[Show abstract][Hide abstract] ABSTRACT: We used molecular dynamics modeling of atomic displacement cascades to characterize the nature of primary radiation damage in 3C–SiC. We demonstrated that the most commonly used interatomic potentials are inconsistent with ab initio calculations of defect energetics. Both the Tersoff potential used in this work and a modified embedded-atom method potential reveal a barrier to recombination of the carbon interstitial and carbon vacancy which is much higher than the density functional theory (DFT) results. The barrier obtained with a newer potential by Gao and Weber is closer to the DFT result. This difference results in significant differences in the cascade production of point defects. We have completed both 10 keV and 50 keV cascade simulations in 3C–SiC at a range of temperatures. In contrast to the Tersoff potential, the Gao-Weber potential produces almost twice as many C vacancies and interstitials at the time of maximum disorder (∼0.2 ps) but only about 25% more stable defects at the end of the simulation. Only about 20% of the carbon defects produced with the Tersoff potential recombine during the in–cascade annealing phase, while about 60% recombine with the Gao-Weber potential. The Gao-Weber potential appears to give a more realistic description of cascade dynamics in SiC, but still has some shortcomings when the defect migration barriers are compared to the ab initio results.
[Show abstract][Hide abstract] ABSTRACT: High energy vibrational scattering in the binary systems UC and US is
measured using time-of-flight inelastic neutron scattering. A clear set of
well-defined peaks equally separated in energy is observed in UC, corresponding
to harmonic oscillations of the light C atoms in a cage of heavy U atoms. The
scattering is much weaker in US and only a few oscillator peaks are visible. We
show how the difference between the materials can be understood by considering
the neutron scattering lengths and masses of the lighter atoms. Monte Carlo ray
tracing is used to simulate the scattering, with near quantitative agreement
with the data in UC, and some differences with US. The possibility of observing
anharmonicity and anisotropy in the potentials of the light atoms is
investigated in UC. Overall the observed data is well accounted for by
considering each light atom as a single atom isotropic quantum harmonic
oscillator.
[Show abstract][Hide abstract] ABSTRACT: High entropy alloys (HEA) have unique properties including the potential to be radiation tolerant. These materials with extreme disorder could resist damage because disorder, stabilized by entropy, is the equilibrium thermodynamic state. Disorder also reduces electron and phonon conductivity keeping the damage energy longer at the deposition locations, eventually favoring defect recombination. In the short time-scales related to thermal spikes induced by collision cascades, phonons become the relevant energy carrier. In this work, we perform a systematic study of phonon thermal conductivity in multiple component solid solutions represented by Lennard-Jones (LJ) potentials. We explore the conditions that minimize phonon mean free path via extreme alloy complexity, by varying the composition and the elements (differing in mass, atomic radii, and cohesive energy). We show that alloy complexity can be tailored to modify the scattering mechanisms that control energy transport in the phonon subsystem. Our analysis provides a qualitative guidance for the selection criteria used in the design of HEA alloys with low phonon thermal conductivity.
Journal of Alloys and Compounds 07/2015; 648:408-413. DOI:10.1016/j.jallcom.2015.06.035 · 3.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Oxide-metal systems are important in many practical applications, and they are undergoing extensive study using a wide range of techniques. The most accurate theoretical approaches are based on density functional theory (DFT), which is limited to ~10(2) atoms. Multi-scale approaches, e.g. DFT + Monte Carlo, are often used to model oxide metal systems at the atomic level. These approaches can qualitatively describe the kinetics of some processes but not the overall stability of individual phases. In this article, we propose a thermodynamic approach to study equilibrium in multi-phase systems, which can be sequentially enhanced by considering different defects and microstructures. We estimate the thermodynamic equilibrium by minimization of the free energy of the whole multi-phase system using a limited set of defects and microstructural objects for which the properties are calculated by DFT. As an example, we consider Y2O3 + bcc Fe with vacancies in both the Y2O3 and bcc Fe phases, Y substitutions and O interstitials in Fe, Fe impurities, and antisite defects in Y2O3. The output of these calculations is the thermal equilibrium concentration of all the defects for a particular temperature and composition. The results obtained confirmed the high temperature stability of yttria in iron. Model development toward more accurate calculations is discussed.
