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2023-01-12 | Hydrogen storage in C14 type TiVZrMnCoFe high entropy alloy | In this present investigation, we discussed the synthesis, microstructure,
and hydrogen storage behavior intermetallic Laves phase in a hexanal
TiVZrMnCoFe high entropy alloy. In this HEA, three elements are hydride-forming
elements and the other three are non-hydride-forming elements (Fe, Mn, Co). The
thermodynamic parameter like enthalpy of mixing was calculated using Meidmas
model.The possibility of developing high entropy Laves phase-based hydrogen
storage materials was advocated. | 2301.04942v1 |
2023-03-27 | Thermoelectric Properties of Mg doped Mercury Selenide HgSe | Using the density functional theory (DFT) in combination with Boltzmann
transport theory, the influence of Mg concentrations (x) doping on the
thermoelectric properties of Hg1-xMgxSe ternary alloys was systematically
investigated. The generalized gradient approximations of Perdew-Burke-Ernzerhof
(GGA-PBE) have been used to illustrate the exchange correlation potential.
Various thermoelectric transport parameters, such as the Seebeck coefficient
(S), the thermal conductivity over relaxation time, the electrical conductivity
over relaxation time, the power factor (PF) and the figure of merit (ZT) have
been deduced and discussed. The obtained results of thermoelectric properties
show that the studied materials can be useful for room temperature
thermoelectric devices. It is also found that Mg compositions can increase the
thermal efficiency of the HgSe alloy. | 2303.15179v1 |
2023-06-12 | Wide Range Thin-FIlm Ceramic Metal-Alloy Thermometers with Low Magnetoresistance | Many thermal measurements in high magnetic fields require thermometers that
are sensitive over a wide temperature range, are low mass, have a rapid thermal
response, and have a minimal, easily correctable magnetoresistance. Here we
report the development of a new granular-metal oxide ceramic composite (cermet)
for this purpose formed by co-sputtering of the metallic alloy nichrome
Ni$_{0.8}$Cr$_{0.2}$ and the insulator silcon dioxide SiO$_2$. The resulting
thin films are sensitive enough to be used from room temperature down to below
100 mK in magnetic fields up to at least 35 tesla. | 2306.06950v2 |
2023-11-22 | Nonrelaxational FMR peak broadening in spatially inhomogeneous films | The modification of magnetic properties in spatially inhomogeneous epitaxial
films of magnetic shape memory alloys in martensitic state with the temperature
variation has been studied. The proposed theoretical model is based on Landau
theory of martensitic transformation and statistical model of martensitic
state. It was shown that that spatial inhomogeneity of the material leads to
the dispersion of local martensitic transformation temperatures resulting in
the variation of local magnetic anisotropy values. This model allows describing
the dramatic ferromagnetic resonance line broadening observed in the
experiments in epitaxial films of magnetic shape memory alloys at low
temperatures. | 2311.13733v1 |
2024-03-06 | Tellurization of Pd(111): absence of PdTe$_2$ but formation of a TePd$_2$ surface alloy | In a recent publication [2D Materials, 8, 045033 (2021), arXiv:2103.11403],
it was reported that the growth of a monolayer PdTe$_2$ in ultra-high vacuum
could be achieved by deposition of tellurium on a palladium (111) crystal
surface and subsequent thermal annealing. By means of low-energy electron
diffraction intensity (LEED-IV) structural analysis, we show that the obtained
$\left(\sqrt{3}\times \sqrt{3} \right)\textrm{R30}^\circ$ superstructure is in
fact a TePd$_2$ surface alloy. Attempts to produce a PdTe$_2$ layer in
ultra-high vacuum by increasing the Te content on the surface were not
successful. | 2403.03564v1 |
1997-06-06 | Exactly Solvable Model of Superconducting Magnetic Alloys | A model describing the Anderson impurity in the Bardeen-Cooper-Schriffer
superconductor is proven to exhibit hidden integrability and is diagonalized
exactly by the Bethe ansatz. | 9706063v1 |
1998-05-05 | Strain-dependent local empirical pseudopotentials for lattice mismatched III-V semiconductors, their alloys, heterostructures and nanostructures | For the latest EPM potentials, please see appendix A in Physical Review B,
59, 15270 (1999) | 9805051v2 |
1998-09-09 | Optimization of alloy-analogy-based approaches to the infinite-dimensional Hubbard model | An analytical expression for the self-energy of the infinite-dimensional
Hubbard model is proposed that interpolates between different exactly solvable
limits. We profit by the combination of two recent approaches that are based on
the alloy-analogy (Hubbard-III) solution: The modified alloy-analogy (MAA)
which focuses on the strong-coupling regime, and the Edwards-Hertz approach
(EHA) which correctly recovers the weak-coupling regime. Investigating the
high-energy expansion of the EHA self-energy, it turns out that the EHA
reproduces the first three exactly known moments of the spectral density only.
This may be insufficient for the investigation of spontaneous magnetism. The
analysis of the high-energy behavior of the CPA self-consistency equation
allows for a new interpretation of the MAA: The MAA is the only (two-component)
alloy-analogy that correctly takes into account the first four moments of the
spectral density. For small U, however, the MAA does not reproduce Fermi-liquid
properties. The defects of the MAA as well as of the EHA are avoided in the new
approach. We discuss the prospects of the theory and present numerical results
in comparison with essentially exact quantum Monte Carlo data. The correct
high-energy behavior of the self-energy is proved to be a decisive ingredient
for a reliable description of spontaneous magnetism. | 9809139v1 |
1998-10-27 | Relationship between resistivity and specific heat in a canonical non-magnetic heavy fermion alloy system: UPt_5-xAu_x | UPt_(5-x)Au_x alloys form in a single crystal structure, cubic AuBe_5-type,
over a wide range of concentrations from x = 0 to at least x = 2.5. All
investigated alloys, with an exception for x = 2.5, were non-magnetic. Their
electronic specific heat coefficient $\gamma$ varies from about 60 (x = 2) to
about 700 mJ/mol K^2 (x = 1). The electrical resistivity for all alloys has a
Fermi-liquid-like temperature variation, \rho = \rho_o + AT^2, in the limit of
T -> 0 K. The coefficient A is strongly enhanced in the heavy-fermion regime in
comparison with normal and transition metals. It changes from about 0.01 (x =
0) to over 2 micro-ohm cm/K^2 (x = 1). A/\gamma^2, which has been postulated to
have a universal value for heavy-fermions, varies from about 10^-6 (x = 0, 0.5)
to 10^-5 micro-ohm cm (mol K/mJ)^2 (x > 1.1), thus from a value typical of
transition metals to that found for some other heavy-fermion metals. This ratio
is unaffected, or only weakly affected, by chemical or crystallographic
disorder. It correlates with the paramagnetic Curie-Weiss temperature of the
high temperature magnetic susceptibility. | 9810361v1 |
2001-01-04 | Formation of hyperfine fields in alloys | This work deals with the analysis of experimental data on the average
magnetization of $Fe_{1-x}Me_x$ (Me=Sn,Si) disordered alloys, the average and
local hyperfine fields (HFF) at the Fe nuclei. The effect of the metalloid
concentration on the HFF is studied with the help of the results of first-
principles calculations of ordered alloys. The disorder is taken into account
by means of model systems. The dependences obtained correspond to those
experimentally observed. Experimental data on the ratio of the average HFF at
Fe nuclei to the average magnetisation in alloys with sp-elements show that the
ratio decreases proportionally with the metalloid concentration. This change in
the ratio is bound up with three factors. First, the contribution of the
valence electron polarization by the neighboring atoms, that is positive
(unlike the polarization by the own magnetic moment), increases with the change
of the disorder degree (increase of concentration). Second, the appearance of
the impurities, i.e. metalloid atoms, in the nearest environment of Fe leads to
the orbital moment increase. And, finally, the change of the disorder degree,
as in the first case, results in an increase in the orbital magnetic moment and
its positive contribution to the HFF. The value and the degree of the influence
of these contributions to the HFF is discussed. | 0101033v1 |
2001-06-24 | The Effect of Lattice Vibrations on Substitutional Alloy Thermodynamics | A longstanding limitation of first-principles calculations of substitutional
alloy phase diagrams is the difficulty to account for lattice vibrations. A
survey of the theoretical and experimental literature seeking to quantify the
impact of lattice vibrations on phase stability indicates that this effect can
be substantial. Typical vibrational entropy differences between phases are of
the order of 0.1 to 0.2 k_B/atom, which is comparable to the typical values of
configurational entropy differences in binary alloys (at most 0.693 k_B/atom).
This paper describes the basic formalism underlying ab initio phase diagram
calculations, along with the generalization required to account for lattice
vibrations. We overview the various techniques allowing the theoretical
calculation and the experimental determination of phonon dispersion curves and
related thermodynamic quantities, such as vibrational entropy or free energy. A
clear picture of the origin of vibrational entropy differences between phases
in an alloy system is presented that goes beyond the traditional bond counting
and volume change arguments. Vibrational entropy change can be attributed to
the changes in chemical bond stiffness associated with the changes in bond
length that take place during a phase transformation. This so-called ``bond
stiffness vs. bond length'' interpretation both summarizes the key phenomenon
driving vibrational entropy changes and provides a practical tool to model
them. | 0106490v1 |
2002-03-22 | Multiple defect model for non-monotonic structure relaxation in binary systems like Pd-Er alloys charged with hydrogen | In binary metallic systems like the Pd--Er alloys charged with hydrogen the
observed structure evolution exhibits complex dynamics. It is characterized by
non-monotonic time variations in an Er-rich fraction respect with an Er-poor
fraction observed experimentally. The present paper proposes a qualitative
model for this non-monotonic structure relaxation. We assume that the alloy
have crystalline defects which trap (or emit) an additional amount of Er atoms.
Hydrogen atoms into the alloy disturb the phase equilibrium as well as change
the defects capacity with respect to Er atoms. Both of these factors lead to
the spatial redistribution of Er atoms and cause the interface between the
Er-rich and the Er-poor phase to move. The competition of diffusion fluxes in
system is responsible for non-monotonic time variations, for example, in the
relative volume of the enriched phase. We have found the conditions when the
interface motion can change its direction several times during the system
relaxation to a new equilibrium state. From our point of view this effect is
the essence of the hydrogen induced non-monotonic relaxation observed in such
systems. The numerical simulation confirms the basic assumptions. | 0203456v2 |
2002-08-09 | Intersubband absorption linewidth in GaAs quantum wells due to scattering by interface roughness, phonons, alloy disorder, and impurities | We calculate the intersubband absorption linewidth in quantum wells (QWs) due
to scattering by interface roughness, LO phonons, LA phonons, alloy disorder,
and ionized impurities, and compare it with the transport energy broadening
that corresponds to the transport relaxation time related to electron mobility.
Numerical calculations for GaAs QWs clarify the different contributions of each
individual scattering mechanism to absorption linewidth and transport
broadening. Interface roughness scattering contributes about an order of
magnitude more to linewidth than to transport broadening, because the
contribution from the intrasubband scattering in the first excited subband is
much larger than that in the ground subband. On the other hand, LO phonon
scattering (at room temperature) and ionized impurity scattering contribute
much less to linewidth than to transport broadening. LA phonon scattering makes
comparable contributions to linewidth and transport broadening, and so does
alloy disorder scattering. The combination of these contributions with
significantly different characteristics makes the absolute values of linewidth
and transport broadening very different, and leads to the apparent lack of
correlation between them when a parameter, such as temperature or alloy
composition, is changed. Our numerical calculations can quantitatively explain
the previously reported experimental results. | 0208195v1 |
2004-07-14 | An ab-initio theoretical investigation of the soft-magnetic properties of permalloys | We study Ni80Fe20-based permalloys with the relativistic spin-polarized
Korringa-Kohn-Rostoker electronic structure method. Treating the compositional
disorder with the coherent potential approximation, we investigate how the
magnetocrystalline anisotropy, K, and magnetostriction, lambda, of Ni-rich
Ni-Fe alloys vary with the addition of small amounts of non-magnetic transition
metals, Cu and Mo. From our calculations we follow the trends in K and lambda
and find the compositions of Ni-Fe-Cu and Ni-Fe-Mo where both are near zero.
These high permeability compositions of Ni-Fe-Cu and Ni-Fe-Mo match well with
those discovered experimentally. We monitor the connection of the magnetic
anisotropy with the number of minority spin electrons, Nmin. By raising Nmin
via artificially increasing the band-filling of Ni80Fe20, we are able to
reproduce the key features that underpin the magnetic softening we find in the
ternary alloys. The effect of band-filling on the dependence of
magnetocrystalline anisotropy on atomic short-range order in Ni80Fe20 is also
studied. Our calculations, based on a static concentration wave theory,
indicate that the susceptibility of the high permeability of the Ni-Fe-Cu and
Ni-Fe-Mo alloys to their annealing conditions is also strongly dependent on the
alloys' compositions. An ideal soft magnet appears from these calculations. | 0407355v1 |
2004-07-27 | Phase-field simulations of solidification in binary and ternary systems using a finite element method | We present adaptive finite element simulations of dendritic and eutectic
solidification in binary and ternary alloys. The computations are based on a
recently formulated phase-field model that is especially appropriate for
modelling non-isothermal solidification in multicomponent multiphase systems.
In this approach, a set of governing equations for the phase-field variables,
for the concentrations of the alloy components and for the temperature has to
be solved numerically, ensuring local entropy production and the conservation
of mass and inner energy. To efficiently perform numerical simulations, we
developed a numerical scheme to solve the governing equations using a finite
element method on an adaptive non-uniform mesh with highest resolution in the
regions of the phase boundaries. Simulation results of the solidification in
ternary Ni$_{60}$Cu$_{40-x}$Cr$_{x}$ alloys are presented investigating the
influence of the alloy composition on the growth morphology and on the growth
velocity. A morphology diagram is obtained that shows a transition from a
dendritic to a globular structure with increasing Cr concentrations.
Furthermore, we comment on 2D and 3D simulations of binary eutectic phase
transformations. Regular oscillatory growth structures are observed combined
with a topological change of the matrix phase in 3D. An outlook for the
application of our methods to describe AlCu eutectics is given. | 0407694v1 |
2005-07-12 | Precipitation in Al-Zr-Sc alloys: a comparison between kinetic Monte Carlo, cluster dynamics and classical nucleation theory | Zr and Sc precipitate in aluminum alloys to form the Al\_3Zr\_xSc\_{1-x}
compound which, for low supersaturations of the solid solution, exhibits the
L1\_2 structure. The aim of the present study is to model at an atomic scale
the kinetics of precipitation and to build mesoscopic models so as to extend
the range of supersaturations and annealing times that can be simulated up to
values of practical interest. In this purpose, we use some ab initio
calculations and experimental data to fit an Ising type model describing
thermodynamics of the Al-Zr-Sc system. Kinetics of precipitation are studied
with a kinetic Monte Carlo algorithm based on an atom-vacancy exchange
mechanism. Cluster dynamics is then used to model at a mesoscopic scale all the
different stages of homogeneous precipitation in the two binary Al-Zr and Al-Sc
alloys. This technique correctly manages to reproduce both the kinetics of
precipitation simulated with kinetic Monte Carlo as well as experimental
observations. Focusing on the nucleation stage, it is shown that classical
theory well applies as long as the short range order tendency of the system is
considered. This allows us to propose an extension of classical nucleation
theory for the ternary Al-Zr-Sc alloy. | 0507259v2 |
2006-03-12 | Kinetic theory of nucleation and coarsening | Classical theory of nucleation based on Becker-Doering equations and
coarsening for a binary alloy. | 0603330v1 |
2006-07-12 | Title Cluster-Variational Treatment of Disordered Mixed Spin Ising Model | A disordered alloy Ap B1-p where both A and B represent the magnetic atoms
with respective spin SA =1/2 and SB =1 and whose magnetic interaction can be
described through Ising Hamiltonian is treated using the cluster-variational
method. In this method it is assumed that the system is built out of building
block which is embedded in an effective field. Taking building block as 4-atom
cluster the approximate free energy of the alloy is then obtained by treating
the interactions between spins within the cluster of all possible
configurations in exact manner and the rest of the interaction by an effective
variational field . The magnetization M and transition temperature Tc are then
calculated for different concentration and exchange parameters (JAA, JBB and
JAB). The magnetization M exhibits different kinds of ferrimagnetic behaviour
depending on concentration and relative strength of intra- and inter-
sub-network exchange interactions. For antiferromagnetic JAB, the sub-network
magnetization saturates and aligned antiferromagnetically at low temperature.
