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Zero Carbon World is a British charity that promotes a net-zero emissions objective by supporting carbon reduction projects. It is a registered in England and Wales and also registered as a Limited liability company .
The objectives of the charity are: [ 1 ]
One way that Zero Carbon World aims to meet its objectives is the donation of Electric Vehicle Charging stations to various organisations around the UK. [ 2 ] [ 3 ] Sites that install donated chargers get added to the charity's ZeroNet EV charger map. [ 4 ]
Zero Carbon World were one of the Sponsors of the 2012 Bath Film Festival [ 5 ] which included a showing of Revenge of the Electric Car . [ 6 ]
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https://en.wikipedia.org/wiki/Zero_Carbon_World
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Modified AMI codes are a digital telecommunications technique to maintain system synchronization . Alternate mark inversion (AMI) line codes are modified by deliberate insertion of bipolar violations . There are several types of modified AMI codes, used in various T-carrier and E-carrier systems.
The clock rate of an incoming T-carrier is extracted from its bipolar line code. Each signal transition provides an opportunity for the receiver to see the transmitter's clock. The AMI code guarantees that transitions are always present before and after each mark (1 bit), but are missing between adjacent spaces (0 bits). To prevent loss of synchronization when a long string of zeros is present in the payload , deliberate bipolar violations are inserted into the line code, to create a sufficient number of transitions to maintain synchronization; this is a form of run length limited coding. The receive terminal equipment recognizes the bipolar violations and removes from the user data the marks attributable to the bipolar violations.
T-carrier was originally developed for voice applications. When voice signals are digitized for transmission via T-carrier, the data stream always includes ample 1 bits to maintain synchronization. (To help this, the μ-law algorithm for digitizing voice signals encodes silence as a continuous stream of 1 bits.) However, when used for the transmission of digital data , the conventional AMI line code may fail to have sufficient marks to permit recovery of the incoming clock, and synchronization is lost. This happens when there are too many consecutive zeros in the user data being transported.
The exact pattern of bipolar violations that is transmitted in any given case depends on the line rate ( i.e. , the level of the line code in the T-carrier hierarchy) and the polarity of the last valid mark in the user data prior to the unacceptably long string of zeros. It would not be useful to have a violation immediately following a mark, as that would not produce a transition. For this reason, all modified AMI codes include a space (0 bit) before each violation mark.
In the descriptions below, " B " denotes a balancing mark with the opposite polarity to that of the preceding mark, while " V " denotes a bipolar violation mark, which has the same polarity as the preceding mark. In order to preserve AMI coding's desirable absence of DC bias , the number of positive marks must equal the number of negative marks. This happens automatically for balancing ( B ) marks, but the line code must ensure that positive and negative violation marks balance each other.
The first technique used to ensure a minimum density of marks was zero code suppression a form of bit stuffing , which set the least significant bit of each 8-bit byte transmitted to a 1. (This bit was already unavailable due to robbed-bit signaling .) This avoided the need to modify the AMI code in any way, but limited available data rates to 56,000 bits per second per DS0 voice channel. Also, the low minimum density of ones (12.5%) sometimes led to increased clock slippage on the span.
Increased demand for bandwidth, and compatibility with the G.703 and ISDN PRI standards which called for 64,000 bits per second, led to this system being superseded by B8ZS.
Commonly used in the North American T1 ( Digital Signal 1 ) 1.544 Mbit/s line code, bipolar with eight-zero substitution (B8ZS) replaces each string of 8 consecutive zeros with the special pattern " 000VB0VB ". Depending on the polarity of the preceding mark, that could be 000+−0−+ or 000−+0+− .
At the North American T2 rate (6.312 Mbit/s), bipolar violations are inserted if 6 or more consecutive zeros occur. This line code is called bipolar with six-zero substitution (B6ZS) , and replaces 6 consecutive zeros with the pattern " 0VB0VB ". Depending on the polarity of the preceding mark, that could be 0+−0−+ or 0−+0+− .
Used in all levels of the European E-carrier system, the high density bipolar of order 3 (HDB3) code replaces any instance of 4 consecutive 0 bits with one of the patterns " 000V " or " B00V ". The choice is made to ensure that consecutive violations are of differing polarity; i.e., separated by an odd number of normal + or − marks.
These rules are applied on the code as it is being built from the original string. Every time there are 4 consecutive zeros in the code they will be replaced by either 000−, 000+, +00+ or −00−. To determine which pattern to use, one must count the number of pluses (+) and the number of minuses (−) since the last violation bit V, then subtract one from the other. If the result is an odd number then 000− or 000+ is used. If the result is an even number then +00+ or −00− is used. To determine which polarity to use, one must look at the pulse preceding the four zeros. If 000V form must be used then V simply copies the polarity of last pulse, if B00V form must be used then B and V chosen will have the opposite polarity of the last pulse.
Here are some examples of bit streams codes with AMI and HDB3. All assume the same starting conditions: the previous 1 bit was −, and the previous violation was an even number of 1 bits ago. (E.g. the preceding bits could have been ++−.)
At the North American T3 rate (44.736 Mbit/s), bipolar violations are inserted if 3 or more consecutive zeros occur. This line code is called bipolar with three-zero substitution (B3ZS) , and is very similar to HDB3. Each run of 3 consecutive zeros is replaced by " 00V " or " B0V ". The choice is made to ensure that consecutive violations are of differing polarity, i.e. separated by an odd number of normal B marks.
Other line codes that have 3 states:
This article incorporates public domain material from Federal Standard 1037C . General Services Administration . Archived from the original on 2022-01-22.
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https://en.wikipedia.org/wiki/Zero_code_suppression
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Zero differential overlap is an approximation in computational molecular orbital theory that is the central technique of semi-empirical methods in quantum chemistry . When computers were first used to calculate bonding in molecules, it was only possible to calculate diatomic molecules. As computers advanced, it became possible to study larger molecules, but the use of this approximation has always allowed the study of even larger molecules. Currently semi-empirical methods can be applied to molecules as large as whole proteins. The approximation involves ignoring certain integrals, usually two-electron repulsion integrals. If the number of orbitals used in the calculation is N, the number of two-electron repulsion integrals scales as N 4 . After the approximation is applied the number of such integrals scales as N 2 , a much smaller number, simplifying the calculation.
If the molecular orbitals Φ i {\displaystyle \mathbf {\Phi } _{i}\ } are expanded in terms of N basis functions, χ μ A {\displaystyle \mathbf {\chi } _{\mu }^{A}\ } as:
where A is the atom the basis function is centred on, and C i μ {\displaystyle \mathbf {C} _{i\mu }\ } are coefficients, the two-electron repulsion integrals are then defined as:
The zero differential overlap approximation ignores integrals that contain the product χ μ A ( 1 ) χ ν B ( 1 ) {\displaystyle \mathbf {\chi } _{\mu }^{A}(1)\mathbf {\chi } _{\nu }^{B}(1)} where μ is not equal to ν . This leads to:
where δ i j = { 0 i ≠ j 1 i = j {\displaystyle \delta _{ij}={\begin{cases}0&i\neq j\\1&i=j\ \end{cases}}}
The total number of such integrals is reduced to N ( N + 1) / 2 (approximately N 2 / 2) from [ N ( N + 1) / 2][ N ( N + 1) / 2 + 1] / 2 (approximately N 4 / 8), all of which are included in ab initio Hartree–Fock and post-Hartree–Fock calculations.
Methods such as the Pariser–Parr–Pople method (PPP) and CNDO/2 use the zero differential overlap approximation completely. Methods based on the intermediate neglect of differential overlap, such as INDO , MINDO , ZINDO and SINDO do not apply it when A = B = C = D , i.e. when all four basis functions are on the same atom. Methods that use the neglect of diatomic differential overlap, such as MNDO , PM3 and AM1 , also do not apply it when A = B and C = D , i.e. when the basis functions for the first electron are on the same atom and the basis functions for the second electron are the same atom.
It is possible to partly justify this approximation, but generally it is used because it works reasonably well when the integrals that remain – ⟨ μ μ | λ λ ⟩ {\displaystyle \langle \mu \mu |\lambda \lambda \rangle } – are parameterised.
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In mathematics , zero dynamics is known as the concept of evaluating the effect of zero on systems. [ 1 ]
The idea was introduced thirty years ago as the nonlinear approach to the concept of transmission of zeros. The original purpose of introducing the concept was to develop an asymptotic stabilization with a set of guaranteed regions of attraction ( semi-global stabilizability ), to make the overall system stable. [ 2 ]
Given the internal dynamics of any system, zero dynamics refers to the control action chosen in which the output variables of the system are kept identically zero. [ 3 ] While, various systems have an equally distinctive set of zeros, such as decoupling zeros, invariant zeros, and transmission zeros. Thus, the reason for developing this concept was to control the non-minimum phase and nonlinear systems effectively. [ 4 ]
The concept is widely utilized in SISO mechanical systems, whereby applying a few heuristic approaches, zeros can be identified for various linear systems. [ 5 ] Zero dynamics adds an essential feature to the overall system’s analysis and the design of the controllers. Mainly its behavior plays a significant role in measuring the performance limitations of specific feedback systems. In a Single Input Single Output system , the zero dynamics can be identified by using junction structure patterns. In other words, using concepts like bond graph models can help to point out the potential direction of the SISO systems. [ 6 ]
Apart from its application in nonlinear standardized systems, similar controlled results can be obtained by using zero dynamics on nonlinear discrete-time systems. In this scenario, the application of zero dynamics can be an interesting tool to measure the performance of nonlinear digital design systems (nonlinear discrete-time systems). [ 7 ]
Before the advent of zero dynamics, the problem of acquiring non-interacting control systems by using internal stability was not specifically discussed. However, with the asymptotic stability present within the zero dynamics of a system, static feedback can be ensured. Such results make zero dynamics an interesting tool to guarantee the internal stability of non-interacting control systems. [ 8 ]
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Zero- to ultralow-field ( ZULF ) NMR is the acquisition of nuclear magnetic resonance (NMR) spectra of chemicals with magnetically active nuclei ( spins 1/2 and greater) in an environment carefully screened from magnetic fields (including from the Earth's field ). ZULF NMR experiments typically involve the use of passive or active shielding to attenuate Earth’s magnetic field. This is in contrast to the majority of NMR experiments which are performed in high magnetic fields provided by superconducting magnets . In ZULF experiments the sample is moved through a low field magnet into the "zero field" region where the dominant interactions are nuclear spin-spin couplings, and the coupling between spins and the external magnetic field is a perturbation to this. There are a number of advantages to operating in this regime: magnetic-susceptibility-induced line broadening is attenuated which reduces inhomogeneous broadening of the spectral lines for samples in heterogeneous environments. Another advantage is that the low frequency signals readily pass through conductive materials such as metals due to the increased skin depth; this is not the case for high-field NMR for which the sample containers are usually made of glass, quartz or ceramic. [ 2 ] High-field NMR employs inductive detectors to pick up the radiofrequency signals, but this would be inefficient in ZULF NMR experiments since the signal frequencies are typically much lower (on the order of hertz to kilohertz). The development of highly sensitive magnetic sensors in the early 2000s including SQUIDs , magnetoresistive sensors , and SERF atomic magnetometers made it possible to detect NMR signals directly in the ZULF regime. Previous ZULF NMR experiments relied on indirect detection where the sample had to be shuttled from the shielded ZULF environment into a high magnetic field for detection with a conventional inductive pick-up coil. One successful implementation was using atomic magnetometers at zero magnetic field working with rubidium vapor cells to detect zero-field NMR. [ 3 ] [ 4 ]
Without a large magnetic field to induce nuclear spin polarization, the nuclear spins must be polarized externally using hyperpolarization techniques. This can be as simple as polarizing the spins in a magnetic field followed by shuttling to the ZULF region for signal acquisition, and alternative chemistry-based hyperpolarization techniques can also be used.
It is sometimes but inaccurately referred to as nuclear quadrupole resonance (NQR). [ 5 ]
Free evolution of nuclear spins is governed by a Hamiltonian ( H ^ {\displaystyle {\hat {H}}} ), which in the case of liquid-state nuclear magnetic resonance may be split into two major terms. The first term ( H ^ z {\displaystyle {\hat {H}}_{z}} ) corresponds to the Zeeman interaction between spins and the external magnetic field, which includes chemical shift ( σ {\displaystyle \sigma } ). The second term ( H ^ J {\displaystyle {\hat {H}}_{J}} ) corresponds to the indirect spin-spin, or J-coupling , interaction.
H ^ = H ^ z + H ^ J {\displaystyle {\hat {H}}={\hat {H}}_{z}+{\hat {H}}_{J}} , where:
H ^ z = − ℏ ∑ a γ a ( 1 − σ a ) I ^ a ⋅ B 0 {\displaystyle {\hat {H}}_{z}=-\hbar \sum _{a}\gamma _{a}(1-\sigma _{a}){\hat {I}}_{a}\cdot B_{0}} , and
H ^ J = − ℏ 2 π ∑ a > b J a b I ^ a ⋅ I ^ b {\displaystyle {\hat {H}}_{J}=-\hbar 2\pi \sum _{a>b}J_{ab}{\hat {I}}_{a}\cdot {\hat {I}}_{b}} .
Here the summation is taken over the whole system of coupled spins; ℏ {\displaystyle \hbar } denotes the reduced Planck constant; γ a {\displaystyle \gamma _{a}} denotes the gyromagnetic ratio of spin a; σ a {\displaystyle \sigma _{a}} denotes the isotropic part of the chemical shift for the a-th spin; I a {\displaystyle I_{a}} denotes the spin operator of the a-th spin; B 0 {\displaystyle B_{0}} is the external magnetic field experienced by all considered spins, and; J a b {\displaystyle J_{ab}} is the J-coupling constant between spins a and b.
Importantly, the relative strength of H ^ z {\displaystyle {\hat {H}}_{z}} and H ^ J {\displaystyle {\hat {H}}_{J}} (and therefore the spin dynamics behavior of such a system) depends on the magnetic field. For example, in conventional NMR, | B 0 | {\displaystyle |B_{0}|} is typically larger than 1 T, so the Larmor frequency ν 0 = − γ B 0 / 2 π {\displaystyle \nu _{0}=-\gamma B_{0}/2\pi } of 1 H exceeds tens of MHz. This is much larger than J {\displaystyle J} -coupling values which are typically Hz to hundreds of Hz. In this limit, H ^ J {\displaystyle {\hat {H}}_{J}} is a perturbation to H ^ z {\displaystyle {\hat {H}}_{z}} . In contrast, at nanotesla fields, Larmor frequencies can be much smaller than J {\displaystyle J} -couplings, and H ^ J {\displaystyle {\hat {H}}_{J}} dominates.
Before signals can be detected in a ZULF NMR experiment, it is first necessary to polarize the nuclear spin ensemble, since the signal is proportional to the nuclear spin magnetization. There are a number of methods to generate nuclear spin polarization. The most common is to allow the spins to thermally equilibrate in a magnetic field, and the nuclear spin alignment with the magnetic field due to the Zeeman interaction leads to weak spin polarization. The polarization generated in this way is on the order of 10 −6 for tesla field-strengths.
An alternative approach is to use hyperpolarization techniques, which are chemical and physical methods to generate nuclear spin polarization. Examples include parahydrogen-induced polarization , spin-exchange optical pumping of noble gas atoms , dissolution dynamic nuclear polarization , and chemically-induced dynamic nuclear polarization .
NMR experiments require creating a transient non-stationary state of the spin system. In conventional high-field experiments, radio frequency pulses tilt the magnetization from along the main magnetic field direction to the transverse plan. Once in the transverse plan, the magnetization is no longer in a stationary state (or eigenstate ) and so it begins to precess about the main magnetic field creating a detectable oscillating magnetic field.
In ZULF experiments, constant magnetic field pulses are used to induce non-stationary states of the spin system. The two main strategies consist of (1) switching of the magnetic field from pseudo-high field to zero (or ultra-low) field, or (2) of ramping down the magnetic field experienced by the spins to zero field in order to convert the Zeeman populations into zero-field eigenstates adiabatically and subsequently in applying a constant magnetic field pulse to induce a coherence between the zero-field eigenstates. In the simple case of a heteronuclear pair of J-coupled spins, both these excitation schemes induce a transition between the singlet and triplet-0 states, which generates a detectable oscillatory magnetic field.
More sophisticated pulse sequences have been reported including selective pulses, [ 6 ] two-dimensional experiments and decoupling schemes. [ 7 ]
NMR signals are usually detected inductively, but the low frequencies of the electromagnetic radiation emitted by samples in a ZULF experiment makes inductive detection impractical at low fields. Hence, the earliest approach for measuring zero-field NMR in solid samples was via field-cycling techniques. [ 8 ] The field cycling involves three steps: preparation, evolution and detection. In the preparation stage, a field is applied in order to magnetize the nuclear spins. Then the field is suddenly switched to zero to initiate the evolution interval and the magnetization evolves under the zero-field Hamiltonian. After a time period, the field is again switched on and the signal is detected inductively at high field. In a single field cycle, the magnetization observed corresponds only to a single value of the zero-field evolution time. The time-varying magnetization can be detected by repeating the field cycle with incremented lengths of the zero-field interval, and hence the evolution and decay of the magnetization is measured point by point. The Fourier transform of this magnetization will result to the zero-field absorption spectrum.
The emergence of highly sensitive magnetometry techniques has allowed for the detection of zero-field NMR signals in situ. Examples include superconducting quantum interference devices ( SQUIDs ), magnetoresistive sensors , and SERF atomic magnetometers . SQUIDs have high sensitivity, but require cryogenic conditions to operate, which makes them practically somewhat difficult to employ for the detection of chemical or biological samples. Magnetoresistive sensors are less sensitive, but are much easier to handle and to bring close to the NMR sample which is advantageous since proximity improves sensitivity. The most common sensors employed in ZULF NMR experiments are optically-pumped magnetometers, which have high sensitivity and can be placed in close proximity to an NMR sample.
The boundaries between zero-, ultralow-, low- and high-field NMR are not rigorously defined, although approximate working definitions are in routine use for experiments involving small molecules in solution. [ 9 ] The boundary between zero and ultralow field is usually defined as the field at which the nuclear spin precession frequency matches the spin relaxation rate, i.e., at zero field the nuclear spins relax faster than they precess about the external field. The boundary between ultralow and low field is usually defined as the field at which Larmor frequency differences between different nuclear spin species match the spin-spin (J or dipolar) couplings, i.e., at ultralow field spin-spin couplings dominate and the Zeeman interaction is a perturbation. The boundary between low and high field is more ambiguous and these terms are used differently depending on the application or research topic. In the context of ZULF NMR, the boundary is defined as the field at which chemical shift differences between nuclei of the same isotopic species in a sample match the spin-spin couplings.
Note that these definitions strongly depend on the sample being studied, and the field regime boundaries can vary by orders of magnitude depending on sample parameters such as the nuclear spin species, spin-spin coupling strengths, and spin relaxation times.
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https://en.wikipedia.org/wiki/Zero_field_NMR
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The zero flag is a single bit flag that is a central feature on most conventional CPU architectures (including x86 , ARM , PDP-11 , 68000 , 6502 , and numerous others). It is often stored in a dedicated register, typically called status register or flag register , along with other flags. The zero flag is typically abbreviated Z or ZF or similar in most documentation and assembly languages.
Along with a carry flag , a sign flag and an overflow flag , the zero flag is used to check the result of an arithmetic operation, including bitwise logical instructions . It is set to 1, or true, if an arithmetic result is zero, and reset otherwise. This includes results which are not stored, as most traditional instruction sets implement the compare instruction as a subtract where the result is discarded. It is also common that processors have a bitwise AND-instruction that does not store the result.
The logical formula of the zero flag for a twos-complement binary operand is NOT(OR(all bits of the operand in question)).
In most processors, the zero flag is mainly used in conditional branch instructions, which alter control flow on previous instruction results, but there are often other uses as well.
In some instruction sets such as the MIPS architecture , a dedicated flag register is not used; jump instructions instead check a register for zero. [ 1 ]
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https://en.wikipedia.org/wiki/Zero_flag
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Zero Liquid Discharge (ZLD) is a classification of water treatment processes intended to reduce wastewater efficiently and produce clean water that is suitable for reuse (e.g., irrigation ). ZLD systems employ wastewater treatment technologies and desalination to purify and recycle virtually all wastewater received. [ 1 ] [ 2 ]
ZLD technologies help industrial facilities meet discharge and water reuse requirements, enabling them to meet government discharge regulations, reach higher water recovery (%), and treat and recover valuable materials from the wastewater streams such as potassium sulfate , caustic soda , sodium sulfate , lithium , and gypsum .
Thermal technologies are the conventional means to achieve ZLD, such as evaporators (for instance multi stage flash distillation ), multi effect distillation , mechanical vapor compression, crystallization , and condensate recovery. ZLD plants produce solid waste .
ZLD processes begin with pre-treatment and evaporation of an industrial effluent until its dissolved solids precipitate . These precipitates are removed and dewatered with a filter press or a centrifuge . The water vapor from evaporation is condensed and returned to the process.
In the last few decades, there has been an effort from the water treatment industry to revolutionize high water recovery and ZLD technologies. [ 3 ]
This has led to processes like electrodialysis , forward osmosis , and membrane distillation .
A quick overview and comparison can be seen in the following representative table: [ 4 ] [ 5 ]
Despite the variable sources of a wastewater stream, a ZLD system is generally comprised by two steps:
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In physics , a zero mode is an eigenvector with a vanishing eigenvalue . [ 1 ]
In various subfields of physics zero modes appear whenever a physical system possesses a certain symmetry. For example, normal modes of multidimensional harmonic oscillator (e.g. a system of beads arranged around the circle, connected with springs) corresponds to elementary vibrational modes of the system. In such a system zero modes typically occur and are related with a rigid rotation around the circle.
The kernel of an operator consists of left zero modes, and the cokernel consists of the right zero modes.
This physics -related article is a stub . You can help Wikipedia by expanding it .
This linear algebra -related article is a stub . You can help Wikipedia by expanding it .
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In algebra , the zero object of a given algebraic structure is, in the sense explained below, the simplest object of such structure. As a set it is a singleton , and as a magma has a trivial structure, which is also an abelian group . The aforementioned abelian group structure is usually identified as addition , and the only element is called zero , so the object itself is typically denoted as {0} . One often refers to the trivial object (of a specified category ) since every trivial object is isomorphic to any other (under a unique isomorphism).
Instances of the zero object include, but are not limited to the following:
These objects are described jointly not only based on the common singleton and trivial group structure, but also because of shared category-theoretical properties .
In the last three cases the scalar multiplication by an element of the base ring (or field) is defined as:
The most general of them, the zero module, is a finitely-generated module with an empty generating set.
For structures requiring the multiplication structure inside the zero object, such as the trivial ring , there is only one possible, 0 × 0 = 0 , because there are no non-zero elements. This structure is associative and commutative . A ring R which has both an additive and multiplicative identity is trivial if and only if 1 = 0 , since this equality implies that for all r within R ,
In this case it is possible to define division by zero , since the single element is its own multiplicative inverse. Some properties of {0} depend on exact definition of the multiplicative identity; see § Unital structures below.
Any trivial algebra is also a trivial ring. A trivial algebra over a field is simultaneously a zero vector space considered below . Over a commutative ring , a trivial algebra is simultaneously a zero module.
The trivial ring is an example of a rng of square zero . A trivial algebra is an example of a zero algebra .
The zero-dimensional vector space is an especially ubiquitous example of a zero object, a vector space over a field with an empty basis . It therefore has dimension zero. It is also a trivial group over addition , and a trivial module mentioned above .
The zero ring, zero module and zero vector space are the zero objects of, respectively, the category of pseudo-rings , the category of modules and the category of vector spaces . However, the zero ring is not a zero object in the category of rings , since there is no ring homomorphism of the zero ring in any other ring.
The zero object, by definition, must be a terminal object, which means that a morphism A → {0} must exist and be unique for an arbitrary object A . This morphism maps any element of A to 0 .
The zero object, also by definition, must be an initial object, which means that a morphism {0} → A must exist and be unique for an arbitrary object A . This morphism maps 0 , the only element of {0} , to the zero element 0 ∈ A , called the zero vector in vector spaces. This map is a monomorphism , and hence its image is isomorphic to {0} . For modules and vector spaces, this subset {0} ⊂ A is the only empty-generated submodule (or 0-dimensional linear subspace ) in each module (or vector space) A .
The {0} object is a terminal object of any algebraic structure where it exists, like it was described for examples above. But its existence and, if it exists, the property to be an initial object (and hence, a zero object in the category-theoretical sense) depend on exact definition of the multiplicative identity 1 in a specified structure.
If the definition of 1 requires that 1 ≠ 0 , then the {0} object cannot exist because it may contain only one element. In particular, the zero ring is not a field . If mathematicians sometimes talk about a field with one element , this abstract and somewhat mysterious mathematical object is not a field.
In categories where the multiplicative identity must be preserved by morphisms, but can equal to zero, the {0} object can exist. But not as initial object because identity-preserving morphisms from {0} to any object where 1 ≠ 0 do not exist. For example, in the category of rings Ring the ring of integers Z is the initial object, not {0} .
If an algebraic structure requires the multiplicative identity, but neither its preservation by morphisms nor 1 ≠ 0 , then zero morphisms exist and the situation is not different from non-unital structures considered in the previous section.
Zero vector spaces and zero modules are usually denoted by 0 (instead of {0} ). This is always the case when they occur in an exact sequence .
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In mathematics , a zero (also sometimes called a root ) of a real -, complex -, or generally vector-valued function f {\displaystyle f} , is a member x {\displaystyle x} of the domain of f {\displaystyle f} such that f ( x ) {\displaystyle f(x)} vanishes at x {\displaystyle x} ; that is, the function f {\displaystyle f} attains the value of 0 at x {\displaystyle x} , or equivalently, x {\displaystyle x} is a solution to the equation f ( x ) = 0 {\displaystyle f(x)=0} . A "zero" of a function is thus an input value that produces an output of 0. [ 1 ]
A root of a polynomial is a zero of the corresponding polynomial function . [ 2 ] The fundamental theorem of algebra shows that any non-zero polynomial has a number of roots at most equal to its degree , and that the number of roots and the degree are equal when one considers the complex roots (or more generally, the roots in an algebraically closed extension ) counted with their multiplicities . [ 3 ] For example, the polynomial f {\displaystyle f} of degree two, defined by f ( x ) = x 2 − 5 x + 6 = ( x − 2 ) ( x − 3 ) {\displaystyle f(x)=x^{2}-5x+6=(x-2)(x-3)} has the two roots (or zeros) that are 2 and 3 . f ( 2 ) = 2 2 − 5 × 2 + 6 = 0 and f ( 3 ) = 3 2 − 5 × 3 + 6 = 0. {\displaystyle f(2)=2^{2}-5\times 2+6=0{\text{ and }}f(3)=3^{2}-5\times 3+6=0.}
If the function maps real numbers to real numbers, then its zeros are the x {\displaystyle x} -coordinates of the points where its graph meets the x -axis . An alternative name for such a point ( x , 0 ) {\displaystyle (x,0)} in this context is an x {\displaystyle x} -intercept .
Every equation in the unknown x {\displaystyle x} may be rewritten as
by regrouping all the terms in the left-hand side. It follows that the solutions of such an equation are exactly the zeros of the function f {\displaystyle f} . In other words, a "zero of a function" is precisely a "solution of the equation obtained by equating the function to 0", and the study of zeros of functions is exactly the same as the study of solutions of equations.
Every real polynomial of odd degree has an odd number of real roots (counting multiplicities ); likewise, a real polynomial of even degree must have an even number of real roots. Consequently, real odd polynomials must have at least one real root (because the smallest odd whole number is 1), whereas even polynomials may have none. This principle can be proven by reference to the intermediate value theorem : since polynomial functions are continuous , the function value must cross zero, in the process of changing from negative to positive or vice versa (which always happens for odd functions).
The fundamental theorem of algebra states that every polynomial of degree n {\displaystyle n} has n {\displaystyle n} complex roots, counted with their multiplicities. The non-real roots of polynomials with real coefficients come in conjugate pairs. [ 1 ] Vieta's formulas relate the coefficients of a polynomial to sums and products of its roots.
There are many methods for computing accurate approximations of roots of functions, the best being Newton's method , see Root-finding algorithm .
For polynomials , there are specialized algorithms that are more efficient and may provide all roots or all real roots; see Polynomial root-finding and Real-root isolation .
Some polynomial, including all those of degree no greater than 4, can have all their roots expressed algebraically in terms of their coefficients; see Solution in radicals .
In various areas of mathematics, the zero set of a function is the set of all its zeros. More precisely, if f : X → R {\displaystyle f:X\to \mathbb {R} } is a real-valued function (or, more generally, a function taking values in some additive group ), its zero set is f − 1 ( 0 ) {\displaystyle f^{-1}(0)} , the inverse image of { 0 } {\displaystyle \{0\}} in X {\displaystyle X} .
Under the same hypothesis on the codomain of the function, a level set of a function f {\displaystyle f} is the zero set of the function f − c {\displaystyle f-c} for some c {\displaystyle c} in the codomain of f . {\displaystyle f.}
The zero set of a linear map is also known as its kernel .
The cozero set of the function f : X → R {\displaystyle f:X\to \mathbb {R} } is the complement of the zero set of f {\displaystyle f} (i.e., the subset of X {\displaystyle X} on which f {\displaystyle f} is nonzero).
In algebraic geometry , the first definition of an algebraic variety is through zero sets. Specifically, an affine algebraic set is the intersection of the zero sets of several polynomials, in a polynomial ring k [ x 1 , … , x n ] {\displaystyle k\left[x_{1},\ldots ,x_{n}\right]} over a field . In this context, a zero set is sometimes called a zero locus .
In analysis and geometry , any closed subset of R n {\displaystyle \mathbb {R} ^{n}} is the zero set of a smooth function defined on all of R n {\displaystyle \mathbb {R} ^{n}} . This extends to any smooth manifold as a corollary of paracompactness .
In differential geometry , zero sets are frequently used to define manifolds . An important special case is the case that f {\displaystyle f} is a smooth function from R p {\displaystyle \mathbb {R} ^{p}} to R n {\displaystyle \mathbb {R} ^{n}} . If zero is a regular value of f {\displaystyle f} , then the zero set of f {\displaystyle f} is a smooth manifold of dimension m = p − n {\displaystyle m=p-n} by the regular value theorem .
For example, the unit m {\displaystyle m} - sphere in R m + 1 {\displaystyle \mathbb {R} ^{m+1}} is the zero set of the real-valued function f ( x ) = ‖ x ‖ 2 − 1 {\displaystyle f(x)=\Vert x\Vert ^{2}-1} .
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A zero register is a processor register that always returns the value zero and has no effect when it is written to. It is found in instruction set architectures including the CDC 6600 , System/360 and ARM64 , among others.
Zero appears as a constant in many instructions, notably "branch if zero", and optimizing these instructions can have a positive benefit on performance. Some architectures accomplish this with dedicated opcodes , specialized variations of their basic instructions. Implementing these requires additional logic in the instruction decoder . The zero register can accomplish the same effect without requiring new opcodes, although at the cost of dedicating a register to this feature, which may have negative impact for architectures with limited number of registers.
The x86 architecture has no zero register, while ARM added a zero register for ARM64. The RISC-V architecture includes one with the register name "x0" and the ABI name "zero"; the reason for this inclusion is stated as "Dedicating a register to zero is surprisingly a large factor in simplifying the RISC-V ISA.
note: this is different to zeroing a register such as by xor as it uses physical hardware.
" [ 1 ]
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Zero sound is the name given by Lev Landau in 1957 to the unique quantum vibrations in quantum Fermi liquids . [ 1 ] The zero sound can no longer be thought of as a simple wave of compression and rarefaction, but rather a fluctuation in space and time of the quasiparticles ' momentum distribution function. As the shape of Fermi distribution function changes slightly (or largely), zero sound propagates in the direction for the head of Fermi surface with no change of the density of the liquid. Predictions and subsequent experimental observations of zero sound [ 2 ] [ 3 ] [ 4 ] was one of the key confirmation on the correctness of Landau's Fermi liquid theory .
The Boltzmann transport equation for general systems in the semiclassical limit gives, for a Fermi liquid,
where f ( p → , x → , t ) = f 0 ( p → ) + δ f ( p → , x → , t ) {\displaystyle f({\vec {p}},{\vec {x}},t)=f_{0}({\vec {p}})+\delta f({\vec {p}},{\vec {x}},t)} is the density of quasiparticles (here we ignore spin ) with momentum p → {\displaystyle {\vec {p}}} and position x → {\displaystyle {\vec {x}}} at time t {\displaystyle t} , and E ( p → , x → , t ) = E 0 ( p → ) + δ E ( p → , x → , t ) {\displaystyle E({\vec {p}},{\vec {x}},t)=E_{0}({\vec {p}})+\delta E({\vec {p}},{\vec {x}},t)} is the energy of a quasiparticle of momentum p → {\displaystyle {\vec {p}}} ( f 0 {\displaystyle f_{0}} and E 0 {\displaystyle E_{0}} denote equilibrium distribution and energy in the equilibrium distribution). The semiclassical limit assumes that f {\displaystyle f} fluctuates with angular frequency ω {\displaystyle \omega } and wavelength λ = 2 π / k {\displaystyle \lambda =2\pi /k} , which are much lower than E F / ℏ {\displaystyle E_{\rm {F}}/\hbar } and much longer than ℏ / p F {\displaystyle \hbar /p_{\rm {F}}} respectively, where E F {\displaystyle E_{\rm {F}}} and p F {\displaystyle p_{\rm {F}}} are the Fermi energy and momentum respectively, around which f {\displaystyle f} is nontrivial. To first order in fluctuation from equilibrium, the equation becomes
When the quasiparticle's mean free path ℓ ≪ λ {\displaystyle \ell \ll \lambda } (equivalently, relaxation time τ ≪ 1 / ω {\displaystyle \tau \ll 1/\omega } ), ordinary sound waves ("first sound") propagate with little absorption. But at low temperatures T {\displaystyle T} (where τ {\displaystyle \tau } and ℓ {\displaystyle \ell } scale as T − 2 {\displaystyle T^{-2}} ), the mean free path exceeds λ {\displaystyle \lambda } , and as a result the collision functional St [ f ] ≈ 0 {\displaystyle {\text{St}}[f]\approx 0} . Zero sound occurs in this collisionless limit.
In the Fermi liquid theory , the energy of a quasiparticle of momentum p → {\displaystyle {\vec {p}}} is
where F {\displaystyle F} is the appropriately normalized Landau parameter, and
The approximated transport equation then has plane wave solutions
with ν ( p ^ ) {\displaystyle \nu ({\hat {p}})} [ 5 ] given by
This functional operator equation gives the dispersion relation for the zero sound waves with frequency ω {\displaystyle \omega } and wave vector k → {\displaystyle {\vec {k}}} . The transport equation is valid in the regime where ℏ ω ≪ E F {\displaystyle \hbar \omega \ll E_{\rm {F}}} and ℏ | k → | ≪ p F {\displaystyle \hbar |{\vec {k}}|\ll p_{\rm {F}}} .
In many systems, F ( p ^ , p ^ ′ ) {\displaystyle F({\hat {p}},{\hat {p}}')} only slowly depends on the angle between p ^ {\displaystyle {\hat {p}}} and p ^ ′ {\displaystyle {\hat {p}}'} . If F {\displaystyle F} is an angle-independent constant F 0 {\displaystyle F_{0}} with F 0 > 0 {\displaystyle F_{0}>0} (note that this constraint is stricter than the Pomeranchuk instability ) then the wave has the form ν ( p ^ ) ∝ ( ω / ( v F p ^ ⋅ k → ) − 1 ) − 1 {\displaystyle \nu ({\hat {p}})\propto ({\omega }/({v_{\rm {F}}{\hat {p}}\cdot {\vec {k}}})-1)^{-1}} and dispersion relation s 2 log s + 1 s − 1 − 1 = 1 / F 0 {\displaystyle {\frac {s}{2}}\log {\frac {s+1}{s-1}}-1=1/F_{0}} where s = ω / k v F {\displaystyle s=\omega /{kv_{\rm {F}}}} is the ratio of zero sound phase velocity to Fermi velocity. If the first two Legendre components of the Landau parameter are significant, F ( p ^ , p ^ ′ ) = F 0 + F 1 p ^ ⋅ p ^ ′ {\displaystyle F({\hat {p}},{\hat {p}}')=F_{0}+F_{1}{\hat {p}}\cdot {\hat {p}}'} and F 1 > 6 {\displaystyle F_{1}>6} , the system also admits an asymmetric zero sound wave solution ν ( p ^ ) ∝ sin ( 2 θ ) / ( s − cos θ ) e i ϕ {\displaystyle \nu ({\hat {p}})\propto {\sin(2\theta )}/({s-\cos {\theta }})e^{i\phi }} (where ϕ {\displaystyle \phi } and θ {\displaystyle \theta } are the azimuthal and polar angle of p ^ {\displaystyle {\hat {p}}} about the propagation direction k ^ {\displaystyle {\hat {k}}} ) and dispersion relation
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In interferometry , the zero spacing flux (ZSF) refers to the estimated integrated flux density within the field of view of an interferometer that would be measured by a (potentially hypothetical) single-dish telescope possessing the same primary beam as the interferometer. This value does not represent a direct measurement, but rather an extrapolation based on both single-dish observation and interferometric data. [ 1 ] Sometimes, in place of single-dish observations, dense array configurations can be used to capture large-scale emission data. [ 2 ]
The ZSF plays a crucial role in the construction of accurate images of astronomical sources, especially those that are extended across the field of view. Because interferometers cannot directly measure visibilities at zero separation between antennas (zero baseline), the ZSF remains unknown. This missing information can lead to a phenomenon known as the "missing flux problem." In the reconstructed image, extended sources appear to be surrounded by a faint, negative halo of brightness. [ 3 ]
By merging single-dish observations with the interferometric data, astronomers can account for the flux density of the largest structures in the sky, which are not captured by interferometers due to their limited spatial resolution. [ 4 ]
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In electrical circuit theory, the zero state response ( ZSR ) is the behaviour or response of a circuit with initial state of zero. The ZSR results only from the external inputs or driving functions of the circuit and not from the initial state.
The total response of the circuit is the superposition of the ZSR and the ZIR, or Zero Input Response. The ZIR results only from the initial state of the circuit and not from any external drive. The ZIR is also called the natural response , and the resonant frequencies of the ZIR are called the natural frequencies . Given a description of a system in the s-domain, the zero-state response can be described as Y(s)=Init(s)/a(s) where a(s) and Init(s) are system-specific.
One example of zero state response being used is in integrator and differentiator circuits. By examining a simple integrator circuit it can be demonstrated that when a function is put into a linear time-invariant (LTI) system, an output can be characterized by a superposition or sum of the Zero Input Response and the zero state response.
A system can be represented as
f ( t ) {\displaystyle f(t)\,} y ( t ) = y ( t 0 ) + ∫ t 0 t f ( τ ) d τ {\displaystyle y(t)=y(t_{0})+\int _{t_{0}}^{t}f(\tau )d\tau }
with the input f ( t ) . {\displaystyle f(t).\ } on the left and the output y ( t ) . {\displaystyle y(t).\ } on the right.
The output y ( t ) . {\displaystyle y(t).\ } can be separated into a zero input and a zero state solution with
y ( t ) = y ( t 0 ) ⏟ Z e r o − i n p u t r e s p o n s e + ∫ t 0 t f ( τ ) d τ ⏟ Z e r o − s t a t e r e s p o n s e . {\displaystyle y(t)=\underbrace {y(t_{0})} _{Zero-input\ response}+\underbrace {\int _{t_{0}}^{t}f(\tau )d\tau } _{Zero-state\ response}.}
The contributions of y ( t 0 ) {\displaystyle y(t_{0})\,} and f ( t ) {\displaystyle f(t)\,} to output y ( t ) {\displaystyle y(t)\,} are additive and each contribution y ( t 0 ) {\displaystyle y(t_{0})\,} and ∫ t 0 t f ( τ ) d τ {\displaystyle \int _{t_{0}}^{t}f(\tau )d\tau } vanishes with vanishing y ( t 0 ) {\displaystyle y(t_{0})\,} and f ( t ) . {\displaystyle f(t).\,}
This behavior constitutes a linear system . A linear system has an output that is a sum of distinct zero-input and zero-state components, each varying linearly, with the initial state of the system and the input of the system respectively.
The zero input response and zero state response are independent of each other and therefore each component can be computed independently of the other.
The Zero State Response ∫ t 0 t f ( τ ) d τ {\displaystyle \int _{t_{0}}^{t}f(\tau )d\tau } represents the system output y ( t ) {\displaystyle y(t)\,} when y ( t 0 ) = 0. {\displaystyle y(t_{0})=0.\,}
When there is no influence from internal voltages or currents due to previously charged components
y ( t ) = ∫ t 0 t f ( τ ) d τ . {\displaystyle y(t)=\int _{t_{0}}^{t}f(\tau )d\tau .\,}
Zero state response varies with the system input and under zero-state conditions we could say that two independent inputs results in two independent outputs:
f 1 ( t ) {\displaystyle f_{1}(t)\,} y 1 ( t ) {\displaystyle y_{1}(t)\,}
and
f 2 ( t ) {\displaystyle f_{2}(t)\,} y 2 ( t ) . {\displaystyle y_{2}(t).\,}
Because of linearity we can then apply the principles of superposition to achieve
K f 1 ( t ) + K f 2 ( t ) {\displaystyle Kf_{1}(t)+Kf_{2}(t)\,} K y 1 ( t ) + K y 2 ( t ) . {\displaystyle Ky_{1}(t)+Ky_{2}(t).\,}
The circuit to the right acts as a simple integrator circuit and will be used to verify the equation y ( t ) = ∫ t 0 t f ( τ ) d τ {\displaystyle y(t)=\int _{t_{0}}^{t}f(\tau )d\tau \,} as the zero state response of an integrator circuit.
Capacitors have the current-voltage relation i ( t ) = C d v d t {\displaystyle i(t)=C{\frac {dv}{dt}}} where C is the capacitance, measured in farads , of the capacitor.
By manipulating the above equation the capacitor can be shown to effectively integrate the current through it. The resulting equation also demonstrates the zero state and zero input responses to the integrator circuit.
First, by integrating both sides of the above equation
∫ a b i ( t ) d t = ∫ a b C d v d t d t . {\displaystyle \int _{a}^{b}i(t)dt=\int _{a}^{b}C{\frac {dv}{dt}}dt.}
Second, by integrating the right side
∫ a b i ( t ) d t = C [ v ( b ) − v ( a ) ] . {\displaystyle \int _{a}^{b}i(t)dt=C[v(b)-v(a)].}
Third, distribute and subtract C v ( a ) {\displaystyle Cv(a)\,} to get
C v ( b ) = C v ( a ) + ∫ a b i ( t ) d t . {\displaystyle Cv(b)=Cv(a)+\int _{a}^{b}i(t)dt.}
Fourth, divide by C {\displaystyle C\,} to achieve
v ( b ) = v ( a ) + 1 C ∫ a b i ( t ) d t . {\displaystyle v(b)=v(a)+{\frac {1}{C}}\int _{a}^{b}i(t)dt.}
By substituting t {\displaystyle t\,} for b {\displaystyle b\,} and t o {\displaystyle t_{o}\,} for a {\displaystyle a\,} and by using the dummy variable τ {\displaystyle \tau \,} as the variable of integration the general equation
v ( t ) = v ( t 0 ) + 1 C ∫ t 0 t i ( τ ) d τ {\displaystyle v(t)=v(t_{0})+{\frac {1}{C}}\int _{t_{0}}^{t}i(\tau )d\tau }
is found.
The general equation can then be used to further demonstrate this verification by using the conditions of the simple integrator circuit above.
By using the capacitance of 1 farad as shown in the integrator circuit above
v ( t ) = v ( t 0 ) + ∫ t 0 t i ( τ ) d τ , {\displaystyle v(t)=v(t_{0})+\int _{t_{0}}^{t}i(\tau )d\tau ,}
which is the equation containing the zero input and zero state response seen at the top of the page.
To verify its zero state linearity set the voltage around the capacitor at time 0 equal to 0, or v ( t 0 ) = 0 {\displaystyle v(t_{0})=0\,} , meaning that there is no initial voltage. This eliminates the first term forming the equation
v ( t ) = ∫ t 0 t i ( τ ) d τ . {\displaystyle v(t)=\int _{t_{0}}^{t}i(\tau )d\tau .\,} .
In accordance with the methods of linear time-invariant systems , by putting two different inputs into the integrator circuit, i 1 ( t ) {\displaystyle i_{1}(t)\,} and i 2 ( t ) {\displaystyle i_{2}(t)\,} , the two different outputs
v 1 ( t ) = ∫ t 0 t i 1 ( τ ) d τ {\displaystyle v_{1}(t)=\int _{t_{0}}^{t}i_{1}(\tau )d\tau \,}
and
v 2 ( t ) = ∫ t 0 t i 2 ( τ ) d τ {\displaystyle v_{2}(t)=\int _{t_{0}}^{t}i_{2}(\tau )d\tau \,}
are found respectively.