[Show abstract][Hide abstract] ABSTRACT: Recently, interest in alnico magnetic alloys has been rekindled due to their potential to substitute for rare-earth based permanent magnets provided modest improvements in their coercivity can be achieved without loss of saturation magnetization. Recent experimental studies have indicated that atomic and magnetic structure of the two phases (one AlNi-based, the other FeCo-based) that comprise these spinodally decomposed alloy is not as simple as previously thought. A key issue that arises is the distribution of Fe, Co, and Ti within the AlNi-based matrix phase. In this paper, we report the results of first-principles calculations of the site preference of ternary alloying additions in DO3 Fe3 Al, Co3 Al, and Ni 3 Al alloys, as models for the aluminide phase. For compound compositions that are Al rich, which correspond to experimental situation, Ti and Fe are found to occupy the α sites, while Co and Ni prefer the γ sites of the DO3 lattice. An important finding is that the magnetic moments of transition metals in Fe3 Al and Co3 Al are ordered ferromagnetically, whereas the Ni 3 Al were found to be nonmagnetic unless the Fe or Co is added as a ternary element.
[Show abstract][Hide abstract] ABSTRACT: A combination of density functional theory (DFT), kinetic Monte Carlo and mean-field rate theory is applied to analyze point defect migration and its effect on the observed growth of hexagonal close-packed (hcp) Zr under 1 MeV electron irradiation. DFT is used to study stability of various configurations of vacancies and self-interstitial atoms (SIAs) and migration barriers. The data are used in kinetic Monte Carlo modeling of defect diffusion at different temperatures. It is found that both defects exhibit anisotropic diffusion, predominantly parallel to the basal planes. The ratio of diffusion coefficients parallel and perpendicular to the basal planes is found to be higher for vacancies as compared to SIAs at temperatures below ∼600 K. This raises doubts that the observed radiation growth in Zr irradiated with 1 MeV electrons, namely positive strains in prismatic and negative strains in basal directions, and void alignment along basal planes, can be accounted for by the anisotropy of point defect diffusion, which predicts opposite strain signs. It is speculated that formation of small SIA clusters with higher diffusion anisotropy may be responsible for the experimental observations.
[Show abstract][Hide abstract] ABSTRACT: The magnetic phase diagrams of models for quasi one-dimensional compounds
belonging to the iron-based superconductors family are presented. The
five-orbital Hubbard model and the real-space Hartree-Fock approximation are
employed, supplemented by density functional theory to obtain the hopping
amplitudes. Phase diagrams are constructed varying the Hubbard $U$ and Hund $J$
couplings and at zero temperature. The study is carried out at electronic
density (electrons per iron) $n = 5.0$, which is of relevance for the already
known material TlFeSe$_2$, and also at $n = 6.0$, where representative
compounds still need to be synthesized. At $n = 5.0$ there is a clear dominance
of staggered spin order along the chain direction. At $n = 6.0$ and the
realistic Hund coupling $J/U = 0.25$, the phase diagram is far richer including
a variety of ``block'' states involving ferromagnetic clusters that are
antiferromagnetically coupled, in qualitative agreement with recent Density
Matrix Renormalization Group calculations for the three-orbital Hubbard model
in a different context. These block states arise from the competition between
ferromagnetic order (induced by double exchange, and prevailing at large $J/U$)
and antiferromagnetic order (dominating at small $J/U$). The density of states
and orbital compositions of the many phases are also provided.
Physical Review B 03/2014; 90(3). DOI:10.1103/PhysRevB.90.035128 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Until the advent of rare earth based magnets Alnico was one of the
highest energy product hard magnets available. Recently, interest in
this system has been rekindled as system whose properties and utility
may be further enhanced but does not contain rare earth elements. Recent
experiments on Alnico alloy suggest that there is no sharp interface
between the disordered bcc FeCo magnetic phase and the ordered B2 NiAl
non-magnetic phase; thereby undermining our understanding of the large
coercivity of this material. By utilizing several electronic structure
methods we first study the issue of the effect of substitutions of
additional elements into B2 NiAl phase. We also calculate the magnetic
moment distribution across the interface and examine the magnetic ground
state. These calculations suggest that the magnetic structure of the
B2-phase as well as the interface in much more complex than previously
thought.
[Show abstract][Hide abstract] ABSTRACT: Mobility of screw dislocations controls low temperature plasticity in
bcc metals including ferritic alloys. Density functional theory (DFT) is
an effective tool in providing parameter-free information on the
energetic and magnetic properties of defects including screw
dislocations. We summarize DFT calculations on atomic properties of
1/2<111> screw dislocations in Fe-Cr system. The periodic
quadrupole approach was applied to model the core dislocation structure,
core interaction with Cr solute atoms and to estimate their effect on
Peierls stress and barrier. The binding energy of Cr impurity atoms with
a screw dislocation and its effect on the dislocation core structure are
discussed and the importance of magnetism in the effects of Cr on screw
dislocation mobility is demonstrated. This work was supported by the
Center for Defect Physics, an Energy Frontier Research Center funded by
the US Department of Energy, Office of Science, Office of Basic Energy
Sciences.