The existence of compensation temperature Tcm, where total magnetization
reverses its direction, depends sensitively on relative values of JAA/ JAB and
JBB/ JAB and p. For B (A)-rich alloy with small JAB, the direction of net
magnetization remains same upto Tc and a maximum of M appears at intermediate T
< Tc when JBB>> JAA (JBB<< JAA).When magnitude of JAB > JAA, JBB, Tc exhibits a
maximum with p. The transition temperature is much less than the mean-field
value for all cases. The magnetic susceptibility for diverges and is
Curie-Wiess like at T >>Tc. The meta-magnetic behaviour at high magnetic field
has been found. Some of these results are in tune with experimental observation
in amorphous rare-earth-transition metal alloys. | 0607299v1 |
2006-12-27 | Calculation of conduction-to-conduction and valence-to-valence transitions between bound states in (In,Ga)As/GaAs quantum dots | We have calculated the conduction-to-conduction and valence-to-valence
absorption spectrum of bound states in (In,Ga)As/GaAs quantum dots charged with
up to three electrons or holes. Several features emerge: (i) In pure
(non-alloyed) InAs/GaAs dots, the 1S-1P_1 and 1S-1P_2 conduction intraband
transitions are fully in-plane polarized along [1\bar 10] and [110],
respectively, while valence transitions are weakly polarized because the hole P
states do not show any in-plane preferential orientation. (ii) In alloyed
In_{0.6}Ga_{0.4}As/GaAs dots the [110] and [1\bar 10] polarization of the
corresponding 1S-1P conduction intraband transitions is weakened since the two
1P states are mixed by alloy fluctuations. The polarization of valence
intraband transitions is insensitive to changes in alloy fluctuations. (iii)
For light polarized along [001], we find a strong valence-to-valence transition
that involves a weakly confined hole state with predominant light-hole
character. (iv) When charging the dots with a few electrons, the conduction
intraband transitions display spectroscopic shifts of ~1-2 meV. These shifts
are a result of correlation effects (captured by configuration-interaction) and
not well described within the Hartree-Fock approximation. (v) When charging the
dots with holes, valence intraband spectra are more complex than the conduction
intraband spectra as hole states are strongly affected by spin-orbit coupling,
and configuration mixing is more pronounced. Spectroscopic shifts can no longer
be identified unambiguously. These predictions could be tested in single-dot
spectroscopy of n-doped and p-doped quantum dots. | 0612648v1 |
2007-02-15 | Real space screened-exchange method for semiconductors and their alloys | This paper has been withdrawn by the author(s), due a crucial error in Eqn.
30. | 0702345v2 |
2007-06-04 | Interplay between thermal percolation and jamming upon dimer adsorption on binary alloys | Using Monte Carlo simulations we study jamming and percolation processes upon
the random sequential adsorption of dimers on binary alloys with different
degrees of structural order. We obtain the equimolar mixtures used as
substrates by applying the isomorphism between an alloy and the Ising model
(conserved order parameter). The annealing temperature $T$ of the mixture then
is a continuous parameter that characterizes the different sets of substrates,
shaping the deposition process. As the alloy undergoes an order-disorder phase
transition at the Onsager critical temperature ($T_{c}$), the jamming and
percolating properties of the dimers deposited over the substrate are subjected
to non-trivial changes. These are reflected in a density-temperature phase
diagram with three well-defined regions. We find that for $T < T^* = 1.22
T_{c}$ the occurrence of jamming prevents the onset of percolating clusters,
while percolation is possible for $T > T^{*}$. Particular attention is focused
close to $T^{*}$, where the interplay between jamming and percolation restricts
fluctuations, forcing exponents seemingly different from the standard
percolation universality class. By analogy with a thermal transition, we study
the onset of percolation by using the {\it temperature} (in this case, the
substrate annealing temperature) as a control parameter. By proposing thermal
scaling Ansatzes we analyze the behavior of the percolation threshold and its
thermally induced fluctuations. Also, the fractal dimension of the percolating
cluster is determined. Based on these measurements and the excellent data
collapsing, we conclude that the universality class of standard percolation is
preserved for all temperatures. | 0706.0562v1 |
2008-07-31 | Non-substitutional single-atom defects in the Ge_(1-x)Sn_x alloy | Ge_(1-x)Sn_x alloys have proved difficult to form at large x, contrary to
what happens with other group IV semiconductor combinations. However, at low x
they are typical examples of well-behaved substitutional compounds, which is
desirable for harnessing the electronic properties of narrow band
semiconductors. In this paper, we propose the appearance of another kind of
single-site defect ($\beta-Sn$), consisting of a single Sn atom in the center
of a Ge divacancy, that may account for these facts. Accordingly, we examine
the electronic and structural properties of these alloys by performing
extensive numerical ab-initio calculations around local defects. The results
show that the environment of the $\beta$ defect relaxes towards a cubic
octahedral configuration, facilitating the nucleation of metallic white tin and
its segregation, as found in amorphous samples. Using the information stemming
from these local defect calculations, we built a simple statistical model to
investigate at which concentration these $\beta$ defects can be formed in
thermal equilibrium. These results agree remarkably well with experimental
findings, concerning the critical concentration above which the homogeneous
alloys cannot be formed at room temperature. Our model also predicts the
observed fact that at lower temperature the critical concentration increases.
We also performed single site effective-field calculations of the electronic
structure, which further support our hypothesis. | 0807.5025v1 |
2009-07-06 | A theoretical study of the cluster glass-Kondo-magnetic disordered alloys | The physics of disordered alloys, such as typically the well known case of
CeNi1-xCux alloys, showing an interplay among the Kondo effect, the spin glass
state and a magnetic order, has been studied firstly within an average
description like in the Sherrington-Kirkpatrick model. Recently, a theoretical
model (PRB 74, 014427 (2006)) involving a more local description of the
intersite interaction has been proposed to describe the phase diagram of
CeNi1-xCux. This alloy is an example of the complex interplay between Kondo
effect and frustration in which there is in particular the onset of a
cluster-glass state. Although the model given in Ref. PRB 74, 014427 (2006) has
reproduced the different phases relatively well, it is not able to describe the
cluster-glass state. We study here the competition between the Kondo effect and
a cluster glass phase within a Kondo Lattice model with an inter-cluster random
Gaussian interaction. The inter-cluster term is treated within the cluster
mean-field theory for spin glasses, while, inside the cluster, an exact
diagonalisation is performed including inter-site ferromagnetic and intra-site
Kondo interactions. The cluster glass order parameters and the Kondo
correlation function are obtained for different values of the cluster size, the
intra-cluster ferromagnetic coupling and the Kondo intra-site coupling. We
obtain, for instance, that the increase of the Kondo coupling tends to destroy
the cluster glass phase. | 0907.0981v1 |
2010-03-26 | Contact superconductivity in In-PbTe junctions | The authors report on electron transport studies on
superconductor-semiconductor hybrid structures of indium and n-type lead
telluride, either in the form of quantum wells or bulk crystals. In-PbTe
contacts form by spontaneous alloying, which occurs already at room
temperature. The alloyed phase penetrates deeply into PbTe and forms metallic
contacts even in the presence of depletion layers at the semiconductor surface.
Although the detailed structure of this phase is unknown, we observe that it
exhibits a superconducting transition at temperatures below 10 K. This causes
such substantial reduction of the contact resistances that they even become
comparable to those predicted for ideal superconductor-normal conductor
contacts. Most importantly, this result indicates that the interface phase in
the superconducting state becomes nearly homogeneous - in contrast to the
structure expected for alloyed contacts. We suggest that the unusual interface
superconductivity is linked to the unique properties of PbTe, namely, its huge
static dielectric constant. Apparently the alloyed interface phase contains
superconducting precipitates randomly distributed within the depletion layers,
and their Coulomb charging energies are extremely small. According to the
existing models of the granular superconductivity, even very weak Josephson
coupling between the neighboring precipitates gives rise to the formation of a
global superconducting phase which explains our observations. | 1003.5140v1 |
2010-06-24 | Kinetics of natural aging in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy | The process of natural aging in pure ternary Al-Mg-Si alloys was studied by
positron annihilation lifetime spectroscopy in real time in order to clarify
the sequence and kinetics of clustering and precipitation. It was found that
natural aging takes place in at least five stages in these alloys, four of
which were directly observed. This is interpreted as the result of complex
interactions between vacancies and solute atoms or clusters. One of the early
stages of positron lifetime evolution coincides with a clustering process
observed by differential scanning calorimetry (DSC) and involves the formation
of a positron trap with \sim 0.200 ns lifetime. In later stages, a positron
trap with a higher lifetime develops in coincidence with the DSC signal of a
second clustering reaction. Mg governs both the kinetics and the lifetime
change in this stage. Within the first 10 min after quenching, a period of
nearly constant positron lifetime was found for those Mg-rich alloys that later
show an insufficient hardness response to artificial aging, the so-called
"negative effect." The various processes observed could be described by two
effective activation energies that were found by varying the aging temperature
from 10 to 37\degree C. | 1006.4778v4 |
2011-10-28 | Self-consistent supercell approach to alloys with local environment effects | We present an efficient and accurate method for calculating electronic
structure and related properties of random alloys with a proper treatment of
local environment effects. The method is a generalization of the locally
self-consistent Green's function (LSGF) technique for the exact muffin-tin
orbital (EMTO) method. An alloy system in the calculations is represented by a
supercell with a certain set of atomic distribution correlation functions. The
Green's function for each atom in the supercell is obtained by embedding the
cluster of neighboring atoms lying within a local interaction zone (LIZ) into
an effective medium and solving the cluster Dyson equation exactly. The key
ingredients of the method are locality, which makes it linearly scaling with
the number of atoms in the supercell, and coherent-potential self-consistency
of the effective medium, which results in a fast convergence of the electronic
structure with respect to the LIZ size. To test the performance and accuracy of
the method, we apply it to two systems: Fe-rich bcc-FeCr random alloy with and
without a short-range order, and a Cr-impurity on the Fe surface. Both cases
demonstrate the importance of taking into account the local environment effects
for correct description of magnetic and bulk properties. | 1110.6354v3 |
2011-11-18 | Tight-binding analysis of the electronic structure of dilute bismide alloys of GaP and GaAs | We develop an atomistic, nearest-neighbor sp3s* tight-binding Hamiltonian to
investigate the electronic structure of dilute bismide alloys of GaP and GaAs.
Using this model we calculate that the incorporation of dilute concentrations
of Bi in GaP introduces Bi-related defect states in the band gap, which
interact with the host matrix valence band edge via a Bi composition dependent
band anti-crossing (BAC) interaction. By extending this analysis to GaBiAs we
demonstrate that the observed strong variation of the band gap Eg and
spin-orbit-splitting (SO) energy with Bi composition can be well explained in
terms of a BAC interaction between the extended states of the GaAs valence band
edge and highly localized Bi-related defect states lying in the valence band,
with the change in Eg also having a significant contribution from a
conventional alloy reduction in the conduction band edge energy. Our calculated
values of Eg and SO are in good agreement with experiment throughout the
investigated composition range x less than 13%. In particular, our calculations
reproduce the experimentally observed crossover to an Eg < SO regime at
approximately 10.5% Bi composition in bulk GaBiAs. Recent x-ray spectroscopy
measurements have indicated the presence of Bi pairs and clusters even for Bi
compositions as low as 2%. We include a systematic study of different Bi
nearest-neighbor environments in the alloy to achieve a quantitative
understanding of the effect of Bi pairing and clustering on the GaBiAs
electronic structure. | 1111.4394v1 |
2012-04-10 | Crystal structure of Cu-Sn-In alloys around the η phase field studied by neutron diffraction | The study of the Cu-Sn-In ternary system has become of great importance in
recent years, due to new environmental regulations forcing to eliminate the use
of Pb in bonding technologies for electronic devices. A key relevant issue
concerns the intermetallic phases which grow in the bonding zone and are
determining in their quality and performance. In this work, we focus in the
{\eta}-phase (Cu2In or Cu6Sn5) that exists in both end binaries and as a
ternary phase. We present a neutron diffraction study of the constitution and
crystallography of a series of alloys around the 60 at.% Cu composition, and
with In contents ranging from 0 to 25 at.%, quenched from 300\degreeC. The
alloys were characterized by scanning electron microscopy, probe microanalysis
and high-resolution neutron diffraction. The Rietveld refinement of neutron
diffraction data allowed to improve the currently available model for site
occupancies in the hexagonal {\eta}-phase in the binary Cu-Sn as well as in
ternary alloys. For the first time, structural data is reported in the ternary
Cu-Sn-In {\eta}-phase as a function of composition, information that is of
fundamental technological importance as well as valuable input data for ongoing
modelisations of the ternary phase diagram. | 1204.2154v1 |
2012-06-20 | The influence of magnetic sublattice dilution on magnetic order in CeNiGe3 and UNiSi2 | Polycrystalline samples of the Y-diluted antiferromagnet CeNiGe3 (T_N = 5.5
K) and Th-diluted ferromagnet UNiSi2 (T_C = 95 K) were studied by means of
x-ray powder diffraction, magnetization and specific heat measurements
performed in a wide temperature range. The lattice parameters of the
Ce1-xYxNiGe3 alloys decrease linearly with increasing the Y content, while the
unit cell volume of U1-xThxNiSi2 increases linearly with rising the Th content.
The ordering temperatures of the systems decrease monotonically with increasing
x down to about 1.2 K in Ce0.4Y0.6NiGe3 and 26 K in U0.3Th0.7NiSi2, forming a
dome of a long-range magnetic order on their magnetic phase diagrams. The
suppression of the magnetic order is associated with distinct broadening of the
anomalies at T_N,C due to crystallographic disorder being a consequence of the
alloying. Below the magnetic percolation threshold xc of about 0.68 and 0.75 in
the Ce- and U-based alloys, respectively, the long-range magnetic order
smoothly evolves into a short-range one, forming a tail on the magnetic phase
diagrams. The observed behaviour Ce1-xYxNiGe3 and U1-xThxNiSi2 is
characteristic of diluted magnetic alloys. (c) 2012 IOP Publishing Ltd. | 1206.4392v1 |
2012-11-02 | Elasticity behavior, phonon spectra, and the pressure-temperature phase diagram of HfTi alloy: A density-functional theory study | The pressure-induced phase transition, elasticity behavior, thermodynamic
properties, and $P\mathtt{-}T$ phase diagram of $\alpha$, $\omega$, and $\beta$
equiatomic HfTi alloy are investigated using first-principles
density-functional theory (DFT). The simulated pressure-induced phase
transition of the alloy follows the sequence of
$\alpha\mathtt{\rightarrow}\omega\mathtt{\rightarrow}\beta$, in agreement with
the experimental results of Hf and Ti metals. Our calculated elastic constants
show that the $\alpha$ and $\omega$ phases are mechanically stable at ambient
pressure, while the $\beta$ phase is unstable, where a critical pressure of
18.5 GPa is predicted for its mechanical stability. All the elastic constants,
bulk modulus, and shear modulus increase upon compression for the three phases
of HfTi. The ductility of the alloy is shown to be well improved with respect
to pure Hf and Ti metals. The Mulliken charge population analysis illustrates
that the increase of the d-band occupancy will stabilize the $\beta$ phase
under pressure. The phonon spectra and phonon density of states are studied
using the supercell approach for the three phases, and the stable nature of
$\alpha$ and $\omega$ phases at ambient pressure are observed, while the
$\beta$ phase is only stable along the [110] direction. With the Gibbs free
energy calculated from DFT-parametrized Debye model as a function of
temperature and pressure, the phase transformation boundaries of the $\alpha$,
$\omega$, and $\beta$ phases of HfTi are identified. | 1211.0375v1 |
2013-04-16 | Modeling the amorphous structure of a mechanically alloyed Ti50Ni25Cu25 alloy using anomalous wide-angle x-ray scattering and reverse Monte Carlo simulations | An amorphous Ti50Ni25Cu25 alloy was produced by 19 h of mechanical alloying.