By using the superposition principle the inputs i 1 ( t ) {\displaystyle i_{1}(t)\,} and i 2 ( t ) {\displaystyle i_{2}(t)\,} can be combined to get a new input
i 3 ( t ) = K 1 i 1 ( t ) + K 2 i 2 ( t ) {\displaystyle i_{3}(t)=K_{1}i_{1}(t)+K_{2}i_{2}(t)\,}
and a new output
v 3 ( t ) = ∫ t 0 t ( K 1 i 1 ( τ ) + K 2 i 2 ( τ ) ) d τ . {\displaystyle v_{3}(t)=\int _{t_{0}}^{t}(K_{1}i_{1}(\tau )+K_{2}i_{2}(\tau ))d\tau .}
By integrating the right side of
v 3 ( t ) = K 1 ∫ t 0 t i 1 ( τ ) d τ + K 2 ∫ t 0 t i 2 ( τ ) d τ , {\displaystyle v_{3}(t)=K_{1}\int _{t_{0}}^{t}i_{1}(\tau )d\tau +K_{2}\int _{t_{0}}^{t}i_{2}(\tau )d\tau ,}
v 3 ( t ) = K 1 v 1 ( t ) + K 2 v 2 ( t ) {\displaystyle v_{3}(t)=K_{1}v_{1}(t)+K_{2}v_{2}(t)\,}
is found, which implies the system is linear at zero state, v ( t 0 ) = 0 {\displaystyle v(t_{0})=0\,} .
This entire verification example could also have been done with a voltage source in place of the current source and an inductor in place of the capacitor. We would have then been solving for a current instead of a voltage.
The circuit analysis method of breaking a system output down into a zero state and zero input response is used industry wide including circuits , control systems , signal processing , and electromagnetics . Also most circuit simulation software , such as SPICE , support the method in one form or another.
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Zero suppression is the removal of redundant zeroes from a number. This can be done for storage, page or display space constraints or formatting reasons, such as making a letter more legible. [ 1 ] [ 2 ] [ 3 ]
One must be careful; in physics and related disciplines, trailing zeros are used to indicate the precision of the number, as an error of ±1 in the last place is assumed. Examples:
It is also a way to store a large array of numbers, where many of the entries are zero. By omitting the zeroes, and instead storing the indices along with the values of the non-zero items, less space may be used in total. It only makes sense if the extra space used for storing the indices (on average) is smaller than the space saved by not storing the zeroes. This is sometimes used in a sparse array . [ citation needed ]
Example:
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Zero trust architecture ( ZTA ) or perimeterless security is a design and implementation strategy of IT systems . The principle is that users and devices should not be trusted by default, even if they are connected to a privileged network such as a corporate LAN and even if they were previously verified.
ZTA is implemented by establishing identity verification, validating device compliance prior to granting access, and ensuring least privilege access to only explicitly-authorized resources. Most modern corporate networks consist of many interconnected zones, cloud services and infrastructure, connections to remote and mobile environments, and connections to non-conventional IT, such as IoT devices.
The traditional approach by trusting users and devices within a notional "corporate perimeter" or via a VPN connection is commonly not sufficient in the complex environment of a corporate network. The zero trust approach advocates mutual authentication , including checking the identity and integrity of users and devices without respect to location, and providing access to applications and services based on the confidence of user and device identity and device status in combination with user authentication . [ 1 ] The zero trust architecture has been proposed for use in specific areas such as supply chains. [ 2 ] [ 3 ]
The principles of zero trust can be applied to data access, and to the management of data. This brings about zero trust data security where every request to access the data needs to be authenticated dynamically and ensure least privileged access to resources. In order to determine if access can be granted, policies can be applied based on the attributes of the data, who the user is, and the type of environment using Attribute-Based Access Control (ABAC) . This zero-trust data security approach can protect access to the data. [ 4 ]
In April 1994, the term "zero trust" was coined by Stephen Paul Marsh in his doctoral thesis on computer security at the University of Stirling . Marsh's work studied trust as something finite that can be described mathematically, asserting that the concept of trust transcends human factors such as morality , ethics , lawfulness , justice , and judgement . [ 5 ]
The problems of the Smartie or M&M model of the network (the precursor description of de-perimeterisation ) was described by a Sun Microsystems engineer in a Network World article in May 1994, who described firewalls' perimeter defence, as a hard shell around a soft centre, like a Cadbury Egg. [ 6 ]
In 2001 the first version of the OSSTMM (Open Source Security Testing Methodology Manual) was released and this had some focus on trust. Version 3 which came out around 2007 has a whole chapter on Trust which says "Trust is a Vulnerability" and talks about how to apply the OSSTMM 10 controls based on Trust levels.
In 2003 the challenges of defining the perimeter to an organisation's IT systems was highlighted by the Jericho Forum of this year, discussing the trend of what was then given the name " de-perimeterisation ".
In response to Operation Aurora , a Chinese APT attack throughout 2009, Google started to implement a zero-trust architecture referred to as BeyondCorp .
In 2010 the term zero trust model was used by analyst John Kindervag of Forrester Research to denote stricter cybersecurity programs and access control within corporations. [ 7 ] [ 8 ] [ 9 ] However, it would take almost a decade for zero trust architectures to become prevalent, driven in part by increased adoption of mobile and cloud services. [ citation needed ]
In 2018, work undertaken in the United States by cybersecurity researchers at NIST and NCCoE led to the publication of NIST SP 800-207 – Zero Trust Architecture. [ 10 ] [ 11 ] The publication defines zero trust (ZT) as a collection of concepts and ideas designed to reduce the uncertainty in enforcing accurate, per-request access decisions in information systems and services in the face of a network viewed as compromised. A zero trust architecture (ZTA) is an enterprise's cyber security plan that utilizes zero trust concepts and encompasses component relationships, workflow planning, and access policies. Therefore, a zero trust enterprise is the network infrastructure (physical and virtual) and operational policies that are in place for an enterprise as a product of a zero trust architecture plan.
There are several ways to implement all the tenets of ZT; a full ZTA solution will include elements of all three:
In 2019 the United Kingdom National Cyber Security Centre (NCSC) recommended that network architects consider a zero trust approach for new IT deployments, particularly where significant use of cloud services is planned. [ 12 ] An alternative but consistent approach is taken by NCSC , in identifying the key principles behind zero trust architectures:
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In computer security , the Zeroday Emergency Response Team ( ZERT ) was a group of volunteer security researchers who produced emergency patches for zero day attack vulnerabilities in proprietary software . They came to public notice in late September 2006 with a patch for that month's Vector Markup Language vulnerability before Microsoft , later producing a patch for older versions of Microsoft Windows which are no longer supported by Microsoft.
The team included several members prominent in antivirus and network security work.
Their manifesto states: "ZERT members work together as a team to release a non-vendor patch when a so-called "0day" (zero-day) exploit appears in the open which poses a serious risk to the public, to the infrastructure of the Internet or both. The purpose of ZERT is not to "crack" products, but rather to "uncrack" them by averting security vulnerabilities in them before they can be widely exploited."
The ZERT website has not been updated since April 2007 and the group is presumed to be inactive.
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Zerodium is an American information security company. The company was founded in 2015 with operations in Washington, D.C. , and Europe . The company develops and acquires zero-day exploits from security researchers.
Zerodium was launched on July 25, 2015 by the founders of Vupen . The company pays bounties for zero-day exploits . A zero-day exploit is a cybersecurity attack that targets security flaws in computer hardware, software or firmware in order to maliciously plant malware, steal data, or damage the program. Bug bounty programs , including Zerodium, pay bounties for knowledge of these security flaws.
Zerodium was the first company to release a full pricing chart for zero-days, ranging from $5,000 to $1,500,000 per exploit. [ 1 ] The company was reported to have spent between $400,000 to $600,000 per month for vulnerability acquisitions in 2015. [ 2 ]
In 2016, the company increased its permanent bug bounty for iOS exploits to $1,500,000. [ 3 ]
In September 2019, Zerodium increased its bounty for Android exploits to $2,500,000, and for the first time the company is paying more for Android exploits than iOS . Payouts for WhatsApp and iMessage have also been increased. The company is now reportedly spending between $1,000,000 to $3,000,000 each month for vulnerability acquisitions. [ 4 ]
In May 2024, Intelligence Online [ 5 ] posted an article titled "France, United States Iconic American vulnerability trader Zerodium to close its doors? " claiming that Zerodium had been absent for quite some time.
In January 2025, Zerodium disabled its website and replaced it with a single page containing their PGP key . [ 5 ]
Reporters Without Borders criticized Zerodium for selling information on exploits used to spy on journalists to foreign governments. [ 6 ]
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Zerologon (formally: CVE - 2020-1472 ) is a privilege elevation vulnerability in Microsoft 's authentication protocol Netlogon Remote Protocol (MS-NRPC) , as implemented in the Windows Client Authentication Architecture and Samba . [ 2 ] The vulnerability was first reported to Microsoft by security researcher Tom Tervoort from Secura on 17 August 2020 and dubbed "Zerologon". [ 1 ] [ 3 ] Zerologon was given a Common Vulnerability Scoring System v3.1 severity ranking of 10 by the U.S. American National Institute of Standards and Technology and a 5.5 by Microsoft. Crowdstrike classifies it as the most severe Active Directory vulnerability of 2020. [ 4 ]
The vulnerability allows from an unauthenticated user of the network to establish an unsafe connection to a Domain Controller (DC) and further impersonate the DC to elevate to domain admin privileges. [ 4 ] It allows attackers to access all valid usernames and passwords in each Microsoft network that they breached. [ 5 ] [ 6 ] This in turn allows them to access additional credentials necessary to assume the privileges of any legitimate user of the network, which in turn can let them compromise Microsoft 365 email accounts. [ 5 ] [ 6 ]
The Netlogon Remote Protocol (MS-NRPC) is a Microsoft protocol used for authentication and secure communication between clients and DCs in a Windows network environment. It facilitates the exchange of authentication data and the establishment of secure channels for communication, enabling clients to authenticate against Active Directory and other network services. The protocol plays a key role in domain join operations, password changes, and other security-related tasks within a Windows domain. [ 7 ]
The original report by Secura explains the exploit in five steps. [ 4 ]
The attack focuses on the DC of a network. MS-NRPC relies on a challenge–response authentication to generate a session key from the shared secret (such as a passphrase ). To authenticate a client, the MS-NRPC client credentials are computed from the session key, an initialization vector (IV), and the client challenge using a less common Advanced Encryption Standard (AES) block cipher mode, namely 8-bit Cipher Feedback Mode (AES-CFB8) . This is, where the vulnerability lies. Due to the randomly chosen server secret, the computations of the session key yield in 1 out of 256 cases a session key that begins with a zero-byte. The session key is then used to encrypt the IV and the client challenge. Since the IV is all-zero by default, the client challenge can be set to an all-zero vector as well and zero-byte beginning of the session key, AES-CFB8 results in an all-zero client credential. The server computed client credentials are then compared to the client sent credentials, which an attacker has also set to all-zero. The client is now authenticated. [ 4 ] [ 3 ]
To circumvent signing and encryption with the session key (which the attacker does not know) that is performed by MS-NRPC, an attacker can disable it by not setting a flag in the authentication RPC call. [ 4 ] [ 3 ]
Another obstacle the attacker must overcome is the so-called authenticator value used by Netlogon, that is required for some calls. This value is computed from an incrementing value held by the client, the client credentials, and a timestamp. If the incrementing value is set to all-zero by the client and the timestamp is also set to all-zero when an RPC call is invoked, the server will set the authenticator to all-zero as well, allowing the attacker to carry out the call. [ 4 ] [ 3 ]
In the penultimate step, the password is set to an empty one, allowing the attacker to follow the normal protocol procedure from this point on. [ 4 ] [ 3 ]
It is possible for the attacker to impersonate not just any user on the domain, but the domain controller itself. Once logged in, the attacker can retrieve hashed credentials from the DC, enabling a Pass the hash attack and ultimately elevating to the domain administrator. [ 4 ] [ 3 ]
Microsoft addressed the Zerologon vulnerability through two security updates. A less strict one in August 2020 and a later one in February 2021 that enforces signing and encryption for MS-NRPC calls by default, with the ability to allow certain devices to handle legacy support. [ 8 ]
In 2020, Zerologon started to be used by sophisticated cyberespionage campaigns of threat groups such as Red Apollo in global attacks against the automotive , engineering and pharmaceutical industry. [ 9 ] Zerologon was also used to hack the Municipal wireless network of Austin, Texas. [ 5 ]
Unusually, Zerologon was the subject of an emergency directive from the United States Cybersecurity and Infrastructure Security Agency . [ 10 ]
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Zerovalent iron (ZVI) is jargon [ clarification needed ] that describes forms of iron metal that are proposed for use in groundwater remediation . [ 1 ] [ 2 ] [ 3 ] [ 4 ]
ZVI operates by electron transfer from Fe 0 toward some organochlorine compounds, a common class of pollutants. The remediation process is proposed to generate Fe 2+ and Cl − and halide-free organic products, all of which are relatively innocuous. [ 5 ] Nanoscale ZVIs (nZVIs) are commonly used in remediation of chlorinated compounds and other pollutants. [ 6 ]
Treatment of many kinds of pollutants has been proposed, but few have been demonstrated in solving environmental challenges.
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ZetaGrid was at one time the largest distributed computing project, designed to explore the non-trivial roots of the Riemann zeta function , checking over one billion roots a day.
Roots of the zeta function are of particular interest in mathematics ; a single root out of alignment would disprove the Riemann hypothesis , with far-reaching consequences for all of mathematics.
The project ended in November 2005 due to instability of the hosting provider. [ 1 ] The first more than 10 13 zeroes were checked. [ 2 ] The project administrator stated that after the results were analyzed, they would be posted on the American Mathematical Society website. [ 3 ] The official status remains unclear, however, as it was never published nor independently verified. This is likely because there was no evidence that each zero was actually computed, as there was no process implemented to check each one as it was calculated. [ 4 ] [ 5 ]
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In mathematics and theoretical physics , zeta function regularization is a type of regularization or summability method that assigns finite values to divergent sums or products, and in particular can be used to define determinants and traces of some self-adjoint operators . The technique is now commonly applied to problems in physics , but has its origins in attempts to give precise meanings to ill-conditioned sums appearing in number theory .
There are several different summation methods called zeta function regularization for defining the sum of a possibly divergent series a 1 + a 2 + ....
One method is to define its zeta regularized sum to be ζ A (−1) if this is defined, where the zeta function is defined for large Re( s ) by
if this sum converges, and by analytic continuation elsewhere.
In the case when a n = n , the zeta function is the ordinary Riemann zeta function . This method was used by Ramanujan to "sum" the series 1 + 2 + 3 + 4 + ⋯ to ζ(−1) = −1/12.
Hawking (1977) showed that in flat space, in which the eigenvalues of Laplacians are known, the zeta function corresponding to the partition function can be computed explicitly. Consider a scalar field φ contained in a large box of volume V in flat spacetime at the temperature T = β −1 . The partition function is defined by a path integral over all fields φ on the Euclidean space obtained by putting τ = it which are zero on the walls of the box and which are periodic in τ with period β . In this situation from the partition function he computes energy, entropy and pressure of the radiation of the field φ . In case of flat spaces the eigenvalues appearing in the physical quantities are generally known, while in case of curved space they are not known: in this case asymptotic methods are needed.
Another method defines the possibly divergent infinite product a 1 a 2 .... to be exp(−ζ′ A (0)). Ray & Singer (1971) used this to define the determinant of a positive self-adjoint operator A (the Laplacian of a Riemannian manifold in their application) with eigenvalues a 1 , a 2 , ...., and in this case the zeta function is formally the trace of A − s . Minakshisundaram & Pleijel (1949) showed that if A is the Laplacian of a compact Riemannian manifold then the Minakshisundaram–Pleijel zeta function converges and has an analytic continuation as a meromorphic function to all complex numbers, and Seeley (1967) extended this to elliptic pseudo-differential operators A on compact Riemannian manifolds. So for such operators one can define the determinant using zeta function regularization. See " analytic torsion ."
Hawking (1977) suggested using this idea to evaluate path integrals in curved spacetimes. He studied zeta function regularization in order to calculate the partition functions for thermal graviton and matter's quanta in curved background such as on the horizon of black holes and on de Sitter background using the relation by the inverse Mellin transformation to the trace of the kernel of heat equations .
The first example in which zeta function regularization is available appears in the Casimir effect, which is in a flat space with the bulk contributions of the quantum field in three space dimensions. In this case we must calculate the value of Riemann zeta function at –3, which diverges explicitly. However, it can be analytically continued to s = –3 where hopefully there is no pole, thus giving a finite value to the expression. A detailed example of this regularization at work is given in the article on the detail example of the Casimir effect , where the resulting sum is very explicitly the Riemann zeta-function (and where the seemingly legerdemain analytic continuation removes an additive infinity, leaving a physically significant finite number).
An example of zeta-function regularization is the calculation of the vacuum expectation value of the energy of a particle field in quantum field theory . More generally, the zeta-function approach can be used to regularize the whole energy–momentum tensor both in flat and in curved spacetime. [1] [2] [3]
The unregulated value of the energy is given by a summation over the zero-point energy of all of the excitation modes of the vacuum:
Here, T 00 {\displaystyle T_{00}} is the zeroth component of the energy–momentum tensor and the sum (which may be an integral) is understood to extend over all (positive and negative) energy modes ω n {\displaystyle \omega _{n}} ; the absolute value reminding us that the energy is taken to be positive. This sum, as written, is usually infinite ( ω n {\displaystyle \omega _{n}} is typically linear in n). The sum may be regularized by writing it as
where s is some parameter, taken to be a complex number . For large, real s greater than 4 (for three-dimensional space), the sum is manifestly finite, and thus may often be evaluated theoretically.
The zeta-regularization is useful as it can often be used in a way such that the various symmetries of the physical system are preserved. Zeta-function regularization is used in conformal field theory , renormalization and in fixing the critical spacetime dimension of string theory .
Zeta function regularization is equivalent to dimensional regularization , see [4] . However, the main advantage of the zeta regularization is that it can be used whenever the dimensional regularization fails, for example if there are matrices or tensors inside the calculations ϵ i , j , k {\displaystyle \epsilon _{i,j,k}}
Zeta-function regularization gives an analytic structure to any sums over an arithmetic function f ( n ). Such sums are known as Dirichlet series . The regularized form
converts divergences of the sum into simple poles on the complex s -plane. In numerical calculations, the zeta-function regularization is inappropriate, as it is extremely slow to converge. For numerical purposes, a more rapidly converging sum is the exponential regularization, given by
This is sometimes called the Z-transform of f , where z = exp(− t ). The analytic structure of the exponential and zeta-regularizations are related. By expanding the exponential sum as a Laurent series
one finds that the zeta-series has the structure
The structure of the exponential and zeta-regulators are related by means of the Mellin transform . The one may be converted to the other by making use of the integral representation of the Gamma function :
which leads to the identity
relating the exponential and zeta-regulators, and converting poles in the s-plane to divergent terms in the Laurent series.
The sum
is sometimes called a heat kernel or a heat-kernel regularized sum ; this name stems from the idea that the ω n {\displaystyle \omega _{n}} can sometimes be understood as eigenvalues of the heat kernel . In mathematics, such a sum is known as a generalized Dirichlet series ; its use for averaging is known as an Abelian mean . It is closely related to the Laplace–Stieltjes transform , in that
where α ( t ) {\displaystyle \alpha (t)} is a step function , with steps of a n {\displaystyle a_{n}} at t = | ω n | {\displaystyle t=|\omega _{n}|} . A number of theorems for the convergence of such a series exist. For example, by the Hardy-Littlewood Tauberian theorem, if [5]
then the series for f ( s ) {\displaystyle f(s)} converges in the half-plane ℜ ( s ) > L {\displaystyle \Re (s)>L} and is uniformly convergent on every compact subset of the half-plane ℜ ( s ) > L {\displaystyle \Re (s)>L} . In almost all applications to physics, one has L = 0 {\displaystyle L=0}
Much of the early work establishing the convergence and equivalence of series regularized with the heat kernel and zeta function regularization methods was done by G. H. Hardy and J. E. Littlewood in 1916 [6] and is based on the application of the Cahen–Mellin integral . The effort was made in order to obtain values for various ill-defined, conditionally convergent sums appearing in number theory .
In terms of application as the regulator in physical problems, before Hawking (1977) , J. Stuart Dowker and Raymond Critchley in 1976 proposed a zeta-function regularization method for quantum physical problems. [7] Emilio Elizalde and others have also proposed a method based on the zeta regularization for the integrals ∫ a ∞ x m − s d x {\displaystyle \int _{a}^{\infty }x^{m-s}dx} , here x − s {\displaystyle x^{-s}} is a regulator and the divergent integral depends on the numbers ζ ( s − m ) {\displaystyle \zeta (s-m)} in the limit s → 0 {\displaystyle s\to 0} see renormalization . Also unlike other regularizations such as dimensional regularization and analytic regularization, zeta regularization has no counterterms and gives only finite results.
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Zeta potential titration is a titration of heterogeneous systems, for example colloids and emulsions . Solids in such systems have very high surface area . This type of titration is used to study the zeta potential of these surfaces under different conditions. Details of zeta potential definition and measuring techniques can be found in the International Standard. [ 1 ]
The iso-electric point is one such property. The iso-electric point is the pH value at which the zeta potential is approximately zero. At a pH near the iso-electric point (± 2 pH units), colloids are usually unstable; the particles tend to coagulate or flocculate . Such titrations use acids or bases as titration reagents . Tables of iso-electric points for different materials are available. [ 2 ] The attached figure illustrates results of such titrations for concentrated dispersions of alumina (4% v/v) and rutile (7% v/v). It is seen that iso-electric point of alumina is around pH 9.3, whereas for rutile it is around pH 4. Alumina is unstable in the pH range from 7 to 11. Rutile is unstable in the pH range from 2 to 6.
Another purpose of this titration is determination of the optimum dose of surfactant for achieving stabilization or flocculation of a heterogeneous system.
In a zeta-potential titration, the Zeta potential is the indicator . Measurement of the zeta potential can be performed using microelectrophoresis , or electrophoretic light scattering , or electroacoustic phenomena . The last method makes possible to perform titrations in concentrated systems, with no dilution.
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The byte is a unit of digital information that most commonly consists of eight bits . Historically, the byte was the number of bits used to encode a single character of text in a computer [ 1 ] [ 2 ] and for this reason it is the smallest addressable unit of memory in many computer architectures . To disambiguate arbitrarily sized bytes from the common 8-bit definition, network protocol documents such as the Internet Protocol ( RFC 791 ) refer to an 8-bit byte as an octet . [ 3 ] Those bits in an octet are usually counted with numbering from 0 to 7 or 7 to 0 depending on the bit endianness .
The size of the byte has historically been hardware -dependent and no definitive standards existed that mandated the size. Sizes from 1 to 48 bits have been used. [ 4 ] [ 5 ] [ 6 ] [ 7 ] The six-bit character code was an often-used implementation in early encoding systems, and computers using six-bit and nine-bit bytes were common in the 1960s. These systems often had memory words of 12, 18, 24, 30, 36, 48, or 60 bits, corresponding to 2, 3, 4, 5, 6, 8, or 10 six-bit bytes, and persisted, in legacy systems, into the twenty-first century. In this era, bit groupings in the instruction stream were often referred to as syllables [ a ] or slab , before the term byte became common.
The modern de facto standard of eight bits, as documented in ISO/IEC 2382-1:1993, is a convenient power of two permitting the binary-encoded values 0 through 255 for one byte, as 2 to the power of 8 is 256. [ 8 ] The international standard IEC 80000-13 codified this common meaning. Many types of applications use information representable in eight or fewer bits and processor designers commonly optimize for this usage. The popularity of major commercial computing architectures has aided in the ubiquitous acceptance of the 8-bit byte. [ 9 ] Modern architectures typically use 32- or 64-bit words, built of four or eight bytes, respectively.
The unit symbol for the byte was designated as the upper-case letter B by the International Electrotechnical Commission (IEC) and Institute of Electrical and Electronics Engineers (IEEE). [ 10 ] Internationally, the unit octet explicitly defines a sequence of eight bits, eliminating the potential ambiguity of the term "byte". [ 11 ] [ 12 ] The symbol for octet, 'o', also conveniently eliminates the ambiguity in the symbol 'B' between byte and bel .
The term byte was coined by Werner Buchholz in June 1956, [ 4 ] [ 13 ] [ 14 ] [ b ] during the early design phase for the IBM Stretch [ 15 ] [ 16 ] [ 1 ] [ 13 ] [ 14 ] [ 17 ] [ 18 ] computer, which had addressing to the bit and variable field length (VFL) instructions with a byte size encoded in the instruction. [ 13 ] It is a deliberate respelling of bite to avoid accidental mutation to bit . [ 1 ] [ 13 ] [ 19 ] [ c ]
Another origin of byte for bit groups smaller than a computer's word size, and in particular groups of four bits , is on record by Louis G. Dooley, who claimed he coined the term while working with Jules Schwartz and Dick Beeler on an air defense system called SAGE at MIT Lincoln Laboratory in 1956 or 1957, which was jointly developed by Rand , MIT, and IBM. [ 20 ] [ 21 ] Later on, Schwartz's language JOVIAL actually used the term, but the author recalled vaguely that it was derived from AN/FSQ-31 . [ 22 ] [ 21 ]
Early computers used a variety of four-bit binary-coded decimal (BCD) representations and the six-bit codes for printable graphic patterns common in the U.S. Army ( FIELDATA ) and Navy . These representations included alphanumeric characters and special graphical symbols. These sets were expanded in 1963 to seven bits of coding, called the American Standard Code for Information Interchange (ASCII) as the Federal Information Processing Standard , which replaced the incompatible teleprinter codes in use by different branches of the U.S. government and universities during the 1960s. ASCII included the distinction of upper- and lowercase alphabets and a set of control characters to facilitate the transmission of written language as well as printing device functions, such as page advance and line feed, and the physical or logical control of data flow over the transmission media. [ 18 ] During the early 1960s, while also active in ASCII standardization, IBM simultaneously introduced in its product line of System/360 the eight-bit Extended Binary Coded Decimal Interchange Code (EBCDIC), an expansion of their six-bit binary-coded decimal (BCDIC) representations [ d ] used in earlier card punches. [ 23 ] The prominence of the System/360 led to the ubiquitous adoption of the eight-bit storage size, [ 18 ] [ 16 ] [ 13 ] while in detail the EBCDIC and ASCII encoding schemes are different.
In the early 1960s, AT&T introduced digital telephony on long-distance trunk lines . These used the eight-bit μ-law encoding . This large investment promised to reduce transmission costs for eight-bit data.
In Volume 1 of The Art of Computer Programming (first published in 1968), Donald Knuth uses byte in his hypothetical MIX computer to denote a unit which "contains an unspecified amount of information ... capable of holding at least 64 distinct values ... at most 100 distinct values. On a binary computer a byte must therefore be composed of six bits". [ 24 ] He notes that "Since 1975 or so, the word byte has come to mean a sequence of precisely eight binary digits...When we speak of bytes in connection with MIX we shall confine ourselves to the former sense of the word, harking back to the days when bytes were not yet standardized." [ 24 ]
The development of eight-bit microprocessors in the 1970s popularized this storage size. Microprocessors such as the Intel 8080 , the direct predecessor of the 8086 , could also perform a small number of operations on the four-bit pairs in a byte, such as the decimal-add-adjust (DAA) instruction. A four-bit quantity is often called a nibble , also nybble , which is conveniently represented by a single hexadecimal digit.
The term octet unambiguously specifies a size of eight bits. [ 18 ] [ 12 ] It is used extensively in protocol definitions.
Historically, the term octad or octade was used to denote eight bits as well at least in Western Europe; [ 25 ] [ 26 ] however, this usage is no longer common. The exact origin of the term is unclear, but it can be found in British, Dutch, and German sources of the 1960s and 1970s, and throughout the documentation of Philips mainframe computers.
The unit symbol for the byte is specified in IEC 80000-13 , IEEE 1541 and the Metric Interchange Format [ 10 ] as the upper-case character B.
In the International System of Quantities (ISQ), B is also the symbol of the bel , a unit of logarithmic power ratio named after Alexander Graham Bell , creating a conflict with the IEC specification. However, little danger of confusion exists, because the bel is a rarely used unit. It is used primarily in its decadic fraction, the decibel (dB), for signal strength and sound pressure level measurements, while a unit for one-tenth of a byte, the decibyte, and other fractions, are only used in derived units, such as transmission rates.
The lowercase letter o for octet is defined as the symbol for octet in IEC 80000-13 and is commonly used in languages such as French [ 27 ] and Romanian , and is also combined with metric prefixes for multiples, for example ko and Mo.
More than one system exists to define unit multiples based on the byte. Some systems are based on powers of 10 , following the International System of Units (SI), which defines for example the prefix kilo as 1000 (10 3 ); other systems are based on powers of two . Nomenclature for these systems has led to confusion. Systems based on powers of 10 use standard SI prefixes ( kilo , mega , giga , ...) and their corresponding symbols (k, M, G, ...). Systems based on powers of 2, however, might use binary prefixes ( kibi , mebi , gibi , ...) and their corresponding symbols (Ki, Mi, Gi, ...) or they might use the prefixes K, M, and G, creating ambiguity when the prefixes M or G are used.
While the difference between the decimal and binary interpretations is relatively small for the kilobyte (about 2% smaller than the kibibyte), the systems deviate increasingly as units grow larger (the relative deviation grows by 2.4% for each three orders of magnitude). For example, a power-of-10-based terabyte is about 9% smaller than power-of-2-based tebibyte.
Definition of prefixes using powers of 10—in which 1 kilobyte (symbol kB) is defined to equal 1,000 bytes—is recommended by the International Electrotechnical Commission (IEC). [ 28 ] The IEC standard defines eight such multiples, up to 1 yottabyte (YB), equal to 1000 8 bytes. [ 29 ] The additional prefixes ronna- for 1000 9 and quetta- for 1000 10 were adopted by the International Bureau of Weights and Measures (BIPM) in 2022. [ 30 ] [ 31 ]
This definition is most commonly used for data-rate units in computer networks , internal bus, hard drive and flash media transfer speeds, and for the capacities of most storage media , particularly hard drives , [ 32 ] flash -based storage, [ 33 ] and DVDs . [ citation needed ] Operating systems that use this definition include macOS , [ 34 ] iOS , [ 34 ] Ubuntu , [ 35 ] and Debian . [ 36 ] It is also consistent with the other uses of the SI prefixes in computing, such as CPU clock speeds or measures of performance .
Prior art, the IBM System 360 and the related tape systems set the byte at 8 bits. [ 37 ] Early 5.25-inch disks used decimal [ dubious – discuss ] even though they used 128-byte and 256-byte sectors. [ 38 ] Hard disks used mostly 256-byte and then 512-byte before 4096-byte blocks became standard. [ 39 ] RAM was always sold in powers of 2. [ citation needed ]
A system of units based on powers of 2 in which 1 kibibyte (KiB) is equal to 1,024 (i.e., 2 10 ) bytes is defined by international standard IEC 80000-13 and is supported by national and international standards bodies ( BIPM , IEC , NIST ). The IEC standard defines eight such multiples, up to 1 yobibyte (YiB), equal to 1024 8 bytes. The natural binary counterparts to ronna- and quetta- were given in a consultation paper of the International Committee for Weights and Measures' Consultative Committee for Units (CCU) as robi- (Ri, 1024 9 ) and quebi- (Qi, 1024 10 ), but have not yet been adopted by the IEC or ISO. [ 40 ]
An alternative system of nomenclature for the same units (referred to here as the customary convention ), in which 1 kilobyte (KB) is equal to 1,024 bytes, [ 41 ] [ 42 ] [ 43 ] 1 megabyte (MB) is equal to 1024 2 bytes and 1 gigabyte (GB) is equal to 1024 3 bytes is mentioned by a 1990s JEDEC standard. Only the first three multiples (up to GB) are mentioned by the JEDEC standard, which makes no mention of TB and larger. While confusing and incorrect, [ 44 ] the customary convention is used by the Microsoft Windows operating system [ 45 ] [ better source needed ] and random-access memory capacity, such as main memory and CPU cache size, and in marketing and billing by telecommunication companies, such as Vodafone , [ 46 ] AT&T , [ 47 ] Orange [ 48 ] and Telstra . [ 49 ]
For storage capacity, the customary convention was used by macOS and iOS through Mac OS X 10.5 Leopard and iOS 10, after which they switched to units based on powers of 10. [ 34 ]
Various computer vendors have coined terms for data of various sizes, sometimes with different sizes for the same term even within a single vendor. These terms include double word , half word , long word , quad word , slab , superword and syllable . There are also informal terms. e.g., half byte and nybble for 4 bits, octal K for 1000 8 .
When I see a disk advertised as having a capacity of one megabyte, what is this telling me? There are three plausible answers, and I wonder if anybody knows which one is correct ... Now this is not a really vital issue, as there is just under 5% difference between the smallest and largest alternatives. Nevertheless, it would [be] nice to know what the standard measure is, or if there is one.
Contemporary [ e ] computer memory has a binary architecture making a definition of memory units based on powers of 2 most practical. The use of the metric prefix kilo for binary multiples arose as a convenience, because 1024 is approximately 1000 . [ 27 ] This definition was popular in early decades of personal computing , with products like the Tandon 5 1 ⁄ 4 -inch DD floppy format (holding 368 640 bytes) being advertised as "360 KB", following the 1024 -byte convention. It was not universal, however. The Shugart SA-400 5 1 ⁄ 4 -inch floppy disk held 109,375 bytes unformatted, [ 51 ] and was advertised as "110 Kbyte", using the 1000 convention. [ 52 ] Likewise, the 8-inch DEC RX01 floppy (1975) held 256 256 bytes formatted, and was advertised as "256k". [ 53 ] Some devices were advertised using a mixture of the two definitions: most notably, floppy disks advertised as "1.44 MB" have an actual capacity of 1440 KiB , the equivalent of 1.47 MB or 1.41 MiB.
In 1995, the International Union of Pure and Applied Chemistry 's (IUPAC) Interdivisional Committee on Nomenclature and Symbols attempted to resolve this ambiguity by proposing a set of binary prefixes for the powers of 1024, including kibi (kilobinary), mebi (megabinary), and gibi (gigabinary). [ 54 ] [ 55 ]
In December 1998, the IEC addressed such multiple usages and definitions by adopting the IUPAC's proposed prefixes (kibi, mebi, gibi, etc.) to unambiguously denote powers of 1024. [ 56 ] Thus one kibibyte (1 KiB) is 1024 1 bytes = 1024 bytes, one mebibyte (1 MiB) is 1024 2 bytes = 1 048 576 bytes, and so on.
In 1999, Donald Knuth suggested calling the kibibyte a "large kilobyte" ( KKB ). [ 57 ]
The IEC adopted the IUPAC proposal and published the standard in January 1999. [ 58 ] [ 59 ] The IEC prefixes are part of the International System of Quantities . The IEC further specified that the kilobyte should only be used to refer to 1000 bytes. [ 60 ]
Lawsuits arising from alleged consumer confusion over the binary and decimal definitions of multiples of the byte have generally ended in favor of the manufacturers, with courts holding that the legal definition of gigabyte or GB is 1 GB = 1 000 000 000 (10 9 ) bytes (the decimal definition), rather than the binary definition (2 30 , i.e., 1 073 741 824 ). Specifically, the United States District Court for the Northern District of California held that "the U.S. Congress has deemed the decimal definition of gigabyte to be the 'preferred' one for the purposes of 'U.S. trade and commerce' [...] The California Legislature has likewise adopted the decimal system for all 'transactions in this state. ' " [ 61 ]
Earlier lawsuits had ended in settlement with no court ruling on the question, such as a lawsuit against drive manufacturer Western Digital . [ 62 ] [ 63 ] Western Digital settled the challenge and added explicit disclaimers to products that the usable capacity may differ from the advertised capacity. [ 62 ] Seagate was sued on similar grounds and also settled. [ 62 ] [ 64 ]
Many programming languages define the data type byte .
The C and C++ programming languages define byte as an "addressable unit of data storage large enough to hold any member of the basic character set of the execution environment" (clause 3.6 of the C standard). The C standard requires that the integral data type unsigned char must hold at least 256 different values, and is represented by at least eight bits (clause 5.2.4.2.1). Various implementations of C and C++ reserve 8, 9, 16, 32, or 36 bits for the storage of a byte. [ 71 ] [ 72 ] [ f ] In addition, the C and C++ standards require that there be no gaps between two bytes. This means every bit in memory is part of a byte. [ 73 ]
Java's primitive data type byte is defined as eight bits. It is a signed data type, holding values from −128 to 127.
.NET programming languages, such as C# , define byte as an unsigned type, and the sbyte as a signed data type, holding values from 0 to 255, and −128 to 127 , respectively.
In data transmission systems, the byte is used as a contiguous sequence of bits in a serial data stream, representing the smallest distinguished unit of data. For asynchronous communication a full transmission unit usually additionally includes a start bit, 1 or 2 stop bits, and possibly a parity bit , and thus its size may vary from seven to twelve bits for five to eight bits of actual data. [ 74 ] For synchronous communication the error checking usually uses bytes at the end of a frame .
Terms used here to describe the structure imposed by the machine design, in addition to bit , are listed below. Byte denotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other than character is used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (i.e., different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined from bite , but respelled to avoid accidental mutation to bit .) A word consists of the number of data bits transmitted in parallel from or to memory in one memory cycle. Word size is thus defined as a structural property of the memory. (The term catena was coined for this purpose by the designers of the Bull GAMMA 60 [ fr ] computer.) Block refers to the number of words transmitted to or from an input-output unit in response to a single input-output instruction. Block size is a structural property of an input-output unit; it may have been fixed by the design or left to be varied by the program.
[...] Most important, from the point of view of editing, will be the ability to handle any characters or digits, from 1 to 6 bits long. Figure 2 shows the Shift Matrix to be used to convert a 60-bit word , coming from Memory in parallel, into characters , or 'bytes' as we have called them, to be sent to the Adder serially. The 60 bits are dumped into magnetic cores on six different levels. Thus, if a 1 comes out of position 9, it appears in all six cores underneath. Pulsing any diagonal line will send the six bits stored along that line to the Adder. The Adder may accept all or only some of the bits. Assume that it is desired to operate on 4 bit decimal digits , starting at the right. The 0-diagonal is pulsed first, sending out the six bits 0 to 5, of which the Adder accepts only the first four (0-3). Bits 4 and 5 are ignored. Next, the 4 diagonal is pulsed. This sends out bits 4 to 9, of which the last two are again ignored, and so on. It is just as easy to use all six bits in alphanumeric work, or to handle bytes of only one bit for logical analysis, or to offset the bytes by any number of bits. All this can be done by pulling the appropriate shift diagonals. An analogous matrix arrangement is used to change from serial to parallel operation at the output of the adder. [...]
byte: A string that consists of a number of bits, treated as a unit, and usually representing a character or a part of a character. NOTES: 1 The number of bits in a byte is fixed for a given data processing system. 2 The number of bits in a byte is usually 8.
We received the following from W Buchholz, one of the individuals who was working on IBM's Project Stretch in the mid 1950s. His letter tells the story. Not being a regular reader of your magazine, I heard about the question in the November 1976 issue regarding the origin of the term "byte" from a colleague who knew that I had perpetrated this piece of jargon [see page 77 of November 1976 BYTE, "Olde Englishe"] . I searched my files and could not locate a birth certificate. But I am sure that "byte" is coming of age in 1977 with its 21st birthday. Many have assumed that byte, meaning 8 bits, originated with the IBM System/360, which spread such bytes far and wide in the mid-1960s. The editor is correct in pointing out that the term goes back to the earlier Stretch computer (but incorrect in that Stretch was the first, not the last, of IBM's second-generation transistorized computers to be developed). The first reference found in the files was contained in an internal memo written in June 1956 during the early days of developing Stretch . A byte was described as consisting of any number of parallel bits from one to six. Thus a byte was assumed to have a length appropriate for the occasion. Its first use was in the context of the input-output equipment of the 1950s, which handled six bits at a time. The possibility of going to 8-bit bytes was considered in August 1956 and incorporated in the design of Stretch shortly thereafter . The first published reference to the term occurred in 1959 in a paper ' Processing Data in Bits and Pieces ' by G A Blaauw , F P Brooks Jr and W Buchholz in the IRE Transactions on Electronic Computers , June 1959, page 121. The notions of that paper were elaborated in Chapter 4 of Planning a Computer System (Project Stretch) , edited by W Buchholz, McGraw-Hill Book Company (1962). The rationale for coining the term was explained there on page 40 as follows: Byte denotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other than character is used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (ie, different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined from bite , but respelled to avoid accidental mutation to bit. ) System/360 took over many of the Stretch concepts, including the basic byte and word sizes, which are powers of 2. For economy, however, the byte size was fixed at the 8 bit maximum, and addressing at the bit level was replaced by byte addressing. Since then the term byte has generally meant 8 bits, and it has thus passed into the general vocabulary. Are there any other terms coined especially for the computer field which have found their way into general dictionaries of English language?
1956 Summer: Gerrit Blaauw , Fred Brooks , Werner Buchholz , John Cocke and Jim Pomerene join the Stretch team. Lloyd Hunter provides transistor leadership. 1956 July [ sic ]: In a report Werner Buchholz lists the advantages of a 64-bit word length for Stretch. It also supports NSA 's requirement for 8-bit bytes. Werner's term "Byte" first popularized in this memo.
NB. This timeline erroneously specifies the birth date of the term "byte" as July 1956 , while Buchholz actually used the term as early as June 1956 .
[...] 60 is a multiple of 1, 2, 3, 4, 5, and 6. Hence bytes of length from 1 to 6 bits can be packed efficiently into a 60-bit word without having to split a byte between one word and the next. If longer bytes were needed, 60 bits would, of course, no longer be ideal. With present applications, 1, 4, and 6 bits are the really important cases. With 64-bit words, it would often be necessary to make some compromises, such as leaving 4 bits unused in a word when dealing with 6-bit bytes at the input and output. However, the LINK Computer can be equipped to edit out these gaps and to permit handling of bytes which are split between words. [...]
[...] The maximum input-output byte size for serial operation will now be 8 bits, not counting any error detection and correction bits. Thus, the Exchange will operate on an 8-bit byte basis, and any input-output units with less than 8 bits per byte will leave the remaining bits blank. The resultant gaps can be edited out later by programming [...]
I came to work for IBM , and saw all the confusion caused by the 64-character limitation. Especially when we started to think about word processing, which would require both upper and lower case. Add 26 lower case letters to 47 existing, and one got 73 -- 9 more than 6 bits could represent. I even made a proposal (in view of STRETCH , the very first computer I know of with an 8-bit byte) that would extend the number of punch card character codes to 256 [1] . Some folks took it seriously. I thought of it as a spoof. So some folks started thinking about 7-bit characters, but this was ridiculous. With IBM's STRETCH computer as background, handling 64-character words divisible into groups of 8 (I designed the character set for it, under the guidance of Dr. Werner Buchholz , the man who DID coin the term "byte" for an 8-bit grouping). [2] It seemed reasonable to make a universal 8-bit character set, handling up to 256. In those days my mantra was "powers of 2 are magic". And so the group I headed developed and justified such a proposal [3]. That was a little too much progress when presented to the standards group that was to formalize ASCII, so they stopped short for the moment with a 7-bit set, or else an 8-bit set with the upper half left for future work. The IBM 360 used 8-bit characters, although not ASCII directly. Thus Buchholz's "byte" caught on everywhere. I myself did not like the name for many reasons. The design had 8 bits moving around in parallel. But then came a new IBM part, with 9 bits for self-checking, both inside the CPU and in the tape drives . I exposed this 9-bit byte to the press in 1973. But long before that, when I headed software operations for Cie. Bull in France in 1965-66, I insisted that 'byte' be deprecated in favor of " octet ". You can notice that my preference then is now the preferred term. It is justified by new communications methods that can carry 16, 32, 64, and even 128 bits in parallel. But some foolish people now refer to a "16-bit byte" because of this parallel transfer, which is visible in the UNICODE set. I'm not sure, but maybe this should be called a " hextet ". But you will notice that I am still correct. Powers of 2 are still magic!
The word byte was coined around 1956 to 1957 at MIT Lincoln Laboratories within a project called SAGE (the North American Air Defense System), which was jointly developed by Rand , Lincoln Labs, and IBM . In that era, computer memory structure was already defined in terms of word size . A word consisted of x number of bits ; a bit represented a binary notational position in a word. Operations typically operated on all the bits in the full word. We coined the word byte to refer to a logical set of bits less than a full word size. At that time, it was not defined specifically as x bits but typically referred to as a set of 4 bits , as that was the size of most of our coded data items. Shortly afterward, I went on to other responsibilities that removed me from SAGE. After having spent many years in Asia, I returned to the U.S. and was bemused to find out that the word byte was being used in the new microcomputer technology to refer to the basic addressable memory unit.
