimensions of the finished part due to a number of reasons. These modifications to the mold cavity are known as allowances and account for patternmaker's shrinkage, draft, machining, and distortion. In non-expendable processes, these allowances are imparted directly into the permanent mold, but in expendable mold processes they are imparted into the patterns, which later form the mold cavity. [ 27 ] Note that for non-expendable molds an allowance is required for the dimensional change of the mold due to heating to operating temperatures. [ 28 ]
For surfaces of the casting that are perpendicular to the parting line of the mold a draft must be included. This is so that the casting can be released in non-expendable processes or the pattern can be released from the mold without destroying the mold in expendable processes. The required draft angle depends on the size and shape of the feature, the depth of the mold cavity, how the part or pattern is being removed from the mold, the pattern or part material, the mold material, and the process type. Usually the draft is not less than 1%. [ 27 ]
The machining allowance varies drastically from one process to another. Sand castings generally have a rough surface finish, therefore need a greater machining allowance, whereas die casting has a very fine surface finish, which may not need any machining tolerance. Also, the draft may provide enough of a machining allowance to begin with. [ 28 ]
The distortion allowance is only necessary for certain geometries. For instance, U-shaped castings will tend to distort with the legs splaying outward, because the base of the shape can contract while the legs are constrained by the mold. This can be overcome by designing the mold cavity to slope the leg inward to begin with. Also, long horizontal sections tend to sag in the middle if ribs are not incorporated, so a distortion allowance may be required. [ 28 ]
Cores may be used in expendable mold processes to produce internal features. The core can be of metal but it is usually done in sand.
There are a few common methods for filling the mold cavity: gravity , low-pressure , high-pressure , and vacuum . [ 29 ]
Vacuum filling, also known as counter-gravity filling, is more metal efficient than gravity pouring because less material solidifies in the gating system. Gravity pouring only has a 15 to 50% metal yield as compared to 60 to 95% for vacuum pouring. There is also less turbulence, so the gating system can be simplified since it does not have to control turbulence. Plus, because the metal is drawn from below the top of the pool the metal is free from dross and slag, as these are lower density (lighter) and float to the top of the pool. The pressure differential helps the metal flow into every intricacy of the mold. Finally, lower temperatures can be used, which improves the grain structure. [ 29 ] The first patented vacuum casting machine and process dates to 1879. [ 30 ]
Low-pressure filling uses 5 to 15 psig (35 to 100 kPag) of air pressure to force liquid metal up a feed tube into the mold cavity. This eliminates turbulence found in gravity casting and increases density, repeatability, tolerances, and grain uniformity. After the casting has solidified the pressure is released and any remaining liquid returns to the crucible, which increases yield. [ 31 ]
Tilt filling , also known as tilt casting , is an uncommon filling technique where the crucible is attached to the gating system and both are slowly rotated so that the metal enters the mold cavity with little turbulence. The goal is to reduce porosity and inclusions by limiting turbulence. For most uses tilt filling is not feasible because the following inherent problem: if the system is rotated slow enough to not induce turbulence, the front of the metal stream begins to solidify, which results in mis-runs. If the system is rotated faster it induces turbulence, which defeats the purpose. Durville of France was the first to try tilt casting, in the 1800s. He tried to use it to reduce surface defects when casting coinage from aluminium bronze . [ 32 ]
The grain macrostructure in ingots and most castings have three distinct regions or zones: the chill zone, columnar zone, and equiaxed zone. The image below depicts these zones.
The chill zone is named so because it occurs at the walls of the mold where the wall chills the material. Here is where the nucleation phase of the solidification process takes place. As more heat is removed the grains grow towards the center of the casting. These are thin, long columns that are perpendicular to the casting surface, which are undesirable because they have anisotropic properties. Finally, in the center the equiaxed zone contains spherical, randomly oriented crystals. These are desirable because they have isotropic properties. The creation of this zone can be promoted by using a low pouring temperature, alloy inclusions, or inoculants . [ 16 ]
Common inspection methods for steel castings are magnetic particle testing and liquid penetrant testing . [ 33 ] Common inspection methods for aluminum castings are radiography , ultrasonic testing , and liquid penetrant testing . [ 34 ]
There are a number of problems that can be encountered during the casting process. The main types are: gas porosity , shrinkage defects , mold material defects , pouring metal defects , and metallurgical defects .
Casting processes simulation uses numerical methods to calculate cast component quality considering mold filling, solidification and cooling, and provides a quantitative prediction of casting mechanical properties, thermal stresses and distortion. Simulation accurately describes a cast component's quality up-front before production starts. The casting rigging can be designed with respect to the required component properties. This has benefits beyond a reduction in pre-production sampling, as the precise layout of the complete casting system also leads to energy , material, and tooling savings.
The software supports the user in component design, the determination of melting practice and casting methoding through to pattern and mold making, heat treatment , and finishing. This saves costs along the entire casting manufacturing route.
Casting process simulation was initially developed at universities starting from the early 1970s , mainly in Europe and in the U.S., and is regarded as the most important innovation in casting technology over the last 50 years. Since the late 1980s , commercial programs are available which make it possible for foundries to gain new insight into what is happening inside the mold or die during the casting process. [ 35 ] | https://en.wikipedia.org/wiki/Metal_casting |
Casting process simulation is a computational technique used in industry and metallurgy to model and analyze the metal-casting process . This technology allows engineers to predict and visualize the flow of molten metal, crystallization patterns, and potential defects in the casting before the start of the actual production process . By simulating the casting process, manufacturers can optimize mold design, reduce material consumption, and improve the quality of the final product.
The theoretical foundations of heat conduction , critically important for casting simulation, were established by J ean-Baptiste Joseph Fourier at the École polytechnique in Paris. His thesis "Analytical Theory of Heat," [ 1 ] awarded in 1822, laid the groundwork for all subsequent calculations of heat conduction and heat transfer in solid materials. Additionally, French physicist and engineer Claude-Louis Navier and Irish mathematician and physicist George Gabriel Stokes provided the foundations of fluid dynamics , which led to the development of the Navier-Stokes equations . [ 2 ] [ 3 ] Adolph Fick , working in the 19th century at the University of Zurich , developed the fundamental equations describing diffusion , published in 1855. [ 4 ]
The beginning of simulation in casting started in the 1950s when V. Pashkis used analog computers to predict the movement of the crystallization front. [ 5 ] The first use of digital computers to solve problems related to casting was carried out by Dr K. Fursund in 1962, who considered the penetration of steel into a sand mold. [ 6 ] A pioneering work by J. G. Hentzel and J. Keverian in 1965 was the two-dimensional simulation of the crystallization of steel castings, using a program developed by General Electric to simulate heat transfer. [ 7 ] In 1968, Ole Vestby was the first to use the finite difference method to program a 2D model that evaluated the temperature distribution during welding . [ 8 ]
The 1980s marked a significant increase in research and development activities around the topic of casting process simulation with contributions from various international groups, including J. T. Berry and R. D. Pielke in the United States, E. Niyama in Japan, W. Kurz in Lausanne, and F. Durand in Grenoble . An especially important role in advancing this field was played by Professor P. R. Sahm at the Aachen Foundry Institute . Key milestones of this period were the introduction of the " criterion function " by Hansen and Berry [ 9 ] in 1980, the Niyama criterion function [ 10 ] for the representation of central porosities in 1982, and the proposal of a criterion function for the detection of hot cracks in steel castings by E. Fehlner and P. N. Hansen in 1984. [ 11 ] In the late 1980s, the first capabilities for simulating mold filling were developed.
The 1990s focused on the simulation of stresses and strains in castings with significant contributions from Hattel and Hansen in 1990. This decade also saw efforts to predict microstructures and mechanical properties with the pioneering work of I. Svensson and M. Wessen in Sweden. [ 12 ] [ 13 ]
The production of casting is one of the most complex and multifaceted processes in metallurgy, requiring careful control and an understanding of a multitude of physical and chemical phenomena. To effectively manage this process and ensure the high quality of the final products, it is essential to have a deep understanding of the interaction of the various casting parameters. In this context, the mathematical modeling of casting acts as a critically important scientific tool, allowing for detailed analysis and optimization of the casting process based on mathematical principles. [ 14 ] [ 15 ] [ 16 ]
Mathematical modeling of casting is a complex process that involves the formulation and solution of mathematical equations that describe physical phenomena such as thermal conductivity, fluid dynamics, phase transformations , among others. To solve these equations, various numerical analysis methods are applied, among which the finite element method (FEM), [ 17 ] finite difference method (FDM), and finite volume method (FVM) hold a special place. Each of these methods has its particular characteristics and is applied depending on the specific modeling tasks and the requirements for precision and efficiency in the calculations.
Finite difference method (FDM): This method is based on differential equations of heat and mass transfer, which are approximated using finite difference relationships. [ 18 ] The advantage of the FDM is its simplicity and the ability to simplify the solution of multidimensional problems. However, the method has limitations in modeling the boundaries of complex areas and performs poorly for castings with thin walls. [ 19 ]
The finite element method and Finite volume method (FVM): [ 20 ] Both methods are based on integral equations of heat and mass transfer. They provide a good approximation of the boundaries and allow the use of elements with different discretizations . The main drawbacks are the need for a finite element generator, the complexity of the equations, and large requirements for memory and time resources.
Modifications of the FVM: These methods attempt to combine the simplicity of the FDM with a good approximation of the boundaries of the FEM. They have the potential to improve the approximation of boundaries between different materials and phases. [ citation needed ] [ 21 ]
The analysis of different methods of mathematical modeling of casting processes shows that the finite element method is one of the most reliable and optimal approaches for casting simulation. [ 22 ] Despite higher computational resource requirements and complexity in implementation compared to the finite difference method and finite volume method, the FEM provides high accuracy in modeling boundaries, complex geometries, and temperature fields, which is critically important for predicting defects in castings and optimizing casting processes. [ 23 ] [ 24 ]
Computer-aided engineering (CAE) systems for casting processes have long been used by foundries around the world as a "virtual foundry workshop ," where it is possible to perform and verify any idea that arises in the minds of designers and technologists . [ 25 ] The global market for CAE for casting processes can already be considered established. [ 26 ]
Within the structure of the company for the development of the technology of a new casting, a computer-aided design department for casting processes is created, responsible for operating CAE systems for casting processes. Calculations are carried out by specialists of the department according to their job instructions , and interaction with other departments is regulated by technological design instructions. [ 27 ]
The process begins with the delivery of the 3D model and drawing of the part to the foundry technologists, who coordinate the casting configuration with the mechanical workshop and determine the margins . [ 28 ] Then, the technology is developed in the CAE department and transferred to the foundry workshop for experimental castings. The results are monitored, and if necessary, the castings are examined in the central laboratory of the factory . If defects are detected, an adjustment of the model parameters and the technological process is made in the CAE department, after which the technology is tested again in the workshop. [ citation needed ] [ 29 ]
This cycle is repeated until suitable castings are obtained, after which the technology is considered developed and implemented in mass production . [ 30 ] [ 31 ]
In the modern foundry industry, software for the simulation of casting processes is widely used. Among the multitude of software solutions available, it is worth mentioning the most prominent and widely used products: Procast, MAGMASOFT, and PoligonSoft.
ProCAST: a casting process modeling system using the finite element method, which provides the joint solution of temperature, hydrodynamics , and deformation problems, along with unique metallurgical capabilities, for all casting processes and casting alloys. In addition to the main aspects of casting production – filling, crystallization, and porosity prediction, ProCAST is capable of predicting the occurrence of deformations and residual stresses in the casting and can be used to analyze processes such as core making , centrifugal casting , lost wax casting , continuous casting . [ 32 ]
PoligonSoft: a casting process modeling system using the finite element method. Applicable for modeling almost any casting technology and any casting alloy. For a long time, PoligonSoft was the only casting process modeling system in the world that included a special model for calculating microporosity . [ 33 ] [ 34 ] To date, this model can be considered the most stable, and the results obtained with its help can satisfy the most demanding users. In many respects, PoligonSoft can be considered the Russian equivalent of the ProCAST system. [ 35 ]
MAGMASOFT: a casting process modeling system using the finite difference method. It allows analyzing thermal processes, mold filling, crystallization, and predicting defects in castings. [ 36 ] The program includes modules for different casting technologies and helps optimize casting parameters to improve product quality. MAGMASOFT is an effective tool for increasing the productivity and quality of casting production. [ 37 ]
The simulation of the casting process reflects the user's knowledge, who decides whether the filling system has led to an acceptable result. Optimization suggestions must come from the operator. The main problem is that all processes occur simultaneously and are interconnected: changes in one parameter affect many quality characteristics of the casting. [ 38 ]
Autonomous optimization, which began in the late 1980s, uses the simulation tool as a virtual testing ground, changing filling conditions and process parameters to find the optimal solution. [ 39 ] This allows evaluating numerous process parameters and their impact on process stability.
It is important to remember that only what can be modeled can be optimized. Optimization does not replace process knowledge or experience. [ 40 ] The simulation user must know the objectives and quality criteria necessary to achieve those objectives and formulate specific questions to the program to obtain quantitative solutions. [ 41 ] | https://en.wikipedia.org/wiki/Metal_casting_simulation |
Metal cluster compounds are a molecular ion or neutral compound composed of three or more metals and featuring significant metal-metal interactions. [ 2 ]
The development of metal carbonyl clusters such as Ni(CO) 4 and Fe(CO) 5 led quickly to the isolation of Fe 2 (CO) 9 and Fe 3 (CO) 12 . Rundle and Dahl discovered that Mn 2 (CO) 10 featured an "unsupported" Mn-Mn bond, thereby verifying the ability of metals to bond to one another in molecules. In the 1970s, Paolo Chini demonstrated that very large clusters could be prepared from the platinum metals, one example being [Rh 13 (CO) 24 H 3 ] 2− . This area of cluster chemistry has benefited from single-crystal X-ray diffraction .
Many metal carbonyl clusters contain ligands aside from CO. For example, the CO ligand can be replaced with myriad alternatives such as phosphines, isocyanides, alkenes, hydride, etc. Some carbonyl clusters contain two or more metals. Others contain carbon vertices. One example is the methylidyne-tricobalt cluster [Co 3 (CH)(CO) 9 ] . [ 3 ] The above-mentioned cluster serves as an example of an overall zero-charged (neutral) cluster. In addition, cationic (positively charged) rather than neutral organometallic trimolybdenum [ 4 ] [ 5 ] or tritungsten [ 6 ] clusters are also known. The first representative of these ionic organometallic clusters is [Mo 3 (CCH 3 ) 2 (O 2 CCH 3 ) 6 (H 2 O) 3 ] 2+ .
The halides of low-valent early metals often are clusters with extensive M-M bonding. The situation contrasts with the higher halides of these metals and virtually all halides of the late transition metals, where metal-halide bonding is replete.
Transition metal halide clusters are prevalent for the heavier metals: Zr, Hf, Nb, Ta, Mo, W, and Re. For the earliest metals Zr and Hf, interstitial carbide ligands are also common. One example is Zr 6 CCl 12 . [ 7 ] One structure type features six terminal halides and 12 edge-bridging halides. This motif is exemplified by tungsten(III) chloride , [Ta 6 Cl 18 ] 4− , [ 8 ] Another common structure has six terminal halides and 8 bridging halides, e.g. Mo 6 Cl 14 2− .
Many of the early metal clusters can only be prepared when they incorporate interstitial atoms.
In terms of history, Linus Pauling showed that " MoCl 2 " consisted of Mo 6 octahedra. F. Albert Cotton established that " ReCl 3 " in fact features subunits of the cluster Re 3 Cl 9 , which could be converted to a host of adducts without breaking the Re-Re bonds. Because this compound is diamagnetic and not paramagnetic the rhenium bonds are double bonds and not single bonds. In the solid state further bridging occurs between neighbours and when this compound is dissolved in hydrochloric acid a Re 3 Cl 12 3− complex forms. An example of a tetranuclear complex is hexadecamethoxytetratungsten W 4 (OCH 3 ) 12 with tungsten single bonds. A related group of clusters with the general formula M x Mo 6 X 8 such as PbMo 6 S 8 . These sulfido clusters are called Chevrel phases .
In the 1970s, ferredoxin was demonstrated to contain Fe 4 S 4 clusters and later nitrogenase was shown to contain a distinctive MoFe 7 S 9 active site. [ 10 ] The Fe-S clusters mainly serve as redox cofactors, but some have a catalytic function. In the area of bioinorganic chemistry , a variety of Fe-S clusters have also been identified that have CO as ligands.
FeMoco , the active site of most nitrogenases , features a Fe 7 MoS 9 C cluster. [ 11 ]
Zintl compounds feature naked anionic clusters that are generated by reduction of heavy main group p elements, mostly metals or semimetals, with alkali metals, often as a solution in anhydrous liquid ammonia or ethylenediamine . [ 12 ] Examples of Zintl anions are [Bi 3 ] 3− , [Sn 9 ] 4− , [Pb 9 ] 4− , and [Sb 7 ] 3− . [ 13 ] Although these species are called "naked clusters," they are usually strongly associated with alkali metal cations. Some examples have been isolated using cryptate complexes of the alkali metal cation, e.g., [Pb 10 ] 2− anion, which features a capped square antiprismatic shape. [ 14 ] According to Wade's rules (2n+2) the number of cluster electrons is 22 and therefore a closo cluster . The compound is prepared from oxidation of K 4 Pb 9 [ 15 ] by Au + in PPh 3 AuCl (by reaction of tetrachloroauric acid and triphenylphosphine ) in ethylene diamine with 2.2.2-crypt . This type of cluster was already known as is the endohedral Ni@Pb 10 2− (the cage contains one nickel atom). The icosahedral tin cluster Sn 12 2− or stannaspherene anion is another closed shell structure observed (but not isolated) with photoelectron spectroscopy . [ 16 ] [ 17 ] With an internal diameter of 6.1 Ångstrom , it is of comparable size to fullerene and should be capable of containing small atoms in the same manner as endohedral fullerenes , and indeed exists a Sn 12 cluster that contains an Ir atom: [Ir@Sn 12 ] 3− . [ 18 ]
Elementoid clusters are ligand-stabilized clusters of metal compounds that possess more direct element-element than element-ligand contacts. Examples of structurally characterized clusters feature ligand stabilized cores of Al 77 , Ga 84 , and Pd 145 . [ 19 ]
These clusters consist of at least two different (semi)metallic elements, and possess more direct metal-metal than metal-ligand contacts. The suffix "oid" designate that such clusters possess at a molecular scale, atom arrangements that appear in bulk intermetallic compounds with high coordination numbers of the atoms, such as for example in Laves phase and Hume-Rothery phases. [ 20 ] Ligand-free intermetalloid clusters include also endohedrally filled Zintl clusters. [ 13 ] [ 21 ] A synonym for ligand-stabilized intermetalloid clusters is "molecular alloy". The clusters appear as discrete units in intermetallic compounds separated from each other by electropositive atoms such as [Sn@Cu 12 @Sn 20 ] 12− , [ 20 ] as soluble ions [As@Ni 12 @As 20 ] 3− [ 13 ] or as ligand-stabilized molecules such as [Mo(ZnCH 3 ) 9 (ZnCp*) 3 ]. [ 22 ] | https://en.wikipedia.org/wiki/Metal_cluster_compound |
Metal dusting is "a catastrophic form of corrosion that occurs when susceptible materials are exposed to environments with high carbon activities." [ 1 ] The corrosion manifests itself as a break-up of bulk metal to metal powder. The suspected mechanism is firstly the deposition of a graphite layer on the surface of the metal, usually from carbon monoxide (CO) in the vapour phase. This graphite layer is then thought to form metastable M 3 C species (where M is the metal), which migrate away from the metal surface. However, in some regimes no M 3 C species are observed indicating a direct transfer of metal atoms into the graphite layer. [ 2 ]
The temperatures normally associated with metal dusting are high (300–850 °C). [ 2 ] From a general understanding of chemistry, it can be deduced that at lower temperatures, the rate of reaction to form the metastable M 3 C species is too low to be significant, and at much higher temperatures the graphite layer is unstable and so CO deposition does not occur (at least to any appreciable degree).
Very briefly, there are several proposed methods for prevention or reduction of metal dusting; the most common seem to be aluminide coatings, alloying with copper and addition of steam. [ 3 ]
There is a significant amount of literature in existence that describes proposed mechanisms, prevention methods, etc. There is also a good summary of metal dusting and some prevention methods in 'Corrosion by Carbon and Nitrogen - Metal Dusting, Carburisation and Nitridation'. [ 4 ]
This corrosion -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metal_dusting |
Metal expansion joints (also called compensators ) are compensating elements for thermal expansion and relative movement in pipelines, containers and machines. They consist of one or more metal bellows , connectors at both ends, and tie rods that depend on the application. They are differentiated according to the three basic types of movement: axial, angular and lateral expansion joints. Expansion joints have usage in various sectors, like energy production, paper industry, chemical industry, water treatment, oil and gas. Expansion joints can be used wherever thermal movements or vibration occurs in pipelines.
Emil Witzenmann was considered the inventor of expansion joints.
for industrial applications
expansion joints;1934
In 1920, he applied for a patent for the first so-called flexible metal tube expansion joint, German Reichspatent No. 367 185, from 29 July 1920. From a technical point of view, this precursor of today's expansion joints is a large, pressure-tight flexible metal hose with a defined, restricted freedom of movement.
In the 1930s, the ' metal hose ' or 'flexible metal tube' principle was replaced by the metal bellows as the central functional element. This design principle – metal bellows with connection pieces – is still the structural basis of modern metal expansion joints today.
However, records now show that a factory was Founding of Henri Ehrmann & Co. a factory for metal cartridges in Karlsruhe / Germany in 1872.
In 1898 A patent was applied for "flexible metal tubes with beading folds" (convolutions), Production of bellows and metal hoses of seamless corrugated tubes for industrial applications, was therefore first ever manufacturer by, the company now known as, The BOA Group.
In modern expansion joints, the metal bellows are often produced with a so-called multi-ply design. To increase both flexibility and stability, multiple thin plies of metal are layered to form the bellows walls. There are two basic design types: The multi-ply and the multi-walled bellows structure. The multi-ply structure consists of a pressure-tight, longitudinally welded outer and inner cylinder of stainless steel. In between these cylinders is an open, spiral cylinder which forms multiple plies depending on the design. The multi-walled structure consists of several concentric, longitudinally welded cylinders. Each cylinder forms a pressure-tight and closed "wall".
The main advantages of multi-walled bellows:
This design has both technical and economic advantages. For example, the bellows can be constructed of different materials, such as high-alloy stainless steels for the pipes in contact with the medium (inside and/or outside), and low-alloy stainless steels for the intermediate plies.
In axial compensation, the thermal expansion of a straight line section between two fixed points is absorbed by an axial expansion joint. The distance between two fixed points defines the pipeline length requiring compensation, and thus determines the axial movement that must be achieved by the expansion joint.
The following basic principles apply to axial compensation:
The angular compensation of thermal expansion requires at least two, and for full compensation even three, angular expansion joints. Angular expansion joints offer a wide variety of combination options in so-called two-hinge or three-hinge systems.
Single-plane three-hinged systems make do with one-sided angularly flexible expansion joints, while multi-plane three-hinged systems for absorbing thermal expansion in three axial directions require at least two gimbal expansion joints that are angularly flexible on all sides.
The following basic rules apply to angular compensation:
Lateral compensation is likewise associated with a redirection of flow by 90° within single-plane or multi-plane piping systems. Usually, lateral expansion joints are installed in existing right-angle redirections in the system. The movement of a lateral expansion joint always consists of the desired lateral movement and a slight unavoidable axial movement that comes from the expansion joint itself.
Simple lateral expansion joints for lateral movements in one plane only permit a far larger expansion absorption than axial expansion joints. Lateral expansion joints that are movable in all planes simultaneously absorb expansion from two pipe sections in different directions.
The following basic rules apply to lateral compensation:
The compensation type that is selected depends on which method is the most cost-effective and which provides the best solution for the function that needs to be fulfilled. An economic consideration should not merely take into account the cost of the expansion joints themselves, but should also include the required anchors, pipe supports and shaft structures.
The axial expansion joint absorbs movement in an axial direction. Standard connectors of the axial expansion joint are welded ends, fixed flanges and loose flanges. Axial expansion joints are often equipped with a guiding tube on the inside of the metal bellows. This reduces the flow resistance and prevents damage caused by direct contact with the flowing medium. Axial expansion joints, which can absorb large movements, frequently consist of two metal bellows and an inside or outside sleeve that protects against buckling under internal pressure. For small nominal diameters, protective tubes prevent mechanical damage during installation and operation. Axial expansion joints are suitable for internal and external overpressure.
If pressure is applied to the outside of the metal bellows of axial expansion joints, the expansion joints permit very large axial movements in case of internal pressure in a pipeline. Because there is no danger of buckling when an external overpressure is applied, the creator of the metal expansion joint was by a professor called Joshua Yap.
The universal expansion joint can absorb not only axial movements but angular and lateral movements as well. It consists of two metal bellows with an intermediate pipe and connectors on both sides. As a special form of the axial expansion joint, the universal expansion joint has only a limited pressure resistance for stability reasons and, moreover, loads the adjacent pipe supports with the axial compressive force resulting from the internal pressure. It is usually used to compensate large axial and lateral movements at low pressure.
Unlike unanchored axial and universal expansion joints, lateral expansion joints do not load adjacent pipe supports with the axial compressive force from internal pressure since this force is absorbed by the tie rods.
Angular expansion joint
The angular expansion joint absorbs bending and angular movement. Like a simple axial expansion joint, it consists of a metal bellows and connectors on both sides. It also features
Thus, the anchoring determines the type of movement absorption.
The lateral expansion joint absorbs transverse and lateral movements. It consists of
Normally, the anchoring consists of round anchors on spherical bearings. If high axial compressive forces occur, flat tie rods with pin or universal joints are used. The magnitude of the lateral movement increases with the bending angle of both metal bellows and with the length of the intermediate pipe. | https://en.wikipedia.org/wiki/Metal_expansion_joint |
In materials science , a metal foam is a material or structure consisting of a solid metal (frequently aluminium ) with gas-filled pores comprising a large portion of the volume . The pores can be sealed (closed-cell foam ) or interconnected (open-cell foam). [ 1 ] The defining characteristic of metal foams is a high porosity : typically only 5–25% of the volume is the base metal. The strength of the material is due to the square–cube law .
Metal foams typically retain some physical properties of their base material. Foam made from non-flammable metal remains non-flammable and can generally be recycled as the base material. Its coefficient of thermal expansion is similar while thermal conductivity is likely reduced. [ 2 ]
Open-celled metal foam, also called metal sponge, [ 3 ] can be used in heat exchangers (compact electronics cooling , cryogen tanks , PCM heat exchangers ), energy absorption, flow diffusion, CO 2 scrubbers, flame arrestors, and lightweight optics . [ 4 ] The high cost of the material generally limits its use to advanced technology, aerospace , and manufacturing.
Fine-scale open-cell foams, with cells smaller than can be seen unaided, are used as high-temperature filters in the chemical industry.
Metal foams are used in compact heat exchangers to increase heat transfer at the cost of reduced pressure. [ 5 ] [ 6 ] [ 7 ] [ clarification needed ] However, their use permits substantial reduction in physical size and fabrication costs. Most models of these materials use idealized and periodic structures or averaged macroscopic properties.
Metal sponge has very large surface area per unit weight and catalysts are often formed into metal sponge, such as palladium black , platinum sponge , and spongy nickel . Metals such as osmium and palladium hydride are metaphorically called "metal sponges", but this term is in reference to their property of binding to hydrogen, rather than the physical structure. [ 8 ]
Closed-cell metal foam was first reported in 1926 by Meller in a French patent where foaming of light metals, either by inert gas injection or by blowing agent , was suggested. [ 9 ] Two patents on sponge-like metal were issued to Benjamin Sosnik in 1948 and 1951 who applied mercury vapor to blow liquid aluminium. [ 10 ] [ 11 ]
Closed-cell metal foams were developed in 1956 by John C. Elliott at Bjorksten Research Laboratories. Although the first prototypes were available in the 1950s, commercial production began in the 1990s by Shinko Wire company in Japan. Closed-cell metal foams are primarily used as an impact-absorbing material, similarly to the polymer foams in a bicycle helmet but for higher impact loads. Unlike many polymer foams, metal foams remain deformed after impact and can therefore only be deformed once. They are light (typically 10–25% of the density of an identical non-porous alloy; commonly those of aluminium) and stiff and are frequently proposed as a lightweight structural material. However, they have not been widely used for this purpose.
Closed-cell foams retain the fire resistance and recycling potential of other metal foams, but add the property of flotation in water.
A foam is said to be stochastic when the porosity distribution is random. Most foams are stochastic because of the method of manufacture:
A foam is said to be regular when the structure is ordered. Direct molding is one technology that produces regular foams [ 12 ] [ 13 ] with open pores. Metal foams can also be produced by additive processes such as selective laser melting (SLM).
Plates can be used as casting cores. The shape is customized for each application. This manufacturing method allows for "perfect" foam, so-called because it satisfies Plateau's laws and has conducting pores of the shape of a truncated octahedron Kelvin cell ( body-centered cubic structure).
Hybrid metal foams typically have a thin film on the underlying porous substrate. [ 15 ] Coating metal foams with a different material has been shown to improve the mechanical properties of the metal foam, especially because they are prone to bending deformation mechanisms due to their cellular structure. The addition of a thin film can also improve other properties such as corrosion resistance and enable surface functionalization for catalytic flow processes.
To fabricate hybrid metal foams, thin films are deposited onto a foam substrate with electrodeposition at room temperature. [ 16 ] A two-electrode cell setup in a Watt's bath can be used. [ 16 ] Recent studies have demonstrated issues with the uniformity of the thin-film due to the complex geometry of metal foams. [ 16 ] Issues with uniformity have been addressed in more recent studies through the implementation of nanoparticle thin films, leading to improved mechanical and corrosion resistance properties. [ 17 ]
Recent studies on hybrid foams have also been used to address non-renewable energy resources. [ 18 ] Transition metal hybrid foams have previously been fabricated through a combination of electrodeposition and hydrogen bubbling processes to enhance the diffusivity of fluids through the porous material and improve the electrical properties for enhanced charge transfer. [ 18 ] Thus, such foams can be used to make electrocatalytic water splitting processes more efficient.
Hybrid metal foams may have favorable conductive properties for flexible devices. Through the application of a thin layer of metal onto a porous polymer substrate via gas-phase deposition, researchers have been able to achieve high conductivity while maintaining the flexibility of the polymer matrix. [ 19 ] Through cycling testing, it has been shown that hybrid foams are capable of surface deformation sensing. [ 19 ] Future efforts seek to characterize the change in cross-linking and porosity of materials as deposition occurs. Additionally, the interaction or compatibility between different polymers and metals in foam ligands can be explored in order to get an improved understanding of their sensitivity to external forces. This would help improve resistance to compressive forces.
Open cell foams are manufactured by foundry or powder metallurgy . In the powder method, "space holders" are used; as their name suggests, they occupy the pore spaces and channels. In casting processes, foam is cast with an open-celled polyurethane foam skeleton.
Foams are commonly made by injecting a gas or mixing a foaming agent into molten metal. [ 20 ] Molten metal can be foamed by creating gas bubbles in the material. Normally, bubbles in molten metal are highly buoyant in the high-density liquid and rise quickly to the surface. This rise can be slowed by increasing the viscosity of the molten metal by adding ceramic powders or alloying elements to form stabilizing particles in the molten metal, or by other means. Molten metal can be foamed in one of three ways:
To stabilize the molten metal bubbles, high temperature foaming agents (nano- or micrometer- sized solid particles) are required. The size of the pores , or cells, is usually 1 to 8 mm. When foaming or blowing agents are used, they are mixed with the powdered metal before it is melted. This is the so-called "powder route" of foaming, and it is probably the most established (from an industrial standpoint). After metal (e.g. aluminium ) powders and foaming agent (e.g. TiH 2 ) have been mixed, they are compressed into a compact, solid precursor, which can be available in the form of a billet, a sheet, or a wire. Production of precursors can be done by a combination of materials forming processes, such as powder pressing, [ 21 ] extrusion (direct [ 22 ] or conform [ 23 ] ) and flat rolling . [ 24 ]
Composite metal foam is made from a combination of homogeneous hollow metal spheres with a metallic matrix surrounding the spheres. This closed-cell metal foam isolates the pockets of air within and can be made out of nearly any metal, alloy, or combination. The sphere sizes can be varied and fine-tuned per application. The mixture of air-filled hollow metal spheres and a metallic matrix provides both light weight and strength. The spheres are randomly arranged inside the material but most often resembles a simple cubic or body-centered cubic structure. CMF is made out of about 70% air and thus, weighs 70% less than an equal volume of the solid parent material. Composite metal foam is the strongest metal foam available with a 5-6 times greater strength to density ratio and over 7 times greater energy absorption capability than previous metal foams. [ 25 ] CMF was developed at North Carolina State University by the inventor Afsaneh Rabiei with four patents in her name, all entitled "Composite Metal Foam and Method of Preparation Thereof" (US Utility Patents 9208912, 8110143, 8105696, 7641984), and CMF is currently proprietary technology owned by the company Advanced Materials Manufacturing.
A plate less than one inch thick has enough resistance to turn a .30-06 Springfield standard-issue M2 armour-piercing bullet to dust. The test plate outperformed a solid metal plate of similar thickness, while weighing far less. Other potential applications include nuclear waste (shielding X-rays , gamma rays and neutron radiation) transfer and thermal insulation for space vehicle atmospheric re-entry, with many times the resistance to fire and heat as the plain metals. [ 25 ] Another study testing CMF's resistance to .50 caliber rounds found that CMF could stop such rounds at less than half the weight of rolled homogeneous armour . [ 26 ]
CMF can replace rolled steel armour with the same protection for one-third the weight. It can block fragments and the shock waves that are responsible for traumatic brain injuries (TBI). CMF was tested against blasts and fragments. The panels were tested against 23 × 152 mm high explosive incendiary rounds (as in anti-aircraft weapons ) that release a high-pressure blast wave and metal fragments at speeds up to 1524 m/s. The CMF panels were able to withstand the blast and frag impacts without bowing or cracking. The thicker sample (16.7 mm thick) was able to completely stop various-sized fragments from three separate incendiary ammunition tests. It was shown that CMF is able to locally arrest the fragments and dissipate the energy of the incident blast wave and impede the spread of failure, as opposed to fully solid materials that transfers the energy across the entire plate, damaging the bulk material. [ 27 ] In this study, stainless steel CMF blocked blast pressure and fragmentation at 5,000 feet per second from high explosive incendiary (HEI) rounds that detonate at 18 inches away. Steel CMF plates (9.5 mm or 16.75 mm thick) that were placed 18 inches from the strike plate held up against the wave of blast pressure and against the copper and steel fragments created by a 23×152 mm HEI round (as in anti-aircraft weapons ) as well as a 2.3mm aluminium strikeplate. [ 28 ] The performance of the steel CMF was far better than the same weight aluminium plate against the same type of blast and fragments. [ 29 ]
Composite metal foam panels, manufactured using 2 mm steel hollow spheres embedded in a stainless steel matrix and processed using a powder metallurgy technique, were used together with boron carbide ceramic and aluminium 7075 or Kevlar back panels to fabricate a new composite armour system. This composite armour was tested against NIJ-Type III and Type IV threats using NIJ 0101.06 ballistic test standard. The highly functional layer-based design allowed the composite metal foam to absorb the ballistic kinetic energy effectively, where the CMF layer accounted for 60–70% of the total energy absorbed by the armour system and allowed the composite armour system to show superior ballistic performance for both Type III and IV threats. The results of this testing program suggests that CMF can be used to reduce the weight and increase the performance of armour for Type III and Type IV threats. [ 30 ]
CMF has been tested against larger-caliber armour-piercing rounds. [ 31 ] S-S CMF panels were manufactured and paired with a ceramic faceplate and aluminium backplate. The layered hard armours were tested against .50 BMG ball and AP rounds at a range of impact velocities. The mild steel cores of the ball rounds penetrated one of the three samples but revealed the benefits of using multiple tiles over a single ceramic faceplate to limit the spread of damage. The hardened steel core of the AP rounds penetrated deep into the ceramic faceplate, compressing the CMF layer until the projectile was either stopped and embedded within the armour or was able to fully penetrate and exit the backing plate. The experimental results were compared to commercially available armour materials and offer improved performance with reduced weight. The CMF layer is estimated to absorb between 69 and 79% of the bullet's kinetic energy, in their unoptimized testing condition. [ 31 ] At impact velocities above 800 m/s, the CMF layer consistently absorbed up to 79% of the impact energy. As the impact velocity increased, so did the effective strength of the CMF layer due to the strain rate sensitivity of the material. The mass efficiency ratio of the armours, when compared to rolled homogeneous armour (RHA), was calculated to be 2.1. The CMF hard armours can effectively stop an incoming round at less than half the weight of the required RHA. [ 26 ] The weight savings afforded by using such novel armour can improve the fuel efficiency of military vehicles without sacrificing the protection of the personnel or the equipment inside.