Anomalous wide angle x-ray scattering data were collected at six energies and
six total scattering factors were obtained. By considering the data collected
at two energies close to the Ni and Cu K edges, two differential anomalous
scattering factors around the Ni and Cu atoms were obtained, showing the
chemical environments around these atoms are different. The eight factors were
used as input data to the reverse Monte Carlo method used to compute the
partial structure factors STi-Ti(K), STi-Cu(K), STi-Ni(K), SCu-Cu(K), SCu-Ni(K)
and SNi-Ni(K). From their Fourier transformation, the partial pair distribution
functions GTi-Ti(r), GTi-Cu(r), GTi-Ni(r), GCu-Cu(r), GCu-Ni(r) and GNi-Ni(r)
were obtained, and the coordination numbers and interatomic atomic distances
for the first neighbors were determined. | 1304.4604v1 |
2013-09-12 | 12-band $\textbf{k}\cdot\textbf{p}$ model for dilute bismide alloys of (In)GaAs derived from supercell calculations | Incorporation of bismuth (Bi) in dilute quantities in (In)GaAs has been shown
to lead to unique electronic properties that can in principle be exploited for
the design of high efficiency telecomm lasers. This motivates the development
of simple models of the electronic structure of these dilute bismide alloys,
which can be used to evaluate their potential as a candidate material system
for optical applications. Here, we begin by using detailed calculations based
on an $sp^{3}s^{*}$ tight-binding model of (In)GaBi$_{x}$As$_{1-x}$ to verify
the presence of a valence band-anticrossing interaction in these alloys. Based
on the tight-binding model the derivation of a 12-band
$\textbf{k}\cdot\textbf{p}$ Hamiltonian for dilute bismide alloys is outlined.
We show that the band structure obtained from the 12-band model is in excellent
agreement with full tight-binding supercell calculations. Finally, we apply the
12-band model to In$_{0.53}$Ga$_{0.47}$Bi$_{x}$As$_{1-x}$ and compare the
calculated variation of the band gap and spin-orbit-splitting to a variety of
spectroscopic measurements performed on a series of MBE-grown
In$_{0.53}$Ga$_{0.47}$Bi$_{x}$As$_{1-x}$/InP layers. | 1309.3305v1 |
2014-01-23 | Magnetization, magnetostriction, and their relationship in Invar Fe$_{1-x}A_{x}$ ($A={\rm Pt},{\rm Ni}$) | A method is proposed for investigating the spontaneous magnetization, the
spontaneous volume magnetostriction, and their relationship in disordered
face-centered-cubic Fe$_{0.72}$Pt$_{0.28}$ and Fe$_{0.65}$Ni$_{0.35}$ in the
temperature interval $0 \leq T/T_{\rm C} < 1$. It relies on the disordered
local moment formalism and the observation that the reduced magnetization in
each of the investigated materials is accurately described by an equation of
the form $M(T)/M(0) = [ 1 -s (T/T_{\rm C})^{3/2}- (1-s)(T/T_{\rm C})^{p}
]^{q}$. The present approach yields interesting results. The alloys at zero
Kelvin share several physical properties: the volume in a partially disordered
local moment state shrinks as the fraction of Fe moments which point down
increases in the interval $0 < x^{{\rm Fe}\downarrow} < 1/2$, following closely
$V(0) - 4 [V(0)-V(1/2)] x^{{\rm Fe}\downarrow} (1-x^{{\rm Fe}\downarrow})$,
while the magnetization collapses, following closely $M(0) - 2 M(0) x^{{\rm
Fe}\downarrow}$; the volume in the homogeneous ferromagnetic state greatly
exceeds that in the disordered local moment state; $x^{{\rm Fe}\downarrow}(0)$
is close to zero. These common properties can account for a variety of
intriguing phenomena displayed by both alloys, including the anomaly in the
magnetostriction at zero Kelvin and, more surprisingly perhaps, the scaling
between the reduced magnetostriction and the reduced magnetization squared
below the Curie temperature. However, the thermal evolution of the fraction of
Fe moments which point down depends strongly on the alloy under consideration.
This, in turn, can explain the observed marked difference in the temperature
dependence of the reduced magnetization between the two alloys. | 1401.6089v2 |
2014-08-19 | Exploring quantum phase transition in Pd_{1-x}Ni_x nanoalloys | Pd$_{1-x}$Ni$_x$ alloy system is an established ideal transition metal system
possessing a composition induced paramagnetic to ferromagnetic quantum phase
transition (QPT) at the critical concentration $x_c \sim$ 0.026 in bulk. A
low-temperature non-Fermi liquid (NFL) behaviour around $x_c$ usually indicates
the presence of quantum criticality (QC) in this system. In this work, we
explore the existence of such a QPT in nanoparticles of this alloy system. We
synthesized single-phase, polydispersed and 40-50 nm mean diameter crystalline
nanoparticles of Pd$_{1-x}$Ni$_x$ alloys, with $x$ near $x_c$ and beyond, by a
chemical reflux method. In addition to the determination of the size,
composition, phase and crystallinity of the alloys by microscopic and
spectroscopic techniques, the existence of a possible QPT was explored by
resistivity and DC magnetization measurements. A dip in the value of the
exponent $n$ near $x_c$, and a concomitant peak in the constant $A$, of the
$AT^n$ dependence of the low temperature ($T$) resistivity indicate the
presence of a quantum-like phase transition in the system. The minimum value of
$n$, however, remains within the Fermi liquid regime ($n >$ 2). The DC
magnetization results suggest an anticipatory presence of a superparamagnetic
to ferromagnetic QPT in the mean-sized nanoparticles. The observation of a
possible quantum critical NFL behaviour ($n <$ 2) through resistivity is argued
to be inhibited by the electron-magnon scatterings present in the smaller
nanoparticles. | 1408.4316v1 |
2015-01-27 | From solid solution to cluster formation of Fe and Cr in $α$-Zr | To understand the mechanisms by which Fe and Cr additions increase the
corrosion rate of irradiated Zr alloys, a combination of experimental (atom
probe tomography, x-ray diffraction and thermoelectric power measurements) and
modelling (density functional theory) techniques are employed to investigate
the non-equilibrium solubility and clustering of Fe and Cr in binary Zr alloys.
Cr occupies both interstitial and substitutional sites in the {\alpha}-Zr
lattice, Fe favours interstitial sites, and a low-symmetry site that was not
previously modelled is found to be the most favourable for Fe. Lattice
expansion as a function of alloying concentration (in the dilute regime) is
strongly anisotropic for Fe additions, expanding the $c$-axis while contracting
the $a$-axis. Defect clusters are observed at higher solution concentrations,
which induce a smaller amount of lattice strain compared to the dilute defects.
In the presence of a Zr vacancy, all two-atom clusters are more soluble than
individual point defects and as many as four Fe or three Cr atoms could be
accommodated in a single Zr vacancy. The Zr vacancy is critical for the
increased solubility of defect clusters, the implications for irradiation
induced microstructure changes in Zr alloys are discussed. | 1501.06732v3 |
2015-03-13 | Broken Time Reversal Symmetry in Superconducting Pr1-xCexPt4Ge12 | We report results of zero-field muon spin relaxation experiments on the
filled-skutterudite superconductors~Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$, $x = 0$,
0.07, 0.1, and 0.2, to investigate the effect of Ce doping on broken
time-reversal symmetry (TRS) in the superconducting state. In these alloys
broken TRS is signaled by the onset of a spontaneous static local magnetic
field~$B_s$ below the superconducting transition temperature. We find that
$B_s$ decreases linearly with $x$ and $\to 0$ at $x \approx 0.4$, close to the
concentration above which superconductivity is no longer observed. The
(Pr,Ce)Pt$_4$Ge$_{12}$ and isostructural (Pr,La)Os$_4$Sb$_{12}$ alloy series
both exhibit superconductivity with broken TRS, and in both the decrease of
$B_s$ is proportional to the decrease of Pr concentration. This suggests that
Pr-Pr intersite interactions are responsible for the broken TRS\@. The two
alloy series differ in that the La-doped alloys are superconducting for all La
concentrations, suggesting that in (Pr,Ce)Pt$_4$Ge$_{12}$ pair-breaking by Ce
doping suppresses superconductivity. For all $x$ the dynamic muon spin
relaxation rate decreases somewhat in the superconducting state. This may be
due to Korringa relaxation by conduction electrons, which is reduced by the
opening of the superconducting energy gap. | 1503.03967v1 |
2015-04-20 | Engineering the electronic bandgaps and band edge positions in carbon-substituted 2D boron nitride: a first-principles investigation | Modification of graphene to open a robust gap in its electronic spectrum is
essential for its use in field effect transistors and photochemistry
applications. Inspired by recent experimental success in the preparation of
homogeneous alloys of graphene and boron nitride (BN), we consider here
engineering the electronic structure and bandgap of C$_{2x}$B$_{1-x}$N$_{1-x}$
alloys via both compositional and configurational modification. We start from
the BN end-member, which already has a large bandgap, and then show that (a)
the bandgap can in principle be reduced to about 2 eV with moderate
substitution of C $(x<0.25)$; and (b) the electronic structure of
C$_{2x}$B$_{1-x}$N$_{1-x}$ can be further tuned not only with composition $x$,
but also with the configuration adopted by C substituents in the BN matrix. Our
analysis, based on accurate screened hybrid functional calculations, provides a
clear understanding of the correlation found between the bandgap and the level
of aggregation of C atoms: the bandgap decreases most when the C atoms are
maximally isolated, and increases with aggregation of C atoms due to the
formation of bonding and anti-bonding bands associated with hybridization of
occupied and empty defect states. We determine the location of valence and
conduction band edges relative to vacuum and discuss the implications on the
potential use of 2D C$_{2x}$B$_{1-x}$N$_{1-x}$ alloys in photocatalytic
applications. Finally, we assess the thermodynamic limitations on the formation
of these alloys using a cluster expansion model derived from first-principles. | 1504.05062v1 |
2015-05-20 | Ab initio prediction of the mechanical properties of alloys: The case of Ni/Mn-doped ferromagnetic Fe | First-principles alloy theory, formulated within the exact muffin-tin
orbitals method in combination with the coherent-potential approximation, is
used to study the mechanical properties of ferromagnetic body-centered cubic
(bcc) Fe$_{1-x}$M$_x$ alloys (M=Mn or Ni, $0\le x \le 0.1$). We consider
several physical parameters accessible from \emph{ab initio} calculations and
their combinations in various phenomenological models to compare the effect of
Mn and Ni on the properties of Fe. Alloying is found to slightly alter the
lattice parameters and produce noticeable influence on elastic moduli. Both Mn
and Ni decrease the surface energy and the unstable stacking fault energy
associated with the $\{110\}$ surface facet and the $\{110\}\langle111\rangle$
slip system, respectively. Nickel is found to produce larger effect on the
planar fault energies than Mn. The semi-empirical ductility criteria by Rice
and Pugh consistently predict that Ni enhances the ductility of Fe but give
contradictory results in the case of Mn doping. The origin of the discrepancy
between the two criteria is discussed and an alternative measure of the
ductile-brittle behavior based on the theoretical cleavage strength and
single-crystal shear modulus $G\{110\}\langle111\rangle$ is proposed. | 1505.05443v1 |
2015-05-31 | Residual stress induced stabilization of martensite phase and its effect on the magneto-structural transition in Mn rich Ni-Mn-In/Ga magnetic shape memory alloys | The irreversibility of the martensite transition in magnetic shape memory
alloys (MSMAs) with respect to external magnetic field is one of the biggest
challenges that limits their application as giant caloric materials. This
transition is a magneto-structural transition that is accompanied with a steep
drop in magnetization (i.e., 'delta M') around the martensite start temperature
(Ms) due to the lower magnetization of the martensite phase. In this
communication, we show that 'delta M' around Ms in Mn rich Ni-Mn based MSMAs
gets suppressed by two orders of magnitude in crushed powders due to the
stabilization of the martensite phase at temperatures well above the Ms and the
austenite finish (Af) temperatures due to residual stresses. Analysis of the
intensities and the FWHM of the x-ray powder diffraction patterns reveals
stabilized martensite phase fractions as 97, 75 and 90% with corresponding
residual microstrains as 5.4, 5.6 and 3% in crushed powders of the three
different Mn rich Ni-Mn alloys, namely, Mn1.8Ni1.8In0.4, Mn1.75Ni1.25Ga and
Mn1.9Ni1.1Ga, respectively. Even after annealing at 773 K, the residual stress
stabilised martensite phase does not fully revert to the equilibrium cubic
austenite phase as the magneto-structural transition is only partially restored
with reduced value of 'delta M'. Our results have very significant bearing on
application of such alloys as inverse magnetocaloric and barocaloric materials. | 1506.00266v1 |
2015-06-17 | Two-dimensional connective nanostructures of electrodeposited Zn on Au(111) induced by spinodal decomposition | Phase-formation of surface alloying by spinodal decomposition has been
studied for the first time at an electrified interface. For this aim Zn was
electrodeposited on Au(111) from the ionic liquid AlCl3-MBIC (58:42) containing
1 mM Zn(II) at different potentials in the underpotential range corresponding
to submonolayer up to monolayer coverage. Structure evolution was observed by
in situ electrochemical scanning tunneling microscopy (STM) at different times
after starting the deposition via potential jumps and at temperatures of 298 K
and 323 K. Spinodal or labyrinth two-dimensional structures predominate at
middle coverage, both in deposition and dissolution experiments. They are
characterized by a length scale of typically 5 nm which has been determined
from the power spectral density of the STM images. Structure formation and
surface alloying is governed by slow kinetics with a rate constant k with
activation energy of 120 meV and preexponential factor of 0.17 Hz. The
evolution of the structural features is described by a continuum model and is
found to be in good agreement with the STM observations. From the experimental
and model calculation results we conclude that the two-dimensional
phase-formation in the Zn on Au(111) system is dominated by surface alloying.