A question-and-answer session at an ACM conference on the history of programming languages included this exchange: [ John Goodenough : You mentioned that the term "byte" is used in JOVIAL . Where did the term come from? ] [ Jules Schwartz (inventor of JOVIAL): As I recall, the AN/FSQ-31 , a totally different computer than the 709 , was byte oriented. I don't recall for sure, but I'm reasonably certain the description of that computer included the word "byte," and we used it. ] [ Fred Brooks : May I speak to that? Werner Buchholz coined the word as part of the definition of STRETCH , and the AN/FSQ-31 picked it up from STRETCH, but Werner is very definitely the author of that word. ] [ Schwartz: That's right. Thank you. ]
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Zettascale computing refers to computing systems capable of calculating at least "10 21 IEEE 754 Double Precision (64-bit) operations (multiplications and/or additions) per second ( zetta FLOPS )". [ 1 ] It is a measure of supercomputer performance, and as of July 2022 [update] is a hypothetical performance barrier. [ 2 ] A zettascale computer system could generate more single floating point data in one second than was stored by the total digital means on Earth in the first quarter of 2011. [ citation needed ]
Floating point operations per second (FLOPS) are one measure of computer performance . FLOPS can be recorded in different measures of precision, however the standard measure (used by the TOP500 supercomputer list) uses 64 bit ( double-precision floating-point format ) operations per second using the High Performance LINPACK (HPLinpack) benchmark . [ 3 ] [ 4 ]
In 2018, Chinese scientists predicted that the first zettascale system will be assembled in 2035. [ 5 ] This forecast looks plausible from a historical point of view as it took some 12 years to progress from the terascale machines (10 12 ) to petascale systems (10 15 ) and then 14 more years to move to exascale computers (10 18 ). [ 5 ]
Scientists forecast that the zettascale systems are likely to be data-centric; this proposition means that the system components will move to the data, not vice versa, as the data volumes in the future are anticipated to be so large that moving data will be too expensive. It is also forecasted that zettascale systems are expected to be decentralized—because such a model can be the shortest route to achieving zettascale performance, with millions of less powerful components linked and working together to form a collective hypercomputer that is more powerful than any single machine. [ 5 ] Such decentralized systems may be designed to mimick complex biologic systems, and the next cybernetic paradigm may be based on liquid cybernetic systems with embodied intelligence solutions. [ 6 ] [ clarification needed ]
China’s National University of Defense Technology propose the following metrics: [ 7 ]
As Moore's law nears its natural limits, supercomputing will face serious physical problems in moving from exascale to zettascale systems, making the decade after 2020 a vital period to develop key high-performance computing techniques. [ 8 ] Many forecasters, including Gordon Moore himself, [ 9 ] expect Moore's law to end by around 2025. [ 10 ] [ 11 ] Another challenge for reaching zettascale performance can be enormous energy consumption. [ 12 ] [ 13 ]
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Zhao Jincai ( Chinese : 赵进才 ; born December 1960) is a Chinese environmental chemist and researcher of the Institute of Chemistry of the Chinese Academy of Sciences . In April 1994, he obtained a doctorate from Meisei University in Japan. [ 1 ] In 2011 he was elected as an academician of CAS. He is a professor at the Institute of Chemistry, CAS ; Deputy Director of Key Laboratory of Photochemistry ; Deputy Director of Science Committee of Molecular Sciences Centre, CAS. [ 2 ] His research mainly focuses on the photocatalytic degradation of toxic and persistent organic pollutants. Zhao introduced TiO 2 -based photocatalyst under visible light , and discovered a new way for oxygen atom transfer during photocatalytic reactions. [ 2 ]
In the early 2000s, Zhao's group reported that visible light can accelerate the degradation of organic pollutants with aqueous
solutions of iron tetrasulfophthalocyanine ([Fe(PcS)]) and H 2 O 2 . They also found out FeBR ( Fe 2+ complex of 2,2′-bipyridine) is efficient in eliminating organic pollutants such as rhodamine B (RhB), malachite green (MG) and N, N-dimethylaniline (DMA). They did several control experiments, in the dark or under irradiation, with or without irradiation. They proposed that when light is introduced, excitation of [Fe III (PcS)] can result in electron transfer from ligand (L) to Fe 3+ , then Fe 3+ can be reduced to Fe 2+ . The Fe 2+ -L complex can react with H 2 O 2 to produce HO . , leading to the degradation of pollutants. [ 3 ] [ 4 ]
Zhao has published over 200 papers in international journals and obtained 20 Chinese invention patents. Zhao won many awards:
2010 Japanese Photochemistry Association Lectureship Award for Asian and OceanianPhotochemist. [ 5 ]
2005 The Second Grade National Prize of Natural Science of China (the first contributor). [ 5 ]
2002 The Award of Excellent Young Scientists of Chinese Academy of Sciences- Bayer (Germany). [ 5 ]
2002 The Fiste Grade Prize of Science and Technology in Beijing (Natural Science). [ 5 ]
2002 The Second Grade Prize Military Science and Technique Progress Prize. [ 5 ]
1999 Be selected into the first and second levels of "the National Hundred, Thousand and Ten Thousand Talent Project". [ 5 ]
1998 The Special Contribution Award from the State Council of China. [ 5 ]
1997 Distinguished Young Scientists of NSFC. [ 5 ]
1999, 2002, 2006 The Award of Excellent Advisor of Graduates of CAS. [ 5 ]
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Zhenghan Wang ( Chinese : 王正汉 ; born April 26, 1965) is a Chinese-American mathematician . He is a principal researcher at Microsoft Station Q , as well as a professor of mathematics at the University of California, Santa Barbara .
Wang graduated with a B.S. and M.S. from the University of Science and Technology of China in 1989 and obtained his Ph.D. in 1993 from UC San Diego under the supervision of Michael Freedman . [ 1 ] From 1993 to 1996 Wang taught as an assistant professor at the University of Michigan and from 1996 to 2007 Wang taught at Indiana University Bloomington . For the majority of this time, Wang specialized in the topology of 4-manifolds . [ 2 ] [ 3 ] [ 4 ]
In 2005, Wang moved to Santa Barbara to serve as a lead scientist in the newly founded research institute Microsoft Station Q. At Station Q, Wang worked with Michael Freedman (the station's director and his former Ph.D. advisor) on the foundations of topological quantum computing . [ 5 ] Since 2012 Wang has served as a full professor at UC Santa Barbara . [ 1 ] From 2013 to 2020 Wang served as a distinguished visiting research chair at the Perimeter Institute for Theoretical Physics as well. [ 1 ] Wang was included in the 2019 class of fellows of the American Mathematical Society . [ 6 ]
Zhenghan Wang's most notable contributions are in the field of topological quantum computation . In a series of early papers with Michael Freedman, Michael J. Larsen , and Alexei Kitaev , Wang established the abstract equivalence of topological quantum computation with the quantum circuit model . [ 7 ] [ 8 ] [ 9 ] [ 10 ] The implication of these works for topological phases is that the Fibonacci anyon model can be used to make a universal quantum computer, and the implication of these works for quantum circuits is the Aharonov–Jones–Landau algorithm . [ 11 ] Wang has also introduced several other schemes for universal topological quantum computation using anyons which are more likely to be experimentally realizable. [ 12 ] [ 13 ] [ 14 ]
Outside of direct applications to topological quantum computing, Wang has made many contributions to the formal algebraic theory of two dimensional topological quantum phases of matter . This includes work on the structure and classification of bosonic topological order (modular tensor categories), [ 15 ] [ 16 ] [ 17 ] [ 18 ] fermionic topological order (super-modular tensor categories), [ 19 ] [ 20 ] [ 21 ] and symmetry-enriched topological order (G-crossed modular tensor categories). [ 22 ] [ 23 ] [ 24 ] Wang has also worked more specifically on the theory of the fractional quantum Hall effect [ 25 ] [ 26 ] [ 27 ] and anyonic chains. [ 28 ] [ 29 ] [ 30 ] [ 31 ]
In addition to his work on two dimensional topological order , Wang has also worked in the theory of three dimensional topological quantum field theory . Here he is most well known introducing the Walker-Wang model along with his coauthor Kevin Walker. [ 32 ] [ 33 ] [ 34 ] This theory has been used to describe the boundaries of topological insulators [ 35 ] and to construct nontrivial quantum cellular automata . [ 36 ] Wang has also made contributions to the theory of three dimensional fracton phases [ 37 ] [ 38 ]
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Zhong Zhong ( Chinese : 中中 ; pinyin : Zhōng Zhōng , born 27 November 2017) and Hua Hua ( Chinese : 华华 ; pinyin : Huá Huá , born 5 December 2017) are a pair of identical crab-eating macaques (also referred to as cynomolgus monkeys) that were created through somatic cell nuclear transfer (SCNT), the same cloning technique that produced Dolly the sheep in 1996. They are the first cloned primates produced by this technique. Unlike previous attempts to clone monkeys, the donated nuclei came from fetal cells , not embryonic cells . [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] The primates were born from two independent surrogate pregnancies at the Institute of Neuroscience of the Chinese Academy of Sciences in Shanghai . [ 6 ]
Since scientists produced the first cloned mammal Dolly the sheep in 1996 using the somatic cell nuclear transfer (SCNT) technique, 23 mammalian species have been successfully cloned, including cattle, cats, dogs, horses and rats. [ 4 ] Using this technique for primates had never been successful and no pregnancy had lasted more than 80 days. The main difficulty was likely the proper programming of the transferred nuclei to support the growth of the embryo. [ 3 ] Tetra (born October 1999), a female rhesus macaque , was created by a team led by Gerald Schatten of the Oregon National Primate Research Center using a different technique, called " embryo splitting ". She is the first cloned primate by artificial twinning, which is a much less complex procedure than the DNA transfer used for the creation of Zhong Zhong and Hua Hua. [ 7 ]
In January 2019, scientists in China reported the creation of five identical cloned gene-edited monkeys, using the same cloning technique that was used with Zhong Zhong and Hua Hua, and the same gene-editing CRISPR - Cas9 technique allegedly used by He Jiankui in creating the first ever gene-modified human babies Lulu and Nana . The monkey clones were made in order to study several medical diseases. [ 8 ] [ 9 ]
Zhong Zhong and Hua Hua were produced by scientists from the Institute of Neuroscience of the Chinese Academy of Sciences in Shanghai , led by Qiang Sun and Mu-ming Poo . [ 1 ] They extracted nuclei from the fibroblasts of an aborted fetal monkey (a crab-eating macaque or Macaca fascicularis ) and inserted them into egg cells (ova) that had had their own nuclei removed. [ 1 ] The team used two enzymes to erase the epigenetic memory of the transferred nuclei of being somatic cells . This crucial reprogramming step allowed the researchers to overcome the main obstacle that had precluded the successful cloning of primates until now. [ 3 ] They then placed 21 of these ova into surrogate mother monkeys, resulting in six pregnancies, two of which produced living animals. [ 1 ] The monkeys were named Zhong Zhong and Hua Hua, a reference to Zhonghua ( Chinese : 中华 , a Chinese name for China ). Although the success rate was still low, the methods could be improved to increase survival rate in the future. [ 3 ] By comparison, the Scotland -based team that created Dolly the sheep in 1996 required 277 attempts and produced only one lamb. [ 10 ]
The scientists also attempted to clone macaques using nuclei from adult donors, which is much more difficult. They implanted 42 surrogates, resulting in 22 pregnancies, but there were still only two infant macaques, and they died soon after birth. [ 1 ]
According to Mu-ming Poo , the principal significance of this event is that it could be used to create genetically identical monkeys for use in animal experiments . Crab-eating macaques are already an established model organism for studies of atherosclerosis , [ 11 ] though Poo chose to emphasize neuroscience , naming Parkinson's disease and Alzheimer's disease when he appeared on the radio news program All Things Considered in January 2018. [ 12 ]
The birth of the two cloned primates also raised concerns from bioethicists. Insoo Hyun of Case Western Reserve University questioned whether this meant that human cloning would be next. Poo told All Things Considered that "Technically speaking one can clone human[s] ... But we're not going to do it. There's absolutely no plan to do anything on humans." [ 12 ]
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Zhongma Fortress ( Chinese : 中馬城) — also Zhong Ma Prison Camp or Unit Tōgō — was a prison camp where the Japanese Kwantung Army carried out covert biological warfare research on human test subjects. Built in Beiyinhe , outside of Harbin , Manchukuo during the Second Sino-Japanese War , the camp served as a center for human subject experimentation and could hold up to 1,000 prisoners at any given time. [ 1 ] In 1937 the prison camp was destroyed and testing operations were transferred to Pingfang under Unit 731 .
In 1930 Doctor Shirō Ishii , an Imperial Japanese Army researcher in biological and chemical warfare , petitioned the Japanese War Ministry to establish a biological weapons program. With the support of Army Minister Sadao Araki and the dean of the Tokyo Army Medical College , Koizumi Chikahiko, a biological weapons program was initiated under a newly formed department of immunology. [ 2 ] Ishii began his research in biological warfare as the head of the "Epidemic Prevention Research Laboratory." [ 3 ] Although protecting Japanese troops from disease was part of the agenda, the laboratory's primary objective was to develop an effective means to spread epidemics. [ 3 ] Encouraged by preliminary results with lab animals, Ishii sought to replicate these outcomes with human trials. Due to containment issues and ethical constraints, human experimentation could not be conducted in his laboratory in Tokyo.
In 1932 the Japanese Imperial Army invaded Manchuria following the Manchurian Incident . The subsequent occupation of Manchuria provided an environment conducive to Ishii's research as human test subjects "could be plucked from the streets like rats." [ 4 ] Ishii relocated his laboratory to a military facility near Harbin. However, the facility's highly populated surroundings threatened to compromise the secrecy of the ongoing human experimentation. [ 5 ] Consequently, a second site, about 100 kilometers to the south of Harbin at the village of Beiyinhe, was selected. Beiyinhe was a diffuse village of about 300 homes known to the local populace as Zhong Ma City. The Imperial Japanese Army cleared out the local inhabitants and burnt down the village, except for a large building suitable for use as a headquarters. [ 3 ]
The prison camp had three-metre-high (9.8 ft) earthen walls topped with electrified barbed wire and a moat with drawbridge surrounded the buildings within. There were hundreds of rooms and smaller surrounding laboratories, office buildings, barracks and dining facilities, warehouses and munitions storage, crematoria , and the prison cells . The Japanese Imperial Army conscripted local Chinese labor for the construction. Due to secrecy, laborers were escorted by armed guards and forced to wear blinders so they could not figure out what they were constructing. Those who worked on the most sensitive areas of the prison camp, such as the inner section of medical laboratories within the prisoners' quarters, were executed once construction was complete to ensure secrecy. [ 3 ] The prisoners brought to Zhongma included common criminals , captured bandits, anti-Japanese partisans, as well as political prisoners and people rounded up on trumped up charges by the Kempeitai .
A variety of medical experiments were conducted on the prisoners within the camp. Prisoners were generally well fed on the usual diet of rice or wheat , meat , fish , and occasionally even alcohol , with the intent of keeping prisoners in their normal state of health at the beginning of experiments. In many cases, prisoners were drained of blood over several days, with careful records kept on their deteriorating physical condition. Others were subject to experiments on nutrient or water deprivation . Prisoners were also injected with microbes and plague bacteria. Data sheets reveal that in at least one case, after prisoners developed a fever of 104 °F (40 °C), they were vivisected while unconscious. [ 3 ]
The average life expectancy of a prisoner at the camp was one month. [ 3 ] Prisoners who survived the experiments, but who were deemed too weak for further tests, were killed. The facility was estimated to have held between 500-600 prisoners at any one time, with a capacity for over 1000. [ 3 ]
In August 1934, [ 6 ] at the time of the traditional summer festival, the prisoners were given a ration of special foods. One prisoner, named Li, managed to overpower his guard, seize the keys and freed about forty of his fellow prisoners. Although their legs were shackled, their arms were free, and the prisoners were able to climb the outside walls. A heavy downpour had knocked out the facility's electricity, deactivating the searchlights and electric fence . Some ten of the escapees were shot by guards while others were recaptured and subjected to sadistic treatment as reprisal, but roughly sixteen managed to escape. Some of the men soon died from exposure, hunger, cold, and the injuries from their experiments but several managed to survive, and spread word of the crimes against humanity being conducted by Shiro and his subordinates. [ 7 ] Although the Kuomintang took no notice of these reports, [ 8 ] Zhongma Fortress was closed down due to the significant publicity, and its activities transferred to a new site closer to Harbin called Pingfang (Heibo), which came to be known as Unit 731 . The testimony of one of the escapees, Ziyang Wang, was collected by Xiao Han, deputy director of the Pingfang museum, in the 1980s. [ 6 ]
The graphic novel Maruta 454 (2010), by Paul-Yanic Laquerre, Song Yang and Pastor, depicts the escape of 12 Chinese prisoners from Unit Tōgō, based on Wang's testimony.
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In organic chemistry , the Ziegler process (also called the Ziegler-Alfol synthesis) is a method for producing fatty alcohols from ethylene using an organoaluminium compound . The reaction produces linear primary alcohols with an even numbered carbon chain. The process uses an aluminum compound to oligomerize ethylene and allow the resulting alkyl group to be oxygenated. The usually targeted products are fatty alcohols, which are otherwise derived from natural fats and oils. Fatty alcohols are used in food and chemical processing. They are useful due to their amphipathic nature. The synthesis route is named after Karl Ziegler , who described the process in 1955. [ 1 ] [ 2 ]
The Ziegler alcohol synthesis involves oligomerization of ethylene using triethylaluminium followed by oxidation . [ 2 ] The triethylaluminium is produced by action of aluminium , ethylene , and hydrogen gas. In the production process, two-thirds of the triethylaluminium produced is recycled back into the reactor, and only one-third is used to produce the fatty alcohols. The recycling step is used to produce triethylaluminium at a higher yield and with less time. Triethylaluminium reacts with ethylene to form higher molecular weight trialkylaluminium. The number of equivalents of ethylene n equals the total number of monomer units being grown on the initial ethylene chains, where (n = x + y + z), and x, y, and z are the number of ethylene units per chain. Trialkylaluminium is oxidized with air to form aluminum alkoxides, and finally hydrolyzed to aluminum hydroxide and the desired alcohols. [ 1 ]
The temperature of the reaction influences the molecular weight of alcohol growth. Temperatures in the range of 60-120°C form higher molecular weight trialkylaluminium while higher temperatures (e.g., 120-150 °C) cause thermal displacement reactions that afford α-olefin chains. Above 150 °C, dimerization of the α-olefins occurs.
Aluminum hydroxide , the byproduct of the synthesis, can be dehydrated to give aluminium oxide , which, at high purities, has a high commercial value. One modification of the Ziegler process is called the EPAL process. In this process, chain growth is optimized to produce alcohols with narrow molecular weight distribution. Synthesis of other alcohols use Ziegler and the updated EPAL process, such as the transalkylation of styrene to form 2-phenylethanol . Diethylaluminum hydride can be employed in place of triethylaluminium. [ 1 ]
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A Ziegler–Natta catalyst , named after Karl Ziegler and Giulio Natta , is a catalyst used in the synthesis of polymers of 1-alkenes ( alpha-olefins ). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility:
Ziegler–Natta catalysts are used to polymerize terminal alkenes (ethylene and alkenes with the vinyl double bond):
The 1963 Nobel Prize in Chemistry was awarded to German Karl Ziegler , for his discovery of first titanium-based catalysts, and Italian Giulio Natta , for using them to prepare stereoregular polymers from propylene . Ziegler–Natta catalysts have been used in the commercial manufacture of various polyolefins since 1956. As of 2010, the total volume of plastics, elastomers, and rubbers produced from alkenes with these and related (especially Phillips) catalysts worldwide exceeds 100 million tonnes. Together, these polymers represent the largest-volume commodity plastics as well as the largest-volume commodity chemicals in the world.
In the early 1950s workers at Phillips Petroleum discovered that chromium catalysts are highly effective for the low-temperature polymerization of ethylene, which launched major industrial technologies culminating in the Phillips catalyst . A few years later, Ziegler discovered that a combination of titanium tetrachloride (TiCl 4 ) and diethylaluminium chloride (Al(C 2 H 5 ) 2 Cl) gave comparable activities for the production of polyethylene. Natta used crystalline α-TiCl 3 in combination with Al(C 2 H 5 ) 3 to produce first isotactic polypropylene . [ 3 ] Usually Ziegler catalysts refer to titanium -based systems for conversions of ethylene and Ziegler–Natta catalysts refer to systems for conversions of propylene .
Also, in the 1960s, BASF developed a gas-phase, mechanically-stirred polymerization process for making polypropylene . In that process, the particle bed in the reactor was either not fluidized or not fully fluidized. In 1968, the first gas-phase fluidized-bed polymerization process, the Unipol process, was commercialized by Union Carbide to produce polyethylene. In the mid-1980s, the Unipol process was further extended to produce polypropylene .
In the 1970s, magnesium chloride (MgCl 2 ) was discovered to greatly enhance the activity of the titanium-based catalysts. These catalysts were so active that the removal of unwanted amorphous polymer and residual titanium from the product (so-called deashing) was no longer necessary, enabling the commercialization of linear low-density polyethylene (LLDPE) resins and allowed the development of fully amorphous copolymers. [ 4 ]
The fluidized-bed process remains one of the two most widely used processes for producing polypropylene . [ 5 ]
Natta first used polymerization catalysts based on titanium chlorides to polymerize propylene and other 1-alkenes. He discovered that these polymers are crystalline materials and ascribed their crystallinity to a special feature of the polymer structure called stereoregularity .
The concept of stereoregularity in polymer chains is illustrated in the picture on the left with polypropylene. Stereoregular poly(1-alkene) can be isotactic or syndiotactic depending on the relative orientation of the alkyl groups in polymer chains consisting of units −[CH 2 −CHR]−, like the CH 3 groups in the figure. In the isotactic polymers, all stereogenic centers CHR share the same configuration. The stereogenic centers in syndiotactic polymers alternate their relative configuration. A polymer that lacks any regular arrangement in the position of its alkyl substituents (R) is called atactic. Both isotactic and syndiotactic polypropylene are crystalline, whereas atactic polypropylene, which can also be prepared with special Ziegler–Natta catalysts, is amorphous. The stereoregularity of the polymer is determined by the catalyst used to prepare it.
The first and dominant class of titanium-based catalysts (and some vanadium -based catalysts) for alkene polymerization can be roughly subdivided into two subclasses:
The overlap between these two subclasses is relatively small because the requirements to the respective catalysts differ widely.
Commercial catalysts are supported by being bound to a solid with a high surface area. Both TiCl 4 and TiCl 3 give active catalysts. [ 6 ] [ 7 ] The support in the majority of the catalysts is MgCl 2 . A third component of most catalysts is a carrier, a material that determines the size and the shape of catalyst particles. The preferred carrier is microporous spheres of amorphous silica with a diameter of 30–40 mm. During the catalyst synthesis, both the titanium compounds and MgCl 2 are packed into the silica pores. All these catalysts are activated with organoaluminum compounds such as Al(C 2 H 5 ) 3 . [ 7 ]
All modern supported Ziegler–Natta catalysts designed for polymerization of propylene and higher 1-alkenes are prepared with TiCl 4 as the active ingredient and MgCl 2 as a support. Another component of all such catalysts is an organic modifier, usually an ester of an aromatic diacid or a diether . The modifiers react both with inorganic ingredients of the solid catalysts as well as with organoaluminum cocatalysts. [ 7 ] These catalysts polymerize propylene and other 1-alkenes to highly crystalline isotactic polymers. [ 6 ] [ 7 ]
A second class of Ziegler–Natta catalysts are soluble in the reaction medium. Traditionally such homogeneous catalysts were derived from metallocenes , but the structures of active catalysts have been significantly broadened to include nitrogen-based ligands.
These catalysts are metallocenes together with a cocatalyst, typically MAO, −[O−Al(CH 3 )] n −. The idealized metallocene catalysts have the composition Cp 2 MCl 2 (M = Ti, Zr , Hf ) such as titanocene dichloride . Typically, the organic ligands are derivatives of cyclopentadienyl . In some complexes, the two cyclopentadiene (Cp) rings are linked with bridges, like −CH 2 −CH 2 − or >SiPh 2 . Depending on the type of their cyclopentadienyl ligands, for example by using an ansa -bridge , metallocene catalysts can produce either isotactic or syndiotactic polymers of propylene and other 1-alkenes. [ 6 ] [ 7 ] [ 9 ] [ 10 ]
Ziegler–Natta catalysts of the third class, non-metallocene catalysts, use a variety of complexes of various metals, ranging from scandium to lanthanoid and actinoid metals, and a large variety of ligands containing oxygen (O 2 ), nitrogen (N 2 ), phosphorus (P), and sulfur (S). The complexes are activated using MAO, as is done for metallocene catalysts.
Most Ziegler–Natta catalysts and all the alkylaluminium cocatalysts are unstable in air, and the alkylaluminium compounds are pyrophoric . The catalysts, therefore, are always prepared and handled under an inert atmosphere.
The structure of active centers in Ziegler–Natta catalysts is well established only for metallocene catalysts. An idealized and simplified metallocene complex Cp 2 ZrCl 2 represents a typical precatalyst. It is unreactive toward alkenes. The dihalide reacts with MAO and is transformed into a metallocenium ion Cp 2 + Zr CH 3 , which is ion-paired to some derivative(s) of MAO. A polymer molecule grows by numerous insertion reactions of C=C bonds of 1-alkene molecules into the Zr–C bond in the ion:
Many thousands of alkene insertion reactions occur at each active center resulting in the formation of long polymer chains attached to the center. The Cossee–Arlman mechanism describes the growth of stereospecific polymers. [ 3 ] [ 11 ] This mechanism states that the polymer grows through alkene coordination at a vacant site at the titanium atom, which is followed by insertion of the C=C bond into the Ti−C bond at the active center.
On occasion, the polymer chain is disengaged from the active centers in the chain termination reaction. Several pathways exist for termination:
Another type of chain termination reaction called a β-hydride elimination reaction also occurs periodically:
Polymerization reactions of alkenes with solid titanium-based catalysts occur at special titanium centers located on the exterior of the catalyst crystallites. Some titanium atoms in these crystallites react with organoaluminum cocatalysts with the formation of Ti–C bonds. The polymerization reaction of alkenes occurs similarly to the reactions in metallocene catalysts:
The two chain termination reactions occur quite rarely in Ziegler–Natta catalysis and the formed polymers have a too high molecular weight to be of commercial use. To reduce the molecular weight, hydrogen is added to the polymerization reaction:
Another termination process involves the action of protic (acidic) reagents, which can be intentionally added or adventitious.
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The Ziff–Gulari–Barshad (ZGB) model is a simple Monte Carlo method for catalytic reactions of oxidation of carbon monoxide to carbon dioxide on a surface using Monte-Carlo methods which captures correctly the essential dynamics: phase transitions between two poisoned states (either CO 2 - or O-poisoned) and a steady-state in between. It is named after Robert M. Ziff, Erdogan Gulari, and Yoav Barshad, who published it in 1986. [ 1 ]
The model consists of three steps:
The simplest implementation considers the catalyst as simple square two-dimensional lattice , but one can also consider other kinds of underlying lattices. [ 2 ] When a gas-phase molecule touches an empty site, adsorption occurs immediately and the chemical reaction is also instantaneous. Furthermore, one assumes that the composition of the gas phase remains constant.
While these requirements would still allow a large number of models and corresponding behaviors, the two special assumptions of the ZGB model are:
(i) CO molecules are adsorbed "standing" with the O touching the surface, and require thus only one free lattice site;
(ii) O 2 molecules are adsorbed "flat" and require thus two adjacent free lattice sites for getting adsorbed.
When the ratio between O 2 and CO in the gas phase is increased, the model shows two phase transitions: A continuous one between a O-poisoned and a
mixed state, and a discontinuous one between the mixed and a CO-poisoned state. The continuous transition belongs to the universality class of directed percolation . [ 3 ] The model was modified several times. [ 4 ] [ 5 ]
This article about statistical mechanics is a stub . You can help Wikipedia by expanding it .
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A zigzag is a pattern made up of small corners at variable angles, though constant within the zigzag, tracing a path between two parallel lines ; it can be described as both jagged and fairly regular.
In geometry , this pattern is described as a skew apeirogon . From the point of view of symmetry , a regular zigzag can be generated from a simple motif like a line segment by repeated application of a glide reflection .
Although the origin of the word is unclear, its first printed appearances were in French-language books and ephemera of the late 17th century. [ 1 ]
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In coding theory , a zigzag code is a type of linear error-correcting code introduced by Ping, Huang & Phamdo (2001) . [ 1 ] They are defined by partitioning the input data into segments of fixed size, and adding sequence of check bits to the data, where each check bit is the exclusive or of the bits in a single segment and of the previous check bit in the sequence.
The code rate is high: J /( J + 1) where J is the number of bits per segment. Its worst-case ability to correct transmission errors is very limited: in the worst case it can only detect a single bit error and cannot correct any errors. However, it works better in the soft-decision model of decoding : its regular structure allows the task of finding a maximum-likelihood decoding or a posteriori probability decoding to be performed in constant time per input bit.
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Zīj as-Sindhind ( Arabic : زيج السندهند الكبير , Zīj as‐Sindhind al‐kabīr , lit. "Great astronomical tables of the Sindhind"; from Sanskrit siddhānta , "system" or "treatise") is a work of zij (astronomical handbook with tables used to calculate celestial positions) brought in the early 770s AD to the court of Caliph al-Mansur in Baghdad from India . Al-Mansur requested an Arabic translation of this work from the Sanskrit. The 8th-century astronomer and translator Muḥammad ibn Ibrāhīm al-Fazārī is known to have contributed to this translation. [ 1 ] In his book Ṭabaqāt al-ʼUmam "Categories of Nations", [ 2 ] Said al-Andalusi informs that others who worked on it include ibn Sa'd and Muhammad ibn Musa al-Khwarizmi . He adds that its meaning is al-dahr al-dahir (infinite time or cyclic time).
This is the first of many Arabic zijs based on the Indian astronomical methods known as the Sindhind. The work contains tables for the movements of the sun, the moon and the five planets known at the time. It consists of approximately 37 chapters on calendar and astronomical calculations and 116 tables with calendar, astronomical and astrological data, as well as a table of sine values. [ citation needed ]
As described by Said al-Andalusi, as-Sindhind divides time into cyclic periods of creation and destruction which are called kalpas .
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In physics , zilch (or zilches) is a set of ten conserved quantities of the source-free electromagnetic field , which were discovered by Daniel M. Lipkin in 1964. [ 1 ] The name refers to the fact that the zilches are only conserved in regions free of electric charge , and therefore have limited physical significance. One of the conserved quantities (Lipkin's Z 0 {\displaystyle Z^{0}} ) has an intuitive physical interpretation and is also known as optical chirality .
In particular, first, Lipkin observed that if he defined the quantities
The free Maxwell equations imply that ∂ 0 Z 0 + ∇ ⋅ Z = 0 {\displaystyle \partial _{0}Z^{0}+\nabla \cdot \mathbf {Z} =0} .
The precedent equation implies that the quantity ∫ Z 0 d 3 x {\displaystyle \int Z^{0}\,d^{3}x} is constant. This time-independent quantity is one of the ten zilches discovered by Lipkin. Nowadays, the quantity ∫ Z 0 d 3 x {\displaystyle \int Z^{0}\,d^{3}x} is widely known as optical chirality (up to a factor of 1/2). [ 2 ]
The quantity Z 0 {\displaystyle {Z}^{0}} is the spatial density of optical chirality, while Z {\displaystyle \mathbf {Z} } is the optical chirality flux . [ 2 ] Generalizing the aforementioned differential conservation law for Z 0 {\displaystyle Z^{0}} , Lipkin found other nine conservation laws, all unrelated to the stress–energy tensor . He collectively named these ten conserved quantities the zilch (nowadays, they are also called the zilches [ 3 ] ) because of the apparent lack of physical significance. [ 1 ] [ 4 ]
The zilch is often described in terms of the zilch tensor, Z ν ρ μ {\displaystyle Z_{\nu \rho }^{\mu }} . The latter can be expressed using the dual electromagnetic tensor F ^ μ ν = ( 1 / 2 ) ϵ μ ν ρ σ F ρ σ {\displaystyle {\hat {F}}^{\mu \nu }=(1/2)\epsilon ^{\mu \nu \rho \sigma }F_{\rho \sigma }} as Z ν ρ μ = F ^ μ λ F λ ν , ρ − F μ λ F ^ λ ν , ρ {\displaystyle Z_{\nu \rho }^{\mu }={\hat {F}}^{\mu \lambda }F_{\lambda \nu ,\rho }-F^{\mu \lambda }{\hat {F}}_{\lambda \nu ,\rho }} . [ 5 ]
The zilch tensor is symmetric under the exchange of its first two indices , μ {\displaystyle \mu } and ν {\displaystyle \nu } , while it is also traceless with respect to any two indices, as well as divergence-free with respect to any index. [ 5 ]
The conservation law ∂ ρ Z μ ν ρ = 0 {\displaystyle \partial _{\rho }Z^{\mu \nu \rho }=0} means that the following ten quantities are time-independent:
It was later demonstrated that Lipkin's zilch is part of an infinite number of zilch-like conserved quantities, a general property of free fields . [ 5 ]
One of the zilches has been rediscovered. This is the zilch called "optical chirality", named by Tang and Cohen, since this zilch determines the degree of chiral asymmetry in the rate of excitation of a small chiral molecule by an incident electromagnetic field. [ 2 ] A further physical insight of optical chirality was offered in 2012; optical chirality is to the curl or time derivative of the electromagnetic field, what helicity, spin and related quantities are to the electromagnetic field itself. [ 6 ] The physical interpretation of all zilches for topologically non-trivial electromagnetic fields was investigated in 2018. [ 3 ]
Since the discovery of the ten zilches in 1964, there is an important open mathematical question concerning their relation with symmetries. (Recently, the full answer to this question seems to have been found [ 7 ] ). This is the question:
What are the symmetries of the standard Maxwell action functional:
S [ A μ ] = − 1 4 ∫ d 4 x F μ ν F μ ν {\displaystyle S[A_{\mu }]=-{\frac {1}{4}}\int d^{4}xF_{\mu \nu }F^{\mu \nu }}
(with F μ ν = ∂ μ A ν − ∂ ν A μ {\displaystyle F_{\mu \nu }=\partial _{\mu }A_{\nu }-\partial _{\nu }A_{\mu }} and A μ {\displaystyle A_{\mu }} is the dynamical field variable) that give rise to the conservation of all zilches using Noether's theorem . Until recently, the answer to this question had been given only for the case of optical chirality by Philbin in 2013. [ 8 ] This open question was also emphasized by Aghapour, Andersson and Rosquist in 2020, [ 9 ] while these authors found the symmetries of the duality-symmetric Maxwell action underlying the conservation of all zilches. (Aghapour, Andersson and Rosquist did not find the symmetries of the standard Maxwell action, but they speculated that such symmetries should exist [ 9 ] ).
There are also earlier works studying the conservation of zilch in the context of duality-symmetric electromagnetism, [ 10 ] but the variational character of the corresponding symmetries was not established.
The full answer to the aforementioned question seems to have been given for the first time in 2022, [ 7 ] where the symmetries of the standard Maxwell action underlying the conservation of all zilches were found. According to this work, there is a hidden invariance algebra of free Maxwell equations in potential form that is related to the conservation of all zilches.
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The Zimbabwe Institution of Engineers is the professional organization of engineers in Zimbabwe . It has graded membership, including student, technician, graduate and corporate membership as well as the status of fellow. [ 1 ]
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Zimbra Collaboration , formerly known as the Zimbra Collaboration Suite ( ZCS ) before 2019, is a collaborative software suite that includes an email server and a web client.
Zimbra was initially developed by LiquidSys, which changed their name to Zimbra, Inc. on 26 July 2005. [ 2 ] The Zimbra Collaboration Suite was first released in 2005. The company was subsequently purchased by Yahoo! on September 17, 2007, [ 3 ] and later sold to VMware on January 12, 2010. [ 4 ] In July 2013, it was sold by VMware to Telligent Systems [ 5 ] which changed its name to Zimbra, Inc. in September 2013. [ 6 ] It was then acquired by Synacor on 18 August 2015. [ 7 ]
According to former Zimbra President and CTO Scott Dietzen, the name for Zimbra is derived from the song " I Zimbra " by Talking Heads . [ 8 ]
The software consists of both client and server components, and at one time also offered a desktop email client, called Zimbra Desktop. Two versions of Zimbra are available: an open-source version, and a commercially supported version ("Network Edition") with closed-source components such as a proprietary Messaging Application Programming Interface connector to Outlook for calendar and contact synchronization. [ 9 ]
The now discontinued Zimbra Desktop was a full-featured free desktop email client . [ 10 ] Development was discontinued under VMware's stewardship in 2013 but was restarted in February 2014, but was ended again by 2019. The web client featured an HTML5 offline mode starting with version 8.5. [ 11 ]
The Zimbra Web Client is a full-featured collaboration suite that supports email and group calendars. At one time it featured document-sharing using an Ajax web interface that enabled tool tips, drag-and-drop items, and right-click menus in the UI. Today it has document sharing, chat, and videoconferencing. Also included are advanced searching capabilities and date relations, online document authoring, "Zimlet" mashups , and a full administration UI. It is written using the Zimbra Ajax Toolkit. [ 12 ]
The Zimbra Server uses several open source projects (see the section, Included open source projects ). It exposes a SOAP application programming interface to all its functionality and is also an IMAP and POP3 server. The server runs on many Linux distributions . [ 13 ] On other, non-Linux operating systems it can be run using a virtual machine and using container technology. It supports CalDAV, CardDAV and SMTP for messaging, LDAP for directory services, and Microsoft Active Directory (AD). Zimbra uses Postfix for its MTA functionality. It includes technology from ClamAV, SpamAssassin and DSPAM for anti-malware features and S/MIME for email signing and encryption. OS X Server support was dropped with version ZCS 7.0.
Zimbra can synchronize mail, contacts, and calendar items with open-source mail clients such as Mozilla Thunderbird and Evolution and also with proprietary clients such as Microsoft Outlook and Apple Mail , either through proprietary connectors or using the ActiveSync protocol, [ 14 ] both available exclusively in the commercially supported version. Zimbra also provides native two-way sync to many mobile devices. [ 9 ]
In 2024, Zimbra was hit by a significant cyber attack [ 15 ] due to a Remote Code Execution (RCE) vulnerability, labeled CVE-2024-45519. The flaw in Zimbra’s postjournal service allowed attackers to send specially crafted emails, enabling remote command execution on affected systems. While the postjournal service isn’t active by default, its use in many setups made Zimbra an attractive target, especially since some instances lacked up-to-date security patches. To mitigate these risks, Zimbra released patches for affected versions, including updates to 9.0.0, 10.0.9, and others.
The closed source variant Network edition is distributed under the Zimbra Network Edition EULA.
Starting with version 8.5 the Zimbra source code is available under the terms of the GNU General Public License version 2 (backend) and the Common Public Attribution License version 1 (frontend). [ 16 ]
Previous versions were released under the Zimbra Public License (ZPL). The Free Software Foundation accepts the license as being a free software license and refers to it as being identical to the Yahoo! Public License with the exception that Zimbra, Inc. provides the license, rather than Yahoo!. [ 17 ]
The Zimbra Server uses open source projects such as: [ 18 ]
It previously used:
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Zimmer's conjecture is a statement in mathematics "which has to do with the circumstances under which geometric spaces exhibit certain kinds of symmetries ." [ 1 ] It was named after the mathematician Robert Zimmer . The conjecture states that there can exist symmetries (specifically higher-rank lattices ) in a higher dimension that cannot exist in lower dimensions.
In 2017, the conjecture was proven by Aaron Brown and Sebastián Hurtado-Salazar of the University of Chicago and David Fisher of Indiana University . [ 1 ] [ 2 ] [ 3 ]
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Zimmerit was a paste-like coating used on mid- and late-war German armored fighting vehicles during World War II . It was used to produce a hard layer covering the metal armor of the vehicle, providing enough separation that magnetically attached anti-tank mines would fail to stick to the vehicle, despite Germany being the only country to use magnetic anti-tank mines in numbers. Zimmerit was often left off late-war vehicles due to the unfounded concern that it could catch fire when hit. [ 1 ] It was developed by the German company Chemische Werke Zimmer & Co ( Berlin ). [ 2 ]
The coating was a barrier that prevented direct contact of magnetic mines with metal surfaces of vehicles. The magnetostatic field decreases very rapidly, with the cube of distance; the non-magnetic coating holds the magnet of the mine too far from the steel of the vehicle for it to adhere. [ 2 ] [ 3 ] The coating was normally ridged to increase the distance between the magnet and the armor even further, as the high points on the pattern increase the effective thickness of the coating while minimising additional weight.
The mixture had the consistency of a thick paste or putty. It was applied to the vehicle, usually at the factory, patterned, and then hardened with blow torches . [ 1 ] There were many variations seen in application designs, from the regular ridge-shaped pattern, to a less common waffle-shaped pattern. The differences mostly related to the factory producing each type of AFV. For example, the waffle pattern was seen almost exclusively on Sturmgeschütz III assault guns. In general, vehicles already in service were not coated with Zimmerit. [ citation needed ]
The German army introduced the Hafthohlladung anti-tank weapon in 1942. This consisted of a shaped charge warhead connected to a metal ring holding three powerful horseshoe magnets . Issued to infantry, the user would run up to the tank and place the device on any surface to which the magnets would stick. The user would then pull the safety pin and run for safety. The magnets not only held the mine to the vehicle but also provided the correct spacing between warhead and armor, allowing the penetrator jet to form properly. Concerned that the simple design could be easily copied in the USSR, or the possibility that many of these weapons could fall into the hands of their enemies, the German army began looking for ways to defeat such a weapon when used against their own vehicles. [ 1 ]
Zimmerit was applied to some tanks and casemate -style closed-top self-propelled guns and tank destroyers produced from December 1943 to 9 September 1944. [ 3 ] It was only rarely applied to open-topped AFVs. The rough appearance of the coating gave a distinct appearance; for one type, e.g. a shingle-like look.
Zimmerit was discontinued from factory application on 9 September 1944 and from field application on 7 October 1944. [ 4 ] This was due to concerns that projectile impacts could ignite it. These proved false, but the order was never rescinded. [ 3 ] [ 4 ] Applying and drying the paste added days to the production of each vehicle, [ 2 ] which was unacceptable as there was a shortage of tanks.
Following the war, the British carried out trials of a similar material on Churchill and Cromwell tanks and some trials were conducted in Canada with a similar material applied to self-propelled guns [ 5 ] but it was not implemented. No similar material was used on post-war tanks as the widespread use of man-portable HEAT rockets such as the bazooka made magnetic mines obsolete.
The paste was composed of the following: [ 2 ] [ 3 ]
In the raw paste, polyvinyl acetate was used in the form of "Mowilith 20", a 50% benzene solution. [ 2 ] During the drying process, the benzene evaporated and the mixture hardened.
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https://en.wikipedia.org/wiki/Zimmerit
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The Zimmermann reagent is used as a simple spot-test used in chromatography to presumptively identify alkaloids , especially benzodiazepines , as well as other compounds. It is therefore used in drugs testing. [ 1 ] [ 2 ] [ 3 ]
It is a two-component reagent, with the first component composed of 1,3-dinitrobenzene (1% w/v) in methanol and the second component composed of 15% potassium hydroxide in water. [ 4 ] [ 5 ] [ 6 ]
One drop of each component is added to the sample being tested and the resulting colour change is observed to give an indication of the identity of the compound.
The reagent works by forming a reddish-purple Meisenheimer complex at C3 for diazepines with a carbonyl at C2 and an alkyl group at N1. [ 7 ] Without these groups it is not possible to form the methylene compound which reacts with dinitrobenzene but triazolo compounds may react.
It is named for the American biochemist Robert Zimmermann (b.1937).
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In statistical mechanics , the Zimm–Bragg model is a helix-coil transition model that describes helix-coil transitions of macromolecules , usually polymer chains. Most models provide a reasonable approximation of the fractional helicity of a given polypeptide ; the Zimm–Bragg model differs by incorporating the ease of propagation (self-replication) with respect to nucleation . It is named for co-discoverers Bruno H. Zimm and J. K. Bragg.
Helix-coil transition models assume that polypeptides are linear chains composed of interconnected segments. Further, models group these sections into two broad categories: coils , random conglomerations of disparate unbound pieces, are represented by the letter 'C', and helices , ordered states where the chain has assumed a structure stabilized by hydrogen bonding , are represented by the letter 'H'. [ 1 ]
Thus, it is possible to loosely represent a macromolecule as a string such as CCCCHCCHCHHHHHCHCCC and so forth. The number of coils and helices factors into the calculation of fractional helicity, θ {\displaystyle \theta \ } , defined as
where
The Zimm–Bragg model takes the cooperativity of each segment into consideration when calculating fractional helicity. The probability of any given monomer being a helix or coil is affected by which the previous monomer is; that is, whether the new site is a nucleation or propagation.
