arXiv subjects

Dane Morgan

Publications and source records attributed to Dane Morgan.

At least 109 records · Page 6Linked to original sources

arXiv2017

Integrated Modeling of Second Phase Precipitation in Cold-Worked 316 Stainless Steels under Irradiation

The current work combines the Cluster Dynamics (CD) technique and CALPHAD-based precipitation modeling to address the second phase precipitation in cold-worked (CW) 316 stainless steels (SS) under irradiation at 300-400 C. CD provides the radiation enhanced diffusion and dislocation evolution as inputs for the precipitation model. The CALPHAD-based precipitation model treats the nucleation, growth and coarsening of precipitation processes based on classical nucleation theory and evolution equations, and simulates the composition, size and size distribution of precipitate phases. We benchmark the model against available experimental data at fast reactor conditions (9.4 x 10^-7 dpa/s and 390 C) and then use the model to predict the phase instability of CW 316 SS under light water reactor (LWR) extended life conditions (7 x 10^-8 dpa/s and 275 C). The model accurately predicts the gamma-prime (Ni3Si) precipitation evolution under fast reactor conditions and that the formation of this phase is dominated by radiation enhanced segregation. The model also predicts a carbide volume fraction that agrees well with available experimental data from a PWR reactor but is much higher than the volume fraction observed in fast reactors. We propose that radiation enhanced dissolution and/or carbon depletion at sinks that occurs at high flux could be the main sources of this inconsistency. The integrated model predicts ~1.2% volume fraction for carbide and ~3.0% volume fraction for gamma-prime for typical CW 316 SS (with 0.054 wt.% carbon) under LWR extended life conditions. This work provides valuable insights into the magnitudes and mechanisms of precipitation in irradiated CW 316 SS for nuclear applications.

cond-mat.mtrl-sci↗

arXiv2017

Robust FCC solute diffusion predictions from ab-initio machine learning methods

We evaluate the performance of four machine learning methods for modeling and predicting FCC solute diffusion barriers. More than 200 FCC solute diffusion barriers from previous density functional theory (DFT) calculations served as our dataset to train four machine learning methods: linear regression (LR), decision tree (DT), Gaussian kernel ridge regression (GKRR), and artificial neural network (ANN). We separately optimize key physical descriptors favored by each method to model diffusion barriers. We also assess the ability of each method to extrapolate when faced with new hosts with limited known data. GKRR and ANN were found to perform the best, showing 0.15 eV cross-validation errors and predicting impurity diffusion in new hosts to within 0.2 eV when given only 5 data points from the host. We demonstrate the success of a combined DFT + data mining approach towards solving materials science challenges and predict the diffusion barrier of all available impurities across all FCC hosts.

cond-mat.mtrl-sci↗

arXiv2017

Optimization of self-interstitial clusters in 3C-SiC with Genetic Algorithm

Under irradiation, SiC develops damage commonly referred to as black spot defects, which are speculated to be self-interstitial atom clusters. To understand the evolution of these defect clusters and their impacts (e.g., through radiation induced swelling) on the performance of SiC in nuclear applications, it is important to identify the cluster composition, structure, and shape. In this work the genetic algorithm code StructOpt was utilized to identify groundstate cluster structures in 3C-SiC. The genetic algorithm was used to explore clusters of up to ~30 interstitials of C-only, Si-only, and Si-C mixtures embedded in the SiC lattice. We performed the structure search using Hamiltonians from both density functional theory and empirical potentials. The thermodynamic stability of clusters was investigated in terms of their composition (with a focus on Si-only, C-only, and stoichiometric) and shape (spherical vs. planar), as a function of the cluster size (n). Our results suggest that large Si-only clusters are likely unstable, and clusters are predominantly C-only for n <= 10 and stoichiometric for n > 10. The results imply that there is an evolution of the shape of the most stable clusters, where small clusters are stable in more spherical geometries while larger clusters are stable in more planar configurations. We also provide an estimated energy vs. size relationship, E(n), for use in future analysis.

cond-mat.mtrl-sci↗

arXiv2017

Iron Partitioning between Ferropericlase and Bridgmanite in the Earth's Lower Mantle

Earth's lower mantle is generally believed to be seismically and chemically homogeneous because most of the key seismic parameters can be explained using a simplified mineralogical model at expected press-temperature conditions. However, recent high-resolution tomographic images have revealed seismic and chemical stratification in the middle-to-lower parts of the lower mantle. Thus far, the mechanism for the compositional stratification and seismic inhomogeneity, especially their relationship with the speciation of iron in the lower mantle, remains poorly understood. We have built a complete and integrated thermodynamic model of iron and aluminum chemistry for lower mantle conditions, and from this model has emerged a stratified picture of the valence, spin and composition profile in the lower mantle. Within this picture the lower mantle has an upper region with Fe3+ enriched bridgmanite with high-spin ferropericlase and metallic Fe, and a lower region with low-spin, iron-enriched ferropericlase coexisting with iron-depleted bridgmanite and almost no metallic Fe. The transition between the regions occurs at a depth of around 1600km and is driven by the spin transition in ferropericlase, which significantly changes the iron partitioning and speciation to one that favors Fe2+ in ferropericlase and suppresses Fe3+ and metallic iron formation These changes lead to lowered bulk sound velocity by 3-4% around the mid-lower mantle and enhanced density by ~1% toward the lowermost mantle. The predicted chemically and seismically stratified lower mantle differs dramatically from the traditional homogeneous model.