The phase separation of a Zn-rich and a Zn-Au alloy phase is governed by 2D
spinodal decomposition. | 1506.05206v2 |
2015-06-22 | Structure of the glass-forming metallic liquids by ab-initio and classical molecular dynamics, a case study: quenching the Cu60Ti20Zr20 alloy | We consider the question of the amorphization of metallic alloys by melt
quenching, as predicted by molecular dynamics simulations with semi-empirical
potentials. The parametrization of the potentials is discussed on the example
of the ternary Cu-Ti-Zr transition metals alloy, using as reference the
ab-initio simulation. The pair structure in the amorphous state is computed
from a potential of the Stillinger Weber form. The transferability of the
parameters during the quench is investigated using two parametrizations: from
solid state data, as usual, and from a new parametrization on the liquid
structure. When the adjustment is made on the pair structure of the liquid, a
satisfactory transferability is found between the pure components and their
alloys. The liquid structure predicted in this way agrees well with experiment,
in contrast with the one obtained using the adjustment on the solid. The final
structure, after quenches down to the amorphous state, determined with the new
set of parameters is shown to be very close to the ab-initio one, the latter
being in excellent agreement with recent X-rays diffraction experiments. The
corresponding critical temperature of the glass transition is estimated from
the behavior of the heat capacity. Discussion of the consistency between the
structures predicted using semi-empirical potentials and ab-initio simulation,
and comparison of different experimental data underlines the question of the
dependence of the final structure on the thermodynamic path followed to reach
the amorphous state. | 1506.06530v3 |
2015-08-04 | Effect of Atomic Size and Valence Electron Concentration on the Formation of fcc or bcc Solid Solid Solutions in High Entropy Alloys | The possibility of solid solution formation in high entropy alloys (HEAs) has
been calculated for alloys with four to seven elements, using a rule previously
reported. Thirty elements were included: transition elements of the fourth,
fifth and sixth periods of the periodic table, and aluminum. A total of
2,799,486 systems were analyzed. The percentage of solid solutions that would
be formed in HEAs decreases from 35.9% to 26.4%, as the number of elements
increases from four to seven. The structure of the solid solutions, fcc, bcc or
a mixture of fcc and bcc, that would be formed, has been predicted using a
previously reported observation. The percentage of systems with fcc or bcc
structure decreases as the number of elements increases from four to seven. The
percentages of solid solutions with fcc, bcc or a mixture of fcc and bcc were
calculated, for alloys with four to seven elements, but maintaining one
constant element. Systems in which the constant element has valence electron
concentration, VEC, from three to four, would have bcc structure in around 50%
of the systems. Systems in which the constant element has VEC from ten to
twelve, around 75% of systems would present fcc structure. | 1508.00935v2 |
2016-01-13 | An Optimal Block Diagonal Preconditioner for Heterogeneous Saddle Point Problems in Phase Separation | The phase separation processes are typically modeled by Cahn-Hilliard
equations. This equation was originally introduced to model phase separation in
binary alloys, where phase stands for concentration of different components in
alloy. When the binary alloy under preparation is subjected to a rapid
reduction in temperature below a critical temperature, it has been
experimentally observed that the concentration changes from a mixed state to a
visibly distinct spatially separated two phase for binary alloy. This rapid
reduction in the temperature, the so-called "deep quench limit", is modeled
effectively by obstacle potential. The discretization of Cahn-Hilliard equation
with obstacle potential leads to a block $2 \times 2$ {\em non-linear} system,
where the $(1,1)$ block has a non-linear and non-smooth term. Recently a
globally convergent Newton Schur method was proposed for the non-linear Schur
complement corresponding to this non-linear system. The proposed method is
similar to an inexact active set method in the sense that the active sets are
first approximately identified by solving a quadratic obstacle problem
corresponding to the $(1,1)$ block of the block $2 \times 2$ system, and later
solving a reduced linear system by annihilating the rows and columns
corresponding to identified active sets. For solving the quadratic obstacle
problem, various optimal multigrid like methods have been proposed. In this
paper, we study a non-standard norm that is equivalent to applying a block
diagonal preconditioner to the reduced linear systems. Numerical experiments
confirm the optimality of the solver and convergence independent of problem
parameters on sufficiently fine mesh. | 1601.03230v1 |
2016-04-13 | A 3D Printed Superconducting Aluminium Microwave Cavity | 3D printing of plastics, ceramics, and metals has existed for several decades
and has revolutionized many areas of manufacturing and science. Printing of
metals in particular has found a number of applications in fields as diverse as
customized medical implants, jet engine bearings, and rapid prototyping in the
automotive industry. Whilst many techniques can be used for 3D printing metals,
they commonly rely on computer controlled melting or sintering of a metal alloy
powder using a laser or electron beam. The mechanical properties of parts
produced in such a way have been well studied, but little attention has been
paid to their electrical properties. Here we show that a microwave cavity
(resonant frequencies 9.9 and 11.2 GHz) 3D printed using an Al-12Si alloy
exhibits superconductivity when cooled below the critical temperature of
aluminium (1.2 K), with a performance comparable to the common 6061 alloy of
aluminium. Superconducting cavities find application in numerous areas of
physics, from particle accelerators to cavity quantum electrodynamics
experiments. The result is achieved even with a very large concentration of
non-superconducting silicon in the alloy of 12.18%, compared to Al-6061, which
has between 0.4 to 0.8%. Our results may pave the way for the possibility of 3D
printing superconducting cavity configurations that are otherwise impossible to
machine. | 1604.04301v2 |
2016-04-21 | A review of the effects of chemical and phase segregation on the mechanical behaviour of multi-phase steels | In the drive towards higher strength alloys, a diverse range of alloying
elements is employed to enhance their strength and ductility. Limited solid
solubility of these elements in steel leads to segregation during casting which
affects the entire down-stream processing and eventually the mechanical
properties of the finished product. Although it is thought that the presence of
continuous bands lead to premature failure, it has not been possible to verify
this link. This poses as increasingly greater risk for higher alloyed, higher
strength steels which are prone to centre-line segregation: it is thus vital to
be able to predict the mechanical behaviour of multi-phase (MP) steels under
loading.
This review covers the microstructure and properties of galvanised advanced
high strength steels with particular emphasis to their use in automotive
applications. In order to understand the origins of banding, the origins of
segregation of alloying elements during casting and partitioning in the solid
state will be discussed along with the effects on the mechanical behaviour and
damage evolution under (tensile) loading. Attention will also be paid to the
application of microstructural models in tailoring the production process to
enable suppression of the effects of segregation upon banding. Finally, the
theory and application of the experimental techniques used in this work to
elucidate the structure and properties will be examined. | 1604.06485v1 |
2016-05-28 | Clock statistics for 1d Schrödinger operators | We study the 1d Schr\"odinger operators with alloy type random supercritical
decaying potential and prove the clock convergence for the local statistics of
eigenvalues. We also consider, besides the standard i.i.d. case, more general
ones with exponentially decaying correlations. | 1605.08825v1 |
2016-06-11 | Random alloy fluctuations and structural inhomogeneities in $c$-plane In$_{x}$Ga$_{1-x}$N quantum wells: theory of ground and excited electron and hole states | We present a detailed theoretical analysis of the electronic structure of
$c$-plane InGaN/GaN quantum wells with indium contents varying between 10\% and
25\%. The electronic structure of the quantum wells is treated by means of an
atomistic tight-binding model, accounting for variations in strain and built-in
field due to random alloy fluctuations. Our analysis reveals strong
localisation effects in the hole states. These effects are found not only in
the ground states, but also the excited states. We conclude that localisation
effects persist to of order 100~meV into the valence band, for as little as
10\% indium in the quantum well, giving rise to a significant density of
localised states. We find, from an examination of the modulus overlap of the
wave functions, that the hole states can be divided into three regimes of
localisation. Our results also show that localisation effects due to random
alloy fluctuations are far less pronounced for electron states. However, the
combination of electrostatic built-in field, alloy fluctuations and structural
inhomogeneities, such as well-width fluctuations, can nevertheless lead to
significant localisation effects in the electron states, especially at higher
indium contents. Overall, our results are indicative of individually localised
electron and hole states, consistent with the experimentally proposed
explanation of time-dependent photoluminescence results in $c$-plane InGaN/GaN
QWs. | 1606.03616v1 |
2016-06-13 | Theoretical analysis of influence of random alloy fluctuations on the opto-electronic properties of site-controlled (111)-oriented InGaAs/GaAs quantum dots | We use an $sp^3d^5s^* $ tight-binding model to investigate the electronic and
optical properties of realistic site-controlled (111)-oriented InGaAs/GaAs
quantum dots. Special attention is paid to the impact of random alloy
fluctuations on key factors that determine the fine-structure splitting in
these systems. Using a pure InAs/GaAs quantum dot as a reference system, we
show that the combination of spin-orbit coupling and biaxial strain effects can
lead to sizeable spin-splitting effects in these systems. Then, a realistic
alloyed InGaAs/GaAs quantum dot with 25\% InAs content is studied. Our analysis
reveals that the impact of random alloy fluctuations on the electronic and
optical properties of (111)-oriented InGaAs/GaAs quantum dots reduces strongly
as the lateral size of the dot increases and approaches realistic sizes. For
instance the optical matrix element shows an almost vanishing anisotropy in the
(111)-growth plane. Furthermore, conduction and valence band mixing effects in
the system under consideration are strongly reduced compared to standard
(100)-oriented InGaAs/GaAs systems. All these factors strongly indicate a
reduced fine structure splitting in site-controlled (111)-oriented InGaAs/GaAs
quantum dots. Thus, we conclude that quantum dots with realistic (50-80~nm)
base length represent promising candidates for polarization entangled photon
generation, consistent with recent experimental data. | 1606.03980v1 |
2016-06-13 | Crystallographic features and state stability of the decagonal quasicrystal in the Al-Co-Cu alloy system | In the Al-Co-Cu alloy system, both the decagonal quasicrystal with the space
group of $P\overline{10}m2$ and its approximant Al$_{13}$Co$_4$ phase with
monoclinic $Cm$ symmetry are present around 20 at.% Co-10 at.% Cu. In this
study, we examined the crystallographic features of prepared Al-(30-x) at.%
Co-x at.% Cu samples mainly by transmission electron microscopy in order to
make clear the crystallographic relation between the decagonal quasicrystal and
the monoclinic Al$_{13}$Co$_4$ structure. The results revealed a coexistence
state consisting of decagonal quasicrystal and approximant Al$_{13}$Co$_4$
regions in Al-20 at.% Co-10 at.% Cu alloy samples. With the help of the
coexistence state, the orientation relationship was established between the
monoclinic Al$_{13}$Co$_4$ structure and the decagonal quasicrystal. In the
determined relationship, the crystallographic axis in the quasicrystal was
found to be parallel to the normal direction of the (010)$_{\rm m}$ plane in
the Al$_{13}$Co$_4$ structure, where the subscript m denotes the monoclinic
system. Based on data obtained experimentally, the state stability of the
decagonal quasicrystal was also examined in terms of the Hume-Rothery (HR)
mechanism on the basis of the nearly-free-electron approximation. It was found
that a model based on the HR mechanism could explain the crystallographic
features such as electron diffraction patterns and atomic arrangements found in
the decagonal quasicrystal. In other words, the HR mechanism is most likely
appropriate for the stability of the decagonal quasicrystal in the Al-Co-Cu
alloy system. | 1606.04043v2 |
2016-11-09 | First-principles Study on Formation of LPSO Structures for Ternary Alloys Revisited from Short-range Order | We investigate the formation of long-period stacking ordered (LPSO) structure
for Mg-Y-Zn ternary alloys based on the short-range order (SRO) tendency of
energetically competitive disordered phases. We find that unisotropic SRO
tendencies for structures with stacking faults cannot be simply interpreted by
arithmetic average of SRO for constituent fcc and hcp stackings, indicating
that the SRO should be significantly affected by periodically-introduced
stacking faults. We also find that SRO for neighboring Y-Zn pair, which should
have positive sign to form specific L12 type cluster found in LPSO, is strongly
affected by the distance between stacking faults: e.g., five atomic layer
distance does not prefer in-plane Y-Zn pair, while seven atomic layer distance
prefer both in- and inter-plane Y-Zn pair. These facts strongly indicate that
ordering tendency for the Mg-Y-Zn alloy is significantly dominated by the
stacking faults as well as their periodicity. We also systematically
investigate correlation between SRO for other Mg-RE-Zn (RE = La, Tb, Dy, Ho,
Er) alloys and the physical property of RE elements. We find that while SRO for
RE-Zn pair does not show effective correlation with atomic radius, it has
strong quadratic correlation with atomic radius considering unisotoropy along
in- and inter-plane directions. | 1611.02866v1 |
2017-03-08 | Chromium-vacancy clusters in dilute bcc Fe-Cr alloys: an ab initio study | Using an ab initio approach, we explore the stability of small vacancy and
vacancy-chromium clusters in dilute body-centred cubic Fe-Cr alloys. To explain
experimental observations described in C.D. Hardie et al., J. Nucl. Mater. 439,
33 (2013) and showing the occurrence of Cr segregation in low-Cr alloys, we
investigate if chromium can form stable bound configurations with vacancies in
alloys with chromium concentration below the low-temperature chromium
solubility limit of 10-11 at. %. We find that a single vacancy can attract up
to four Cr atoms in the most energetically favourable cluster configuration.
The binding energy of a cluster containing a single vacancy and from one to
eight Cr atoms can be well described by a linear function of the number of
chromium atoms in the second, third and fifth nearest neighbour coordination.
The magnetic origin of the binding energy trend is confirmed by a correlation
between the average value of the magnetic moment of a Cr atom and the binding
energy. Similar trends are also found for di-vacancy-Cr clusters, confirming
that they likely also characterise larger systems not yet accessible to ab
initio calculations. The ratio of the binding energy to the number of Cr atoms
increased more than twice in the di-vacancy case in comparison with a single
vacancy case. | 1703.02767v1 |
2017-11-14 | Cluster dynamics modeling of Mn-Ni-Si precipitates in ferritic-martensitic steel under irradiation | Mn-Ni-Si precipitates (MNSPs) are known to be responsible for
irradiation-induced hardening and embrittlement in structural alloys used in
nuclear reactors. Studies have shown that precipitation of the MNSPs in 9-Cr
ferritic-martensitic (F-M) alloys, such as T91, is strongly associated with
heterogeneous nucleation on dislocations, coupled with radiation-induced solute
segregation to these sinks. Therefore it is important to develop advanced
predictive models for Mn-Ni-Si precipitation in F-M alloys under irradiation
based on an understanding of the underlying mechanisms. Here we use a cluster
dynamics model, which includes multiple effects of dislocations, to study the
evolution of MNSPs in a commercial F-M alloy T91. The model predictions are
calibrated by data from proton irradiation experiments at 400 {\deg}C.
Radiation induced solute segregation at dislocations is evaluated by a
continuum model that is integrated into the cluster dynamics simulations,
including the effects of dislocations as heterogeneous nucleation sites. The
result shows that MNSPs in T91 are primarily irradiation-induced and, in
particular, both heterogeneous nucleation and radiation-induced segregation at
dislocations are necessary to rationalize the experimental observations. | 1711.05008v1 |
2017-12-18 | Large-scale atomistic simulations demonstrate dominant alloy disorder effects in GaBi$_x$As$_{1-x}$/GaAs multiple quantum wells | Bismide semiconductor materials and heterostructures are considered a
promising candidate for the design and implementation of photonic,
thermoelectric, photovoltaic, and spintronic devices. This work presents a
detailed theoretical study of the electronic and optical properties of
strongly-coupled GaBi$_x$As$_{1-x}$/GaAs multiple quantum well (MQW)
structures. Based on a systematic set of large-scale atomistic tight-binding
calculations, our results reveal that the impact of atomic-scale fluctuations
in alloy composition is stronger than the inter-well coupling effect, and plays
an important role in the electronic and optical properties of MQW structures.