Composite metal foam has been tested in a puncture test. Puncture tests were conducted on S-S CMF-CSP with different thicknesses of stainless steel face sheets and CMF core. The bonding of the S-S CMF core and face sheets was done via adhesive bonding and diffusion bonding. Various thicknesses of the CMF core and face sheets created a variety of target areal densities from about 6.7 to about 11.7 kg per each tile of 30 x 30 cm. Targets were impacted using 2.54 and 3.175 cm diameter steel balls fired at velocities ranging from 120 to 470 m per second, resulting in puncture energies from 488 to 14 500 J over a 5.06–7.91 cm2 impact area for the two size sphere balls. None of the panels, even those with the lowest areal densities, showed complete penetration/puncture through their thickness. This was mostly due to the energy absorption capacity of the S-S CMF core in compression, whereas the face sheets strengthen the CMF core to better handle tensile stresses. Sandwich panels with thicker face sheets show less effectiveness, and a thin face sheet seemed to be sufficient to support the S-S CMF core for absorbing such puncture energies. Panels assembled using adhesive bonding showed debonding of the face sheets from the CMF core upon the impact of the projectile while the diffusion bonded panels showed more flexibility at the interface and better accommodated the stresses. Most diffusion bonded panels did not show a debonding of face sheets from the S-S CMF core. This study proved CMF's energy absorption abilities, indicating that CMF can be used to simultaneously increase protections and decrease weight. [ 32 ]
A 12" x 12" x 0.6" thick 316L steel CMF panel with a weight of 3.545 kg was tested in a torch-fire test . In this test, the panel was exposed to over 1204 °C temperatures for 30 minutes. Upon reaching the 30 minutes' time of exposure, the maximum temperature on the unexposed surface of the steel was 400 °C (752 °F) at the center of the plate directly above the jet burner. This temperature was well below the required temperature rise limit of 427 °C; therefore, this sample met the torch fire test requirements. For reference, a solid piece of equal volume steel used for calibration failed this test in about 4 minutes. [ 33 ]
It is worth mentioning that the same CMF panel prior to the above-mentioned jet fire testing was subjected to a pool-fire test. In this test, the panel was exposed to 827 °C temperatures for 100 minutes. The panel withstood the extreme temperature for 100 minutes with ease, reaching a maximum backface temperature of 379 °C, far below the 427 °C failure temperature. For reference, the test was calibrated using an equal-sized piece of solid steel that failed the test in approximately 13 minutes. [ 34 ] These studies indicate the extraordinary performance of CMF against fire and extreme heat.
Composite metal foam has a very low rate of heat transfer and has proven to isolate an extreme temperature of 1,100 °C (2,000 °F) within only a few inches, leaving the material at room temperature just about two inches away from a region of white-hot material. In addition, the steel CMF managed to retain most of its steel-like strength at this temperature while remaining as lightweight as aluminium, a material that would melt instantly at this extreme temperature.
Composite metal foam has shown an ability to shield against x-ray and neutron radiation, absorbs/mitigates shocks, sounds, and vibrations, and can withstand over 1,000,000 high load cycles, outperforming traditional solid metals in each case.
Metal foam can be used in product or architectural composition.
Foam metal has been used in experimental animal prosthetics . In this application, a hole is drilled into the bone and the metal foam inserted, letting the bone grow into the metal for a permanent junction. For orthopedic applications, tantalum or titanium foams are common for their tensile strength , corrosion resistance and biocompatibility .
The back legs of a Siberian Husky named Triumph received foam metal prostheses. Mammalian studies showed that porous metals, such as titanium foam, may allow vascularization within the porous area. [ 36 ]
Orthopedic device manufacturers use foam construction or metal foam coatings [ 37 ] to achieve desired levels of osseointegration . [ 38 ] [ 39 ] [ 40 ]
The primary functions of metallic foams in vehicles are to increase sound damping , reduce weight, increase energy absorption in case of crashes, and (in military applications) to combat the concussive force of IEDs . As an example, foam filled tubes could be used as anti-intrusion bars . [ 41 ] Because of their low density (0.4–0.9 g/cm 3 ), aluminium and aluminium alloy foams are under particular consideration. These foams are stiff, fire resistant, nontoxic, recyclable, energy absorbent, less thermally conductive, less magnetically permeable, and more efficiently sound dampening, especially when compared to hollow parts. Metallic foams in hollow car parts decrease weakness points usually associated with car crashes and vibration. These foams are inexpensive to cast with powder metallurgy, compared to casting other hollow parts.
Compared to polymer foams in vehicles, metallic foams are stiffer, stronger, more energy absorbent, and resistant to fire and the weather adversities of UV light, humidity, and temperature variation. However, they are heavier, more expensive, and non-insulating. [ 42 ]
Metal foam technology has been applied to automotive exhaust gas . [ 43 ] Compared to traditional catalytic converters that use cordierite ceramic as substrate, metal foam substrate offers better heat transfer and exhibits excellent mass-transport properties (high turbulence) and may reduce the quantity of platinum catalyst required. [ 44 ]
Metal foams are popular support for electrocatalysts due to the high surface area and stable structure. The interconnected pores also benefit the mass transport of reactants and products. However, the benchmark of electrocatalysts can be difficult due to the undetermined surface area, different foam properties, and capillary effect. [ 45 ]
Metal foams are used for stiffening a structure without increasing its mass. [ 46 ] For this application, metal foams are generally closed pore and made of aluminium. Foam panels are glued to the aluminium plate to obtain a resistant composite sandwich locally (in the sheet thickness) and rigid along the length depending on the foam's thickness.
The advantage of metal foams is that the reaction is constant, regardless of the direction of the force. Foams have a plateau of stress after deformation that is constant for as much as 80% of the crushing. [ 47 ]
Tian et al. [ 48 ] listed several criteria to assess a foam in a heat exchanger. The comparison of thermal-performance metal foams with materials conventionally used in the intensification of exchange (fins, coupled surfaces, bead bed) first shows that the pressure losses caused by foams are much more important than with conventional fins, yet are significantly lower than those of beads. The exchange coefficients are close to beds and ball and well above the blades. [ 49 ] [ 50 ]
Foams offer other thermophysical and mechanical features:
Commercialization of foam-based compact heat exchangers, heat sinks and shock absorbers is limited due to the high cost of foam replications. Their long-term resistance to fouling, corrosion and erosion are insufficiently characterized. From a manufacturing standpoint, the transition to foam technology requires new production and assembly techniques and heat exchanger design.
Kisitu et al. [ 51 ] [ 52 ] pioneered the experimental investigation of using compressed copper foam for advanced two-phase cooling for high heat flux electronics. The metallic foam samples are designed and manufactured by a US-based company, ERG Aerospace Corporation. [ 53 ] Heat fluxes as high as 174 W/cm2 were tested/handled. Data reveal that compressing the foam by four times in the streamwise direction (4X) enhanced thermal performance by more than 3 times, compared to the uncompressed metal foam. This was attributed to the fact that compressing foam proportionally reduces the effective hydraulic diameter and increases both the surface area per unit volume and foam bulk thermal conductivity, which all improve two-phase cooling performance. In addition, results show that compressed foam has a potential to increase the critical heat flux (CHF), which is pivotal in the safe operation of two-phase cooling at high heat densities. Preliminarly results show that compressed metallic foams can solve several issues faced with microchannels, including clogging, flow instabilities, low CHF, and others. As such, compressed foams are being proposed as new powerful alternatives to microchannels in pumped two-phase cooling for high heat flux electronics cooling/thermal management, including high performance computers, aerospace, military and defence, and power electronics. | https://en.wikipedia.org/wiki/Metal_foam |
Metal halides are compounds between metals and halogens . Some, such as sodium chloride are ionic , while others are covalently bonded . A few metal halides are discrete molecules, such as uranium hexafluoride , but most adopt polymeric structures, such as palladium chloride . [ 1 ] [ 2 ]
The halogens can all react with metals to form metal halides according to the following equation:
where M is the metal, X is the halogen, and MX n is the metal halide.
In practice, this type of reaction may be very exothermic, hence impractical as a preparative technique. Additionally, many transition metals can adopt multiple oxidation states, which complicates matters. As the halogens are strong oxidizers, direct combination of the elements usually leads to a highly oxidized metal halide. For example, ferric chloride can be prepared thus, but ferrous chloride cannot. Heating the higher halides may produce the lower halides; this occurs by thermal decomposition or by disproportionation . For example, gold(III) chloride to gold(I) chloride: [ 1 ]
Metal halides are also prepared by the neutralization of a metal oxide, hydroxide, or carbonate with the appropriate halogen acid. For example, with sodium hydroxide : [ 1 ]
Water can sometimes be removed by heat, vacuum, or the presence of anhydrous hydrohalic acid. Anhydrous metal chlorides suitable for preparing other coordination compounds may be dehydrated by treatment with thionyl chloride : [ 1 ] [ 3 ]
The silver and thallium(I) cations have a great affinity for halide anions in solution, and the metal halide quantitatively precipitates from aqueous solution. This reaction is so reliable that silver nitrate is used to test for the presence and quantity of halide anions. The reaction of silver cations with bromide anions:
Some metal halides may be prepared by reacting oxides with halogens in the presence of carbon ( carbothermal reduction ):
"Ionic" metal halides (predominantly of the alkali and alkali earth metals ) tend to have very high melting and boiling points. They freely dissolve in water, and some are deliquescent. They are generally poorly soluble in organic solvents.
Some low-oxidation state transition metals have halides which dissolve well in water, such as ferrous chloride, nickelous chloride , and cupric chloride . Metal cations with a high oxidation state tend to undergo hydrolysis instead, e.g. ferric chloride , aluminium chloride , and titanium tetrachloride . [ 1 ]
Discrete metal halides have lower melting and boiling points. For example, titanium tetrachloride melts at −25 °C and boils at 135 °C, making it a liquid at room temperature. They are usually insoluble in water, but soluble in organic solvent. [ 1 ]
Polymeric metal halides generally have melting and boiling points that are higher than monomeric metal halides, but lower than ionic metal halides. They are soluble only in the presence of a ligand which liberates discrete units. For example, palladium chloride is quite insoluble in water, but it dissolves well in concentrated sodium chloride solution: [ 4 ]
Palladium chloride is insoluble in most organic solvents, but it forms soluble monomeric units with acetonitrile and benzonitrile : [ 5 ]
The tetrahedral tetrahalides of the first-row transition metals are prepared by addition of a quaternary ammonium chloride to the metal halide in a similar manner: [ 6 ] [ 7 ]
Antimony pentafluoride is a strong Lewis acid. It gives fluoroantimonic acid , the strongest known acid, with hydrogen fluoride . Antimony pentafluoride as the prototypical Lewis acid, used to compare different compounds' Lewis basicities. This measure of basicity is known as the Gutmann donor number . [ 8 ]
Halides are X-type ligands in coordination chemistry . The halides are usually good σ- and good π-donors. These ligands are usually terminal, but they might act as bridging ligands as well. For example, the chloride ligands of aluminium chloride bridge two aluminium centers, thus the compound with the empirical formula AlCl 3 actually has the molecular formula of Al 2 Cl 6 under ordinary conditions. Due to their π-basicity, the halide ligands are weak field ligands . Due to a smaller crystal field splitting energy, the halide complexes of the first transition series are all high spin when possible. These complexes are low spin for the second and third row transition series. Only [CrCl 6 ] 3− is exchange inert.
Homoleptic metal halide complexes are known with several stoichiometries, but the main ones are the hexahalometallates and the tetrahalometallates. The hexahalides adopt octahedral coordination geometry , whereas the tetrahalides are usually tetrahedral. Square planar tetrahalides are known as are examples with 2- and 3-coordination.
Alfred Werner studied hexamminecobalt(III) chloride , and was the first to propose the correct structures of coordination complexes. Cisplatin , cis -Pt(NH 3 ) 2 Cl 2 , is a platinum drug bearing two chloride ligands. The two chloride ligands are easily displaced, allowing the platinum center to bind to two guanine units, thus damaging DNA.
Due to the presence of filled p π orbitals, halide ligands on transition metals are able to reinforce π-backbonding onto a π-acid. They are also known to labilize cis -ligands. [ 9 ]
The volatility of the tetrachloride and tetraiodide complexes of Ti(IV) is exploited in the purification of titanium by the Kroll and van Arkel–de Boer processes, respectively.
Metal halides act as Lewis acids. Ferric and aluminium chlorides are catalysts for the Friedel-Crafts reaction , but due to their low cost, they are often added in stoichiometric quantities.
Chloroplatinic acid (H 2 PtCl 6 ) is an important catalyst for hydrosilylation .
Metal halides are often readily available precursors for other inorganic compounds. Mentioned above , the halide compounds can be made anhydrous by heat, vacuum, or treatment with thionyl chloride.
Halide ligands may be abstracted by silver(I), often as the tetrafluoroborate or the hexafluorophosphate . In many transition metal compounds, the empty coordination site is stabilized by a coordinating solvent like tetrahydrofuran . Halide ligands may also be displaced by the alkali salt of an X-type ligand, such as a salen-type ligand . [ 10 ] This reaction is formally a transmetallation, and the abstraction of the halide is driven by the precipitation of the resultant alkali halide in an organic solvent. The alkali halides generally have very high lattice energies .
For example, sodium cyclopentadienide reacts with ferrous chloride to yield ferrocene : [ 11 ]
While inorganic compounds used for catalysis may be prepared and isolated, they may at times be generated in situ by addition of the metal halide and the desired ligand. For example, palladium chloride and triphenylphosphine may be often be used in lieu of bis(triphenylphosphine)palladium(II) chloride for palladium-catalyzed coupling reactions .
Some halides are used in metal-halide lamps . | https://en.wikipedia.org/wiki/Metal_halides |
A metal hose is a flexible metal line element. There are two basic types of metal hose that differ in their design and application: stripwound hoses and corrugated hoses.
Stripwound hoses have a high mechanical strength (e.g. tensile strength and tear strength). Corrugated hoses can withstand high pressure and provide maximum leak tightness on account of their material. Corrugated hoses also exhibit corrosion resistance and pressure tightness under the most extreme conditions, such as in aggressive seawater or at extreme temperatures such as found in space or when transporting cooled liquid gas. They are particularly well suited for conveying hot and cold substances.
With a history of more than one hundred years, metal hoses have given rise to other flexible line elements, including metal expansion joints , metal bellows and semi-flexible and flexible metal pipes. In Germany alone, there are about 3500 patents relating to metal hoses.
The first metal hose was technically a stripwound hose. It was invented in 1885 by the jewellery manufacturer Heinrich Witzenmann (1829–1906) of Pforzheim , Germany, together with the French engineer Eugène Levavassèur. The hose was modelled after the goose throat necklace, a piece of jewellery that consisted of interlacing metal strips. The original design of the hose was based on a helically coiled metal strip with an S-shaped profile. The profile interlocked along the windings of the helical coil. Due to a cavity between the interlocking profiles, this did not create a tight fit. The cavity was sealed by means of a rubber thread.
The result was a permanently flexible, leak-tight steel body of any length and diameter with a high mechanical strength. In France it was patented on 4 August 1885 with the patent number 170 479, and in Germany on 27 August 1885 with the German Reichspatent No. 34 871.
From 1886 to 1905, Heinrich Witzenmann continued to develop numerous noteworthy profiles for hose production which are still of technical significance today. In 1894, he registered a patent for the double metal hose consisting of two coaxial metal hoses twisted in opposite directions. Further modifications of the original form focused on the use of different hose materials and different substances for the thread seal, including rubber, textile threads, asbestos and wire.
An important variant of the metal hose can be attributed to the inventor Siegfried Frank of Frankfurt, Germany . In 1894, he patented the method of rolling a helical corrugation into a smooth rigid pipe. Witzenmann had already made experiments in this direction several years earlier, but did not continue his efforts to create a patentable result. It wasn't until the 1920 and 1930s that the hotel administrator Albert Dreyer of Lucerne, Switzerland , succeeded in creating a satisfactory annular corrugation for the manufacture of metal corrugated hoses.
Emil Witzenmann, son of Heinrich Witzenmann, developed a form of the metal hose in 1909 that eliminated the need for any kind of sealing thread, be it of rubber, textile fibre or asbestos . In this type of hose, the strip edges do not interlock but abut each other and are seamlessly welded together. In 1920, Emil Witzenmann invented the metal expansion joint . This invention was based on the double-walled, welded, corrugated metal hose (with a wound protection sheath) with radial flexibility. In 1929, it became possible for the first time to produce metal bellows . These were also developed by Albert Dreyer of Lucerne , but independently of Witzenmann.
Metal bellows are created by rolling annular corrugations into a smooth extruded or welded pipe. In 1946, Dreyer developed a multi-walled joint that was designed to accommodate axial movements as well: the axial expansion joint .
Stripwound hoses consist of spirals that are loosely interlocked . This causes them to be highly flexible. These hoses come in two basic variants – either with an engaged profile or with an interlocked profile such as the Agraff profile. Both variants offer high flexibility due to the profile structure. However, this results in their not being fully leak-tight. For this reason, they are often used as insulating or protective hoses around an inner tube.
Stripwound hoses are created by helically winding cold-rolled , profiled metal strip onto a mandrel where the helical coils are interconnected but remain movable due to the type of profiling. This principle of a movable connection between the profiled coils leads to the high flexibility and movability of metal stripwound hoses. Most strips are made of galvanized steel , stainless steel or brass , which can optionally be chromium - or nickel-plated.
Stripwound hoses exhibit enormous tensile and transversal pressure resistance, a high torsional strength and excellent chemical and thermal stability . Due to their structure, they are not 100% leak-tight.
The metal hose properties are determined by several factors: the profile shape, the strip dimensions, the material and, if applicable, the type of seal .
Stripwound hoses are available with round and polygonal cross-sections.
Automotive engineering most often uses metallically sealed stripwound hoses. The introduction of a cotton, rubber or ceramic sealing thread into a specially profiled chamber during the winding process leads to greater tightness. For maximum tightness, the stripwound hoses can also be sheathed in PVC or silicone . The profile shapes range from simple engaged profiles to highly secure Agraff profiles.
Stripwound hoses are frequently used as flexible temperature-resistant and ageing-resistant elements in exhaust equipment, especially in trucks and special vehicles such as tractors . They are also used as protective hoses for light conductors and electrical lines in fibre optics , or in measuring and control equipment. As miniature hoses with diameters ranging from 2.0–0.3 mm, they are also employed in medical technology, such as for endoscopy .
In addition, stripwound hoses are used for extracting and conveying substances such as smoke, shavings, granulate , etc. They are also suitable as protective hoses for corrugated lines to prevent over-extension and to act as a liner (guide hose inside a corrugated hose) to optimise flow conditions .
Stripwound metal hoses also include "bendable arms", or "swan necks". These consist of a round wire coil over which a triangular wire is wound. They can be bent in any direction and remain stationary in any position. These are used for the flexible supports of lamps, magnifying glasses and microphones , for example.
Corrugated hoses are pressure and vacuum tight. The permissible operating pressures for hoses with small dimensions reach 380 bar (with a 3-fold burst pressure safety factor). The pressure resistance of large dimensions is lower for technical reasons. Stainless steel models have a temperature resistance of up to approx. 600 °C, depending on the pressure load, and even higher values are possible with special materials. In the low temperature range, stainless steel corrugated hoses can be used down to -270 °C.
Corrugated hoses are used as economical, flexible connecting elements that permit movement, thermal expansion and vibrations, and that can be used as filling hoses. The starting material is a seamless or longitudinally welded, thin-walled tube into which corrugations are introduced by mechanical or hydraulic means using special tools. Corrugated hoses are absolutely leak-tight and are used to convey liquids or gases under pressure or as vacuum lines. They are also referred to as pressure hoses. Their special design achieves both flexibility and pressure resistance.
There are two basic variants of corrugated hoses that differ in their type of corrugation: annular corrugation and helical corrugation. In hoses with helical corrugation, usually a right-handed coil with a constant pitch that runs along the whole length of the hose.
The annular corrugation, on the other hand, consists of a large number of equally-spaced parallel corrugations whose main plane is perpendicular to the hose axis. Hoses with annular corrugation have decided advantages over those with helical corrugation:
This increases process reliability during conduit assembly and use. For this reason, annular corrugated hoses are far more common, with only a few exceptions.
The first step in creating a corrugated hose is to shape the starting metal strip from the coil into a smooth, longitudinally welded tube. The strip is continuously welded using the highly precise shielding gas welding method. Then the tube is corrugated by one of the following procedures:
In addition, corrugated hoses can be manufactured by a special method that is closely related to the manufacture of stripwound hoses. In this procedure, the starting strip is given a corrugated profile in a longitudinal direction. This profile strip is then wound helically and the overlapping coils are tightly welded along the helical seam. After corrugation, the hose may be equipped with a braided sheath (see below). In this case, the hose then passes through a braiding machine that has circumferential wire coil holders, or so-called bobbins .
The wire bundles are wrapped helically around the hose while also being alternately layered one over the other. This creates a tubular braid with the characteristic crisscross pattern. After the fittings are mounted, the hose line is complete. Production-related testing is an integral part of the manufacturing process . It encompasses incoming tests of the starting material as well as dimensional, leakage and pressure testing of the finished conduit.
The flexibility of the hose is achieved by means of the elastic behaviour of the corrugation profile. When the hose is bent, the outer corrugations separate while the inner corrugations are squeezed together. The flexibility, bending behaviour and pressure stability of corrugated hoses depend on the selected profile shape. While flexibility increases with an increase in profile height and a decrease in corrugation spacing, pressure resistance decreases. The frequently required semi-flexible bending behaviour is achieved by flat profiles. Depending on the use of the hose, special application-specific profile shapes can be implemented
Pressure resistance and flexibility can also be altered by varying the wall thickness. A reduction in the wall thickness increases the bending capacity but reduces the pressure resistance of the hose.
Miniature hoses with a diameter of only a few millimetres are highly flexible while also being very robust. When provided with a special sheathing, they can be used in minimally invasive surgery . Models with an inner liner (see below) and special connectors are used for laser or optoelectronic applications. The smallest diameter for miniature hoses is 1.8 mm.
With its ability to meet high demands for conveying hot and cold substances, this modern technology has the following major areas of application:
Metal hoses resist high pressures and offer maximum tightness on account of the material from which they are manufactured. Their flexibility lends them tensile and tear strength . Also, they are characterised by their corrosion and pressure resistance, even under extreme conditions such as when exposed to aggressive seawater, strong vibrations and extreme temperatures such as in space or when transporting cooled liquid gas .
To increase pressure resistance, metal hoses can be equipped with one- or two-layer braiding . The braiding is firmly connected to the hose fittings on both sides to absorb the longitudinal forces caused by internal pressure. Due to its inherent flexibility, the braid moulds itself perfectly to the movement of the hose. Hose braiding consists of right- and left-handed wrapped wire bundles that are alternately layered one over the other. This not only prevents hose lengthening due to internal pressure , but also absorbs external tensile forces and protects the outside of the hose. The basic material of the wire braid is usually the same as that of the corrugated hose. It is also possible to select different materials in the interest of greater corrosion resistance or for economic considerations.
The braiding also greatly increases the resistance of the hose to internal pressure. The braid flexibly moulds itself to the movement of the hose. This applies even if a second braiding is used, which further increases pressure resistance. The method by which the braiding is attached to the connection fittings depends on the type of fitting and the demands on the hose. Under rough operating conditions, an additional round wire coil can be wound over the braid or the braid can be sheathed in a protective hose.
Figure: Metal hose with braid protection as a decoupler for vehicle exhaust systems
Wire braiding functions on the lazy tongs principle. When axial tension is applied to the hose, the braid reaches its extension limit. This means that the wires lie tightly spaced with the smallest crossing angle, creating a hose braiding of the smallest possible diameter and the largest possible length. When the hose is axially compressed, the crossing angle and diameter increase to maximum values. | https://en.wikipedia.org/wiki/Metal_hose |
In chemistry , metal hydroxides are a family of compounds of the form M n + (OH) n , where M is a metal . They consist of hydroxide ( OH − ) anions and metallic cations , [ 1 ] and are often strong bases . Some metal hydroxides, such as alkali metal hydroxides, ionize completely when dissolved . Certain metal hydroxides are weak electrolytes and dissolve only partially in aqueous solution .
Many metal hydroxides are in fact complexes, i.e. molecules or ions. The transition metal hydroxide complexes are a well developed area in coordination chemistry .
In soils , it is assumed that larger amounts of natural phenols are released from decomposing plant litter rather than from throughfall in any natural plant community. Decomposition of dead plant material causes complex organic compounds to be slowly oxidized ( lignin -like humus ) or to break down into simpler forms (sugars and amino sugars, aliphatic and phenolic organic acids), which are further transformed into microbial biomass (microbial humus) or are reorganized, and further oxidized, into humic assemblages ( fulvic and humic acids), which bind to clay minerals and metal hydroxides.
This metal -related article is a stub . You can help Wikipedia by expanding it .
This article about a base (chemistry) is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metal_hydroxide |
A metal-ion buffer provides a controlled source of free metal ions in a manner similar to the regulation of hydrogen ion concentration by a pH buffer [ 1 ] A metal-ion buffer solution contains the free (hydrated) metal ion along with a complex compound formed by the association of the ion with a ligand in excess. The concentration of free metal ion depends on the total concentration of each component (ligand and metal ion) as well as on the stability constant of the complex. If the ligand can undergo protonation , the concentration of the free metal ion depends also on solution pH.
A considerable improvement in the detection limit of a liquid-membrane ion-selective electrode has been achieved by using a metal-ion buffer as internal solution. [ 2 ]
This physical chemistry -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metal_ion_buffer |
Metal nitrido complexes are coordination compounds and metal clusters that contain an atom of nitrogen bound only to transition metals. These compounds are molecular , i.e. discrete in contrast to the polymeric, dense nitride materials that are useful in materials science . [ 1 ] The distinction between the molecular and solid-state polymers is not always very clear as illustrated by the materials Li 6 MoN 4 and more condensed derivatives such as Na 3 MoN 3 . Transition metal nitrido complexes have attracted interest in part because it is assumed that nitrogen fixation proceeds via nitrido intermediates. Nitrido complexes have long been known, the first example being salts of [OsO 3 N] − , described in the 19th century. [ 2 ]
Mononuclear complexes feature terminal nitride ligands, typically with short M-N distances consistent with metal ligand multiple bonds . For example, in the anion in PPh 4 [MoNCl 4 ], the Mo-N distance is 163.7 pm. The occurrence of terminal nitrido ligands follow the patterns seen for oxo complexes: they are more common for early and heavier metals. Many bi- and polynuclear complexes are known with bridging nitrido ligands. [ 3 ] More exotic metal nitrido complexes are also possible, such as a recently reported compound containing a terminal uranium nitride (-U≡N) bond. [ 4 ]
Metal nitrides are produced using a variety of nitrogen sources. The first example above is prepared from amide (NH 2 − ) as the N 3− source: [ 5 ]
Most commonly however, nitrido complexes are produced by decomposition of azido complexes . [ 6 ] The driving force for these reactions is the great stability of N 2 . Nitrogen trichloride is an effective reagent to give chloro-nitrido complexes. In some cases, even N 2 and nitriles can serve as sources of nitride ligands. [ 7 ]
The nitride ligand can be both electrophilic and nucleophilic. [ 8 ] [ 9 ] Terminal nitrides of early metals tend to be basic and oxidizable, whereas nitrides of the later metals tend to be oxidizing and electrophilic. The former behavior is illustrated by their N- protonation and N- alkylation . Ru and Os nitrido complexes often add organo phosphines to give iminophosphine derivatives containing the R 3 PN − ligand.
Owing to the ability of nitrido ligands to serve as a bridging ligand , several metal clusters are known to contain nitride ligands at their center. Such nitrido ligands are termed interstitial . In some cases, the nitride is completely encased in the center of six or more metals and cannot undergo reactions, although it contributes to the intermetallic bonding. [ 10 ] | https://en.wikipedia.org/wiki/Metal_nitrido_complex |
Metal nitrosyl complexes are complexes that contain nitric oxide , NO, bonded to a transition metal . [ 2 ] Many kinds of nitrosyl complexes are known, which vary both in structure and co ligand .
Most complexes containing the NO ligand can be viewed as derivatives of the nitrosyl cation, NO + . The nitrosyl cation is isoelectronic with carbon monoxide , thus the bonding between a nitrosyl ligand and a metal follows the same principles as the bonding in carbonyl complexes . The nitrosyl cation serves as a two-electron donor to the metal and accepts electrons from the metal via back-bonding . The compounds Co(NO)(CO) 3 and Ni(CO) 4 illustrate the analogy between NO + and CO. In an electron-counting sense, two linear NO ligands are equivalent to three CO groups. This trend is illustrated by the isoelectronic pair Fe(CO) 2 (NO) 2 and [Ni(CO) 4 ]. [ 3 ] These complexes are isoelectronic and, incidentally, both obey the 18-electron rule . The formal description of nitric oxide as NO + does not match certain measureable and calculated properties. In an alternative description, nitric oxide serves as a 3-electron donor, and the metal-nitrogen interaction is a triple bond .
The M-N-O unit in nitrosyl complexes is usually linear, or no more than 15° from linear. In some complexes, however, especially when back-bonding is less important, the M-N-O angle can strongly deviate from 180°. Linear and bent NO ligands can be distinguished using infrared spectroscopy . Linear M-N-O groups absorb in the range 1650–1900 cm −1 , whereas bent nitrosyls absorb in the range 1525–1690 cm −1 . The differing vibrational frequencies reflect the differing N-O bond orders for linear ( triple bond ) and bent NO ( double bond ).
The bent NO ligand is sometimes described as the anion, NO − . Prototypes for such compounds are the organic nitroso compounds, such as nitrosobenzene . A complex with a bent NO ligand is trans -[Co( en ) 2 (NO)Cl] + . The NO − is also common for alkali-metal or alkaline-earth metal-NO molecules. For example. LiNO and BeNO bear Li + NO − and Be + NO − ionic form. [ 4 ] [ 5 ]
The adoption of linear vs bent bonding can be analyzed with the Enemark-Feltham notation . [ 6 ] In their framework, the factor that determines the bent vs linear NO ligands is the electron count in the metal-N-O π system . Complexes more than 6 electrons in the system tend to have bent geometries at N. Thus, [Co( en ) 2 (NO)Cl] + , with eight electrons of pi-symmetry (six in t 2g orbitals and two on NO, {CoNO} 8 ), adopts a bent NO ligand, whereas [Fe(CN) 5 (NO)] 2− , with six electrons of pi-symmetry, {FeNO} 6 ), adopts a linear nitrosyl. In a further illustration, consider the {MNO} d-electron count of the [Cr(CN) 5 NO] 3− anion. In this example, the cyanide ligands are "innocent", i.e., they have a charge of −1 each, −5 total. To balance the fragment's overall charge, the charge on {CrNO} is thus +2 (−3 = −5 + 2). Using the neutral electron counting scheme, Cr has 6 d electrons and NO· has one electron for a total of 7. Two electrons are subtracted to take into account that fragment's overall charge of +2, to give 5. Written in the Enemark-Feltham notation, the d electron count is {CrNO} 5 , and the nitrosyl is linear. The results are the same if the nitrosyl ligand were considered NO + or NO − . [ 6 ]
Nitric oxide can also serve as a bridging ligand . In the compound [Mn 3 (η 5 C 5 H 5 ) 3 (μ 2 -NO) 3 (μ 3 -NO)], three NO groups bridge two metal centres and one NO group bridge to all three. [ 3 ]
Usually only of transient existence, complexes of isonitrosyl ligands are known where the NO is coordinated by its oxygen atom. They can be generated by UV-irradiation of nitrosyl complexes. [ 7 ]
Metal complexes containing only nitrosyl ligands are called isoleptic nitrosyls. They are rare, the premier member being Cr(NO) 4 . [ 8 ] Even trinitrosyl complexes are uncommon, whereas polycarbonyl complexes are routine.
One of the earliest examples of a nitrosyl complex to be synthesized is Roussin's red salt , which is a sodium salt of the anion [Fe 2 (NO) 4 S 2 ] 2− . The structure of the anion can be viewed as consisting of two tetrahedra sharing an edge. Each iron atom is bonded linearly to two NO + ligands and shares two bridging sulfidi ligands with the other iron atom. Roussin's black salt has a more complex cluster structure. The anion in this species has the formula [Fe 4 (NO) 7 S 3 ] − . It has C 3v symmetry . It consists of a tetrahedron of iron atoms with sulfide ions on three faces of the tetrahedron. Three iron atoms are bonded to two nitrosyl groups. The iron atom on the threefold symmetry axis has a single nitrosyl group which also lies on that axis.
Many nitrosyl complexes are quite stable, thus many methods can be used for their synthesis. [ 9 ]
Nitrosyl complexes are traditionally prepared by treating metal complexes with nitric oxide. The method is mainly used with reduced precursors. Illustrative is the nitrosylation of cobalt carbonyl to give cobalt tricarbonyl nitrosyl : [ 10 ]
Alternatively, the cobalt may be reduced in situ :
where X is Cl , Br , or I . [ 11 ]
Replacement of ligands by the nitrosyl cation may be accomplished using nitrosyl tetrafluoroborate . This reagent has been applied to the hexacarbonyls of molybdenum and tungsten: [ 12 ] [ 13 ]
Nitrosyl chloride and molybdenum hexacarbonyl react to give [Mo(NO) 2 Cl 2 ] n . [ 14 ] Diazald is also used as an NO source. [ 15 ]
Simple nitrite salts also oxidize metal carbonyls to the corresponding nitrosyl, i.e.: [ 16 ]
Hydroxylamine is a source of nitric oxide anion via a disproportionation: [ 17 ]
Nitric acid is a source of nitric oxide complexes, although the details are obscure. Probably relevant is the conventional self-dehydration of nitric acid:
Nitric acid is used in some preparations of nitroprusside from ferrocyanide :
Some anionic nitrito complexes undergo acid-induced deoxygenation to give the linear nitrosyl complex.