By convention, a coil unit ('C') is always of statistical weight 1. Addition of a helix state ('H') to a previously coiled state (nucleation) is assigned a statistical weight σ s {\displaystyle \sigma s\ } , where σ {\displaystyle \sigma \ } is the nucleation parameter and s {\displaystyle s\ } is the equilibrium constant
Adding a helix state to a site that is already a helix (propagation) has a statistical weight of s {\displaystyle s\ } . For most proteins ,
which makes the propagation of a helix more favorable than nucleation of a helix from coil state. [ 2 ]
From these parameters, it is possible to compute the fractional helicity θ {\displaystyle \theta \ } . The average helicity ⟨ i ⟩ {\displaystyle \left\langle i\right\rangle \ } is given by
where q {\displaystyle q\ } is the partition function given by the sum of the probabilities of each site on the polypeptide. The fractional helicity is thus given by the equation
The Zimm–Bragg model is equivalent to a one-dimensional Ising model and has no long-range interactions, i.e., interactions between residues well separated along the backbone; therefore, by the famous argument of Rudolf Peierls , it cannot undergo a phase transition .
The statistical mechanics of the Zimm–Bragg model [ 3 ] may be solved exactly using the transfer-matrix method . The two parameters of the Zimm–Bragg model are σ, the statistical weight for nucleating a helix and s , the statistical weight for propagating a helix. These parameters may depend on the residue j ; for example, a proline residue may easily nucleate a helix but not propagate one; a leucine residue may nucleate and propagate a helix easily; whereas glycine may disfavor both the nucleation and propagation of a helix. Since only nearest-neighbour interactions are considered in the Zimm–Bragg model, the full partition function for a chain of N residues can be written as follows
where the 2x2 transfer matrix W j of the j th residue equals the matrix of statistical weights for the state transitions
The row-column entry in the transfer matrix equals the statistical weight for making a transition from state row in residue j − 1 to state column in residue j . The two states here are helix (the first) and coil (the second). Thus, the upper left entry s is the statistical weight for transitioning from helix to helix, whereas the lower left entry σs is that for transitioning from coil to helix.
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https://en.wikipedia.org/wiki/Zimm–Bragg_model
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The Zinagizado is an electrochemical process to provide a ferrous metal material with anti- corrosive properties. [ 1 ] It involves the application of a constant electric current through a circuit to break the bonds and these are attached to the metal to be coated by forming a surface coating . [ 2 ] The alloy used is called Zinag (Zn-Al-Ag); this alloy has excellent mechanical and corrosive properties, so the piece will have increased by 60% of life. [ 3 ]
The deposition of Zinag provides environmental protection against corrosion and can be used in covering all kinds of steel metallic materials in contact with a corrosive medium. The anti-corrosive property has been obtained by the corrosion resistance of zinc achieved by the aluminium and silver addition, which is cathodically respect to the iron and steel. Cathodic protection
This process is an innovation by Said Robles Casolco and Adrianni Zanatta. [ 4 ]
Patent called: Zinagizado as corrosion process for metals by electrolytic method. No. MX/a/2010/009200, IMPI-Mexico.
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https://en.wikipedia.org/wiki/Zinagizado
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Zinc-finger nucleases ( ZFNs ) are artificial restriction enzymes generated by fusing a zinc finger DNA-binding domain to a DNA-cleavage domain . Zinc finger domains can be engineered to target specific desired DNA sequences and this enables zinc-finger nucleases to target unique sequences within complex genomes . By taking advantage of endogenous DNA repair machinery, these reagents can be used to precisely alter the genomes of higher organisms. Alongside CRISPR/Cas9 and TALEN , ZFN is a prominent tool in the field of genome editing .
It was initially created by researcher Srinivasan Chandrasegaran .
The DNA-binding domains of individual ZFNs typically contain between three and six individual zinc finger repeats and can each recognize between 9 and 18 basepairs. If the zinc finger domains perfectly recognize a 3 basepair DNA sequence, they can generate a 3-finger array that can recognize a 9 basepair target site. Other procedures can utilize either 1-finger or 2-finger modules to generate zinc-finger arrays with six or more individual zinc fingers. The main drawback with this procedure is the specificities of individual zinc fingers can overlap and can depend on the context of the surrounding zinc fingers and DNA. Without methods to account for this "context dependence", the standard modular assembly procedure often fails. [ 1 ]
Numerous selection methods have been used to generate zinc-finger arrays capable of targeting desired sequences. Initial selection efforts utilized phage display to select proteins that bound a given DNA target from a large pool of partially randomized zinc-finger arrays. More recent efforts have utilized yeast one-hybrid systems, bacterial one-hybrid and two-hybrid systems, and mammalian cells. A promising new method to select novel zinc-finger arrays utilizes a bacterial two-hybrid system and has been dubbed "OPEN" by its creators. [ 2 ] This system combines pre-selected pools of individual zinc fingers that were each selected to bind a given triplet and then utilizes a second round of selection to obtain 3-finger arrays capable of binding a desired 9-bp sequence. This system was developed by the Zinc-Finger Consortium as an alternative to commercial sources of engineered zinc-finger arrays.
(see: Zinc finger chimera for more info on zinc finger selection techniques)
The non-specific cleavage domain from the type IIs restriction endonuclease FokI is typically used as the cleavage domain in ZFNs. [ 4 ] This cleavage domain must dimerize in order to cleave DNA [ 5 ] and thus a pair of ZFNs are required to target non-palindromic DNA sites. Standard ZFNs fuse the cleavage domain to the C-terminus of each zinc finger domain. To let the two cleavage domains dimerize and cleave DNA, the two individual ZFNs must bind opposite strands of DNA with their C-termini a certain distance apart. The most commonly used linker sequences between the zinc finger domain and the cleavage domain requires the 5′ edge of each binding site to be separated by 5 to 7 bp. [ 6 ]
Several different protein engineering techniques have been employed to improve both the activity and specificity of the nuclease domain used in ZFNs. Directed evolution has been employed to generate a FokI variant with enhanced cleavage activity that the authors dubbed "Sharkey". [ 7 ] Structure-based design has also been employed to improve the cleavage specificity of FokI by modifying the dimerization interface so that only the intended heterodimeric species are active. [ 8 ] [ 9 ] [ 10 ] [ 11 ]
Zinc finger nucleases are useful to manipulate the genomes of many plants and animals including arabidopsis , [ 12 ] [ 13 ] tobacco , [ 14 ] [ 15 ] soybean , [ 16 ] corn , [ 17 ] Drosophila melanogaster , [ 18 ] C. elegans , [ 19 ] Platynereis dumerilii , [ 20 ] sea urchin , [ 21 ] silkworm , [ 22 ] zebrafish , [ 23 ] frogs , [ 24 ] mice , [ 25 ] rats , [ 26 ] rabbits , [ 27 ] pigs , [ 28 ] cattle , [ 29 ] and
various types of mammalian cells. [ 30 ] Zinc finger nucleases have also been used in a mouse model of haemophilia [ 31 ] and a clinical trial found CD4+ human T-cells with the CCR5 gene disrupted by zinc finger nucleases to be safe as a potential treatment for HIV/AIDS . [ 32 ] ZFNs are also used to create a new generation of genetic disease models called isogenic human disease models .
ZFNs can be used to disable dominant mutations in heterozygous individuals by producing double-strand breaks (DSBs) in the DNA (see Genetic recombination ) in the mutant allele, which will, in the absence of a homologous template, be repaired by non-homologous end-joining (NHEJ). NHEJ repairs DSBs by joining the two ends together and usually produces no mutations, provided that the cut is clean and uncomplicated. In some instances, however, the repair is imperfect, resulting in deletion or insertion of base-pairs, producing frame-shift and preventing the production of the harmful protein. [ 33 ] Multiple pairs of ZFNs can also be used to completely remove entire large segments of genomic sequence. [ 34 ] To monitor the editing activity, a PCR of the target area amplifies both alleles and, if one contains an insertion, deletion, or mutation, it results in a heteroduplex single-strand bubble that cleavage assays can easily detect.
ZFNs have also been used to modify disease-causing alleles in triplet repeat disorders. Expanded CAG/CTG repeat tracts are the genetic basis for more than a dozen inherited neurological disorders including Huntington's disease, myotonic dystrophy, and several spinocerebellar ataxias. It has been demonstrated in human cells that ZFNs can direct double-strand breaks (DSBs) to CAG repeats and shrink the repeat from long pathological lengths to short, less toxic lengths. [ 35 ]
Recently, a group of researchers have successfully applied the ZFN technology to genetically modify the gol pigment gene and the ntl gene in zebrafish embryo. Specific zinc-finger motifs were engineered to recognize distinct DNA sequences. The ZFN-encoding mRNA was injected into one-cell embryos and a high percentage of animals carried the desired mutations and phenotypes. Their research work demonstrated that ZFNs can specifically and efficiently create heritable mutant alleles at loci of interest in the germ line, and ZFN-induced alleles can be propagated in subsequent generations.
Similar research of using ZFNs to create specific mutations in zebrafish embryo has also been carried out by other research groups. The kdr gene in zebra fish encodes for the vascular endothelial growth factor-2 receptor. Mutagenic lesions at this target site was induced using ZFN technique by a group of researchers in US. They suggested that the ZFN technique allows straightforward generation of a targeted allelic series of mutants; it does not rely on the existence of species-specific embryonic stem cell lines and is applicable to other vertebrates, especially those whose embryos are easily available; finally, it is also feasible to achieve targeted knock-ins in zebrafish, therefore it is possible to create human disease models that are heretofore inaccessible.
ZFNs are also used to rewrite the sequence of an allele by invoking the homologous recombination (HR) machinery to repair the DSB using the supplied DNA fragment as a template. The HR machinery searches for homology between the damaged chromosome and the extra-chromosomal fragment and copies the sequence of the fragment between the two broken ends of the chromosome, regardless of whether the fragment contains the original sequence. If the subject is homozygous for the target allele, the efficiency of the technique is reduced since the undamaged copy of the allele may be used as a template for repair instead of the supplied fragment.
The success of gene therapy depends on the efficient insertion of therapeutic genes at an appropriate chromosomal target site within the human genome , without causing cell injury, oncogenic mutations, or an immune response . The construction of plasmid vectors is simple and straightforward. Custom-designed ZFNs that combine the non-specific cleavage domain (N) of Fok I endonuclease with zinc-finger proteins (ZFPs) offer a general way to deliver a site-specific DSB to the genome, and stimulate local homologous recombination by several orders of magnitude. This makes targeted gene correction or genome editing a viable option in human cells. Since ZFN-encoding plasmids could be used to transiently express ZFNs to target a DSB to a specific gene locus in human cells, they offer an excellent way for targeted delivery of the therapeutic genes to a pre-selected chromosomal site. The ZFN-encoding plasmid-based approach has the potential to circumvent all the problems associated with the viral delivery of therapeutic genes. [ 36 ] The first therapeutic applications of ZFNs are likely to involve ex vivo therapy using a patient's own stem cells. After editing the stem cell genome, the cells could be expanded in culture and reinserted into the patient to produce differentiated cells with corrected functions. Initial targets likely include the causes of monogenic diseases, such as the IL2Rγ gene and the β-globin gene for gene correction and CCR5 gene for mutagenesis and disablement. [ 33 ]
If the zinc finger domains are not specific enough for their target site or they do not target a unique site within the genome of interest, off-target cleavage may occur. Such off-target cleavage may lead to the production of enough double-strand breaks to overwhelm the repair machinery and, as a consequence, yield chromosomal rearrangements and/or cell death. Off-target cleavage events may also promote random integration of donor DNA. [ 33 ] Two separate methods have been demonstrated to decrease off-target cleavage for 3-finger ZFNs that target two adjacent 9-basepair sites. [ 37 ] Other groups use ZFNs with 4, 5 or 6 zinc fingers that target longer and presumably rarer sites and such ZFNs could theoretically yield less off-target activity. A comparison of a pair of 3-finger ZFNs and a pair of 4-finger ZFNs detected off-target cleavage in human cells at 31 loci for the 3-finger ZFNs and at 9 loci for the 4-finger ZFNs. [ 38 ] Whole genome sequencing of C. elegans modified with a pair of 5-finger ZFNs found only the intended modification and a deletion at a site "unrelated to the ZFN site" indicating this pair of ZFNs was capable of targeting a unique site in the C. elegans genome. [ 19 ]
As with many foreign proteins inserted into the human body, there is a risk of an immunological response against the therapeutic agent and the cells in which it is active. Since the protein must be expressed only transiently, however, the time over which a response may develop is short. [ 33 ]
Liu et al. respectively target ZFNickases to the endogenous b-casein(CSN2) locus stimulates lysostaphin and human lysozyme gene addition by homology-directed repair and derive secrete lysostaphin cows. [ 39 ] [ 40 ]
The ability to precisely manipulate the genomes of plants and animals has numerous applications in basic research, agriculture, and human therapeutics. Using ZFNs to modify endogenous genes has traditionally been a difficult task due mainly to the challenge of generating zinc finger domains that target the desired sequence with sufficient specificity. Improved methods of engineering zinc finger domains and the availability of ZFNs from a commercial supplier now put this technology in the hands of increasing numbers of researchers. Several groups are also developing other types of engineered nucleases including engineered homing endonucleases [ 41 ] [ 42 ] and nucleases based on engineered TAL effectors . [ 43 ] [ 44 ] TAL effector nucleases (TALENs) are particularly interesting because TAL effectors appear to be very simple to engineer [ 45 ] [ 46 ] and TALENs can be used to target endogenous loci in human cells. [ 47 ] But to date no one has reported the isolation of clonal cell lines or transgenic organisms using such reagents. One type of ZFN, known as SB-728-T, has been tested for potential application in the treatment of HIV. [ 48 ]
Zinc-finger nickases (ZFNickases) are created by inactivating the catalytic activity of one ZFN monomer in the ZFN dimer required for double-strand cleavage. [ 49 ] ZFNickases demonstrate strand-specific nicking activity in vitro and thus provide for highly specific single-strand breaks in DNA. [ 49 ] These SSBs undergo the same cellular mechanisms for DNA that ZFNs exploit, but they show a significantly reduced frequency of mutagenic NHEJ repairs at their target nicking site. This reduction provides a bias for HR-mediated gene modifications. ZFNickases can induce targeted HR in cultured human and livestock cells, although at lower levels than corresponding ZFNs from which they were derived because nicks can be repaired without genetic alteration. [ 39 ] [ 50 ] A major limitation of ZFN-mediated gene modifications is the competition between NHEJ and HR repair pathways. Regardless of the presence of a DNA donor construct, both repair mechanisms can be activated following DSBs induced by ZFNs. Thus, ZFNickases is the first plausible attempt at engineering a method to favor the HR method of DNA repair as opposed to the error-prone NHEJ repair. By reducing NHEJ repairs, ZFNickases can thereby reduce the spectrum of unwanted off-target alterations. The ease by which ZFNickases can be derive from ZFNs provides a great platform for further studies regarding the optimization of ZFNickases and possibly increasing their levels of targeted HR while still maintain their reduced NHEJ frequency.
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Zinc amalgam is a solution of zinc in mercury. In practice the term refers to particles of zinc with a surface coating of the amalgam . A gray solid, it is typically used for reduction . It is written as Zn(Hg) in reactions. [ 1 ] It is usually prepared by treating an aqueous suspension of zinc with mercuric chloride . Some zinc chloride is produced in the process.
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Zinc azide Zn(N 3 ) 2 is an inorganic compound composed of zinc cations ( Zn 2+ ) and azide anions ( N − 3 ). It is a white, explosive solid that can be prepared by the protonolysis of diethylzinc with hydrazoic acid : [ 1 ]
Zinc azide is a coordination polymer which crystallizes in three polymorphs , all of which feature tetrahedral zinc centers and bridging azide ligands. α- Zn(N 3 ) 2 crystallizes in the monoclinic space group and is stable, while the other two polymorphs are metastable . P 2 1 / n . β- Zn(N 3 ) 2 is trigonal, space group P 3 2 21, and γ- Zn(N 3 ) 2 is monoclinic, space group C 2.
It is easily hydrolyzed, and attempts to prepare it in aqueous solution resulted in the precipitation of basic azides Zn(OH) 2− x (N 3 ) x ( x = 0.9–1.0). Both the α- and β-forms were found to be very friction- and shock-sensitive, violently exploding in blue flashes, but can be made to decompose slowly by gentle heating, giving off nitrogen gas. In a sealed glass tube with inert atmosphere, this yields zinc nitride , Zn 3 N 2 . [ 1 ]
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Zinc chloride is an inorganic chemical compound with the formula ZnCl 2 · n H 2 O, with n ranging from 0 to 4.5, forming hydrates . Zinc chloride, anhydrous and its hydrates, are colorless or white crystalline solids, and are highly soluble in water . Five hydrates of zinc chloride are known, as well as four polymorphs of anhydrous zinc chloride. [ 5 ]
All forms of zinc chloride are deliquescent . They can usually be produced by the reaction of zinc or its compounds with some form of hydrogen chloride . Anhydrous zinc compound is a Lewis acid , readily forming complexes with a variety of Lewis bases. Zinc chloride finds wide application in textile processing, metallurgical fluxes , chemical synthesis of organic compounds , such as benzaldehyde , and processes to produce other compounds of zinc. [ 5 ]
Zinc chloride has long been known but currently practiced industrial applications all evolved in the latter half of 20th century. [ 5 ]
An amorphous cement formed from aqueous zinc chloride and zinc oxide was first investigated in 1855 by Stanislas Sorel . Sorel later went on to investigate the related magnesium oxychloride cement , which bears his name. [ 6 ]
Dilute aqueous zinc chloride was used as a disinfectant under the name "Burnett's Disinfecting Fluid". [ 7 ] From 1839 Sir William Burnett promoted its use as a disinfectant as well as a wood preservative. The Royal Navy conducted trials into its use as a disinfectant in the late 1840s, including during the cholera epidemic of 1849 ; and at the same time experiments were conducted into its preservative properties as applicable to the shipbuilding and railway industries. Burnett had some commercial success with his eponymous fluid. Following his death however, its use was largely superseded by that of carbolic acid and other proprietary products. [ 8 ]
Unlike other metal dichlorides, zinc dichloride adopts several crystalline forms ( polymorphs ). Four polymorph are known: α, β, γ, and δ. Each features Zn 2+ centers surrounded in a tetrahedral manner by four chloride ligands. [ 9 ]
Here a , b , and c are lattice constants, Z is the number of structure units per unit cell, and ρ is the density calculated from the structure parameters. [ 10 ] [ 11 ] [ 12 ]
The orthorhombic form (δ) rapidly changes to another polymorph upon exposure to the atmosphere. A possible explanation is that the OH − ions originating from the absorbed water facilitate the rearrangement. [ 9 ] Rapid cooling of molten ZnCl 2 gives a glass . [ 13 ]
Molten ZnCl 2 has a high viscosity at its melting point and a comparatively low electrical conductivity, which increases markedly with temperature. [ 14 ] [ 15 ] As indicated by a Raman scattering study, the viscosity is explained by the presence of polymers,. [ 16 ] Neutron scattering study indicated the presence of tetrahedral ZnCl 4 centers, which requires aggregation of ZnCl 2 monomers as well. [ 17 ]
A variety of hydrated zinc chloride are known: ZnCl 2 (H 2 O) n with n = 1, 1.33, 2.5, 3, and 4.5. [ 18 ] The 1.33-hydrate, previously thought to be the hemitrihydrate, consists of trans -Zn(H 2 O) 4 Cl 2 centers with the chlorine atoms connected to repeating ZnCl 4 chains. The hemipentahydrate, structurally formulated [Zn(H 2 O) 5 ][ZnCl 4 ], consists of Zn(H 2 O) 5 Cl octahedrons where the chlorine atom is part of a [ZnCl 4 ] 2- tetrahedera. The trihydrate consists of distinct hexaaquozinc(II) cations and tetrachlorozincate anions; formulated [Zn(H 2 O) 6 ][ZnCl 4 ]. Finally, the heminonahydrate, structurally formulated [Zn(H 2 O) 6 ][ZnCl 4 ]·3H 2 O also consists of distinct hexaaquozinc(II) cations and tetrachlorozincate anions like the trihydrate but has three extra water molecules. These hydrates can be produced by evaporation of aqueous solutions of zinc chloride at different temperatures. [ 19 ] [ 20 ]
Historically, zinc chlorides are prepared from the reaction of hydrochloric acid with zinc metal or zinc oxide. Aqueous acids cannot be used to produce anhydrous zinc chloride. According to an early procedure, a suspension of powdered zinc in diethyl ether is treated with hydrogen chloride, followed by drying [ 21 ] The overall method remains useful in industry, but without the solvent: [ 5 ]
Aqueous solutions may be readily prepared similarly by treating Zn metal, zinc carbonate, zinc oxide, and zinc sulfide with hydrochloric acid: [ 22 ]
Hydrates can be produced by evaporation of an aqueous solution of zinc chloride. The temperature of the evaporation determines the hydrates For example, evaporation at room temperature produces the 1.33-hydrate. [ 19 ] [ 23 ] Lower evaporation temperatures produce higher hydrates. [ 20 ]
Commercial samples of zinc chloride typically contain water and products from hydrolysis as impurities. Laboratory samples may be purified by recrystallization from hot dioxane . Anhydrous samples can be purified by sublimation in a stream of hydrogen chloride gas, followed by heating the sublimate to 400 °C in a stream of dry nitrogen gas. [ 24 ] A simple method relies on treating the zinc chloride with thionyl chloride . [ 25 ]
A number of salts containing the tetrachlorozincate anion, [ZnCl 4 ] 2− , are known. [ 14 ] "Caulton's reagent", V 2 Cl 3 ( thf ) 6 ] [Zn 2 Cl 6 ] , which is used in organic chemistry, is an example of a salt containing [Zn 2 Cl 6 ] 2− . [ 26 ] [ 27 ] The compound Cs 3 ZnCl 5 contains tetrahedral [ZnCl 4 ] 2− and Cl − anions, [ 9 ] so, the compound is not caesium pentachlorozincate, but caesium tetrachlorozincate chloride. No compounds containing the [ZnCl 6 ] 4− ion (hexachlorozincate ion) have been characterized. [ 9 ] The compound ZnCl 2 ·0.5HCl·H 2 O crystallizes from a solution of ZnCl 2 in hydrochloric acid . It contains a polymeric anion (Zn 2 Cl − 5 ) n with balancing monohydrated hydronium ions, H 5 O + 2 ions. [ 9 ]
The adduct with thf ZnCl 2 (thf) 2 illustrates the tendency of zinc chloride to form 1:2 adducts with weak Lewis bases . Being soluble in ethers and lacking acidic protons, this complex is used in the synthesis of organozinc compounds . [ 29 ] A related 1:2 complex is ZnCl 2 (NH 2 OH) 2 (zinc dichloride di(hydroxylamine)). Known as Crismer's salt, this complexes releases hydroxylamine upon heating. [ 30 ] The distinctive ability of aqueous solutions of ZnCl 2 to dissolve cellulose is attributed to the formation of zinc-cellulose complexes, illustrating the stability of its adducts. [ 31 ] Cellulose also dissolves in molten ZnCl 2 hydrate. [ 32 ] Overall, this behavior is consistent with Zn 2+ as a hard Lewis acid.
When solutions of zinc chloride are treated with ammonia , diverse ammine complexes are produced. In addition to the tetrahedral 1:2 complex ZnCl 2 (NH 3 ) 2 . [ 33 ] [ 34 ] the complex Zn(NH 3 ) 4 Cl 2 ·H 2 O also has been isolated. The latter contains the [Zn(NH 3 ) 6 ] 2+ ion,. [ 9 ] The species in aqueous solution have been investigated and show that [Zn(NH 3 ) 4 ] 2+ is the main species present with [Zn(NH 3 ) 3 Cl] + also present at lower NH 3 :Zn ratio. [ 35 ]
Zinc chloride dissolves readily in water to give ZnCl x (H 2 O) 4− x species and some free chloride. [ 36 ] [ 37 ] [ 38 ] Aqueous solutions of ZnCl 2 are acidic: a 6 M aqueous solution has a pH of 1. [ 18 ] The acidity of aqueous ZnCl 2 solutions relative to solutions of other Zn 2+ salts (say the sulfate) is due to the formation of the tetrahedral chloro aqua complexes such as [ZnCl 3 (H 2 O)] − . [ 39 ] Most metal dichlorides form octahedral complexes, with stronger O-H bonds. The combination of hydrochloric acid and ZnCl 2 gives a reagent known as " Lucas reagent ". Such reagents were once used as a test for primary alcohols. Similar reactions are the basis of industrial routes from methanol and ethanol respectively to methyl chloride and ethyl chloride . [ 40 ]
In alkali solution, zinc chloride converts to various zinc hydroxychlorides. These include [Zn(OH) 3 Cl] 2− , [Zn(OH) 2 Cl 2 ] 2− , [Zn(OH)Cl 3 ] 2− , and the insoluble Zn 5 (OH) 8 Cl 2 ·H 2 O . The latter is the mineral simonkolleite . [ 41 ] When zinc chloride hydrates are heated, hydrogen chloride evolves and hydroxychlorides result. [ 42 ]
In aqueous solution ZnCl 2 , as well as other halides (bromide, iodide), behave interchangeably for the preparation of other zinc compounds. These salts give
precipitates of zinc carbonate when treated with aqueous carbonate sources: [ 5 ]
Ninhydrin reacts with amino acids and amines to form a colored compound "Ruhemann's purple" (RP). Spraying with a zinc chloride solution, which is colorless, forms a 1:1 complex RP: ZnCl(H 2 O) 2 , which is more readily detected as it fluoresces more intensely than RP. [ 43 ]
Anhydrous zinc chloride melts and even boils without any decomposition up to 900 °C. When zinc metal is dissolved in molten ZnCl 2 at 500–700 °C, a yellow diamagnetic solution is formed consisting of the Zn 2+ 2 , which has zinc in the oxidation state +1. The nature of this dizinc dication has been confirmed by Raman spectroscopy . [ 18 ] Although Zn 2+ 2 is unusual, mercury, a heavy congener of zinc, forms a wide variety of Hg 2+ 2 salts.
In the presence of oxygen, zinc chloride oxidizes to zinc oxide above 400 °C. Again, this observation indicates the nonoxidation of Zn 2+ . [ 44 ]
Concentrated aqueous zinc chloride dissolves zinc oxide to form zinc hydroxychloride, which is obtained as colorless crystals: [ 45 ]
The same material forms when hydrated zinc chloride is heated. [ 46 ]
The ability of zinc chloride to dissolve metal oxides (MO) [ 47 ] is relevant to the utility of ZnCl 2 as a flux for soldering . It dissolves passivating oxides, exposing the clean metal surface. [ 47 ]
Zinc chloride is an occasional laboratory reagent often as a Lewis acid . A dramatic example is the conversion of methanol into hexamethylbenzene using zinc chloride as the solvent and catalyst: [ 48 ]
This kind of reactivity has been investigated for the valorization of C1 precursors. [ 49 ]
Examples of zinc chloride as a Lewis acid include the Fischer indole synthesis : [ 50 ]
Related Lewis-acid behavior is illustrated by a traditional preparation of the dye fluorescein from phthalic anhydride and resorcinol , which involves a Friedel-Crafts acylation . [ 51 ] This transformation has in fact been accomplished using even the hydrated ZnCl 2 sample shown in the picture above. Many examples describe the use of zinc chloride in Friedel-Crafts acylation reactions. [ 52 ] [ 53 ]
Zinc chloride also activates benzylic and allylic halides towards substitution by weak nucleophiles such as alkenes : [ 54 ]
In similar fashion, ZnCl 2 promotes selective Na[BH 3 (CN)] reduction of tertiary, allylic or benzylic halides to the corresponding hydrocarbons. [ 24 ]
Zinc enolates , prepared from alkali metal enolates and ZnCl 2 , provide control of stereochemistry in aldol condensation reactions. This control is attributed to chelation at the zinc. In the example shown below, the threo product was favored over the erythro by a factor of 5:1 when ZnCl 2 . [ 55 ]
Being inexpensive and anhydrous, ZnCl 2 is a widely used for the synthesis of many organozinc reagents, such as those used in the palladium catalyzed Negishi coupling with aryl halides or vinyl halides . The prominence of this reaction was highlighted by the award of the 2010 Nobel Prize in Chemistry to Ei-ichi Negishi . [ 56 ]
Rieke zinc , a highly reactive form of zinc metal, is generated by reduction of zinc dichloride with lithium . Rieke Zn is useful for the preparation of polythiophenes [ 57 ] and for the Reformatsky reaction . [ 58 ]
Zinc chloride is used as a catalyst or reagent in diverse reactions conducted on an industrial scale. Benzaldehyde, 20,000 tons of which is produced annually in Western countries, is produced from inexpensive toluene by exploiting the catalytic properties of zinc dichloride. This process begins with the chlorination of toluene to give benzal chloride . In the presence of a small amount of anhydrous zinc chloride, a mixture of benzal chloride are treated continuously with water according to the following stoichiometry: [ 59 ]
Similarly zinc chloride is employed in hydrolysis of benzotrichloride, the main route to benzoyl chloride . It serves as a catalyst for the production of methylene-bis(dithiocarbamate). [ 5 ]
The use of zinc chloride as a flux, sometimes in a mixture with ammonium chloride (see also Zinc ammonium chloride ), involves the production of HCl and its subsequent reaction with surface oxides.
Zinc chloride forms two salts with ammonium chloride: [NH 4 ] 2 [ZnCl 4 ] and [NH 4 ] 3 [ZnCl 4 ]Cl , which decompose on heating liberating HCl, just as zinc chloride hydrate does. The action of zinc chloride/ammonium chloride fluxes, for example, in the hot-dip galvanizing process produces H 2 gas and ammonia fumes. [ 60 ]
Relevant to its affinity for these paper and textiles, ZnCl 2 is used as a fireproofing agent and in the process of making Vulcanized fibre , which is made by soaking paper in concentrated zinc chloride. [ 61 ] [ 62 ] Zinc chloride is also used as a deodorizing agent and to make zinc soaps . [ 5 ]
Zinc and chloride are essential for life. Zn 2+ is a component of several enzymes , e.g., carboxypeptidase and carbonic anhydrase . Thus, aqueous solutions of zinc chlorides are rarely problematic as an acute poison. [ 5 ] Anhydrous zinc chloride is however an aggressive Lewis acid as it can burn skin and other tissues. Ingestion of zinc chloride, often from soldering flux , requires endoscopic monitoring. [ 63 ] Another source of zinc chloride is zinc chloride smoke mixture ("HC") used in smoke grenades . Containing zinc oxide, hexachloroethane and aluminium powder release zinc chloride, carbon and aluminium oxide smoke, an effective smoke screen . [ 64 ] Such smoke screens can lead to fatalities. [ 65 ]
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Zinc hydride is an inorganic compound with the chemical formula Zn H 2 . It is a white, odourless solid which slowly decomposes into its elements at room temperature; despite this it is the most stable of the binary first row transition metal hydrides . A variety of coordination compounds containing Zn–H bonds are used as reducing agents , [ 1 ] but ZnH 2 itself has no common applications.
Zinc(II) hydride was first synthesized in 1947 by Hermann Schlesinger , via a reaction between dimethylzinc Zn(CH 3 ) 2 and lithium aluminium hydride Li[AlH 4 ] ; [ 2 ] a process which was somewhat hazardous due to the pyrophoric nature of Zn(CH 3 ) 2 .
Later methods were predominantly salt metathesis reactions between zinc halides and alkali metal hydrides, which are significantly safer. [ 3 ] [ 4 ] Examples include:
Small quantities of gaseous zinc(II) hydride have also been produced by laser ablation of zinc under a hydrogen atmosphere [ 5 ] [ 6 ] and other high energy techniques. These methods have been used to assess its gas phase properties.
New evidence suggests that in zinc(II) hydride, elements form a one-dimensional network ( polymer ), being connected by covalent bonds . [ 7 ] Other lower metal hydrides polymerise in a similar fashion (cf. aluminium hydride ). Solid zinc(II) hydride is the irreversible autopolymerisation product of the molecular form, and the molecular form cannot be isolated in concentration. Solubilising zinc(II) hydride in non-aqueous solvents, involve adducts with molecular zinc(II) hydride, such as ZnH 2 ·H 2 in liquid hydrogen.
Zinc(II) hydride slowly decomposes to metallic zinc and hydrogen gas at room temperature, with decomposition becoming rapid if it is heated above 90°C. [ 8 ]
It is readily oxidised and is sensitive to both air and moisture; being hydrolysed slowly by water but violently by aqueous acids, [ 3 ] which indicates possible passivation via the formation of a surface layer of ZnO . Despite this older samples may be pyrophoric. [ 3 ] Zinc hydride can therefore be considered metastable at best, however it is still the most stable of all the binary first row transition metal hydrides (cf. titanium(IV) hydride ).
Molecular zinc(II) hydride, ZnH 2 , has been identified as a volatile product of the acidified reduction of zinc ions with sodium borohydride . [ citation needed ] This reaction is similar to the acidified reduction with lithium aluminium hydride , however a greater fraction of the generated zinc(II) hydride is in the molecular form. This can be attributed to a slower reaction rate, which prevents a polymerising concentration of building over the progression of the reaction. This follows earlier experiments in direct synthesis from the elements. The reaction of excited zinc atoms with molecular hydrogen in the gas phase was studied by Breckenridge et al using laserpump-probe techniques. [ citation needed ] Owing to its relative thermal stability, molecular zinc(II) hydride is included in the short list of molecular metal hydrides, which have been successfully identified in the gas phase (that is, not limited to matrix isolation).
The average Zn–H bond energy was recently calculated to be 51.24 kcal mol −1 , while the H–H bond energy is 103.3 kcal mol −1 . [ citation needed ] Therefore, the overall reaction is nearly ergoneutral.
Molecular zinc hydride in the gas phase was found to be linear with a Zn–H bond length of 153.5 pm. [ 9 ]
The molecule can be found a singlet ground state of 1 Σ g + .
Quantum chemical calculations predict the molecular form to exist in a doubly hydrogen-bridged, dimeric groundstate, with little or no formational energy barrier . [ citation needed ] The dimer can be called di-μ-hydrido-bis(hydridozinc), per IUPAC additive nomenclature.
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Zinc is an essential trace element for humans [ 1 ] [ 2 ] [ 3 ] and other animals, [ 4 ] for plants [ 5 ] and for microorganisms . [ 6 ] Zinc is required for the function of over 300 enzymes and 1000 transcription factors , [ 3 ] and is stored and transferred in metallothioneins . [ 7 ] [ 8 ] It is the second most abundant trace metal in humans after iron and it is the only metal which appears in all enzyme classes . [ 5 ] [ 3 ]
In proteins, zinc ions are often coordinated to the amino acid side chains of aspartic acid , glutamic acid , cysteine and histidine . The theoretical and computational description of this zinc binding in proteins (as well as that of other transition metals ) is difficult. [ 9 ]
Roughly 2–4 grams of zinc [ 10 ] are distributed throughout the human body. Most zinc is in the brain, muscle, bones, kidney, and liver, with the highest concentrations in the prostate and parts of the eye. [ 11 ] Semen is particularly rich in zinc, a key factor in prostate gland function and reproductive organ growth. [ 12 ]
Zinc homeostasis of the body is mainly controlled by the intestine. Here, ZIP4 and especially TRPM7 were linked to intestinal zinc uptake essential for postnatal survival. [ 13 ] [ 14 ]
In humans, the biological roles of zinc are ubiquitous. [ 15 ] [ 2 ] It interacts with "a wide range of organic ligands ", [ 15 ] and has roles in the metabolism of RNA and DNA, signal transduction , and gene expression . It also regulates apoptosis . A review from 2015 indicated that about 10% of human proteins (~3000) bind zinc, [ 16 ] in addition to hundreds more that transport and traffic zinc; a similar in silico study in the plant Arabidopsis thaliana found 2367 zinc-related proteins. [ 5 ]
In the brain , zinc is stored in specific synaptic vesicles by glutamatergic neurons and can modulate neuronal excitability. [ 2 ] [ 3 ] [ 17 ] It plays a key role in synaptic plasticity and so in learning. [ 2 ] [ 18 ] Zinc homeostasis also plays a critical role in the functional regulation of the central nervous system . [ 2 ] [ 17 ] [ 3 ] Dysregulation of zinc homeostasis in the central nervous system that results in excessive synaptic zinc concentrations is believed to induce neurotoxicity through mitochondrial oxidative stress (e.g., by disrupting certain enzymes involved in the electron transport chain , including complex I , complex III , and α-ketoglutarate dehydrogenase ), the dysregulation of calcium homeostasis, glutamatergic neuronal excitotoxicity , and interference with intraneuronal signal transduction . [ 2 ] [ 19 ] L- and D-histidine facilitate brain zinc uptake. [ 20 ] SLC30A3 is the primary zinc transporter involved in cerebral zinc homeostasis. [ 2 ]
Zinc is an efficient Lewis acid , making it a useful catalytic agent in hydroxylation and other enzymatic reactions. [ 21 ] The metal also has a flexible coordination geometry , which allows proteins using it to rapidly shift conformations to perform biological reactions. [ 22 ] Two examples of zinc-containing enzymes are carbonic anhydrase and carboxypeptidase , which are vital to the processes of carbon dioxide ( CO 2 ) regulation and digestion of proteins, respectively. [ 23 ]
In vertebrate blood, carbonic anhydrase converts CO 2 into bicarbonate and the same enzyme transforms the bicarbonate back into CO 2 for exhalation through the lungs. [ 24 ] Without this enzyme, this conversion would occur about one million times slower [ 25 ] at the normal blood pH of 7 or would require a pH of 10 or more. [ 26 ] The non-related β-carbonic anhydrase is required in plants for leaf formation, the synthesis of indole acetic acid (auxin) and alcoholic fermentation . [ 27 ]
Carboxypeptidase cleaves peptide linkages during digestion of proteins. A coordinate covalent bond is formed between the terminal peptide and a C=O group attached to zinc, which gives the carbon a positive charge. This helps to create a hydrophobic pocket on the enzyme near the zinc, which attracts the non-polar part of the protein being digested. [ 23 ]
Zinc has been recognized as a messenger, able to activate signalling pathways. Many of these pathways provide the driving force in aberrant cancer growth. They can be targeted through ZIP transporters . [ 28 ]
Zinc serves a purely structural role in zinc fingers , twists and clusters. [ 29 ] Zinc fingers form parts of some transcription factors , which are proteins that recognize DNA base sequences during the replication and transcription of DNA . Each of the nine or ten Zn 2+ ions in a zinc finger helps maintain the finger's structure by coordinately binding to four amino acids in the transcription factor. [ 25 ]
In blood plasma , zinc is bound to and transported by albumin (60%, low-affinity) and transferrin (10%). [ 10 ] Because transferrin also transports iron, excessive iron reduces zinc absorption, and vice versa. A similar antagonism exists with copper. [ 30 ] The concentration of zinc in blood plasma stays relatively constant regardless of zinc intake. [ 21 ] Cells in the salivary gland, prostate, immune system, and intestine use zinc signaling to communicate with other cells. [ 31 ]
Zinc may be held in metallothionein reserves within microorganisms or in the intestines or liver of animals. [ 32 ] Metallothionein in intestinal cells is capable of adjusting absorption of zinc by 15–40%. [ 33 ] However, inadequate or excessive zinc intake can be harmful; excess zinc particularly impairs copper absorption because metallothionein absorbs both metals. [ 34 ]
The human dopamine transporter contains a high affinity extracellular zinc binding site which, upon zinc binding, inhibits dopamine reuptake and amplifies amphetamine -induced dopamine efflux in vitro . [ 35 ] [ 36 ] [ 37 ] The human serotonin transporter and norepinephrine transporter do not contain zinc binding sites. [ 37 ] Some EF-hand calcium binding proteins such as S100 or NCS-1 are also able to bind zinc ions. [ 38 ]
The U.S. Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for zinc in 2001. The current EARs for zinc for women and men ages 14 and up is 6.8 and 9.4 mg/day, respectively. The RDAs are 8 and 11 mg/day. RDAs are higher than EARs so as to identify amounts that will cover people with higher-than-average requirements. RDA for pregnancy is 11 mg/day. RDA for lactation is 12 mg/day. For infants up to 12 months, the RDA is 3 mg/day. For children ages 1–13 years, the RDA increases with age from 3 to 8 mg/day. As for safety, the IOM sets Tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient. In the case of zinc the adult UL is 40 mg/day (lower for children). Collectively the EARs, RDAs, AIs and ULs are referred to as Dietary Reference Intakes (DRIs). [ 21 ]
The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL are defined the same as in the United States. For people ages 18 and older, the PRI calculations are complex, as the EFSA has set higher and higher values as the phytate content of the diet increases. For women, PRIs increase from 7.5 to 12.7 mg/day as phytate intake increases from 300 to 1200 mg/day; for men, the range is 9.4 to 16.3 mg/day. These PRIs are higher than the U.S. RDAs. [ 39 ] The EFSA reviewed the same safety question and set its UL at 25 mg/day, which is much lower than the U.S. value. [ 40 ]
For U.S. food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of Daily Value (%DV). For zinc labeling purposes, 100% of the Daily Value was 15 mg, but on May 27, 2016, it was revised to 11 mg. [ 41 ] [ 42 ] A table of the old and new adult daily values is provided at Reference Daily Intake .
Animal products such as meat, fish, shellfish, fowl, eggs, and dairy contain zinc. The concentration of zinc in plants varies with the level in the soil. With adequate zinc in the soil, the food plants that contain the most zinc are wheat (germ and bran) and various seeds, including sesame , poppy , alfalfa , celery , and mustard . [ 43 ] Zinc is also found in beans , nuts , almonds , whole grains , pumpkin seeds , sunflower seeds , and blackcurrant . [ 44 ]
Other sources include fortified food and dietary supplements in various forms. A 1998 review concluded that zinc oxide , one of the most common supplements in the United States, and zinc carbonate are nearly insoluble and poorly absorbed in the body. [ 45 ] This review cited studies that found lower plasma zinc concentrations in the subjects who consumed zinc oxide and zinc carbonate than in those who took zinc acetate and sulfate salts. [ 45 ] For fortification, however, a 2003 review recommended cereals (containing zinc oxide) as a cheap, stable source that is as easily absorbed as the more expensive forms. [ 46 ] A 2005 study found that various compounds of zinc, including oxide and sulfate, did not show statistically significant differences in absorption when added as fortificants to maize tortillas. [ 47 ]
Nearly two billion people in the developing world are deficient in zinc. Groups at risk include children in developing countries and the elderly with chronic illnesses. [ 48 ] In children, it causes an increase in infection and diarrhea and contributes to the death of about 800,000 children worldwide per year. [ 15 ] The World Health Organization advocates zinc supplementation for severe malnutrition and diarrhea. [ 49 ] Zinc supplements help prevent disease and reduce mortality, especially among children with low birth weight or stunted growth. [ 49 ] However, zinc supplements should not be administered alone, because many in the developing world have several deficiencies, and zinc interacts with other micronutrients. [ 50 ] While zinc deficiency is usually due to insufficient dietary intake, it can be associated with malabsorption , acrodermatitis enteropathica , chronic liver disease, chronic renal disease, sickle cell disease, diabetes, malignancy, and other chronic illnesses. [ 48 ]
In the United States, a federal survey of food consumption determined that for women and men over the age of 19, average consumption was 9.7 and 14.2 mg/day, respectively. For women, 17% consumed less than the EAR, for men 11%. The percentages below EAR increased with age. [ 51 ] The most recent published update of the survey (NHANES 2013–2014) reported lower averages – 9.3 and 13.2 mg/day – again with intake decreasing with age. [ 52 ]
Symptoms of mild zinc deficiency are diverse. [ 21 ] Clinical outcomes include depressed growth, diarrhea, impotence and delayed sexual maturation, alopecia , eye and skin lesions, impaired appetite, altered cognition, impaired immune functions, defects in carbohydrate utilization, and reproductive teratogenesis . [ 21 ] Zinc deficiency depresses immunity, [ 53 ] but excessive zinc does also. [ 10 ]
Despite some concerns, [ 54 ] western vegetarians and vegans do not suffer any more from overt zinc deficiency than meat-eaters. [ 55 ] Major plant sources of zinc include cooked dried beans, sea vegetables, fortified cereals, soy foods, nuts, peas, and seeds. [ 54 ] However, phytates in many whole-grains and fibers may interfere with zinc absorption and marginal zinc intake has poorly understood effects. The zinc chelator phytate , found in seeds and cereal bran , can contribute to zinc malabsorption. [ 48 ] Some evidence suggests that more than the US RDA (8 mg/day for adult women; 11 mg/day for adult men) may be needed in those whose diet is high in phytates, such as some vegetarians. [ 54 ] The European Food Safety Authority (EFSA) guidelines attempt to compensate for this by recommending higher zinc intake when dietary phytate intake is greater. [ 39 ] These considerations must be balanced against the paucity of adequate zinc biomarkers , and the most widely used indicator, plasma zinc, has poor sensitivity and specificity . [ 56 ]
Zinc can be present in six different forms in soil namely; water soluble zinc, exchangeable zinc, organically bound zinc, carbonate bound zinc, aluminium and manganese oxide bound zinc and residual fractions of zinc. [ 57 ]
In toxic conditions, species of Calluna , Erica and Vaccinium can grow in zinc-metalliferous soils, because translocation of toxic ions is prevented by the action of ericoid mycorrhizal fungi . [ 58 ]
Zinc deficiency appears to be the most common micronutrient deficiency in crop plants; it is particularly common in high-pH soils. [ 59 ] Zinc-deficient soil is cultivated in the cropland of about half of Turkey and India, a third of China, and most of Western Australia. Substantial responses to zinc fertilization have been reported in these areas. [ 5 ] Plants that grow in soils that are zinc-deficient are more susceptible to disease. Zinc is added to the soil primarily through the weathering of rocks, but humans have added zinc through fossil fuel combustion, mine waste, phosphate fertilizers, pesticide ( zinc phosphide ), limestone, manure, sewage sludge, and particles from galvanized surfaces. Excess zinc is toxic to plants, although zinc toxicity is far less widespread. [ 5 ]
Zinc (Zn), alongside Magnesium (Mg) and Iron (Fe), constitutes one of the three families of biodegradable metals. [ 60 ] Zinc, as an abundant trace element, ranks sixth among all the essential metallic elements crucial for sustaining life within the human body. [ 61 ] Zinc exhibits an intermediate biodegradation rate, falling between that of Fe (relatively slow) and Mg (relatively high) which positions it as a promising material for use in biodegradable implants. [ 62 ] [ 63 ] [ 64 ]
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Zinc perchlorate is the inorganic compound with the chemical formula Zn(ClO 4 ) 2 which forms the hexahydrate. [ 1 ] [ 2 ]
Zinc perchlorate can be prepared by dissolving zinc oxide or zinc carbonate in perchloric acid : [ 3 ]
The compound decomposes when heated to high temperatures and may explode if heated too strongly.