Independent of QW geometry parameters, alloy disorder leads to a strong
confinement of charge carriers, a large broadening of the hole energies, and a
red shift in the ground-state transition wavelength. Polarisation-resolved
optical transition strengths exhibit a striking effect of disorder, where the
inhomogeneous broadening could exceed an order of magnitude for MQWs, in
comparison to a factor of about three for single quantum wells. The strong
influence of alloy disorder effects persists when small variations in the size
and composition of MQWs typically expected in a realistic experimental
environment are considered. The presented results highlight the limited scope
of continuum methods and emphasise on the need for large-scale atomistic
approaches to design devices with tailored functionalities based on the novel
properties of bismide materials. | 1712.06720v2 |
2018-01-16 | On the asymmetry of the forward and reverse martensitic transformations in shape memory alloys | Differential Scanning Calorimetric, DSC, runs taken during martensitic phase
transformations in shape memory alloys, often look differently during cooling
and heating. Similar asymmetry is observed e.g. for the numbers of hits or the
critical exponents of energy and amplitude distributions (\epsilon and \alpha,
respectively) in acoustic emission measurements. It is illustrated that, in
accordance with empirical correlations, the above asymmetry of acoustic noises
can be classified into two groups: the relative changes of the exponents during
cooling and heating (\gamma \epsilon=(\epsilon h-\epsilon c)/\epsilon c as well
as \gamma \alpha=(\alpha h-\alpha c)/\alpha c)) are either positive or
negative. For positive \gamma values the number of hits and the total energy of
acoustic emission are larger for cooling, and the situation is just the reverse
for negative asymmetry. Our interpretation is based on the different ways of
relaxation of the elastic strain energy during cooling as well as heating. It
is illustrated that if the relaxed fraction of the total elastic strain energy
(which would be stored without relaxations) during cooling is larger than the
corresponding relaxed fraction during heating, then the asymmetry is positive.
Magnetic emission noises, accompanied with martensitic phase transformations in
ferromagnetic alloys, show similar asymmetry than those observed for thermal
(DSC) and acoustic noises and depends on the constant external magnetic field
too. | 1801.05229v1 |
2018-06-03 | Tunable stacking fault energies by tailoring local chemical order in CrCoNi medium-entropy alloys | High-entropy alloys (HEAs) are an intriguing new class of metallic materials
due to their unique mechanical behavior. Achieving a detailed understanding of
structure-property relationships in these materials has been challenged by the
compositional disorder that underlies their unique mechanical behavior.
Accordingly, in this work, we employ first-principles calculations to
investigate the nature of local chemical order and establish its relationship
to the intrinsic and extrinsic stacking fault energy (SFE) in CrCoNi
medium-entropy solid-solution alloys, whose combination of strength, ductility
and toughness properties approach the best on record. We find that the average
intrinsic and extrinsic SFE are both highly tunable, with values ranging from
-43 mJ.m-2 to 30 mJ.m-2 and from -28 mJ.m-2 to 66 mJ.m-2, respectively, as the
degree of local chemical order increases. The state of local ordering also
strongly correlates with the energy difference between the face-centered cubic
(fcc) and hexagonal-close packed (hcp) phases, which affects the occurrence of
transformation-induced plasticity. This theoretical study demonstrates that
chemical short-range order is thermodynamically favored in HEAs and can be
tuned to affect the mechanical behavior of these alloys. It thus addresses the
pressing need to establish robust processing-structure-property relationships
to guide the science-based design of new HEAs with targeted mechanical
behavior. | 1806.00718v1 |
2018-11-05 | Outstanding Radiation Resistance of Tungsten-based High Entropy Alloys | A novel W-based refractory high entropy alloy with outstanding radiation
resistance has been developed. The alloy was grown as thin films showing a
bimodal grain size distribution in the nanocrystalline and ultrafine regimes
and a unique 4 nm lamella-like structure revealed by atom probe tomography
(APT). Transmission electron microscopy (TEM) and X-ray diffraction show an
underlying body-centered cubic crystalline structure with certain black spots
appearing after thermal annealing at elevated temperatures. Thorough analysis
based on TEM and APT correlated the black spots with second phase particles
rich in Cr and V. After both in situ and ex situ irradiation, these
precipitates evolve to quasi-spherical particles with no sign of
irradiation-created dislocation loops even after 8 dpa at either room
temperature or 1073 K. Furthermore, nanomechanical testing shows a large
hardness of 14 GPa in the as-deposited samples, with a slight increase after
thermal annealing and almost negligible irradiation hardening. Theoretical
modeling based on ab initio methodologies combined with Monte Carlo techniques
predicts the formation of Cr and V rich second phase particles and points at
equal mobilities of point defects as the origin of the exceptional radiation
tolerance. The fact that these alloys are suitable for bulk production coupled
with the exceptional radiation and mechanical properties makes them ideal
structural materials for applications requiring extreme conditions. | 1811.01915v1 |
2018-11-22 | Nonlinearity in Canonical Ensemble for Multicomponent Alloys Revisited from Structural Degree of Freedoms | For classical discrete system under constant composition typically referred
to substitutional alloys, we examine local nonlinearity in canonical average
phi . We have respectively investigated the local and global behavior of
nonlinearity through previously-introduced vector field A and through tropical
limit of the vector field. While these studies indicated the importance of
constraints to structural degree of freedoms (SDFs) for global nonlinearity, it
has been still unclear how the constraints to SDF affects local nonlinearity.
Based on statistical manifold, we make intuitive bridge between the SDF-based
information and local nonlinearity, decomposing the local nonlinearity into two
(for binary alloys with pair correlations) or three (for otherwise)
contributions in terms of the Kullback-Leibler divergence, where this
decomposition is independent of temperature and many-body interaction, and is
defined on individual configuration. We also find that we can provide
A-dependent as well as A-independent decomposition of the local nonlinearity,
where non-separability in SDFs and its nonadditive character is independent of
A, which indicates that information about evolution of the vector field should
be required to address the non-separability and nonadditivity. The present work
enables to quantify how configuration-dependent constraints to SDF affect local
nonlinearity in canonical average for multicomponent alloys. | 1811.09612v5 |
2018-11-23 | 3D Deep Learning with voxelized atomic configurations for modeling atomistic potentials in complex solid-solution alloys | The need for advanced materials has led to the development of complex,
multi-component alloys or solid-solution alloys. These materials have shown
exceptional properties like strength, toughness, ductility, electrical and
electronic properties. Current development of such material systems are
hindered by expensive experiments and computationally demanding
first-principles simulations. Atomistic simulations can provide reasonable
insights on properties in such material systems. However, the issue of
designing robust potentials still exists. In this paper, we explore a deep
convolutional neural-network based approach to develop the atomistic potential
for such complex alloys to investigate materials for insights into controlling
properties. In the present work, we propose a voxel representation of the
atomic configuration of a cell and design a 3D convolutional neural network to
learn the interaction of the atoms. Our results highlight the performance of
the 3D convolutional neural network and its efficacy in machine-learning the
atomistic potential. We also explore the role of voxel resolution and provide
insights into the two bounding box methodologies implemented for voxelization. | 1811.09724v1 |
2019-11-02 | Direct Bandgap Emission from Hexagonal Ge and SiGe Alloys | Silicon crystallized in the usual cubic (diamond) lattice structure has
dominated the electronics industry for more than half a century. However, cubic
silicon (Si), germanium (Ge) and SiGe-alloys are all indirect bandgap
semiconductors that cannot emit light efficiently. Accordingly, achieving
efficient light emission from group-IV materials has been a holy grail in
silicon technology for decades and, despite tremendous efforts, it has remained
elusive. Here, we demonstrate efficient light emission from direct bandgap
hexagonal Ge and SiGe alloys. We measure a subnanosecond,
temperature-insensitive radiative recombination lifetime and observe a similar
emission yield to direct bandgap III-V semiconductors. Moreover, we demonstrate
how by controlling the composition of the hexagonal SiGe alloy, the emission
wavelength can be continuously tuned in a broad range, while preserving a
direct bandgap. Our experimental findings are shown to be in excellent
quantitative agreement with the ab initio theory. Hexagonal SiGe embodies an
ideal material system to fully unite electronic and optoelectronic
functionalities on a single chip, opening the way towards novel device concepts
and information processing technologies. | 1911.00726v1 |
2020-01-17 | Methods of electron transport in ab initio theory of spin stiffness | We present an ab initio theory of the spin-wave stiffness tensor for ordered
and disordered itinerant ferromagnets with pair exchange interactions derived
from a method of infinitesimal spin rotations. The resulting formula bears an
explicit form of a linear-response coefficient which involves one-particle
Green's functions and effective velocity operators encountered in a recent
theory of electron transport. Application of this approach to ideal metal
crystals yields more reliable values of the spin stiffness than traditional
ill-convergent real-space lattice summations. The formalism can also be
combined with the coherent potential approximation for an effective-medium
treatment of random alloys, which leads naturally to an inclusion of
disorder-induced vertex corrections to the spin stiffness. The calculated
concentration dependence of the spin-wave stiffness of random fcc Ni-Fe alloys
can be ascribed to a variation of the reciprocal value of alloy magnetization.
Calculations for random iron-rich bcc Fe-Al alloys reveal that their spin-wave
stiffness is strongly reduced owing to the atomic ordering; this effect takes
place due to weakly coupled local magnetic moments of Fe atoms surrounded by a
reduced number of Fe nearest neighbors. | 2001.06279v2 |
2020-01-30 | Lattice dynamics of $\textit{Pnma}$ Sn(S$_{1-x}$Se$_{x}$) solid solutions: energetics, phonon spectra and thermal transport | Alloying is widely used as a means to fine-tune the properties of
thermoelectric materials by reducing the lattice thermal conductivity. However,
the effects of compositional variation on the lattice dynamics of alloy systems
are not well understood, due in part to the difficulty of building realistic
first-principles models of structurally-complex solid solutions. This work
builds on our previous study of Sn$_{n}$(S$_{1-x}$Se$_{x}$)$_{m}$ solid
solutions [Gunn $\textit{et al.}$, $\textit{Chem. Mater.}$ $\textbf{31}$,
$\textit{10}$, 3672, $\textbf{2019}$] to explore the lattice dynamics of the
$\textit{Pnma}$ Sn(S$_{1-x}$Se$_{x}$) system, which has been widely studied for
potential thermoelectric applications. We find that the vibrational internal
energy and entropy have a large quantitative impact on the mixing free energy
and are likely to be particularly important in alloy systems with competing
phases. The thermodynamically-averaged phonon dispersions and density of states
curves show that alloying preserves the structure of the low-frequency bands of
modes associated with the Sn sublattice but broadens the high-frequency
chalcogen bands into a near-continuous spectrum at the 50/50 mixed composition.
This results in a general reduction in the phonon mode group velocities and an
increase in the number of energy-conserving scattering channels for
heat-carrying low-frequency modes, which is consistent with the decrease in
thermal conductivity observed in experimental measurements. Finally, we discuss
some of the limitations of our first-principles modelling approach and propose
methods to address these in future studies. | 2001.11311v1 |
2021-07-20 | Controlled synthesis of MoxW1-xTe2 atomic layers with emergent quantum states | Recently, new states of matter like superconducting or topological quantum
states were found in transition metal dichalcogenides (TMDs) and manifested
themselves in a series of exotic physical behaviors. Such phenomena have been
demonstrated to exist in a series of transition metal tellurides including
MoTe2, WTe2 and alloyed MoxW1-xTe2. However, the behaviors in the alloy system
have been rarely addressed due to their difficulty in obtaining atomic layers
with controlled composition, albeit the alloy offers a great platform to tune
the quantum states. Here, we report a facile CVD method to synthesize the
MoxW1-xTe2 with controllable thickness and chemical composition ratios. The
atomic structure of monolayer MoxW1-xTe2 alloy was experimentally confirmed by
scanning transmission electron microscopy (STEM). Importantly, two different
transport behaviors including superconducting and Weyl semimetal (WSM) states
were observed in Mo-rich Mo0.8W0.2Te2 and W-rich Mo0.2W0.8Te2 samples
respectively. Our results show that the electrical properties of MoxW1-xTe2 can
be tuned by controlling the chemical composition, demonstrating our
controllable CVD growth method is an efficient strategy to manipulate the
physical properties of TMDCs. Meanwhile, it provides a perspective on further
comprehension and shed light on the design of device with topological
multicomponent TMDCs materials. | 2107.09482v1 |
2018-07-18 | Multi-Objective Bayesian Materials Discovery: Application on the Discovery of Precipitation Strengthened NiTi Shape Memory Alloys through Micromechanical Modeling | In this study, a framework for the multi-objective materials discovery based
on Bayesian approaches is developed. The capabilities of the framework are
demonstrated on an example case related to the discovery of precipitation
strengthened NiTi shape memory alloys with up to three desired properties. In
the presented case the framework is used to carry out an efficient search of
the shape memory alloys with desired properties while minimizing the required
number of computational experiments. The developed scheme features a Bayesian
optimal experimental design process that operates in a closed loop. A Gaussian
process regression model is utilized in the framework to emulate the response
and uncertainty of the physical/computational data while the sequential
exploration of the materials design space is carried out by using an optimal
policy based on the expected hyper-volume improvement acquisition function.
This scalar metric provides a measure of the utility of querying the materials
design space at different locations, irrespective of the number of objectives
in the performed task. The framework is deployed for the determination of the
composition and microstructure of precipitation-strengthened NiTi shape memory
alloys with desired properties, while the materials response as a function of
microstructure is determined through a thermodynamically-consistent
micromechanical model. | 1807.06868v1 |
2019-01-21 | Solid-State Thermal Energy Storage Using Reversible Martensitic Transformations | The identification and use of reversible Martensitic transformations,
typically described as shape memory transformations, as a new class of
solid-solid phase change material is experimentally demonstrated here for the
first time. To prove this claim, time-domain thermoreflectance,
frequency-domain thermoreflectance, and differential scanning calorimetry
studies were conducted on commercial NiTi alloys to quantify thermal
conductivity and latent heat. Additional Joule-heating experiments demonstrate
successful temperature leveling during transient heating and cooling in a
simulated environment. Compared to standard solid-solid materials and
solid-liquid paraffin, these experimental results show that shape memory alloys
provide up to a two order of magnitude higher Figure of Merit. Beyond these
novel experimental results, a comprehensive review of >75 binary NiTi and
NiTi-based ternary and quaternary alloys in the literature shows that shape
memory alloys can be tuned in a wide range of transformation temperatures (from
-50 to 330{deg}C), latent heats (from 9.1 to 35.1 J/g), and thermal
conductivities (from 15.6 to 28 W/mK). This can be accomplished by changing the
Ni and Ti balance, introducing trace elements, and/or by thermomechanical
processing. Combining excellent corrosion resistance, formability, high
strength and ductility, high thermal performance, and tunability, SMAs
represent an exceptional phase change material that circumvents many of the
scientific and engineering challenges hindering progress in this field. | 1901.06990v1 |
2019-04-16 | A phase-field study of elastic stress effects on phase separation in ternary alloys | Most of the commercially important alloys are multicomponent, producing
multiphase microstructures as a result of processing. When the coexisting
phases are elastically coherent, the elastic interactions between these phases
play a major role in the development of microstructures. To elucidate the key
effects of elastic stress on microstructural evolution when more than two
misfitting phases are present in the microstructure, we have developed a
microelastic phase-field model in two dimensions to study phase separation in
ternary alloy system. Numerical solutions of a set of coupled Cahn-Hilliard
equations for the composition fields govern the spatiotemporal evolution of the
three-phase microstructure. The model incorporates coherency strain
interactions between the phases using Khachaturyan's microelasticity theory. We
systematically vary the misfit strains (magnitude and sign) between the phases
along with the bulk alloy composition to study their effects on the
morphological development of the phases and the resulting phase separation
kinetics. We also vary the ratio of interfacial energies between the phases to
understand the interplay between elastic and interfacial energies on
morphological evolution. The sign and degree of misfit affect strain
partitioning between the phases during spinodal decomposition, thereby
affecting their compositional history and morphology. Moreover, strain
partitioning affects solute partitioning and alters the kinetics of coarsening
of the phases. The phases associated with higher misfit strain appear coarser
and exhibit wider size distribution compared to those having lower misfit. When
the interfacial energies satisfy complete wetting condition, phase separation
leads to development of stable core-shell morphology depending on the misfit
between the core (wetted) and the shell (wetting) phases. | 1904.07401v1 |
2019-07-27 | Elastic stiffness tensors of Zr-$x$Nb alloy in presence of defects: A molecular dynamics study | In a nuclear reactor, the Zr-$x$Nb alloy, which is used as a structural
material in the core region, is irradiated by energetic particles that cause
the atoms to be displaced from their lattice sites and giving rise to crystal
defects. The local changes in the atomic arrangements lead to local
deformations of the solid and thereby changes of its local mechanical
properties. Understanding the mechanisms behind this evolution in the core
region of a reactor, and its monitoring or controlling is a critical task in
nuclear industry. In this work, using extensive molecular dynamics simulations,
we have studied the effects of radiation damage on the local mechanical
properties of Zr-$x$Nb alloy. In the first step, the effect of Nb-concentration
on the mechanical stability of homogeneous Zr-$x$Nb alloy is investigated. In
the second step, we have studied the local changes of the elastic constants due
to local changes of the microstructure. These local changes include presence
and accumulation of vacancies in the form of dislocation loops or voids,
accumulation of Nb atoms in the form of clusters of different morphologies.