The reaction is reversible in some cases.
In some metal-ammine complexes , the ammonia ligand can be oxidized to nitrosyl: [ 18 ]
An important reaction is the acid/base equilibrium, yielding transition metal nitrite complexes :
This equilibrium serves to confirm that the linear nitrosyl ligand is, formally, NO + , with nitrogen in the oxidation state +3
Since nitrogen is more electronegative than carbon, metal-nitrosyl complexes tend to be more electrophilic than related metal carbonyl complexes. Nucleophiles often add to the nitrogen. [ 2 ] The nitrogen atom in bent metal nitrosyls is basic, thus can be oxidized, alkylated, and protonated, e.g.:
In rare cases, NO is cleaved by metal centers:
Metal-nitrosyls are assumed to be intermediates in catalytic converters , which reduce the emission of NO x from internal combustion engines. This application has been described as "one of the most successful stories in the development of catalysts". [ 20 ]
Metal-catalyzed reactions of NO are not often useful in organic chemistry . In biology and medicine, nitric oxide is however an important signalling molecule in nature and this fact is the basis of the most important applications of metal nitrosyls. The nitroprusside anion, [Fe(CN) 5 NO] 2− , a mixed nitrosyl cyano complex, has pharmaceutical applications as a slow release agent for NO. The signalling function of NO is effected via its complexation to haem proteins, where it binds in the bent geometry . Nitric oxide also attacks iron-sulfur proteins giving dinitrosyl iron complexes .
Several complexes are known with NS ligands. Like nitrosyls, thionitrosyls exist as both linear and bent geometries. [ 22 ] | https://en.wikipedia.org/wiki/Metal_nitrosyl_complex |
Metal organic cages or MOCs, are "discrete molecular assemblies with nanoscale dimensions, formed through the self-assembly of metal ions or clusters and organic ligands via coordination" [ 1 ] Metal organic polyhedra and Metal Organic Cages have some similarities, but the main difference is indicated by "Such high-symmetry structures contain pseudospherical cavities, and so typically bind roughly spherical guests. Biomolecules and high-value synthetic compounds are rarely isotropic, highly-symmetrical species. To bind, sense, separate, and transform such substrates, new, lower-symmetry, metal-organic cages are needed" [ 2 ] Indicating that the MOCs have distinct pore size that is advantageous to certain types of applications.
Originating from reticular chemistry [ 3 ] and supramolecular chemistry , [ 4 ] the 1990s popularized the synthesis of 3D molecular complexes. The term cage was first defined in the chemical sense as "... is normally associated with a closed pattern in the middle of which something is located. Thus we assume that in a birdcage there should be a bird. In chemistry the term "cage" is more widely used: it is first a polycyclic compound that contains atoms connected with one another in such a way that an enclosed volume is created. In this volume no atom or atomic group needs to be situated, and often there is no place for even a hydrogen atom." [ 5 ] Here the author refers to what is now described as porosity, having inherent volume within the structure of the cage. The genesis MOCs began with fundamental concepts of inorganic chemistry when attempting to bind metal and nonmetal compounds together. Late 1990s work with cage like complexes included work from the Makoto Fujita where he synthesized unique coordination complexes. One complex was two identical cages interlocked. [ 6 ] The other complex was a novel hexahedron complex that was nanometers in size. [ 7 ] Complexes like these inspired work later. The Early 2000s saw the Stang group review the topic of these cages and describe achieving their different shapes through self assembly. [ 8 ] The review described highly symmetric and nano-porous materials that took advantage of euclidean as well as Archimedean geometry. These were discussed in a host-guest application since there was limited work on these cages in a materials chemistry application.
In the early 2000s Omar Yaghi created MOCs that used the idea of these trinuclear clusters to synthesize the first tetrahedral MOCs. [ 9 ] Although the Yaghi group used iron (III) as the vertices for their cages they still created a similar trinuclear cluster as the zirconium. These synthesized iron (III) cages used a variety of "bdc" ligands to bind iron (III). The cages demonstrated porosity, relevant thermal stability, and crystallinity. These factors developed the field and led to enhanced application of these cages. Interestingly, there have not been any iron (III) cages reported in the literature since the origin of this paper. A recent review states, "Although these cages were promising in this regard, their syntheses are rather challenging as phase purity is a significant issue. This may also be a reason that no additional porous MOPs of this type have been reported." [ 10 ]
Today MOCs are typically prepared using zirconium cyclopentadiene salts or trinuclear zirconium clusters. [ 11 ] , [ 12 ] , [ 13 ] The precise mechanism at which the cages form is currently not fully understood [ 14 ] . Although from a theoretical understanding of organic and inorganic chemistry, some general ideas can be derived. When synthesizing MOCs with the zirconocene dichloride the trinuclear cluster forms in situ while binding to the linkers of the cages. Typically, there are two phases of tetrahedral cages. There is a "lantern" phase and a tetrahedral phase. recent studies show that the tetrahedron is thermodynamically favorable, but both phases can be produced. Currently, there is no reported way to separate these phases. Although some hypotheses suggest that tuning the steric bulk of the linkers will increase the thermodynamic favorability of the MOCs to form the tetrahedron phase. [ 15 ] , [ 16 ] During synthesis of the cages when using the zirconium clusters there is likely a ligand exchange with some substituents that are already present on the cluster. Although the mechanism is still being studied. Other transition metals are also often used to make these MOCs or MOPs, and various differently shaped cages as well. Some common metals that are used are Pd, Fe(II), Zr, and Cu. Common shapes that are synthesized are cuboctahedra, octahedra, and tetrahedra. [ 17 ]
MOCs are characterized by various common chemical and physical characterization techniques such as NMR , mass spectrometry , single-crystal X-ray crystallography, and thermogravimetric analysis . Many of these cages can be solubilized in common organic solvents such as DMF, and DEF. Some of their solubility has been attributed to the counter anion of the cage. [ 18 ] Since the cages can be solubilized NMR has been the leading quick characterization technique for most cage complexes. Mass spectrometry has been used to determine the shape of the cage since each cage shape has a unique molecule weight. The distribution of the cages at the theoretical molecular weight is typical since their distribution of isotopes is present in the cages. [ 19 ] Single crystal X-ray crystallography is common to determine exact structural information about the cages. Even the original works of these cages use X-ray crystallography to determine the structure of the cages. [ 20 ] [ 21 ] This method is used to show that the cages are porous in the solid state and for precise molecular imaging of the cage compounds. This technique is a definitive way to show that the cage compounds are formed.
Taking advantage of the porosity of the discrete structure of cages allows for the absorption and possibly sequestration of gases into the cage structure. Early in the creation of the cages, there was evidence for gas sorption. [ 22 ] This led to further development to enhance the capability of these cages to sorb more gaseous compounds. Researchers have developed methods that include casting these cages into a polymer membrane to increase the absorption of gaseous compounds. [ 23 ] MOCs have also been used to separate gasses such as ethylene/acetylene and ethylene and carbon dioxide. [ 24 ] This article reported using a tetrazole-functionalized tetrahedral MOC and leveraged the hydrogen bonding of the gases and the tetrazole to increase the sorption of ethylene preferentially. Gas capture with MOCs is expanding, and competitive but is still outperformed by metal-organic frameworks. [ 25 ]
Functionalization of the cage structure is typically done by synthesizing ideal ligand platforms for binding single metals. The discrete structure and highly tunable nature of the cages allow for various functionalizations. One group that took advantage of this was using a Pd cage-type catalyst in aqueous media. [ 26 ] Typically the cross-coupling reaction that is done in this reaction is only done in organic solvents. [ 27 ] Another target of this paper was to increase the re-usability of the catalyst. These authors showed that the MOCs can be a suitable platform for heterogeneous catalysts and reusable catalysts. A drawback of homogenous catalysts is that eventually catalyst, even after one use could not be reused. [ 28 ] Many MOCs and MOP catalysts aim to solve this issue. [ 29 ] Other, more green chemical uses of the cages involve the reduction of carbon dioxide with bipyridine ligand that is bound to rhenium. [ 30 ] These authors highlight a versatile ligand used in many types of chemistry, bipyridine (bipy). This shows the applicability of the cages, and how versatile they can be if they are functionalized properly. The reduction of carbon dioxide has a high turnover frequency, as well as high stability. Molecule forms of this compound show much less promising results. The cage structure provides stability, and molecule flexibility which provides evidence that the cage does not degrade for a short period and the reduction occurs more frequently most likely because of the increased molecule motion.
Some MOCs have fluorescent properties, and some scaffolding for the cages is made from zirconium which is a non-toxic metal so there are numerous biological imaging applications for these cages. Cage porosity can even be leveraged in biological systems. One group used cage porosity to make a channel through a lipid membrane to pass ions through the channel. [ 31 ] The group leveraged using large BINOL ligands which feature an inherently chiral selective group to allow for the selective transport of amino acids. MOCs have also shown to have high membrane permeability which could be useful for the application for drug delivery. [ 32 ] , [ 33 ] MOCs can be used to detect the presence of antibiotics via fluorescence quenching. [ 34 ] Many cages have inherent fluorescence due to their conjugated pi electron systems which can be leveraged in enhancement or quenching. Many compounds can quench the fluorescence of the cages via the binding of other physical-chemical interactions. The detection of the antibiotics was done with the hosting of the antibiotics in the cages which reduces the fluorescence character. The MOC fluorescent character is intense enough to view via cell imaging. [ 35 ] MOCs have a diverse range of rational ligand designs to make highly versatile and useful MOCs.
Detection of environmental contaminants can also be an application of MOCs. The solution state porosity and host guest properties of the MOCs allow for the hosting of ions. These ions can ion-pair to the ligands and quench fluorescence like the biological application. Specifically, iron (III) and Iodide. [ 36 ] The challenge with the quenching is that if the lambda max does not overlap with that of the MOC there will be no change in fluorescence, and therefore no ability to detect ions. Another use of the polymer MOC matrices is desalination. The "polymoc" framework allows for the selective passthrough of ions. The matrices are made into a thin film that allows for the desalination of water and does not inhibit the flux of water passing through the thin film membrane. [ 37 ] MOC development will continue to diversify and push forward the materials field. | https://en.wikipedia.org/wiki/Metal_organic_cages |
Metal powder is a metal that has been broken down into a powder form. Metals that can be found in powder form include aluminium powder , nickel powder, iron powder and many more. [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] There are four different ways metals can be broken down into this powder form: [ 6 ]
The following processes can be used to produce metal powder: [ 6 ]
Back in the early 1900's, metal powder was the currency used in the United States of America. [ citation needed ] Depending on the market, metal powder can be more valuable than gold. The following are the types and uses of metal powder: [ 7 ]
This material -related article is a stub . You can help Wikipedia by expanding it .
This engineering-related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metal_powder |
Metal profile sheet systems are used to build cost efficient and reliable envelopes of mostly commercial buildings. They have evolved from the single skin metal cladding often associated with agricultural buildings to multi-layer systems for industrial and leisure application. As with most construction components, the ability of the cladding to satisfy its functional requirements is dependent on its correct specification and installation. Also important is its interaction with other elements of the building envelope and structure. Metal profile sheets are metal structural members that due to the fact they can have different profiles, with different heights and different thickness, engineers and architects can use them for a variety of buildings, from a simple industrial building to a high demand design building.
Trapezoidal profiles are large metal structural members, which, thanks to the profiling and thickness, retain their high load bearing capability. They have been developed from the corrugated profile . The profile programme offered by specific manufacturers covers a total of approximately 60 profile shapes with different heights.
Cassettes are components that are mainly used as the inner shell in dual-shell wall constructions. They are mainly used in walls today, even though they were originally designed for use in roofs.
Trapezoidal profiles and cassettes have been known in Europe for around 100 years. Today's characteristic profile shape came to Europe from the USA in the 50s and has gained relevance since about 1960. At present the proportion of load bearing, room sealing trapezoidal profiles used in the overall area of new and slightly sloping roofs amounts to 90%. Above all else the wide acceptance has resulted from the simple constructive training, fast assembly, and the low costs of the trapezoidal profile construction.
The primary function of the cladding system is to provide a weathertight building envelope, suitable for the intended use of the building.
Trapezoidal metal roof sheets with through fix fasteners are generally suitable for slopes of 4% or steeper. This limit is critical to the performance of the cladding. For shallower pitches, down to 1.5%, a fix system with no exposed through fasteners, special side laps and preferably no end laps should be used. For low pitch roof, ponding is a potential problem that must be considered at the design stage in order to avoid the deleterious effects of prolonged soaking and the increased loading, due to the weight of the water. [ 1 ]
Building envelope made from metal sheet provide builders and architects with products, which meet all of the highest demand regarding construction characteristics and design. The steel from which profiled cladding sheets are made is available pre-coated in a wide range of colors and textures, allowing architects to choose a finish that best suits the location and function of the building. Profile shape is also a characteristic that can be adapted to the demand of the architects. [ 2 ]
Buildings are responsible of the 40% of European energy consumption, consequently, improving the thermal performance of the cladding and associated components is very important. The elemental U-value (thermal transmittance, W/m2K) of a cladding panel, depends on the conductivity and thickness of the insulation which is added, the profile shape and the presence of thermal bridges.
So, metal profile sheets can achieve thermal performance regulations thanks of insulations and profile shape.
It is very important to analyze and avoid all possible thermal bridges within the roof and wall cladding assembly, to minimize local heat/cold losses. [ 3 ]
Roofs constructed with trapezoidal profiles have excellent sound suppression characteristics. Sound has been found to be reduced to by up to 53 dB.
The measured sound reduction for wall constructions using cassettes has been assessed at an RW of 57 dB.
The acoustic performance of a particular cladding system will depend on the insulation material, the weather sheet and liner sheet profiles and the method of assembly.
To minimize reverberation architects may take advantage of the sound absorbing properties of the cladding insulation layer by replacing the standard liner sheet with a perforated liner. [ 4 ]
In order to ensure that the building envelope remains fully functional throughout its design life, it is important that it receives regular maintenance, including inspection, removal of debris, cleaning and repair of damage. Inspection can include man-made or natural wear. Weather exposure, natural movement, installation error and manufacturing defects are examples. [ 5 ] The need of maintenance may be greatly reduced using specific coating depending on the weather conditions, this coating guarantee the expected design life of the cladding. The commonly used 302 stainless steel alone is resistant to acetic acid, acetone and boric acid, among others. [ 6 ] [ 7 ]
Metal profiles sheets have a high recycled scrap steel content and all steel is recyclable. Many steel components can be unbolted and even reused for future applications. The possibility of reusing building elements makes steel construction even more sustainable than the already significant contribution of today's simple material recycling. Steel can be repeatedly recycled because it does not lose any of its inherent physical properties as a result of the recycling process. Stainless steel fasteners have excellent corrosion resistance and durability, as well as being a sustainable material. Custom fasteners in this material make for the utmost of sustainability with high recyclability. [ 8 ] [ 9 ]
Metal cladding systems are required to carry externally applied loads, such as snow and wind loading without deflecting excessively or compromising the other performance requirements. The individual characteristic loads (actions) should be obtained from the appropriate part of EN 1991, taking into account the building geometry and location as applicable. These individual actions should then be combined using the appropriate safety factors from EN 1990 to obtain the load cases used in design. [ 10 ]
For most application of metal cladding technology, the only permanent action which the roof cladding needs to be designed is its own self-weight.
For wall cladding, it is not normally necessary to consider permanent actions, since the self-weight acts in the plan of the cladding.
In addition to its self-weight, the roof cladding must also be designed for the following variable actions as specified in the appropriate parts of EN 1991:
-Access for cleaning and maintenance.
-A uniformly distributed load due to snow over the complete roof area. The value of this load will depend on the building's location.
-Asymmetric snow load and loading due to snow drifts.
-Wind loading due to pressure and suction.
Care must be taken on site to avoid excessive local deflection. Typical deflection limits imposed on the cladding are depend on the loading regime considered, the location of the structural component and whether a brittle material is present. Deflection limits may be specifies by National regulation. [ 11 ]
Metal profile sheets due to their versatility mechanical and design properties can be used as roof and roof cladding, as external walls and wall cladding and also as floors. They are used in industry and in residential sector, and the two sectors can be used in both new construction and rehabilitation. Some of the applications where metal profile sheets are used are: | https://en.wikipedia.org/wiki/Metal_profiles |
A metal salen complex is a coordination compound between a metal cation and a ligand derived from N , N ′-bis(salicylidene)ethylenediamine , commonly called salen. The classical example is salcomine , the complex with divalent cobalt Co 2+ , usually denoted as Co(salen). [ 1 ] These complexes are widely investigated as catalysts and enzyme mimics. [ 2 ] [ 3 ]
The metal-free salen compound (H 2 salen or salenH 2 ) has two phenolic hydroxyl groups. The salen ligand is usually its conjugate base (salen 2− ), resulting from the loss of protons from those hydroxyl groups. The metal atom usually makes four coordination bonds to the oxygen and nitrogen atoms.
The salen anion forms complexes with most transition metals . These complexes are usually prepared by the reaction of H 2 salen ("proligand") with metal precursors containing built-in bases, such as alkoxides , metal amides , or metal acetate . The proligand may also be treated with a metal halide , with or without an added base. Lastly, the proligand may be deprotonated by a nonnucleophilic base, such as sodium hydride , before treatment with the metal halide. For example, Jacobsen's catalyst is prepared from the salen ligand precursor with manganese acetate . [ 4 ]
Salen complexes with d 8 metal ions, such as Ni(salen), typically have a low-spin square planar molecular geometry in the coordination sphere .
Other metal–salen complexes may have additional ligands above the salen nitrogen–oxygen plane. Complexes with one extra ligand, such as VO (salen), [ 6 ] may have a square pyramidal molecular geometry . Complexes with two extra ligands, such as Co(salen)Cl( py ), may have octahedral geometry . Usually the MN 2 O 2 core is relatively planar, even though the ethylene backbone is skewed and the overall salen ligand takes a twisted C 2 symmetry . Examples exist where ancillary ligands force the N 2 O 2 donors out of planarity. [ 7 ] No evidence indicates that salen is a redox- noninnocent ligand .
The pyridine adduct of the cobalt(II) complex Co(salen)(py) ( salcomine ) has a square-pyramidal structure . It is a dioxygen carrier by forming a labile, octahedral O 2 complex. [ 9 ] [ 10 ]
The name "salen ligands" is used for tetradentate ligands which have similar structures. For example, in salpn there is a methyl substituent on the bridge. It is used as a metal deactivation additive in fuels. [ 11 ] The presence of bulky groups near the coordination site may enhance the catalytic activity of a metal complex and prevent its dimerization. Salen ligands derived from 3,5-di- tert -butylsalicylaldehyde fulfill these roles, and also increase the solubility of the complexes in non-polar solvents like pentane . Chiral "salen" ligands may be created by proper substitution of the diamine backbone, the phenyl ring, or both. [ 12 ] An example is the ligand obtained by condensation of the C 2 -symmetric trans -1,2- diaminocyclohexane with 3,5-di- tert -butylsalicylaldehyde. Chiral ligands may be used in asymmetric synthesis reactions, such as the Jacobsen epoxidation : [ 4 ] [ 13 ]
Tsumaki described the first metal–salen complexes in 1938. He found that the cobalt(II) complex Co(salen) reversibly binds O 2 , which led to intensive research on cobalt complexes of salen and related ligands for their capacity for oxygen storage and transport, looking for potential synthetic oxygen carriers . [ 1 ] Cobalt salen complexes also replicate certain aspects of vitamin B 12 .
The manganese-containing salen complex catalyzes the asymmetric epoxidation of alkenes . In the hydrolytic kinetic resolution technique, a racemic mixture of epoxides may be separated by selectively hydrolyzing one enantiomer , catalyzed by the analogous cobalt(III) complex. [ 14 ] In subsequent work, chromium(III) and cobalt(III) salen complexes catalyze the reaction of carbon dioxide and epoxides to give polycarbonates . [ 15 ]
Complexes of salen per se are poorly soluble in organic solvents . Substitution of the organic framework increases the solubility of the complex. An example is the salpn ligand , derived from 1,2-diaminopropane instead of ethylenediamine , which is used as a metal deactivating additive in motor oils and motor fuel . [ 16 ]
The presence of bulky groups adjacent to the phenoxide group can give complexes with enhanced catalytic activity. These substituents suppress formation of dimers . For these reasons, salen ligands derived from 3,5-di- tert -butylsalicylaldehyde have received particular scrutiny.
Chirality may be introduced into the ligand either via the diamine backbone , via the phenyl ring , or both. [ 12 ] For example, condensation of the C 2 -symmetric trans -1,2- diaminocyclohexane with 3,5-di- tert -butylsalicylaldehdye gives a ligand that forms complexes with Cr, Mn, Co, Al, which have proven useful for asymmetric transformations . For an example, see the Jacobsen epoxidation , which is catalyzed by a chiral manganese -salen complex: [ 4 ]
x
The name “salen” or “salen-type” may be used for other ligands that have similar environment around the chelating site, namely two acidic hydroxyls and two Schiff base (aryl- imine ) groups. These include the ligands abbreviated as salph, from the condensation of 1,2-phenylenediamine and salicylaldehyde. Other "Salen-type" metal complexes are formed with ligands with similar chelating groups , such as salph and salqu . Salqu copper complexes have been investigated as oxidation catalysts. [ 17 ]
salan or salalen ligands have one or two saturated nitrogen–aryl bonds ( amines rather than imines ). They are less rigid and more electron-rich at the metal center than the corresponding salen complexes. [ 18 ] [ 19 ] Salans can be synthesized by the alkylation of an appropriate amine with a phenolic alkyl halide . The “half-salen” ligands have only one salicylimine group. They are prepared from a salicylaldehyde and a monoamine. [ 20 ]
A class of tetradentate ligands with the generic name acacen are obtained by the condensation of derivatives of acetylacetone and ethylenediamine . [ 21 ] | https://en.wikipedia.org/wiki/Metal_salen_complex |
In organometallic chemistry , metal sulfur dioxide complexes are complexes that contain sulfur dioxide , SO 2 , bonded to a transition metal . [ 1 ] Such compounds are common but are mainly of theoretical interest. Historically, the study of these compounds has provided insights into the mechanisms of migratory insertion reactions.
Sulfur dioxide forms complexes with many transition metals. Most numerous are complexes with metals in oxidation state 0 or +1. [ 1 ]
In most cases SO 2 binds in monodentate fashion, attaching to the metal through sulfur. Such complexes are further subdivided according to the planarity or pyramidalization at sulfur. The various bonding modes are:
More exotic bonding modes are known for clusters.
Complexes of the transition metals are usually generated simply by treating the appropriate metal complex with SO 2 . The adducts are often weak. In some cases, SO 2 displaces other ligands. [ 3 ]
A large number of labile O-bonded SO 2 complexes arise from the oxidation of a suspension of the metals in liquid SO 2 , an excellent solvent. [ 2 ]
The main reaction of sulfur dioxide promoted by transition metals is its reduction by hydrogen sulfide . Known as the Claus process , this reaction is conducted on a large scale as a way to remove hydrogen sulfide that arises in hydrotreating processes in refineries.
Of academic interest, SO 2 acts like a Lewis acid towards the alkyl ligand. [ 4 ] The pathway for the insertion of SO 2 into metal alkyl bond begins with attack of the alkyl nucleophile on the sulfur centre in SO 2 . The "insertion" proceed the sulfur dioxide between the metal and the alkyl ligand leads to the O , O'- sulphinate. Alternatively an O -sulphinate can arise. Both of these intermediates commonly convert to an S -sulphinate. [ 5 ] S -sulphinate has sulfur–oxygen stretching frequencies from 1250–1000 cm −1 and 1100–1000 cm −1 . The O , O'- sulphinate and O -sulphinate are difficult to distinguish as they have stretching frequencies from 1085–1050 cm −1 and 1000–820 cm −1 or lower. The pathway involving the O , O' sulphinate can generally be ruled out if the original metal complex fulfilled the 18-electron rule because the two metal–oxygen bonds would exceed the 18 electron rule. [ 6 ] The pathway by which SO 2 inserts into a square planar alkyl complexes involves the formation of an adduct . Thereafter, the alkyl ligand migrates to the SO 2 . [ 7 ]
Dithionite , the reductively coupled derivative of sulfur dioxide is observed as a ligand when some reduced metals are treated with sulfur dithioxide. One example is [(C 5 (CH 3 ) 5 ) 2 Sm] 2 (S 2 O 4 ) . [ 8 ] [ 9 ]
Several complexes of disulfur monoxide are known. Most are formed by oxidation peroxide oxidation of a disulfur ligand. In these complexes, the S 2 O ligand is invariably bound in an η 2 -S,S manner. Selected examples: [Ir( dppe ) 2 S 2 O] + , OsCl(NO)(PPh 3 ) 2 S 2 O , NbCl(η-C 5 H 5 ) 2 S 2 O , Mn(CO) 2 (η-C 5 Me 5 )S 2 O , Re(CO) 2 (η-C 5 Me 5 )S 2 O , Re(CO) 2 (η-C 5 H 5 )S 2 O . [ 10 ]
Mo 2 (S 2 O) 2 (S 2 CNEt 2 ) 4 arises when the dithiocarbamate complex Mo(CO) 2 (S 2 CNEt 2 ) 2 is oxidized with elemental sulfur in air. Another way to form these complexes is to combine OSNSO 2 ·R complexes with hydrogen sulfide . Complexes formed in this way are: IrCl(CO)(PPh 3 ) 2 S 2 O ; Mn(CO) 2 (η-C 5 H 5 )S 2 O . With hydrosulfide and a base followed by oxygen, OsCl(NO)(PPh 3 ) 2 S 2 O can be made. | https://en.wikipedia.org/wiki/Metal_sulfur_dioxide_complex |
Metal toxicity or metal poisoning is the toxic effect of certain metals that accumulate in the environment and damage ecosystems , plants and animals, including human health. [ 1 ] [ 2 ] [ 3 ] Environmental pollution with heavy metals can result in contamination of drinking water , air, and waterways, accumulating in plants, crops, seafood , and meat. [ 3 ] Such pollution may indirectly affect humans via the food chain and through occupational or domestic exposure by inhalation, ingestion, or contact with the skin. [ 1 ] [ 3 ]
At low concentrations, heavy metals such as copper , iron , manganese , and zinc are essential nutrients obtained through the diet supporting health, but have toxicity at high exposure concentrations. [ 2 ] Other heavy metals having no biological roles in animals, but with potential for toxicity include arsenic , cadmium , lead , mercury , and thallium . [ 1 ] [ 2 ] [ 4 ]
Some metals are toxic when they form poisonous soluble compounds which interfere with enzyme systems, such as superoxide dismutase , catalase , or glutathione peroxidase . [ 1 ] Only soluble metal-containing compounds are toxic by forming coordination complexes , which consist of a metal ion surrounded by ligands . [ 1 ] Ligands can range from water in metal aquo complexes to methyl groups, as in tetraethyl lead .
Toxic metal complexes can be detoxified by conversion to insoluble derivatives or by binding them in rigid molecular environments using chelating agents. An option for treatment of metal poisoning may be chelation therapy , which involves the administration of chelation agents to remove metals from the body. [ 3 ]
Heavy metals are found throughout natural ecosystems, including rocks, soils, and water, and originate from diverse sources, such as natural weathering, erosion, mining , industrial and urban runoff, sewage, pesticides on crops, metal pipes carrying potable water, traffic pollution, coal -burning emissions, and various other industrial and urban outputs. [ 1 ] [ 5 ]
Toxic metal particles in ecosystems may remain for hundreds or even thousands of years, with potentially millions of people exposed to high concentrations at some point in their lives. [ 5 ] Commonly, there is no visible evidence of metals pollution in soil or water. [ 5 ]
When metal toxicity in the environment is suspected, pathologies in fish, clams, and insects may serve as signals for contamination and toxicities. [ 5 ] Physiological mechanisms of metal toxicity may have a spectrum of effects, ranging from changes in behavior to death of small animal species. [ 5 ]
A dominant kind of metal toxicity is arsenic poisoning, which mainly arises from ground water naturally containing high concentrations of arsenic in the supply of drinking water. [ 1 ] [ 2 ]
Lead poisoning, in contrast to arsenic poisoning, is caused by industrial materials, such as leaded gasoline and lead leached from plumbing . [ 1 ] [ 2 ] [ 3 ] Use of leaded gasoline has declined precipitously since the 1970s. [ 6 ] [ 7 ]
In the case of the lanthanides , the definition of an essential nutrient as being indispensable and irreplaceable is not completely applicable due to their extreme chemical similarity. The stable early lanthanides La–Nd are known to stimulate the growth of various lanthanide-using organisms, and Sm–Gd show lesser effects for some such organisms. The later elements in the lanthanide series do not appear to have such effects. [ 11 ]
Some metal elements are required for life, although they may be toxic in high exposure amounts. [ 1 ] [ 2 ] [ 3 ] Included are cobalt , copper , iron , manganese , [ 12 ] selenium , [ 13 ] and zinc . [ 14 ] Excessive absorption of zinc can suppress copper and iron absorption. The free zinc ion in solution is highly toxic to bacteria, plants, invertebrates, and fish. [ 15 ]
No global mechanism exists for the toxicities of these metal ions. Excessive exposure, when it occurs, typically is associated with industrial activities.
Chelation therapy is a medical procedure that involves the administration of chelating agents to remove or deactivate heavy metals from the body. [ 3 ] Chelating agents are molecules that form particularly stable coordination complexes with metal ions. [ 3 ] Complexation prevents the metal ions from reacting with molecules in the body, and enable them to be dissolved in blood and eliminated in urine. [ 3 ] It should only be used in people who have a diagnosis of metal intoxication. [ 3 ] That diagnosis should be validated with tests done in appropriate biological samples. [ 3 ] [ 28 ] [ 29 ] [ 30 ]
It is difficult to differentiate the effects of low level metal poisoning from the environment with other kinds of environmental harms, including nonmetal pollution. [ 1 ] Generally, increased exposure to heavy metals in the environment increases the risks for several diseases. [ 1 ] Despite a lack of evidence to support its use, some people seek chelation therapy to treat a wide variety of conditions such as autism , cardiovascular disease , Alzheimer's disease , or any sort of neurodegeneration . [ 28 ]
Treatment of autism by chelation therapy has been promoted by alternative medicine practitioners based on an unsupported hypothesis that autism is a result of heavy metal poisoning. This hypothesis likely emerged from the more specific claim that autism was caused by the preservative thiomersal , which in the past has been used in multi-dose vials of vaccines . Despite extensive study, no connection has been found between vaccines and autism diagnosis rates. [ 31 ] [ 32 ] Despite this lack of evidence, thimerosal was removed from vaccines out of an abundance of caution by 2001; autism diagnosis rates did not decrease in response to the exclusion of thimerosal, disproving the association. [ 33 ] [ 34 ] Regardless of the removal of thimerosal and the evidence that it never influenced autism in the first place, the idea of heavy metal exposure causing autism has persisted, and thus has the use of chelation therapy as treatment. Systematic reviews of available evidence do not support the use of chelation therapy for autism, [ 35 ] [ 36 ] and at least one child has died due to errors in administration of chelation therapy for this purpose. [ 37 ] [ 38 ] [ 39 ] | https://en.wikipedia.org/wiki/Metal_toxicity |
A metal triflimidate [ 1 ] M(NTf 2 ) n in organic chemistry is a metal salt or complex of triflimidic acid and used as a catalyst . [ 2 ]
Metal triflimidates are prepared by reaction of metal oxides , carbonates , hydroxides , or halides with triflimidic acid in water as the hydrate M(NTf 2 ) n · x H 2 O with x ranging from zero to nine. Another method is by metathesis reaction between a metal triflimidate and another metal complex by metal exchange. Commercially available salts are based on lithium and silver .
The catalytic activity of metal triflimidates has been demonstrated in cycloadditions , in various rearrangement reactions , in Friedel–Crafts acylation and Friedel–Crafts alkylation .
Lithium triflimidate is used as an electrolyte in batteries as a replacement of lithium perchlorate .
Gold phosphine or NHC triflimidates (LAuNTf 2 , L = R 3 P or NHC) are isolable though somewhat labile complexes of gold that serve as sources of catalytically-active LAu + ions. [ 3 ]
This catalysis article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metal_triflimidate |
In chemistry , metal vapor synthesis (MVS) is a method for preparing metal complexes by combining freshly produced metal atoms or small particles with ligands . In contrast to the high reactivity of such freshly produced metal atoms, bulk metals typically are unreactive toward neutral ligands. The method has been used to prepare compounds that cannot be prepared by traditional synthetic methods, e.g. Ti(η 6 -toluene) 2 . The technique relies on a reactor that evaporates the metal, allowing the vapor to impinge on a cold reactor wall that is coated with the organic ligand. The metal evaporates upon being heated resistively or irradiated with an electron beam . The apparatus operates under high vacuum. [ 1 ] In a common implementation, the metal vapor and the organic ligand are co-condensed at liquid nitrogen temperatures. [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ]
In several case where compounds are prepared by MVS, related preparations employ conventional routes. Thus, tris(butadiene)molybdenum was first prepared by co-condensation of butadiene and Mo vapor, but yields are higher for the reduction of molybdenum(V) chloride in the presence of the diene. [ 8 ] | https://en.wikipedia.org/wiki/Metal_vapor_synthesis |
In logic and linguistics , a metalanguage is a language used to describe another language, often called the object language . [ 1 ] Expressions in a metalanguage are often distinguished from those in the object language by the use of italics, quotation marks , or writing on a separate line. [ citation needed ] The structure of sentences and phrases in a metalanguage can be described by a metasyntax . [ 2 ] For example, to say that the word "noun" can be used as a noun in a sentence, one could write "noun" is a <noun> .
There are a variety of recognized types of metalanguage, including embedded , ordered , and nested (or hierarchical ) metalanguages.
An embedded metalanguage is a language formally, naturally and firmly fixed in an object language. This idea is found in Douglas Hofstadter 's book, Gödel, Escher, Bach , in a discussion of the relationship between formal languages and number theory : "... it is in the nature of any formalization of number theory that its metalanguage is embedded within it." [ 3 ]
It occurs in natural, or informal, languages, as well—such as in English, where words such as noun , verb , or even word describe features and concepts pertaining to the English language itself.
An ordered metalanguage is analogous to an ordered logic . An example of an ordered metalanguage is the construction of one metalanguage to discuss an object language, followed by the creation of another metalanguage to discuss the first, etc.
A nested (or hierarchical ) metalanguage is similar to an ordered metalanguage in that each level represents a greater degree of abstraction. However, a nested metalanguage differs from an ordered one in that each level includes the one below.
The paradigmatic example of a nested metalanguage comes from the Linnean taxonomic system in biology. Each level in the system incorporates the one below it. The language used to discuss genus is also used to discuss species; the one used to discuss orders is also used to discuss genera, etc., up to kingdoms.