Like most other perchlorates such as copper perchlorate and lead perchlorate , zinc perchlorate is prone to deliquescence .
Zinc perchlorate can form complexes with ligands such as 8-aminoquinoline , tricarbohydrazide, and tetraphenylethylene tetratriazole. [ 4 ]
The compound forms a hexahydrate Zn(ClO 4 ) 2 ·6 H 2 O . [ 5 ] [ 6 ]
Zinc perchlorate forms a hygroscopic colorless solid, odorless, soluble in water and low-weight alcohols.
Zinc perchlorate is used as an oxidizing agent and catalyst.
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Zinc peroxide (ZnO 2 ) is a chemical compound of zinc that appears as a bright yellow powder at room temperature. It was historically used as a surgical antiseptic . More recently zinc peroxide has also been used as an oxidant in explosives and pyrotechnic mixtures. Its properties have been described as a transition between ionic and covalent peroxides. [ 3 ]
Zinc peroxide can be synthesized through the reaction of zinc chloride and hydrogen peroxide . [ 4 ]
According to X-ray crystallography , the compound consists of octahedral Zn(II) centers bonded to six distinct peroxide (O 2 2- ) ligands. The overall motif is very similar to that for iron pyrite (FeS 2 ). The structure, with intact O-O bonds, makes clear that this material is a peroxide, not a dioxide.
The treatment of burrowing ulcers in the abdominal wall with zinc peroxide was first recorded in 1933 and throughout the 1940s ZnO 2 was used as a disinfectant in surgical . [ 5 ] Zinc peroxide was, however, deemed ineffective against certain bacterial strains, such as Streptococcus viridans , staphylococcus aureus , E. coli , B. proteus , and B. pyocyoneus .
Zinc peroxide is hazardous in case of skin contact, of eye contact, or inhalation. [ 6 ]
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Zinc pest (from German Zinkpest "zinc plague"), also known as zinc rot , mazak rot and zamak rot , is a destructive, intercrystalline corrosion process of zinc alloys containing lead impurities. [ 1 ] While impurities of the alloy are the primary cause of the problem, environmental conditions such as high humidity (greater than 65%) may accelerate the process. [ 2 ] [ 3 ]
It was first discovered to be a problem in 1923, [ 1 ] and primarily affects die-cast zinc articles that were manufactured during the 1920s through 1950s. The New Jersey Zinc Company developed zamak alloys in 1929 using 99.99% pure zinc metal to avoid the problem, and articles made after 1960 are usually considered free of the risk of zinc pest since the use of purer materials and more controlled manufacturing conditions make zinc pest degradation unlikely. [ 2 ]
Affected objects may show surface irregularities such as small cracks and fractures, blisters or pitting. Over time, the material slowly expands, cracking, buckling and warping in an irreversible process that makes the object exceedingly brittle and prone to fracture, and can eventually shatter the object, destroying it altogether. Due to the expansion process, attached normal material may also be damaged. The occurrence and severity of zinc pest in articles made of susceptible zinc alloys depends both on the concentration of lead impurities in the metal and on the storage conditions of the article in the ensuing decades. Zinc pest is dreaded by collectors of vintage die-cast model trains , toys, or radios, because rare or otherwise valuable items can inescapably be rendered worthless as the process of zinc pest destroys them. Because castings of the same object were usually made from various batches of metal over the production process, some examples of a given toy or model may survive today completely unaffected, while other identical examples may have completely disintegrated. It has also affected carburetors, hubcaps, door handles and automobile trim on cars of the 1920s and 1930s.
Since the 1940s, some model railroad hobbyists have claimed, [ citation needed ] with varying degrees of success, that a method of "pickling" zinc alloy parts by soaking them in vinegar or oxalic acid solution for several minutes before painting and assembling them could prevent or delay the effects of zinc pest.
Engine parts of older vehicles or airplanes, and military medals made of zinc alloys, may also be affected. In addition, the post-1982 copper-plated zinc Lincoln cents have been known to be affected.
Zinc pest is not related to tin pest , and is also different from a superficial white corrosion oxidation process (" Weissrost ") that affects some zinc articles.
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Zinc refining is the process of purifying zinc to special high grade (SHG) zinc, which is at least 99.995% pure. [ 1 ] This process is not usually required when smelting of zinc is done through electrolysis processes, but is needed when zinc is produced by pyrometallurgical processes, because it is only 98.5% pure.
There are various refining methods, but the refluxing process is the most commonly used.
High purity zinc is required industrially to avoid zinc pest , a slow distortion and cracking of zinc die castings caused by impurities precipitating out.
The New Jersey Zinc Company invented this process in 1930.
The process take advantage of the relatively low boiling point of zinc (907 °C (1,665 °F)) as compared to the impurities being removed in the first "column": iron and aluminium . Therefore, in the first column the zinc is heated above its boiling point and allowed to rise to a condenser. The iron and aluminium impurities sink to the bottom in the form of a solid or liquid. There are still lead and cadmium vapor impurities. In order to remove the lead 2-3% of the vapor is condensed, which draws the majority of the lead out of the vapor; down to the point where it is only 0.003% of the total contents. Finally the vapor is pumped into the cadmium column where it is cooled to an intermediate temperature below the boiling point zinc, but still above the boiling point of cadmium (767 °C (1,413 °F)). The zinc leaves out the bottom as a refined liquid, while the cadmium leaves out the top as vapor.
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Zinc smelting is the process of converting zinc concentrates ( ores that contain zinc) into pure zinc. Zinc smelting has historically been more difficult than the smelting of other metals, e.g. iron , because in contrast, zinc has a low boiling point . At temperatures typically used for smelting metals, zinc is a gas that will escape from a furnace with the flue gas and be lost, unless specific measures are taken to prevent it.
The most common zinc concentrate processed is zinc sulfide , [ 1 ] which is obtained by concentrating sphalerite via froth flotation . Secondary (recycled) zinc material, such as zinc oxide, is also processed with the zinc sulfide. [ 2 ] Approximately 30% of all zinc produced is from recycled sources. [ 3 ]
There are two methods of smelting zinc: the pyrometallurgical process and the electrolysis process. [ 2 ] Both methods are still used. [ 2 ] [ 4 ] Both of these processes share the same first step: roasting.
Roasting is a process of oxidizing zinc sulfide concentrates at high temperatures into an impure zinc oxide, called "Zinc Calcine". The chemical reactions that take place are as follows:
Approximately 90% of zinc in concentrates are oxidized to zinc oxide. However, at the roasting temperatures around 10% of the zinc reacts with the iron impurities of the zinc sulfide concentrates to form zinc ferrite . A byproduct of roasting is sulfur dioxide , which is further processed into sulfuric acid , a commodity . [ 2 ] The linked refinery flow sheet shows a schematic of Noranda's eastern Canadian zinc roasting operation [ 5 ]
The process of roasting varies based on the type of roaster used. There are three types of roasters: multiple-hearth, suspension, and fluidized-bed. [ 1 ]
In a multiple-hearth roaster, the concentrate drops through a series of 9 or more hearths stacked inside a brick-lined cylindrical column. As the feed concentrate drops through the furnace, it is first dried by the hot gases passing through the hearths and then oxidized to produce calcine. The reactions are slow and can be sustained only by the addition of fuel. Multiple hearth roasters are unpressurized and operate at about 690 °C (1,270 °F). Operating time depends upon the composition of concentrate and the amount of the sulfur removal required. Multiple hearth roasters have the capability of producing a high-purity calcine. [ 1 ]
In a suspension roaster, the concentrates are blown into a combustion chamber very similar to that of a pulverized coal furnace. The roaster consists of a refractory-lined cylindrical steel shell, with a large combustion space at the top and 2 to 4 hearths in the lower portion, similar to those of a multiple hearth furnace. Additional grinding, beyond that required for a multiple hearth furnace, is normally required to ensure that heat transfer to the material is sufficiently rapid for the desulfurization and oxidation reactions to occur in the furnace chamber. Suspension roasters are unpressurized and operate at about 980 °C (1,800 °F). [ 1 ]
In a fluidized-bed roaster, finely ground sulfide concentrates are suspended and oxidized in feedstock bed supported on an air column. As in the suspension roaster, the reaction rates for desulfurization are more rapid than in the older multiple-hearth processes. Fluidized-bed roasters operate under a pressure slightly lower than atmospheric and at temperatures averaging 1,000 °C (1,830 °F). In the fluidized-bed process, no additional fuel is required after ignition has been achieved. The major advantages of this roaster are greater throughput capacities, greater sulfur removal capabilities, and lower maintenance. [ 1 ]
The electrolysis process, also known as the hydrometallurgical process, Roast-Leach-Electrowin (RLE) process, or electrolytic process, is more widely used than the pyrometallurgical processes. [ 2 ]
The electrolysis process consists of 4 steps: leaching, purification, electrolysis, and melting and casting.
The basic leaching chemical formula that drives this process is:
This is achieved in practice through a process called double leaching. The calcine is first leached in a neutral or slightly acidic solution (of sulfuric acid) in order to leach the zinc out of the zinc oxide. The remaining calcine is then leached in strong sulfuric acid to leach the rest of the zinc out of the zinc oxide and zinc ferrite. The result of this process is a solid and a liquid; the liquid contains the zinc and is often called leach product; the solid is called leach residue and contains precious metals (usually lead and silver) which are sold as a by-product. There is also iron in the leach product from the strong acid leach, which is removed in an intermediate step, in the form of goethite , jarosite , and haematite . There is still cadmium , copper , arsenic , antimony , cobalt , germanium , nickel , and thallium in the leach product. Therefore, it needs to be purified. [ 1 ] [ 2 ]
The purification process utilizes the cementation process to further purify the zinc. It uses zinc dust and steam to remove copper, cadmium, cobalt, and nickel, which would interfere with the electrolysis process. After purification, concentrations of these impurities are limited to less than 0.05 milligram per liter (4×10 −7 pound per U.S. gallon). Purification is usually conducted in large agitated tanks. The process takes place at temperatures ranging from 40 to 85 °C (104 to 185 °F), and pressures ranging from atmospheric to 2.4 atm (240 kPa) (absolute scale). The by-products are sold for further refining. [ 1 ] [ 2 ]
The zinc sulfate solution must be very pure for electrowinning to be at all efficient. Impurities can change the decomposition voltage enough to where the electrolysis cell produces largely hydrogen gas rather than zinc metal. [ 6 ]
Zinc is extracted from the purified zinc sulfate solution by electrowinning , which is a specialized form of electrolysis. The process works by passing an electric current through the solution in a series of cells. This causes the zinc to deposit on the cathodes ( aluminium sheets) and oxygen to form at the anodes. Sulfuric acid is also formed in the process and reused in the leaching process. Every 24 to 48 hours, each cell is shut down, the zinc-coated cathodes are removed and rinsed, and the zinc is mechanically stripped from the aluminium plates. [ 1 ] [ 2 ]
Electrolytic zinc smelters contain as many as several hundred cells. A portion of the electrical energy is converted into heat, which increases the temperature of the electrolyte. Electrolytic cells operate at temperature ranges from 30 to 35 °C (86 to 95 °F) and at atmospheric pressure. A portion of the electrolyte is continuously circulated through the cooling towers both to cool and concentrate the electrolyte through evaporation of water. The cooled and concentrated electrolyte is then recycled to the cells. [ 1 ] This process accounts for approximately one-third of all the energy usage when smelting zinc. [ 2 ]
There are two common processes for electrowinning the metal: the low current density process, and the Tainton high current density process. The former uses a 10% sulfuric acid solution as the electrolyte, with current density of 270–325 amperes per square meter. The latter uses 22–28% sulfuric acid solution as the electrolyte with a current density of about 1,000 amperes per square metre. The latter gives better purity and has higher production capacity per volume of electrolyte, but has the disadvantage of running hotter and being more corrosive to the vessel in which it is done. In either of the electrolytic processes, each metric ton of zinc production expends about 3,900 kW⋅h (14 GJ ) of electric power. [ 6 ]
Depending on the type of end-products produced, the zinc cathodes coming out of the electro-winning plant can undergo an additional transformation step in a foundry. Zinc cathodes are melted in induction furnaces and cast into marketable products such as ingots. Other metals and alloy components may be added to produce zinc containing alloys used in die-casting or general galvanization applications. Finally, molten zinc may be transported to nearby conversion plants or third parties using specially-designed insulated containers.
There are also several pyrometallurgical processes that reduce zinc oxide using carbon, then distil the metallic zinc from the resulting mix in an atmosphere of carbon monoxide. The major downfall of any of the pyrometallurgical process is that it is only 98% pure; a standard composition is 1.3% lead, 0.2% cadmium, 0.03% iron, and 98.5% zinc. [ 7 ] This may be pure enough for galvanization, but not enough for die casting alloys, which requires special high-grade zinc (99.995% pure). [ 7 ] In order to reach this purity the zinc must be refined .
The four types of commercial pyrometallurgical processes are the St. Joseph Minerals Corporation's (electrothermic) process, the blast furnace process, the New Jersey Zinc continuous vertical-retort process, and the Belgian-type horizontal retort process.
This process was developed by the St. Joseph Mineral Company in 1930, and is the only pyrometallurgical process still used in the US to smelt zinc. The advantage of this system is that it is able to smelt a wide variety of zinc-bearing materials, including electric arc furnace dust. [ 1 ] The disadvantage of this process is that it is less efficient than the electrolysis process. [ 2 ]
The process begins with a downdraft sintering operation. The sinter, which is a mixture of roaster calcine and EAF (electric arc furnace) calcine, is loaded onto a gate type conveyor and then combustions gases are pumped through the sinter. The carbon in the combustion gases react with some impurities, such as lead, cadmium, and halides. These impurities are driven off into filtration bags. The sinter after this process, called product sinter, usually has a composition of 48% zinc, 8% iron, 5% aluminium, 4% silicon, 2.5% calcium, and smaller quantities of magnesium, lead, and other metals. The sinter product is then charged with coke into an electric retort furnace. A pair of graphite electrodes from the top and bottom of the furnace produce current flow through the mixture. The coke provides electrical resistance to the mixture in order to heat the mixture to 1,400 °C (2,550 °F) and produce carbon monoxide. These conditions allow for the following chemical reaction to occur:
The zinc vapour and carbon dioxide pass to a vacuum condenser, where zinc is recovered by bubbling through a molten zinc bath. Over 95% of the zinc vapour leaving the retort is condensed to liquid zinc. The carbon dioxide is regenerated with carbon, and the carbon monoxide is recycled back to the retort furnace. [ 1 ]
This process was developed by the National Smelting Company at Avonmouth Docks , England , in order to increase production, increase efficiency, and decrease labour and maintenance costs. L. J. Derham proposed using a spray of molten lead droplets to rapidly cool and absorb the zinc vapour, despite the high concentration of carbon dioxide. The mixture is then cooled, where the zinc separates from the lead. The first plant using this design opened up in 1950. One of the advantages of this process is that it can co-produce lead bullion and copper dross. In 1990, it accounted for 12% of the world's zinc production.
The process starts by charging solid sinter and heated coke into the top of the blast furnace. Preheated air at 190 to 1,050 °C (370 to 1,920 °F) is blown into the bottom of the furnace. Zinc vapour and sulfides leave through the top and enter the condenser. Slag and lead collect at the bottom of the furnace and are tapped off regularly. The zinc is scrubbed from the vapour in the condenser via liquid lead. The liquid zinc is separated from the lead in the cooling circuit. Approximately 5,000 metric tons (5,500 short tons ) of lead are required each year for this process, however this process recovers 25% more lead from the starting ores than other processes.
The New Jersey Zinc process [ 8 ] is no longer used to produce primary zinc in the U.S., in Europe and Japan, but it still is used to treat secondary operations. This process peaked in 1960, when it accounted for 5% of world zinc production. A modified version of this process is still used at a Huludao plant in China (originally established by the Japanese in 1937), which produced 65,000 metric tons per year as of 1991 [ 7 ] and increased capacity to at least 210,000 t/year by 2023. [ 9 ]
This process begins by roasting concentrates that are mixed with coal and briquetted in two stages. The briquettes are then heated in an autogenous coker at 700 °C (1,292 °F) and then charged into the retort. There are three reasons to briquette the calcine: to ensure free downward movement of the charge; to permit heat transfer across a practical size cross-section; to allow adequate porosity for the passage of reduced zinc vapour to the top of the retort. The reduced zinc vapour that is collected at the top of the retort is then condensed to a liquid. [ 7 ]
Overpelt improved upon this design by using only one large condensation chamber, instead of many small ones, as it was originally designed. This allowed for the carbon monoxide to be recirculated into the furnaces for heating the retorts. [ 7 ]
This process was licensed to the Imperial Smelting Corporation (ISC), based in Avonmouth , England, which had a large vertical retort (VR) plant in production for many years. It was used until the mid-1970s when it was superseded by the company's Imperial Smelting Furnace (ISF) plant. The VR plant was demolished in 1975.
This process was the main process used in Britain from the mid-19th century until 1951. [ 7 ] [ 10 ] The process was very inefficient as it was designed as a small scale batch operation. Each retort only produced 40 kilograms (88 lb) so companies would put them together in banks and used one large gas burner to heat all of them. [ 10 ] The Belgian process requires redistillation to remove impurities of lead, cadmium, iron, copper, and arsenic. [ 6 ]
The first production of zinc in quantity seems to have been in India starting from 12th century and later in China from 16th century. [ 11 ] In India, zinc was produced at Zawar from the 12th to the 18th centuries, although some zinc artifacts appear to have been made during classical antiquity in Europe . [ 12 ] The sphalerite ore found here was presumably converted to zinc oxide via roasting, although no archaeological evidence of this has been found. Smelting is thought to have been done in sealed cylindrical clay retorts which were packed with a mixture of roasted ore, dolomite , and an organic material, perhaps cow dung , and then placed vertically in a furnace and heated to around 1100 °C. Carbon monoxide produced by the charring of the organic material would have reduced the zinc oxide to zinc vapour, which then liquefied in a conical clay condenser at the bottom of the retort, dripping down into a collection vessel. Over the period 1400–1800, production is estimated to have been about 200 kg/day. [ 13 ] Zinc was also smelted in China from the mid-sixteenth century on. [ 14 ]
Large-scale zinc production in Europe began with William Champion , who patented a zinc distillation process in 1738. [ 15 ] In Champion's process, zinc ore (in this case, the carbonate, ZnCO 3 ) was sealed in large reduction pots with charcoal and heated in a furnace. The zinc vapor then descended through an iron condensing pipe until reaching a water-filled vessel at the bottom. [ 16 ] Champion set up his first zinc works in Bristol , England, but soon expanded to Warmley and by 1754 had built four zinc furnaces there. [ 17 ] Although Champion succeeded in producing about 200 tons of zinc, [ 17 ] his business plans were not successful and he was bankrupt by 1769. [ 16 ] However, zinc smelting continued in this area until 1880. [ 17 ]
Early European zinc production also took place in Silesia , in Carinthia , and in Liège , Belgium . In the Carinthian process, used in works established in 1798 by Bergrath Dillinger, a wood-fueled furnace heated a large number of small vertical retorts, [ 20 ] and zinc vapor then dropped through a ceramic pipe into a common condensation chamber below. This process was out of use by 1840. The Belgian and Silesian processes both used horizontal retorts. [ 21 ] In Silesia, Johann Ruhberg built a furnace to distill zinc in 1799, at first using pots but later changing to flat-bottomed retorts called "muffles", attached to horizontal tubes bent downwards in which the zinc condensed. The Silesian process eventually merged with the Belgian process. This process, developed by Jean-Jacques Daniel Dony , was introduced 1805–1810, and used retorts with a cylindrical cross-section. [ 20 ] [ 21 ] Condensers were horizontal clay tubes extending from the ends of the retorts. [ 22 ] The merged "Belgo-Silesian" horizontal retort process was widely adopted in Europe by the third quarter of the 19th century, and later in the United States. [ 21 ]
Experimental attempts to extract zinc via electrolysis begun in the 19th century, but the only commercially successful process before 1913 was through electrolysis of an aqueous zinc chloride solution, [ 23 ] a process used in Great Britain and Austria . The Anaconda Copper Company , at Anaconda , Montana , and the Consolidated Mining and Smelting Company , at Trail , British Columbia , both built successful electrolytic plants in 1915 using the currently used zinc sulfate process. [ 24 ] This method has continued to grow in importance and in 1975 accounted for 68% of world zinc production. [ 25 ]
The continuous vertical retort process was introduced in 1929 by the New Jersey Zinc Company. This process used a retort with silicon carbide walls, around 9 meters high and with a cross section of 2 by 0.3 meters. The walls of the retort were heated to 1300 °C and briquettes consisting of sintered zinc ore, coke, coal, and recycled material were fed into the top of the retort. Gaseous zinc was drawn off from the top of the column and, after a 20-hour journey through the retort, spent briquettes were removed from the bottom. [ 26 ] To condense the gaseous zinc, the company first used a simple brick chamber with carborundum baffles, but efficiency was poor. During the 1940s a condenser was developed which condensed the zinc vapor on a spray of liquid zinc droplets, thrown up by an electrical impeller. [ 27 ]
The electrothermic process, developed by the St. Joseph's Lead Company , was somewhat similar. [ 26 ] [ 28 ] The first commercial plant using this process was built in 1930 at the present site of Josephtown , Pennsylvania . The electrothermic furnace was a steel cylinder around 15 meters high and 2 meters in diameter, lined with firebrick. A mixture of sintered ore and coke was fed into the top of the furnace, and a current of 10,000–20,000 amperes, at a potential difference of 240 volts, was applied between carbon electrodes in the furnace, raising the temperature to 1200–1400 °C. [ 26 ] [ 28 ] An efficient condenser was devised for this process from 1931–1936; it consisted of a bath of liquid zinc which the exhaust gases were drawn through by suction. The zinc content of the gas stream was absorbed into the liquid bath. [ 27 ]
The blast-furnace process was developed starting in 1943 at Avonmouth, England by the Imperial Smelting Corporation , [ 29 ] which became part of Rio Tinto Zinc in 1968. [ 30 ] It uses a spray of molten lead droplets to condense the zinc vapor. [ 31 ]
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2.3.3 European Community (EC) Number
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2.3.3 European Community (EC) Number
Zinc sulfide (or zinc sulphide ) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite . Although this mineral is usually black because of various impurities, the pure material is white, and it is widely used as a pigment. In its dense synthetic form, zinc sulfide can be transparent , and it is used as a window for visible optics and infrared optics.
ZnS exists in two main crystalline forms . This dualism is an example of polymorphism . In each form, the coordination geometry at Zn and S is tetrahedral. The more stable cubic form is known also as zinc blende or sphalerite . The hexagonal form is known as the mineral wurtzite , although it also can be produced synthetically. [ 2 ] The transition from the sphalerite form to the wurtzite form occurs at around 1020 °C .
Zinc sulfide, with addition of a few ppm of a suitable activator , exhibits strong phosphorescence . The phenomenon was described by Nikola Tesla in 1893, [ 3 ] and is currently used in many applications, from cathode-ray tubes through X-ray screens to glow in the dark products. When silver is used as activator, the resulting color is bright blue, with maximum at 450 nanometers . Using manganese yields an orange-red color at around 590 nanometers. Copper gives a longer glow, and it has the familiar greenish glow-in-the-dark. Copper-doped zinc sulfide ("ZnS plus Cu") is used also in electroluminescent panels. [ 4 ] It also exhibits phosphorescence due to impurities on illumination with blue or ultraviolet light.
Zinc sulfide is also used as an infrared optical material, transmitting from visible wavelengths to just over 12 micrometers . It can be used planar as an optical window or shaped into a lens . It is made as microcrystalline sheets by the synthesis from hydrogen sulfide gas and zinc vapour, and this is sold as FLIR -grade (Forward Looking Infrared), where the zinc sulfide is in a milky-yellow, opaque form. This material when hot isostatically pressed (HIPed) can be converted to a water-clear form known as Cleartran (trademark). Early commercial forms were marketed as Irtran-2 but this designation is now obsolete.
Zinc sulfide is a common pigment , sometimes called sachtolith. When combined with barium sulfate, zinc sulfide forms lithopone . [ 5 ]
Fine ZnS powder is an efficient photocatalyst , which produces hydrogen gas from water upon illumination. Sulfur vacancies can be introduced in ZnS during its synthesis; this gradually turns the white-yellowish ZnS into a brown powder, and boosts the photocatalytic activity through enhanced light absorption. [ 1 ]
Both sphalerite and wurtzite are intrinsic, wide- bandgap semiconductors . These are prototypical II-VI semiconductors , and they adopt structures related to many of the other semiconductors, such as gallium arsenide . The cubic form of ZnS has a band gap of about 3.54 electron volts at 300 kelvins , but the hexagonal form has a band gap of about 3.91 electron volts. ZnS can be doped as either an n-type semiconductor or a p-type semiconductor .
The phosphorescence of ZnS was first reported by the French chemist Théodore Sidot in 1866. His findings were presented by A. E. Becquerel , who was renowned for the research on luminescence . [ 6 ] ZnS was used by Ernest Rutherford and others in the early years of nuclear physics as a scintillation detector, because it emits light upon excitation by x-rays or electron beam , making it useful for X-ray screens and cathode-ray tubes. [ 7 ] This property made zinc sulfide useful in the dials of radium watches.
Zinc sulfide is usually produced from waste materials from other applications. Typical sources include smelter, slag, and pickle liquors. [ 5 ] As an example, the synthesis of ammonia from methane requires a priori removal of hydrogen sulfide impurities in the natural gas, for which zinc oxide is used. This scavenging produces zinc sulfide:
Crude zinc sulfide can be produced by igniting a mixture of zinc and sulfur . [ 8 ] More conventionally, ZnS is prepared by treating a mildly acidic solution of Zn 2+ salts with H 2 S : [ 9 ]
This reaction is the basis of a gravimetric analysis for zinc. [ 10 ]
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The Zincke nitration is a nitration reaction in which a bromine is replaced by a nitro group on an electron-rich aryl compound such as a phenol or cresol . Typical reagents are nitrous acid or sodium nitrite . The reaction is a manifestation of nucleophilic aromatic substitution and is named after Theodor Zincke , who first reported it in 1900. [ 1 ] [ 2 ]
Two examples: [ 3 ]
and: [ 4 ]
The Zincke nitration should not be confused with the Zincke–Suhl reaction or the Zincke reaction .
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The Zincke reaction is an organic reaction , named after Theodor Zincke , in which a pyridine is transformed into a pyridinium salt by reaction with 2,4-dinitro-chlorobenzene and a primary amine . [ 1 ] [ 2 ] [ 3 ] [ 4 ]
The Zincke reaction should not be confused with the Zincke-Suhl reaction or the Zincke nitration . Furthermore, the Zincke reaction has nothing to do with the chemical element zinc .
The first reaction is the formation of the N -2,4-dinitrophenyl-pyridinium salt ( 2 ). This salt is typically isolated and purified by recrystallization .
Upon heating a primary amine with the N -2,4-dinitrophenyl-pyridinium salt ( 2 ), the addition of the amine leads to the opening of the pyridinium ring. A second addition of amine leads to the displacement of 2,4-dinitroaniline ( 5 ) and formation of the König salt [ 5 ] ( 6a and 6b ). The trans-cis-trans isomer of the König salt ( 6a ) can react by either sigmatropic rearrangement or nucleophilic addition of a zwitterionic intermediate to give cyclized intermediate ( 7 ). [ 6 ] This has been suggested to be the rate-determining step . [ 7 ] [ 8 ] After proton transfer and amine elimination , the desired pyridinium ion ( 9 ) is formed.
This mechanism can be referred to as an instance of the ANRORC mechanism : nucleophilic addition (A N ), ring opening and ring closing .
In one solid-phase synthesis application, the amine is covalently attached to Wang resin . [ 9 ]
Another example is the synthesis of a chiral isoquinolinium salt. [ 10 ]
With secondary amines and not primary amines the Zincke reaction takes on a different shape forming so-called Zincke aldehydes in which the pyridine ring is ring-opened with the terminal iminium group hydrolyzed to an aldehyde : [ 4 ]
This variation has been applied in the synthesis of novel indoles : [ 11 ]
with cyanogen bromide mediated pyridine activation.
In 2006 and again in 2007 the Zincke reaction was rediscovered by a research group from Japan [ 12 ] and a group from the USA. [ 13 ] Both groups claimed the synthesis of a 12 membered diaza annulene (structure 1 ) from an N-aryl pyridinium chloride and an amine , an aniline in the case of the Japanese group (depicted below) and an aliphatic amine (anticipating surfactant properties) in the case of the American group.
In a letter to Angewandte Chemie , the German chemist Manfred Christl [ 14 ] pointed out not only that the alleged new chemistry was in fact 100-year-old Zincke chemistry but also that the proposed structure for the reaction product was not the 12 membered ring but the 6 membered pyridinium salt (structure 2 ). Initially both groups conceded that they had ignored existing literature on Zincke but held on to the annulene structure based on their electrospray ionization (ESI) results which according to them clearly showed dimer. In his letter Christl remarked that in ESI measurements association of molecules is a common phenomenon. In addition, he noted similarities in melting point and NMR spectroscopy .
As of December 2007 the Japanese group retracted its paper in Organic Letters due to uncertainties regarding what products are formed in the reaction described and the US group added a correction to theirs in the Angewandte Chemie stating they wish(ed) to alter the proposed structure of (the) annulene . [ 15 ] The issue did receive some media coverage: [ 16 ] [ 17 ]
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The Zincke–Suhl reaction is a special case of a Friedel-Crafts alkylation and was first described by Theodor Zincke and Suhl in 1906. [ 1 ] [ 2 ] [ 3 ] Unlike the traditional Friedel-Crafts reaction, the reduction of the phenyl ring leads to a higher energy final product that can be used as starting material in the dienol–benzene rearrangement , among other reactions.
The classic example of this reaction is the conversion of p-cresol to a cyclohexadienone (with the aid of aluminium chloride as a catalyst and tetrachloromethane as a solvent ). Melvin Newman, a scientist from the U.S. intensively studied the reaction in the 1950s and reported several improved procedures as well as mechanistic studies. Since then, studies investigating the impact of alternate reagents have been conducted by others. [ 4 ] [ 5 ]
Aluminum chloride plays a range of functions in this reaction, [ 1 ] first reacting with p-cresol to form phenoxy aluminum chloride along with a molecule of hydrogen chloride . Additionally, aluminum chloride activates a molecule of tetrachloride that in turn is subjected to a nucleophilic attack by the phenoxy aluminum chloride. Subsequently, the product interacts with aluminum chloride and tetrachloride again to form a loose complex. Finally, the product is treated with water, resulting in the final product.
During a series of tests, Newman found that utilization of carbon disulfide as a solvent was demonstrated to improve the yield. [ 1 ] As an example, addition of a solution of p-cresol and carbon disulfide to a suspension of aluminum chloride and carbon disulfide resulted in a 20% improvement in yield.
Zincke-Suhl products can be used as starting reagents for the dienol benzene rearrangement. This is an important reaction for artificially producing biologically relevant molecules including the A ring of steroids. [ 6 ]
[ 7 ] [ 8 ]
Perhaps the most intriguing application of the Zincke–Suhl reaction is its potential following von Auwers rearrangements . Demeunier and Jaeckh described how such rearrangements may result in the formation of high energy intermediates. [ 9 ] For example, reformation of the aromatic ring from the semibenzene (cyclohexadienone) follows an energy drop of just under 36 kcal/mol. Such staunch drops have been experimentally shown to lead to efficient aromatization with high yields.
[ 9 ]
Other products including dioxocins and polymers can form depending upon the location of the initial addition of carbon tetrachloride. [ 2 ] [ 4 ] Furthermore, changes to the reagents such as the use of chloroform instead of carbon tetrachloride can form additional products. [ 4 ] [ 5 ]
Above: Ortho-addition of tetrachloride to phenoxy aluminum chloride can produce 6,12-diphenyl-2,8-dimethyl-6,12-epoxy-6 H , 12 H -dibenzo[ b,f ][1,5] dioxocin, a high-melting, white polymer.
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Zinc–copper couple is an alloy of zinc and copper that is employed as a reagent in organic synthesis . The “couple” was popularized after the report by Simmons and Smith, published in 1959, on its application as an activated source of zinc required for formation of an organozinc reagent in the Simmons–Smith cyclopropanation of alkenes . [ 1 ] The couple has been widely applied as a reagent in other reactions requiring activated zinc metal. Zinc–copper couple does not refer to a rigorously defined chemical structure or alloy composition. The couple may contain varying proportions of copper and zinc; the zinc content is typically greater than 90%, although an alloy containing similar proportions of zinc and copper is used in some cases. The couple is frequently prepared as a darkly-colored powder and is slurried in an ethereal solvent prior to being used in slight excess relative to the substrate. Activation of zinc by copper is essential to the couple’s utility, but the origin of this effect is poorly documented. It is speculated that copper enhances reactivity of zinc at the surface of the alloy. [ 2 ]
Zinc–copper couple has been prepared by numerous methods, which vary mainly with respect to the source of copper, but also by the ratio of copper to zinc, the physical state of the zinc (e.g. powder or granules), the use of protic acids and other additives, and temperature of the preparation. Most often the couple is generated and isolated prior to use, but routes have been described to storable forms of the alloy. Most methods involve reduction of an oxidized copper species with zinc, which is used in excess.
An early method for the synthesis of zinc–copper couple entailed treatment of a mixture of zinc dust and copper(II) oxide with hydrogen gas at 500 °C. [ 1 ] A more convenient and cheaper method proceeds by treatment of zinc powder with hydrochloric acid and copper(II) sulfate . [ 3 ] Treatment of zinc powder with copper(II) acetate monohydrate in hot acetic acid is reportedly highly reproducible. [ 4 ] The couple may also be generated in situ by reaction of one equivalent of zinc dust with one equivalent of copper(I) chloride (or copper powder) in refluxing ether . [ 5 ]
The choice of method is dictated primarily by the application. The development of newer methods was motivated by the need for zinc–copper couple with reproducible behavior.
Zinc–copper couple has found widespread use in organic synthesis, especially in the Simmons–Smith cyclopropanation of alkenes. In this process, the couple (typically a slurry in an ethereal solvent) reacts with methylene iodide to generate iodomethylzinc iodide, which is the intermediate responsible for cyclopropanation.
The couple has also been employed to generate alkyl zinc reagents for conjugate addition , as a dehalogenating reagent, as a promoter of reductive coupling of carbonyl compounds , and to reduce electron-deficient alkenes and alkynes . Sonication has been employed to enhance the rate of the zinc–copper couple-mediated cycloaddition of α,α’-dibromo ketones to 1,3-dienes . [ 6 ]
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For chemical reactions , the zinc–zinc oxide cycle or Zn–ZnO cycle is a two step thermochemical cycle based on zinc and zinc oxide [ 1 ] for hydrogen production [ 2 ] with a typical efficiency around 40%. [ 3 ]
The thermochemical two-step water splitting process uses redox systems: [ 4 ]
For the first endothermic step concentrating solar power is used in which zinc oxide is thermally dissociated at 1,900 °C (3,450 °F) into zinc and oxygen. In the second non-solar exothermic step zinc reacts at 427 °C (801 °F) with water and produces hydrogen and zinc oxide. The temperature level is realized by using a solar power tower and a set of heliostats to collect the solar thermal energy .
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Zineb is the chemical compound with the formula {Zn[S 2 CN(H)CH 2 CH 2 N(H)CS 2 ]} n . Structurally, it is classified as a coordination polymer and a dithiocarbamate complex . This pale yellow solid is used as fungicide . [ 2 ]
It is produced by treating ethylene bis(dithiocarbamate) sodium salt, "nabam", with zinc sulfate . This procedure can be carried out by mixing nabam and zinc sulfate in a spray tank. [ 3 ] Its uses include control of downy mildews , rusts , and redfire disease. [ 2 ] In the US it was once registered as a "General Use Pesticide", however all registrations were voluntarily cancelled following an EPA special review. [ 3 ] It continues to be used in many other countries.
Zineb is a polymeric complex of zinc with a dithiocarbamate . [ 2 ] The polymer is composed of Zn(dithiocarbamate) 2 subunits linked by an ethylene (-CH 2 CH 2 -) backbone. [ 4 ] A reference compound is [Zn(S 2 CNEt 2 ) 2 ] 2 , which features a pair of tetrahedral Zn centers bridged by one sulfur center. [ 5 ]
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Zinin reaction or Zinin reduction involves reduction of nitro aromatic compounds to the amines using sodium sulfide . [ 1 ] It is used to convert nitrobenzenes to anilines . [ 2 ] [ 3 ] The reaction selectively reduces nitro groups in the presence of other easily reduced functional groups (e.g., aryl halides and C=C bonds) are present in the molecule.
The reaction requires water, with thiosulfate being formed as a by-product. A possible stoichiometry for the reaction is:
Mechanistic studies have implicated a role for disulfide that is generated in situ . Nitrosobenzenes (ArNO) and phenylhydroxylamine (ArNHOH) are probable intermediates. [ 4 ]
Dinitrobenzenes can often be reduced selectively to the nitroaniline, [ 5 ] for example in the synthesis of 3-nitroaniline from 1,3-dinitrobenzene
The reaction was discovered by a Russian organic chemist Nikolay Zinin (Russian: Николай Николаевич Зинин) (25 August 1812, Shusha – 18 February 1880, Saint Petersburg).
This chemical reaction article is a stub . You can help Wikipedia by expanding it .
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Zinovy Reichstein (born 1961) is a Russian-born American mathematician . He is a professor at the University of British Columbia in Vancouver .
He studies mainly algebra , algebraic geometry and algebraic groups . He introduced (with Joe P. Buhler ) the concept of essential dimension . [ 1 ]
In high school, Reichstein participated in the national mathematics olympiad in Russia and was the third highest scorer in 1977 and second highest scorer in 1978.
Because of the Antisemitism in the Soviet Union at the time, Reichstein was not accepted to Moscow University, even though he had passed the special math entrance exams. He attended a semester of college at Russian University of Transport instead.
His family then decided to emigrate, arriving in Vienna, Austria, in August 1979 and New York, United States in the fall of 1980. Reichstein worked as a delivery boy for a short period of time in New York. He was then accepted to and attended California Institute of Technology for his undergraduate studies. [ 2 ]
Reichstein received his PhD degree in 1988 from Harvard University under the supervision of Michael Artin . Parts of his thesis entitled "The Behavior of Stability under Equivariant Maps" were published in the journal Inventiones Mathematicae . [ 3 ]
As of 2011, he is on the editorial board of the mathematics journal Transformation groups . [ 4 ]
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In chemistry , a Zintl phase is a product of a reaction between a group 1 ( alkali metal ) or group 2 ( alkaline earth metal ) and main group metal or metalloid (from groups 13, 14, 15, or 16). It is characterized by intermediate metallic / ionic bonding. Zintl phases are a subgroup of brittle , high-melting intermetallic compounds that are diamagnetic or exhibit temperature-independent paramagnetism and are poor conductors or semiconductors . [ 1 ]
This type of solid is named after German chemist Eduard Zintl who investigated them in the 1930s. The term "Zintl Phases" was first used by Laves in 1941. [ 2 ] In his early studies, Zintl noted that there was an atomic volume contraction upon the formation of these products and realized that this could indicate cation formation. He suggested that the structures of these phases were ionic, with complete electron transfer from the more electropositive metal to the more electronegative main group element. [ 1 ] The structure of the anion within the phase is then considered on the basis of the resulting electronic state. These ideas are further developed in the Zintl-Klemm-Busmann concept, where the polyanion structure should be similar to that of the isovalent element. Further, the anionic sublattice can be isolated as polyanions (Zintl ions) in solution and are the basis of a rich subfield of main group inorganic chemistry.
A "Zintl Phase" was first observed in 1891 by M. Joannis, who noted an unexpected green colored solution after dissolving lead and sodium in liquid ammonia , indicating the formation of a new product. [ 3 ] It was not until many years later, in 1930, that the stoichiometry of the new product was identified as Na 4 Pb 9 4− by titrations performed by Zintl et al.; [ 4 ] and it was not until 1970 that the structure was confirmed by crystallization with ethylenediamine (en) by Kummer. [ 5 ]
In the intervening years and in the years since, many other reaction mixtures of metals were explored to provide a great number of examples of this type of system. There are hundreds of both compounds composed of group 14 elements and group 15 elements, plus dozens of others beyond those groups, all spanning a variety of different geometries. [ 6 ] Corbett has contributed improvements to the crystallization of Zintl ions by demonstrating the use of chelating ligands , such as cryptands , as cation sequestering agents. [ 7 ]
More recently, Zintl phase and ion reactivity in more complex systems, with organic ligands or transition metals, have been investigated, as well as their use in practical applications, such as for catalytic purposes or in materials science.
Zintl phases are intermetallic compounds that have a pronounced ionic bonding character. They are made up of a polyanionic substructure and group 1 or 2 counter ions, and their structure can be understood by a formal electron transfer from the electropositive element to the more electronegative element in their composition. Thus, the valence electron concentration (VEC) of the anionic element is increased, and it formally moves to the right in its row of the periodic table. Generally the anion does not reach an octet , so to reach that closed shell configuration , bonds are formed. The structure can be explained by the 8-N rule (replacing the number of valence electrons, N, by VEC), making it comparable to an isovalent element. [ 8 ] The formed polyanionic substructures can be chains (two-dimensional), rings, and other two-or three-dimensional networks or molecule-like entities.
The Zintl line is a hypothetical boundary drawn between groups 13 and 14. It separates the columns based on the tendency for group 13 elements to form metals when reacted with electropositive group 1 or 2 elements and for group 14 and above to form ionic solids. [ 9 ] The 'typical salts' formed in these reactions become more metallic as the main group element becomes heavier. [ 8 ]
Zintl phases can be prepared in regular solid state reactions, usually performed under an inert atmosphere or in a molten salt solution . Typical solid state methods include direct reduction of corresponding oxides in solution phase reactions in liquid ammonia or mercury. The product can be purified in some cases via zone refining , though often careful annealing will result in large single crystals of a desired phase. [ 8 ]
Many of the usual methods are useful for determining physical and structural properties of Zintl phases. Some Zintl phases can be decomposed into a Zintl ion—the polyanion that composes the anionic substructure of the phase—and counter ion, which can be studied as described below. The heat of formation of these phases can be evaluated. Often their magnitude is comparable to those of salt formation, providing evidence for the ionic character of these phases. [ 8 ] Density measurements indicate a contraction of the product compared to reactants, similarly indicating ionic bonding within the phase. [ 10 ] X-ray spectroscopy gives additional information about the oxidation state of the elements, and correspondingly the nature of their bonding. Conductivity and magnetization measurements can also be taken. Finally, the structure of a Zintl phase or ion is most reliably confirmed via X-ray crystallography .