This study covers both cases of $T=0^\circ$K and finite temperatures up to
$T=600^\circ$K. | 1907.11977v2 |
2019-10-29 | Inward-growth plating of lithium driven by solid-solution based alloy phase for highly reversible lithium metal anode | Lithium metal batteries (LMB) are vital devices for high-energy-density
energy storage, but Li metal anode is highly reactive with electrolyte and
forms uncontrolled dendrite that can cause undesirable parasitic reactions thus
poor cycling stability and raise safety concerns. Despite remarkable progress
made to partly solve these issues, the Li metal still plate at the
electrode/electrolyte interface where the parasitic reactions and dendrite
formation invariably occur. Here we demonstrate the inward-growth plating of Li
into a metal foil while avoiding surface deposition, which is driven by the
reversible solid-solution based alloy phase change. Lithiation of the solid
solution alloy phase facilitates the freshly generated Li atoms at the surface
to sink into the foil, while the reversible alloy phase change is companied by
the dealloying reaction during delithiation, which extracts Li atoms from
inside of the foil. The yielded dendrite free Li anode produces an enhanced
Coulombic efficiency of 99.5 plus or minus 0.2% with a reversible capacity of
1660 mA h $g^{-1}$ (3.3 mA h cm$^{-2}$). | 1910.13159v1 |
2012-09-25 | Sb concentration dependent structural and resistive properties of polycrystalline Bi-Sb alloys | Polycrystalline Bi-Sb alloys have been synthesized over a wide range of
antimony concentration (8 at% to 20 at%) by solid state reaction method. In
depth structural analysis using X-Ray diffraction (XRD) and temperature
dependent resistivity measurement of synthesized samples have been performed.
XRD data confirmed single phase nature of polycrystalline samples and revealed
that complete solid solution is formed between bismuth and antimony. Rietveld
refinement technique, utilizing MAUD software, has been used to perform detail
structural analysis of the samples and lattice parameters of synthesized Bi-Sb
alloys have been estimated. Lattice parameter and unit cell volume decreases
monotonically with increasing antimony content. The variation of lattice
parameters with antimony concentration depicts a distinct slope change beyond
12 at% Sb content sample. Band gap has been estimated from the thermal
variation of resistivity data, with the 12% Sb content sample showing maximum
value. It has been observed that, with increasing antimony concentration the
transition from direct to indirect gap semiconductor is intimately related to
the variation of the estimated lattice parameters. Band diagram for the
polycrystalline Bi-Sb alloy system has also been proposed. | 1209.5506v1 |
2014-06-01 | Statistical Thermodynamics and Ordering Kinetics of D019-Type Phase: Application of the Models for H.C.P.-Ti-Al Alloy | Using the self-consistent field approximation, the static concentration waves
approach and the Onsager-type kinetics equations, the descriptions of both the
statistical thermodynamics and the kinetics of an atomic ordering of D019 phase
are developed and applied for h.c.p.-Ti-Al alloy. The model of order-disorder
phase transformation describes the phase transformation of h.c.p. solid
solution into the D019 phase. Interatomic-interaction parameters are estimated
for both approximations: one supposes temperature-independent
interatomic-interaction parameters, while the other one includes the
temperature dependence of interchange energies for Ti-Al alloy. The partial
Ti-Al phase diagrams (equilibrium compositions of the coexistent ordered and
disordered phases) are evaluated for both cases. The equation for the time
dependence of D019- type long-range order (LRO) parameter is analyzed. The
curves (showing the LRO parameter evolution) are obtained numerically for both
temperature-independent interaction energies and temperature-dependent ones.
Temperature dependence of the interatomic-interaction energies accelerates the
LRO relaxation and diminishes a spread of the values of instantaneous and
equilibrium LRO parameters versus the temperature. Both
statistical-thermodynamics and kinetics results show that equilibrium LRO
parameter for a non-stoichiometry (where an atomic fraction of alloying
component is more than 0.25) can be higher than for a stoichiometry at high
temperatures. The experimental phase diagram confirms the predicted (ordered or
disordered) states for h.c.p.-Ti-Al. | 1406.0162v1 |
2014-06-28 | Superdiffusive heat conduction in semiconductor alloys -- I. Theoretical foundations | Semiconductor alloys exhibit a strong dependence of effective thermal
conductivity on measurement frequency. So far this quasi-ballistic behaviour
has only been interpreted phenomenologically, providing limited insight into
the underlying thermal transport dynamics. Here, we show that quasi-ballistic
heat conduction in semiconductor alloys is governed by L\'evy superdiffusion.
By solving the Boltzmann transport equation (BTE) with ab initio phonon
dispersions and scattering rates, we reveal a transport regime with fractal
space dimension $1 < \alpha < 2$ and superlinear time evolution of mean square
energy displacement $\sigma^2(t) \sim t^{\beta} (1 < \beta < 2)$. The
characteristic exponents are directly interconnected with the order $n$ of the
dominant phonon scattering mechanism $\tau \sim \omega^{-n} (n>3)$ and
cumulative conductivity spectra $\kappa_{\Sigma}(\tau;\Lambda)\sim
(\tau;\Lambda)^{\gamma}$ resolved for relaxation times or mean free paths
through simple relations $\alpha = 3-\beta = 1 + 3/n = 2 - \gamma$. The
quasi-ballistic transport inside alloys is no longer governed by Brownian
motion, but instead dominated by L\'evy dynamics. This has important
implications for the interpretation of thermoreflectance (TR) measurements with
modified Fourier theory. Experimental $\alpha$ values for InGaAs and SiGe,
determined through TR analysis with a novel L\'evy heat formalism, match ab
initio BTE predictions within a few percent. Our findings lead to a deeper and
more accurate quantitative understanding of the physics of nanoscale heat flow
experiments. | 1406.7341v2 |
2018-02-27 | Computational study on microstructure evolution and magnetic property of laser additively manufactured magnetic materials | Additive manufacturing (AM) offers an unprecedented opportunity for the quick
production of complex shaped parts directly from a powder precursor. But its
application to functional materials in general and magnetic materials in
particular is still at the very beginning. Here we present the first attempt to
computationally study the microstructure evolution and magnetic properties of
magnetic materials (e.g. Fe-Ni alloys) processed by selective laser melting
(SLM). SLM process induced thermal history and thus the residual stress
distribution in Fe-Ni alloys are calculated by finite element analysis (FEA).
The evolution and distribution of the $\gamma$-Fe-Ni and FeNi$_3$ phase
fractions were predicted by using the temperature information from FEA and the
output from CALculation of PHAse Diagrams (CALPHAD). Based on the relation
between residual stress and magnetoelastic energy, magnetic properties of SLM
processed Fe-Ni alloys (magnetic coercivity, remanent magnetization, and
magnetic domain structure) are examined by micromagnetic simulations. The
calculated coercivity is found to be in line with the experimentally measured
values of SLM-processed Fe-Ni alloys. This computation study demonstrates a
feasible approach for the simulation of additively manufactured magnetic
materials by integrating FEA, CALPHAD, and micromagnetics. | 1802.09821v2 |
2018-08-22 | Record-High Superconductivity in Niobium-Titanium Alloy | Here we report the observation of extraordinary superconductivity in a
pressurized commercial niobium-titanium alloy. We find that its zero-resistance
superconductivity persists from ambient pressure to the pressure as high as
261.7 GPa, a record high pressure up to which a known superconducting state can
continuously survives. Remarkably, at such an ultra-high pressure, although the
ambient pressure volume is shrunk by 45% without structural phase transition,
the superconducting transition temperature (TC) increases to ~19.1 K from ~9.6
K, and the critical magnetic field (HC2) at 1.8 K has been enhanced to 19 T
from 15.4 T. These results set new records for both of the TC and the HC2 among
all the known alloy superconductors composed of only transition metal elements.
The remarkable high pressure superconducting properties observed in the NbTi
alloy not only expand our knowledge on this important commercial superconductor
but also are helpful for a better understanding on the superconducting
mechanism. | 1808.07215v3 |
2018-12-12 | Kinetic Monte Carlo simulations of vacancy diffusion in non-dilute Ni-X (X=Re,W,Ta) alloys | The mobility of vacancies in alloys may limit dislocation climb. Using a
combined density functional theory and kinetic Monte Carlo approach we
investigate vacancy diffusion in Ni-Re, Ni-W, and Ni-Ta binary alloys up to 10
at.% solute concentration. We introduce an interaction model that takes into
account the chemical environment close to the diffusing atom to capture the
effect of solute-host and solute-solute interactions on the diffusion barriers.
In contrast to an ideal solid solution it is not only the diffusion barrier of
the solute atom that influences the vacancy mobility, but primarily the change
in the host diffusion barriers due to the presence of solute atoms. This is
evidenced by the fact that the observed vacancy slowdown as a function of
solute concentration is larger in Ni-W than in Ni-Re, even though Re is a
slower diffuser than W. To model diffusion in complex, non-dilute alloys an
explicit treatment of interaction energies is thus unavoidable. In the context
of Ni-based superalloys two conclusions can be drawn from our kinetic Monte
Carlo simulations: the observed slowdown in vacancy mobility is not sufficient
to be the sole cause for the so-called Re-effect; and assuming a direct
correlation between vacancy mobility, dislocation climb, and creep strength the
experimentally observed similar effect of W and Re in enhancing creep strength
can be confirmed. | 1812.04989v1 |
2018-12-12 | Thermodynamic Stability and Structural Insights for CH$_3$NH$_3$Pb$_{1-x}$Si$_x$I$_3$, CH$_3$NH$_3$Pb$_{1-x}$Ge$_x$I$_3$, and CH$_3$NH$_3$Pb$_{1-x}$Sn$_x$I$_3$ Hybrid Perovskite Alloys: A Statistical Approach from First Principles Calculations | The recent reaching of 20% of conversion efficiency by solar cells based on
metal hybrid perovskites (MHP), e.g., the methylammonium (MA) lead iodide,
CH3NH3PbI3 (MAPbI3), has excited the scientific community devoted to the
photovoltaics materials. However, the toxicity of Pb is a hindrance for large
scale commercial of MHP and motivates the search of another congener
eco-friendly metal. Here, we employed first-principles calculations via density
functional theory combined with the generalized quasichemical approximation to
investigate the structural, thermodynamic, and ordering properties of
MAPb1-xSixI3, MAPb1-xGexI3, and MAPb1-xSnxI3 alloys as pseudo-cubic structures.
The inclusion of a smaller second metal, as Si and Ge, strongly affects the
structural properties, reducing the cavity volume occupied by the organic
cation and limitating the free orientation under high temperature effects.
Unstable and metaestable phases are observed at room temperature for
MAPb1-xSixI3, whereas MAPb1-xGexI3 is energetically favored for Pb-rich in
ordered phases even at very low temperatures. Conversely, the high miscibility
of Pb and Sn into MAPb1-xSnxI3 yields an alloy energetically favored as a
pseudo-cubic random alloy with tunable properties at room temperature. | 1812.05150v1 |
2019-02-05 | The Dynamics of Magnetism in Fe-Cr Alloys with Cr Clustering | The dynamics of magnetic moments in iron-chromium alloys with different
levels of Cr clustering show unusual features resulting from the fact that even
in a perfect body-centred cubic structure, magnetic moments experience
geometric magnetic frustration resembling that of a spin glass. Due to the long
range exchange coupling and configuration randomness, magnetic moments of Cr
solutes remain non-collinear at all temperatures. To characterise magnetic
properties of Fe-Cr alloys, we explore the temperature dependence of
magnetisation, susceptibility, Curie temperature and spin-spin correlations
with spatial resolution. The static and dynamic magnetic properties are
correlated with the microstructure of Fe-Cr, where magnetisation and
susceptibility are determined by the size of Cr precipitates at nominal Cr
concentrations. The Curie temperature is always maximised when the solute
concentration of Cr in the $\alpha$ phase is close to 5 to 6 at.\%, and the
susceptibility of Fe atoms is always enhanced at the boundary between a
precipitate and solid solution. Interaction between Cr and Fe stimulates
magnetic disorder, lowering the effective Curie temperature. Dynamic simulation
of evolution of magnetic correlations shows that the spin-spin relaxation time
in Fe-Cr alloys is in the 20 to 40 ps range. | 1902.01645v1 |
2019-06-18 | Machine-learned Interatomic Potentials for Alloys and Alloy Phase Diagrams | We introduce machine-learned potentials for Ag-Pd to describe the energy of
alloy configurations over a wide range of compositions. We compare two
different approaches. Moment tensor potentials (MTP) are polynomial-like
functions of interatomic distances and angles. The Gaussian Approximation
Potential (GAP) framework uses kernel regression, and we use the Smooth Overlap
of Atomic Positions (SOAP) representation of atomic neighbourhoods that
consists of a complete set of rotational and permutational invariants provided
by the power spectrum of the spherical Fourier transform of the neighbour
density. Both types of potentials give excellent accuracy for a wide range of
compositions and rival the accuracy of cluster expansion, a benchmark for this
system. While both models are able to describe small deformations away from the
lattice positions, SOAP-GAP excels at transferability as shown by sensible
transformation paths between configurations, and MTP allows, due to its lower
computational cost, the calculation of compositional phase diagrams. Given the
fact that both methods perform as well as cluster expansion would but yield
off-lattice models, we expect them to open new avenues in computational
materials modeling for alloys. | 1906.07816v2 |
2020-04-14 | Templated Dewetting-Alloying of NiCu Bilayers on TiO$_2$ Nanotubes Enables Efficient Noble Metal-Free Photocatalytic $H_2$ Evolution | Photocatalytic $H_2$ evolution reactions on pristine TiO$_2$ is characterized
by low efficiencies due to trapping and recombination of charge carriers, and
due to a sluggish kinetics of electron transfer. Noble metal (mainly Pt, Pd,
Au) nanoparticles are typically decorated as cocatalyst on the TiO$_2$ surface
to reach reasonable photocatalytic yields. However, owing to the high cost of
noble metals, alternative metal cocatalysts are being developed. Here we
introduce an approach to fabricate an efficient noble metal free photocatalytic
platform for $H_2$ evolution based on alloyed NiCu cocatalytic nanoparticles at
the surface of anodic TiO$_2$ nanotube arrays. NiCu bilayers are deposited onto
the TiO$_2$ nanotubes by plasma sputtering. A subsequent thermal treatment is
carried out that leads to dewetting, that is, owing to surface diffusion the Ni
and Cu sputtered layers simultaneously mix with each other and split into NiCu
nanoparticles at the nanotube surface. The approach allows for a full control
over key features of the alloyed nanoparticles such as their composition, work
function and cocatalytic ability towards $H_2$ generation. Dewetted-alloyed
cocatalytic nanoparticles composed of equal Ni and Cu amounts not only are
significantly more reactive than pure Ni or Cu nanoparticles, but also lead to
$H_2$ generation rates that can be comparable to those obtained by conventional
noble metal (Pt) decoration of TiO$_2$ nanotube arrays. | 2004.06561v1 |
2020-04-27 | Understanding Mechanical Properties and Failure Mechanism of Germanium-Silicon Alloy at Nanoscale | We used molecular dynamics (MD) simulations to investigate the mechanical
properties of cubic zinc blende (ZB) Si0.5Ge0.5 alloy nanowire (NW). Tersoff
potential is employed to elucidate the effect of nanowire size, crystal
orientations, and temperature on the material properties. We found that the
reduction in the cross-sectional area results in lower ultimate tensile
strength and Youngs modulus of this alloy which can be attributed to the
increased surface to volume ratio. The [111] oriented Si0.5Ge0.5 NW exhibits
the highest fracture strength compared to other crystal orientations but [110]
orientation possesses the highest fracture toughness. The effect of temperature
depicts an inverse relationship with the ultimate tensile strength and Youngs
modulus. The increased temperature facilitates the failure of the material,
thus degrades the materials strength. Our study reveals that the vacancy
defects introduced via removal of either Si or Ge atoms exhibit similar
behavior, and with the increase in vacancy concentration, both ultimate tensile
strength and Youngs modulus reduces linearly. We further illustrate the failure
characteristics of Si0.5Ge0.5 NW at two extremely low and high temperatures.