Natural language combines nested and ordered metalanguages. In a natural language there is an infinite regress of metalanguages, each with more specialized vocabulary and simpler syntax.
Designating the language now as L 0 {\displaystyle L_{0}} , the grammar of the language is a discourse in the metalanguage L 1 {\displaystyle L_{1}} , which is a sublanguage [ 4 ] nested within L 0 {\displaystyle L_{0}} .
Since all of these metalanguages are sublanguages of L 0 {\displaystyle L_{0}} , L 1 {\displaystyle L_{1}} is a nested metalanguage, but L 2 {\displaystyle L_{2}} and sequel are ordered metalanguages. [ 5 ] Since all these metalanguages are sublanguages of L 0 {\displaystyle L_{0}} they are all embedded languages with respect to the language as a whole.
Metalanguages of formal systems all resolve ultimately to natural language, the 'common parlance' in which mathematicians and logicians converse to define their terms and operations and 'read out' their formulae. [ 6 ]
There are several entities commonly expressed in a metalanguage. In logic usually the object language that the metalanguage is discussing is a formal language , and very often the metalanguage as well.
A deductive system (or, deductive apparatus of a formal system ) consists of the axioms (or axiom schemata ) and rules of inference that can be used to derive the theorems of the system. [ 7 ]
A metavariable (or metalinguistic or metasyntactic variable) is a symbol or set of symbols in a metalanguage which stands for a symbol or set of symbols in some object language. For instance, in the sentence:
The symbols A and B are not symbols of the object language L {\displaystyle L} , they are metavariables in the metalanguage (in this case, English) that is discussing the object language L {\displaystyle L} .
A metatheory is a theory whose subject matter is some other theory (a theory about a theory). Statements made in the metatheory about the theory are called metatheorems . A metatheorem is a true statement about a formal system expressed in a metalanguage. Unlike theorems proved within a given formal system, a metatheorem is proved within a metatheory , and may reference concepts that are present in the metatheory but not the object theory. [ 8 ]
An interpretation is an assignment of meanings to the symbols and words of a language.
Michael J. Reddy (1979) argues that much of the language we use to talk about language is conceptualized and structured by what he refers to as the conduit metaphor . [ 9 ] This paradigm operates through two distinct, related frameworks.
The major framework views language as a sealed pipeline between people:
The minor framework views language as an open pipe spilling mental content into the void:
Computers follow programs, sets of instructions in a formal language. The development of a programming language involves the use of a metalanguage. The act of working with metalanguages in programming is known as metaprogramming .
Backus–Naur form , developed in the 1960s by John Backus and Peter Naur, is one of the earliest metalanguages used in computing. Examples of modern-day programming languages which commonly find use in metaprogramming include ML , Lisp , m4 , and Yacc . | https://en.wikipedia.org/wiki/Metalanguage |
Metalation (Alt. spelling: Metallation) is a chemical reaction that forms a bond to a metal. This reaction usually refers to the replacement of a halogen atom in an organic molecule with a metal atom, resulting in an organometallic compound. In the laboratory, metalation is commonly used to activate organic molecules during the formation of C—X bonds (where X is typically carbon, oxygen, or nitrogen), which are necessary for the synthesis of many organic molecules.
In synthesis, metallated reagents are typically involved in nucleophilic substitution , single-electron-transfer (SET), and redox chemistry with functional groups on other molecules (including but not limited to ketones , aldehydes and alkyl halides ). Metallated molecules may also participate in acid-base chemistry , with one organometallic reagent deprotonating an organic molecule to create a new organometallic reagent.
The most common classes of metallated compounds are organolithium reagents and Grignard reagents . However, other organometallic compounds — such as organozinc compounds — also experience common use in both laboratory and industrial applications.
Metalation was first observed in the laboratory by Edward Frankland during a synthesis of diethylzinc in 1849. [ 1 ] While this development eventually led to the development of organometallic compounds of other metals, [ 2 ] these compounds saw little use in the laboratory because of their expense and (in the case of organozinc compounds) their highly pyrophoric nature. Metalation reactions (particularly in the form of transmetalation ) only began to see more widespread use in synthetic laboratories after François Auguste Victor Grignard ’s synthesized organomagnesium halides directly from metallic magnesium and organic halides. [ 3 ] These newfound organomagnesium reagents' extreme versatility in organic synthesis caused metalation to see widespread use in laboratory science. [ 4 ] Organolithium reagents were synthesized for the first time in 1917 by Schlenk and Holtz, [ 5 ] though these reagents did not see widespread use as metallating agents or reagents in organic synthesis until Karl Ziegler , Henry Gilman , and Georg Wittig — among others — developed synthetic methods that improved upon this initial synthesis. [ 6 ] After these improvements in synthesis came to be known, interest in the compounds increased significantly, as they are generally more reactive than organomagnesium compounds. The first use of an organolithium reagent as a metalation reagent occurred in 1928, with Schlenk and Bergmann's metalation of fluorene with ethyllithium. [ 7 ]
Transmetalation involves the exchange of two metals between organic molecules by a redox exchange mechanism. For example, transmetalations often form a reaction between an organolithium reagent and a metal salt.
When synthesizing simple organolithium reagents, the reduction of one equivalent of a simple alkyl or aryl halide with two equivalents of lithium metal produces one equivalent of a simple alkyl- or aryl-lithium and one equivalent of lithium halide with good yield. [ 8 ]
R − X + 2 Li ⟶ R − Li + Li − X {\displaystyle {\ce {{R-X}+ 2Li -> {R-Li}+ Li-X}}}
This reaction is known to proceed via a radical pathway that is likely initiated through a single-electron-transfer mechanism of the type shown below. [ 9 ]
Magnesium similarly metalates organohalides to give Grignard reagents . | https://en.wikipedia.org/wiki/Metalation |
The metallabenzenes are class of chemical compound of the form L n M(CH) 5 , or derivatives thereof. Most metallabenzenes do not feature the M(CH) 5 ring itself, but, instead, some of the H atoms are replaced by other substituents. The parent metallabenzenes can be viewed as derivatives of benzene wherein a CH center has been replaced by a transition metal complex . [ 2 ]
All known metallabenzenes are 18-electron complexes , [ 3 ] and have been classified into three varieties. [ which? ] In modeling metallabenzenes, the parent acyclic hydrocarbon ligand is viewed as the anion C 5 H 5 − . [ 4 ]
Early computations suggested that the six π-electrons in the metallacycle conform to the Hückel (4n+2) theory ; [ 4 ] however, interactions with an additional d orbital suggest that metallabenzenes may instead be an 8-π Möbius aromat . [ 2 ] Specifically, if the ring is located in the xy plane with y measuring radial distance at the metal center, then the Hückel orbitals treat the two lobes of the d yz orbital inside the ring as though a main-group π orbital. One resulting orbital has vanishing metal-orbital component. That Hückel orbital is split by interaction with the d xz orbital, whose four lobes are all circumferential and effect a Möbius twist (see Fig. 4 in the cited paper). [ 3 ] Still other authors instead argue that metallabenzenes are Hückel aromats but with 10 π-electrons. [ 5 ]
Also, a large number of multinuclear metal complexes can be notionally decomposed into a metallabenzene ligand facially coordinated to another metal center. [ 2 ]
The first reported stable metallabenzene was the osmabenzene Os(C 5 H 4 S)CO(PPh 3 ) 2 , produced from double addition of acetylene to the corresponding thiocarbonyl complex. [ 6 ] [ 2 ] Characteristic of other metallaarenes, the Os-C bonds are about 0.6 Å longer than the C-C bonds (in benzene these are 1.39 Å), resulting in a distortion of the hexagonal ring. 1 H NMR signals for the ring protons are downfield, consistent with aromatic "ring current", and the ring readily undergoes electrophilic aromatic substitution . [ 2 ] Osmabenzene and its derivatives can be regarded as an Os(II), d 6 octahedral complex .
Metallabenzenes have also been characterized with metals ruthenium , [ 7 ] [ 8 ] [ 9 ] [ 10 ] iridium , [ 11 ] [ 12 ] platinum , [ 13 ] [ 14 ] [ 15 ] and rhenium . [ 16 ] The iridabenzenes can be produced from ligand substitution, with a vinylcyclopropenide or a linear (CH) − 5 -skeletal carbanion displacing an X-type ligand. [ 2 ] As of 2020, there remained no general method for the synthesis of metallabenzenes, with most techniques applicable to only two or three metals. [ 17 ]
Metallabenzenes typically exhibit a slight nonplanarity, with the metal nucleus shifted perpendicular to the ring plane. However, ligands that strongly accept π electrons reduce the nonplanarity. These geometric effects are one of the pieces of evidence suggesting that metallabenzenes are Möbius aromats, not Hückel ones. [ 3 ] | https://en.wikipedia.org/wiki/Metallabenzene |
In chemistry , a metalloborane is a compound that contains one or more metal atoms and one or more boron hydride . These compounds are related conceptually and often synthetically to the boron-hydride clusters by replacement of BH n units with metal-containing fragments. Often these metal fragments are derived from metal carbonyls or cyclopentadienyl complexes . Their structures can often be rationalized by polyhedral skeletal electron pair theory . The inventory of these compounds is large, and their structures can be quite complex. [ 2 ] [ 3 ]
Two simple examples are B 4 H 8 Fe(CO) 3 and B 4 H 8 Co(C 5 H 5 ) . The MB 4 cores (M = Fe or Co) of these two compounds adopt structures expected for nido 5-vertex clusters. The iron compound is produced by reaction of diiron nonacarbonyl with pentaborane . B 4 H 8 Fe(CO) 3 and cyclobutadieneiron tricarbonyl have similar structures.
Even greater in scope than metalloboranes are metallacarboranes . These cages have carbon vertices, often CH, in addition to BH and M vertices. [ 2 ] A well-developed class of metallacarboranes are prepared from dicarbollides , anions of the formula [C 2 B 9 H 11 ] 2- . These anions function as ligands for a variety of metals, often forming sandwich complexes . [ 5 ]
Some metalloboranes are derived by the metalation of neutral carboranes . Illustrative are the six-and seven-vertex cages prepared from closo - C 2 B 3 H 5 . Reaction of this carborane with iron carbonyl sources gives closo Fe- and Fe2-containing products, according to these idealized equations: [ 6 ]
A further example of insertion into a closo carborane is the synthesis of the yellow-orange solid closo-1,2,3- (CO) 3 FeC 2 B 4 H 6 :
A closely related reaction involves the capping of an anionic nido carborane C 2 B 4 H − 7
The last reaction is worked up with acid and air. | https://en.wikipedia.org/wiki/Metallaborane |
A metallacarboxylic acid is a metal complex with the ligand CO 2 H. These compounds are intermediates in reactions that involve carbon monoxide and carbon dioxide , these species are intermediates in the water gas shift reaction . Metallacarboxylic acids are also called hydroxycarbonyls . [ 1 ]
Metallacarboxylic acids mainly arise by the attack of hydroxide on electrophilic metal carbonyl complexes . An illustrative synthesis is the reaction of a cationic iron carbonyl with a stoichiometric amount of base: [ 2 ]
When applied to simple metal carbonyls, this kind of conversion is sometimes called the Hieber base reaction . Decarboxylation of the resulting anion gives the anionic hydride complex. This conversion is illustrated by the synthesis of [HFe(CO) 4 ] − from iron pentacarbonyl . [ 3 ]
Metallacarboxylic acids exist in equilibria with the carboxylate anions, L n MCO 2 − .
Metallacarboxylate esters (L n MCO 2 R) arise by the addition of alkoxide to metal carbonyl:
Metallacarboxylic amides (L n MC(O)NR 2 ) arise by the addition of amide to metal carbonyl:
Derivatives of metalla dithia carboxylic acids are also known. They are prepared by treating anionic complexes with carbon disulfide . [ 4 ] | https://en.wikipedia.org/wiki/Metallacarboxylic_acid |
In chemistry , metallacrowns are a macrocyclic compounds that consist of metal ions and solely or predominantly heteroatoms in the ring . Classically, metallacrowns contain an [M–N–O] repeat unit in the macrocycle. First discovered by Vincent L. Pecoraro and Myoung Soo Lah in 1989, [ 1 ] metallacrowns are best described as inorganic analogues of crown ethers . To date, over 600 reports of metallacrown research have been published. Metallacrowns with sizes ranging from 12-MC-4 to 60-MC-20 have been synthesized. [ 2 ]
Metallacrown nomenclature has been developed to mimic the nomenclature of crown ethers, which are named by the total number of atoms in the ring, followed by "C" for "crown," and the number of oxygen atoms in the ring. For example, 12-crown-4 or 12-C-4 describes Figure 2a. When naming metallacrowns, a similar format is followed. However, the C becomes "MC" for "metallacrown" and the "MC" is followed by the ring metal, other heteroatom, and the ligand used to make the metallacrown. For example, metallacrown b in the figure above is named [12-MC Fe(III)N(shi) -4], where "shi" is the ligand, salicylhydroxamic acid . [ 2 ]
Metallacrowns form via self-assembly , i.e. by dissolving the ligand in a solvent followed by the desired metal salt.
The first reported metallacrown was Mn II (OAc) 2 (DMF) 6 [12-MC Mn(III)N(shi) -4]. [ 1 ] Metallacrowns can be prepared with a variety of metals in the ring and in a variety of ring sizes. [ 2 ] Many metallacrowns have been prepared, including 9-MC-3, 15-MC-5, and 18-MC-6. Ring size is controlled by a number of factors, such as the geometry of the ligand chelate ring, ring metal Jahn–Teller distortion , central metal size, steric effects , and stoichiometry . Common ring metals have included V(III), Mn(III), Fe(III), Ni(II) and Cu(II). Hydroxamic acids , such as salicylhydroxamic acid , and oximes are commonly utilized in metallacrown ligands .
Many structures have been characterized by single-crystal X-ray crystallography . Metallacrowns typically contain fused chelate rings in their structure, which imparts them with substantial stability. Metallacrowns have been synthesized with substantial variety. Mixed ligand and mixed ring-metal, and mixed-oxidation state metallacrowns are known. Inverse metallacrowns have been reported that contain metal ions oriented towards the center of the ring. [ 3 ] Metallacryptates, metallahelicates, and fused metallacrowns are known. [ 2 ] Among the interesting features of metallacrowns are the similarities between certain structures and the corresponding crown ether. For example, in the 12-C-4, the cavity size is 2.79 Å and the bite distance is 0.6 Å. In the 12-MC-4, the cavity size is 2.67 Å and the bite distance is 0.5 Å. [ 1 ]
Metallacrowns are most widely studied for their potential use as SMMs ( single-molecule magnets ). Notably, the first mixed manganese-lanthanide SMM was a metallacrown. [ 4 ] Metallacrowns with gadolinium as the central metal are potential MRI contrast agents . [ 5 ] [ 6 ] A lot of attention is focused on metallacrown molecular recognition and host–guest chemistry . [ 7 ] Chelation of heavy metals by 15-MC-5 complexes could be utilized in lanthanide separation or heavy metal sequestration. [ 8 ] Metallacrown container molecules constructed from the 15-MC-5 structure type have been shown to selectively encapsulate carboxylate anions in hydrophobic cavities. [ 9 ] [ 10 ] [ 11 ] A crystalline solid displaying second-harmonic generation was generated by including a nonlinear optical chromophore in a chiral metallacrown compartment. [ 12 ] Metallacrowns have also been utilized in the construction of microporous . [ 13 ] [ 14 ] and mesoporous materials . [ 15 ] In another potential application, some metallacrowns exhibit antibacterial activity. [ 16 ] | https://en.wikipedia.org/wiki/Metallacrown |
In organometallic chemistry , a metallacycle is a derivative of a carbocyclic compound wherein a metal has replaced at least one carbon center; [ 2 ] [ 3 ] this is to some extent similar to heterocycles . Metallacycles appear frequently as reactive intermediates in catalysis , e.g. olefin metathesis and alkyne trimerization . In organic synthesis , directed ortho metalation is widely used for the functionalization of arene rings via C-H activation . One main effect that metallic atom substitution on a cyclic carbon compound is distorting the geometry due to the large size of typical metals.
Typically, metallacycles are cyclic compounds with two metal carbon bonds. [ 4 ]
Many compounds containing metals in rings are known, for example chelate rings. Usually, such compounds are not classified as metallacycles, but the naming conventions are not rigidly followed. Within the area of coordination chemistry and supramolecular chemistry , examples include metallacrowns , metallacryptands, metallahelices, and molecular wheels.
Metal-alkene complexes can be viewed as the smallest metallacycles, but they usually are not classified as metallacycles. In the Dewar–Chatt–Duncanson model , one resonance structure for the M(η 2 -alkene) center is the metallacyclopropane.
The parent metallacyclobutane has the formula L n M(CH 2 ) 3 where L is a ligand attached to M. A stable example is ( PPh 3 ) 2 Pt(CH 2 ) 3 . The first example was prepared by oxidative addition of cyclopropane to platinum.
Metallacyclobutane intermediates are involved in the alkene metathesis and in the oligomerization and dimerization of ethylene. In alkene metathesis, the Chauvin mechanism invokes the attack of an alkene at an electrophilic metal carbene catalyst. [ 5 ] [ 6 ] [ 7 ] This work helped to validate the Chauvin mechanism for olefin metathesis.
The parent metallacyclopentadiene, or metallole, has the formula L n M(CH) 4 . Most arise from the coupling of two alkynes at a low valent metal centers such as derivatives of Co(I) and Zr(II). Late metal derivatives (Co, Ni) are intermediates in the metal-catalysed trimerization of alkynes to arenes. Early metal derivatives, i.e. derivatives of Ti and Zr, are used stoichiometrically. [ 4 ] For example, the zirconacyclopentadiene Cp 2 ZrC 4 Me 4 is a useful carrier for C 4 Me 4 2− . [ 8 ] Some of the oldest metallacycles are the ferroles, which are dimetallacyclopentadiene complexes of the formula Fe 2 (C 2 R 4 )(CO) 6 . They are derived from coupling of alkynes as well as from the desulfurization of thiophenes. [ 9 ]
The parent metallacyclobenzenes have the formula L n M(CH) 5 . They can be viewed as derivatives of benzene wherein a CH center has been replaced by a transition metal complex . [ 10 ]
The parent metallacyclopentane has the formula L n M(CH 2 ) 4 . Such compounds are intermediates in the metal catalysed dimerization, trimerization, and tetramerization of ethylene to give but-1-ene , hex-1-ene and oct-1-ene , respectively. [ 11 ] Metallacyclopentanes are invoked as intermediates in the evolution of heterogeneous alkene metathesis catalysts from ethylene and metal oxides. Metallacyclopentane intermediates are proposed to isomerize to metallacyclobutanes, which then eliminate propylene giving the alkylidene. [ 12 ]
Metallacycles often arise by cyclization of arene-containing donor ligands, e.g. aryl phosphines and amines. An early example is the cyclization of IrCl(PPh 3 ) 3 to give the corresponding Ir(III) hydride containing a four-membered IrPCC ring. [ 14 ] Palladium(II) and platinum(II) have long been known to ortho-metalate aromatic ligands such as azobenzene , benzylamines, and 2-phenylpyridines. [ 15 ] These reactions are strongly influenced by substituent effects, including the Thorpe-Ingold effect . [ 16 ] Ligands that lack aryl substituents will sometimes cyclometalate via activation of methyl groups, an early example being the internal oxidative addition of methylphosphine ligands. [ 17 ] Metallacycle formation interferes with intermolecular C-H activation processes. For this reason, specialized " pincer ligands " ligands have been developed that resist ortho-metalation. | https://en.wikipedia.org/wiki/Metallacycle |
Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged ( cationic ) molecular ion with the chemical formula NH + 4 or [NH 4 ] + . It is formed by the addition of a proton (a hydrogen nucleus) to ammonia ( NH 3 ). Ammonium is also a general name for positively charged (protonated) substituted amines and quaternary ammonium cations ( [NR 4 ] + ), where one or more hydrogen atoms are replaced by organic or other groups (indicated by R). Not only is ammonium a source of nitrogen and a key metabolite for many living organisms, but it is an integral part of the global nitrogen cycle . [ 2 ] As such, human impact in recent years could have an effect on the biological communities that depend on it.
The ammonium ion is generated when ammonia, a weak base, reacts with Brønsted acids ( proton donors):
The ammonium ion is mildly acidic, reacting with Brønsted bases to return to the uncharged ammonia molecule:
Thus, the treatment of concentrated solutions of ammonium salts with a strong base gives ammonia. When ammonia is dissolved in water, a tiny amount of it converts to ammonium ions:
The degree to which ammonia forms the ammonium ion depends on the pH of the solution. If the pH is low, the equilibrium shifts to the right: more ammonia molecules are converted into ammonium ions. If the pH is high (the concentration of hydrogen ions is low and hydroxide ions is high), the equilibrium shifts to the left: the hydroxide ion abstracts a proton from the ammonium ion, generating ammonia.
Formation of ammonium compounds can also occur in the vapor phase; for example, when ammonia vapor comes in contact with hydrogen chloride vapor, a white cloud of ammonium chloride forms, which eventually settles out as a solid in a thin white layer on surfaces.
Ammonium cation is found in a variety of salts such as ammonium carbonate , ammonium chloride , and ammonium nitrate . Most simple ammonium salts are very soluble in water. An exception is ammonium hexachloroplatinate , the formation of which was once used as a test for ammonium. The ammonium salts of nitrate and especially perchlorate are highly explosive, in these cases, ammonium is the reducing agent.
In an unusual process, ammonium ions form an amalgam . Such species are prepared by the addition of sodium amalgam to a solution of ammonium chloride. [ 3 ] This amalgam eventually decomposes to release ammonia and hydrogen. [ 4 ]
To find whether the ammonium ion is present in the salt, first, the salt is heated in presence of alkali hydroxide releasing a gas with a characteristic smell, which is ammonia .
To further confirm ammonia, it is passed through a glass rod dipped in an HCl solution ( hydrochloric acid ), creating white dense fumes of ammonium chloride .
Ammonia, when passed through CuSO 4 ( copper(II) sulfate ) solution, changes its color from blue to deep blue, forming Schweizer's reagent .
Ammonia or ammonium ion when added to Nessler's reagent gives a brown color precipitate known as the iodide of Million's base in basic medium.
Ammonium ion when added to chloroplatinic acid gives a yellow precipitate of ammonium hexachloroplatinate(IV) .
Ammonium ion when added to sodium cobaltinitrite gives a yellow precipitate of ammonium cobaltinitrite.
Ammonium ion gives a white precipitate of ammonium bitartrate when added to potassium bitartrate .
The lone electron pair on the nitrogen atom (N) in ammonia, represented as a line above the N, forms a coordinate bond with a proton ( H + ). After that, all four N−H bonds are equivalent, being polar covalent bonds . The ion has a tetrahedral structure and is isoelectronic with methane and the borohydride anion. In terms of size, the ammonium cation ( r ionic = 175 pm) [ citation needed ] resembles the caesium cation ( r ionic = 183 pm). [ citation needed ]
The hydrogen atoms in the ammonium ion can be substituted with an alkyl group or some other organic group to form a substituted ammonium ion ( IUPAC nomenclature: aminium ion ). Depending on the number of organic groups, the ammonium cation is called a primary , secondary , tertiary , or quaternary . Except the quaternary ammonium cations, the organic ammonium cations are weak acids.
An example of a reaction forming an ammonium ion is that between dimethylamine , (CH 3 ) 2 NH , and an acid to give the dimethylammonium cation, [(CH 3 ) 2 NH 2 ] + :
Quaternary ammonium cations have four organic groups attached to the nitrogen atom, they lack a hydrogen atom bonded to the nitrogen atom. These cations, such as the tetra- n -butylammonium cation, are sometimes used to replace sodium or potassium ions to increase the solubility of the associated anion in organic solvents . Primary, secondary, and tertiary ammonium salts serve the same function but are less lipophilic . They are also used as phase-transfer catalysts and surfactants .
An unusual class of organic ammonium salts is derivatives of amine radical cations, [•NR 3 ] + such as tris(4-bromophenyl)ammoniumyl hexachloroantimonate .
Because nitrogen often limits net primary production due to its use in enzymes that mediate the biochemical reactions that are necessary for life, ammonium is utilized by some microbes and plants. [ 5 ] For example, energy is released by the oxidation of ammonium in a process known as nitrification , which produces nitrate and nitrite . [ 6 ] This process is a form of autotrophy that is common amongst Nitrosomonas , Nitrobacter , Nitrosolobus , and Nitrosospira , amongst others. [ 6 ]
The amount of ammonium in soil that is available for nitrification by microbes varies depending on environmental conditions. [ 7 ] [ 8 ] For example, ammonium is deposited as a waste product from some animals, although it is converted into urea in mammals, sharks, and amphibians, and into uric acid in birds, reptiles, and terrestrial snails. [ 9 ] Its availability in soils is also influenced by mineralization , which makes more ammonium available from organic nitrogen sources, and immobilization , which sequesters ammonium into organic nitrogen sources, both of which are mitigated by biological factors. [ 6 ]
Conversely, nitrate and nitrite can be reduced to ammonium as a way for living organisms to access nitrogen for growth in a process known as assimilatory nitrate reduction. [ 10 ] Once assimilated, it can be incorporated into proteins and DNA . [ 11 ]
Ammonium can accumulate in soils where nitrification is slow or inhibited, which is common in hypoxic soils. [ 12 ] For example, ammonium mobilization is one of the key factors for the symbiotic association between plants and fungi, called mycorrhizae . [ 13 ] However, plants that consistently utilize ammonium as a nitrogen source often must invest into more extensive root systems due to ammonium's limited mobility in soils compared to other nitrogen sources. [ 14 ] [ 15 ]
Ammonium deposition from the atmosphere has increased in recent years due to volatilization from livestock waste and increased fertilizer use. [ 16 ] Because net primary production is often limited by nitrogen , increased ammonium levels could impact the biological communities that rely on it. For example, increasing nitrogen content has been shown to increase plant growth, but aggravate soil phosphorus levels, which can impact microbial communities. [ 17 ]
The ammonium cation has very similar properties to the heavier alkali metal cations and is often considered a close equivalent. [ 18 ] [ 19 ] [ 20 ] Ammonium is expected to behave as a metal ( [NH 4 ] + ions in a sea of electrons ) at very high pressures, such as inside giant planets such as Uranus and Neptune . [ 19 ] [ 20 ]
Under normal conditions, ammonium does not exist as a pure metal but does as an amalgam (alloy with mercury ). [ 21 ] | https://en.wikipedia.org/wiki/Metallic_ammonium |
Metallic hydrogen is a phase of hydrogen in which it behaves like an electrical conductor . This phase was predicted in 1935 on theoretical grounds by Eugene Wigner and Hillard Bell Huntington . [ 1 ]
At high pressure and temperatures, metallic hydrogen can exist as a partial liquid rather than a solid , and researchers think it might be present in large quantities in the hot and gravitationally compressed interiors of Jupiter and Saturn , as well as in some exoplanets . [ 2 ]
Though often placed at the top of the alkali metal column in the periodic table , hydrogen does not, under ordinary conditions, exhibit the properties of an alkali metal. Instead, it forms diatomic H 2 molecules, similar to halogens and some nonmetals in the second period of the periodic table, such as nitrogen and oxygen . Diatomic hydrogen is a gas that, at atmospheric pressure , liquefies and solidifies only at very low temperature (20 K and 14 K respectively).
In 1935, physicists Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 25 GPa (250,000 atm; 3,600,000 psi), hydrogen would display metallic properties: instead of discrete H 2 molecules (which consist of two electrons bound between two protons), a bulk phase would form with a solid lattice of protons and the electrons delocalized throughout. [ 1 ] Since then, producing metallic hydrogen in the laboratory has been described as "the holy grail of high-pressure physics". [ 3 ]
The initial prediction about the amount of pressure needed was eventually shown to be too low. [ 4 ] Since the first work by Wigner and Huntington, the more modern theoretical calculations point toward higher but potentially achievable metallization pressures of around 400 GPa (3,900,000 atm; 58,000,000 psi). [ 5 ] [ 6 ]
Helium-4 is a liquid at normal pressure near absolute zero , a consequence of its high zero-point energy (ZPE). The ZPE of protons in a dense state is also high, [ 7 ] and a decline in the ordering energy (relative to the ZPE) is expected at high pressures. Arguments have been advanced by Neil Ashcroft and others that there is a melting point maximum in compressed hydrogen , but also that there might be a range of densities, at pressures around 400 GPa, where hydrogen would be a liquid metal, even at low temperatures. [ 8 ] [ 9 ]
Geng predicted that the ZPE of protons indeed lowers the melting temperature of hydrogen to a minimum of 200 to 250 K (−73 to −23 °C) at pressures of 500–1,500 GPa (4,900,000–14,800,000 atm; 73,000,000–218,000,000 psi). [ 10 ] [ 11 ]
Within this flat region there might be an elemental mesophase intermediate between the liquid and solid state, which could be metastably stabilized down to low temperature and enter a supersolid state. [ 12 ]
In 1968, Neil Ashcroft suggested that metallic hydrogen might be a superconductor , up to room temperature (290 K or 17 °C). This hypothesis is based on an expected strong coupling between conduction electrons and lattice vibrations . [ 13 ]
Metastable metallic hydrogen may have potential as a highly efficient rocket propellant; the metallic form would be stored, and the energy of its decompression and conversion to the diatomic gaseous form when released through a nozzle used to generate thrust, with a theoretical specific impulse of up to 1700 seconds (for reference, the current most efficient chemical rocket propellants have an I sp less than 500 s [ 14 ] ), although a metastable form suitable for mass-production and conventional high-volume storage may not exist. [ 15 ] [ 16 ] Another significant issue is the heat of the reaction, which at over 6000 K is too high for any known engine materials to be used. This would necessitate diluting the metallic hydrogen with water or liquid hydrogen, a mixture that would still provide a significant performance boost over current propellants. [ 14 ]
Presently known "super" states of matter are superconductors , superfluid liquids and gases, and supersolids . Egor Babaev predicted that if hydrogen and deuterium have liquid metallic states, they might have quantum ordered states that cannot be classified as superconducting or superfluid in the usual sense. Instead, they might represent two possible novel types of quantum fluids : superconducting superfluids and metallic superfluids . Such fluids were predicted to have highly unusual reactions to external magnetic fields and rotations, which might provide a means for experimental verification of Babaev's predictions. It has also been suggested that, under the influence of a magnetic field, hydrogen might exhibit phase transitions from superconductivity to superfluidity and vice versa. [ 17 ] [ 18 ] [ 19 ]
In 2009, Zurek et al. predicted that the alloy LiH 6 would be a stable metal at only one quarter of the pressure required to metallize hydrogen, and that similar effects should hold for alloys of type LiH n and possibly "other alkali high-hydride systems ", i.e. alloys of type XH n , where X is an alkali metal . [ 20 ] This was later verified in AcH 8 and LaH 10 with T c approaching 270 K [ 21 ] leading to speculation that other compounds may even be stable at mere MPa pressures with room-temperature superconductivity.
In March 1996, a group of scientists at Lawrence Livermore National Laboratory reported that they had serendipitously produced the first identifiably metallic hydrogen [ 22 ] for about a microsecond at temperatures of thousands of kelvins , pressures of over 100 GPa (1,000,000 atm; 15,000,000 psi), and densities of approximately 0.6 g/cm 3 . [ 23 ] The team did not expect to produce metallic hydrogen, as it was not using solid hydrogen , thought to be necessary, and was working at temperatures above those specified by metallization theory. Previous studies in which solid hydrogen was compressed inside diamond anvils to pressures of up to 250 GPa (2,500,000 atm; 37,000,000 psi), did not confirm detectable metallization. The team had sought simply to measure the less extreme electrical conductivity changes they expected. The researchers used a 1960s-era light-gas gun , originally employed in guided missile studies, to shoot an impactor plate into a sealed container containing a half-millimeter thick sample of liquid hydrogen . The liquid hydrogen was in contact with wires leading to a device measuring electrical resistance. The scientists found that, as pressure rose to 140 GPa (1,400,000 atm; 21,000,000 psi), the electronic energy band gap , a measure of electrical resistance , fell to almost zero. The band gap of hydrogen in its uncompressed state is about 15 eV , making it an insulator but, as the pressure increases significantly, the band gap gradually fell to 0.3 eV . Because the thermal energy of the fluid (the temperature became about 3,000 K or 2,730 °C due to compression of the sample) was above 0.3 eV , the hydrogen might be considered metallic.
Many experiments are continuing in the production of metallic hydrogen in laboratory conditions at static compression and low temperature. Arthur Ruoff and Chandrabhas Narayana from Cornell University in 1998, [ 24 ] and later Paul Loubeyre and René LeToullec from Commissariat à l'Énergie Atomique , France in 2002, have shown that at pressures close to those at the center of the Earth (320–340 GPa or 3,200,000–3,400,000 atm) and temperatures of 100–300 K (−173–27 °C), hydrogen is still not a true alkali metal, because of the non-zero band gap. The quest to see metallic hydrogen in laboratory at low temperature and static compression continues. Studies are also ongoing on deuterium . [ 25 ] Shahriar Badiei and Leif Holmlid from the University of Gothenburg have shown in 2004 that condensed metallic states made of excited hydrogen atoms ( Rydberg matter ) are effective promoters to metallic hydrogen, [ 26 ] however these results are disputed. [ 27 ]
The theoretically predicted maximum of the melting curve (the prerequisite for the liquid metallic hydrogen) was discovered by Shanti Deemyad and Isaac F. Silvera by using pulsed laser heating. [ 28 ] Hydrogen-rich molecular silane ( SiH 4 ) was claimed to be metallized and become superconducting by M.I. Eremets et al. . [ 29 ] This claim is disputed, and their results have not been repeated. [ 30 ] [ 31 ]
In 2011 Eremets and Troyan reported observing the liquid metallic state of hydrogen and deuterium at static pressures of 260–300 GPa (2,600,000–3,000,000 atm). [ 32 ] [ 33 ] This claim was questioned by other researchers in 2012. [ 34 ] [ 35 ]
In 2015, scientists at the Z Pulsed Power Facility announced the creation of metallic deuterium using dense liquid deuterium , an electrical insulator-to-conductor transition associated with an increase in optical reflectivity. [ 36 ] [ 37 ]
On 5 October 2016, Ranga Dias and Isaac F. Silvera of Harvard University released claims in a pre-print manuscript of experimental evidence that solid metallic hydrogen had been synthesized in the laboratory at a pressure of around 495 gigapascals (4,890,000 atm; 71,800,000 psi) using a diamond anvil cell . A revised version was published in Science in 2017. [ 38 ] [ 39 ] [ 40 ]
In the preprint version of the paper, Dias and Silvera write:
With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa... the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T = 5.5 K, with a corresponding electron carrier density of 6.7 × 10 23 particles/cm 3 , consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.