An illustrative example: There are two types of Zintl ions in K 12 Si 17 ; 2x Si 4− 4 (pseudo P 4 ,
or according to Wade's rules , 12 = 2n + 4 skeletal-electrons corresponding to a nido -form of a trigonal-bipyramid ) and 1x Si 4− 9 (according to Wade's rules , 22 = 2n + 4 skeletal-electrons corresponding to a nido -form of a bicapped square antiprism )
Examples from Müller's 1973 review paper with known structures are listed in the table below. [ 8 ]
Li 9 Al 4 LiGa LiIn LiTl
Li 7 Si 2 Li 22 Ge 5
Li 9 Ge 4 Li 22 Sn 5
Li 2 Sn 5
Li 3 As LiAs Li 3 Sb Li 3 Bi LiBi
Li 2 Se Li 2 Te
LiBr LiI
NaIn Na 2 Tl (the polyanion is tetrahedral (Tl 4 ) 8- Concept Tl 2- ~ P)
NaTl (See Figure)
Na x Si 136 (x ≤ 11)
Na 8 Si NaGe Na 15 Pb 4 Na 13 Pb 5
Na 9 Pb 4 NaPb
Na 3 As Na 3 Sb NaSb Na 3 Bi NaBi
Na 2 Se Na 2 Se 2
Na 2 Te
NaBr NaI
K 8 Si 46 K 8 Ge 46 K 8 Sn 46
KPb KPb 2
K 3 As K 3 Sb KBi 2
K 2 S 2 K 2 Se
K 2 Se 2 K 2 Te
KBr KI
RbGe RbSn RbPb
Rb 3 As Rb 3 Bi RbBi 2
RbBr RbI
CsGe CsSn CsPb
Cs 3 Sb Cs 3 Bi CsBi 2
Cs 2 NaAs 7 (See Figure)
CsBr CsI
Mg 5 Ga 2 Mg 2 Ga
MgGa MgGa 2 Mg 2 Ga 5 Mg 3 In
Mg 5 In 2 Mg 2 In
MgIn MgIn 5 Mg 2 Tl MgTl
Mg 2 Ge Mg 2 Sn
Mg 2 Pb
Mg 3 As 2 Mg 3 Sb 2
Mg 3 Bi 2
MgSe MgTe MgTe 2
MgBr 2 MgI 2
CaAl 4 CaGa CaGa 2 CaGa 4 CaIn CaIn 2 CaTl CaTl 3
CaSi 2 Ca 7 Ge
Ca 2 Ge CaGe 2 Ca 3 Sn Ca 2 Sn
CaSn CaSn 3 Ca 3 Pb Ca 2 Pb
Ca 5 Pb 3
CaSe CaTe
CaBr 2 CaI 2
SrGa 2 SrGa 4 SrIn 2 SrTl SrTl 2 SrTl 3
SrSi Sr 4 Si 7
SrSi 2 Sr 3 Ge 4
SrGe 2 SrSn SrPb 3
Sr 3 P 14 SrBi 3
SrSe SrTe
SrBr 2 SrI 2
BaGa 2 BaGa 4 BaIn 2 BaIn 4 BaTl 2
BaSi Ba 3 Si 4
BaSi 2 Ba 2 Ge
BaGe BaGe 2 BaSn Ba 5 Pb 3
BaPb BaPb 3
BaP 3 BaBi 3
BaS 3 BaSe BaTe
BaBr 2 BaI 2
There are examples of a new class of compounds that, on the basis of their chemical formulae, would appear to be Zintl phases, e.g., K 8 In 11 , which is metallic and paramagnetic. Molecular orbital calculations have shown that the anion is (In 11 ) 7− and that the extra electron is distributed over the cations and, possibly, the anion antibonding orbitals . [ 13 ] Another exception is the metallic InBi. InBi fulfills the Zintl phase requisite of element-element bonds but not the requisite of the polyanionic structure fitting a normal valence compound, i.e., the Bi–Bi polyanionic structure does not correspond to a normal valence structure such as the diamond Tl − in NaTl. [ 14 ]
Zintl phases that contain molecule-like polyanions will often separate into its constituent anions and cations in liquid ammonia, ethylenediamene, crown ethers , or cryptand solutions. Therefore, they are referred to as Zintl ions. The term 'clusters' is also used to emphasize them as groups with homonuclear bonding. The structures can be described by Wade's rules and occupy an area of transition between localized covalent bonds and delocalized skeletal bonding. [ 15 ] Beyond the "aesthetic simplicity and beauty of their structures" and distinctive electronic properties, Zintl ions are also of interest in synthesis because of their unique and unpredictable behavior in solution. [ 15 ]
The largest subcategory of Zintl ions is homoatomic clusters of group 14 or 15 elements. Some examples are listed below. [ 15 ] [ 16 ]
Many examples similarly exist for heteroatomic clusters where the polyanion is composed of greater than one main group element. Some examples are listed below. [ 15 ] [ 16 ] Zintl ions are also capable of reacting with ligands and transition metals, and further 'heteroatomic examples are discussed below (intermetalloid clusters). In some solvents, atoms exchange can occur between heteroatomic clusters. [ 17 ] Additionally, it is notable that fewer large cluster examples exist. [ 15 ]
Zintl ions are typically prepared through one of two methods. The first is a direct reduction route performed at low temperature. In this method, dry ammonia is condensed over a mixture of the two (or more) metals under inert atmosphere. The reaction initially produces solvated electrons in ammonia that reduce the more electronegative element over the course of the reaction. This reaction can be monitored by a color change from blue (solvated electrons) to the color of the Zintl phase. The second is method, performed at higher temperatures, is to dissolve a Zintl phase in liquid ammonia or other polar aprotic solvent like ethylenediamine (on rare occasions DMF or pyridine is used). [ 18 ] Some Zintl ions, such as Si and Ge based ions, can only be prepared via this indirect method because they cannot be reduced at low temperatures. [ 18 ]
The structure of Zintl ions can be confirmed through x-ray crystallography. Corbett has also improved the crystallization of Zintl ions by demonstrating the use of chelating ligands such as cryptands, as cation sequestering agents. [ 7 ]
Many of the main group elements have NMR active nuclei, thus NMR experiments are also valuable for gaining structural and electronic information; they can reveal information about the flexibility of clusters. For example, differently charged species can be present in solution because the polyanions are highly reduced and may be oxidized by solvent molecules. NMR experiments have shown a low barrier to change and thus similar energies for different states. [ 17 ] NMR is also useful for gaining information about the coupling between individual atoms of the polyanion and with the counter-ion, a coordinated transition metal, or ligand. Nucleus independent chemical shifts can also be an indicator for 3D aromaticity, which causes magnetic shielding at special points.
Additionally, EPR can be used to measure paramagnetic in relevant clusters, of which there are a number of examples of the [E 9 ] 3− type, among others. [ 19 ]
As highly reduced species in solution, Zintl ions offer many and often unexpected, reaction possibilities, and their discrete nature positions them as potentially important starting materials in inorganic synthesis. [ 18 ]
In solution, individual Zintl ions can react with each other to form oligomers and polymers . In fact, anions with high nuclearity can be viewed as oxidative coupling products of monomers. [ 20 ] After oxidation, the clusters may sometimes persist as radicals that can be used as precursors in other reactions. Zintl ions can oxidize without the presence of specific oxidizing agents through solvent molecules or impurities, for example in the presence of cryptand, which is often used to aid crystallization. [ 15 ]
Zintl ion clusters can be functionalized with a variety of ligands in a similar reaction to their oligomerization. As such, functionalization competes with those reactions and both can be observed to occur. Organic groups, for example phenyl , TMS , and bromomethane , form exo bonds to the electronegative main group atoms. These ligands can also stabilize high nuclearity clusters, in particular heteroatomic examples. [ 15 ]
Similarly in solids, Zintl phases can incorporate hydrogen. Such Zintl phase hydrides can be either formed by direct synthesis of the elements or element hydrides in a hydrogen atmosphere or by a hydrogenation reaction of a pristine Zintl phase. Since hydrogen has a comparable electronegativity as the post-transition metal it is incorporated as part of the polyanionic spatial structure. There are two structural motifs present. A monatomic hydride can be formed occupying an interstitial site that is coordinated by cations exclusively ( interstitial hydride ) or it can bind covalently to the polyanion (polyanionic hydride). [ 21 ]
The Zintl ion itself can also act as a ligand in transition metal complexes. This reactivity is usually seen in clusters composed of greater than 9 atoms, and it is more common for group 15 clusters. A change in geometry often accompanies complexation; however zero electrons are contributed from the metal to the complex, so the electron count with respect to Wade's rules does not change. [ 15 ] In some cases the transition metal will cap the face of the cluster. Another mode of reaction is the formation of endohedral complexes where the metal is encapsulated inside the cluster. These types of complexes lend themselves to comparison with the solid state structure of the corresponding Zintl phase. [ 16 ] These reactions tend to be unpredictable and highly dependent on temperature, among other reaction conditions.
The geometry and bonding of a Zintl ion cannot be easily described by classical two electron two center bonding theories ; however the geometries Zintl ions can be well described by Wade’s rules of boranes. Wade’s rules offer an alternative model for the relationship between geometry and electron count in delocalized electron deficient systems. The rules were developed to predict the geometries of boranes from the number of electrons and can be applied to these polyanions by replacing the BH unit with a lone pair. [ 18 ] Some unique clusters of Ge occur in non- deltahedral shapes that cannot be described by Wade’s rules. The rules also become more convoluted in intermetallic clusters with transition metals and consideration needs to be taken for the location of the additional electrons.
The Zintl-Klemm-Busmann concept describes how in an anionic cluster, the atoms arrange in typical geometries found for the element to the right of it on the periodic table. So “the anionic lattice is isometric with elemental lattices having the same number of valence electrons.” [ 8 ] In this formulation, the average charge on each atom of the cluster can be calculated by:
anion valence + cation valence n a = VEC {\displaystyle {\frac {{\text{anion valence}}+{\text{cation valence}}}{n_{a}}}={\text{VEC}}}
where n a is number of anion atoms and VEC is the valence electron concentration per anion atom, then:
8 − VEC = number of bonds per anion atom {\displaystyle 8-{\text{VEC}}={\text{number of bonds per anion atom}}} . [ 18 ]
The number of bonds per anion predicts structure based on isoelectronic neighbor. This rule is also referred to as the 8 - N rule and can also be written as:
n e + b a − b c n a = 8 {\displaystyle {\frac {n_{e}+b_{a}-b_{c}}{n_{a}}}=8} .
Not all phases follow the Zintl-Klemm-Busmann concept, particularly when there is a high content of either the electronegative or electropositive element. There are still other examples where this does not apply. [ 8 ]
Wade's rules are successful in describing the geometry of the anionic sublattice of Zintl phases and of Zintl ions but not the electronic structure. Other 'spherical shell models' with spherical harmonic wave functions for molecular orbitals—analogous to atomic orbitals—that describe the clusters as pseduo elements. The Jellium model uses a spherical potential from the nuclei to give orbitals with global nodal properties. Again, this formulates the cluster as a 'super atom' with an electron configuration comparable to a single atom. The model is best applied to spherically symmetric systems, and two examples for which it works well are the icosahedral Al 13 − and [Sn@Cu 12 @Sn 20 ] 12− clusters. [ 22 ] [ 23 ] DFT or ab initio molecular orbital calculations similarly treat the clusters with atomic, and correspondingly label them S, P, D etc. These closed shell configurations have prompted some investigation of 3D aromaticity . This concept was first suggested for fullerenes and corresponds to a 2(N+1) 2 rule in the spherical shell model. An indicator of this phenomenon is a negative Nucleus Independent Chemical Shift (NICS) values of the center of the cluster or of certain additional high symmetry points. [ 16 ]
Some Zintl ions show the ability to activate small molecules. One example from Dehnen and coworkers is the capture of O 2 by the intermetallic cluster [Bi 9 {Ru(cod)} 2 ] 3− . [ 24 ] Another ruthenium intertermetallic cluster, [Ru@Sn 9 ] 6− , was used as a precursor to selectively disperse the CO 2 hydrogenation catalyst Ru-SnOx onto CeO 2 , resulting in nearly 100% CO selectivity for methanation . [ 25 ]
In materials science , Ge 9 4− has been used as a source of Ge in lithium ion batteries , where is can be deposited in a microporous layer of alpha-Ge. [ 26 ] The discrete nature of Zintl ions opens the possibility for the bottom up synthesis of nanostructured semiconductors and the surface modification of solids. [ 15 ] The oxidation and polymerization of Zintl ions may also be a source of new materials. For example, polymerization of Ge clusters was used to create guest free germanium clathrate , in other words a particular, pure Ge. [ 27 ]
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Zion Tse is a professor in robotics, and the director of Centre for Bioengineering at the School of Engineering and Materials Science , Queen Mary, University of London . [ 1 ]
Tse received his Ph.D degree in mechatronics in medicine from Imperial College London in England. [ 2 ] [ 3 ] He then received training and worked at the National Institutes of Health in Bethesda, Maryland, Harvard University in Boston, US, and the University of York , England. [ 4 ]
Currently, Tse is a professor in robotics, and the director of Centre for Bioengineering at the School of Engineering and Materials Science , Queen Mary, University of London . [ 1 ]
Most of Tse's academic and professional experience has been in Digital Health, Surgical Robotics, and AI Medical Imaging. He has developed and tested a broad range of medical technologies in his career, most of which have been applied in clinical patient trials. [ 5 ] His research bridges Engineering and Medicine, connecting multidisciplinary teams of medical doctors, researchers and engineers. Tse has published internationally circulated journal papers, articles at international conferences, and robotic and mechatronic patents. [ 6 ]
Intelligent robotics, vision, sensory processing, AI healthcare, imaging and medical assistive robots
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ZipcodeZoo was a free, online encyclopedia intended to document all living species and infraspecies known to science. It was compiled from existing databases. It offered one page for each living species, supplementing text with video, sound, and images where available. ZipcodeZoo was integrated into an app called Lookup Life. [ 1 ] As of 2019 the site no longer works.
ZipcodeZoo was an online database that collected the natural history , classification , species characteristics, conservation biology , and distribution information of thousands of species and infraspecies . It included over 800,000 photographs , 50,000 videos , 160,000 sound clips , and 3.2 million maps describing nearly 3.2 million species and infraspecies. Its content is now only available on the Internet Archive
The site and its sister site lookup.life included a number of specialized search functions, such as identifying a bird species from its color, shape and other traits, including where it was seen; or generating a list of plants or animals likely to be found in or near a specific location (a zipcode, state, country, latitude/longitude, etc.). The searches could be restricted to specific taxa, or broad categories like reptiles or fish. A sound trainer could play multiple bird song recordings simultaneously.
ZipcodeZoo drew on the Catalogue of Life for its basic species list, the Global Biodiversity Information Facility for its maps, Flickr for many of its photos, YouTube for videos, Xeno-canto for some of its sound recordings, the IUCN for conservation status, and other major sources. [ 2 ]
All the pages were published under one of the Creative Commons licenses.
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The Zippe-type centrifuge is a gas centrifuge designed to enrich the rare fissile isotope uranium-235 ( 235 U) from the mixture of isotopes found in naturally occurring uranium compounds. The isotopic separation is based on the slight difference in mass of the isotopes. The Zippe design was originally developed in the Soviet Union by a team led by 60 Austrian and German scientists and engineers captured after World War II , working in detention. In the West (and now generally) the type is known by the name of the man who recreated the technology after his return to the West in 1956, based on his recollection of his work in (and contributions to) the Soviet program, Gernot Zippe . To the extent that it might be referred to in Soviet/Russian usage by any one person's name, it was known (at least at a somewhat earlier stage of development) as a Kamenev centrifuge (after Evgeni Kamenev). [ 1 ] [ 2 ]
Natural uranium consists of three isotopes ; the majority (99.274%) is U-238 , while approximately 0.72% is U-235 , fissile by thermal neutrons, and the remaining 0.0055% is U-234 . If natural uranium is enriched to 3% U-235 , it can be used as fuel for light water nuclear reactors . If it is enriched to 90% uranium-235, it can be used for nuclear weapons .
Enriching uranium is difficult because the isotopes are practically identical in chemistry and very similar in weight: U-235 is only 1.26% lighter than U-238 (note this applies only to uranium metal). Centrifuges need to work with a fluid rather than a solid, and this process used gaseous uranium hexafluoride . The relative mass difference between 235 UF 6 and 238 UF 6 is less than 0.86%. Separation efficiency in a centrifuge depends on absolute mass difference. Separation of uranium isotopes requires a centrifuge that can spin at 1,500 revolutions per second (90,000 rpm ). If we assume a rotor diameter of 20 cm (as in some modern centrifuges [ 3 ] ), this would correspond to a centripetal acceleration of around 900,000 x g [ 4 ] (around 42 times the max speed of a standard, lab benchtop microcentrifuge [ 5 ] and between 0.9 and 9 times the max speed of a standard lab ultracentrifuge [ 6 ] ) or a linear speed of greater than Mach 2 in air (Mach 1 = sound velocity, in air ca. 340 m/s) and much more in UF 6 . For comparison, automatic washing machines operate at only about 12 to 25 revolutions per second (720–1500 rpm) during the spin cycle, while turbines in automotive turbochargers can run up to around 2500–3333 revolutions per second (150,000–200,000 rpm). [ 7 ] [ 8 ]
A Zippe-type centrifuge [ 9 ] has a hollow, cylindrical rotor filled with gaseous uranium hexafluoride (UF 6 ). A rotating magnetic field at the bottom of the rotor, as used in an electric motor , is able to spin it quickly enough that the UF 6 is thrown towards the outer wall, with the 238 UF 6 enriched in the outermost layer and the 235 UF 6 enriched at the inside of this layer. The centrifugal force creates a pressure gradient: On the axis of the centrifuge there is practically vacuum, so that no mechanical feedthrough or seal is needed for the gas inlet and outlets; near the wall the UF 6 reaches its saturation pressure, which in turn limits the rotation speed, because condensation must be avoided. In the so-called countercurrent centrifuge, the bottom of the gaseous mix can be heated, producing convection currents . But the countercurrent is usually stimulated mechanically by the scoop collecting the enriched fraction. In such a way, the enrichment in each horizontal layer is repeated (and thus multiplied) in the next layer, similarly as in column distillation . One scoop is behind a perforated baffle that rotates with the centrifuge; it collects the 238 UF 6 -rich fraction. The other scoop is without baffle. It slows down the gas rotation and thus increases the pressure towards the inside, so that also the 235 UF 6 -rich fraction can be collected without pumping. [ 1 ] [ 9 ] Each centrifuge has one inlet on the axis and two output lines, one collecting the gas at the bottom and one at the top.
Quantitatively, the radial pressure (or density) distribution can be given by [ 9 ] p ( r ) = p ( R ) exp ( − ( M ω 2 / 2 k T ) ( R 2 − r 2 ) ) {\displaystyle p(r)=p(R)\exp(-(M\omega ^{2}/2kT)(R^{2}-r^{2}))}
where p is the pressure, r the variable radius and R its maximum, M the molecular mass, ω the angular velocity, k the Boltzmann constant and T the temperature. (This equation is similar to the barometric formula .) Writing this equation for both isotopes and dividing, gives the ( r -dependent) isotope ratio. It only contains Δ M (not the relative mass difference Δ M/M ) in the exponent. The radial enrichment factor then results by dividing through the initial isotope ratio. To calculate the total enrichment in a countercurrent centrifuge of height H , one has to add a factor of H /( R √2) in the exponent.
According to Glaser, [ 3 ] early centrifuges had rotor diameters of 7.4 to 15 cm and lengths of 0.3 to 3.2 m, and the peripheral speed was 350 to 500 m/s. The modern centrifuge TC-21 of Urenco has a diameter of 20 cm and a length of more than 5 m, spinning with 770 m/s. Centrus (formerly Usec) plans a centrifuge with 60 cm diameter, 12 m height and 900 m/s peripheral speed.
A countercurrent of the gas is stimulated either mechanically or (less preferred) by a temperature gradient between the top and bottom of the rotor. With a countercurrent-to-feed ratio of 4, Glaser [ 3 ] calculates a separation factor of 1,74 for a TC-21 centrifuge of 5 m height. Lowering this ratio (by increasing the feed) decreases the separation factor but increases the throughput and thus the productivity.
To reduce friction, the rotor spins in a vacuum . Part of the rotor with the near-by housing acts as a molecular pump, which maintains the vacuum. A magnetic bearing holds the top of the rotor steady, and the only physical contact (necessary only during start-up) is the conical jewel bearing on which the rotor sits. [ 1 ] [ 9 ] Both bearings contain measures for damping vibrations. The three gas lines enter the rotor on its axis.
After the scientists were released from Soviet captivity in 1956, [ 1 ] Gernot Zippe was surprised to find that engineers in the West were years behind in their centrifuge technology. He was able to reproduce his design at the University of Virginia in the United States , publishing the results, even though the Soviets had confiscated his notes. Zippe left the United States when he was effectively barred from continuing his research: The Americans classified the work as secret, requiring him either to become a U.S. citizen (he refused), return to Europe, or abandon his research. [ 1 ] He returned to Europe where, during the 1960s, he and his colleagues made the centrifuge more efficient by changing the material of the rotor from aluminium to maraging steel , an alloy with a longer fatigue life and longer breaking length, which allowed higher speed. This improved centrifuge design was long used by the commercial company Urenco to produce enriched uranium fuel for nuclear power stations . [ 1 ] More recently, they use (e.g. in their model TC-21) carbon fiber reinforced walls. [ 3 ]
The exact details of advanced Zippe-type centrifuges are closely guarded secrets. For example, the efficiency of the centrifuges is improved by increasing their speed of rotation. To do so, stronger materials, such as carbon fiber -reinforced composite materials , are used; but details of the material and its protection against chemical attacks are proprietary. Such are also the various techniques that are used to avoid forces causing destructive (bending) vibrations: Lengthening of a (countercurrent) centrifuge improves the enrichment exponentially. [ 9 ] But it also decreases the vibrational frequency of mechanical resonances, which increases the danger of catastrophic failure during start-up (as happened during the Stuxnet event in Iran). Interrupting the cylindrical rotor by flexible bellows controls the low-frequency vibrations, and careful speed control during start-up helps to ensure that the centrifuge does not operate too long at speeds where resonance is a problem. But more (proprietary) measures seem necessary. Therefore, Russia stayed with "subcritical" centrifuges (i.e., with small lengths around 0.5–1 m), whereas those of Urenco have lengths up to 10 m.
The Zippe-type centrifuge is difficult to build successfully and requires carefully machined parts. However, compared to other enrichment methods , it is much cheaper and is faster to set up, consumes much less energy and requires little area for the plant. Therefore, it can be built in relative secrecy. This makes it ideal for covert nuclear-weapons programs and increases the risk of nuclear proliferation . [ 3 ] Centrifuge cascades also have much less material held in the machine at any time than gaseous diffusion plants .
Pakistan's atomic bomb program developed the P1 and P2 centrifuges based on early designs of Urenco; [ 3 ] the first two centrifuges that Pakistan deployed in larger numbers but reduce it after 1981 based on estimation require for critical mass. The P1 centrifuge uses an aluminum rotor, and the P2 centrifuge uses a maraging steel rotor, [ 3 ] which is stronger, spins faster, and enriches more uranium per machine than the P1. In Pakistan, the Zippe-type centrifuge had a local designation and was known as Centrifuge Khan (after Abdul Qadeer Khan ). : 151 [ 10 ]
Russian sources dispute the account of Soviet centrifuge development given by Gernot Zippe. They cite Max Steenbeck as the German scientist in charge of the German part of the Soviet centrifuge effort, which was started by German refugee Fritz Lange [ ru ] in the 1930s. The Soviets credit Steenbeck, Isaac Kikoin and Evgeni Kamenev [ ru ] with originating different valuable aspects of the design. They state Zippe was engaged in building prototypes for the project for two years from 1953. Since the centrifuge project was top secret the Soviets did not challenge any of Zippe's claims at the time. [ 2 ]
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Zirconium silicon sulfide ( ZrSiS ) is a crystalline layered Dirac semi-metal compound of zirconium , silicon and sulfur . [ 1 ] Its crystals are made from planes of five single-atom layers of each element in the order S-Zr-Si-Zr-S, with the single element planes connected to their neighbors by van der Waals forces . [ 1 ] [ 2 ]
Semi-Dirac fermions were first observed within ZrSiS. [ 3 ] [ 4 ]
This physics -related article is a stub . You can help Wikipedia by expanding it .
This chemistry -related article is a stub . You can help Wikipedia by expanding it .
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Zirconocene is a hypothetical compound with 14 valence electrons , which has not been observed or isolated. It is an organometallic compound consisting of two cyclopentadienyl rings bound on a central zirconium atom. A crucial question in research is what kind of ligands can be used to stabilize the Cp 2 Zr II metallocene fragment to make it available for further reactions in organic synthesis. [ 1 ]
In contrast to sandwich compounds that have parallel cyclopentadienyl rings bound on opposite sides of the metal atom, such as ferrocene , zirconocene and other group 4 metallocenes are bent . Without stabilizing ligands, the Cp 2 Zr II fragment is unstable and dimerizes to form a fulvalene complex. [ 2 ]
In 1954, Wilkinson and Birmingham described zirconocene dihalides Cp 2 ZrX 2 with X=Cl or Br, as some of the earliest examples of organozirconium compounds. [ 2 ] The chemistry of Cp 2 Zr II -compounds was explored more extensively in the 1980s by Negishi , Takahashi, Buchwald , and others. [ 3 ] In the 1990s, Rosenthal synthesized zirconocene reagents using bis(trimethylsilyl)acetylene as stabilizing ligand. This novel zirconocene source offers a number of compelling advantages over previously used reagents and broadens the range of possible reactions. [ 1 ] The chemistry of Cp 2 Zr II -compounds is still a rapid growing area with zirconium being ranked among the most widely used transition metals in organic synthesis. [ 3 ]
The unstable 14-electron Cp 2 Zr II -compound is generally non-existent, but can be generated using ligands that stabilize the metallocene fragment. Optimally, these ligands can be quantitatively released under mild conditions. [ 1 ]
One option is the usage of π-acceptor ligands like carbon monoxide . Furthermore, a reaction with trimethylphosphine yields Cp 2 Zr II -complex as illustrated below. [ 2 ]
In the synthesis of the Negishi reagent , treatment of zirconocene dichloride in tetrahydrofuran with two equivalents of n -butyllithium at −78 °C gives (1-butene)zirconocene, which is represented by the resonance structures A and B . [ 4 ]
If bis(trimethylsilyl)acetylene is used instead of n -butyllithium, a higher yield is can be obtained. In this case, zirconocene complexes are synthesized to Rosenthal's reagent , represented by the resonance structures A and B . This reagent is stable at room temperature, can be stored under an inert atmosphere and allows a more precise control over the stoichiometry of reactions as it can be formed quantitatively. [ 5 ] A fine tune of the general reaction shown below is feasible by using different substituted cyclopentadienyl ligands as well as additional ligands (e. g. THF , pyridine ). Instead of zirconium used as central atom, an analogous reaction with titanium is possible, too. [ 6 ]
The highly reactive Cp 2 Zr II compound possesses one lone electron pair and two vacant valence orbitals . Therefore, it can be compared to carbenes in terms of its reactivity. [ 1 ] Typical reactions of in situ generated zirconocenes are coupling or insertion to form metallacycles . These reactions have been observed upon addition of carbon monoxide , ketones , nitriles , alkynes and other substances and led to five-, seven- and nine-membered metallacycles. [ 7 ]
Zirconocene coupling and insertion are used extensively to generate functionalized organic compounds. Taking Rosenthal's reagent, high yields of predictable macrocyclic products can be obtained. These macrocycles are applicated in numerous ways, such as host–guest chemistry , chemical sensing , catalysis , and materials science . [ 8 ] Moreover, with zirconocene complexes, the synthesis of so far unknown heterometallacycles and synthetically challenging organic structures can be realized by novel C-C coupling of nitriles. [ 9 ]
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Zircotec is a high temperature coating and heat barrier manufacturer, based in Abingdon near Oxford , England . It uses plasma-sprayed ceramic materials to provide thermal and abrasive resistance to components – with a focus on automotive exhaust systems . Its best-known products include coloured thermal barrier coatings and Zircoflex – a flexible ceramic heatshield .
Zircotec began life as part of the United Kingdom Atomic Energy Authority , where its high temperature coatings and heat barrier processes were originally developed for the nuclear industry . It was based at the Atomic Energy Research Establishment near Harwell , Oxfordshire . At the time, this was the main centre for atomic energy research and development in the United Kingdom . [ 1 ]
In 1994, Zircotec's thermal barrier coatings were first used in a motorsport application. These coatings were applied to the exhaust systems of Subaru rally cars to lower in-cabin temperatures. After initial success, these high temperature coatings were then used on a variety of other vehicles, including Formula One cars and trucks.
Zircotec was bought by a venture capital fund in 2003. Subsequently, Zircotec shifted its focus from the nuclear industry, towards general industry and automotive applications. As a result, Zircotec developed Thermohold coatings for high performance automotive and classic car applications. Their main Thermohold coating was called Performance White, a white dual-layer plasma-sprayed ceramic.
In July 2007, Terry Graham was appointed as the new managing director. [ 2 ] Later that year, high performance sportscar manufacturer Koenigsegg announced it would be using Zircotec coatings on its CCX supercar. [ 3 ] Additionally, in October, Zircotec became involved in the world land speed steam-car record attempt, supplying its coatings for thermal protection of sensitive components – this attempt was successful a year later. [ 4 ] [ 5 ]
In June 2008, Zircotec developed a plasma-sprayed ceramic coating specifically for composite materials . Predominantly aimed at motorsport and high-performance car applications, coated composites could now function at temperatures above their melting points. [ 6 ]
In September 2008, Zircotec launched its Performance Colours range. This range is based on Zircotec's Performance White coating, but has an additional coloured finish, offering a more robust and maintainable finish. Zircotec initially released thirteen different colours, with plans to increase this variety over the next few years.
Zircotec's directors completed a management buyout early in 2009. [ 7 ]
In 2010, the company's headquarters were relocated to the nearby town of Abingdon , Oxfordshire . The move was completed to provide increased production capacity and to accommodate future business growth. [ 8 ]
In 2011, Zircotec developed the world's first flexible ceramic heatshield , named Zircoflex. [ citation needed ]
In July 2019, Zircotec experienced a fire at its premises and temporarily relocated to a nearby facility. [ 9 ]
Zircotec announced an investment of £2.5m into a new 20,000 square foot premises in Abingdon which was completed by October 2019. This offers a ten-fold increase in capacity with a significant increase in workforce [ 10 ]
In March 2021, the release of a new heatshield product – ZircoFlex SHIELD – was announced. [ 11 ]
In June 2022, Zircotec procured a further 10,000 square foot building which will be dedicated to producing technology for low and zero carbon vehicles. [ 12 ]
In 2024, Zircotec began leading the CeraBEV (Ceramics for Battery Electric Vehicles) project, a £1 million Advanced Propulsion Centre -funded collaboration with Cranfield University . The project aims to develop a single-layer ceramic coating that provides both dielectric insulation and flame resistance, enabling the use of lightweight materials such as aluminium and polymer composites in EV battery enclosures and cooling plates. [ 13 ]
Zircotec develops thermal coatings and heat shielding materials for use in high-temperature environments. Its main products include:
Zircotec works closely with motorsport teams from various categories, including Formula One , the British Touring Car Championship and NASCAR .
Since 2018, Zircotec has maintained a technical partnership with Power Maxed Racing . [ 18 ]
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The Zisman plot the graphical method of the Zisman theory or the Zisman method for characterizing the wettability of a solid surface [1] , named for the American chemist and geophysicist, William Albert Zisman (1905–1986). It is a prominent Sessile drop technique used for characterizing liquid-surface interactions based on the contact angle of a single drop of liquid sitting on the solid surface.
In 1964, William Zisman published an article in the ACS publications on the "Relation of the Equilibrium Contact Angle to Liquid and Solid Constitution". [ 1 ] It was in this article where he used what we call today as the Zisman plot. The Zisman plot is used to very quickly give a quantitative measurement of wettability, also known as the critical surface tension, γ C , of a solid surface by measuring the liquid contact angle as shown in Figure 1. Taking the cosine of said angle and then graphing it against the surface tension of the liquid wetting the solid substrate yields the critical surface tension. Wettability is a measure of how well a liquid spreads and how complete the contact of the liquid is across the surface of a solid interface. A small contact angle indicates good wettability, while a large contact angle indicates poor wettability. The critical surface tension is the highest liquid surface tension that can completely wet a specific solid surface. For adhesive bonding complete wetting is used to maximize the adhesive joint strength.
Even though this relationship is empirical and less precise than the surface tension of a homologous series of liquids, it is very useful considering it is a parameter of the solid surface. This method is especially used to compare and measure the critical surface tension of low-energy solids (mainly plastics) very quickly and easily. Figure 4 in ZIsman's published article from 1964 [ 1 ] shows the critical surface tension as a measure of wettability of Polyethylene. Zisman published this analysis in 1964 and used a variety of nonhomologous liquids to measure the critical surface tension of Polyethylene to be around 35 dynes per centimeter as shown by the intercept at x=1 in Figure 4. Figure 12 in Zisman's 1964 article [ 1 ] shows that different solids can also be plotted on the same graph to easily compare the critical solid surface tensions of a variety of plastic substrates including very different polymers such as teflon, acid monolayers, and esters. The ZIsman Plot proved to be a breakthrough which allowed for a very efficient way to measure wettability of a solid which helped to spawn the work of Dann in the late 1960s. [ 2 ] Dann characterized the critical surface tensions of a variety of polymeric materials using the Zisman Plot. In modern days, David and Neumann in an investigation of contact angle on low-energy surfaces. [ 3 ] However, today some different variations of the Zisman plot exist because the dependent variable is unitless being since it is cosine of the contact angle for the liquid.
For adhesive bonding of materials, wetting of the surface, which can be measured by the contact angle, is critical to successful adhesive application. To determine how well a liquid wets a solid surface is proportional to the contact angle from the liquid while on the solid. This is determined by the respective surface tensions of the solid and liquid. William Zisman's contribution to adhesives in the way of his Zisman Plot, which has a variation today that graphs 1-cos(θ SL ) vs γ L . In this variation the X-intercept gives the critical surface tension of the liquid needed to effectively wet the solid surface. There are two steps when graphing the data which are to neglect all the points around zero on the y-axis to initially plot the line of best fit to find γ c ; however, when graphing the line initially if a point near 0 lands to the right of the intersection redo the regression including that point to make the measurement of the critical surface tension, γ c , more accurate. A table of variables and an example can be seen below.
In this example we will use the five liquids in the Table 2 (Liquid Data) to find the critical wetting surface tension needed to effectively wet PC (polycarbonate) using the Zisman Plot.
The data of the liquids given from the table above is then graphed on the Zisman Plot (Figure 2) with the independent variable as the surface tension of the liquid in dynes/cm and the dependent variable as 1-cos(θ SL ). There also are different variations of the Zisman plot since the Y-axis is unitless as seen in Table 1 and as mentioned above.
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Liquids 1 and 2 fully wet the surface as shown by their low contact angles, so they should be neglected when first drawing the line of best fit to find the critical liquid surface tension needed to effectively wet the PC surface, γ C , which is simply the x-intercept of the best fit line for the Zisman Plot. To find the best fit line a least squares regression is recommended by using a computer program such as Microsoft Excel , Minitab , Matlab , or it can also be done using a modern graphing calculator such as a TI-84. This was done with the data from Table 1 and the fit data for liquids 3,4, and 5 can be seen on Figure 3.
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The x-intercept lands at 39.5 dynes per centimeter (This can be calculated by setting y equal to zero and solving for x) which is less than that of liquid 2, 42.9 dynes per centimeter; therefore, a more accurate measurement of the critical liquid surface tension needed to effectively wet the surface of PC can be obtained by including liquid 2 when making the line of best fit, as seen in Figure 4.
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The x-intercept here lands at 42.1 dynes per centimeter (This can be calculated by setting y equal to zero and solving for x), indicating the critical liquid surface tension for PC.
William Zisman's contribution of what is called today as the Zisman Plot revolutionized the world of adhesive bonding and surface chemistry by giving a fast, effective, and quantitative way to measure the wettability or critical surface tension of a solid. This spawned the work of many others over the past few decades. This spans from Dann's work in the late 1960s to David and Neumann's work in 2014. The Zisman Plot is still used today, and it has many variations since the y-axis is unitless and can be found more easily and accurately using modern regression software packages.
William Zisman
Contact angle
Adhesion
Wettability
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Živojin Jocić (1870–1914) was a Serbian organic chemist . The eponymous Jocic reaction (also called the Jocic–Reeve reaction) is a name reaction that involves nucleophilic displacement of the hydroxyl group in a 1,1,1-trichloro-2-hydroxyalkyl structure with concomitant conversion of the trichloromethyl portion to a carboxylic acid or similar functional group .
At the turn of the century, Živojin Jocić worked as an assistant at the University of Petrograd in Imperial Russia . In a relatively short time – between 1897 and 1911 – he published a large number of papers in organic chemistry , for the most part dealing with the synthesis of acetylene hydrocarbons and synthesis by means of Grignard reagent .
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Zlatko Boško Tešanović (August 1, 1956 – July 26, 2012) was a Yugoslav-American theoretical condensed-matter physicist , whose work focused mainly on the high-temperature superconductors (HTS) and related materials.
His particular research interests were in the areas of theoretical condensed matter physics, revolving primarily around iron- and copper-based high-temperature superconductors, quantum Hall effects (QHE), superconductivity and strongly correlated electron materials . His broad knowledge of condensed matter physics, his deep understanding of the effects of strong magnetic fields , and his talent for exposition were influential. [ 2 ] [ 3 ] [ 4 ]
He was born in Sarajevo , former Yugoslavia (present Bosnia and Herzegovina ). In 1979, he received a B.Sci. in physics from the University of Sarajevo . He then received a Fulbright Fellowship and attended the University of Minnesota, where he earned a Ph.D. in physics in 1985. He became a naturalized American citizen .
He worked as a professor of physics at Johns Hopkins University (JHU) in the Henry A. Rowland Department of Physics and Astronomy in Baltimore from July 1987 until his death on July 26, 2012. Previously, he served as director of the TIPAC Theory Center at JHU. [ 4 ]
He was a foreign member of the Royal Norwegian Society of Sciences and Letters and a fellow of the APS Division of Condensed Matter Physics (DCMP). He served as a member of the committee to Assess the Current Status and Future Direction of High Magnetic Field Science in the United States , and contributed strongly to it, until his death. [ 5 ] [ 2 ]
Among his graduate students are: [ 6 ]
He gave more than 100 invited talks at scientific meetings, including major international conferences. He has authored and published more than 125 scientific papers, and a book entitled: [ 2 ] [ 4 ]
He received grants from the Department of Energy , and the National Science Foundation awarded him a post-doctoral fellowship that enabled him to spend two years studying at Harvard University . [ 4 ]
He died on July 26, 2012, at the age of 55 of an "apparent" heart attack at the George Washington University Hospital in Washington, D.C., after collapsing at Reagan National Airport . [ 4 ]
On March 23, 2013, the Johns Hopkins University Department of Physics and Astronomy organised a memorial symposium as a tribute to him. A number of distinguished speakers have been invited to highlight Zlatko's scientific accomplishments. [ 7 ]
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Zinc acetate is a salt with the formula Zn(CH 3 CO 2 ) 2 , which commonly occurs as the dihydrate Zn(CH 3 CO 2 ) 2 ·2H 2 O. Both the hydrate and the anhydrous forms are colorless solids that are used as dietary supplements. When used as a food additive , it has the E number E650.
Zinc acetate is a component of some medicines, e.g., lozenges for treating the common cold . [ 1 ] Zinc acetate can also be used as a dietary supplement. [ 2 ] As an oral daily supplement it is used to inhibit the body's absorption of copper as part of the treatment for Wilson's disease . [ 3 ] Zinc acetate is also sold as an astringent in the form of an ointment, a topical lotion, or combined with an antibiotic such as erythromycin for the topical treatment of acne. [ 4 ] It is commonly sold as a topical anti-itch ointment.
Zinc acetate is used as the catalyst for the industrial production of vinyl acetate from acetylene : CH 3 CO 2 H + C 2 H 2 → CH 3 CO 2 CH=CH 2 . Approximately 1/3 of the worlds production uses this route, which because of its environmental impact, is mainly practiced in countries with relaxed environmental regulations such as China. [ 5 ]
Zinc acetates are prepared by the action of acetic acid on zinc carbonate or zinc metal. Treatment of zinc nitrate with acetic anhydride is an alternative route. [ 6 ]
In anhydrous zinc acetate the zinc is coordinated to four oxygen atoms to give a tetrahedral environment, these tetrahedral polyhedra are then interconnected by acetate ligands to give a range of polymeric structures. [ 7 ] [ 8 ] [ 9 ]
In the dihydrate, zinc is octahedral, wherein both acetate groups are bidentate. [ 10 ] [ 11 ]
Heating Zn(CH 3 CO 2 ) 2 in a vacuum results in a loss of acetic anhydride , leaving a residue of "basic zinc acetate," with the formula Zn 4 O(CH 3 CO 2 ) 6 . It can also be prepared by a reaction of glacial acetic acid with zinc oxide . [ 12 ] The cluster compound has a tetrahedral structure with an oxide ligand at its center [ 13 ] Basic zinc acetate is a common precursor to metal-organic frameworks (MOFs).
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Zinc chlorate ( Zn ( Cl O 3 ) 2 ) is an inorganic chemical compound .
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Zinc azide Zn(N 3 ) 2 is an inorganic compound composed of zinc cations ( Zn 2+ ) and azide anions ( N − 3 ). It is a white, explosive solid that can be prepared by the protonolysis of diethylzinc with hydrazoic acid : [ 1 ]
Zinc azide is a coordination polymer which crystallizes in three polymorphs , all of which feature tetrahedral zinc centers and bridging azide ligands. α- Zn(N 3 ) 2 crystallizes in the monoclinic space group and is stable, while the other two polymorphs are metastable . P 2 1 / n . β- Zn(N 3 ) 2 is trigonal, space group P 3 2 21, and γ- Zn(N 3 ) 2 is monoclinic, space group C 2.
It is easily hydrolyzed, and attempts to prepare it in aqueous solution resulted in the precipitation of basic azides Zn(OH) 2− x (N 3 ) x ( x = 0.9–1.0). Both the α- and β-forms were found to be very friction- and shock-sensitive, violently exploding in blue flashes, but can be made to decompose slowly by gentle heating, giving off nitrogen gas. In a sealed glass tube with inert atmosphere, this yields zinc nitride , Zn 3 N 2 . [ 1 ]
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Zinc nitrate is an inorganic chemical compound with the formula Zn(NO 3 ) 2 . This colorless, crystalline salt is highly deliquescent . It is typically encountered as a hexahydrate Zn(NO 3 ) 2 ·6H 2 O . It is soluble in both water and alcohol.
Zinc nitrate is usually prepared by dissolving zinc metal, zinc oxide , or related materials in nitric acid :
These reactions are accompanied by the hydration of the zinc nitrate.
The anhydrous salt arises by the reaction of anhydrous zinc chloride with nitrogen dioxide : [ 1 ]
Treatment of zinc nitrate with acetic anhydride gives zinc acetate. [ 2 ]
On heating, zinc nitrate undergoes thermal decomposition to form zinc oxide , nitrogen dioxide and oxygen :
Aqueous zinc nitrate contains aquo complexes [Zn(H 2 O) 6 ] 2+ and [Zn(H 2 O) 4 ] 2+ . [ 3 ] and, thus, this reaction may be better written as the reaction of the aquated ion with hydroxide through donation of a proton, as follows.
Zinc nitrate has no large scale application but is used on a laboratory scale for the synthesis of coordination polymers . [ 4 ] Its controlled decomposition to zinc oxide has also been used for the generation of various ZnO based structures, including nanowires. [ 5 ]
It is used as a corrosion inhibitor. [ 6 ]
It can be used as a mordant in dyeing . An example reaction gives a precipitate of zinc carbonate :
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Zinc acetate is a salt with the formula Zn(CH 3 CO 2 ) 2 , which commonly occurs as the dihydrate Zn(CH 3 CO 2 ) 2 ·2H 2 O. Both the hydrate and the anhydrous forms are colorless solids that are used as dietary supplements. When used as a food additive , it has the E number E650.
Zinc acetate is a component of some medicines, e.g., lozenges for treating the common cold . [ 1 ] Zinc acetate can also be used as a dietary supplement. [ 2 ] As an oral daily supplement it is used to inhibit the body's absorption of copper as part of the treatment for Wilson's disease . [ 3 ] Zinc acetate is also sold as an astringent in the form of an ointment, a topical lotion, or combined with an antibiotic such as erythromycin for the topical treatment of acne. [ 4 ] It is commonly sold as a topical anti-itch ointment.