The intrinsic failure characteristics of Si0.5Ge0.5 alloy is found to be
insensitive to the temperature. Interestingly, at both temperatures, with the
increasing strain, the cross-section of Si0.5Ge0.5 eventually resembles a neck
as typically observed in ductile materials, although the NW failure is brittle
in nature. Overall, this work offers a new perspective on understanding
material properties and failure characteristics of ZB Si0.5Ge0.5 NW that will
be a guide for designing Si-Ge based nanodevices. | 2004.13445v1 |
2020-05-17 | Solving the issues of multicomponent diffusion in an equiatomic NiCoFeCr medium entropy alloy | Estimating the diffusion coefficients experimentally in a four-component
inhomogeneous alloy following the conventional diffusion couple method by
intersecting three couples at the same composition is difficult unless a small
composition range of constant diffusivity is identified. Additionally, the
intrinsic diffusion coefficients of the components cannot be estimated in a
system with more than two components. To solve these issues, we have followed
the pseudo-binary and pseudo-ternary diffusion couple methods for estimating
the diffusion coefficients at the equiatomic composition of NiCoFeCr medium
entropy alloy. Along with the pseudo-binary interdiffusion coefficients, we
have estimated the intrinsic diffusion coefficients of all the components by
designing the pseudo-binary couples such that Ni and Co develop the diffusion
profiles keeping Fe and Cr constant in one couple and Fe and Cr develop the
diffusion profiles keeping Ni and Co constant in another couple. Subsequently,
we have proposed the relations for calculating the tracer diffusion
coefficients utilizing the thermodynamic details. We have found a good match
with the data estimated directly following the radiotracer method at the
equiatomic composition. Following, we have produced three pseudo-ternary
diffusion couples intersecting at the compositions close to the equiatomic
composition. The main pseudo-ternary interdiffusion coefficients of Fe are
found to be higher than Ni and Co. Therefore, we have estimated different types
of diffusion coefficients highlighting the complex diffusion process in the
four-component NiCoFeCr medium entropy alloy. | 2005.08160v1 |
2020-08-01 | Electrical and thermal transport properties of medium-entropy SiyGeySnx alloys | Electrical and thermal transport properties of disordered materials have long
been of both theoretical interest and engineering importance. As a new class of
materials with an intrinsic compositional disorder, high/medium-entropy alloys
(HEAs/MEAs) are being immensely studied mainly for their excellent mechanical
properties. By contrast, electrical and thermal transport properties of
HEAs/MEAs are less well studied. Here we investigate these two properties of
silicon (Si)-germanium (Ge)-tin (Sn) MEAs, where we keep the same content of Si
and Ge while increasing the content of Sn from 0 to 1/3 to tune the
configurational entropy and thus the degree of compositional disorder. We
predict all SiyGeySnx MEAs to be semiconductors with a wide range of bandgaps
from near-infrared (0.28 eV) to visible (1.11 eV) in the light spectrum. We
find that the bandgaps and effective carrier masses decrease with increasing Sn
content. As a result, increasing the compositional disorder in SiyGeySnx MEAs
enhances their electrical conductivity. For the thermal transport properties of
SiyGeySnx MEAs, our molecular dynamics simulations show an opposite trend in
the thermal conductivity of these MEAs at room temperature, which decreases
with increasing compositional disorder, owing to enhanced Anderson localization
and strong phonon-phonon anharmonic interactions. The enhanced electrical
conductivity and weakened thermal conductivity make SiyGeySnx MEAs with high Sn
content promising functional materials for thermoelectric applications. Our
work demonstrates that HEAs/MEAs not only represent a new class of structural
alloys but also a novel category of functional alloys with unique electrical
and thermal transport properties. | 2008.00352v1 |
2020-08-12 | Phase stability of Au-Li binary systems studied using neural network potential | The miscibility of Au and Li exhibits a potential application as an adhesion
layer and electrode material in secondary batteries. Here, to explore alloying
properties, we constructed a neural network potential (NNP) of Au-Li binary
systems based on density functional theory (DFT) calculations. To accelerate
construction of NNPs, we proposed an efficient and inexpensive method of
structural dataset generation. The predictions by the constructed NNP on
lattice parameters and phonon properties agree well with those obtained by DFT
calculations. We also investigated the mixing energy of Au$_{1-x}$Li$_{x}$ with
fine composition grids, showing excellent agreement with DFT verifications. We
found the existence of various compositions with structures on and slightly
above the convex hull, which can explain the lack of consensus on the Au-Li
stable phases in previous studies. Moreover, we newly found
Au$_{0.469}$Li$_{0.531}$ as a stable phase, which has never been reported
elsewhere. Finally, we examined the alloying process starting from the phase
separated structure to the complete mixing phase. We found that when multiple
adjacent Au atoms dissolved into Li, the alloying of the entire Au/Li interface
started from the dissolved region. This paper demonstrates the applicability of
NNPs toward miscible phases and provides the understanding of the alloying
mechanism. | 2008.05094v1 |
2020-08-16 | High-throughput computational characterization of two-dimensional compositionally complex transition-metal chalcogenide alloys | Two-dimensional (2D) binary transition-metal chalcogenides (TMCs) like
molybdenum disulfide exhibits excellent properties as materials for light
adsorption devices. Alloying binary TMCs can form 2D compositionally complex
TMC alloys (CCTMCAs) that possess remarkable properties from the constituent
TMCs. We adopt a high-throughput workflow performing density functional theory
(DFT) calculations based on the virtual crystal approximation (VCA) model
(VCA-DFT). We test the workflow by predicting properties including in-plane
lattice constants, band gaps, effective masses, spin-orbit coupling (SOC), and
band alignments of the Mo-W-S-Se, Mo-W-S-Te, and Mo-W-Se-Te 2D CCTMCAs. We
validate the VCA-DFT results by computing the same properties using unit cells
and supercells of selected compositions. The VCA-DFT results of the
abovementioned five properties are comparable to that of DFT calculations, with
some inaccuracies in several properties of MoSTe and WSTe. Moreover, 2D CCTMCAs
can form type II heterostructures as used in photovoltaics. Finally, we use
Mo0.5W0.5SSe, Mo0.5W0.5STe, and Mo0.5W0.5SeTe 2D CCTMCAs to demonstrate the
room-temperature entropy-stabilized alloys. They also exhibit high electrical
conductivities at 300K, promising for light adsorption devices. Our work shows
that the high-throughput workflow using VCA-DFT calculations provides a
tradeoff between efficiency and accuracy, opening up opportunities in the
computational design of other 2D CCTMCAs for various applications. | 2008.06838v1 |
2020-08-17 | Enhancement of spin Hall conductivity in W-Ta alloy | Generating pure spin currents via the spin Hall effect in heavy metals has
been an active topic of research in the last decade. In order to reduce the
energy required to efficiently switch neighbouring ferromagnetic layers for
applications, one should not only increase the charge- to-spin conversion
efficiency but also decrease the longitudinal resistivity of the heavy metal.
In this work, we investigate the spin Hall conductivity in W_{1-x}Ta_{x} /
CoFeB / MgO (x = 0 - 0.2) using spin torque ferromagnetic resonance
measurements. Alloying W with Ta leads to a factor of two change in both the
damping-like effective spin Hall angle (from - 0.15 to - 0.3) and longitudinal
resistivity (60 - 120 {\mu}W cm). At 11% Ta concentration, a remarkably high
spin Hall angle value of - 0.3 is achieved with a low longitudinal resistivity
100 {\mu}W cm, which could lead to a very low power consumption for this
W-based alloy. This work demonstrates sputter-deposited W-Ta alloys could be a
promising material for power-efficient spin current generation. | 2008.07572v1 |
2021-03-03 | On the early stages of precipitation during direct ageing of Alloy 718 | The Ni-based superalloy Alloy 718 is used in aircraft engines as
high-pressure turbine discs and must endure challenging demands on
high-temperature yield strength, creep-, and oxidation-resistance. Nanoscale
$\gamma^{\prime}$- and $\gamma^{\prime \prime}$-precipitates commonly found in
duplet and triplet co-precipitate morphologies provide high-temperature
strength under these harsh operating conditions. Direct ageing of Alloy 718 is
an attractive alternative manufacturing route known to increase the yield
strength at 650 $^{\deg}$C by at least +10 $\%$, by both retaining high
dislocation densities and changing the nanoscale co-precipitate morphology.
However, the detailed nucleation and growth mechanisms of the duplet and
triplet co-precipitate morphologies of $\gamma^{\prime}$ and $\gamma^{\prime
\prime}$ during the direct ageing process remain unknown. We provide a
correlative high-resolution microscopy approach using transmission electron
microscopy, high-angle annular dark-field imaging, and atom probe microscopy to
reveal the early stages of precipitation during direct ageing of Alloy 718.
Quantitative stereological analyses of the $\gamma^{\prime}$- and
$\gamma^{\prime \prime}$-precipitate dispersions as well as their chemical
compositions have allowed us to propose a qualitative model of the
microstructural evolution. It is shown that fine $\gamma^{\prime}$- and
$\gamma^{\prime \prime}$-precipitates nucleate homogeneously and grow
coherently. However, $\gamma^{\prime \prime}$-precipitates also nucleate
heterogeneously on dislocations and experience accelerated growth due to Nb
pipe diffusion. Moreover, the co-precipitation reactions are largely influenced
by solute availability and the potential for enrichment of Nb and rejection of
Al+Ti. | 2103.02763v1 |
2021-03-18 | In Operando magnetometry study on the charge storage mechanism of SnCo alloy lithium ion batteries | In view of the long-standing controversy over the reversibility of transition
metals in Sn-based alloys as anode for Li-ion batteries, an in situ real-time
magnetic monitoring method was used to investigate the evolution of Sn-Co
intermetallic during the electrochemical cycling. Sn-Co alloy film anodes with
different compositions were prepared via magnetron sputtering without using
binders and conductive additives. The magnetic responses showed that the Co
particles liberated by Li insertion recombine fully with Sn during the
delithiation to reform Sn-Co intermetallic into stannum richer phases Sn7Co3.
However, as the Co content increases, it can only recombine partially with Sn
into cobalt richer phases Sn3Co7. The unconverted Co particles may form a dense
barrier layer and prevent the full reaction of Li with all the Sn in the anode,
leading to lower capacities. These critical results shed light on understanding
the reaction mechanism of transition metals, and provide valuable insights
toward the design of high-performance Sn alloy based anodes. | 2103.10141v1 |
2022-02-08 | Kinetic Monte Carlo Modelling of Nano-oxide Precipitation and its Associated Stability under Neutron Irradiation for the Fe-Ti-Y-O system | While developing nuclear materials, predicting their behavior under long-term
irradiation regimes spanning decades poses a significant challenge. We
developed a novel Kinetic Monte Carlo (KMC) model to explore the precipitation
behavior of Y-Ti-O oxides along grain boundaries within nanostructured ferritic
alloys (NFA). This model also assessed the response of the oxides to neutron
irradiation, even up simulated radiation damage levels in the desired long dpa
range for reactor components. Our simulations investigated how temperature and
grain boundary sinks influenced the oxide characteristics of a 12YWT-like alloy
during heat treatments at 1023 K, 1123 K, and 1223 K. The oxide characteristics
observed in our simulations were in good agreement with existing literature.