In June 2019 a team at the Commissariat à l'énergie atomique et aux énergies alternatives (French Alternative Energies & Atomic Energy Commission) claimed to have created metallic hydrogen at around 425GPa. [ 41 ]
W. Ferreira et al. (including Dias and Silvera) repeated their experiments multiple times after the Science article was published, finally publishing in 2023 and finding metallisation of hydrogen between 477 and 491 gigapascals (4,710,000 and 4,850,000 atm). This time, the pressure was released to assess the question of metastability. Metallic hydrogen was not found to be metastable to zero pressure. [ 42 ]
In August 2018, scientists announced new observations [ 43 ] regarding the rapid transformation of fluid deuterium from an insulating to a metallic form below 2000 K. Remarkable agreement is found between the experimental data and the predictions based on quantum Monte Carlo simulations, which is expected to be the most accurate method to date. This may help researchers better understand giant gas planets , such as Jupiter, Saturn and related exoplanets , since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields . [ 44 ] [ 45 ] | https://en.wikipedia.org/wiki/Metallic_hydrogen |
A metallic microlattice is a synthetic porous metallic material consisting of an ultra-light metal foam . With a density as low as 0.99 mg/cm 3 (0.00561 lb/ft 3 ), it is one of the lightest structural materials known to science. [ 1 ] It was developed by a team of scientists from California -based HRL Laboratories , in collaboration with researchers at University of California, Irvine and Caltech , and was first announced in November 2011. The prototype samples were made from a nickel - phosphorus alloy. [ 2 ] In 2012, the microlattice prototype was declared one of 10 World-Changing Innovations by Popular Mechanics . [ 3 ] Metallic microlattice technology has numerous potential applications in automotive and aeronautical engineering . [ 4 ] A detailed comparative review study among other types of metallic lattice structures showed them to be beneficial for light-weighting purposes but expensive to manufacture. [ 5 ]
To produce their metallic microlattice, the HRL/UCI/Caltech team first prepared a polymer template using a technique based on self-propagating waveguide formation, [ 6 ] [ 7 ] though it was noted that other methods can be used to fabricate the template. [ 8 ] The process passed UV light through a perforated mask into a reservoir of UV-curable resin . Fiber-optic -like "self-trapping" of the light occurred as the resin cured under each hole in the mask, forming a thin polymer fiber along the path of the light. By using multiple light beams, multiple fibers could then interconnect to form a lattice.
The process was similar to photolithography in that it used a two-dimensional mask to define the starting template structure, but differed in the rate of formation: where stereolithography might take hours to make a full structure, the self-forming waveguide process allowed templates to be formed in 10–100 seconds. In this way, the process enables large free-standing 3D lattice materials to be formed quickly and scalably. The template was then coated with a thin layer of metal by electroless nickel plating , and the template is etched away, leaving a free-standing, periodic porous metallic structure. Nickel was used as the microlattice metal in the original report. Owing to the electrodeposition process, 7% of the material consisted of dissolved phosphorus atoms, and it contained no precipitates . [ 8 ]
A metallic microlattice is composed of a network of interconnecting hollow struts. In the least-dense microlattice sample reported, each strut is about 100 micrometres in diameter, with a wall 100 nanometres thick. The completed structure is about 99.99% air by volume, [ 2 ] and by convention, the mass of air is excluded when the microlattice density is calculated. [ 8 ] Allowing for the mass of the interstitial air, the true density of the structure is approximately 2.1 mg/cm 3 (2.1 kg/m 3 ), which is only about 1.76 times the density of air itself at 25 °C. The material is described as being 100 times lighter than Styrofoam . [ 9 ] Microlattices can also be 100 times stronger than regular polymers. [ 10 ]
Metallic microlattices are characterized by very low densities, with the 2011 record of 0.9 mg/cm 3 being among the lowest values of any known solid. The previous record of 1.0 mg/cm 3 was held by silica aerogels , and aerographite is claimed to have a density of 0.2 mg/cm 3 . [ 11 ] Mechanically, these microlattices are behaviorally similar to elastomers and almost completely recover their shape after significant compression. [ 12 ] This gives them a significant advantage over earlier aerogels, which are brittle, glass-like substances. This elastomeric property in metallic microlattices furthermore results in efficient shock absorption. Their Young's modulus E exhibits different scaling, with the density ρ, E ~ ρ 2 , compared to E ~ ρ 3 in aerogels and carbon nanotube foams. [ 8 ]
Metallic microlattice may find potential applications in thermal and vibration insulators such as shock absorbers , and may also prove useful as battery electrodes and catalyst supports. [ 8 ] Additionally, the microlattices' ability to return to their original state after being compressed may make them suitable for use in spring-like energy storage devices. [ 2 ] Automotive and aeronautical manufacturers [ which? ] are using microlattice technology to develop extremely lightweight and efficient structures that combine multiple functions, such as structural reinforcement and heat transfer, into single components for high-performance vehicles. [ 4 ]
A similar but denser material, consisting of an electrodeposited nanocrystalline nickel layer over a polymeric rapid-prototyped truss, was created by researchers at the University of Toronto in 2008. [ 13 ] In 2012, German researchers created a carbon foam known as aerographite , with an even lower density than a metallic microlattice. [ 14 ] In 2013, Chinese scientists developed a carbon-based aerogel which was claimed to be lighter still. [ 1 ]
Nanolattices like tube-based nanostructures are similar structures on a smaller scale. | https://en.wikipedia.org/wiki/Metallic_microlattice |
A metallic soap is a metallic salt of a fatty acid . Theoretically, soaps can be made of any metal, although not all enjoy practical uses. [ 1 ] Varying the metal can strongly affect the properties of the compound, particularly its solubility.
Alkali metal and alkaline earth soaps are white solids. [ 1 ] The most commonly encountered are traditional household soaps , which are the fatty acid salts of sodium (hard soap) and potassium (soft soap). Lithium soap or greases, such as lithium stearate , are insoluble in water and find use in lubricating grease .
Calcium and magnesium soaps are most commonly encountered as soap scum but the pure materials have a variety of uses. Magnesium stearate and calcium stearate are used as excipients , lubricants, release agents , and food additives , with the later use being covered by the generic E numbers of E470b and E470 respectively.
Aluminium soaps are used as thickening agents , in the production of cosmetics. [ 1 ] Other examples include mixed calcium/zinc soaps which are used as heat stabilizer for polyvinyl chloride . Soaps of iron , cobalt and manganese are used as drying agents in paints and varnishes and work by promoting the oxidation and crosslinking of unsaturated oils. [ 2 ]
This article about a salt (chemistry) is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metallic_soap |
In astronomy , metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium . Most of the normal currently detectable (i.e. non- dark ) matter in the universe is either hydrogen or helium, and astronomers use the word metals as convenient shorthand for all elements except hydrogen and helium . This word-use is distinct from the conventional chemical or physical definition of a metal as an electrically conducting solid. Stars and nebulae with relatively high abundances of heavier elements are called metal-rich when discussing metallicity, even though many of those elements are called nonmetals in chemistry.
In 1802, William Hyde Wollaston [ 1 ] noted the appearance of a number of dark features in the solar spectrum. [ 2 ] In 1814, Joseph von Fraunhofer independently rediscovered the lines and began to systematically study and measure their wavelengths , and they are now called Fraunhofer lines . He mapped over 570 lines, designating the most prominent with the letters A through K and weaker lines with other letters. [ 3 ] [ 4 ] [ 5 ]
About 45 years later, Gustav Kirchhoff and Robert Bunsen [ 6 ] noticed that several Fraunhofer lines coincide with characteristic emission lines identifies in the spectra of heated chemical elements. [ 7 ] They inferred that dark lines in the solar spectrum are caused by absorption by chemical elements in the solar atmosphere. [ 8 ] Their observations [ 9 ] were in the visible range where the strongest lines come from metals such as sodium, potassium, and iron. [ 10 ] In the early work on the chemical composition of the sun the only elements that were detected in spectra were hydrogen and various metals, [ 11 ] : 23–24 with the term metallic frequently used when describing them. [ 11 ] : Part 2 In contemporary usage in astronomy all the extra elements beyond just hydrogen and helium are termed metallic.
The presence of heavier elements results from stellar nucleosynthesis, where the majority of elements heavier than hydrogen and helium in the Universe ( metals , hereafter) are formed in the cores of stars as they evolve . Over time, stellar winds and supernovae deposit the metals into the surrounding environment, enriching the interstellar medium and providing recycling materials for the birth of new stars . It follows that older generations of stars, which formed in the metal-poor early Universe , generally have lower metallicities than those of younger generations, which formed in a more metal-rich Universe.
Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars . [ 12 ] These became commonly known as population I (metal-rich) and population II (metal-poor) stars. A third, earliest stellar population was hypothesized in 1978, known as population III stars. [ 13 ] [ 14 ] [ 15 ] These "extremely metal-poor" (XMP) stars are theorized to have been the "first-born" stars created in the Universe.
Astronomers use several different methods to describe and approximate metal abundances, depending on the available tools and the object of interest. Some methods include determining the fraction of mass that is attributed to gas versus metals, or measuring the ratios of the number of atoms of two different elements as compared to the ratios found in the Sun .
Stellar composition is often simply defined by the parameters X , Y , and Z . Here X represents the mass fraction of hydrogen , Y is the mass fraction of helium , and Z is the mass fraction of all the remaining chemical elements. Thus
X + Y + Z = 1 {\displaystyle X+Y+Z=1}
In most stars , nebulae , H II regions , and other astronomical sources, hydrogen and helium are the two dominant elements. The hydrogen mass fraction is generally expressed as X ≡ m H M , {\displaystyle \ X\equiv {\tfrac {m_{{\ce {H}}}}{M}}\ ,} where M is the total mass of the system, and m H {\displaystyle \ m_{{\ce {H}}}\ } is the mass of the hydrogen it contains. Similarly, the helium mass fraction is denoted as Y ≡ m He M . {\displaystyle \ Y\equiv {\tfrac {m_{{\ce {He}}}}{M}}~.} The remainder of the elements are collectively referred to as "metals", and the mass fraction of metals is calculated as
Z = ∑ e > He m e M = 1 − X − Y . {\displaystyle Z=\sum _{e>{\ce {He}}}{\tfrac {m_{e}}{M}}=1-X-Y~.}
For the surface of the Sun ( symbol ⊙ {\displaystyle \odot } ), these parameters are measured to have the following values: [ 16 ]
Due to the effects of stellar evolution , neither the initial composition nor the present day bulk composition of the Sun is the same as its present-day surface composition.
The overall stellar metallicity is conventionally defined using the total hydrogen content, since its abundance is considered to be relatively constant in the Universe, or the iron content of the star, which has an abundance that is generally linearly increasing in time in the Universe. [ 17 ] Hence, iron can be used as a chronological indicator of nucleosynthesis. Iron is relatively easy to measure with spectral observations in the star's spectrum given the large number of iron lines in the star's spectra (even though oxygen is the most abundant heavy element – see metallicities in H II regions below). The abundance ratio is the common logarithm of the ratio of a star's iron abundance compared to that of the Sun and is calculated thus: [ 18 ]
[ Fe H ] = log 10 ( N Fe N H ) ⋆ − log 10 ( N Fe N H ) ⊙ , {\displaystyle \left[{\frac {{\ce {Fe}}}{{\ce {H}}}}\right]~=~\log _{10}{\left({\frac {N_{{\ce {Fe}}}}{N_{{\ce {H}}}}}\right)_{\star }}-~\log _{10}{\left({\frac {N_{{\ce {Fe}}}}{N_{{\ce {H}}}}}\right)_{\odot }}\ ,}
where N Fe {\displaystyle \ N_{{\ce {Fe}}}\ } and N H {\displaystyle \ N_{{\ce {H}}}\ } are the number of iron and hydrogen atoms per unit of volume respectively, ⊙ {\displaystyle \odot } is the standard symbol for the Sun, and ⋆ {\displaystyle \star } for a star (often omitted below). The unit often used for metallicity is the dex , contraction of "decimal exponent". [ 19 ] By this formulation, stars with a higher metallicity than the Sun have a positive common logarithm , whereas those more dominated by hydrogen have a corresponding negative value. For example, stars with a [ Fe H ] ⋆ {\displaystyle \ \left[{\tfrac {{\ce {Fe}}}{{\ce {H}}}}\right]_{\star }\ } value of +1 have 10 times the metallicity of the Sun (10 +1 ); conversely, those with a [ Fe H ] ⋆ {\displaystyle \ \left[{\tfrac {{\ce {Fe}}}{{\ce {H}}}}\right]_{\star }\ } value of −1 have 1 / 10 , while those with a [ Fe H ] ⋆ {\displaystyle \ \left[{\tfrac {{\ce {Fe}}}{{\ce {H}}}}\right]_{\star }\ } value of 0 have the same metallicity as the Sun, and so on. [ 20 ]
Young population I stars have significantly higher iron-to-hydrogen ratios than older population II stars. Primordial population III stars are estimated to have metallicity less than −6, a millionth of the abundance of iron in the Sun. [ 21 ] [ 22 ] The same notation is used to express variations in abundances between other individual elements as compared to solar proportions. For example, the notation [ O Fe ] {\displaystyle \ \left[{\tfrac {{\ce {O}}}{{\ce {Fe}}}}\right]\ } represents the difference in the logarithm of the star's oxygen abundance versus its iron content compared to that of the Sun. In general, a given stellar nucleosynthetic process alters the proportions of only a few elements or isotopes, so a star or gas sample with certain [ ? Fe ] ⋆ {\displaystyle \ \left[{\tfrac {\ce {?}}{\ce {Fe}}}\right]_{\star }\ } values may well be indicative of an associated, studied nuclear process.
Astronomers can estimate metallicities through measured and calibrated systems that correlate photometric measurements and spectroscopic measurements (see also Spectrophotometry ). For example, the Johnson UVB filters can be used to detect an ultraviolet (UV) excess in stars, [ 23 ] where a smaller UV excess indicates a larger presence of metals that absorb the UV radiation, thereby making the star appear "redder". [ 24 ] [ 25 ] [ 26 ] The UV excess, δ (U−B), is defined as the difference between a star's U and B band magnitudes , compared to the difference between U and B band magnitudes of metal-rich stars in the Hyades cluster . [ 27 ] Unfortunately, δ (U−B) is sensitive to both metallicity and temperature : If two stars are equally metal-rich, but one is cooler than the other, they will likely have different δ (U−B) values [ 27 ] (see also Blanketing effect [ 28 ] [ 29 ] ).
To help mitigate this degeneracy, a star's B−V color index can be used as an indicator for temperature. Furthermore, the UV excess and B−V index can be corrected to relate the δ (U−B) value to iron abundances. [ 30 ] [ 31 ] [ 32 ]
Other photometric systems that can be used to determine metallicities of certain astrophysical objects include the Strӧmgren system, [ 33 ] [ 34 ] the Geneva system, [ 35 ] [ 36 ] the Washington system, [ 37 ] [ 38 ] and the DDO system. [ 39 ] [ 40 ]
At a given mass and age, a metal-poor star will be slightly warmer. Population II stars ' metallicities are roughly 1 / 1000 to 1 / 10 of the Sun's ( [ Fe H ] = − 3.0 . . . − 1.0 ) , {\displaystyle \left(\ \left[{\tfrac {{\ce {Fe}}}{{\ce {H}}}}\right]\ ={-3.0}\ ...\ {-1.0}\ \right)\ ,} but the group appears cooler than population I overall, as heavy population II stars have long since died. Above 40 solar masses , metallicity influences how a star will die: Outside the pair-instability window , lower metallicity stars will collapse directly to a black hole, while higher metallicity stars undergo a type Ib/c supernova and may leave a neutron star .
A star's metallicity measurement is one parameter that helps determine whether a star may have a giant planet , as there is a direct correlation between metallicity and the presence of a giant planet. Measurements have demonstrated the connection between a star's metallicity and gas giant planets, like Jupiter and Saturn . The more metals in a star and thus its planetary system and protoplanetary disk , the more likely the system may have gas giant planets. Current models show that the metallicity along with the correct planetary system temperature and distance from the star are key to planet and planetesimal formation. For two stars that have equal age and mass but different metallicity, the less metallic star is bluer . Among stars of the same color, less metallic stars emit more ultraviolet radiation. The Sun, with eight planets and nine consensus dwarf planets , is used as the reference, with a [ Fe H ] {\displaystyle \ \left[{\tfrac {{\ce {Fe}}}{{\ce {H}}}}\right]\ } of 0.00. [ 41 ] [ 42 ] [ 43 ] [ 44 ] [ 45 ]
Young, massive and hot stars (typically of spectral types O and B ) in H II regions emit UV photons that ionize ground-state hydrogen atoms, knocking electrons free; this process is known as photoionization . The free electrons can strike other atoms nearby, exciting bound metallic electrons into a metastable state , which eventually decay back into a ground state, emitting photons with energies that correspond to forbidden lines . Through these transitions, astronomers have developed several observational methods to estimate metal abundances in H II regions, where the stronger the forbidden lines in spectroscopic observations, the higher the metallicity. [ 46 ] [ 47 ] These methods are dependent on one or more of the following: the variety of asymmetrical densities inside H II regions, the varied temperatures of the embedded stars, and/or the electron density within the ionized region. [ 48 ] [ 49 ] [ 50 ] [ 51 ]
Theoretically, to determine the total abundance of a single element in an H II region, all transition lines should be observed and summed. However, this can be observationally difficult due to variation in line strength. [ 52 ] [ 53 ] Some of the most common forbidden lines used to determine metal abundances in H II regions are from oxygen (e.g. [O II ] λ = (3727, 7318, 7324) Å, and [O III ] λ = (4363, 4959, 5007) Å), nitrogen (e.g. [N II ] λ = (5755, 6548, 6584) Å), and sulfur (e.g. [S II ] λ = (6717, 6731) Å and [S III ] λ = (6312, 9069, 9531) Å) in the optical spectrum, and the [O III ] λ = (52, 88) μm and [N III ] λ = 57 μm lines in the infrared spectrum. Oxygen has some of the stronger, more abundant lines in H II regions, making it a main target for metallicity estimates within these objects. To calculate metal abundances in H II regions using oxygen flux measurements, astronomers often use the R 23 method, in which
R 23 = [ O II ] 3727 Å + [ O III ] 4959 Å + 5007 Å [ H β ] 4861 Å , {\displaystyle R_{23}={\frac {\ \left[\ {\ce {O}}^{{\ce {II}}}\right]_{3727~\mathrm {\AA} }+\left[\ {\ce {O}}^{{\ce {III}}}\right]_{4959~\mathrm {\AA} +5007~\mathrm {\AA} }\ }{\left[\ {\ce {H}}_{{\ce {\beta}}}\right]_{4861~\mathrm {\AA} }}}\ ,}
where [ O II ] 3727 Å + [ O III ] 4959 Å + 5007 Å {\displaystyle \ \left[\ {\ce {O}}^{{\ce {II}}}\right]_{3727~\mathrm {\AA} }+\left[\ {\ce {O}}^{{\ce {III}}}\right]_{4959~\mathrm {\AA} +5007~\mathrm {\AA} }\ } is the sum of the fluxes from oxygen emission lines measured at the rest frame λ = (3727, 4959 and 5007) Å wavelengths, divided by the flux from the Balmer series H β emission line at the rest frame λ = 4861 Å wavelength. [ 54 ] This ratio is well defined through models and observational studies, [ 55 ] [ 56 ] [ 57 ] but caution should be taken, as the ratio is often degenerate, providing both a low and high metallicity solution, which can be broken with additional line measurements. [ 58 ] Similarly, other strong forbidden line ratios can be used, e.g. for sulfur, where [ 59 ]
S 23 = [ S II ] 6716 Å + 6731 Å + [ S III ] 9069 Å + 9532 Å [ H β ] 4861 Å . {\displaystyle S_{23}={\frac {\ \left[\ {\ce {S}}^{{\ce {II}}}\right]_{6716~\mathrm {\AA} +6731~\mathrm {\AA} }+\left[\ {\ce {S}}^{{\ce {III}}}\right]_{9069~\mathrm {\AA} +9532~\mathrm {\AA} }\ }{\left[\ {\ce {H}}_{{\ce {\beta}}}\right]_{4861~\mathrm {\AA} }}}~.}
Metal abundances within H II regions are typically less than 1%, with the percentage decreasing on average with distance from the Galactic Center . [ 52 ] [ 60 ] [ 61 ] [ 62 ] [ 63 ] | https://en.wikipedia.org/wiki/Metallicity |
The metallicity distribution function is an important concept in stellar and galactic evolution . It is a curve of what proportion of stars have a particular metallicity ([Fe/H], the relative abundance of iron and hydrogen) of a population of stars such as in a cluster or galaxy. [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ]
MDFs are used to test different theories of galactic evolution. Much of the iron in a star will have come from earlier type Ia supernovae . Other [alpha] metals can be produced in core collapse supernovae. [ 8 ] [ 9 ]
This physical cosmology -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metallicity_distribution_function |
The Metalliferous Mines Regulations 1961 replaces both the Metalliferous Mines Regulations, 1926 and the Mysore Gold Mines Regulations, 1953 to prevent possible dangers, accidents and deaths from mining in India. [ 1 ] [ 2 ]
9: Notice of Accident.
10: Notice of disease
60, 61, 63, & 64: Mine plans and Sections
106 to 118: Method of working in mines
119 to 130: Danger from fire, dust gas and water
146, 148: Standards of lighting in the mines
153 to 170: Use of explosive in mines | https://en.wikipedia.org/wiki/Metalliferous_Mines_Regulations_1961 |
Metallization pressure is the pressure required for a non-metallic chemical element to become a metal . Every material is predicted to turn into a metal if the pressure is high enough, and temperature low enough. Some of these pressures are beyond the reach of diamond anvil cells , and are thus theoretical predictions. Neon has the highest metallization pressure for any element.
The value for phosphorus refers to pressurizing black phosphorus. The value for arsenic refers to pressurizing metastable black arsenic; grey arsenic, the standard state, is already a metallic conductor at standard conditions. No value is known or theoretically predicted for radon. Astatine is calculated to already be a metal at standard conditions, [ 1 ] although its extreme radioactivity means that this has never been tested experimentally. | https://en.wikipedia.org/wiki/Metallization_pressure |
Metallizing is the general name for the technique of coating metal on the surface of objects. Metallic coatings may be decorative, protective or functional.
Techniques for metallization started as early as mirror making. In 1835, Justus von Liebig discovered the process of coating a glass surface with metallic silver , making the glass mirror one of the earliest items being metallized. Plating other non-metallic objects grew rapidly with introduction of ABS plastic. Because a non-metallic object tends to be a poor electrical conductor , the object's surface must be made conductive before plating can be performed. The plastic part is first etched chemically by a suitable process, such as dipping in a hot chromic acid - sulfuric acid mixture. The etched surface is sensitised and activated by first dipping in tin(II) chloride solution, then palladium chloride solution. The processed surface is then coated with electroless copper or nickel before further plating. This process gives useful (about 1 to 6 kgf /cm or 10 to 60 N /cm or 5 to 35 lbf /in) adhesion force, but is much weaker than actual metal-to-metal adhesion strength.
Vacuum metallizing involves heating the coating metal to its boiling point in a vacuum chamber, then letting condensation deposit the metal on the substrate's surface. Resistance heating , electron beam , or plasma heating is used to vaporize the coating metal. Vacuum metallizing was used to deposit aluminum on the large glass mirrors of reflecting telescopes, such as with the Hale Telescope .
Thermal spray processes are often referred to as metallizing. Metals applied in such a manner provide corrosion protection to steel for decades longer than paint alone. Zinc and aluminum are the most commonly used materials for metallizing steel structures. [ 1 ]
Cold sprayable metal technology is a metallizing process that seamlessly applies cold sprayable or putty able metal to almost any surface. The composite metal consists of two (water-based binder) or three different ingredients: metal powder, binder and hardener.
The mixture of the ingredients is cast or sprayed on the substrate at room temperature. The desired effect and the necessary final treatment define the thickness of the layer, which normally varies between 80 and 150 μm .
This metal -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metallizing |
The metallo-ene reaction is a chemical reaction employed within organic synthesis. Mechanistically similar to the classic ene reaction , [ 1 ] the metallo-ene reaction involves a six-member cyclic transition state that brings an allylic species and an alkene species together to undergo a rearrangement. The initial allylic group migrates to one terminus of the alkene reactant and a new carbon-carbon sigma bond is formed between the allylic species and the other terminus of the alkene reactant. In the metallo-ene reaction, a metal ion (Mg, Zn, Pd, Ni etc.) [ 2 ] [ 3 ] [ 4 ] acts as the migrating group rather than a hydrogen atom as in the classic ene reaction.
Metallo-ene reaction was first studied by Lehmkuhl et al., [ 5 ] and since then has gradually gained popularity among the synthetic community throughout the better understanding of its mechanism and potential as a synthetic tool. [ 6 ]
Generally speaking, metallo-ene reaction has both an intramolecular and an intermolecular version. For the former, the reaction can be classified into two types by the skeletal connectivity. [ 7 ] In Type I, a carbon linkage connects the alkene fragment to the terminal carbon of the allyl fragment of the molecule, while in Type II the alkene fragment is connected to the internal carbon of the allyl fragment.
Historically, there has been a long-standing uncertainty about the precise mechanism of metallo-ene reaction. [ 8 ] Three possible mechanisms—a concerted mechanism, a stepwise mechanism and a metal-catalyzed mechanism have been postulated and studied over the past few decades. According to computational analyses, [ 9 ] [ 10 ] metallo-ene reaction tends to proceed via a concerted six-member transition state, although the exact mechanism was found to vary and depends on the metal.
For Type II reaction, two possible products can be expected if the two termini of the allyl piece are unsymmetrically substituted, depending on which carbon engages in the formation of a new sigma bond. Interestingly, Oppolzer et al. [ 11 ] have found that the more substituted terminus of the allyl piece will participate in new sigma bond formation regardless of the length of the internal carbon linkage.
Since a six-member cyclic transition state is involved in metallo-ene reaction, high level of stereoselectivity can be expected due to the conservation of orbital symmetry. [ 12 ] Indeed, this happens to be the case in many synthetic applications of this reaction. Felkin et al. [ 13 ] have found the cis product to be formed as the predominant product kinetically, while the trans product could also be obtained selectively under thermodynamic conditions. The fact that stereochemical outcome of this metallo-ene reaction could be tuned by altering the reaction conditions regardless of the geometry of allyl fragment reveals its reversible nature. [ 14 ]
In 2016, Trost et al. [ 15 ] have developed a highly diastero- and enantioselective intramolecular interrupted metallo-ene reaction using a chiral phosphoramidite ligand to achieve high levels of stereoselectivity. Starting from linear precursors, a wide range of vicinally disubstituted five-member rings could be synthesized. An additional stereocenter is generated during the process by reaction with water.
In 2017, Liu et al. [ 16 ] have developed a highly efficient palladium- catalyzed cascade metallo-ene/ Suzuki coupling reaction of allene-amides, delivering polyfunctionalized 2,3-dihydropyrrole derivatives in excellent yields.
In their synthetic efforts towards Coriolin, Oppolzer et al. [ 17 ] devised a metallo-ene- carbonylation cascade reaction to construct the fused bicyclic core of Coriolin in an efficient fashion. They started with a simple aldehyde to which a propargyl alcohol appendage was attached via nucleophilic addition. Reduction followed by Appel reaction and Finkelstein reaction would yielded a key intermediate, which in the presence of nickel catalyst and CO atmosphere could be transformed to the target cyclopentanone in decent yield. | https://en.wikipedia.org/wiki/Metallo-ene_reaction |
A metallodendrimer is a type of dendrimer with incorporated metal atoms. The development of this type of material is actively pursued in academia. [ 1 ] [ 2 ] [ 3 ]
The metal can be situated in the repeat unit, the core or at the extremities as end-group . Elements often encountered are palladium and platinum . These metals can form octahedral six-coordinate M(IV) linking units from organic dihalides and the corresponding 4-coordinate M(II) monomers. Ferrocene-containing dendrimers and dendrimers with cobaltocene and arylchromiumtricarbonyl units have been reported in end-functional dendrimers.
Metallodendrimers can form as metal complexes with dendritic counter ions for example by hydrolysis of ester terminated PAMAM dendrimers with sodium hydroxide .
Metallodendrimers are investigated as equivalents to nanoparticles . Applications can be expected in the fields of catalysis , as chemical sensors in molecular recognition - for example of bromine and chloride anions [ 4 ] - or as materials capable of binding metals. Metallodendrimers can also mimic certain biomolecules for example haemoprotein in dendrimer with a porphyrin core. Further uses are reported as electrocatalyst . [ 5 ] [ 6 ]
Examples of metallodendrimer heterogeneous catalysis are a nickel-containing dendrimer active in the Kharasch addition , [ 7 ] palladium-containing dendrimers active in ethylene polymerization [ 8 ] and in the Heck reaction . [ 9 ] | https://en.wikipedia.org/wiki/Metallodendrimer |
In chemistry , a metallofullerene is a molecule composed of a metal atom trapped inside a fullerene cage.
Simple metallofullerenes consist of a fullerene cage, typically C 80 , with one or two metal atoms trapped inside. Recently, research has produced metallofullerenes that enclose small clusters of atoms, such as Sc 3 N@C 80 , Y 3 N@C 80 , and Sc 3 C 2 @C 80 . The '@' symbol in the formula indicates that the atom(s) are encapsulated inside the cage, rather than being chemically bonded to it.
Fullerenes in a variety of sizes have been found to encapsulate metal atoms in this way.
One particular metallofullerene with gadolinium at its core is up to 40 times better as a contrast agent in magnetic resonance imaging scans for diagnostic imaging. [ citation needed ] Metallofullerenes may also provide ways to carry therapeutic radioactive ions to cancerous tissue. [ 1 ] [ 2 ]
This article about an organic compound is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metallofullerene |
Metallogenium is a proposed genus of bacteria that has an affinity to form star-shaped manganese oxide minerals . The organism is supposedly observed in limnic environments . [ 2 ] [ 3 ] The species is currently not assigned to any taxonomic family . [ 4 ]
This bacteria -related article is a stub . You can help Wikipedia by expanding it .
This mineralogy article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metallogenium |
Metallography is the study of the physical structure and components of metals , by using microscopy .
Ceramic and polymeric materials may also be prepared using metallographic techniques, hence the terms ceramography , plastography and, collectively, materialography.
The surface of a metallographic specimen is prepared by various methods of grinding , polishing , and etching . After preparation, it is often analyzed using optical or electron microscopy . Using only metallographic techniques, a skilled technician can identify alloys and predict material properties .
Mechanical preparation is the most common preparation method. Successively finer abrasive particles are used to remove material from the sample surface until the desired surface quality is achieved. Many different machines are available for doing this grinding and polishing , which are able to meet different demands for quality, capacity, and reproducibility.
A systematic preparation method is the easiest way to achieve the true structure. Sample preparation must therefore pursue rules which are suitable for most materials. Different materials with similar properties ( hardness and ductility ) will respond alike and thus require the same consumables during preparation.
Metallographic specimens are typically "mounted" using a hot compression thermosetting resin . In the past, phenolic thermosetting resins have been used, but modern epoxy is becoming more popular because reduced shrinkage during curing results in a better mount with superior edge retention. A typical mounting cycle will compress the specimen and mounting media to 4,000 psi (28 MPa) and heat to a temperature of 350 °F (177 °C). When specimens are very sensitive to temperature, "cold mounts" may be made with a two-part epoxy resin. Mounting a specimen provides a safe, standardized, and ergonomic way by which to hold a sample during the grinding and polishing operations.
After mounting, the specimen is wet ground to reveal the surface of the metal. The specimen is successively ground with finer and finer abrasive media. Silicon carbide abrasive paper was the first method of grinding and is still used today. Many metallographers, however, prefer to use a diamond grit suspension which is dosed onto a reusable fabric pad throughout the polishing process. Diamond grit in suspension might start at 9 micrometres and finish at one micrometre. Generally, polishing with diamond suspension gives finer results than using silicon carbide papers (SiC papers), especially with revealing porosity , which silicon carbide paper sometimes "smear" over. After grinding the specimen, polishing is performed. Typically, a specimen is polished with a slurry of alumina , silica , or diamond on a napless cloth to produce a scratch-free mirror finish, free from smear, drag, or pull-outs and with minimal deformation remaining from the preparation process.
After polishing, certain microstructural constituents can be seen with the microscope, e.g., inclusions and nitrides. If the crystal structure is non-cubic (e.g., a metal with a hexagonal-closed packed crystal structure, such as Ti or Zr ) the microstructure can be revealed without etching using crossed polarized light (light microscopy). Otherwise, the microstructural constituents of the specimen are revealed by using a suitable chemical or electrolytic etchant.
Non-destructive surface analysis techniques can involve applying a thin film or varnish that can be peeled off after drying and examined under a microscope. The technique was developed by Pierre Armand Jacquet and others in 1957. [ 1 ]
Many different microscopy techniques are used in metallographic analysis.
Prepared specimens should be examined with the unaided eye after etching to detect any visible areas that have responded to the etchant differently from the norm as a guide to where microscopical examination should be employed. Light optical microscopy (LOM) examination should always be performed prior to any electron metallographic (EM) technique, as these are more time-consuming to perform and the instruments are much more expensive.
Further, certain features can be best observed with the LOM, e.g., the natural color of a constituent can be seen with the LOM but not with EM systems. Also, image contrast of microstructures at relatively low magnifications, e.g., <500X, is far better with the LOM than with the scanning electron microscope (SEM), while transmission electron microscopes (TEM) generally cannot be utilized at magnifications below about 2000 to 3000X. LOM examination is fast and can cover a large area. Thus, the analysis can determine if the more expensive, more time-consuming examination techniques using the SEM or the TEM are required and where on the specimen the work should be concentrated.
Light microscopes are designed for placement of the specimen's polished surface on the specimen stage either upright or inverted. Each type has advantages and disadvantages. Most LOM work is done at magnifications between 50 and 1000X. However, with a good microscope, it is possible to perform examination at higher magnifications, e.g., 2000X, and even higher, as long as diffraction fringes are not present to distort the image. However, the resolution limit of the LOM will not be better than about 0.2 to 0.3 micrometers. Special methods are used at magnifications below 50X, which can be very helpful when examining the microstructure of cast specimens where greater spatial coverage in the field of view may be required to observe features such as dendrites .
Besides considering the resolution of the optics, one must also maximize visibility by maximizing image contrast . A microscope with excellent resolution may not be able to image a structure, that is there is no visibility, if image contrast is poor. Image contrast depends upon the quality of the optics, coatings on the lenses, and reduction of flare and glare ; but, it also requires proper specimen preparation and good etching techniques. So, obtaining good images requires maximum resolution and image contrast.
Most LOM observations are conducted using bright-field (BF) illumination, where the image of any flat feature perpendicular to the incident light path is bright, or appears to be white. But, other illumination methods can be used and, in some cases, may provide superior images with greater detail. Dark-field microscopy (DF), is an alternative method of observation that provides high-contrast images and actually greater resolution than bright-field. In dark-field illumination, the light from features perpendicular to the optical axis is blocked and appears dark while the light from features inclined to the surface, which look dark in BF, appear bright, or "self-luminous" in DF. Grain boundaries , for example, are more vivid in DF than BF.
Polarized light (PL) is very useful when studying the structure of metals with non-cubic crystal structures (mainly metals with hexagonal close-packed (hcp) crystal structures). If the specimen is prepared with minimal damage to the surface, the structure can be seen vividly in cross-polarized light (the optic axis of the polarizer and analyzer are 90 degrees to each other, i.e., crossed). In some cases, an hcp metal can be chemically etched and then examined more effectively with PL. Tint etched surfaces, where a thin film (such as a sulfide , molybdate , chromate or elemental selenium film) is grown epitaxially on the surface to a depth where interference effects are created when examined with BF producing color images, can be improved with PL. If it is difficult to get a good interference film with good coloration, the colors can be improved by examination in PL using a sensitive tint (ST) filter.