Zinc acetate is used as the catalyst for the industrial production of vinyl acetate from acetylene : CH 3 CO 2 H + C 2 H 2 → CH 3 CO 2 CH=CH 2 . Approximately 1/3 of the worlds production uses this route, which because of its environmental impact, is mainly practiced in countries with relaxed environmental regulations such as China. [ 5 ]
Zinc acetates are prepared by the action of acetic acid on zinc carbonate or zinc metal. Treatment of zinc nitrate with acetic anhydride is an alternative route. [ 6 ]
In anhydrous zinc acetate the zinc is coordinated to four oxygen atoms to give a tetrahedral environment, these tetrahedral polyhedra are then interconnected by acetate ligands to give a range of polymeric structures. [ 7 ] [ 8 ] [ 9 ]
In the dihydrate, zinc is octahedral, wherein both acetate groups are bidentate. [ 10 ] [ 11 ]
Heating Zn(CH 3 CO 2 ) 2 in a vacuum results in a loss of acetic anhydride , leaving a residue of "basic zinc acetate," with the formula Zn 4 O(CH 3 CO 2 ) 6 . It can also be prepared by a reaction of glacial acetic acid with zinc oxide . [ 12 ] The cluster compound has a tetrahedral structure with an oxide ligand at its center [ 13 ] Basic zinc acetate is a common precursor to metal-organic frameworks (MOFs).
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Brianyoungite is a secondary zinc carbonate mineral . The Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) classifies it as a carbonate with the formula Zn 3 (CO 3 )(OH) 4 , [ 1 ] but sulfate groups SO 4 also occupy the carbonate CO 3 positions, in the ratio of about one sulfate to three carbonates, [ 3 ] so other sources give the formula as Zn 3 (CO 3 ,SO 4 )(OH) 4 , and Gaines et al. classify the mineral as a compound carbonate. [ 7 ] It is
similar in appearance to hydrozincite , another zinc carbonate. [ 5 ] It was discovered in 1991 and designated IMA1991-053. [ 5 ] In 1993 it was named "brianyoungite" after Brian Young (born 1947), a field geologist with the British Geological Survey , who provided the first specimens. [ 4 ] [ 7 ]
The mineral occurs as tiny rosettes less than 100 μm across, composed of thin blades just one or two micrometers across, elongated parallel to the b crystal axis , and tapering to a sharp point. [ 3 ] The crystals are white and transparent to translucent, with a vitreous lustre and a white streak .
The mineral belongs in the orthorhombic crystal system , or the monoclinic with β (the angle between the a and c crystal axes) close to 90 o . [ 3 ] The space group is unknown, but assumed to be either P2 1 /m, P21 or P2221. [ 4 ] [ 5 ] The structure is similar to that of hydrozincite . [ 7 ] There are four formula units per unit cell (Z = 4) and the lengths of the sides of the unit cell are a = 15.724 Å , b = 6.256 Å and c = 5.427 Å. [ 3 ]
Brianyoungite is a soft mineral with Mohs hardness similar to halite , only 2 to 2 + 1 ⁄ 2 according to some sources, [ 6 ] [ 5 ] but others say that the hardness is not determinable. [ 3 ] [ 4 ] It is fairly dense, with specific gravity 3.93 to 4.09, similar to that of celestine . Cleavage is perfect perpendicular to the a crystal axis (perfect on {100}) and possible perpendicular to the c crystal axis (possible on {001}). [ 3 ] [ 4 ] It is readily soluble with effervescence in acids. [ 3 ]
The mineral is biaxial , with refractive indices n ω = 1.635 and n ε = 1.650 and maximum birefringence δ = 1.635. [ 5 ] It exhibits straight extinction . [ 3 ] It is not fluorescent. [ 3 ]
The type locality is the Bloomsberry Horse level of the Brownley Hill mine, Nenthead , Alston Moor District, North Pennines, North and Western Region (Cumberland), Cumbria, England. [ 5 ] The type material is conserved at the Royal Museum of Scotland , Edinburgh, Scotland, 1992.17.1–8. [ 4 ] Brianyoungite occurs with gypsum on rubbly limestone in the oxidised zone of Brownley Hill Mine, and on specimens from the nearby Smallcleugh mine. [ 3 ] It may be a secondary post-mining mineral. [ 6 ] [ 4 ] At the type locality it is associated with gypsum , smithsonite , pyrite and goethite . [ 4 ]
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Brianyoungite is a secondary zinc carbonate mineral . The Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) classifies it as a carbonate with the formula Zn 3 (CO 3 )(OH) 4 , [ 1 ] but sulfate groups SO 4 also occupy the carbonate CO 3 positions, in the ratio of about one sulfate to three carbonates, [ 3 ] so other sources give the formula as Zn 3 (CO 3 ,SO 4 )(OH) 4 , and Gaines et al. classify the mineral as a compound carbonate. [ 7 ] It is
similar in appearance to hydrozincite , another zinc carbonate. [ 5 ] It was discovered in 1991 and designated IMA1991-053. [ 5 ] In 1993 it was named "brianyoungite" after Brian Young (born 1947), a field geologist with the British Geological Survey , who provided the first specimens. [ 4 ] [ 7 ]
The mineral occurs as tiny rosettes less than 100 μm across, composed of thin blades just one or two micrometers across, elongated parallel to the b crystal axis , and tapering to a sharp point. [ 3 ] The crystals are white and transparent to translucent, with a vitreous lustre and a white streak .
The mineral belongs in the orthorhombic crystal system , or the monoclinic with β (the angle between the a and c crystal axes) close to 90 o . [ 3 ] The space group is unknown, but assumed to be either P2 1 /m, P21 or P2221. [ 4 ] [ 5 ] The structure is similar to that of hydrozincite . [ 7 ] There are four formula units per unit cell (Z = 4) and the lengths of the sides of the unit cell are a = 15.724 Å , b = 6.256 Å and c = 5.427 Å. [ 3 ]
Brianyoungite is a soft mineral with Mohs hardness similar to halite , only 2 to 2 + 1 ⁄ 2 according to some sources, [ 6 ] [ 5 ] but others say that the hardness is not determinable. [ 3 ] [ 4 ] It is fairly dense, with specific gravity 3.93 to 4.09, similar to that of celestine . Cleavage is perfect perpendicular to the a crystal axis (perfect on {100}) and possible perpendicular to the c crystal axis (possible on {001}). [ 3 ] [ 4 ] It is readily soluble with effervescence in acids. [ 3 ]
The mineral is biaxial , with refractive indices n ω = 1.635 and n ε = 1.650 and maximum birefringence δ = 1.635. [ 5 ] It exhibits straight extinction . [ 3 ] It is not fluorescent. [ 3 ]
The type locality is the Bloomsberry Horse level of the Brownley Hill mine, Nenthead , Alston Moor District, North Pennines, North and Western Region (Cumberland), Cumbria, England. [ 5 ] The type material is conserved at the Royal Museum of Scotland , Edinburgh, Scotland, 1992.17.1–8. [ 4 ] Brianyoungite occurs with gypsum on rubbly limestone in the oxidised zone of Brownley Hill Mine, and on specimens from the nearby Smallcleugh mine. [ 3 ] It may be a secondary post-mining mineral. [ 6 ] [ 4 ] At the type locality it is associated with gypsum , smithsonite , pyrite and goethite . [ 4 ]
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Zinc phosphate is an inorganic compound with the formula Zn 3 ( PO 4 ) 2 . This white powder is widely used as a corrosion resistant coating on metal surfaces either as part of an electroplating process or applied as a primer pigment (see also red lead ). It has largely displaced toxic materials based on lead or chromium, and by 2006 it had become the most commonly used corrosion inhibitor. [ 1 ] [ 2 ] Zinc phosphate coats better on a crystalline structure than bare metal, so a seeding agent is often used as a pre-treatment. One common agent is sodium pyrophosphate . [ 3 ]
Natural forms of zinc phosphate include minerals hopeite and parahopeite . A somewhat similar mineral is natural hydrous zinc phosphate called tarbuttite , Zn 2 (PO 4 )(OH). Both are known from oxidation zones of Zn ore beds and were formed through oxidation of sphalerite by the presence of phosphate-rich solutions. The anhydrous form has not yet been found naturally.
Zinc phosphate cement is the classic dental cement par excellence. It is commonly used for luting permanent metal and zirconium dioxide [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ] [ 9 ] restorations and as a base for dental restorations . Zinc phosphate cement is used for cementation of inlays , crowns , bridges , and orthodontic appliances and occasionally as a temporary restoration .
It is prepared by mixing zinc oxide (ZnO) and magnesium oxide (MgO) powders with a liquid consisting principally of phosphoric acid , water, and buffers . It is the standard cement to measure against. It has the longest track record of use in dentistry.
In recent years, newer adhesive cements on a different chemical basis have been added (e.g. glass ionomer cement ), but they have not displaced the classic phosphate cement, which continues to hold its own in the dental market with its simple and safe processing and good price-performance ratio. Zinc phosphate cement has only a low flexural strength and it does not stick to the dentin (it is a cement and not an adhesive).
Zinc phosphate cement has high compressive strength, low film thickness, minimal setting shrinkage and thermal expansion and is biocompatible. Compared to other luting materials such as glass ionomer cement or composites, zinc phosphate cement is less sensitive to moisture. The excess produced during the cementation of dental restorations can be easily removed.
Zinc phosphate cement has a high adhesive capacity to the tooth, metal, or even zirconium oxide.
Despite its strong acidity, zinc phosphate cement does not damage the pulp (or the tooth nerve) during the setting phase. It is therefore used as liner to protect the pulp under composite fillings.
Well-known dental brands in Germany and the world for zinc phosphate cement are Harvard cement and Hoffmann's cement. Otto Hoffmann invented this cement in 1892 and had it patented. Until the beginning of the First World War, he had a worldwide monopoly position with his cement.
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it .
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Zinc nitride ( Zn 3 N 2 ) is an inorganic compound of zinc and nitrogen , usually obtained as (blue)grey crystals. It is a semiconductor. In pure form, it has the anti- bixbyite structure.
Zinc nitride can be obtained by thermally decomposing zincamide (zinc diamine) [ 3 ] in an anaerobic environment, at temperatures in excess of 200 °C . The by-product of the reaction is ammonia . [ 4 ]
3 Zn( NH 2 ) 2 → Zn 3 N 2 + 4 NH 3
It can also be formed by heating zinc to 600 °C in a current of ammonia; the by-product is hydrogen gas . [ 3 ] [ 5 ]
3 Zn + 2 NH 3 → Zn 3 N 2 + 3 H 2
The decomposition of Zinc Nitride into the elements at the same temperature is a competing reaction. [ 6 ] At 700 °C Zinc Nitride decomposes. [ 1 ] It has also been made by producing an electric discharge between zinc electrodes in a nitrogen atmosphere. [ 6 ] [ 7 ] Thin films have been produced by chemical vapour deposition of Bis(bis(trimethylsilyl)amido]zinc with ammonia gas onto silica or ZnO coated alumina at 275 to 410 °C. [ 8 ]
The crystal structure is anti- isomorphous with Manganese(III) oxide . ( bixbyite ). [ 2 ] [ 7 ] The heat of formation is c. 24 kilocalories (100 kJ) per mol. [ 7 ] It is a semiconductor with a reported bandgap of c. 3.2eV, [ 9 ] however, a thin zinc nitride film prepared by electrolysis of molten salt mixture containing Li 3 N with a zinc electrode showed a band-gap of 1.01 eV. [ 10 ]
Zinc nitride reacts violently with water to form ammonia and zinc oxide . [ 3 ] [ 4 ]
Zn 3 N 2 + 3 H 2 O → 3 ZnO + 2 NH 3
Zinc nitride reacts with lithium (produced in an electrochemical cell) by insertion. The initial reaction is the irreversible conversion into LiZn in a matrix of beta - Li 3 N . These products then can be converted reversibly and electrochemically into LiZnN and metallic Zn. [ 11 ] [ 12 ]
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Zinc phosphide ( Zn 3 P 2 ) is an inorganic chemical compound . It is a grey solid, although commercial samples are often dark or even black. It is used as a rodenticide . [ 5 ] Zn 3 P 2 is a II-V semiconductor with a direct band gap of 1.5 eV [ 6 ] and may have applications in photovoltaic cells . [ 7 ] A second compound exists in the zinc-phosphorus system, zinc diphosphide (ZnP 2 ) .
Zinc phosphide can be prepared by the reaction of zinc with phosphorus ; however, for critical applications, additional processing to remove arsenic compounds may be needed. [ 8 ]
Another method of preparation include reacting tri-n-octylphosphine with dimethylzinc . [ 9 ]
Zinc phosphide reacts with water to produce highly toxic phosphine (PH 3 ) and zinc hydroxide (Zn(OH) 2 ):
Zn 3 P 2 has a room-temperature tetragonal form that converts to a cubic form at around 845 °C. [ 10 ] In the room-temperature form there are discrete P atoms, zinc atoms are tetrahedrally coordinated and phosphorus six coordinate, with zinc atoms at 6 of the vertices of a distorted cube. [ 11 ]
The crystalline structure of zinc phosphide is very similar to that of cadmium arsenide (Cd 3 As 2 ), zinc arsenide (Zn 3 As 2 ) and cadmium phosphide (Cd 3 P 2 ). These compounds of the Zn-Cd-P-As quaternary system exhibit full continuous solid-solution. [ 12 ]
Zinc phosphide is an ideal candidate for thin film photovoltaic applications, for it has strong optical absorption and an almost ideal band gap (1.5eV). In addition to this, both zinc and phosphorus are found abundantly in the Earth's crust, meaning that material extraction cost is low compared with that of other thin film photovoltaics . Both zinc and phosphorus are also nontoxic, which is not the case for other common commercial thin film photovoltaics, like cadmium telluride . [ 13 ]
Researchers at the University of Alberta were the first to successfully synthesize colloidal zinc phosphide. Before this, researchers were able to create efficient solar cells from bulk zinc phosphide, but their fabrication required temperatures greater than 850 °C or complicated vacuum deposition methods. By contrast, colloidal zinc phosphide nanoparticles , contained in a zinc phosphide “ink”, allows for inexpensive, easy large-scale production, by means of slot-die coating or spray coating. [ 14 ]
The testing and development of these zinc phosphide thin films is still in its early stages, but early results have been positive. Prototype heterojunction devices fabricated from zinc phosphide nanoparticle ink exhibited a rectification ratio of 600 and photosensitivity with an on/off ratio near 100. These are both acceptable suitability benchmarks for solar cells. Development still needs to be made on optimizing the nanoparticle ink formation and device architecture before commercialization is possible, but commercial spray-on zinc phosphide solar cells may be possible within ten years. [ 15 ]
Metal phosphides have been used as rodenticides . A mixture of food and zinc phosphide is left where the rodents can eat it. The acid in the digestive system of the rodent reacts with the phosphide to generate toxic phosphine gas. This method of vermin control has possible use in places where rodents are immune to other common poisons. Other pesticides similar to zinc phosphide are aluminium phosphide and calcium phosphide .
Zinc phosphide is typically added to rodent baits in amount of around 0.75-2%. Such baits have a strong, pungent garlic -like odor characteristic of phosphine liberated by hydrolysis . The odor attracts rodents, but has a repulsive effect on other animals; However, birds, notably wild turkeys , are not sensitive to the smell. The baits have to contain sufficient amount of zinc phosphide in sufficiently attractive food in order to kill rodents in a single serving; a sublethal dose may cause aversion towards zinc phosphide baits encountered by surviving rodents in the future.
Rodenticide-grade zinc phosphide usually comes as a black powder containing 75% of zinc phosphide and 25% of antimony potassium tartrate , an emetic to cause vomiting if the material is accidentally ingested by humans or domestic animals. However, it is still effective against rats, mice, guinea pigs and rabbits, none of which have a vomiting reflex. [ 17 ]
The New Zealand Environmental Protection Authority has approved the import and manufacture of Microencapsulated Zinc Phosphide (MZP Paste) for the ground control of possums . The application was made by Pest Tech Limited, with support from Connovation Ltd, Lincoln University and the Animal Health Board . It will be used as an additional vertebrate poison in certain situations. Unlike 1080 poison , it cannot be used for aerial application. [ 18 ]
Zinc phosphide is highly toxic, especially when ingested or inhaled. The reason for its toxicity is the release of phosphorus compounds, usually phosphine , when it reacts with water and acids. Phosphine is very toxic and, with trace amounts of P 2 H 4 , pyrophoric . Phosphine is also denser than air and may remain close to the ground without sufficient ventilation .
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Zinc bromide ( Zn Br 2 ) is an inorganic compound with the chemical formula Zn Br 2 . It is a colourless salt that shares many properties with zinc chloride (ZnCl 2 ), namely a high solubility in water forming acidic solutions, and good solubility in organic solvents. It is hygroscopic and forms a dihydrate ZnBr 2 ·2H 2 O. [ 2 ]
ZnBr 2 · 2H 2 O is prepared by treating zinc oxide or zinc metal with hydrobromic acid . [ 1 ]
The anhydrous material can be produced by dehydration of the dihydrate with hot CO 2 or by reaction of zinc metal and bromine. [ 2 ] Sublimation in a stream of hydrogen bromide also gives the anhydrous derivative. [ 1 ]
ZnBr 2 crystallizes in the same structure as ZnI 2 : four tetrahedral Zn centers share three vertices to form “super-tetrahedra” of nominal composition {Zn 4 Br 10 } 2− , which are linked by their vertices to form a three-dimensional structure. [ 3 ] The dihydrate ZnBr 2 · 2H 2 O can be described as ([Zn(H 2 O) 6 ] 2+ ) 2 ([Zn 2 Br 6 ] 2- ). [ 4 ]
Gaseous ZnBr 2 is linear in accordance with VSEPR theory with a Zn-Br bond length of 221 pm. [ 5 ]
Zinc bromide is mainly used in servicing oil and natural gas wells, solutions of zinc bromide are used to displace drilling mud when transitioning from the drilling phase to the completion phase or in well workover operations. The extremely dense brine solution gives the fluid its weight of 20 pounds/gallon, which makes it especially useful in holding back flammable oil and gas particles in high pressure wells. However, the high acidity and osmolarity cause corrosion and handling problems. Crews must be issued slicker suits and rubber boots because the fluid is so dehydrating. [ 6 ] [ 2 ]
It is the electrolyte in the zinc bromide battery .
Zinc bromide solutions can be used as a transparent shield against radiation . The space between two glass panes is filled with a strong aqueous solution of zinc bromide with a very high density , to be used as a window on a hot cell . This type of window has the advantage over lead glass in that it will not darken as a result of exposure to radiation. All glass will darken slowly over time due to radiation, however this is especially true in a hot cell, where exceptional levels of radiation are present. The advantage of an aqueous salt solution is that any radiation damage will last less than a millisecond , so the shield will undergo self-repair. [ 7 ]
In organic chemistry anhydrous ZnBr 2 is sometimes used as a Lewis acid .
Safety considerations are similar to those for zinc chloride, for which the toxic dose for humans is 3–5 g. [ 2 ]
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Zinc chloride is an inorganic chemical compound with the formula ZnCl 2 · n H 2 O, with n ranging from 0 to 4.5, forming hydrates . Zinc chloride, anhydrous and its hydrates, are colorless or white crystalline solids, and are highly soluble in water . Five hydrates of zinc chloride are known, as well as four polymorphs of anhydrous zinc chloride. [ 5 ]
All forms of zinc chloride are deliquescent . They can usually be produced by the reaction of zinc or its compounds with some form of hydrogen chloride . Anhydrous zinc compound is a Lewis acid , readily forming complexes with a variety of Lewis bases. Zinc chloride finds wide application in textile processing, metallurgical fluxes , chemical synthesis of organic compounds , such as benzaldehyde , and processes to produce other compounds of zinc. [ 5 ]
Zinc chloride has long been known but currently practiced industrial applications all evolved in the latter half of 20th century. [ 5 ]
An amorphous cement formed from aqueous zinc chloride and zinc oxide was first investigated in 1855 by Stanislas Sorel . Sorel later went on to investigate the related magnesium oxychloride cement , which bears his name. [ 6 ]
Dilute aqueous zinc chloride was used as a disinfectant under the name "Burnett's Disinfecting Fluid". [ 7 ] From 1839 Sir William Burnett promoted its use as a disinfectant as well as a wood preservative. The Royal Navy conducted trials into its use as a disinfectant in the late 1840s, including during the cholera epidemic of 1849 ; and at the same time experiments were conducted into its preservative properties as applicable to the shipbuilding and railway industries. Burnett had some commercial success with his eponymous fluid. Following his death however, its use was largely superseded by that of carbolic acid and other proprietary products. [ 8 ]
Unlike other metal dichlorides, zinc dichloride adopts several crystalline forms ( polymorphs ). Four polymorph are known: α, β, γ, and δ. Each features Zn 2+ centers surrounded in a tetrahedral manner by four chloride ligands. [ 9 ]
Here a , b , and c are lattice constants, Z is the number of structure units per unit cell, and ρ is the density calculated from the structure parameters. [ 10 ] [ 11 ] [ 12 ]
The orthorhombic form (δ) rapidly changes to another polymorph upon exposure to the atmosphere. A possible explanation is that the OH − ions originating from the absorbed water facilitate the rearrangement. [ 9 ] Rapid cooling of molten ZnCl 2 gives a glass . [ 13 ]
Molten ZnCl 2 has a high viscosity at its melting point and a comparatively low electrical conductivity, which increases markedly with temperature. [ 14 ] [ 15 ] As indicated by a Raman scattering study, the viscosity is explained by the presence of polymers,. [ 16 ] Neutron scattering study indicated the presence of tetrahedral ZnCl 4 centers, which requires aggregation of ZnCl 2 monomers as well. [ 17 ]
A variety of hydrated zinc chloride are known: ZnCl 2 (H 2 O) n with n = 1, 1.33, 2.5, 3, and 4.5. [ 18 ] The 1.33-hydrate, previously thought to be the hemitrihydrate, consists of trans -Zn(H 2 O) 4 Cl 2 centers with the chlorine atoms connected to repeating ZnCl 4 chains. The hemipentahydrate, structurally formulated [Zn(H 2 O) 5 ][ZnCl 4 ], consists of Zn(H 2 O) 5 Cl octahedrons where the chlorine atom is part of a [ZnCl 4 ] 2- tetrahedera. The trihydrate consists of distinct hexaaquozinc(II) cations and tetrachlorozincate anions; formulated [Zn(H 2 O) 6 ][ZnCl 4 ]. Finally, the heminonahydrate, structurally formulated [Zn(H 2 O) 6 ][ZnCl 4 ]·3H 2 O also consists of distinct hexaaquozinc(II) cations and tetrachlorozincate anions like the trihydrate but has three extra water molecules. These hydrates can be produced by evaporation of aqueous solutions of zinc chloride at different temperatures. [ 19 ] [ 20 ]
Historically, zinc chlorides are prepared from the reaction of hydrochloric acid with zinc metal or zinc oxide. Aqueous acids cannot be used to produce anhydrous zinc chloride. According to an early procedure, a suspension of powdered zinc in diethyl ether is treated with hydrogen chloride, followed by drying [ 21 ] The overall method remains useful in industry, but without the solvent: [ 5 ]
Aqueous solutions may be readily prepared similarly by treating Zn metal, zinc carbonate, zinc oxide, and zinc sulfide with hydrochloric acid: [ 22 ]
Hydrates can be produced by evaporation of an aqueous solution of zinc chloride. The temperature of the evaporation determines the hydrates For example, evaporation at room temperature produces the 1.33-hydrate. [ 19 ] [ 23 ] Lower evaporation temperatures produce higher hydrates. [ 20 ]
Commercial samples of zinc chloride typically contain water and products from hydrolysis as impurities. Laboratory samples may be purified by recrystallization from hot dioxane . Anhydrous samples can be purified by sublimation in a stream of hydrogen chloride gas, followed by heating the sublimate to 400 °C in a stream of dry nitrogen gas. [ 24 ] A simple method relies on treating the zinc chloride with thionyl chloride . [ 25 ]
A number of salts containing the tetrachlorozincate anion, [ZnCl 4 ] 2− , are known. [ 14 ] "Caulton's reagent", V 2 Cl 3 ( thf ) 6 ] [Zn 2 Cl 6 ] , which is used in organic chemistry, is an example of a salt containing [Zn 2 Cl 6 ] 2− . [ 26 ] [ 27 ] The compound Cs 3 ZnCl 5 contains tetrahedral [ZnCl 4 ] 2− and Cl − anions, [ 9 ] so, the compound is not caesium pentachlorozincate, but caesium tetrachlorozincate chloride. No compounds containing the [ZnCl 6 ] 4− ion (hexachlorozincate ion) have been characterized. [ 9 ] The compound ZnCl 2 ·0.5HCl·H 2 O crystallizes from a solution of ZnCl 2 in hydrochloric acid . It contains a polymeric anion (Zn 2 Cl − 5 ) n with balancing monohydrated hydronium ions, H 5 O + 2 ions. [ 9 ]
The adduct with thf ZnCl 2 (thf) 2 illustrates the tendency of zinc chloride to form 1:2 adducts with weak Lewis bases . Being soluble in ethers and lacking acidic protons, this complex is used in the synthesis of organozinc compounds . [ 29 ] A related 1:2 complex is ZnCl 2 (NH 2 OH) 2 (zinc dichloride di(hydroxylamine)). Known as Crismer's salt, this complexes releases hydroxylamine upon heating. [ 30 ] The distinctive ability of aqueous solutions of ZnCl 2 to dissolve cellulose is attributed to the formation of zinc-cellulose complexes, illustrating the stability of its adducts. [ 31 ] Cellulose also dissolves in molten ZnCl 2 hydrate. [ 32 ] Overall, this behavior is consistent with Zn 2+ as a hard Lewis acid.
When solutions of zinc chloride are treated with ammonia , diverse ammine complexes are produced. In addition to the tetrahedral 1:2 complex ZnCl 2 (NH 3 ) 2 . [ 33 ] [ 34 ] the complex Zn(NH 3 ) 4 Cl 2 ·H 2 O also has been isolated. The latter contains the [Zn(NH 3 ) 6 ] 2+ ion,. [ 9 ] The species in aqueous solution have been investigated and show that [Zn(NH 3 ) 4 ] 2+ is the main species present with [Zn(NH 3 ) 3 Cl] + also present at lower NH 3 :Zn ratio. [ 35 ]
Zinc chloride dissolves readily in water to give ZnCl x (H 2 O) 4− x species and some free chloride. [ 36 ] [ 37 ] [ 38 ] Aqueous solutions of ZnCl 2 are acidic: a 6 M aqueous solution has a pH of 1. [ 18 ] The acidity of aqueous ZnCl 2 solutions relative to solutions of other Zn 2+ salts (say the sulfate) is due to the formation of the tetrahedral chloro aqua complexes such as [ZnCl 3 (H 2 O)] − . [ 39 ] Most metal dichlorides form octahedral complexes, with stronger O-H bonds. The combination of hydrochloric acid and ZnCl 2 gives a reagent known as " Lucas reagent ". Such reagents were once used as a test for primary alcohols. Similar reactions are the basis of industrial routes from methanol and ethanol respectively to methyl chloride and ethyl chloride . [ 40 ]
In alkali solution, zinc chloride converts to various zinc hydroxychlorides. These include [Zn(OH) 3 Cl] 2− , [Zn(OH) 2 Cl 2 ] 2− , [Zn(OH)Cl 3 ] 2− , and the insoluble Zn 5 (OH) 8 Cl 2 ·H 2 O . The latter is the mineral simonkolleite . [ 41 ] When zinc chloride hydrates are heated, hydrogen chloride evolves and hydroxychlorides result. [ 42 ]
In aqueous solution ZnCl 2 , as well as other halides (bromide, iodide), behave interchangeably for the preparation of other zinc compounds. These salts give
precipitates of zinc carbonate when treated with aqueous carbonate sources: [ 5 ]
Ninhydrin reacts with amino acids and amines to form a colored compound "Ruhemann's purple" (RP). Spraying with a zinc chloride solution, which is colorless, forms a 1:1 complex RP: ZnCl(H 2 O) 2 , which is more readily detected as it fluoresces more intensely than RP. [ 43 ]
Anhydrous zinc chloride melts and even boils without any decomposition up to 900 °C. When zinc metal is dissolved in molten ZnCl 2 at 500–700 °C, a yellow diamagnetic solution is formed consisting of the Zn 2+ 2 , which has zinc in the oxidation state +1. The nature of this dizinc dication has been confirmed by Raman spectroscopy . [ 18 ] Although Zn 2+ 2 is unusual, mercury, a heavy congener of zinc, forms a wide variety of Hg 2+ 2 salts.
In the presence of oxygen, zinc chloride oxidizes to zinc oxide above 400 °C. Again, this observation indicates the nonoxidation of Zn 2+ . [ 44 ]
Concentrated aqueous zinc chloride dissolves zinc oxide to form zinc hydroxychloride, which is obtained as colorless crystals: [ 45 ]
The same material forms when hydrated zinc chloride is heated. [ 46 ]
The ability of zinc chloride to dissolve metal oxides (MO) [ 47 ] is relevant to the utility of ZnCl 2 as a flux for soldering . It dissolves passivating oxides, exposing the clean metal surface. [ 47 ]
Zinc chloride is an occasional laboratory reagent often as a Lewis acid . A dramatic example is the conversion of methanol into hexamethylbenzene using zinc chloride as the solvent and catalyst: [ 48 ]
This kind of reactivity has been investigated for the valorization of C1 precursors. [ 49 ]
Examples of zinc chloride as a Lewis acid include the Fischer indole synthesis : [ 50 ]
Related Lewis-acid behavior is illustrated by a traditional preparation of the dye fluorescein from phthalic anhydride and resorcinol , which involves a Friedel-Crafts acylation . [ 51 ] This transformation has in fact been accomplished using even the hydrated ZnCl 2 sample shown in the picture above. Many examples describe the use of zinc chloride in Friedel-Crafts acylation reactions. [ 52 ] [ 53 ]
Zinc chloride also activates benzylic and allylic halides towards substitution by weak nucleophiles such as alkenes : [ 54 ]
In similar fashion, ZnCl 2 promotes selective Na[BH 3 (CN)] reduction of tertiary, allylic or benzylic halides to the corresponding hydrocarbons. [ 24 ]
Zinc enolates , prepared from alkali metal enolates and ZnCl 2 , provide control of stereochemistry in aldol condensation reactions. This control is attributed to chelation at the zinc. In the example shown below, the threo product was favored over the erythro by a factor of 5:1 when ZnCl 2 . [ 55 ]
Being inexpensive and anhydrous, ZnCl 2 is a widely used for the synthesis of many organozinc reagents, such as those used in the palladium catalyzed Negishi coupling with aryl halides or vinyl halides . The prominence of this reaction was highlighted by the award of the 2010 Nobel Prize in Chemistry to Ei-ichi Negishi . [ 56 ]
Rieke zinc , a highly reactive form of zinc metal, is generated by reduction of zinc dichloride with lithium . Rieke Zn is useful for the preparation of polythiophenes [ 57 ] and for the Reformatsky reaction . [ 58 ]
Zinc chloride is used as a catalyst or reagent in diverse reactions conducted on an industrial scale. Benzaldehyde, 20,000 tons of which is produced annually in Western countries, is produced from inexpensive toluene by exploiting the catalytic properties of zinc dichloride. This process begins with the chlorination of toluene to give benzal chloride . In the presence of a small amount of anhydrous zinc chloride, a mixture of benzal chloride are treated continuously with water according to the following stoichiometry: [ 59 ]
Similarly zinc chloride is employed in hydrolysis of benzotrichloride, the main route to benzoyl chloride . It serves as a catalyst for the production of methylene-bis(dithiocarbamate). [ 5 ]
The use of zinc chloride as a flux, sometimes in a mixture with ammonium chloride (see also Zinc ammonium chloride ), involves the production of HCl and its subsequent reaction with surface oxides.
Zinc chloride forms two salts with ammonium chloride: [NH 4 ] 2 [ZnCl 4 ] and [NH 4 ] 3 [ZnCl 4 ]Cl , which decompose on heating liberating HCl, just as zinc chloride hydrate does. The action of zinc chloride/ammonium chloride fluxes, for example, in the hot-dip galvanizing process produces H 2 gas and ammonia fumes. [ 60 ]
Relevant to its affinity for these paper and textiles, ZnCl 2 is used as a fireproofing agent and in the process of making Vulcanized fibre , which is made by soaking paper in concentrated zinc chloride. [ 61 ] [ 62 ] Zinc chloride is also used as a deodorizing agent and to make zinc soaps . [ 5 ]
Zinc and chloride are essential for life. Zn 2+ is a component of several enzymes , e.g., carboxypeptidase and carbonic anhydrase . Thus, aqueous solutions of zinc chlorides are rarely problematic as an acute poison. [ 5 ] Anhydrous zinc chloride is however an aggressive Lewis acid as it can burn skin and other tissues. Ingestion of zinc chloride, often from soldering flux , requires endoscopic monitoring. [ 63 ] Another source of zinc chloride is zinc chloride smoke mixture ("HC") used in smoke grenades . Containing zinc oxide, hexachloroethane and aluminium powder release zinc chloride, carbon and aluminium oxide smoke, an effective smoke screen . [ 64 ] Such smoke screens can lead to fatalities. [ 65 ]
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Zinc chromate , Zn Cr O 4 , is a chemical compound , a salt containing the chromate anion , appearing as odorless yellow powder or yellow-green crystals, but, when used for coatings, pigments are often added. [ 2 ] [ 3 ] [ 4 ] It is used industrially in chromate conversion coatings , having been developed by the Ford Motor Company in the 1920s. [ 5 ]
A process known as the Cronak process is used to create zinc chromate for use in industry. This process is done by putting zinc or a zinc plated metal in a solution of sodium dichromate and sulfuric acid for a few seconds. [ 6 ] Zinc chromate can also be synthesized by using neutral potassium chromate (K 2 CrO 4 ) and zinc sulfate (ZnSO 4 ), which forms a precipitate. [ 7 ]
K 2 CrO 4 + ZnSO 4 → ZnCrO 4 + K 2 SO 4
Zinc chromate's main use is in industrial painting as a coating over iron or aluminium materials. [ 8 ] It was used extensively on aircraft by the US military, especially during the 1930s and 1940s. It is also used in a variety of paint coatings for the aerospace and automotive industries. [ 9 ] Its use as a corrosion-resistant agent was applied to aluminium alloy parts first in commercial aircraft, and then in military ones. During the 1940 and 1950s it was typically found as the "paint" in the wheel wells of retractable landing gear on US military aircraft to protect the aluminium from corrosion. This compound was a useful coating because it is an anti-corrosive and anti-rust primer. [ 8 ] Since it is highly toxic, it also destroys organic growth on the surface. Zinc chromate is also used in spray paints, artists' paints, pigments in varnishes, and in making linoleum. [ 5 ]
When used as a pigment, it is known as Zinc Yellow, [ 2 ] Buttercup Yellow or Yellow 36. [ 10 ] It is rarely used in art because the pigment degenerates into a brown color. This effect can be seen in Georges Seurat's famous painting A Sunday Afternoon on the Island of La Grande Jatte . [ 11 ] The degradation of zinc yellow in Seurat's painting was thoroughly investigated [ 12 ] and these findings were subsequently employed in a digital rejuvenation [ 13 ] of the painting. [ 14 ] [ 15 ]
Zinc chromate putty was used as sealant in addition to two O-rings between sections of the failed solid rocket booster on Space Shuttle Challenger . Blowholes in this putty may have been a minor contributor to its catastrophic loss . [ 16 ]
Recent studies have shown that not only is zinc chromate highly toxic, it is also a carcinogen because it contains Cr(VI) . [ 17 ] Exposure to zinc chromate can cause tissue ulceration and cancer. [ 1 ] [ 3 ] A study published in the British Journal of Industrial Medicine showed a significant correlation between the use of zinc chromate and lead chromate in factories and the number of cases in lung cancer experienced by the workers. [ 18 ] Because of its toxicity the use of zinc chromate has greatly diminished in recent years. [ citation needed ]
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Zinc fluoride is an inorganic chemical compound with the chemical formula Zn F 2 . It is encountered as the anhydrous form and also as the tetrahydrate , ZnF 2 ·4H 2 O (rhombohedral crystal structure). [ 2 ] It has a high melting point and has the rutile structure containing 6 coordinate zinc, which suggests appreciable ionic character in its chemical bonding. [ 3 ] Unlike the other zinc halides, ZnCl 2 , ZnBr 2 and ZnI 2 , it is not very soluble in water. [ 3 ]
Like some other metal difluorides, ZnF 2 crystallizes in the rutile structure, which features octahedral Zn cations and trigonal planar fluorides . [ 4 ]
Zinc fluoride can be synthesized several ways.
Zinc fluoride can be hydrolysed by hot water to form the zinc hydroxide fluoride, Zn(OH)F. [ 5 ]
The salt is believed to form both a tetrahydrate and a dihydrate. [ 6 ]
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Zinc hydride is an inorganic compound with the chemical formula Zn H 2 . It is a white, odourless solid which slowly decomposes into its elements at room temperature; despite this it is the most stable of the binary first row transition metal hydrides . A variety of coordination compounds containing Zn–H bonds are used as reducing agents , [ 1 ] but ZnH 2 itself has no common applications.
Zinc(II) hydride was first synthesized in 1947 by Hermann Schlesinger , via a reaction between dimethylzinc Zn(CH 3 ) 2 and lithium aluminium hydride Li[AlH 4 ] ; [ 2 ] a process which was somewhat hazardous due to the pyrophoric nature of Zn(CH 3 ) 2 .
Later methods were predominantly salt metathesis reactions between zinc halides and alkali metal hydrides, which are significantly safer. [ 3 ] [ 4 ] Examples include:
Small quantities of gaseous zinc(II) hydride have also been produced by laser ablation of zinc under a hydrogen atmosphere [ 5 ] [ 6 ] and other high energy techniques. These methods have been used to assess its gas phase properties.
New evidence suggests that in zinc(II) hydride, elements form a one-dimensional network ( polymer ), being connected by covalent bonds . [ 7 ] Other lower metal hydrides polymerise in a similar fashion (cf. aluminium hydride ). Solid zinc(II) hydride is the irreversible autopolymerisation product of the molecular form, and the molecular form cannot be isolated in concentration. Solubilising zinc(II) hydride in non-aqueous solvents, involve adducts with molecular zinc(II) hydride, such as ZnH 2 ·H 2 in liquid hydrogen.
Zinc(II) hydride slowly decomposes to metallic zinc and hydrogen gas at room temperature, with decomposition becoming rapid if it is heated above 90°C. [ 8 ]
It is readily oxidised and is sensitive to both air and moisture; being hydrolysed slowly by water but violently by aqueous acids, [ 3 ] which indicates possible passivation via the formation of a surface layer of ZnO . Despite this older samples may be pyrophoric. [ 3 ] Zinc hydride can therefore be considered metastable at best, however it is still the most stable of all the binary first row transition metal hydrides (cf. titanium(IV) hydride ).
Molecular zinc(II) hydride, ZnH 2 , has been identified as a volatile product of the acidified reduction of zinc ions with sodium borohydride . [ citation needed ] This reaction is similar to the acidified reduction with lithium aluminium hydride , however a greater fraction of the generated zinc(II) hydride is in the molecular form. This can be attributed to a slower reaction rate, which prevents a polymerising concentration of building over the progression of the reaction. This follows earlier experiments in direct synthesis from the elements. The reaction of excited zinc atoms with molecular hydrogen in the gas phase was studied by Breckenridge et al using laserpump-probe techniques. [ citation needed ] Owing to its relative thermal stability, molecular zinc(II) hydride is included in the short list of molecular metal hydrides, which have been successfully identified in the gas phase (that is, not limited to matrix isolation).
The average Zn–H bond energy was recently calculated to be 51.24 kcal mol −1 , while the H–H bond energy is 103.3 kcal mol −1 . [ citation needed ] Therefore, the overall reaction is nearly ergoneutral.
Molecular zinc hydride in the gas phase was found to be linear with a Zn–H bond length of 153.5 pm. [ 9 ]
The molecule can be found a singlet ground state of 1 Σ g + .
Quantum chemical calculations predict the molecular form to exist in a doubly hydrogen-bridged, dimeric groundstate, with little or no formational energy barrier . [ citation needed ] The dimer can be called di-μ-hydrido-bis(hydridozinc), per IUPAC additive nomenclature.
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Zinc iodide is the inorganic compound with the formula ZnI 2 . It exists both in anhydrous form and as a dihydrate. Both are white and readily absorb water from the atmosphere. It has no major application.
It can be prepared by the direct reaction of zinc and iodine in water [ 1 ] [ 2 ] or refluxing ether : [ 3 ]
Absent a solvent, the elements do not combine directly at room temperature. [ 4 ]
The structure of solid ZnI 2 is unusual relative to the dichloride. While zinc centers are tetrahedrally coordinated, as in ZnCl 2 , groups of four of these tetrahedra share three vertices to form “super-tetrahedra” of composition {Zn 4 I 10 }, which are linked by their vertices to form a three-dimensional structure. [ 5 ] These "super-tetrahedra" are similar to the P 4 O 10 structure. [ 5 ] [ 6 ]
Molecular ZnI 2 is linear as predicted by VSEPR theory with a Zn-I bond length of 238 pm. [ 5 ]
In aqueous solution the following have been detected: Zn(H 2 O) 6 2+ , [ZnI(H 2 O) 5 ] + , tetrahedral ZnI 2 (H 2 O) 2 , ZnI 3 (H 2 O) − , and ZnI 4 2− . [ 7 ]
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Zinc molybdate is an inorganic compound with the formula Zn MoO 4 . It is used as a white pigment , which is also a corrosion inhibitor . A related pigment is sodium zinc molybdate, Na 2 Zn(MoO 4 ) 2 . [ 4 ] The material has also been investigated as an electrode material. [ 5 ]
In terms of its structure, the Mo(VI) centers are tetrahedral and the Zn(II) centers are octahedral. [ 2 ]
The LD50 (oral, rats) is 11,500 mg/kg. [ 4 ] While highly soluble molybdates like e.g. sodium molybdate are toxic in higher doses, zinc molybdate is essentially non-toxic because of its insolubility in water. Molybdates possess a lower toxicity than chromates or lead salts and are therefore seen as an alternative to these salts for corrosion inhibition.
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it .
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Fume: TWA 5 mg/m 3 ST 10 mg/m 3 [ 2 ]
Zinc oxide is an inorganic compound with the formula Zn O . It is a white powder which is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements , rubbers, plastics, ceramics, glass, cement, lubricants, [ 12 ] paints, sunscreens, ointments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants, semi conductors, [ 13 ] and first-aid tapes. Although it occurs naturally as the mineral zincite , most zinc oxide is produced synthetically. [ 14 ]
Early humans probably used zinc compounds in processed [ 14 ] and unprocessed forms, as paint or medicinal ointment; however, their composition is uncertain. The use of pushpanjan , probably zinc oxide, as a salve for eyes and open wounds is mentioned in the Indian medical text the Charaka Samhita , thought to date from 500 BC or before. [ 15 ] Zinc oxide ointment is also mentioned by the Greek physician Dioscorides (1st century AD). [ 16 ] Galen suggested treating ulcerating cancers with zinc oxide, [ 17 ] as did Avicenna in his The Canon of Medicine . It is used as an ingredient in products such as baby powder and creams against diaper rashes , calamine cream, anti- dandruff shampoos , and antiseptic ointments. [ 18 ]
The Romans produced considerable quantities of brass (an alloy of zinc and copper ) as early as 200 BC by a cementation process where copper was reacted with zinc oxide. [ 19 ] The zinc oxide is thought to have been produced by heating zinc ore in a shaft furnace. This liberated metallic zinc as a vapor, which then ascended the flue and condensed as the oxide. This process was described by Dioscorides in the 1st century AD. [ 20 ] Zinc oxide has also been recovered from zinc mines at Zawar in India , dating from the second half of the first millennium BC. [ 16 ]
From the 12th to the 16th century, zinc and zinc oxide were recognized and produced in India using a primitive form of the direct synthesis process. From India, zinc manufacturing moved to China in the 17th century. In 1743, the first European zinc smelter was established in Bristol , United Kingdom. [ 21 ] Around 1782, Louis-Bernard Guyton de Morveau proposed replacing lead white pigment with zinc oxide. [ 22 ]
The main usage of zinc oxide (zinc white) was in paints and as an additive to ointments. Zinc white was accepted as a pigment in oil paintings by 1834 but it did not mix well with oil. This problem was solved by optimizing the synthesis of ZnO. In 1845, Edme-Jean Leclaire in Paris was producing the oil paint on a large scale; by 1850, zinc white was being manufactured throughout Europe. The success of zinc white paint was due to its advantages over the traditional white lead : zinc white is essentially permanent in sunlight, it is not blackened by sulfur-bearing air, it is non-toxic and more economical. Because zinc white is so "clean" it is valuable for making tints with other colors, but it makes a rather brittle dry film when unmixed with other colors. For example, during the late 1890s and early 1900s, some artists used zinc white as a ground for their oil paintings. These paintings developed cracks over time. [ 23 ]
In recent times, most zinc oxide has been used in the rubber industry to resist corrosion . In the 1970s, the second largest application of ZnO was photocopying . High-quality ZnO produced by the "French process" was added to photocopying paper as a filler. This application was soon displaced by titanium . [ 24 ]
Pure ZnO is a white powder. However, in nature, it occurs as the rare mineral zincite , which usually contains manganese and other impurities that confer a yellow to red color. [ 25 ]
Crystalline zinc oxide is thermochromic , changing from white to yellow when heated in air and reverting to white on cooling. [ 26 ] This color change is caused by a small loss of oxygen to the environment at high temperatures to form the non-stoichiometric Zn 1+x O, where at 800 °C, x = 0.00007. [ 26 ]
Zinc oxide is an amphoteric oxide . It is nearly insoluble in water, but it will dissolve in most acids , such as hydrochloric acid: [ 27 ]
Solid zinc oxide will also dissolve in alkalis to give soluble zincates: [ 27 ]
ZnO reacts slowly with fatty acids in oils to produce the corresponding carboxylates , such as oleate or stearate . When mixed with a strong aqueous solution of zinc chloride , ZnO forms cement-like products best described as zinc hydroxy chlorides. [ 28 ] This cement was used in dentistry. [ 29 ]
ZnO also forms cement-like material when treated with phosphoric acid ; related materials are used in dentistry. [ 29 ] A major component of zinc phosphate cement produced by this reaction is hopeite , Zn 3 (PO 4 ) 2 ·4H 2 O. [ 30 ]
ZnO decomposes into zinc vapor and oxygen at around 1975 °C with a standard oxygen pressure. In a carbothermic reaction , heating with carbon converts the oxide into zinc vapor at a much lower temperature (around 950 °C). [ 27 ]
Zinc oxide crystallizes in two main forms , hexagonal wurtzite [ 31 ] and cubic zincblende . The wurtzite structure is most stable at ambient conditions and thus most common. The zincblende form can be stabilized by growing ZnO on substrates with cubic lattice structure. In both cases, the zinc and oxide centers are tetrahedral , the most characteristic geometry for Zn(II). ZnO converts to the rocksalt motif at relatively high pressures about 10 GPa. [ 13 ]
Hexagonal [ 32 ] and zincblende polymorphs have no inversion symmetry (reflection of a crystal relative to any given point does not transform it into itself). [ 33 ] This and other lattice symmetry properties result in piezoelectricity of the hexagonal [ 32 ] and zincblende [ 33 ] ZnO, and pyroelectricity of hexagonal ZnO. [ 34 ]
The hexagonal structure has a point group 6 mm ( Hermann–Mauguin notation ) or C 6v ( Schoenflies notation ), and the space group is P6 3 mc or C 6v 4 . The lattice constants are a = 3.25 Å and c = 5.2 Å; their ratio c/a ~ 1.60 is close to the ideal value for hexagonal cell c/a = 1.633. [ 35 ] As in most group II-VI materials, the bonding in ZnO is largely ionic (Zn 2+ O 2− ) with the corresponding radii of 0.074 nm for Zn 2+ and 0.140 nm for O 2− . This property accounts for the preferential formation of wurtzite rather than zinc blende structure, [ 36 ] as well as the strong piezoelectricity of ZnO. Because of the polar Zn−O bonds, zinc and oxygen planes are electrically charged. To maintain electrical neutrality, those planes reconstruct at atomic level in most relative materials, but not in ZnO – its surfaces are atomically flat, stable and exhibit no reconstruction. [ 37 ] However, studies using wurtzoid structures explained the origin of surface flatness and the absence of reconstruction at ZnO wurtzite surfaces [ 38 ] in addition to the origin of charges on ZnO planes.