[Show abstract][Hide abstract] ABSTRACT: Classical Molecular Dynamic (MD) simulations characterizing extended
defects typically require millions of atoms. First principles
calculations employed to understand these defect systems at an
electronic level cannot, and should not deal with such large numbers of
atoms. We present an efficient coarse graining (CG) approach to
calculate local electronic properties of large MD-generated structures
from the first principles. We used the Locally Self-consistent Multiple
Scattering (LSMS) method for two types of iron defect structures 1)
screw-dislocation dipoles and 2) radiation cascades. The multiple
scattering equations are solved at fewer sites using the CG. The atomic
positions were determined by MD with an embedded atom force field. The
local moments in the neighborhood of the defect cores are calculated
with first-principles based on full local structure information, while
atoms in the rest of the system are modeled by representative atoms with
approximated properties. This CG approach reduces computational costs
significantly and makes large-scale structures amenable to first
principles study. Work is sponsored by the USDoE, Office of Basic Energy
Sciences, ``Center for Defect Physics,'' an Energy Frontier Research
Center. This research used resources of the Oak Ridge Leadership
Computing Facility at the ORNL, which is supported by the Office of
Science of the USDoE under Contract No. DE-AC05-00OR22725.
[Show abstract][Hide abstract] ABSTRACT: Mobile defects such as dislocations and crowdions respond to gradients
of strain, temperature, concentration, and applied field, thereby,
determining a material's viability in particular applications. In Fe,
defects affect the magnetic state of the surrounding atoms. We discuss
the defect-induced changes in magnetic moment magnitude and orientation,
magnetic anisotropy and magnetic interactions. These quantities are
calculated (density functional theory (DFT)) for defect models ranging
in size from a few hundred to a few thousand. Comparisons are made
between different DFT methods. The importance of magnetism to the
response of defects to gradients is discussed.
[Show abstract][Hide abstract] ABSTRACT: The alignment of vacancy loops and voids along basal planes observed in irradiated Zr and Zr alloys requires anisotropic point-defect transport with a dominant contribution along the basal plane. For neutron irradiation, this can be explained by one-dimensional mobility of self-interstitial atom (SIA) clusters, but experiments with electron irradiation indicate unambiguously that even single SIA should exhibit anisotropic diffusion. No experimental information is available on SIA properties in Zr and the previous ab initio calculations did not provide any evidence of anisotropic diffusion mechanisms. An extensive investigation of SIAs in Zr has been performed from first principles using two different codes. It was demonstrated that the simulation cell size, type of pseudopotential, exchange-correlation functional and the c/a ratio are crucially important for determining the properties of interstitials in hcp Zr. The most stable SIA configurations lie in the basal plane, which should lead to SIA diffusion mainly along basal planes.
[Show abstract][Hide abstract] ABSTRACT: Classical Molecular Dynamics (MD) simulations characterizing
dislocations and radiation damage typically treat
105-107 atoms. First principles techniques
employed to understand systems at an atomistic level are not practical
for such large systems consisting of millions of atoms. We present an
efficient coarse grained (CG) approach to calculate local electronic and
magnetic properties of large MD-generated structures from the first
principles.
Local atomic magnetic moments in crystalline Fe are perturbed by the
presence of radiation generated vacancies and interstitials. The effects
are most pronounced near the defect cores and decay slowly as the strain
field of the defects decrease with distance. We develop the CG technique
based on the Locally Self-consistent Multiple Scattering (LSMS) method
that exploits the near-sightedness of the electron Green function. The
atomic positions were determined by MD with an embedded atom force
field.
The local moments in the neighborhood of the defect cores are calculated
with first-principles based on full local structure information. Atoms
in the rest of the system are modeled by representative atoms with
approximated properties. The calculations result in local moments near
the defect centers with first-principles accuracy, while capturing
coarse-grained details of local moments at greater length scales. This
CG approach makes these large scale structures amenable to first
principles study.