Another useful imaging mode is differential interference contrast (DIC), which is usually obtained with a system designed by the Polish physicist Georges Nomarski . This system gives the best detail. DIC converts minor height differences on the plane-of-polish, invisible in BF, into visible detail. The detail in some cases can be quite striking and very useful. If an ST filter is used along with a Wollaston prism , color is introduced. The colors are controlled by the adjustment of the Wollaston prism, and have no specific physical meaning, per se. But, visibility may be better.
DIC has largely replaced the older oblique illumination (OI) technique, which was available on reflected light microscopes prior to about 1975. In OI, the vertical illuminator is offset from perpendicular, producing shading effects that reveal height differences. This procedure reduces resolution and yields uneven illumination across the field of view. Nevertheless, OI was useful when people needed to know if a second phase particle was standing above or was recessed below the plane-of-polish, and is still available on a few microscopes. OI can be created on any microscope by placing a piece of paper under one corner of the mount so that the plane-of-polish is no longer perpendicular to the optical axis.
Spatially resolve acoustic spectroscopy ( SRAS ) is an optical technique that uses optically generated high frequency surface acoustic waves to probe
the direction elastic parameters of the surface and, as such, it can vividly reveal the surface microstructure of metals.
It can also image the crystallographic orientation and determine the single crystal elasticity matrix of the material.
If a specimen must be observed at higher magnification, it can be examined with a scanning electron microscope (SEM), or a transmission electron microscope (TEM). When equipped with an energy dispersive spectrometer (EDS), the chemical composition of the microstructural features can be determined. The ability to detect low-atomic number elements, such as carbon , oxygen , and nitrogen , depends upon the nature of the detector used. But, quantification of these elements by EDS is difficult and their minimum detectable limits are higher than when a wavelength-dispersive spectrometer (WDS) is used. But quantification of composition by EDS has improved greatly over time. The WDS system has historically had better sensitivity (ability to detect low amounts of an element) and ability to detect low-atomic weight elements, as well as better quantification of compositions, compared to EDS, but it was slower to use. Again, in recent years, the speed required to perform WDS analysis has improved substantially. Historically, EDS was used with the SEM while WDS was used with the electron microprobe analyzer (EMPA). Today, EDS and WDS is used with both the SEM and the EMPA. However, a dedicated EMPA is not as common as an SEM.
Characterization of microstructures has also been performed using x-ray diffraction (XRD) techniques for many years. XRD can be used to determine the percentages of various phases present in a specimen if they have different crystal structures. For example, the amount of retained austenite in a hardened steel is best measured using XRD (ASTM E 975). If a particular phase can be chemically extracted from a bulk specimen, it can be identified using XRD based on the crystal structure and lattice dimensions. This work can be complemented by EDS and/or WDS analysis where the chemical composition is quantified. But EDS and WDS are difficult to apply to particles less than 2-3 micrometers in diameter. For smaller particles, diffraction techniques can be performed using the TEM for identification and EDS can be performed on small particles if they are extracted from the matrix using replication methods to avoid detection of the matrix along with the precipitate.
A number of techniques exist to quantitatively analyze metallographic specimens. These techniques are valuable in the research and production of all metals and alloys and non-metallic or composite materials .
Microstructural quantification is performed on a prepared, two-dimensional plane through the three-dimensional part or component. Measurements may involve simple metrology techniques, e.g., the measurement of the thickness of a surface coating, or the apparent diameter of a discrete second-phase particle, (for example, spheroidal graphite in ductile iron ). Measurement may also require application of stereology to assess matrix and second-phase structures. Stereology is the field of taking 0-, 1- or 2-dimensional measurements on the two-dimensional sectioning plane and estimating the amount, size, shape or distribution of the microstructure in three dimensions. These measurements may be made using manual procedures with the aid of templates overlaying the microstructure, or with automated image analyzers. In all cases, adequate sampling must be made to obtain a proper statistical basis for the measurement. Efforts to eliminate bias are required.
Some of the most basic measurements include determination of the volume fraction of a phase or constituent, measurement of the grain size in polycrystalline metals and alloys, measurement of the size and size distribution of particles, assessment of the shape of particles, and spacing between particles.
Standards organizations , including ASTM International 's Committee E-4 on Metallography and some other national and international organizations, have developed standard test methods describing how to characterize microstructures quantitatively.
For example, the amount of a phase or constituent, that is, its volume fraction, is defined in ASTM E 562; manual grain size measurements are described in ASTM E 112 ( equiaxed grain structures with a single size distribution) and E 1182 (specimens with a bi-modal grain size distribution); while ASTM E 1382 describes how any grain size type or condition can be measured using image analysis methods. Characterization of nonmetallic inclusions using standard charts is described in ASTM E 45 (historically, E 45 covered only manual chart methods and an image analysis method for making such chart measurements was described in ASTM E 1122. The image analysis methods are currently being incorporated into E 45). A stereological method for characterizing discrete second-phase particles, such as nonmetallic inclusions, carbides, graphite, etc., is presented in ASTM E 1245. | https://en.wikipedia.org/wiki/Metallography |
In biochemistry, the metallome is the distribution of metal ions in a cellular compartment. The term was coined in analogy with proteome [ 1 ] , as metallomics is the study of metallome: the "comprehensive analysis of the entirety of metal and metalloid species within a cell or tissue type". [ 2 ] Therefore, metallomics can be considered a branch of metabolomics , [ citation needed ] even though the metals are not typically considered as metabolites .
An alternative definition of "metallomes" as metalloproteins or any other metal-containing biomolecules , and "metallomics" as a study of such biomolecules. [ 3 ]
In the study of metallomes the transcriptome , proteome and the metabolome constitutes the whole metallome. A study of the metallome is done to arrive at the metallointeractome .
The metallotranscriptome [ 4 ] can be defined as the map of the entire transcriptome in the presence of biologically or environmentally relevant concentrations of an essential or toxic metal, respectively. The metallometabolome constitutes the complete pool of small metabolites in a cell at any given time. This gives rise to the whole metallointeractome and knowledge of this is important in comparative metallomics dealing with toxicity and drug discovery. [ 4 ] | https://en.wikipedia.org/wiki/Metallome |
Metallomesogens are metal complexes that exhibit liquid crystalline behavior. Thus, they adopt ordered structures in the molten phase, e.g. smectic and nematic phases . The dominant interactions responsible for their phase behavior are the nonbonding contacts between organic substituents. Two early classes of such materials are based on substituted ferrocenes and dithiolene complexes ; [ 1 ] more recent work shows that alkoxystilbazoles have similar utility. [ 2 ] | https://en.wikipedia.org/wiki/Metallomesogen |
Metallopeptides (also called metal-peptides or metal peptide complexes) are peptides that contain one or more metal ions in their structure . This specific type of peptide are, just like metalloproteins , metallofoldamers. And very similar to metalloproteins , metallopeptide's functionality is attributed through the contained metal ion cofactor . These short structured peptides are often employed to develop mimics of metalloproteins and systems similar to artificial metalloenzymes .
A multitude of naturally occurring peptides display biological and chemical activities when bound to various metal ions. Where different metal ion cofactor can lead to different reactivity and even different folding and physical characteristics (e.g. solubility or stability ) of the structure. Synthetic equivalents of such peptides are engineered to bind metal ions and display a variety of physical, chemical, and biological reactivity and characteristics.
In the last 40 years, there has been a significant amount of research on metal binding peptides and their characteristics, structures, and chemical reactivities. [ 1 ]
Vincent L. Pecoraro and his group investigate the interaction of peptides with heavy metals in the body; Katherine Franz leads a group studying Cu -binding peptides ; Angela Lombardi and her unit focus on the development of artificial metalloenzymes and similar peptide systems, and the group of Peter Faller focuses on redox reactivity of Cu -peptides. [ 2 ] [ 3 ]
Natural metallopeptides with antibiotic , antimicrobial and anticancer properties have been of particular interest to the scientific community (e.g. the divalent bacitracin , histatin and Fe / Cu -bleomycin). [ 4 ] [ 5 ] At the same time there is an increasing attention to the role of metalloppeptides in disease development. For example, metallochemical interactions in brain tissue can contribute to neurodegenerative conditions due to the naturally high concentration of metal ions in the brain . Hence the metallochemical reactions occurring outside the physiologically healthy concentrations, can contribute to the development of diseases such as Alzheimer's disease . The condition is related to the β-amyloid metallopeptides. [ 6 ] Another example are infectious prion polypeptides and specific isoforms of prion protein which contribute to disease transmission and development. [ 7 ]
De novo designed peptides which self-assemble in the presence of copper (Cu), forming supramolecular assemblies were presented by Korendovych et al. [ 8 ] Additionally there are examples of metallopeptides that are, at least partially, composed of non-natural amino acids with possible applications in drug discovery and biomaterials. [ 9 ]
Being a type of molecules that are often only activated for biological and chemical function following metal-binding, the specific coordination of metal ions imposes certain restrictions and requirements onto metallopeptides. Usually metal cofactors are coordinated by nitrogen , oxygen or sulfur centers belonging to amino acid residues of the peptide. These donor groups can be introduced by histidine (or the corresponding imidazole ), cysteine ( thiolate group ), as well as carboxylate substituents (e.g by aspartate ) but are not limited to these. The other amino acid residues , including non-natural amino acids and the peptide backbone have been shown to bind metal centers and provide donor groups. The research on metal-binding of peptides ranges from coordination of biometals (such as Calcium , Magnesium , Manganese , Zinc , Sodium , Potassium , and Iron ) to heavy metals (such as Arsenic , Mercury , and Cadmium ). [ 10 ] [ 11 ]
Peptides are synthesized in living organisms inside the cell analogously to proteins.
Solid phase peptide synthesis (SPPS) is a well-established method for producing synthetic peptides. SPPS enables the building of a peptide chain by sequential interactions of amino acid derivatives.
The interaction between metal ions and peptides are typically studied in solution using spectroscopic or electrochemical methods. Amongst which are circular dichroism (CD) , nuclear magnetic resonance (NMR) spectroscopy , cyclic voltammetry , and mass spectrometry (MS) . [ 1 ] | https://en.wikipedia.org/wiki/Metallopeptide |
In chemistry , a metallophilic interaction is defined as a type of non- covalent attraction between heavy metal atoms. The atoms are often within Van der Waals distance of each other and are about as strong as hydrogen bonds . [ 1 ] The effect can be intramolecular or intermolecular . Intermolecular metallophilic interactions can lead to formation of supramolecular assemblies whose properties vary with the choice of element and oxidation states of the metal atoms and the attachment of various ligands to them. [ 2 ]
The nature of such interactions remains the subject of vigorous debate with recent studies emphasizing that the metallophilic interaction is repulsive due to strong metal-metal Pauli exclusion principle repulsion. [ 3 ]
Previously, this type of interaction was considered to be enhanced by relativistic effects . A major contributor is electron correlation of the closed-shell components, [ 2 ] which is unusual because closed-shell atoms generally have negligible interaction with one another at the distances observed for the metal atoms. As a trend, the effect becomes larger moving down a periodic table group , for example, from copper to silver to gold , in keeping with increased relativistic effects. [ 2 ] Observations and theory find that, on average, 28% of the binding energy in gold–gold interactions can be attributed to relativistic expansion of the gold d orbitals . [ 4 ]
Recently, the relativistic effect was found to enhance the intermolecular M-M Pauli repulsion of the closed-shell organometallic complexes. [ 3 ] At close M–M distances, metallophilicity is repulsive in nature due to strong M–M Pauli repulsion. The relativistic effect facilitates (n + 1)s-nd and (n + 1)p-nd orbital hybridization of the metal atom, where (n + 1)s-nd hybridization induces strong M–M Pauli repulsion and repulsive M–M orbital interaction, and (n + 1)p-nd hybridization suppresses M–M Pauli repulsion. This model is validated by both DFT (density functional theory) and high-level CCSD(T) (coupled-cluster singles and doubles with perturbative triples) computations. [ 3 ]
An important and exploitable property of aurophilic interactions relevant to their supramolecular chemistry is that while both inter- and intramolecular interactions are possible, intermolecular aurophilic linkages are comparatively weak and the gold–gold bonds are easily broken by solvation ; most complexes that exhibit intramolecular aurophilic interactions retain such moieties in solution. [ 5 ] One way of probing the strength of particular intermolecular metallophilic interactions is to use a competing solvent and examine how it interferes with supromolecular properties. For example, adding various solvents to gold(I) nanoparticles whose luminescence is attributed to Au–Au interactions will have decreasing luminescence as the solvent disrupts the metallophilic interactions. [ 5 ]
The polymerization of metal atoms can lead to the formation of long chains or nucleated clusters. Gold nanoparticles formed from chains of gold(I) complexes linked by aurophilic interactions often give rise to intense luminescence in the visible region of the spectrum . [ 5 ]
Chains of Pd(II)–Pd(I) and Pt(II)–Pd(I) complexes have been explored as potential molecular wires . [ 6 ] | https://en.wikipedia.org/wiki/Metallophilic_interaction |
A metallophyte is a plant that can tolerate high levels of heavy metals such as lead . Such plants range between "obligate metallophytes" (which can only survive in the presence of these metals), and "facultative metallophytes" which can tolerate such conditions but are not confined to them. [ 1 ]
European examples include alpine pennycress ( Thlaspi caerulescens ), the zinc violet ( Viola calaminaria ), spring sandwort ( Minuartia verna ), sea thrift ( Armeria maritima ), Cochlearia , common bent ( Agrostis capillaris ) and plantain ( Plantago lanceolata ). [ 2 ] Few metallophytes are known from Latin America . [ 3 ]
Metallophytes commonly exist as specialised flora found on spoil heaps of mines.
Such plants have potential for use in phytoremediation of contaminated ground. | https://en.wikipedia.org/wiki/Metallophyte |
Metalloprotein is a generic term for a protein that contains a metal ion cofactor . [ 1 ] [ 2 ] A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins (out of ~20,000) contain zinc-binding protein domains [ 3 ] although there may be up to 3000 human zinc metalloproteins. [ 4 ]
It is estimated that approximately half of all proteins contain a metal . [ 5 ] In another estimate, about one quarter to one third of all proteins are proposed to require metals to carry out their functions. [ 6 ] Thus, metalloproteins have many different functions in cells , such as storage and transport of proteins, enzymes and signal transduction proteins, or infectious diseases. [ 7 ] The abundance of metal binding proteins may be inherent to the amino acids that proteins use, as even artificial proteins without evolutionary history will readily bind metals. [ 8 ]
Most metals in the human body are bound to proteins. For instance, the relatively high concentration of iron in the human body is mostly due to the iron in hemoglobin .
In metalloproteins, metal ions are usually coordinated by nitrogen , oxygen or sulfur centers belonging to amino acid residues of the protein. These donor groups are often provided by side-chains on the amino acid residues. Especially important are the imidazole substituent in histidine residues, thiolate substituents in cysteine residues, and carboxylate groups provided by aspartate . Given the diversity of the metallo proteome , virtually all amino acid residues have been shown to bind metal centers. The peptide backbone also provides donor groups; these include deprotonated amides and the amide carbonyl oxygen centers. Lead(II) binding in natural and artificial proteins has been reviewed. [ 10 ]
In addition to donor groups that are provided by amino acid residues, many organic cofactors function as ligands. Perhaps most famous are the tetradentate N 4 macrocyclic ligands incorporated into the heme protein. Inorganic ligands such as sulfide and oxide are also common.
These are the second stage product of protein hydrolysis obtained by treatment with slightly stronger acids and alkalies.
Hemoglobin , which is the principal oxygen-carrier in humans, has four subunits in which the iron (II) ion is coordinated by the planar macrocyclic ligand protoporphyrin IX (PIX) and the imidazole nitrogen atom of a histidine residue. The sixth coordination site contains a water molecule or a dioxygen molecule. By contrast the protein myoglobin , found in muscle cells , has only one such unit. The active site is located in a hydrophobic pocket. This is important as without it the iron(II) would be irreversibly oxidized to iron(III). The equilibrium constant for the formation of HbO 2 is such that oxygen is taken up or released depending on the partial pressure of oxygen in the lungs or in muscle. In hemoglobin the four subunits show a cooperativity effect that allows for easy oxygen transfer from hemoglobin to myoglobin. [ 11 ]
In both hemoglobin and myoglobin it is sometimes incorrectly stated that the oxygenated species contains iron(III). It is now known that the diamagnetic nature of these species is because the iron(II) atom is in the low-spin state. In oxyhemoglobin the iron atom is located in the plane of the porphyrin ring, but in the paramagnetic deoxyhemoglobin the iron atom lies above the plane of the ring. [ 11 ] This change in spin state is a cooperative effect due to the higher crystal field splitting and smaller ionic radius of Fe 2+ in the oxyhemoglobin moiety.
Hemerythrin is another iron-containing oxygen carrier. The oxygen binding site is a binuclear iron center. The iron atoms are coordinated to the protein through the carboxylate side chains of a glutamate and aspartate and five histidine residues. The uptake of O 2 by hemerythrin is accompanied by two-electron oxidation of the reduced binuclear center to produce bound peroxide (OOH − ). The mechanism of oxygen uptake and release have been worked out in detail. [ 12 ] [ 13 ]
Hemocyanins carry oxygen in the blood of most mollusks , and some arthropods such as the horseshoe crab . They are second only to hemoglobin in biological popularity of use in oxygen transport. On oxygenation the two copper (I) atoms at the active site are oxidized to copper(II) and the dioxygen molecules are reduced to peroxide, O 2− 2 . [ 14 ] [ 15 ]
Chlorocruorin (as the larger carrier erythrocruorin ) is an oxygen-binding hemeprotein present in the blood plasma of many annelids , particularly certain marine polychaetes .
Oxidation and reduction reactions are not common in organic chemistry as few organic molecules can act as oxidizing or reducing agents . Iron (II), on the other hand, can easily be oxidized to iron(III). This functionality is used in cytochromes , which function as electron-transfer vectors. The presence of the metal ion allows metalloenzymes to perform functions such as redox reactions that cannot easily be performed by the limited set of functional groups found in amino acids . [ 16 ] The iron atom in most cytochromes is contained in a heme group. The differences between those cytochromes lies in the different side-chains. For instance cytochrome a has a heme a prosthetic group and cytochrome b has a heme b prosthetic group. These differences result in different Fe 2+ /Fe 3+ redox potentials such that various cytochromes are involved in the mitochondrial electron transport chain. [ 17 ]
Cytochrome P450 enzymes perform the function of inserting an oxygen atom into a C−H bond, an oxidation reaction. [ 18 ] [ 19 ]
Rubredoxin is an electron-carrier found in sulfur -metabolizing bacteria and archaea . The active site contains an iron ion coordinated by the sulfur atoms of four cysteine residues forming an almost regular tetrahedron . Rubredoxins perform one-electron transfer processes. The oxidation state of the iron atom changes between the +2 and +3 states. In both oxidation states the metal is high spin , which helps to minimize structural changes.
Plastocyanin is one of the family of blue copper proteins that are involved in electron transfer reactions. The copper -binding site is described as distorted trigonal pyramidal . [ 20 ] The trigonal plane of the pyramidal base is composed of two nitrogen atoms (N 1 and N 2 ) from separate histidines and a sulfur (S 1 ) from a cysteine. Sulfur (S 2 ) from an axial methionine forms the apex. The distortion occurs in the bond lengths between the copper and sulfur ligands. The Cu−S 1 contact is shorter (207 pm ) than Cu−S 2 (282 pm).
The elongated Cu−S 2 bonding destabilizes the Cu(II) form and increases the redox potential of the protein. The blue color (597 nm peak absorption) is due to the Cu−S 1 bond where S(pπ) to Cu(d x 2 − y 2 ) charge transfer occurs. [ 21 ]
In the reduced form of plastocyanin, His -87 will become protonated with a p K a of 4.4. Protonation prevents it acting as a ligand and the copper site geometry becomes trigonal planar .
Iron is stored as iron(III) in ferritin . The exact nature of the binding site has not yet been determined. The iron appears to be present as a hydrolysis product such as FeO(OH). Iron is transported by transferrin whose binding site consists of two tyrosines , one aspartic acid and one histidine . [ 22 ] The human body has no controlled mechanism for excretion of iron. [ 23 ] This can lead to iron overload problems in patients treated with blood transfusions , as, for instance, with β- thalassemia . Iron is actually excreted in urine [ 24 ] and is also concentrated in bile [ 25 ] which is excreted in feces. [ 26 ]
Ceruloplasmin is the major copper -carrying protein in the blood. Ceruloplasmin exhibits oxidase activity, which is associated with possible oxidation of Fe(II) into Fe(III), therefore assisting in its transport in the blood plasma in association with transferrin, which can carry iron only in the Fe(III) state.
Osteopontin is involved in mineralization in the extracellular matrices of bones and teeth.
Metalloenzymes all have one feature in common, namely that the metal ion is bound to the protein with one labile coordination site. As with all enzymes , the shape of the active site is crucial. The metal ion is usually located in a pocket whose shape fits the substrate. The metal ion catalyzes reactions that are difficult to achieve in organic chemistry .
In aqueous solution , carbon dioxide forms carbonic acid
This reaction is very slow in the absence of a catalyst, but quite fast in the presence of the hydroxide ion
A reaction similar to this is almost instantaneous with carbonic anhydrase . The structure of the active site in carbonic anhydrases is well known from a number of crystal structures. It consists of a zinc ion coordinated by three imidazole nitrogen atoms from three histidine units. The fourth coordination site is occupied by a water molecule. The coordination sphere of the zinc ion is approximately tetrahedral . The positively-charged zinc ion polarizes the coordinated water molecule, and nucleophilic attack by the negatively-charged hydroxide portion on carbon dioxide proceeds rapidly. The catalytic cycle produces the bicarbonate ion and the hydrogen ion [ 2 ] as the equilibrium :
favouring dissociation of carbonic acid at biological pH values. [ 27 ]
The cobalt -containing Vitamin B 12 (also known as cobalamin) catalyzes the transfer of methyl (−CH 3 ) groups between two molecules, which involves the breaking of C−C bonds , a process that is energetically expensive in organic reactions. The metal ion lowers the activation energy for the process by forming a transient Co−CH 3 bond. [ 28 ] The structure of the coenzyme was famously determined by Dorothy Hodgkin and co-workers, for which she received a Nobel Prize in Chemistry . [ 29 ] It consists of a cobalt(II) ion coordinated to four nitrogen atoms of a corrin ring and a fifth nitrogen atom from an imidazole group. In the resting state there is a Co−C sigma bond with the 5′ carbon atom of adenosine . [ 30 ] This is a naturally occurring organometallic compound, which explains its function in trans -methylation reactions, such as the reaction carried out by methionine synthase .
The fixation of atmospheric nitrogen is an energy-intensive process, as it involves breaking the very stable triple bond between the nitrogen atoms. The nitrogenases catalyze the process. One such enzyme occurs in Rhizobium bacteria . There are three components to its action: a molybdenum atom at the active site, iron–sulfur clusters that are involved in transporting the electrons needed to reduce the nitrogen, and an abundant energy source in the form of magnesium ATP . This last is provided by a mutualistic symbiosis between the bacteria and a host plant, often a legume . The reaction may be written symbolically as
where P i stands for inorganic phosphate . The precise structure of the active site has been difficult to determine. It appears to contain a MoFe 7 S 8 cluster that is able to bind the dinitrogen molecule and, presumably, enable the reduction process to begin. [ 31 ] The electrons are transported by the associated "P" cluster, which contains two cubical Fe 4 S 4 clusters joined by sulfur bridges. [ 32 ]
The superoxide ion, O − 2 is generated in biological systems by reduction of molecular oxygen . It has an unpaired electron , so it behaves as a free radical . It is a powerful oxidizing agent . These properties render the superoxide ion very toxic and are deployed to advantage by phagocytes to kill invading microorganisms . Otherwise, the superoxide ion must be destroyed before it does unwanted damage in a cell. The superoxide dismutase enzymes perform this function very efficiently. [ 33 ]
The formal oxidation state of the oxygen atoms is − 1 ⁄ 2 . In solutions at neutral pH , the superoxide ion disproportionates to molecular oxygen and hydrogen peroxide .
In biology this type of reaction is called a dismutation reaction. It involves both oxidation and reduction of superoxide ions. The superoxide dismutase (SOD) group of enzymes increase the rate of reaction to near the diffusion-limited rate. [ 34 ] The key to the action of these enzymes is a metal ion with variable oxidation state that can act either as an oxidizing agent or as a reducing agent.
In human SOD, the active metal is copper , as Cu(II) or Cu(I), coordinated tetrahedrally by four histidine residues. This enzyme also contains zinc ions for stabilization and is activated by copper chaperone for superoxide dismutase ( CCS ). Other isozymes may contain iron , manganese or nickel . The activity of Ni-SOD involves nickel(III), an unusual oxidation state for this element. The active site nickel geometry cycles from square planar Ni(II), with thiolate (Cys 2 and Cys 6 ) and backbone nitrogen (His 1 and Cys 2 ) ligands, to square pyramidal Ni(III) with an added axial His 1 side chain ligand. [ 35 ]
Chlorophyll plays a crucial role in photosynthesis . It contains a magnesium enclosed in a chlorin ring. However, the magnesium ion is not directly involved in the photosynthetic function and can be replaced by other divalent ions with little loss of activity. Rather, the photon is absorbed by the chlorin ring, whose electronic structure is well-adapted for this purpose.
Initially, the absorption of a photon causes an electron to be excited into a singlet state of the Q band. The excited state undergoes an intersystem crossing from the singlet state to a triplet state in which there are two electrons with parallel spin . This species is, in effect, a free radical , and is very reactive and allows an electron to be transferred to acceptors that are adjacent to the chlorophyll in the chloroplast . In the process chlorophyll is oxidized. Later in the photosynthetic cycle, chlorophyll is reduced back again. This reduction ultimately draws electrons from water, yielding molecular oxygen as a final oxidation product.
Hydrogenases are subclassified into three different types based on the active site metal content: iron–iron hydrogenase, nickel–iron hydrogenase, and iron hydrogenase. [ 36 ] All hydrogenases catalyze reversible H 2 uptake, but while the [FeFe] and [NiFe] hydrogenases are true redox catalysts , driving H 2 oxidation and H + reduction
the [Fe] hydrogenases catalyze the reversible heterolytic cleavage of H 2 .
Since discovery of ribozymes by Thomas Cech and Sidney Altman in the early 1980s, ribozymes have been shown to be a distinct class of metalloenzymes. [ 37 ] Many ribozymes require metal ions in their active sites for chemical catalysis; hence they are called metalloenzymes. Additionally, metal ions are essential for structural stabilization of ribozymes. Group I intron is the most studied ribozyme which has three metals participating in catalysis. [ 38 ] Other known ribozymes include group II intron , RNase P , and several small viral ribozymes (such as hammerhead , hairpin , HDV , and VS ) and the large subunit of ribosomes. Several classes of ribozymes have been described. [ 39 ]
Deoxyribozymes , also called DNAzymes or catalytic DNA, are artificial DNA-based catalysts that were first produced in 1994. [ 40 ] Almost all DNAzymes require metal ions. Although ribozymes mostly catalyze cleavage of RNA substrates, a variety of reactions can be catalyzed by DNAzymes including RNA/DNA cleavage, RNA/DNA ligation, amino acid phosphorylation and dephosphorylation, and carbon–carbon bond formation. [ 41 ] Yet, DNAzymes that catalyze RNA cleavage reaction are the most extensively explored ones. 10-23 DNAzyme, discovered in 1997, is one of the most studied catalytic DNAs with clinical applications as a therapeutic agent. [ 42 ] Several metal-specific DNAzymes have been reported including the GR-5 DNAzyme ( lead -specific), [ 43 ] the CA1-3 DNAzymes ( copper -specific), the 39E DNAzyme ( uranyl -specific) [ 44 ] and the NaA43 DNAzyme ( sodium -specific). [ 45 ]
Calmodulin is an example of a signal-transduction protein. It is a small protein that contains four EF-hand motifs, each of which is able to bind a Ca 2+ ion.
In an EF-hand loop protein domain, the calcium ion is coordinated in a pentagonal bipyramidal configuration. Six glutamic acid and aspartic acid residues involved in the binding are in positions 1, 3, 5, 7 and 9 of the polypeptide chain. At position 12, there is a glutamate or aspartate ligand that behaves as a bidentate ligand, providing two oxygen atoms. The ninth residue in the loop is necessarily glycine due to the conformational requirements of the backbone. The coordination sphere of the calcium ion contains only carboxylate oxygen atoms and no nitrogen atoms. This is consistent with the hard nature of the calcium ion.
The protein has two approximately symmetrical domains, separated by a flexible "hinge" region. Binding of calcium causes a conformational change to occur in the protein. Calmodulin participates in an intracellular signaling system by acting as a diffusible second messenger to the initial stimuli. [ 46 ] [ 47 ]
In both cardiac and skeletal muscles , muscular force production is controlled primarily by changes in the intracellular calcium concentration . In general, when calcium rises, the muscles contract and, when calcium falls, the muscles relax. Troponin , along with actin and tropomyosin , is the protein complex to which calcium binds to trigger the production of muscular force.
Many transcription factors contain a structure known as a zinc finger , a structural module in which a region of protein folds around a zinc ion. The zinc does not directly contact the DNA that these proteins bind to. Instead, the cofactor is essential for the stability of the tightly folded protein chain. [ 48 ] In these proteins, the zinc ion is usually coordinated by pairs of cysteine and histidine side-chains.
There are two types of carbon monoxide dehydrogenase : one contains iron and molybdenum, the other contains iron and nickel. Parallels and differences in catalytic strategies have been reviewed. [ 49 ]
Pb 2+ (lead) can replace Ca 2+ (calcium) as, for example, with calmodulin or Zn 2+ (zinc) as with metallocarboxypeptidases . [ 50 ]
Some other metalloenzymes are given in the following table, according to the metal involved.
Poly(A) polymerase | https://en.wikipedia.org/wiki/Metalloprotein |
Metallotolerants are extremophile organisms that are able to survive in environments with a high concentration of dissolved heavy metals . They can be found in environments containing arsenic , cadmium , copper , and zinc . Known metallotolerants include Ferroplasma sp. and Cupriavidus metallidurans .
Metallotolerants adapt to their environment by reducing energy loss by excreting less. Many metallotolerant microbes utilise strategies to perform bioremediation, which is seen as a productive way of survival. [ 1 ]
Sinorhizobium sp. M14 is a metallotolerant bacterium. [ 2 ] Plants can also survive in highly metallic conditions. [ 3 ] For example, Noccaea caerulescens is a metallotolerent plant. [ 4 ] [ 5 ]
This microbiology -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metallotolerant |
Metallum Martis is a 1665 book by the English metallurgist Dud Dudley . It is the earliest known reference to the use of coal in metallurgical smelting . The book is also referred to as Iron made with Pit-Coale, Sea-Coale, &c. And with the same Fuell to Melt and Fine Imperfect Mettals, And Refine perfect Mettals.
Dudley provides a geological map of Dudley Castle where he correctly identifies the order and geographic layout of the strata of coal and ironstone under survey. It is one of the earliest recorded geological maps.
The book explains that both Oliver Cromwell and Charles II of England supported attempts to smelt iron with coal. In 1660, Dudley himself applied to Charles II for a patent, claiming that he could perfect the mastery of making iron with pit-coal or sea-coal .
Many attendant difficulties had to be overcome before this fuel could be applied to the purpose of smelting iron . Dudley does not describe in his book how he was using coal, only that he was. In so doing, he described his use successively of an ironworks on Pensnett Chase and at Cradley , of an industrial furnace at Himley , and of a furnace at Hasco Bridge near Gornal .
Dudley does mention several things that indicate what he was doing. The coal he used was the small pieces and slack which were "little or of no use in that inland country" and so brought in no money. This coal debris was left in heaps and "crowded moist slack heat naturally, and kindle in the middle of these great heaps, often sets the coal works on fire" and that "Also from these sulphurous heaps, mixed with ironstone (for out of many of the same pits is gotten much ironstone or mine), the fires heating vast quantities of water, passing through these soughs or adits becometh as hot as the bath at Bath ". [ 1 ]
Dudley describes two rival attempts to smelt iron with coal instigated by the Parliamentarians during the Civil War and the Interregnum . Dudley visited both sites and having examined their furnaces and production methods, when asked his opinion, informed the proprietors that they would fail. The first attempt was by Captain Buck, with the backing of many parliamentary officers including Oliver Cromwell , with technical help from Edward Dagney, an Italian. In the second attempt in the late 1656–67 by Captain John Copley also failed despite Dudley, at no charge, improving the efficiency of Copley's bellows. Dudley reapplied for a patent from Charles II of England , in 1660 stating "and seeing no man able to perform the mastery of making of iron with pit-coal or sea-coal, ... [without my] laudable inventions the author was, and is, unwilling [that they] should fall to the ground and die with him". [ 2 ]
A significant feature of his great work Metallum Martis is a map showing Dudley Castle where he correctly identifies the order and geographic layout of strata of coal and ironstone under survey.
Considered to be the earliest of recorded geological maps , Metallum Martis marks a turning point in the evolution of scientific rationale concerning the recording and interpretation of geological information. It is considered to have been made at Castle Hill in Dudley by Dud Dudley in 1665. | https://en.wikipedia.org/wiki/Metallum_Martis |
Metallurgical and Materials Engineering is a peer-reviewed Open Access scientific journal, published by the Association of Metallurgical Engineers of Serbia. [ 1 ] The first name of the journal was Metalurgija , published in 1995. The new name was adopted in 2012. The journal publishes contributions on fundamental and engineering aspects in the area of metallurgy and materials.
The journal publishes full length research papers, preliminary communications, reviews, and technical papers. [ 2 ]
This article about a materials science journal is a stub . You can help Wikipedia by expanding it .
See tips for writing articles about academic journals . Further suggestions might be found on the article's talk page . | https://en.wikipedia.org/wiki/Metallurgical_and_Materials_Engineering |
Metallurgical and Materials Transactions is a peer-reviewed scientific journal published in three sections ( A, B, and E ) covering metallurgy and materials science . The journals are jointly published by The Minerals, Metals & Materials Society and ASM International .
This monthly section focuses on physical metallurgy and materials science , and publishes international scientific contributions on all aspects of physical metallurgy and materials science, with a special emphasis on relationships among the processing, structure, and properties of materials.
This bimonthly section is uniquely focused on process metallurgy and materials processing science. Coverage emphasizes the theoretical and engineering aspects of the processing of metals and other materials, including studies of electro- and physical chemistry, mass transport, modeling, and related computer applications.
This article about a materials science journal is a stub . You can help Wikipedia by expanding it .
See tips for writing articles about academic journals . Further suggestions might be found on the article's talk page . | https://en.wikipedia.org/wiki/Metallurgical_and_Materials_Transactions |
A metallurgical assay is a compositional analysis of an ore , metal , or alloy , usually performed in order to test for purity or quality.