Furthermore, the impact of grain boundaries on precipitation was found to be
minimal. The resulting oxide configurations and positions were used in
subsequent simulations that exposed them to simulated neutron irradiation to a
total accumulated dose of 8 dpa at three temperatures: 673 K, 773 K, and 873 K,
and at dose rates of 10^(-3), 10^(-4), and 10^(-5) dpa/s. This demonstrated the
expected inverse relationship between oxide size and dose rate. In a long-term
irradiation simulation at 873 K and 10^(-3) dpa/s was taken out to 66 dpa and
found the oxides in the vicinity of the grain boundary were more susceptible to
dissolution. Additionally, we conducted irradiation simulations of a 14YWT-like
alloy to reproduce findings from neutron irradiation experiments. The larger
oxides in the 14YWT-like alloy did not dissolve and displayed stability similar
to the experimental results. | 2202.03641v3 |
2022-02-28 | Influence of hydrogen vacancy interactions on natural and artificial ageing of an AlMgSi alloy | The influence of hydrogen on the structural evolutions of an Al-Mg-Si alloy
during natural and artificial ageing was investigated experimentally. The aim
of the study was a better understanding of interactions between hydrogen and
crystalline defects and especially vacancies. Experimental data demonstrate
that during natural ageing in hydrogen environment, the hardening response is
delayed. This is attributed to a slower recovery of excess vacancies linked to
a lower mobility. To confirm and quantify the influence of hydrogen on the
vacancy migration energy, artificial ageing was carried out in conditions where
the vacancy concentration is constant. Hence, long-time annealing treatments
were carried out to investigate the influence of hydrogen on the coarsening of
rod-shaped precipitates. Using transmission electron microscopy and atom probe
tomography, it was demonstrated that the precipitate volume fraction and
composition are unchanged under H2 atmosphere but the coarsening kinetic is
significantly reduced. This leads to a delayed softening, in good agreement
with theoretical estimates. Thus, even a low concentration of hydrogen in solid
solution significantly affects the mobility of alloying elements in the
aluminium matrix. This is the result of hydrogen-vacancy interactions that lead
to an increase of the vacancy migration energy. Based on classical coarsening
theories, it was possible to demonstrate that this increase is of about 5% for
a concentration of hydrogen close to the vacancy concentration. | 2202.13745v1 |
2022-03-10 | 3D Nanoscale Mapping of Short-Range Order in GeSn Alloys | GeSn on Si has attracted much research interest due to its tunable direct
bandgap for mid-infrared applications. Recently, short-range order (SRO) in
GeSn alloys has been theoretically predicted, which profoundly impacts the band
structure. However, characterizing SRO in GeSn is challenging. Guided by
physics-informed Poisson statistical analyses of Kth-nearest neighbors (KNN) in
atom probe tomography, a new approach is demonstrated here for 3D nanoscale SRO
mapping and semi-quantitative strain mapping in GeSn. For GeSn with ~14 at.%
Sn, the SRO parameters of Sn-Sn 1NN in 10x10x10 nm$^{3}$ nanocubes can deviate
from that of the random alloys by $\pm$15%. The relatively large fluctuation of
the SRO parameters contributes to band-edge softening observed optically. Sn-Sn
1NN also tends to be more favored towards the surface, less favored under
strain relaxation or tensile strain, while almost independent of local Sn
composition. An algorithm based on least square fit of atomic positions further
verifies this Poisson-KNN statistical method. Compared to existing macroscopic
spectroscopy or electron microscopy techniques, this new APT statistical
analysis uniquely offers 3D SRO mapping at nanoscale resolution in a relatively
large volume with millions of atoms. It can also be extended to investigate SRO
in other alloy systems. | 2203.05667v1 |
2022-03-14 | Nontrivial doping evolution of electronic properties in Ising-superconducting alloys | Transition metal dichalcogenides offer unprecedented versatility to engineer
2D materials with tailored properties to explore novel structural and
electronic phase transitions. In this work, we present the atomic-scale
evolution of the electronic ground state of a monolayer of
Nb$_{1-\delta}$Mo$_{\delta}$Se$_2$ across the entire alloy composition range (0
< ${\delta}$ < 1) using low-temperature (300 mK) scanning tunneling microscopy
and spectroscopy (STM/STS). In particular, we investigate the atomic and
electronic structure of this 2D alloy throughout the metal to semiconductor
transition (monolayer NbSe$_2$ to MoSe$_2$). Our measurements let us extract
the effective doping of Mo atoms, the bandgap evolution and the band shifts,
which are monotonic with ${\delta}$. Furthermore, we demonstrate that
collective electronic phases (charge density wave and superconductivity) are
remarkably robust against disorder. We further show that the superconducting TC
changes non-monotonically with doping. This contrasting behavior in the normal
and superconducting state is explained using first-principles calculations. We
show that Mo doping decreases the density of states at the Fermi level and the
magnitude of pair-breaking spin fluctuations as a function of Mo content. Our
results paint a detailed picture of the electronic structure evolution in 2D
TMD alloys, which is of utmost relevance for future 2D materials design. | 2203.07432v1 |
2022-03-17 | In situ characterization of vacancy ordering in Ge-Sb-Te phase-change memory alloys | Tailoring the degree of structural disorder in Ge-Sb-Te alloys is important
for the development of non-volatile phase-change memory and neuro-inspired
computing. Upon crystallization from the amorphous phase, these alloys form a
cubic rocksalt-like structure with a high content of intrinsic vacancies.
Further thermal annealing results in a gradual structural transition towards a
layered structure and an insulator-to-metal transition. In this work, we
elucidate the atomic-level details of the structural transition in crystalline
GeSb2Te4 by in situ high-resolution transmission electron microscopy (HRTEM)
experiments and ab initio density functional theory (DFT) calculations,
providing a comprehensive real-time and real-space view of the vacancy ordering
process. We also discuss the impact of vacancy ordering on altering the
electronic and optical properties of GeSb2Te4, which is relevant to multilevel
storage applications. The phase evolution paths in Ge-Sb-Te alloys are
illustrated using a summary diagram, which serves as a guide for designing
phase-change memory devices. | 2203.09310v2 |
2022-03-21 | Sm-Co-based amorphous alloy films for zero-field operation of transverse thermoelectric generation | Transverse thermoelectric generation using magnetic materials is essential to
develop active thermal engineering technologies, for which the improvements of
not only the thermoelectric output but also applicability and versatility are
required. In this study, using combinatorial material science and lock-in
thermography technique, we have systematically investigated the transverse
thermoelectric performance of Sm-Co-based alloy films. The high-throughput
material investigation revealed the best Sm-Co-based alloys with the large
anomalous Nernst effect (ANE) as well as the anomalous Ettingshausen effect
(AEE). In addition to ANE/AEE, we discovered unique and superior material
properties in these alloys: the amorphous structure, low thermal conductivity,
and large in-plane coercivity and remanent magnetization. These properties make
it advantageous over conventional materials to realize heat flux sensing
applications based on ANE, as our Sm-Co-based films can generate thermoelectric
output without an external magnetic field. Importantly, the amorphous nature
enables the fabrication of these films on various substrates including flexible
sheets, making the large-scale and low-cost manufacturing easier. Our
demonstration will provide a pathway to develop flexible transverse
thermoelectric devices for smart thermal management. | 2203.10737v2 |
2022-03-22 | Silver-Gold Bimetallic Alloy versus Core-Shell Nanoparticles: Implications for Plasmonic Enhancement and Photothermal Applications | Bimetallic plasmonic nanoparticles enable tuning of the optical response and
chemical stability by variation of the composition. The present numerical
simulation study compares Ag-Au alloy, Ag@Au core-shell, and Au@Ag core-shell
bimetallic plasmonic nanoparticles of both spherical and anisotropic
(nanotriangle and nanorods) shapes. By studying both spherical and anisotropic
(with LSPR in the near-infrared region) shapes, cases with and without
interband transitions of Au can be decoupled. Explicit comparisons are
facilitated by numerical models supported by careful validation and examination
of optical constants of Au-Ag alloys reported in literature. Although both
Au-Ag core-shell and alloy nanoparticles exhibit an intermediary optical
response between that of pure Ag and Au nanoparticles, there are noticeable
differences in the spectral characteristics. Also, the effect of the bimetallic
constitution in anisotropic nanoparticles is starkly different from that in
spherical nanoparticles due to the absence of Au interband transitions in the
former case. In general, the improved chemical stability of Ag nanoparticles by
incorporation of Au comes with a cost of reduction in plasmonic enhancement,
also applicable to anisotropic nanoparticles with a weaker effect. A
photothermal heat transfer study confirms that increased absorption by the
incorporation of Au in spherical Ag nanoparticles also results in an increased
steady state temperature. On the other hand, anisotropic nanoparticles are
inherently better absorbers, hence better photothermal sources and their
photothermal properties are apparently not strongly affected by the
incorporation of one metal in the other. | 2203.11682v1 |
2022-03-28 | The influence of alloying on slip intermittency and the implications for dwell fatigue in titanium | Dwell fatigue, the reduction in fatigue life experienced by titanium alloys
due to holds at stresses as low as 60% of yield, has been implicated in several
uncontained jet engine failures. Dislocation slip has long been observed to be
an intermittent, scale-bridging phenomenon, similar to that seen in earthquakes
but at the nanoscale, leading to the speculation that large stress bursts might
promote the initial opening of a crack. Here we observe such stress bursts at
the scale of individual grains in situ, using high energy X-ray diffraction
microscopy in Ti-7Al-O alloys. This shows that the detrimental effect of
precipitation of ordered Ti_3Al is to increase the magnitude of rare pri<a> and
bas<a> slip bursts associated with slip localisation. In contrast, the addition
of trace O interstitials is beneficial, reducing the magnitude of bas<a> slip
bursts and increasing the homogeneity between basal and prismatic <a> slip.
This is further evidence that the formation of long paths for easy basal plane
slip localisation should be avoided when engineering titanium alloys against
dwell fatigue. | 2203.14615v3 |
2022-03-29 | Realistic micromagnetic description of all-optical ultrafast switching processes in ferrimagnetic alloys | Both helicity-independent and helicity-dependent all-optical switching
processes driven by single ultrashort laser pulse have been experimentally
demonstrated in ferrimagnetic alloys as GdFeCo. Although the switching has been
previously reproduced by atomistic simulations, the lack of a robust
micromagnetic framework for ferrimagnets limits the predictions to small
nano-systems, whereas the experiments are usually performed with lasers and
samples of tens of micrometers. Here we develop a micromagnetic model based on
the extended Landau-Lifshitz-Bloch equation, which is firstly validated by
directly reproducing atomistic results for small samples and uniform laser
heating. After that, the model is used to study ultrafast single shot
all-optical switching in ferrimagnetic alloys under realistic conditions. We
find that the helicity-independent switching under a linearly polarized laser
pulse is a pure thermal phenomenon, in which the size of inverted area directly
correlates with the maximum electron temperature in the sample. On the other
hand, the analysis of the helicity-dependent processes under circular polarized
pulses in ferrimagnetic alloys with different composition indicates qualitative
differences between the results predicted by the magnetic circular dichroism
and the ones from inverse Faraday effect. Based on these predictions, we
propose experiments that would allow to resolve the controversy over the
physical phenomenon that underlies these helicity-dependent all optical
processes. | 2203.15460v1 |
2016-03-15 | The two gap transitions in Ge$_{1-x}$Sn$_x$: effect of non-substitutional complex defects | The existence of non-substitutional $\beta$-Sn defects in Ge$_{1-x}$Sn$_{x}$
was confirmed by emission channeling experiments [Decoster et al., Phys. Rev. B
81, 155204 (2010)], which established that although most Sn enters
substitutionally ($\alpha$-Sn) in the Ge lattice, a second significant fraction
corresponds to the Sn-vacancy defect complex in the split-vacancy configuration
( $\beta$-Sn ), in agreement with our previous theoretical study [Ventura et
al., Phys. Rev. B 79, 155202 (2009)]. Here, we present our electronic structure
calculation for Ge$_{1-x}$Sn$_{x}$, including substitutional $\alpha$-Sn as
well as non-substitutional $\beta$-Sn defects. To include the presence of
non-substitutional complex defects in the electronic structure calculation for
this multi-orbital alloy problem, we extended the approach for the purely
substitutional alloy by Jenkins and Dow [Jenkins and Dow, Phys. Rev. B 36, 7994
(1987)]. We employed an effective substitutional two-site cluster equivalent to
the real non-substitutional $\beta$-Sn defect, which was determined by a
Green's functions calculation. We then calculated the electronic structure of
the effective alloy purely in terms of substitutional defects, embedding the
effective substitutional clusters in the lattice. Our results describe the two
transitions of the fundamental gap of Ge$_{1-x}$Sn$_{x}$ as a function of the
total Sn-concentration: namely from an indirect to a direct gap, first, and the
metallization transition at higher $x$. They also highlight the role of
$\beta$-Sn in the reduction of the concentration range which corresponds to the
direct-gap phase of this alloy, of interest for optoelectronics applications. | 1603.04802v1 |
2017-02-19 | Atomic-ordering-induced quantum phase transition between topological crystalline insulator and Z2 topological insulator | Topological phase transition in a single material usually refers to
transitions between a trivial band insulator and a topological Dirac phase, but
the transition may also occur between different classes of topological Dirac
phases. However, it is a fundamental challenge to realize quantum transition
between Z2 nontrivial topological insulator (TI) and topological crystalline
insulator (TCI) in one material because Z2 TI and TCI are hardly both co-exist
in a single material due to their contradictory requirement on the number of
band inversions. The Z2 TIs must have an odd number of band inversions over all
the time-reversal invariant momenta, whereas, the newly discovered TCIs, as a
distinct class of the topological Dirac materials protected by the underlying
crystalline symmetry, owns an even number of band inversions. Here, take
PbSnTe2 alloy as an example, we show that at proper alloy composition the
atomic-ordering is an effective way to tune the symmetry of the alloy so that
we can electrically switch between TCI phase and Z2 TI phase when the alloy is
ordered from a random phase into a stable CuPt phase. Our results suggest that
atomic-ordering provides a new platform to switch between different topological
phases. | 1702.05697v1 |
2019-03-22 | Structure and mechanical behavior of ultrafine-grained aluminum-iron alloy stabilized by nanoscaled intermetallic particles | Ultrafine-grained aluminum alloys offer interesting multifunctional
properties with a combination of high strength, low electrical resistivity, and
low density. However, due to thermally induced grain coarsening, they typically
suffer from an intrinsic poor thermal stability. To overcome this drawback, an
Al-2%Fe alloy has been selected because of the low solubility of Fe in Al and
their highly positive enthalpy of mixing leading to the formation of stable
intermetallic particles. The two-phase alloy has been processed by severe
plastic deformation to achieve simultaneously submicrometer Al grains and a
uniform distribution of nanoscaled intermetallic particles. The influence of
the level of deformation on the microstructure has been investigated thanks to
transmission electron microscopy and atom probe tomography and it is shown that
for the highest strain a partial dissolution of the metastable Al6Fe particle
occurred leading to the formation of a Fe super saturated solid solution. The
thermal stability, and especially the precipitation of particles from the
ultrafine-grained solid solution and the way they pin grain boundaries has been
investigated both from static annealing and in-situ transmission electron
microscopy experiments. The correlation between microstructural features and
microhardness has been established to identify the various strengthening
contributions. Finally, it is 2 shown that ultrafine grained high purity Al
with less than 0.01 at. % Fe in solid solution could preserve a grain size only
300nm after 1h at 250$^\circ$C. | 1903.09391v1 |
2019-03-27 | Predicting magnetization of ferromagnetic binary Fe alloys from chemical short range order | Among the ferromagnetic binary alloys, body centered cubic (bcc) Fe-Co is the
one showing the highest magnetization. It is known experimentally that ordered
Fe-Co structures show a larger magnetization than the random solid solutions
with the same Co content. In this work, based on density functional theory
(DFT) studies, we aim at a quantitative prediction of this feature, and point
out the role of the orbital magnetic moments. Then, we introduce a DFT-based
analytical model correlating local magnetic moments and chemical compositions
for Co concentrations ranging from 0 to 70 at.%. It is also extended to predict
the global magnetization of both ordered and disordered structures at given
concentration and chemical short range orders. The latter model is particularly
useful for interpreting experimental data. Based on these models, we note that
the local magnetic moment of a Fe atom is mainly dictated by the Co
concentration in its first two neighbor shells. The detailed local arrangement
of the Co atoms has a minor effect. These simple models can fully reproduce the
difference in magnetization between the ordered and disordered Fe-Co alloys
between 30% and 70% Co, in good agreement with experimental data. Finally, we
show that a similar model can be established for another bcc binary Fe alloy,
the Fe-Ni, also presenting ferromagnetic interactions between atoms. | 1903.11468v1 |
2019-03-27 | Design, Fabrication and Characterization of FeAl-based Metallic-Intermetallic Laminate (MIL) Composites | FeAl-based MIL composites of various iron alloys were fabricated with an
innovative 'multiple-thin-foil' configuration and 'two-stage reaction'
strategy. | 1903.11724v1 |
2019-03-29 | Fe alloy slurry and a compacting cumulate pile across Earth's inner-core boundary | Seismic observations show a reduced compressional-wave gradient at the base
of the outer core relative to the preliminary reference Earth model and seismic
wave asymmetry between the east-west hemispheres at the top of the inner core.
Here, we propose a model for the inner core boundary (ICB), where a slurry
layer forms through fractional crystallization of an Fe alloy at the base of
the outer core (F layer) above a compacting cumulate pile at the top of the
inner core (F' layer). Using recent mineral physics data, we show that
fractional crystallization of an Fe alloy (e.g., Fe-Si-O) with light element
partitioning can explain the observed reduced velocity gradient in the F layer,
in cases with a solid fraction of ~15(5)% in liquid with a compositional
gradient due to preferential light element partitioning into liquid. The
compacting cumulate pile in the F' layer may exhibit lateral variations in
thickness between the east-west hemispheres due to lateral variations of
large-scale heat flow in the outer core, which may explain the east-west
asymmetry observed in the seismic velocity. Our interpretations suggest that
the inner core with solid Fe alloy has a high shear viscosity of ~10^23 Pa s. | 1903.12574v2 |
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