cond-mat.mtrl-sci↗

arXiv2017

Nanoscale Voltage Enhancement at Cathode Interfaces in Li-ion Batteries

Interfaces are ubiquitous in Li-ion battery electrodes, occurring across compositional gradients, regions of multiphase intergrowths, and between electrodes and solid electrolyte interphases or protective coatings. However, the impact of these interfaces on Li energetics remains largely unknown. In this work, we calculated Li intercalation-site energetics across cathode interfaces and demonstrated the physics governing these energetics on both sides of the interface. We studied the olivine/olivine-structured LixFePO4/LixMPO4 (x=0 and 1, M=Co, Ti, Mn) and layered/layered-structured LiNiO2/TiO2 interfaces to explore different material structures and transition metal elements. We found that across an interface from a high- to low-voltage material the Li voltage remains constant in the high-voltage material and decays approximately linearly in the low-voltage region, approaching the Li voltage of the low-voltage material. This effect ranges from 0.5-9nm depending on the interfacial dipole screening. This effect provides a mechanism for a high-voltage material at an interface to significantly enhance the Li intercalation voltage in a low-voltage material over nanometer scale. We showed that this voltage enhancement is governed by a combination of electron transfer (from low- to high-voltage regions), strain and interfacial dipole screening. We explored the implications of this voltage enhancement for a novel heterostructured-cathode design and redox pseudocapacitors.

cond-mat.mtrl-sci↗

arXiv2017

Continuum model for hydrogen pickup in Zirconium alloys of LWR fuel cladding

A continuum model for calculating the time-dependent hydrogen pickup fractions in various Zirconium alloys under steam and pressured water oxidation has been developed in this study. Using only one fitting parameter, the effective hydrogen gas partial pressure at the oxide surface, a qualitative agreement is obtained between the predicted and previously measured hydrogen pickup fractions. The calculation results therefore demonstrate that H diffusion through the dense oxide layer plays an important role in the hydrogen pickup process. The limitations and possible improvement of the model are also discussed.

cond-mat.mtrl-sci↗

arXiv2017

Counterintuitive Reconstruction of the Polar O-Terminated ZnO Surface With Zinc Vacancies and Hydrogen

Understanding the structure of ZnO surface reconstructions and their resultant properties is crucial to the rational design of ZnO-containing devices ranging from optoelectronics to catalysts. Here, we are motivated by recent experimental work which showed a new surface reconstruction containing Zn vacancies ordered in a Zn(3x3) pattern in the subsurface of (0001)-O terminated ZnO. A reconstruction with Zn vacancies on (0001)-O is surprising and counterintuitive because Zn vacancies enhance the surface dipole rather than reduce it. In this work, we show using Density Functional Theory (DFT) that subsurface Zn vacancies can form on (0001)-O when coupled with adsorption of surface H and are in fact stable under a wide range of common conditions. We also show these vacancies have a significant ordering tendency and that Sb-doping created subsurface inversion domain boundaries (IDBs) enhances the driving force of Zn vacancy alignment into large domains of the Zn(3x3) reconstruction.

cond-mat.mtrl-sci↗

arXiv2016

Understanding and reducing deleterious defects in metastable alloy GaAsBi

Technological applications of novel metastable materials are frequently inhibited by abundant defects residing in these materials. Using first-principles methods we investigate the point defect thermodynamics and phase segregation in the technologically-important metastable alloy GaAsBi. Our calculations predict defect energy levels in good agreement with abundant previous experiments and clarify the defect structures giving rise to these levels. We find that vacancies in some charge states become metastable or unstable with respect to antisite formation, and this instability is a general characteristic of zincblende semiconductors with small ionicity. The dominant point defects degrading electronical and optical performances are predicted to be As$_G$$_a$, Bi$_G$$_a$, Bi$_G$$_a$+Bi$_A$$_s$, As$_G$$_a$+Bi$_A$$_s$, V$_G$$_a$ and V$_G$$_a$+Bi$_A$$_s$, of which the first-four and second-two defects are minority-electron and minority-hole traps, respectively. V$_G$$_a$ is also found to play a critical role in controlling the metastable Bi supersaturation through mediating Bi diffusion and clustering. To reduce the influences of these deleterious defects, we suggest shifting the growth away from As-rich condition and/or using hydrogen passivation to reduce the minority-carrier traps. We expect this work to aid in the applications of GaAsBi to novel electronic and optoelectronic devices, and shine a light on controlling the deleterious defects in other metastable materials.