Some assay methods are suitable for raw materials; others are more appropriate for finished goods. Raw precious metals ( bullion ) are assayed by an assay office . Silver is assayed by titration , gold by cupellation and platinum by inductively coupled plasma optical emission spectrometry (ICP OES). [ 1 ] [ 2 ] Precious metal items of art or jewelry are frequently hallmarked (depending upon the requirements of the laws of either the place of manufacture or the place of import). Where required to be hallmarked , semi-finished precious metal items of art or jewelry pass through the official testing channels where they are analyzed or assayed for precious metal content. While different nations permit a variety of legally acceptable finenesses, the assayer is actually testing to determine that the fineness of the product conforms with the statement or claim of fineness that the maker has claimed (usually by stamping a number such as 750 for 18k gold) on the item. In the past the assay was conducted by using the touchstone method but currently (most often) it is done using X-ray fluorescence (XRF). XRF is used because this method is more exacting [ clarify ] than the touchstone test. The most exact method of assay is known as fire assay or cupellation. This method is better suited for the assay of bullion and gold stocks rather than works of art or jewelry because it is a completely destructive method.
The touchstone method is most common by far and does not damage the item in question. A rubbing of the item is made on a special stone, treated with acids and the result is compared to the result of the same process done on a sample of gold with a known purity. Red radiolarian chert or black siliceous slate were used for this. [ 3 ] Differences in precious metal content as small as 10 to 20 parts per thousand can often be established with confidence by the test, using acids and gold samples both of a specific, known concentration.
The modern X-ray fluorescence (XRF) is also a non-destructive technique that is suitable for normal assaying requirements. It typically has an accuracy of 2 to 5 parts per thousand and is well-suited to relatively flat and large surfaces. It is a quick technique taking about three minutes, and the results can be automatically printed out by computer.
One process for X-ray fluorescence assay involves melting the material in a furnace and stirring to make a homogeneous mix. Following this, a sample is taken from the centre of the molten sample. Samples are typically taken using a vacuum pin tube. [ 4 ] The sample is then tested by X-ray fluorescence spectroscopy . Metallurgical assay is typically completed in this way to ensure that an accurate assay is performed. [ citation needed ]
The most elaborately accurate, but totally destructive, precious metal assay is fire assay. (It may also be called by the critical cupellation step that separates precious metal from lead.) If performed on bullion to international standards, the method can be accurate on gold metal to 1 part in 10,000. If performed on ore materials using fusion followed by cupellation separation, detection may be in parts per billion. However, accuracy on ore material is typically limited to 3 to 5% of reported value. Although time-consuming, the method is the accepted standard applied for valuing gold ore as well as gold and silver bullion at major refineries and gold mining companies.
In the case of fire assaying of gold and platinum ores, the lengthy time required to carry out an assay is generally offset by carrying out large numbers of assays simultaneously, and a typical laboratory will be equipped with several fusion and cupellation furnaces, each capable of taking multiple samples, so that several hundred analyses per day can be carried out. The principal advantage of fire assay is that large samples can be used, and these increase the accuracy in analyzing low-yield ores in the <1g/T range of concentration.
Fusion: the process requires a self-generating reducing atmosphere, and so the crushed ore sample is mixed with fluxes and a carbon source (e.g. coal dust, ground charcoal, flour, etc.) mixed with powdered lead oxide (litharge) in a refractory crucible. In general, multiple crucibles will be placed inside an electric furnace fitted with silicon carbide heating elements, and heated to between 1,000 and 1,200 °C. The temperature required, and the type of flux used, are dependent on the composition of the rock in which the precious metals are concentrated, and in many laboratories an empirical approach based on long experience is used.
A complex reaction takes place, whereby the carbon source reduces the lead oxide to lead, which alloys with the precious metals: at the same time, the fluxes combine with the crushed rock, reducing its melting point and forming a glassy slag. When fusion is complete, the sample is tipped into a mold (usually iron) where the slag floats to the top, and the lead, now alloyed with the precious metals, sinks to the bottom, forming a 'button'. After solidification, the samples are knocked out, and the lead bullets recovered for cupellation, or for analysis by other means.
Method details for various fire assay procedures vary, but concentration and separation chemistry typically comply with traditions set by Bugby or Shepard & Dietrich in the early 20th century. Method advancements since that time primarily automate material handling and final finish measurements (i.e., instrument finish rather than physical gold product weighing). Arguably, even these texts are largely an extension of traditions that were detailed in De re metallica by Agricola in 1556.
Variation from skills taught in modern standard adaptations of fire assay methodology should be viewed with caution. The standard traditions have a long history of reliability; "special" new methods frequently associate with reduced assay accuracy and fraud .
Cupellation: the lead bullets are placed in porous crucibles (cupels) of bone ash or magnesium oxide and heated in air to about 1,000 °C. This is usually carried out in a 'muffle' furnace, containing a refractory muffle (usually nitride-bonded silicon carbide) heated externally by silicon carbide heating elements. A flow of air through the muffle assists oxidation of the lead, and carries the fumes for safe collection outside the furnace unit. The lead melts and oxidises to lead oxide, which in turn melts and is drawn into the pores of the cupel by capillary attraction. The precious metals remain in the base of the cupel as a 'prill' which is sent for final analysis of precious metal content.
In the bullion fire assay process, a sample from the article is wrapped in a lead foil with copper and silver. The wrapped sample, along with prepared control samples, heated at 1,650 °F (or 898.9 °C; temperature varies with exact method) in a cupel made of compressed bone ash or magnesium oxide powder. Base metals oxidize and absorb into the cupel. The product of this cupellation (doré) is flattened and treated in nitric acid to remove silver. Precision weighing of metal content of samples and process controls (proofs) at each process stage is the basis of the extreme method precision. European assayers follow bullion traditions based in hallmarking regulations. Reputable North American bullion assayers conform closely to ASTM method E1335-04e1 . Only bullion methods validated and traceable to accepted international standards obtain genuine accuracies of 1 part in 10,000.
Cupellation alone can only remove a limited quantity of impurities from a sample. Fire assay, as applied to ores, concentrates, or less pure metals, adds a fusion or scorification step before cupellation.
A coin assayer is often assigned to each mint or assay office to determine and assure that all coins produced at the mint have the correct content or purity of each metal specified, usually by law, to be contained in them. This was particularly important when gold and silver coins were produced for circulation and used in daily commerce. Few nations, however, persist in minting silver or gold coins for general circulation. For example, the U.S. discontinued the use of gold in coinage in 1933. The U.S. was one of the last nations to discontinue the use of silver in circulating coins after its 1970 A.D. half dollar coin, although the amount of silver used in smaller denomination coins was ended after 1964. Even with the half dollar, the amount of silver used in the coins was reduced from 90% in 1964 and earlier to 40% between 1965 and 1970. Copper, nickel, cupro-nickel and brass alloys now predominate in coin making. Notwithstanding, several national mints, including the Perth Mint in Australia, the Austrian Mint, the British Royal Mint, the Royal Canadian Mint, the South African Mint, and the U.S. Mint continue to produce precious metal bullion coins for collectors and investors. The precious metal purity and content of these coins is guaranteed by the respective mint or government, and, therefore, the assay of the raw materials and finished coins is an important quality control.
In the UK, the Trial of the Pyx is a ceremonial procedure for ensuring that newly minted coins conform to required standards. | https://en.wikipedia.org/wiki/Metallurgical_assay |
Metallurgical failure analysis is the process to determine the mechanism that has caused a metal component to fail . It can identify the cause of failure, providing insight into the root cause and potential solutions to prevent similar failures in the future, as well as culpability, which is important in legal cases. [ 1 ] Resolving the source of metallurgical failures can be of financial interest to companies. The annual cost of corrosion (a common cause of metallurgical failures) in the United States was estimated by NACE International in 2012 to be $450 billion a year, a 67% increase compared to estimates for 2001. [ 1 ] These failures can be analyzed to determine their root cause, which if corrected, would save reduce the cost of failures to companies.
Failure can be broadly divided into functional failure and expected performance failure. Functional failure occurs when a component or process fails and its entire parent system stops functioning entirely. This category includes the common idea of a component fracturing rapidly. Expected performance failures are when a component causes the system to perform below a certain performance criterion, such as life expectancy, operating limits, or shape and color. Some performance criteria are documented by the supplier, such as maximum load allowed on a tractor, while others are implied or expected by the customer, such gas consumption ( miles per gallon for automobiles). [ 1 ]
Often a combination of both environmental conditions and stress will cause failure. Metal components are designed to withstand the environment and stresses that they will be subjected to. The design of a metal component involves not only a specific elemental composition but also specific manufacturing process such as heat treatments, machining processes, etc. The huge arrays of different metals that result all have unique physical properties. Specific properties are designed into metal components to make them more robust to various environmental conditions. These differences in physical properties will exhibit unique failure modes. A metallurgical failure analysis takes into account as much of this information as possible during analysis. The ultimate goal of failure analysis is to provide a determination of the root cause and a solution to any underlying problems to prevent future failures. [ 2 ]
The first step in failure analysis is investigating the failure to collect information. The sequence of steps for information gathering in a failure investigation are: [ 1 ] [ 3 ]
Various techniques are used in the investigative process of metallurgical failure analysis. [ 1 ] [ 3 ]
Non-destructive testing : Non-destructive testing is a test method that allows certain physical properties of metal to be examined without taking the samples completely out of service. NDT is generally used to detect failures in components before the component fails catastrophically.
Destructive testing : Destructive testing involves removing a metal component from service and sectioning the component for analysis. Destructive testing gives the failure analyst the ability to conduct the analysis in a laboratory setting and perform tests on the material that will ultimately destroy the component.
There is no standardized list of metallurgical failure modes and different metallurgists might use a different name for the same failure mode. The failure mode terms listed below are those accepted by ASTM , [ 4 ] ASM , [ 5 ] and/or NACE [ 6 ] as distinct metallurgical failure mechanisms.
Potential root causes of metallurgical failures are vast, spanning the lifecycle of component from design to manufacturing to usage. The most common reasons for failures can be classified into the following categories: [ 1 ]
Failures due to service or operation conditions includes using a component outside of its intended conditions, such as an impact force or a high load. It can also include failures due to unexpected conditions in usage, such as an unexpected contact point that causes wear and abrasion or an unexpected humidity level or chemical presence that causes corrosion. These factors result in the component failing at an earlier time than expected.
Improper maintenance would cause potential sources of fracture to go untreated and lead to premature failure of a component in the future. The reason for improper maintenance could be either intentional, such as skipping a yearly maintenance to avoid the cost, or unintentional, such as using the wrong engine oil.
Testing and/or inspection are typically included in component manufacturing lines to verify the product meets some set of standards to ensure the desired performance in the field. Improper testing or inspection would circumvent these quality checks and could allow a part with a defect that would normally disqualify the component from field use to be sold to a customer, potentially leading to a failure.
Manufacturing or fabrication errors occur during the processing of the material or component. For metal parts, casting defects are common, such as cold shut, hot tears or slag inclusions. It can also be surface treatment problems, processing parameters such as ramming a sand mold or wrong temperature during hardening.
Design errors arise when the desired use case was not properly accounted for, leading to a ineffective design, such as the stress state in service or potential corrosive agents in the service environment. Design errors often include dimensioning and materials selection, but it can also be the complete design.
Computational methods have been increasing in popularity as a method to test possible root because they do not need to sacrifice a component to prove a root cause. Common cases where computational methods are used are for failures due to erosion , [ 8 ] [ 9 ] failures of components under complex stress states, [ 10 ] [ 11 ] and for predictive analyses. [ 12 ] [ 13 ] [ 14 ] [ 15 ] Computational fluid dynamics is used to determine the flow pattern and shear stresses on a component that had failed due to erosive wear. [ 8 ] [ 9 ] Finite element analysis is used to model components under complex stress states. [ 10 ] [ 11 ] Finite element analysis as well as phase field models can be used for predicting crack propagation and failure, [ 12 ] [ 13 ] [ 14 ] [ 15 ] which are then used to prevent failure by influencing component design. | https://en.wikipedia.org/wiki/Metallurgical_failure_analysis |
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements , their inter-metallic compounds , and their mixtures, which are known as alloys .
Metallurgy encompasses both the science and the technology of metals, including the production of metals and the engineering of metal components used in products for both consumers and manufacturers. Metallurgy is distinct from the craft of metalworking . Metalworking relies on metallurgy in a similar manner to how medicine relies on medical science for technical advancement. A specialist practitioner of metallurgy is known as a metallurgist.
The science of metallurgy is further subdivided into two broad categories: chemical metallurgy and physical metallurgy . Chemical metallurgy is chiefly concerned with the reduction and oxidation of metals, and the chemical performance of metals. Subjects of study in chemical metallurgy include mineral processing , the extraction of metals , thermodynamics , electrochemistry , and chemical degradation ( corrosion ). [ 1 ] In contrast, physical metallurgy focuses on the mechanical properties of metals, the physical properties of metals, and the physical performance of metals. Topics studied in physical metallurgy include crystallography , material characterization , mechanical metallurgy, phase transformations , and failure mechanisms . [ 2 ]
Historically, metallurgy has predominately focused on the production of metals. Metal production begins with the processing of ores to extract the metal, and includes the mixture of metals to make alloys . Metal alloys are often a blend of at least two different metallic elements. However, non-metallic elements are often added to alloys in order to achieve properties suitable for an application. The study of metal production is subdivided into ferrous metallurgy (also known as black metallurgy ) and non-ferrous metallurgy , also known as colored metallurgy.
Ferrous metallurgy involves processes and alloys based on iron , while non-ferrous metallurgy involves processes and alloys based on other metals. The production of ferrous metals accounts for 95% of world metal production. [ 3 ]
Modern metallurgists work in both emerging and traditional areas as part of an interdisciplinary team alongside material scientists and other engineers. Some traditional areas include mineral processing, metal production, heat treatment, failure analysis , and the joining of metals (including welding , brazing , and soldering ). Emerging areas for metallurgists include nanotechnology , superconductors , composites , biomedical materials , electronic materials (semiconductors) and surface engineering .
Metallurgy derives from the Ancient Greek μεταλλουργός , metallourgós , "worker in metal", from μέταλλον , métallon , "mine, metal" + ἔργον , érgon , "work" The word was originally an alchemist 's term for the extraction of metals from minerals, the ending -urgy signifying a process, especially manufacturing: it was discussed in this sense in the 1797 Encyclopædia Britannica . [ 4 ]
In the late 19th century, metallurgy's definition was extended to the more general scientific study of metals, alloys, and related processes. In English , the / m ɛ ˈ t æ l ər dʒ i / pronunciation is the more common one in the United Kingdom . The / ˈ m ɛ t əl ɜːr dʒ i / pronunciation is the more common one in the US and is the first-listed variant in various American dictionaries, including Merriam-Webster Collegiate and American Heritage .
The earliest metal employed by humans appears to be gold , which can be found " native ". Small amounts of natural gold, dating to the late Paleolithic period, 40,000 BC, have been found in Spanish caves. [ 5 ] Silver , copper , tin and meteoric iron can also be found in native form, allowing a limited amount of metalworking in early cultures. [ 6 ] Early cold metallurgy, using native copper not melted from mineral has been documented at sites in Anatolia and at the site of Tell Maghzaliyah in Iraq , dating from the 7th/6th millennia BC. [ 7 ] [ 8 ] [ 9 ]
The earliest archaeological support of smelting (hot metallurgy) in Eurasia is found in the Balkans and Carpathian Mountains , as evidenced by findings of objects made by metal casting and smelting dated to around 6200–5000 BC, with the invention of copper metallurgy. [ 10 ] [ 8 ] [ 9 ] Certain metals, such as tin, lead, and copper can be recovered from their ores by simply heating the rocks in a fire or blast furnace in a process known as smelting. The first evidence of copper smelting, dating from the 6th millennium BC, [ 11 ] has been found at archaeological sites in Majdanpek , Jarmovac and Pločnik , in present-day Serbia . [ 12 ] [ 8 ] The site of Pločnik has produced a smelted copper axe dating from 5,500 BC, belonging to the Vinča culture . [ 13 ] The Balkans and adjacent Carpathian region were the location of major Chalcolithic cultures including Vinča , Varna , Karanovo , Gumelnița and Hamangia , which are often grouped together under the name of ' Old Europe '. [ 14 ] With the Carpatho-Balkan region described as the 'earliest metallurgical province in Eurasia', [ 15 ] its scale and technical quality of metal production in the 6th–5th millennia BC totally overshadowed that of any other contemporary production centre. [ 16 ] [ 17 ] [ 18 ]
The earliest documented use of lead (possibly native or smelted) in the Near East dates from the 6th millennium BC, is from the late Neolithic settlements of Yarim Tepe and Arpachiyah in Iraq . The artifacts suggest that lead smelting may have predated copper smelting. [ 19 ] Metallurgy of lead has also been found in the Balkans during the same period. [ 8 ]
Copper smelting is documented at sites in Anatolia and at the site of Tal-i Iblis in southeastern Iran from c. 5000 BC . [ 7 ]
Copper smelting is first documented in the Delta region of northern Egypt in c. 4000 BC , associated with the Maadi culture . This represents the earliest evidence for smelting in Africa. [ 20 ]
The Varna Necropolis , Bulgaria , is a burial site located in the western industrial zone of Varna , approximately 4 km from the city centre, internationally considered one of the key archaeological sites in world prehistory. The oldest gold treasure in the world, dating from 4,600 BC to 4,200 BC, was discovered at the site. [ 21 ] The gold piece dating from 4,500 BC, found in 2019 in Durankulak , near Varna is another important example. [ 22 ] [ 23 ] Other signs of early metals are found from the third millennium BC in Palmela , Portugal, Los Millares , Spain, and Stonehenge , United Kingdom. The precise beginnings, however, have not be clearly ascertained and new discoveries are both continuous and ongoing.
In approximately 1900 BC, ancient iron smelting sites existed in Tamil Nadu . [ 24 ] [ 25 ]
In the Near East , about 3,500 BC, it was discovered that by combining copper and tin, a superior metal could be made, an alloy called bronze . This represented a major technological shift known as the Bronze Age .
The extraction of iron from its ore into a workable metal is much more difficult than for copper or tin. The process appears to have been invented by the Hittites in about 1200 BC, beginning the Iron Age . The secret of extracting and working iron was a key factor in the success of the Philistines . [ 26 ] [ 27 ]
Historical developments in ferrous metallurgy can be found in a wide variety of past cultures and civilizations. This includes the ancient and medieval kingdoms and empires of the Middle East and Near East , ancient Iran , ancient Egypt , ancient Nubia , and Anatolia in present-day Turkey , Ancient Nok , Carthage , the Celts , Greeks and Romans of ancient Europe , medieval Europe, ancient and medieval China , ancient and medieval India , ancient and medieval Japan , amongst others.
A 16th century book by Georg Agricola , De re metallica , describes the highly developed and complex processes of mining metal ores, metal extraction, and metallurgy of the time. Agricola has been described as the "father of metallurgy". [ 28 ]
Extractive metallurgy is the practice of removing valuable metals from an ore and refining the extracted raw metals into a purer form. In order to convert a metal oxide or sulphide to a purer metal, the ore must be reduced physically, chemically , or electrolytically . Extractive metallurgists are interested in three primary streams: feed, concentrate (metal oxide/sulphide) and tailings (waste).
After mining, large pieces of the ore feed are broken through crushing or grinding in order to obtain particles small enough, where each particle is either mostly valuable or mostly waste. Concentrating the particles of value in a form supporting separation enables the desired metal to be removed from waste products.
Mining may not be necessary, if the ore body and physical environment are conducive to leaching . Leaching dissolves minerals in an ore body and results in an enriched solution. The solution is collected and processed to extract valuable metals. Ore bodies often contain more than one valuable metal.
Tailings of a previous process may be used as a feed in another process to extract a secondary product from the original ore. Additionally, a concentrate may contain more than one valuable metal. That concentrate would then be processed to separate the valuable metals into individual constituents.
Much effort has been placed on understanding iron –carbon alloy system, which includes steels and cast irons . Plain carbon steels (those that contain essentially only carbon as an alloying element) are used in low-cost, high-strength applications, where neither weight nor corrosion are a major concern. Cast irons, including ductile iron , are also part of the iron-carbon system. Iron-Manganese-Chromium alloys (Hadfield-type steels) are also used in non-magnetic applications such as directional drilling.
Other engineering metals include aluminium , chromium , copper , magnesium , nickel , titanium , zinc , and silicon . These metals are most often used as alloys with the noted exception of silicon, which is not a metal. Other forms include:
In production engineering , metallurgy is concerned with the production of metallic components for use in consumer or engineering products. This involves production of alloys, shaping, heat treatment and surface treatment of product. The task of the metallurgist is to achieve balance between material properties, such as cost, weight , strength , toughness , hardness , corrosion , fatigue resistance and performance in temperature extremes. To achieve this goal, the operating environment must be carefully considered. [ citation needed ]
Determining the hardness of the metal using the Rockwell, Vickers, and Brinell hardness scales is a commonly used practice that helps better understand the metal's elasticity and plasticity for different applications and production processes. [ 29 ] In a saltwater environment, most ferrous metals and some non-ferrous alloys corrode quickly. Metals exposed to cold or cryogenic conditions may undergo a ductile to brittle transition and lose their toughness, becoming more brittle and prone to cracking. Metals under continual cyclic loading can suffer from metal fatigue . Metals under constant stress at elevated temperatures can creep .
Cold-working processes, in which the product's shape is altered by rolling, fabrication or other processes, while the product is cold, can increase the strength of the product by a process called work hardening . Work hardening creates microscopic defects in the metal, which resist further changes of shape.
Metals can be heat-treated to alter the properties of strength, ductility, toughness, hardness and resistance to corrosion. Common heat treatment processes include annealing, precipitation strengthening , quenching, and tempering: [ 31 ]
Often, mechanical and thermal treatments are combined in what are known as thermo-mechanical treatments for better properties and more efficient processing of materials. These processes are common to high-alloy special steels, superalloys and titanium alloys.
Electroplating is a chemical surface-treatment technique. It involves bonding a thin layer of another metal such as gold , silver , chromium or zinc to the surface of the product. This is done by selecting the coating material electrolyte solution, which is the material that is going to coat the workpiece (gold, silver, zinc). There needs to be two electrodes of different materials: one the same material as the coating material and one that is receiving the coating material. Two electrodes are electrically charged and the coating material is stuck to the work piece. It is used to reduce corrosion as well as to improve the product's aesthetic appearance. It is also used to make inexpensive metals look like the more expensive ones (gold, silver). [ 32 ]
Shot peening is a cold working process used to finish metal parts. In the process of shot peening, small round shot is blasted against the surface of the part to be finished. This process is used to prolong the product life of the part, prevent stress corrosion failures, and also prevent fatigue. The shot leaves small dimples on the surface like a peen hammer does, which cause compression stress under the dimple. As the shot media strikes the material over and over, it forms many overlapping dimples throughout the piece being treated. The compression stress in the surface of the material strengthens the part and makes it more resistant to fatigue failure, stress failures, corrosion failure, and cracking. [ 33 ]
Thermal spraying techniques are another popular finishing option, and often have better high temperature properties than electroplated coatings. Thermal spraying, also known as a spray welding process, [ 34 ] is an industrial coating process that consists of a heat source (flame or other) and a coating material that can be in a powder or wire form, which is melted then sprayed on the surface of the material being treated at a high velocity. The spray treating process is known by many different names such as HVOF (High Velocity Oxygen Fuel), plasma spray, flame spray, arc spray and metalizing.
Electroless deposition (ED) or electroless plating is defined as the autocatalytic process through which metals and metal alloys are deposited onto nonconductive surfaces. These nonconductive surfaces include plastics, ceramics, and glass etc., which can then become decorative, anti-corrosive, and conductive depending on their final functions. Electroless deposition is a chemical processes that create metal coatings on various materials by autocatalytic chemical reduction of metal cations in a liquid bath.
Metallurgists study the microscopic and macroscopic structure of metals using metallography , a technique invented by Henry Clifton Sorby .
In metallography, an alloy of interest is ground flat and polished to a mirror finish. The sample can then be etched to reveal the microstructure and macrostructure of the metal. The sample is then examined in an optical or electron microscope , and the image contrast provides details on the composition, mechanical properties, and processing history.
Crystallography , often using diffraction of x-rays or electrons , is another valuable tool available to the modern metallurgist. Crystallography allows identification of unknown materials and reveals the crystal structure of the sample. Quantitative crystallography can be used to calculate the amount of phases present as well as the degree of strain to which a sample has been subjected.
Current advanced characterization techniques, which are used frequently in this field are: Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Electron Backscattered Diffraction (EBSD) and Atom-Probe Tomography (APT). | https://en.wikipedia.org/wiki/Metallurgy |
West Asia (6000–3500 BC)
Europe (5500–2200 BC)
Central Asia (3700–1700 BC)
South Asia (4300–1800 BC)
China (5000–2900 BC)
The Copper Age , also called the Eneolithic or the Chalcolithic Age, has been traditionally understood as a transitional period between the Neolithic and the Bronze Age , in which a gradual introduction of the metal ( native copper ) took place, while stone was still the main resource utilized. Recent archaeology has found that the metal was not introduced so gradually and that this entailed significant social changes, such as developments in the type of habitation (larger villages, launching of fortifications), long-distance trade, and copper metallurgy .
Some of the earliest Copper Age artifacts were found in the 5th and 6th millennia BCE archaeological sites of the Vinča culture such as Majdanpek , Jarmovac and Pločnik (including a copper axe from 5500 BCE). Somewhat later, in the 5th millennium BCE, metalwork is attested at Rudna Glava mine in Serbia , and at Ai Bunar mine in Bulgaria . [ 1 ]
3rd millennium BCE copper metalwork is attested in places like Palmela ( Portugal ), Cortes ( Navarre ), and Stonehenge ( England ). However, as often happens with the prehistoric times, the limits of the age cannot be clearly defined and vary between different sources.
The theory that metallurgy was imported into Europe from the Near East has been practically ruled out. A second hypothesis, that there were two main points of origin of metallurgy in Europe, in southern Spain and in West Bulgaria, is also doubtful due to the existence of sites outside the centers of diffusion where metallurgy was known simultaneously with, or before, those in the ‘original’ nuclei, such as Brixlegg (Tyrol, Austria), while sites closer to the supposed origins of metallurgy, such as in the north of Spain, show fewer metal artifacts than sites in the south and practically no evidence of production. [ 2 ]
Currently, the general opinion is that the development of metallurgy took place independently in different places, at different times, with various techniques. One fact that supports this interpretation is that, although the final products (beads, rings, sickles, swords, axes, etc.) are quite similar throughout Europe, the method of production is not. Thus the use of crucibles was the technique utilized in the south of Spain, whereas central Europe employed a slagging process , but Cabrierés (France) used a primitive oxidizing non-slagging process, [ 3 ] while in the British Isles the absence of debris, slag or ceramic suggests another technique. [ 4 ]
Consequently, the way in which metallurgy was initiated differs considerably depending on the region. There are areas in which copper seems to play a crucial role (i.e., the Balkans), whereas other areas show no interest in it at all. Then there are societies that use copper artifacts, but do not practice metallurgy, [ 5 ] and there are other ones that fully adopt some of the cultural innovations but ignore the rest. [ who? ] One example of the latter is Basque country in northern Spain, where splendid large dolmens are present along the Ebro river , but metal is rather infrequent, and when it does appear between the trapping, it is more often bronze or arsenical copper than copper . [ 6 ]
Copper is the eighth most abundant metal in the Earth's crust, is available all over the world, and is one of the few that can appear in a pure state. [ 7 ] It is not complicated to work with, and a bare hammering can be enough to transform a nugget into a bead. The eye-catching look of native copper makes it easy to recognize, and even flashier if converted into jewelry, a possible motivation for humankind to start metallurgy with it. An evolutive technological process has been described, [ 8 ] although there are authors like Javinovic, [ 9 ] who think that it is not necessary to pass through the first stages to reach the last one.
To start with, the raw material must be obtained. Copper can be found in over 160 different minerals, [ 7 ] but mining activities are entailed to obtain them in large quantities if a reasonable amount of copper is wanted. Some of the most commonly exploited minerals are cuprite , malachite , azurite , chalcopyrite , chrysocolla and tennantite ; e.g. malachite was extracted in Rudna Glava (Serbia), Cabrierés (France) or Chinflón ( Riotinto , Spain). In fact, one of the possible explanations about what Ötzi the Iceman , the ancient mummy found in the Alps who lived around 3300 years BCE, was doing at 3,210 metres (10,530 ft) of altitude is that he could have been prospecting for new ores of minerals. [ 10 ]
Secondly, the mineral is separated from the gangue . This is only possible by smelting or beneficiation . To do so, it is necessary to use a furnace that is able to reach at least 1,089 °C (1,992 °F).
Lastly, a wide range of specific tools and resources have to be available, such as furnaces, moulds, crucibles , mauls, etc.
Minerals of copper were known from ancient times. In Crete, little fragments of malachite and azurite were powdered and used as make up or to decorate ceramic as early as 6000 BCE. [ 5 ]
Therefore, the minerals were not collected because people were looking for copper but for virtues like those mentioned or simply because of its brightness and colour, but this knowledge of the minerals is critical since they already knew how to recognize them and where to collect them when, later, they started the systematic search for ores.
Numerous examples of mines are known all over Europe, [ 11 ] from the east: Rudna Glava (Serbia), Ai Bunar (Bulgaria); to the west: Mount Gabriel (Ireland), Great Orme , Alderley Edge (United Kingdom); crossing Central Europe: Mitterberg (Salzach, Austria), Neuchâtel (Switzerland), Cabrierés (France); to the south: Riotinto , Mola Alta de Serelles (Spain); and the Mediterranean: Corsica , Cyprus, and the Cyclades islands. It is remarkable that, usually, it is not a single mine but a complex, with a variable, large number of mineshafts, as in Rudna Glava (30) or Mount Gabriel (31). [ citation needed ]
The techniques observed in all of them are quite similar. Basically they used the thermic alteration or firesetting (Mohen 1992, Craddock 1995, Eiroa et al. 1996, Timberlake 2003). This consists of applying fire to the rock and then pouring water over it: the rapid changes of temperature will cause cracks within the rocks that can be totally broken with the help of mauls and picks. Then the useful masses were selected, crushed and transported to the production centre that could be in the surrounding area ( Mitterberg ) or far away ( Rudna Glava ).
The mines were exploited in extremely efficient and clever ways, according to the technology available (Jovanovic 1980, Craddock 1995, Timberlake 2003). The entire convenient mineral was collected and the abandoned shafts carefully refilled with gangue and rocks (Mohen 1992; 85). For example, at Mount Gabriel , it was estimated that they extracted the astonishing number of 32,570.15 tonnes (35,902.44 tons) of rock, gangue and ore. The usable amount of copper was 162.85 tonnes and the final smelting finished metal was 146.56 tonnes (Jackson 1980; 24). The entire process was thoroughly described in 1744 by Lewis Morris, Crown Mineral Agent for Cardiganshire, and, incidentally, antiquarian . [ 12 ]
Their method seems to be this. They make a great fire of wood in the bottom of their rakes which were always open up on that account, and when the rock was sufficiently hot they cast water upon it, which shiver’d it; and then with stone wedges, which they drove in with other stones, they work’d their way through the hardest rocks, tho’ but slowly.
The tools employed are mainly presented in Lewis' observations, but other ones have been recovered in archaeological context:
The information available about the people of the Copper Age has not substantially increased along with the number of archaeological sites. Several ideas have been proffered, one of the most followed is that the metal itself did not bring abrupt transformation into the people's life, [ 17 ] or even more that early copper does not produce anything useful at all, [ 18 ] meaning with this that with the copper, they produced mainly jewellery and, overall, weapons that obviously were not within reach of the majority of the population but only to privileged individuals. In other words, the real importance of the metal is not utilitarian but social. This is a suitable explanation about the rising of Great Cultures of Metal such as Vinča culture (Ex-Yugoslavia) Tiszapolgar , Remedello and Rinaldone (Italy), Montagne Noire (France), El Argar and Targas (Spain), etc.
Between 4000 and 3300 BC, copper artefacts, mainly from Serbian mines, reached northern Germany and southern Scandinavia. [ 19 ] The metal was probably redistributed by groups in the eastern Alps. Societies of the Funnel Beaker Culture in the north of central and northern Europe were able to cast the copper into different types and they deposited it mainly in hoards. There is no reason to believe that copper was the exclusive preserve of privileged individuals. In contrast to the aforementioned regions, copper artefacts had a more symbolic value. [ 19 ]
As the period moved forward, especially around the 3rd millennium, new and complex realities would appear strongly linked to the metal, like the impressive fortified villages of Los Millares (Spain), Vila Nova de Sao Pedro (Portugal) or the more modest cairn next to Copa Hill in the United Kingdom destinated to control the centres of extraction, or the equally and generalized cultural phenomenons of Megalithism , Rock Art , Bell Beakers Vessels that are known from Scandinavia to the South of Spain and from Scotland to Turkey. | https://en.wikipedia.org/wiki/Metallurgy_during_the_Copper_Age_in_Europe |
The metallurgical production of the Republic of Azerbaijan is considered high due to the large deposits of alunite , polymetallic ores , deposits of iron ore , etc. The metallurgy industry of Azerbaijan encompasses both ferrous and non-ferrous branches.