ZnO is a wide-band gap semiconductor of the II-VI semiconductor group . The native doping of the semiconductor due to oxygen vacancies or zinc interstitials is n-type. [ 13 ]
ZnO is a relatively soft material with approximate hardness of 4.5 on the Mohs scale . [ 12 ] Its elastic constants are smaller than those of relevant III-V semiconductors, such as GaN . The high heat capacity and heat conductivity, low thermal expansion and high melting temperature of ZnO are beneficial for ceramics. [ 24 ] The E2 optical phonon in ZnO exhibits an unusually long lifetime of 133 ps at 10 K. [ 39 ]
Among the tetrahedrally bonded semiconductors, it has been stated that ZnO has the highest piezoelectric tensor, or at least one comparable to that of GaN and AlN . [ 40 ] This property makes it a technologically important material for many piezoelectrical applications, which require a large electromechanical coupling. Therefore, ZnO in the form of thin film has been one of the most studied and used resonator materials for thin-film bulk acoustic resonators . [ 41 ]
Favourable properties of zinc oxide include good transparency, high electron mobility , wide band gap , and strong room-temperature luminescence . Those properties make ZnO valuable for a variety of emerging applications: transparent electrodes in liquid crystal displays , [ 42 ] energy-saving or heat-protecting windows, [ 25 ] and electronics as thin-film transistors and light-emitting diodes . [ 43 ]
ZnO has a relatively wide direct band gap of ~3.3 eV at room temperature. Advantages associated with a wide band gap include higher breakdown voltages , ability to sustain large electric fields, lower electronic noise , and high-temperature and high-power operation. The band gap of ZnO can further be tuned to ~3–4 eV by its alloying with magnesium oxide or cadmium oxide . [ 13 ] Due to this large band gap, there have been efforts to create visibly transparent solar cells utilising ZnO as a light absorbing layer. However, these solar cells have so far proven highly inefficient. [ 44 ]
Most ZnO has n -type character, even in the absence of intentional doping . Nonstoichiometry is typically the origin of n-type character, but the subject remains controversial. [ 45 ] An alternative explanation has been proposed, based on theoretical calculations, that unintentional substitutional hydrogen impurities are responsible. [ 46 ] Controllable n-type doping is easily achieved by substituting Zn with group-III elements such as Al, Ga, In or by substituting oxygen with group-VII elements chlorine or iodine . [ 47 ]
Reliable p-type doping of ZnO remains difficult. This problem originates from low solubility of p-type dopants and their compensation by abundant n-type impurities. This problem is observed with GaN and ZnSe . Measurement of p-type in "intrinsically" n-type material is complicated by the inhomogeneity of samples. [ 48 ]
Current limitations to p-doping limit electronic and optoelectronic applications of ZnO, which usually require junctions of n-type and p-type material. Known p-type dopants include group-I elements Li, Na, K; group-V elements N, P and As; as well as copper and silver. However, many of these form deep acceptors and do not produce significant p-type conduction at room temperature. [ 13 ]
Electron mobility of ZnO strongly varies with temperature and has a maximum of ~2000 cm 2 /(V·s) at 80 K. [ 49 ] Data on hole mobility are scarce with values in the range 5–30 cm 2 /(V·s). [ 50 ]
ZnO discs, acting as a varistor , are the active material in most surge arresters . [ 51 ] [ 52 ]
Zinc oxide is noted for its strongly nonlinear optical properties, especially in bulk. The nonlinearity of ZnO nanoparticles can be fine-tuned according to their size. [ 53 ]
For industrial use, ZnO is produced at levels of 10 5 tons per year [ 25 ] by three main processes: [ 24 ]
In the indirect or French process, metallic zinc is melted in a graphite crucible and vaporized at temperatures above 907 °C (typically around 1000 °C). Zinc vapor reacts with the oxygen in the air to give ZnO, [ 54 ] accompanied by a drop in its temperature and bright luminescence. Zinc oxide particles are transported into a cooling duct and collected in a bag house. This indirect method was popularized by Edme Jean LeClaire of Paris in 1844 and therefore is commonly known as the French process. Its product normally consists of agglomerated zinc oxide particles with an average size of 0.1 to a few micrometers. By weight, most of the world's zinc oxide is manufactured via French process. [ citation needed ]
The direct or American process starts with diverse contaminated zinc composites, such as zinc ores or smelter by-products. The zinc precursors are reduced ( carbothermal reduction ) by heating with a source of carbon such as anthracite to produce zinc vapor, which is then oxidized as in the indirect process. Because of the lower purity of the source material, the final product is also of lower quality in the direct process as compared to the indirect one. [ 54 ]
A small amount of industrial production involves wet chemical processes, which start with aqueous solutions of zinc salts, from which zinc carbonate or zinc hydroxide is precipitated. The solid precipitate is then calcined at temperatures around 800 °C. [ citation needed ]
Numerous specialised methods exist for producing ZnO for scientific studies and niche applications. These methods can be classified by the resulting ZnO form (bulk, thin film, nanowire ), temperature ("low", that is close to room temperature or "high", that is T ~ 1000 °C), process type (vapor deposition or growth from solution) and other parameters. [ citation needed ]
Large single crystals (many cubic centimeters) can be grown by the gas transport (vapor-phase deposition), hydrothermal synthesis , [ 37 ] [ 55 ] [ 56 ] or melt growth. [ 7 ] However, because of the high vapor pressure of ZnO, growth from the melt is problematic. Growth by gas transport is difficult to control, leaving the hydrothermal method as a preference. [ 7 ] Thin films can be produced by a variety of methods including chemical vapor deposition , [ 57 ] metalorganic vapour phase epitaxy , electrodeposition , sputtering , spray pyrolysis, thermal oxidation , [ 58 ] sol–gel synthesis, atomic layer deposition , and pulsed laser deposition . [ 59 ]
Zinc oxide can be produced in bulk by precipitation from zinc compounds, mainly zinc acetate , in various solutions, such as aqueous sodium hydroxide or aqueous ammonium carbonate . [ 60 ] Synthetic methods characterized in literature since the year 2000 aim to produce ZnO particles with high surface area and minimal size distribution, including precipitation, mechanochemical , sol-gel, microwave , and emulsion methods. [ 61 ]
Nanostructures of ZnO can be synthesized into a variety of morphologies, including nanowires, nanorods , tetrapods, nanobelts, nanoflowers, nanoparticles, etc. Nanostructures can be obtained with most above-mentioned techniques, at certain conditions, and also with the vapor–liquid–solid method . [ 37 ] [ 62 ] [ 63 ] The synthesis is typically carried out at temperatures of about 90 °C, in an equimolar aqueous solution of zinc nitrate and hexamine , the latter providing the basic environment. Certain additives, such as polyethylene glycol or polyethylenimine, can improve the aspect ratio of the ZnO nanowires. [ 64 ] Doping of the ZnO nanowires has been achieved by adding other metal nitrates to the growth solution. [ 65 ] The morphology of the resulting nanostructures can be tuned by changing the parameters relating to the precursor composition (such as the zinc concentration and pH) or to the thermal treatment (such as the temperature and heating rate). [ 66 ]
Aligned ZnO nanowires on pre-seeded silicon , glass , and gallium nitride substrates have been grown using aqueous zinc salts such as zinc nitrate and zinc acetate in basic environments. [ 67 ] Pre-seeding substrates with ZnO creates sites for homogeneous nucleation of ZnO crystal during the synthesis. Common pre-seeding methods include in-situ thermal decomposition of zinc acetate crystallites, spin coating of ZnO nanoparticles, and the use of physical vapor deposition methods to deposit ZnO thin films. [ 68 ] [ 69 ] Pre-seeding can be performed in conjunction with top down patterning methods such as electron beam lithography and nanosphere lithography to designate nucleation sites prior to growth. Aligned ZnO nanowires can be used in dye-sensitized solar cells and field emission devices. [ 70 ] [ 71 ]
The applications of zinc oxide powder are numerous, and the principal ones are summarized below. Most applications exploit the reactivity of the oxide as a precursor to other zinc compounds. For material science applications, zinc oxide has high refractive index , high thermal conductivity, binding, antibacterial and UV-protection properties. Consequently, it is added into materials and products including plastics, ceramics, glass, cement, [ 72 ] rubber, lubricants, [ 12 ] paints, ointments, adhesive, sealants, concrete manufacturing, pigments, foods, batteries, ferrites , and fire retardants. [ 73 ]
Between 50% and 60% of ZnO use is in the rubber industry. [ 74 ] Zinc oxide along with stearic acid is used in the sulfur vulcanization of rubber. [ 24 ] [ 75 ] ZnO additives in the form of nanoparticles are used in rubber as a pigment [ 76 ] and to enhance its durability, [ 77 ] and have been used in composite rubber materials such as those based on montmorillonite to impart germicidal properties. [ 78 ]
Ceramic industry consumes a significant amount of zinc oxide, in particular in ceramic glaze and frit compositions. The relatively high heat capacity, thermal conductivity and high temperature stability of ZnO coupled with a comparatively low coefficient of expansion are desirable properties in the production of ceramics. ZnO affects the melting point and optical properties of the glazes, enamels, and ceramic formulations. Zinc oxide as a low expansion, secondary flux improves the elasticity of glazes by reducing the change in viscosity as a function of temperature and helps prevent crazing and shivering. By substituting ZnO for BaO and PbO, the heat capacity is decreased and the thermal conductivity is increased. Zinc in small amounts improves the development of glossy and brilliant surfaces. However, in moderate to high amounts, it produces matte and crystalline surfaces. With regard to color, zinc has a complicated influence. [ 74 ]
Zinc oxide as a mixture with about 0.5% iron(III) oxide (Fe 2 O 3 ) is called calamine and is used in calamine lotion , a topical skin treatment. [ 79 ] Historically, the name calamine was ascribed to a mineral that contained zinc used in powdered form as medicine, [ 80 ] but it was determined in 1803 that ore described as calamine was actually a mixture of the zinc minerals smithsonite and hemimorphite . [ 81 ]
Zinc oxide is widely used to treat a variety of skin conditions, including atopic dermatitis , contact dermatitis , itching due to eczema , diaper rash and acne . [ 82 ] It is used in products such as baby powder and barrier creams to treat diaper rashes , calamine cream, anti- dandruff shampoos , and antiseptic ointments. [ 18 ] [ 83 ] It is often combined with castor oil to form an emollient and astringent , zinc and castor oil cream, commonly used to treat infants. [ 84 ] [ 85 ]
It is also a component in tape (called "zinc oxide tape") used by athletes as a bandage to prevent soft tissue damage during workouts. [ 86 ]
Zinc oxide is used in mouthwash products and toothpastes as an anti-bacterial agent proposed to prevent plaque and tartar formation, [ 87 ] and to control bad breath by reducing the volatile gases and volatile sulfur compounds (VSC) in the mouth. [ 88 ] Along with zinc oxide or zinc salts, these products also commonly contain other active ingredients, such as cetylpyridinium chloride , [ 89 ] xylitol , [ 90 ] hinokitiol , [ 91 ] essential oils and plant extracts . [ 92 ] [ 93 ] Powdered zinc oxide has deodorizing and antibacterial properties. [ 94 ]
ZnO is added to cotton fabric, rubber, oral care products, [ 95 ] [ 96 ] and food packaging. [ 97 ] [ 98 ] Enhanced antibacterial action of fine particles compared to bulk material is not exclusive to ZnO and is observed for other materials, such as silver . [ 99 ] The mechanism of ZnO's antibacterial effect has been variously described as the generation of reactive oxygen species , the release of Zn 2+ ions, and a general disturbance of the bacterial cell membrane by nanoparticles. [ 100 ]
Zinc oxide is used in sunscreen to absorb ultraviolet light . [ 82 ] It is the broadest spectrum UVA and UVB absorber [ 101 ] [ 102 ] that is approved for use as a sunscreen by the U.S. Food and Drug Administration (FDA), [ 103 ] and is completely photostable. [ 104 ] When used as an ingredient in sunscreen, zinc oxide blocks both UVA (320–400 nm) and UVB (280–320 nm) rays of ultraviolet light . Zinc oxide and the other most common physical sunscreen, titanium dioxide , are considered to be nonirritating, nonallergenic, and non- comedogenic . [ 105 ] Zinc from zinc oxide is, however, slightly absorbed into the skin. [ 106 ]
Many sunscreens use nanoparticles of zinc oxide (along with nanoparticles of titanium dioxide) because such small particles do not scatter light and therefore do not appear white. The nanoparticles are not absorbed into the skin more than regular-sized zinc oxide particles are [ 107 ] and are only absorbed into the outermost layer of the skin but not into the body. [ 107 ]
When mixed with eugenol , zinc oxide eugenol is formed, which has applications as a restorative and prosthodontic in dentistry . [ 29 ] [ 108 ]
Zinc oxide is added to many food products, including breakfast cereals , as a source of zinc, a necessary nutrient . Zinc may be added to food in the form of zinc oxide nanoparticles , or as zinc sulfate , zinc gluconate , zinc acetate , or zinc citrate . [ 109 ] Some foods also include trace amounts of ZnO even if it is not intended as a nutrient. [ 110 ]
Zinc oxide (zinc white) is used as a pigment in paints and is more opaque than lithopone , but less opaque than titanium dioxide . [ 14 ] It is also used in coatings for paper. Chinese white is a special grade of zinc white used in artists' pigments. [ 111 ] The use of zinc white as a pigment in oil painting started in the middle of 18th century. [ 112 ] It has partly replaced the poisonous lead white and was used by painters such as Böcklin , Van Gogh , [ 113 ] Manet , Munch and others. It is also a main ingredient of mineral makeup (CI 77947). [ 114 ]
Micronized and nano-scale zinc oxide provides strong protection against UVA and UVB ultraviolet radiation , and are consequently used in sunscreens , [ 115 ] and also in UV-blocking sunglasses for use in space and for protection when welding , following research by scientists at Jet Propulsion Laboratory ( JPL ). [ 116 ]
Paints containing zinc oxide powder have long been utilized as anticorrosive coatings for metals. They are especially effective for galvanized iron. Iron is difficult to protect because its reactivity with organic coatings leads to brittleness and lack of adhesion. Zinc oxide paints retain their flexibility and adherence on such surfaces for many years. [ 73 ]
ZnO highly n-type doped with aluminium , gallium , or indium is transparent and conductive ( transparency ~90%, lowest resistivity ~10 −4 Ω·cm [ 117 ] ). ZnO:Al coatings are used for energy-saving or heat-protecting windows. The coating lets the visible part of the spectrum in but either reflects the infrared (IR) radiation back into the room (energy saving) or does not let the IR radiation into the room (heat protection), depending on which side of the window has the coating. [ 25 ]
Plastics, such as polyethylene naphthalate (PEN), can be protected by applying zinc oxide coating. The coating reduces the diffusion of oxygen through PEN. [ 118 ] Zinc oxide layers can also be used on polycarbonate in outdoor applications. The coating protects polycarbonate from solar radiation, and decreases its oxidation rate and photo-yellowing. [ 119 ]
Zinc oxide depleted in 64 Zn (the zinc isotope with atomic mass 64) is used in corrosion prevention in nuclear pressurized water reactors . The depletion is necessary, because 64 Zn is transformed into radioactive 65 Zn under irradiation by the reactor neutrons. [ 120 ]
Zinc oxide (ZnO) is used as a pretreatment step to remove hydrogen sulfide (H 2 S) from natural gas following hydrogenation of any sulfur compounds prior to a methane reformer , which can poison the catalyst. At temperatures between about 230–430 °C (446–806 °F), H 2 S is converted to water by the following reaction: [ 121 ]
ZnO has wide direct band gap (3.37 eV or 375 nm at room temperature). Therefore, its most common potential applications are in laser diodes and light emitting diodes (LEDs). [ 124 ] Moreover, ultrafast nonlinearities and photoconductive functions have been reported in ZnO. [ 125 ] Some optoelectronic applications of ZnO overlap with that of GaN , which has a similar band gap (~3.4 eV at room temperature). Compared to GaN, ZnO has a larger exciton binding energy (~60 meV, 2.4 times of the room-temperature thermal energy), which results in bright room-temperature emission from ZnO. ZnO can be combined with GaN for LED-applications. For instance, a transparent conducting oxide layer and ZnO nanostructures provide better light outcoupling. [ 126 ] Other properties of ZnO favorable for electronic applications include its stability to high-energy radiation and its ability to be patterned by wet chemical etching. [ 127 ] Radiation resistance [ 128 ] makes ZnO a suitable candidate for space applications. Nanostructured ZnO is an effective medium both in powder and polycrystalline forms in random lasers , [ 129 ] due to its high refractive index and aforementioned light emission properties. [ 130 ]
Zinc oxide is used in semiconductor gas sensors for detecting airborne compounds such as hydrogen sulfide , nitrogen dioxide , and volatile organic compounds . ZnO is a semiconductor that becomes n-doped by adsorption of reducing compounds , which reduces the detected electrical resistance through the device, in a manner similar to the widely used tin oxide semiconductor gas sensors. It is formed into nanostructures such as thin films, nanoparticles , nanopillars , or nanowires to provide a large surface area for interaction with gasses. The sensors are made selective for specific gasses by doping or surface-attaching materials such as catalytic noble metals. [ 131 ] [ 132 ]
Aluminium-doped ZnO layers are used as transparent electrodes . The components Zn and Al are much cheaper and less toxic compared to the generally used indium tin oxide (ITO). One application which has begun to be commercially available is the use of ZnO as the front contact for solar cells or of liquid crystal displays . [ 42 ]
Transparent thin-film transistors (TTFT) can be produced with ZnO. As field-effect transistors, they do not need a p–n junction, [ 133 ] thus avoiding the p-type doping problem of ZnO. Some of the field-effect transistors even use ZnO nanorods as conducting channels. [ 134 ]
The piezoelectricity in textile fibers coated in ZnO have been shown capable of fabricating "self-powered nanosystems" with everyday mechanical stress from wind or body movements. [ 135 ] [ 136 ]
ZnO, both in macro- [ 137 ] and nano- [ 138 ] scales, could in principle be used as an electrode in photocatalysis , mainly as an anode [ 139 ] in green chemistry applications. As a photocatalyst, ZnO reacts when exposed to UV radiation [ 137 ] and is used in photodegradation reactions to remove organic pollutants from the environment. [ 140 ] [ 141 ] It is also used to replace catalysts used in photochemical reactions that would ordinarily require costly or inconvenient reaction conditions with low yields . [ 137 ]
The pointed tips of ZnO nanorods could be used as field emitters . [ 142 ]
ZnO is a promising anode material for lithium-ion battery because it is cheap, biocompatible, and environmentally friendly. ZnO has a higher theoretical capacity (978 mAh g −1 ) than many other transition metal oxides such as CoO (715 mAh g −1 ), NiO (718 mAh g −1 ) and CuO (674 mAh g −1 ). [ 143 ] ZnO is also used as an electrode in supercapacitors. [ 144 ]
As a food additive , zinc oxide is on the U.S. Food and Drug Administration 's list of generally recognized as safe substances. [ 145 ]
Zinc oxide itself is non-toxic; it is hazardous, however, to inhale high concentrations of zinc oxide fumes, such as those generated when zinc or zinc alloys are melted and oxidized at high temperature. This problem occurs while melting alloys containing brass because the melting point of brass is close to the boiling point of zinc. [ 146 ] Inhalation of zinc oxide, which may occur when welding galvanized (zinc-plated) steel , can result in a malady called metal fume fever . [ 146 ]
In sunscreen formulations that combined zinc oxide with small-molecule UV absorbers, UV light caused photodegradation of the small-molecule absorbers and toxicity in embryonic zebrafish assays. [ 147 ]
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Zinc peroxide (ZnO 2 ) is a chemical compound of zinc that appears as a bright yellow powder at room temperature. It was historically used as a surgical antiseptic . More recently zinc peroxide has also been used as an oxidant in explosives and pyrotechnic mixtures. Its properties have been described as a transition between ionic and covalent peroxides. [ 3 ]
Zinc peroxide can be synthesized through the reaction of zinc chloride and hydrogen peroxide . [ 4 ]
According to X-ray crystallography , the compound consists of octahedral Zn(II) centers bonded to six distinct peroxide (O 2 2- ) ligands. The overall motif is very similar to that for iron pyrite (FeS 2 ). The structure, with intact O-O bonds, makes clear that this material is a peroxide, not a dioxide.
The treatment of burrowing ulcers in the abdominal wall with zinc peroxide was first recorded in 1933 and throughout the 1940s ZnO 2 was used as a disinfectant in surgical . [ 5 ] Zinc peroxide was, however, deemed ineffective against certain bacterial strains, such as Streptococcus viridans , staphylococcus aureus , E. coli , B. proteus , and B. pyocyoneus .
Zinc peroxide is hazardous in case of skin contact, of eye contact, or inhalation. [ 6 ]
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Zinc sulfate is an inorganic compound with the formula ZnSO 4 . It forms hydrates ZnSO 4 · n H 2 O, where n can range from 0 to 7. All are colorless solids. The most common form includes water of crystallization as the heptahydrate, [ 4 ] with the formula Zn SO 4 ·7H 2 O . As early as the 16th century it was prepared on a large scale, and was historically known as "white vitriol " [ 5 ] (the name was used, for example, in 1620s by the collective writing under the pseudonym of Basil Valentine ). [ citation needed ] Zinc sulfate and its hydrates are colourless solids.
The main application of the heptahydrate is as a coagulant in the production of rayon . It is also a precursor to the pigment lithopone . It is also used as an electrolyte for zinc electroplating , as a mordant in dyeing, and as a preservative for skins and leather.
Zinc sulfate is used to supply zinc in animal feeds, fertilizers, toothpaste, and agricultural sprays. Zinc sulfate, [ 6 ] like many zinc compounds, can be used to control moss growth on roofs. [ 7 ]
Zinc sulfate can be used to supplement zinc in the brewing process. Zinc is a necessary nutrient for optimal yeast health and performance, although it is not a necessary supplement for low-gravity beers, as the grains commonly used in brewing already provide adequate zinc. It is a more common practice when pushing yeast to their limit by increasing alcohol content beyond their comfort zone. Before modern stainless steel, brew Kettles, fermenting vessels and after wood, zinc was slowly leached by the use of copper kettles. A modern copper immersion chiller is speculated to provide trace amounts of zinc; thus care must be taken when adding supplemental zinc so as not to cause excess. Side effects include "...increased acetaldehyde and fusel alcohol production due to high yeast growth when zinc concentrations exceed 5 ppm. Excess zinc can also cause soapy or goaty flavors." [ 8 ] [ 9 ] [ 10 ]
Zinc sulfate is a potent inhibitor of sweetness perception for most sweet-tasting substances. [ 11 ]
It is used as a dietary supplement to treat zinc deficiency and to prevent the condition in those at high risk. [ 12 ] Side effects of excess supplementation may include abdominal pain , vomiting , headache , and tiredness . [ 13 ] it is also used together with oral rehydration therapy (ORT) and an astringent . [ 4 ]
Zinc sulfate is produced by treating virtually any zinc-containing material (metal, minerals, oxides) with sulfuric acid. [ 4 ]
Specific reactions include the reaction of the metal with aqueous sulfuric acid :
Pharmaceutical-grade zinc sulfate is produced by treating high-purity zinc oxide with sulfuric acid:
In aqueous solution, all forms of zinc sulfate behave identically. These aqueous solutions consist of the metal aquo complex [Zn(H 2 O) 6 ] 2+ and SO 2− 4 ions. Barium sulfate forms when these solutions are treated with solutions of barium ions:
With a reduction potential of −0.76 V, zinc(II) reduces only with difficulty.
When heated above 680 °C, zinc sulfate decomposes into sulfur dioxide gas and zinc oxide fume, both of which are hazardous. [ 14 ]
The heptahydrate is isostructural with ferrous sulfate heptahydrate. The solid consists of [Zn(H 2 O) 6 ] 2+ ions interacting with sulfate and one water of crystallization by hydrogen bonds. Anhydrous zinc sulfate is isomorphous with anhydrous copper(II) sulfate . It exists as the mineral zincosite . [ 15 ] A monohydrate is known. [ 16 ] The hexahydrate is also recognized. [ 17 ]
As a mineral, ZnSO 4 ·7H 2 O is known as goslarite . Zinc sulfate occurs as several other minor minerals, such as zincmelanterite , (Zn,Cu,Fe)SO 4 ·7H 2 O (structurally different from goslarite). Lower hydrates of zinc sulfate are rarely found in nature: (Zn,Fe)SO 4 ·6H 2 O (bianchite), (Zn,Mg)SO 4 ·4H 2 O (boyleite), and (Zn,Mn)SO 4 ·H 2 O ( gunningite ).
Zinc sulfate powder is an eye irritant. Ingestion of trace amounts is considered safe, and zinc sulfate is added to animal feed as a source of essential zinc, at rates of up to several hundred milligrams per kilogram of feed. Excess ingestion results in acute stomach distress, with nausea and vomiting appearing at 2–8 mg/kg of body weight. [ 18 ] Nasal irrigation with a solution of zinc sulfate has been found to be able to damage the olfactory sense nerves and induce anosmia in a number of different species, including humans. [ 19 ]
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Zinc telluride is a binary chemical compound with the formula ZnTe. This solid is a semiconductor material with a direct band gap of 2.26 eV . [ 2 ] It is usually a p-type semiconductor . Its crystal structure is cubic , like that for sphalerite and diamond . [ 1 ]
ZnTe has the appearance of grey or brownish-red powder, or ruby-red crystals when refined by sublimation. Zinc telluride typically has a cubic (sphalerite, or " zincblende ") crystal structure, but can be also prepared as rocksalt crystals or in hexagonal crystals ( wurtzite structure). Irradiated by a strong optical beam burns in presence of oxygen. Its lattice constant is 0.6101 nm, allowing it to be grown with or on aluminium antimonide , gallium antimonide , indium arsenide , and lead selenide . With some lattice mismatch, it can also be grown on other substrates such as GaAs , [ 4 ] and it can be grown in thin-film polycrystalline (or nanocrystalline) form on substrates such as glass, for example, in the manufacture of thin-film solar cells . In the wurtzite (hexagonal) crystal structure, it has lattice parameters a = 0.427 and c = 0.699 nm. [ 5 ]
Zinc telluride can be easily doped , and for this reason it is one of the more common semiconducting materials used in optoelectronics . ZnTe is important for development of various semiconductor devices , including blue LEDs , laser diodes , solar cells , and components of microwave generators. It can be used for solar cells , for example, as a back-surface field layer and p-type semiconductor material for a CdTe /ZnTe structure [ 6 ] or in PIN diode structures.
The material can also be used as a component of ternary semiconductor compounds, such as Cd x Zn (1-x) Te (conceptually a mixture composed from the end-members ZnTe and CdTe), which can be made with a varying composition x to allow the optical bandgap to be tuned as desired.
Zinc telluride together with lithium niobate is often used for generation of pulsed terahertz radiation in time-domain terahertz spectroscopy and terahertz imaging . When a crystal of such material is subjected to a high-intensity light pulse of subpicosecond duration, it emits a pulse of terahertz frequency through a nonlinear optical process called optical rectification . [ 7 ] Conversely, subjecting a zinc telluride crystal to terahertz radiation causes it to show optical birefringence and change the polarization of a transmitting light, making it an electro-optic detector.
Vanadium -doped zinc telluride, "ZnTe:V", is a non-linear optical photorefractive material of possible use in the protection of sensors at visible wavelengths. ZnTe:V optical limiters are light and compact, without complicated optics of conventional limiters. ZnTe:V can block a high-intensity jamming beam from a laser dazzler , while still passing the lower-intensity image of the observed scene. It can also be used in holographic interferometry , in reconfigurable optical interconnections , and in laser optical phase conjugation devices. It offers superior photorefractive performance at wavelengths between 600 and 1300 nm, in comparison with other III-V and II-VI compound semiconductors . By adding manganese as an additional dopant (ZnTe:V:Mn), its photorefractive yield can be significantly increased.
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The zodi is a unit of zodiacal dust . [ 1 ] One zodi is the amount of zodiacal dust in the inner Solar System . [ 2 ] This dust absorbs light from the Sun and re-radiates it as thermal radiation . The luminosity of the zodiacal dust in the Solar System is about 10 − 7 {\displaystyle 10^{-7}} relative to the luminosity of the Sun, [ 3 ] and in practice, this is the observable characteristic defining one zodi.
This astronomy -related article is a stub . You can help Wikipedia by expanding it .
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The zodiacal light (also called false dawn [ 2 ] [ 3 ] [ 4 ] [ 5 ] when seen before sunrise ) is a faint glow of diffuse sunlight scattered by interplanetary dust . Brighter around the Sun , it appears in a particularly dark night sky to extend from the Sun's direction in a roughly triangular shape along the zodiac , and appears with less intensity and visibility along the whole ecliptic as the zodiacal band . [ 6 ] Zodiacal light spans the entire sky and contributes [ 7 ] to the natural light of a clear and moonless night sky. A related phenomenon is gegenschein (or counterglow ), sunlight backscattered from the interplanetary dust, which appears directly opposite to the Sun as a faint but slightly brighter oval glow.
Zodiacal light contributes [ 8 ] to the natural light of the sky, though since zodiacal light is very faint, it is often outshone and rendered invisible by moonlight or light pollution .
The interplanetary dust in the Solar System forms a thick, pancake-shaped cloud called the zodiacal cloud which straddles the ecliptic plane. The particle sizes range from 10 to 300 micrometres , implying masses from one nanogram to tens of micrograms . [ 9 ] [ 10 ]
The Pioneer 10 and Helios spacecraft observations in the 1970s revealed zodiacal light to be scattered by the interplanetary dust cloud in the Solar System. [ 11 ] [ 12 ] Analysis of images of impact debris from the Juno spacecraft shows that the distribution of the dust extends from Earth's orbit to the 4:1 orbital resonance with Jupiter at 2.06 AU , and suggests that the dust is from Mars. [ 13 ] However, no other dedicated dust instrumentation on Pioneer 10 , Pioneer 11 , Galileo , Ulysses , nor Cassini found an indication that Mars is a significant source of dust besides comets and asteroids. [ 11 ] [ 14 ] [ 15 ] [ 16 ]
In the mid-latitudes, the zodiacal light is best observed in the western sky in the spring after the evening twilight has completely disappeared, or in the eastern sky in the autumn just before the morning twilight appears. The zodiacal light appears as a column, brighter at the horizon and tilted at the angle of the ecliptic. The light scattered from extremely small dust particles is strongly forward scattering , although the zodiacal light actually extends all the way around the sky, hence it is brightest when observing at a small angle with the Sun. This is why it is most clearly visible near sunrise or sunset when the Sun is blocked, but the dust particles nearest the line of sight to the Sun are not. The dust band that causes the zodiacal light is uniform across the whole ecliptic.
The dust further from the ecliptic is almost undetectable except when viewed at a small angle with the Sun. Thus it is possible to see more of the width at small angles toward the Sun, and it appears wider near the horizon, closer to the Sun under the horizon.
The source of the dust has been long debated. Until recently, it was thought that the dust originated from the tails of active comets and from collisions between asteroids in the asteroid belt . [ 18 ] Many of our meteor showers have no known active comet parent bodies. Over 85 percent of the dust is attributed to occasional fragmentations of Jupiter-family comets that are nearly dormant . [ 19 ] Jupiter-family comets have orbital periods of less than 20 years [ 20 ] and are considered dormant when not actively outgassing, but may do so in the future. [ 21 ] The first fully dynamical model of the zodiacal cloud demonstrated that only if the dust was released in orbits that approach Jupiter, is it stirred up enough to explain the thickness of the zodiacal dust cloud. The dust in meteoroid streams is much larger, 300 to 10,000 micrometres in diameter, and falls apart into smaller zodiacal dust grains over time.
The Poynting–Robertson effect forces the dust into more circular (but still elongated) orbits, while spiralling slowly into the Sun. Hence a continuous source of new particles is needed to maintain the zodiacal cloud. Cometary dust and dust generated by collisions among the asteroids are believed to be mostly responsible for the maintenance of the dust cloud producing the zodiacal light and the gegenschein .
Particles can be reduced in size by collisions or by space weathering. When ground down to sizes less than 10 micrometres, the grains are removed from the inner Solar System by solar radiation pressure. The dust is then replenished by the infall from comets. Zodiacal dust around nearby stars is called exozodiacal dust ; it is a potentially important source of noise in attempts to directly image extrasolar planets . It has been pointed out that this exozodiacal dust, or hot debris disks, can be an indicator of planets, as planets tend to scatter the comets to the inner Solar System.
In 2015, new results from the secondary ion dust spectrometer COSIMA on board the ESA/Rosetta orbiter confirmed that the parent bodies of interplanetary dust are most probably Jupiter-family comets such as comet 67P/Churyumov–Gerasimenko . [ 23 ] Data from the Juno mission indicate that the dust close to Earth has a local origin in the inner Solar System, best fitting the planet Mars as a source. [ 24 ]
Zodiacal light is produced by sunlight reflecting off dust particles in the Solar System known as cosmic dust . Consequently, its spectrum is the same as the solar spectrum. The material producing the zodiacal light is located in a lens-shaped volume of space centered on the sun and extending well out beyond the orbit of Earth. This material is known as the interplanetary dust cloud . Since most of the material is located near the plane of the Solar System, the zodiacal light is seen along the ecliptic . The amount of material needed to produce the observed zodiacal light is quite small. If it were in the form of 1 mm particles, each with the same albedo (reflecting power) as the Moon , each particle would be 8 km from its neighbors. The gegenschein may be caused by particles directly opposite the Sun as seen from Earth, which would be in full phase .
According to Nesvorný and Jenniskens, when the dust grains are as small as about 150 micrometres in size, they will hit the Earth at an average speed of 14.5 km/s, many as slowly as 12 km/s. If so, they pointed out, this comet dust can survive entry in partially molten form, accounting for the unusual attributes of the micrometeorites collected in Antarctica, which do not resemble the larger meteorites known to originate from asteroids . In recent years, observations by a variety of spacecraft have shown significant structure in the zodiacal light including dust bands associated with debris from particular asteroid families and several cometary trails.
According to Alexander von Humboldt 's Kosmos , Mesoamericans were aware of the zodiacal light before 1500. [ 25 ] It was perhaps first reported in print by Joshua Childrey in 1661. The phenomenon was investigated by the astronomer Giovanni Domenico Cassini in 1683. According to some sources, he explained it by dust particles around the Sun. [ 26 ] [ 27 ] Other sources state that it was first explained this way by Nicolas Fatio de Duillier , in 1684, [ 28 ] whom Cassini advised to study the zodiacal light. [ 25 ]
The Islamic prophet Muhammad described zodiacal light in reference to the timing of the five daily prayers , calling it the "false dawn" ( الفجر الكاذب al-fajr al-kādhib ). Muslim oral tradition preserves numerous sayings, or hadith , in which Muhammad describes the difference between the light of false dawn, appearing in the sky long after sunset, and the light of the first band of horizontal light at sunrise, the "true dawn" ( الفجر الصادق al-fajr al-sādiq ). According to the vast majority of Muslim scholars, navigation twilight is considered the true dawn. Practitioners of Islam use Muhammad's descriptions of zodiacal light to avoid errors in determining the timing of fasting and daily prayers. [ 29 ] [ 30 ] [ 31 ]
In 2007, Brian May , lead guitarist with the band Queen , completed his thesis , A Survey of Radial Velocities in the Zodiacal Dust Cloud , thirty-six years after abandoning it to pursue a career in music. [ 32 ] He was able to submit it only because of the minimal amount of research on the topic undertaken during the intervening years. May described the subject as being one that became "trendy" again in the 2000s. [ 33 ]
Other planets, like Venus or Mercury, [ 34 ] have shown to have rings of interplanetary dust in their orbital spaces.
Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow ( → ) may be read as "within" or "part of".
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Zofenopril ( INN ) is a medication that protects the heart and helps reduce high blood pressure . It is an angiotensin-converting enzyme (ACE) inhibitor . [ 1 ]
In small studies, zofenopril appeared significantly more effective in reducing hypertension than two older antihypertensive drugs , atenolol and enalapril , and was associated with fewer adverse effects . [ 2 ] [ 3 ]
Zofenopril is a prodrug with zofenoprilat as the active metabolite . [ 4 ]
It was patented in 1978 and approved for medical use in 2000. [ 5 ]
This drug article relating to the cardiovascular system is a stub . You can help Wikipedia by expanding it .
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Zoghman Mebkhout (born 1949 [ 1 ] ) (زغمان مبخوت) is a French - Algerian mathematician. He is known for his work in algebraic analysis , geometry and representation theory , more precisely on the theory of D -modules .
Mebkhout is currently a research director at the French National Centre for Scientific Research [ 2 ] and in 2002 Zoghman received the Servant Medal from the CNRS a prize given every two years with an amount of €10,000.
In September 1979 Mebkhout presented the Riemann–Hilbert correspondence , [ 3 ] which is a generalization of Hilbert's twenty-first problem to higher dimensions. The original setting was for Riemann surfaces , where it was about the existence of regular differential equations with prescribed monodromy groups .
In higher dimensions, Riemann surfaces are replaced by complex manifolds of dimension > 1. Certain systems of partial differential equations (linear and having very special properties for their solutions) and possible monodromies of their solutions correspond. [ 4 ] An independent proof of this result was presented by Masaki Kashiwara in April 1980. [ 5 ]
Zoghman is now largely known as a specialist in D -modules theory. [ 6 ]
Zoghman is one of the first modern international-caliber North-African mathematicians. A symposium in Spain was held on his sixtieth birthday. He was invited to the Institute for Advanced Study [ 7 ] and gave a recent talk at Institut Fourier. [ 8 ]
In his quasi-autobiographical text Récoltes et semailles Alexander Grothendieck wrote extensively about what he for a time thought of as gross mistreatment of Mebkhout, in particular in the context of attribution of credit for the formulation and proof of the Riemann-Hilbert correspondence . However, in May 1986, after being contacted by a number of mathematicians involved in the matter, Grothendieck retracted his former viewpoints (that had been based on direct testimony of Mebkhout) in a number of additions to the manuscript. They were not included in the first edition of the book by Éditions Gallimard , but were added in the later "augmented" edition. [ 9 ]
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In mathematics, particularly in differential geometry , a Zoll surface , named after Otto Zoll , is a surface homeomorphic to the 2-sphere , equipped with a Riemannian metric all of whose geodesics are closed and of equal length. While the usual unit-sphere metric on S 2 obviously has this property, it also has an infinite-dimensional family of geometrically distinct deformations that are still Zoll surfaces. In particular, most Zoll surfaces do not have constant curvature .
Zoll, a student of David Hilbert , discovered the first non-trivial examples.
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In number theory , Zolotarev's lemma states that the Legendre symbol
for an integer a modulo an odd prime number p , where p does not divide a , can be computed as the sign of a permutation:
where ε denotes the signature of a permutation and π a is the permutation of the nonzero residue classes mod p induced by multiplication by a .
For example, take a = 2 and p = 7. The nonzero squares mod 7 are 1, 2, and 4, so (2|7) = 1 and (6|7) = −1. Multiplication by 2 on the nonzero numbers mod 7 has the cycle decomposition (1,2,4)(3,6,5), so the sign of this permutation is 1, which is (2|7). Multiplication by 6 on the nonzero numbers mod 7 has cycle decomposition (1,6)(2,5)(3,4), whose sign is −1, which is (6|7).
In general, for any finite group G of order n , it is straightforward to determine the signature of the permutation π g made by left-multiplication by the element g of G . The permutation π g will be even, unless there are an odd number of orbits of even size. Assuming n even, therefore, the condition for π g to be an odd permutation, when g has order k , is that n / k should be odd, or that the subgroup < g > generated by g should have odd index .
We will apply this to the group of nonzero numbers mod p , which is a cyclic group of order p − 1. The j th power of a primitive root modulo p will have index the greatest common divisor
The condition for a nonzero number mod p to be a quadratic non-residue is to be an odd power of a primitive root.
The lemma therefore comes down to saying that i is odd when j is odd, which is true a fortiori , and j is odd when i is odd, which is true because p − 1 is even ( p is odd).
Zolotarev's lemma can be deduced easily from Gauss's lemma and vice versa . The example
i.e. the Legendre symbol ( a / p ) with a = 3 and p = 11, will illustrate how the proof goes. Start with the set {1, 2, . . . , p − 1} arranged as a matrix of two rows such that the sum of the two elements in any column is zero mod p , say:
Apply the permutation U : x ↦ a x ( mod p ) {\displaystyle U:x\mapsto ax{\pmod {p}}} :
The columns still have the property that the sum of two elements in one column is zero mod p . Now apply a permutation V which swaps any pairs in which the upper member was originally a lower member:
Finally, apply a permutation W which gets back the original matrix:
We have W −1 = VU . Zolotarev's lemma says ( a / p ) = 1 if and only if the permutation U is even. Gauss's lemma says ( a/p ) = 1 iff V is even. But W is even, so the two lemmas are equivalent for the given (but arbitrary) a and p .
This interpretation of the Legendre symbol as the sign of a permutation can be extended to the Jacobi symbol
where a and n are relatively prime integers with odd n > 0: a is invertible mod n , so multiplication by a on Z / n Z is a permutation and a generalization of Zolotarev's lemma is that the Jacobi symbol above is the sign of this permutation.
For example, multiplication by 2 on Z /21 Z has cycle decomposition (0)(1,2,4,8,16,11)(3,6,12)(5,10,20,19,17,13)(7,14)(9,18,15), so the sign of this permutation is (1)(−1)(1)(−1)(−1)(1) = −1 and the Jacobi symbol (2|21) is −1. (Note that multiplication by 2 on the units mod 21 is a product of two 6-cycles, so its sign is 1. Thus it's important to use all integers mod n and not just the units mod n to define the right permutation.)
When n = p is an odd prime and a is not divisible by p , multiplication by a fixes 0 mod p , so the sign of multiplication by a on all numbers mod p and on the units mod p have the same sign. But for composite n that is not the case, as we see in the example above.
This lemma was introduced by Yegor Ivanovich Zolotarev in an 1872 proof of quadratic reciprocity .
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