Journal of Physics Conference Series 12/2012; 402. DOI:10.1088/1742-6596/402/1/012011
[Show abstract][Hide abstract] ABSTRACT: Several transition metals were examined to evaluate their potential for improving the ductility of tungsten. The dislocation core structure and Peierls stress and barrier of 1/2〈111〉 screw dislocations in binary tungsten-transition metal alloys (W(1-x)TM(x)) were investigated using density functional theory calculations. The periodic quadrupole approach was applied to model the structure of the 1/2〈111〉 dislocation. Alloying with transition metals was modeled using the virtual crystal approximation and the applicability of this approach was assessed by calculating the equilibrium lattice parameter and elastic constants of the tungsten alloys. Reasonable agreement was obtained with experimental data and with results obtained from the conventional supercell approach. Increasing the concentration of a transition metal from the VIIIA group, i.e. the elements in columns headed by Fe, Co and Ni, leads to reduction of the C' elastic constant and increase of the elastic anisotropy A = C(44)/C'. Alloying W with a group VIIIA transition metal changes the structure of the dislocation core from symmetric to asymmetric, similarly to results obtained for W(1-x)Re(x) alloys in the earlier work of Romaner et al (2010 Phys. Rev. Lett. 104 195503). In addition to a change in the core symmetry, the values of the Peierls stress and barrier are reduced. The latter effect could lead to increased ductility in a tungsten-based alloy. Our results demonstrate that alloying with any of the transition metals from the VIIIA group should have a similar effect to alloying with Re.
[Show abstract][Hide abstract] ABSTRACT: Under the DOE Deep Burn program TRISO fuel is being investigated as a fuel form for consuming plutonium and minor actinides, and for greater efficiency in uranium utilization. The result will thus be to drive TRISO particulate fuel to very high burn-ups. In the current effort the various phenomena in the TRISO particle are being modeled using a variety of techniques. The chemical behavior is being treated utilizing thermochemical analysis to identify phase formation/transformation and chemical activities in the particle, including kernel migration. Density functional theory is being used to understand fission product diffusion within the plutonia oxide kernel, the fission product's attack on the SiC coating layer, as well as fission product diffusion through an alternative coating layer, ZrC. Finally, a multiscale approach is being used to understand thermal transport, including the effect of radiation damage induced defects, in a model SiC material. (C) 2012 Elsevier B.V. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: The vibrational excitations of crystalline solids corresponding to acoustic or optic one-phonon modes appear as sharp features in measurements such as neutron spectroscopy. In contrast, many-phonon excitations generally produce a complicated, weak and featureless response. Here we present time-of-flight neutron scattering measurements for the binary solid uranium nitride, showing well-defined, equally spaced, high-energy vibrational modes in addition to the usual phonons. The spectrum is that of a single atom, isotropic quantum harmonic oscillator and characterizes independent motions of light nitrogen atoms, each found in an octahedral cage of heavy uranium atoms. This is an unexpected and beautiful experimental realization of one of the fundamental, exactly solvable problems in quantum mechanics. There are also practical implications, as the oscillator modes must be accounted for in the design of generation IV nuclear reactors that plan to use uranium nitride as a fuel.
[Show abstract][Hide abstract] ABSTRACT: We present the growths and detailed thermodynamic and transport
measurements on single crystals of the recently discovered binary
intermetallic superconductors, SrSn4 and BaSn5.
Their superconducting transition temperatures Tc are found to
be 4.8 K and 4.4 K respectively. Both materials are strongly-coupled,
possibly multi-band superconductors. Hydrostatic pressure causes a
decrease in the superconducting transition temperature at the rate of
-0.068 K/kbar for SrSn4, and -0.053 K/kbar for
BaSn5. Band structure and upper superconducting critical
field anisotropy of SrSn4 suggest complex, multi-sheet Fermi
surface formed by four bands. De Hass-van Alphen oscillations are
observed in BaSn5, which indicates a more complex topology of
Fermi surface.
[Show abstract][Hide abstract] ABSTRACT: The anisotropic physical properties of single crystals of orthorhombic PtSn4 are reported for magnetic fields up to 140 kOe, applied parallel and perpendicular to the crystallographic b axis. The magnetic susceptibility has an approximately temperature-independent behavior and reveals an anisotropy between the ac plane and b axis. Clear de Haas-van Alphen oscillations in fields as low as 5 kOe and at temperatures as high as 30 K were detected in magnetization isotherms. The thermoelectric power and resistivity of PtSn4 show the strong temperature and magnetic field dependencies. A change of the thermoelectric power at H=140 kOe is observed as high as ≃50 μV/K. Single crystals of PtSn4 exhibit very large transverse magnetoresistance of ≃5×105% for the ac plane and of ≃1.4×105% for the b axis resistivity at 1.8 K and 140 kOe, as well as pronounced Shubnikov de Haas oscillations. The magnetoresistance of PtSn4 appears to obey Kohler's rule in the temperature and field range measured. The Hall resistivity shows a linear temperature dependence at high temperatures followed by a sign reversal around 25 K which is consistent with thermoelectric power measurements. The observed quantum oscillations and band structure calculations indicate that PtSn4 has three-dimensional Fermi surfaces.