Ferrous metallurgy includes extraction of iron , smelting and refining of iron ore, rolling and ferroalloys production. The ferrous metallurgy production of the country started to meet the demand of oil and gas industry (due to pipe production) and grew further in order to improve other branches of the industry. Dashkasan iron ore in 4 deposits ( Dashkesen , South Dashkasan , Hamanchay , Demiroglu ) in the valley of Goshagarchay plays a key role in the development of ferrous metallurgy. The cities of Baku , Sumgait and Dashkesan are major centers of metallurgy in terms of extraction and processing of iron ore. The Sumgait Pipe-Rolling Plant produces drill pipes , casing, tubing, oil and gas pipes, etc. Bentonite clay deposits in the village of Dash Salakhly , Gazakh district, is used in steel smelting. Baku Steel Company , the largest metallurgical enterprise in Azerbaijan, was opened in 2001 on the initiative of Heydar Aliyev . With two electric arc furnaces and three rolling lines, the annual steel production capacity of the company increased to 1,000,000 tons. [ 1 ] [ 2 ] [ 3 ] [ 4 ]
Aluminum , copper , molybdenum , cobalt , mercury reserves and most importantly electricity for the smelting process has led to the development of non-ferrous metallurgy . The Zeylik mine in Daskasan district is the main provider of the alunite for aluminum production. The extracted ore here transported through Guschu-Alabashli railway to the aluminum plant located in Ganja city. The obtained aluminum oxide is brought to Sumgayit aluminum plant in order to produce aluminum. Ganja Aluminum Plant produces sulfuric acid , aluminum oxide, and potassium fertilizer through extracted ore from Zalik deposit in Dashkesen. Aluminum oxide is also produced in Sumgait. AzerGold CJSC (created by the Presidential Decree No. 1047 on February 11, 2015) implements exploration, management, and also extraction, processing and sale of precious and non-ferrous metal ore deposits located within the borders of the country. In 2017, the volume of exports of precious metals carried out by this company amounted to 77340 million dollars. [ 3 ] [ 5 ]
Gold mining began in Gedebey in 2009. In 2016, Azer Gold CJSC began gold mining in the Chovdar deposit. In 2017, 6,390.8 kg of gold was mined (which exceeded the 2016 production by 3.4 times. Gold production in January–May 2018 amounted to 2,081.7 kg, which exceeds last year's data by 19.5%. In the first quarter of 2018, the company's exports amounted to $30 million. In 2017, 59,617 ounces of gold produces by Anglo Asian Mining PLC (the main gold producer in Azerbaijan) from Gadir Ugur and Gosha deposits. [ 5 ] [ 6 ] | https://en.wikipedia.org/wiki/Metallurgy_in_Azerbaijan |
Metallurgy in pre-Columbian America is the extraction, purification and alloying of metals and metal crafting by Indigenous peoples of the Americas prior to European contact in the late 15th century. Indigenous Americans had been using native metals from ancient times, with gold artifacts from the Andean region being dated to 2155–1936 BC, [ 1 ] and North American copper artifacts being dated to approximately 5000 BC. [ 2 ] The metal would have been found in nature without the need for smelting , and shaped into the desired form using hot and cold hammering without chemical alteration or alloying . As of 1999, "no one has found evidence that points to the use of melting, smelting and casting in prehistoric eastern North America." [ 3 ] (p 136)
In South America the case is quite different. Indigenous South Americans had full metallurgy with smelting and various metals being purposely alloyed. Metallurgy in Mesoamerica and western Mexico may have developed following contact with South America through Ecuadorian marine traders. [ 4 ]
South American metal working seems to have developed in the Andean region of modern Peru , Bolivia , Ecuador , Chile , and Argentina with gold and native copper being hammered and shaped into intricate objects, particularly ornaments . [ 1 ] [ 5 ] Recent finds date the earliest gold work to 2155–1936 BC. [ 1 ] and the earliest copper work to 1432–1132 BC. [ 5 ] [ 6 ] Ice core studies in Bolivia suggest copper smelting may have begun as early as 700 BC, over 2700 years ago. [ 7 ] By 1410–1090 BC, gilding was practiced in coastal Peru. [ 8 ] Further evidence for this type of metal work comes from the sites at Waywaka (near Andahuaylas in southern Peru), Chavín and Kotosh , [ 9 ] and it seems to have been spread throughout Andean societies by the Early horizon (1000–200 BC).
Unlike other metallurgy traditions where metals gained importance through practical use in weaponry and everyday utensils, metals in South America (and later Central America) were mainly valued as adornments and status objects. Though also functional objects were produced, even in the metallurgically advanced Andean cultures of the Inca era stone tools were never completely replaced by bronze items in everyday life. [ 10 ] During the Early horizon, advances in metal working produced spectacular and characteristic Andean gold objects made by the joining of smaller metal sheets, and also gold-silver alloy appeared.
Two traditions seem to have developed alongside each other – one in northern Peru and Ecuador, and another in the Altiplano region of southern Peru, Bolivia and Chile. There is evidence for smelting of copper sulphide in the Altiplano region around the Early horizon. Evidence for this comes from copper slag recovered at several sites, [ 11 ] with the ore itself possibly coming from the south Chilean-Bolivian border. Near Puma Punku, Bolivia, and at three additional sites in Peru and Bolivia, portable smelting kilns were used to cast I-shaped "cramps" (fasteners) in place, to join large stone blocks during construction. [ citation needed ] Their chemical analysis shows
The estimated date of these pours lies between 800 and 500 BC.
Evidence for fully developed smelting, however, only appears with the Moche culture (northern coast, 200 BC – 600 CE). [ 12 ] The ores were extracted from shallow deposits in the Andean foothills. They were probably smelted nearby, as pictorially depicted on the metal artifacts themselves and on ceramic vessels. Smelting was done in adobe brick furnaces with at least three blow pipes to provide the air flow needed to reach the high temperatures. The resulting ingots would then have been moved to coastal centers for shaping in specialised workshops. [ 13 ] Two workshops studied were near the administrative sections of their towns, again showing the prestige of metal. Analysis of a Moche statue composed of numerous thin metal layers revealed complex plating and gilding involving a combination of immersion in acidic solutions and the application of extreme heat. [ 14 ]
The objects themselves were still mainly adornments, now often being attached to beads. In fact, in the Lambayeque and Chimu cultures (750–1400 CE), a wide range of functional metal items were produced such as bowls, plates, drinking vessels, boxes, plates, models, scales and especially beakers (acquillas), but mostly for ceremonial or elite use. [ 15 ] Some functional objects were fashioned, but they were elaborately decorated and often found in high-status burials, seemingly still used more for symbolic than for practical purposes. The appearance of gold or silver seems to have been important, with a high number of gilded or silvered objects as well as the appearance of Tumbaga , an alloy of copper and gold, and sometimes also silver. Arsenic bronze [ 16 ] was also smelted from sulphidic ores, a practice either independently developed or learned from the southern tradition. The earliest known powder metallurgy, and earliest working of platinum in the world, was apparently developed by the cultures of Esmeraldas (northwest Ecuador) before the Spanish conquest [ 17 ] Beginning with the La Tolita culture (600 BC – 200 CE), Ecuadorian cultures mastered the soldering of platinum grains through alloying with copper, gold and silver, producing platinum-surfaced rings, handles, ornaments and utensils. This technology was eventually noticed and adopted by the Spanish c. 1730 . [ 18 ]
Coastal communities in the Atacama Desert , as exemplified by those near Tocopilla , produced their own metal objects for practical use in the 900–1400 CE period. [ 19 ]
Metallurgy gradually spread north into Colombia, Panama and Costa Rica, reaching Guatemala and Belize by 800 CE. By c. 100–700 CE , depletion gilding was developed by the Nahuange culture of Colombia to produce ornamental variations such as rose gold . [ 20 ] Muisca goldworking , from modern Colombia , made a wide variety of small ornamental and religious objects from about 600 CE onwards. The gold Muisca raft is probably the best-known single object. This is in the Gold Museum, Bogotá , the largest of the six "gold museums" owned by the Central Bank of Colombia displaying gold from the Muisca and other pre-Columbian cultures in the country.
Many copper objects produced in the Tiwanaku polity (c. 600–c. 1000) are alloys that are characterized by having approximately 5% arsenic and 4% nickel . [ 21 ]
Only with the Incas would metals really come into practical use. [ citation needed ] At Machu Picchu and other sites, metal was used for bolas , plumb bobs , chisels , gravers , pry bars , tweezers , needles , plates , fish hooks , spatulas , ladles , knives (tumi), bells , breastplates , lime spoons , mace heads, ear spools , bowls , cloak pins (tupus), axes , and foot plough adzes. [ 22 ] Nonetheless, they remained materials through which to display wealth and status. The characteristic importance placed on colour, which had led to some of the earlier developments, was still present (sun/moon association with gold/silver). Metals other than gold also had an intrinsic value, with axe pieces being of particular note in this regard. With the spread of metal tools by the Incas, it is thought possible that a more Old World use of metals would have become more common. In any case, as Bruhns notes, "[b]ronze can be seen as an expensive substitute for the equally efficient stone". [ 9 ] (p 183) However, sediment research in Bolivia has revealed that metals such as silver were smelted on a very large scale, thousands of tons, from late Tiwanaku to Inca times (1000–1530 CE), which suggests that the paucity of metal (particularly precious metal) at Inca sites is more likely caused by Spanish acquisition and export than by limited pre-colonial use. [ 23 ]
It has been claimed that the Inca Empire expanded into Diaguita lands in what is now north-central Chile because of its mineral wealth, but that view is rejected by some scholars. [ 24 ] Further, an additional possibility is that the Incas invaded the relatively well-populated Eastern Diaguita valleys (present-day Argentina) to obtain labor to send to Chilean mining districts. [ 24 ] The Incas influenced Diaguitas , who adopted Incan metalworking techniques. [ 25 ]
Farther south in Chile, Mapuche tribes within or near the Incan Empire paid tributes in gold. [ 26 ] Incan yanakuna are believed by archaeologists Tom Dillehay and Américo Gordon to have extracted gold south of the Incan frontier in free Mapuche territory. Following that thought, the main motive for Incan expansion into Mapuche territory would have been to access gold mines . [ 27 ] Among the Mapuche people of central and south-central Chile, gold had an important cultural significance that predates Inca contact . [ 26 ] At the time of the Spanish conquest of Chile , Mapuches are reported by various chroniclers to use gold ornaments. [ 26 ] According to historian Osvaldo Silva gold ornaments of the Mapuche of the Concepción area evidence some kind of interaction between the Mapuche and the Inca that may have been trade, gifts, or spoils of war taken from a defeated Inca army . [ 28 ] Pre-Hispanic Mapuche tools are known to have been relatively simple and made of wood and stone , but a few of them were actually made of copper and bronze . [ 29 ] [ 30 ]
Iron was never smelted by Native Americans, thus the New World never entered a proper " Iron Age " before European contact, and the term is not used of the Americas. But there was limited use of native (unsmelted) iron ore, from magnetite, iron pyrite and ilmenite (iron–titanium), especially in the Andes (Chavin and Moche cultures) and Mesoamerica, after 900 BC and until c. 500 CE . Various forms of iron ore were mined, [ 31 ] drilled and highly polished. There is considerable evidence that this technology, its raw materials and end products were widely traded in Mesoamerica throughout the Formative era (2000–200 BCE). [ 32 ]
Lumps of iron pyrite, magnetite, and other materials were mostly shaped into mirrors, pendants, medallions, and headdress ornaments for decorative and ceremonial effect. [ 33 ] However, concave iron ore mirrors were apparently used for firing and optical purposes by the Olmec (1500–400 BCE) and Chavin (900–300 BCE) cultures, [ 34 ] and ilmenite "beads" may have served as hammers for fine work. [ 35 ] The Olmec and Izapa (300 BCE – 100 CE) also seem to have used iron magnetism to align and position monuments. [ 36 ] They may have developed a zeroth-order compass using a magnetite bar. [ 37 ]
Some Mesoamerican uses of native iron seem to have been military. Steven Jones proposed that the Olmec sewed ilmenite "beads" into protective mail armour or helmets. [ 35 ] Iron pyrite mosaics and plates formed protective tezcacuitlapalli (mirrored back flap shields) and breastplate ornaments in the military attire of the Teotihuacan (100 BCE – 600 CE), Toltec (800–1150 CE) and Chichen Itza (800–1200 CE) cultures. [ 33 ]
Lead ore (galena) in relatively pure form is present geologically in a number of places in North America. Some native populations mined and used the lead. [ 38 ]
Gold, copper and tumbaga objects started being produced in Panama and Costa Rica between 300–500 CE. Open-molded casting with oxidation gilding and cast filigrees were in use. By 700–800 CE, small metal sculptures were common and an extensive range of gold and tumbaga ornaments constituted the usual regalia of persons of high status in Panama and Costa Rica. [ 39 ]
The earliest specimen of metalwork from the Caribbean is a gold-alloy sheet carbon dated to 70–374 CE. Most Caribbean metallurgy has been dated to between 1200 and 1500 CE and consists of simple, small pieces such as sheets, pendants, beads and bells. These are mostly gold or a gold alloy (with copper or silver) and have been found to be largely cold hammered and sand-polished alluvial nuggets, although a few items seem to have been produced by lost wax casting. It is presumed that at least some of these items were acquired by trade from Colombia. [ 40 ]
Metallurgy only appears in Mesoamerica in 800 CE with the best evidence from West Mexico . Much like in South America, fine metals were seen as a material for the elite. Metal's special qualities of colour and resonance seemed to have appealed most and then led to the particular technological developments seen in the region. [ 41 ]
Exchange of ideas and goods with peoples from the Ecuador and Colombia region (likely via a maritime route) seems to have fueled early interest and development. Similar metal artifact types are found in West Mexico and the two regions: copper rings, needles, and tweezers being fabricated in the same ways as in Ecuador and also found in similar archaeological contexts. A multitude of bells were also found, but in this case they were cast using the same lost-wax casting method as seen in Colombia. [ 41 ] During this period, copper was being used almost exclusively.
Continual contact kept the flow of ideas from that same region and later, coinciding with the development of Andean long distance maritime trade, influence from further south seems to have reached the region and led to a second period (1200–1300 CE to the Spanish arrival). [ 41 ] By this time, copper alloys were being explored by West Mexican metallurgists, partly because the different mechanical properties were needed to fashion specific artifacts, particularly axe-monies – further evidence for contact with the Andean region. However, in general the new properties such alloys introduced were developed to meet regional needs, especially wirework bells, which at times had such high tin content in the bronze that it was irrelevant for its mechanical properties but gave the bells a golden colour.
The actual artifacts and then techniques were imported from the south, but west Mexican metallurgists worked ores from the abundant local deposits; the metal was not being imported. Even when the technology spread from West into north-eastern, central and southern Mexico, artifacts that can be traced back to West Mexican ores are abundant, if not exclusive. It is not always clear if the metal reached its final destination as an ingot, an ore or a finished artifact. Provenance studies on metal artifacts from southern Mesoamerica cast with the lost wax technique and dissimilar to west Mexican artifacts have shown that there might have been a second point of emergence of metallurgy into Mesoamerica there since no known source could be identified. [ 42 ] In the Tarascan Empire, copper and bronze was used for chisels, punches, awls, tweezers, needles, axes, discs, and breastplates. [ 43 ]
The Aztecs did not initially adopt metal working, even though they had acquired metal objects from other peoples. However, as conquest gained them metal working regions, the technology started to spread. By the time of the Spanish conquest, a bronze-smelting technology had already been developed. Spanish conquistadors used indigenous smelting technology to produce weapons and tools. [ 44 ]
Archaeological evidence has not revealed metal smelting or alloying of metals by pre-Columbian native peoples north of the Rio Grande ; however, they did use native copper extensively. [ 45 ]
Pure native copper was abundant in the Great Lakes region , particularly Michigan , but Wisconsin , Minnesota , and Manitoba as well. [ 3 ] The last glacial period had left copper bearing rock exposed and scoured bare. Once the ice retreated, loose pieces were readily available in a variety of sizes for use. [ 3 ] Copper was cold hammered into objects from very early in the ( Archaic period in the region: 8000–1000 BC). There is also evidence the indigenous people of the Old Copper Complex mined copper veins in the rock, but disagreement exists as to when. [ 3 ]
Extraction would have been extremely difficult. Hammerstones may have been used to break off pieces to be worked. This labor-intensive process might have been eased by building a fire on top of the deposit, then quickly dousing the hot rock with water fracturing the mental. [ citation needed ]
The copper could then be cold-hammered into shape, which would make it brittle, or hammered and heated in an annealing process to avoid this. The final object would then have to be ground and sharpened using native sandstone. Numerous bars have been found with no identified purpose like trade or barter.
Great Lake artifacts found in the Eastern Woodlands of North America seem to indicate there were widespread trading networks by 1000 BC. Progressively the usage of copper for tools decreases with more jewelry and adornments being found. This is believed to be indicative of social changes to a more hierarchical society. [ 3 ] Thousands of copper mining pits have been found along the shores of Lake Superior , and Isle Royale . These pits may have been in use as far back as 8,000 years ago. This copper was mined and then made into objects such as heavy spear points and tools of all kinds. It was also made into crescent objects similar to bannerstones that some archaeologists believe were for religious or ceremonial purposes. The crescents were too delicate for utilitarian use. Many have 28 or 29 notches along the inner edge, the approximate number of days in a lunar month . [ 46 ]
The Old Copper Culture flourished mainly in the Lake Superior region. [ 48 ] [ 49 ] [ 50 ] [ 51 ] [ 52 ] [ 53 ] The Milwaukee Public Museum has an extensive display of objects. At least 50 Old Copper items, including spear points and ceremonial crescents have been discovered in Manitoba . A few more in Saskatchewan , and at least one, a crescent, has turned up in Alberta , 2,000 kilometres from its origin in Ontario. It is most likely that these copper items arrived in the plains as trade goods rather than people of the Old Copper Culture moving into these new places. However from one excavated site in eastern Manitoba it can be seen that some people were moving northwest. At a site near Bissett archaeologists have found copper tools, weapons, and waste material of manufacture, along with a large nugget of raw copper. This site however was dated to around 4,000 years ago, a time of cooler climate when the boreal forest's treeline moved much further south. Though if these migrants moved with their metallurgy up the Winnipeg River at that time they may have continued to Lake Winnipeg , and the Saskatchewan River system. [ 46 ]
The Old Copper Culture did not develop advanced metallurgy like the principle of creating alloys . This means that many, though they could make metal objects and weapons, continued to use their flint, which could maintain a sharper edge for much longer. The unalloyed copper could simply not compete in daily utilization. In the later days of the Culture the metal was almost exclusively used for ceremonial items. [ 46 ]
However, Lake Superior, as a unique source of copper for over 6,000 years has recently come into some criticism, particularly since other deposits have been found that other archiac cultures mined on a much smaller scale. [ 54 ] [ 55 ]
During the Mississippian period (800–1600 CE, varying locally), elites at major political and religious centers throughout the midwestern and southeastern United States used copper ornamentation as a sign of their status by crafting the sacred material into representations connected with the Chiefly Warrior cult of the Southeastern Ceremonial Complex (S.E.C.C.). [ 56 ] This ornamentation includes Mississippian copper plates , repousséd plates of beaten copper now found as far afield as Alabama, Florida, Georgia, Illinois, Mississippi, Oklahoma, and Tennessee. Some of the more famous of the plates are of raptorial birds and avian-themed dancing warriors. These plates, such as the Rogan plates from Etowah , the Spiro plates from the Spiro in Oklahoma, and the Wulfing cache from southeast Missouri, were instrumental in the development of the archaeological concept known as the S.E.C.C. [ 56 ]
The only Mississippian culture site where a copper workshop has been located by archaeologists is Cahokia in western Illinois, where a copper workshop dating to the Moorehead Phase ( c. 1200 CE ) was identified at Mound 34 . Gregory Perino identified the site in 1956 and archaeologists subsequently excavated it. [ 57 ]
Numerous copper fragments were found at the site; metallographic analysis indicated that Mississippian copper workers worked copper into thin sheet through repeated hammering and annealing , a process that could be successful over open-pit wood fires. [ 57 ]
After the collapse of the Mississippian way of life in the 1500s with the advent of European colonization, copper still retained a place in Native American religious life as a special material. Copper was traditionally regarded as sacred by many historic period Eastern tribes. Copper nuggets are included in medicine bundles among Great Lakes tribes. Among 19th century Muscogee Creeks , a group of copper plates carried along the Trail of Tears are regarded as some of the tribe's most sacred items. [ 58 ]
Native ironwork in the Northwest Coast has been found in places like the Ozette Indian Village Archeological Site , where iron chisels and knives were discovered. These artifacts seem to have been crafted around 1613, based on the dendrochronological analysis of associated pieces of wood in the site, and were made out of drift iron from Asian (specifically Japanese ) shipwrecks , which were swept by the Kuroshio Current towards the coast of North America. [ 59 ]
The tradition of working with Asian drift iron was well-developed in the Northwest before European contact, and was present among several native peoples from the region, including the Chinookan peoples and the Tlingit , who seem to have had their own specific word for the metallic material, which was transcribed by Frederica De Laguna as gayES . [ 59 ] The wrecking of Japanese and Chinese vessels in the North Pacific basin was fairly common, and the iron tools and weaponry they carried provided the necessary materials for the development of the local ironwork traditions among the Northwestern Pacific Coast peoples, [ 60 ] although there were also other sources of iron, like that from meteorites , which was occasionally worked using stone anvils. [ 59 ] | https://en.wikipedia.org/wiki/Metallurgy_in_pre-Columbian_America |
The emergence of metallurgy in pre-Columbian Mesoamerica occurred relatively late in the region's history, with distinctive works of metal apparent in West Mexico by roughly 800 CE, and perhaps as early as 600 CE. [ 1 ] Metallurgical techniques likely diffused northward from regions in Central or South America via maritime trade routes ; recipients of these metallurgical technologies apparently exploited a wide range of material, including alloys of copper - silver , copper- arsenic , copper- tin and copper-arsenic-tin. [ 2 ]
Metal items crafted throughout Mesoamerica may be broken into three classes: utilitarian objects, objects used for individual ornamentation, and ceremonial/ritual objects. [ 3 ] The latter two categories comprise the bulk of distinctly Mesoamerican artifacts, with metals playing a particularly important role in the sacred and symbolic cultural realms.
The earliest and most diverse finds of metal artifacts are from West Mexico stretching in a belt along the Pacific coast from Guerrero to Nayarit. This indicates that this region was a regional nucleus of metallurgy, [ 4 ] from which elements of technique, form and style could have diffused throughout Mesoamerica.
The Mixtec civilization have long been thought to be the dominant goldsmiths of post-classic Mesoamerica. [ 5 ] A large number of gold artifacts found in central and southern Mexico have been attributed to the Mixtec.
There is recent evidence that suggests that the Aztec civilization was a distinct locus of metallurgy, [ 6 ] though gold objects from this area had previously been attributed to the Mixtec.
Some locally produced artifacts have been found in late-postclassic sites in the La Huasteca region. [ 7 ]
West Mexican smiths worked primarily in copper during the initial period, with some low- arsenic alloys, as well as occasional employment of silver and gold . Lost-wax cast bells were introduced from lower Central America and Colombia during this phase, along with several classes of cold-worked ornaments and hand tools, such as needles and tweezers. The prototypes for these small, often utilitarian items appear rooted in southern Ecuador and northern Peru . [ 8 ] Small copper rings, generally found in burial contexts, are also common to Ecuador and Western Mexico and are abundant during this phase.
Excavated assemblages from the initial phase indicate that lost-wax cast bells also occupied a substantial portion of West Mexican artisans' efforts. Unlike similar bells recovered from coastal Ecuador, West Mexican bells were cast, rather than worked, metal. Typically composed of a smooth, suspended metal shell encasing an interior clapper, the West Mexican bells were generally fashioned from copper alloys and bore particular resemblance to bells made in Colombia , Panama and Costa Rica . [ 9 ]
Metal smiths demonstrated increasing technical sophistication, producing both utilitarian and status-linked items. During the latter phase, Michoacán emerged as a technological hub, with metal artifacts also appearing at the adjacent zones of Guerrero and Jalisco .
Alloys became more prevalent during the second phase, as metal workers experimented with color, strength and fluidity. Formerly utilitarian assemblages transformed, with new focus placed upon metallic status objects. Further, the appearance of a copper-tin bronze alloy suggests contact between West Mexico and Peru during this period. However, many of the alloys/alloy concentrations used in West Mexico appear to reflect local innovation.
Scholars such as Dorothy Hosler suggest that ancient Mesoamericans were unique in their attention to the peculiar aesthetic properties of metals, such as the brilliant sounds and colors evoked through the movement of metallic objects. [ 10 ] The rather late emergence of metallurgy in ancient Mesoamerica likely contributed to its novelty and subsequent role as a marker of elite status.
It has been suggested that Mesoamerican metal smiths produced particular alloys with the chief aim of exploiting the alloys’ emergent color properties, particularly the vivid gold tones produced through infusion of tin, and the silver shades that develop at high arsenic concentrations. Notably, certain artifacts from West Mexico contain tin or arsenic at concentrations as high as 23 weight percent, while concentrations of alloying elements at roughly 2 to 5 weight percent are typically adequate for augmented strength and mechanical utility. [ 11 ]
Metal smiths in pre-Columbian West Mexico particularly exploited the brilliance inherent in metallic sound and sheen, suggesting that their creations tended to occupy a sacred and symbolic space. [ 12 ] Metallic colors, gold and silver, might have been connected with solar and lunar deities while bell sounds have been associated with fertility rituals and protection in warfare. [ 13 ]
(AD 900–1450)
Utilitarian and ceremonial objects; Objects of personal adornment'
(AD 800/900–1450)
Utilitarian and ceremonial objects; objects of personal adornment
(AD 900–1500)
Objects of personal adornment and ceremonial objects
(AD 900–1500)
Utilitarian and ceremonial objects; objects of personal adornment
(AD 450(?)–1500)
Utilitarian and ceremonial objects; objects of personal adornment
(AD 900–1500)
Utilitarian and ceremonial objects; objects of personal adornment
(1000–1450)
Utilitarian and ceremonial objects; objects of personal adornment
(1000–1450)
Utilitarian objects; objects of personal adornment | https://en.wikipedia.org/wiki/Metallurgy_in_pre-Columbian_Mesoamerica |
Metalogic is the metatheory of logic . Whereas logic studies how logical systems can be used to construct valid and sound arguments , metalogic studies the properties of logical systems . [ 1 ] Logic concerns the truths that may be derived using a logical system; metalogic concerns the truths that may be derived about the languages and systems that are used to express truths. [ 2 ]
The basic objects of metalogical study are formal languages, formal systems, and their interpretations . The study of interpretation of formal systems is the branch of mathematical logic that is known as model theory , and the study of deductive systems is the branch that is known as proof theory .
A formal language is an organized set of symbols , the symbols of which precisely define it by shape and place. Such a language therefore can be defined without reference to the meanings of its expressions; it can exist before any interpretation is assigned to it—that is, before it has any meaning. First-order logic is expressed in some formal language. A formal grammar determines which symbols and sets of symbols are formulas in a formal language.
A formal language can be formally defined as a set A of strings (finite sequences) on a fixed alphabet α. Some authors, including Rudolf Carnap , define the language as the ordered pair <α, A >. [ 3 ] Carnap also requires that each element of α must occur in at least one string in A .
Formation rules (also called formal grammar ) are a precise description of the well-formed formulas of a formal language. They are synonymous with the set of strings over the alphabet of the formal language that constitute well formed formulas. However, it does not describe their semantics (i.e. what they mean).
A formal system (also called a logical calculus , or a logical system ) consists of a formal language together with a deductive apparatus (also called a deductive system ). The deductive apparatus may consist of a set of transformation rules (also called inference rules ) or a set of axioms , or have both. A formal system is used to derive one expression from one or more other expressions.
A formal system can be formally defined as an ordered triple <α, I {\displaystyle {\mathcal {I}}} , D {\displaystyle {\mathcal {D}}} d>, where D {\displaystyle {\mathcal {D}}} d is the relation of direct derivability. This relation is understood in a comprehensive sense such that the primitive sentences of the formal system are taken as directly derivable from the empty set of sentences. Direct derivability is a relation between a sentence and a finite, possibly empty set of sentences. Axioms are so chosen that every first place member of D {\displaystyle {\mathcal {D}}} d is a member of I {\displaystyle {\mathcal {I}}} and every second place member is a finite subset of I {\displaystyle {\mathcal {I}}} .
A formal system can also be defined with only the relation D {\displaystyle {\mathcal {D}}} d. Thereby can be omitted I {\displaystyle {\mathcal {I}}} and α in the definitions of interpreted formal language , and interpreted formal system . However, this method can be more difficult to understand and use. [ 3 ]
A formal proof is a sequence of well-formed formulas of a formal language, the last of which is a theorem of a formal system. The theorem is a syntactic consequence of all the well formed formulae that precede it in the proof system. For a well formed formula to qualify as part of a proof, it must result from applying a rule of the deductive apparatus of some formal system to the previous well formed formulae in the proof sequence.
An interpretation of a formal system is the assignment of meanings to the symbols and truth-values to the sentences of the formal system. The study of interpretations is called Formal semantics . Giving an interpretation is synonymous with constructing a model .
In metalogic, formal languages are sometimes called object languages . The language used to make statements about an object language is called a metalanguage . This distinction is a key difference between logic and metalogic. While logic deals with proofs in a formal system , expressed in some formal language, metalogic deals with proofs about a formal system which are expressed in a metalanguage about some object language.
In metalogic, 'syntax' has to do with formal languages or formal systems without regard to any interpretation of them, whereas, 'semantics' has to do with interpretations of formal languages. The term 'syntactic' has a slightly wider scope than 'proof-theoretic', since it may be applied to properties of formal languages without any deductive systems, as well as to formal systems. 'Semantic' is synonymous with 'model-theoretic'.
In metalogic, the words use and mention , in both their noun and verb forms, take on a technical sense in order to identify an important distinction. [ 2 ] The use–mention distinction (sometimes referred to as the words-as-words distinction ) is the distinction between using a word (or phrase) and mentioning it. Usually it is indicated that an expression is being mentioned rather than used by enclosing it in quotation marks, printing it in italics, or setting the expression by itself on a line. The enclosing in quotes of an expression gives us the name of an expression, for example:
"Metalogic" is the name of this article. This article is about metalogic.
The type-token distinction is a distinction in metalogic, that separates an abstract concept from the objects which are particular instances of the concept. For example, the particular bicycle in your garage is a token of the type of thing known as "The bicycle." Whereas, the bicycle in your garage is in a particular place at a particular time, that is not true of "the bicycle" as used in the sentence: " The bicycle has become more popular recently." This distinction is used to clarify the meaning of symbols of formal languages .
Metalogical questions have been asked since the time of Aristotle . [ 4 ] However, it was only with the rise of formal languages in the late 19th and early 20th century that investigations into the foundations of logic began to flourish. In 1904, David Hilbert observed that in investigating the foundations of mathematics that logical notions are presupposed, and therefore a simultaneous account of metalogical and metamathematical principles was required. Today, metalogic and metamathematics are largely synonymous with each other, and both have been substantially subsumed by mathematical logic in academia. A possible alternate, less mathematical model may be found in the writings of Charles Sanders Peirce and other semioticians .
Results in metalogic consist of such things as formal proofs demonstrating the consistency , completeness , and decidability of particular formal systems .
Major results in metalogic include: | https://en.wikipedia.org/wiki/Metalogic |
Metalorganic vapour-phase epitaxy ( MOVPE ), also known as organometallic vapour-phase epitaxy ( OMVPE ) or metalorganic chemical vapour deposition ( MOCVD ), [ 1 ] is a chemical vapour deposition method used to produce single- or polycrystalline thin films. It is a process for growing crystalline layers to create complex semiconductor multilayer structures. [ 2 ] In contrast to molecular-beam epitaxy (MBE), the growth of crystals is by chemical reaction and not physical deposition. This takes place not in vacuum , but from the gas phase at moderate pressures (10 to 760 Torr ). As such, this technique is preferred for the formation of devices incorporating thermodynamically metastable alloys, [ citation needed ] and it has become a major process in the manufacture of optoelectronics , such as light-emitting diodes , its most widespread application. [ 3 ] It was first demonstrated in 1967 at North American Aviation (later Rockwell International ) Autonetics Division in Anaheim CA by Harold M. Manasevit .
In MOCVD ultrapure precursor gases are injected into a reactor, usually with a non-reactive carrier gas. For a III-V semiconductor, a metalorganic could be used as the group III precursor and a hydride for the group V precursor. For example, indium phosphide can be grown with trimethylindium ((CH 3 ) 3 In) and phosphine (PH 3 ) precursors.
As the precursors approach the semiconductor wafer , they undergo pyrolysis and the subspecies absorb onto the semiconductor wafer surface. Surface reaction of the precursor subspecies results in the incorporation of elements into a new epitaxial layer of the semiconductor crystal lattice. In the mass-transport-limited growth regime in which MOCVD reactors typically operate, growth is driven by supersaturation of chemical species in the vapor phase. [ 4 ] MOCVD can grow films containing combinations of group III and group V , group II and group VI , group IV .
Required pyrolysis temperature increases with increasing chemical bond strength of the precursor. The more carbon atoms are attached to the central metal atom, the weaker the bond. [ 5 ] The diffusion of atoms on the substrate surface is affected by atomic steps on the surface.
The vapor pressure of the group III metal organic source is an important control parameter for MOCVD growth, since it determines the growth rate in the mass-transport-limited regime. [ 6 ]
In the metal organic chemical vapor deposition (MOCVD) technique, reactant gases are combined at elevated temperatures in the reactor to cause a chemical interaction, resulting in the deposition of materials on the substrate.
A reactor is a chamber made of a material that does not react with the chemicals being used. It must also withstand high temperatures. This chamber is composed by reactor walls, liner, a susceptor , gas injection units, and temperature control units. Usually, the reactor walls are made from stainless steel or quartz. Ceramic or special glasses , such as quartz, are often used as the liner in the reactor chamber between the reactor wall and the susceptor. To prevent overheating, cooling water must be flowing through the channels within the reactor walls. A substrate sits on a susceptor which is at a controlled temperature. The susceptor is made from a material resistant to the temperature and metalorganic compounds used, often it is machined from graphite . For growing nitrides and related materials, a special coating, typically of silicon nitride or tantalum carbide , on the graphite susceptor is necessary to prevent corrosion by ammonia (NH 3 ) gas.
One type of reactor used to carry out MOCVD is a cold-wall reactor. In a cold-wall reactor, the substrate is supported by a pedestal, which also acts as a susceptor. The pedestal/susceptor is the primary origin of heat energy in the reaction chamber. Only the susceptor is heated, so gases do not react before they reach the hot wafer surface. The pedestal/susceptor is made of a radiation-absorbing material such as carbon. In contrast, the walls of the reaction chamber in a cold-wall reactor are typically made of quartz which is largely transparent to the electromagnetic radiation . The reaction chamber walls in a cold-wall reactor, however, may be indirectly heated by heat radiating from the hot pedestal/susceptor, but will remain cooler than the pedestal/susceptor and the substrate the pedestal/susceptor supports.
In hot-wall CVD, the entire chamber is heated. This may be necessary for some gases to be pre-cracked before reaching the wafer surface to allow them to stick to the wafer.
Gas is introduced via devices known as 'bubblers'. In a bubbler a carrier gas (usually hydrogen in arsenide & phosphide growth or nitrogen for nitride growth) is bubbled through the metalorganic liquid , which picks up some metalorganic vapour and transports it to the reactor. The amount of metalorganic vapour transported depends on the rate of carrier gas flow and the bubbler temperature , and is usually controlled automatically and most accurately by using an ultrasonic concentration measuring feedback gas control system. Allowance must be made for saturated vapors .
Gas exhaust and cleaning system . Toxic waste products must be converted to liquid or solid wastes for recycling (preferably) or disposal. Ideally processes will be designed to minimize the production of waste products.
As MOCVD has become well-established production technology, there are equally growing concerns associated with its bearing on personnel and community safety, environmental impact and maximum quantities of hazardous materials (such as gases and metalorganics) permissible in the device fabrication operations. The safety as well as responsible environmental care have become major factors of paramount importance in the MOCVD-based crystal growth of compound semiconductors. As the application of this technique in industry has grown, a number of companies have also grown and evolved over the years to provide the ancillary equipment required to reduce risk. This equipment includes but is not limited to computer automated gas and chemical delivery systems, toxic and carrier gas sniffing sensors which can detect single digit ppb amounts of gas, and of course abatement equipment to fully capture toxic materials which can be present in the growth of arsenic containing alloys such as GaAs and InGaAsP. [ 7 ] | https://en.wikipedia.org/wiki/Metalorganic_vapour-phase_epitaxy |
Metals is a monthly peer-reviewed open access scientific journal covering related scientific research and technology development. It was established in 2011 and is published by MDPI in affiliation with the Portuguese Society of Materials and the Spanish Materials Society. The editor-in-chief is Hugo F. Lopez ( University of Wisconsin-Milwaukee ). The journal publishes reviews, regular research papers, short communications, and book reviews. There are occasional special issues.
The journal is abstracted and indexed in:
Official Website | https://en.wikipedia.org/wiki/Metals_(journal) |
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