cond-mat.mtrl-sci↗

arXiv2016

The MAterials Simulation Toolkit (MAST) for atomistic modeling of defects and diffusion

The MAterials Simulation Toolkit (MAST) is a workflow manager and post-processing tool for ab initio defect and diffusion workflows. MAST codifies research knowledge and best practices for such workflows, and allows for the generation and management of easily modified and reproducible workflows, where data is stored along with workflow information for data provenance tracking. MAST is open-source and available for download (see PDF for links).

cond-mat.mtrl-sci↗

arXiv2016

First-principles Predictions of Electronic Properties of GaAs1-x-yPyBix and GaAs1-x-yPyBix-based Heterojunctions

Significant efficiency droop is a major concern for light-emitting diodes and laser diodes operating at high current density. Recent study has suggested that heavily Bi-alloyed GaAs can decrease the non-radiative Auger recombination and therefore alleviate the efficiency droop. Using density functional theory, we studied a newly fabricated quaternary alloy, GaAs1-x-yPyBix, which can host significant amounts of Bi, through calculations of its band gap, spin-orbit splitting, and band offsets with GaAs. We found that the band gap changes of GaAs1-x-yPyBix relative to GaAs are determined mainly by the local structural changes around P and Bi atoms rather than their electronic structure differences. To obtain alloy with lower Auger recombination than GaAs bulk, we identified the necessary constraints on the compositions of P and Bi. Finally, we demonstrated that GaAs/GaAs1-x-yPyBix heterojunctions with potentially low Auger recombination can exhibit small lattice mismatch and large enough band offsets for strong carrier confinement. This work shows that the electronic properties of GaAs1-x-yPyBix are potentially suitable for high-energy infrared light-emitting diodes and laser diodes with improved efficiency.

cond-mat.mtrl-sci↗

arXiv2016

Factors controlling oxygen migration barriers in perovskites

Perovskites with fast oxygen ion conduction can enable technologies like solid oxide fuel cells. One component of fast oxygen ion conduction is low oxygen migration barrier. Here we apply ab initio methods on over 40 perovskites to produce a database of oxygen migration barriers ranging from 0.2 to 1.6 eV. Mining the database revealed that systems with low barriers also have low metal-oxygen bond strength, as measured by oxygen vacancy formation energy and oxygen p-band center energy. These correlations provide a powerful descriptor for the development of new oxygen ion conductors and may explain the poor stability of some of the best oxygen conducting perovskites under reducing conditions. Other commonly-cited measures of space, volume, or structure ideality showed only weak correlation with migration barrier. The lowest migration barriers (< 0.5 eV) belong to perovskites with non-transition-metal B-site cations, and may require vacancy-creation strategies that involve no dopants or low-association dopants for optimal performance.

cond-mat.mtrl-sci↗

arXiv2016

Accelerated atomistic simulation study on the stability and mobility of carbon tri-interstitial cluster in cubic SiC

Using a combination of kinetic Activation Relaxation Technique with empirical potential and ab initio based climbing image nudged elastic band method, we perform an extensive search of the migration and rotation paths of the most stable carbon tri-interstitial cluster in cubic SiC. Our research reveals paths with the lowest energy barriers to migration, rotation, and dissociation of the most stable cluster. The kinetic properties of the most stable cluster, including its mobility, rotation behavior at different temperatures and stability against high temperature annealing, are discussed based on the calculated transition barriers. In addition to fundamental insights, our study provides a methodology for investigation of other extended defects in a technologically important material.

cond-mat.mtrl-sci↗

arXiv2016

Radiation-induced mobility of small defect clusters in covalent materials

Although defect clusters are detrimental to electronic and mechanical properties of semiconductor materials, annihilation of such clusters is limited by their lack of thermal mobility due to high migration barriers. Here, we find that small clusters in bulk SiC (a covalent material of importance for both electronic and nuclear applications) can become mobile at room temperature under the influence of electron radiation. So far, direct observation of radiation-induced diffusion of defect clusters in bulk materials has not been demonstrated yet. This finding was made possible by low angle annular dark field (LAADF) scanning transmission electron microscopy (STEM) combined with non-rigid registration technique to remove sample instability, which enables atomic resolution imaging of small migrating defect clusters. We show that the underlying mechanism of this athermal diffusion is ballistic collision between incoming electrons and cluster atoms. Our findings suggest that defect clusters may be mobile under certain irradiation conditions, changing current understanding of cluster annealing process in irradiated covalent materials.

cond-mat.mtrl-sci↗

arXiv2016

Strain Effects on Oxygen Migration in Perovskites

Fast oxygen transport materials are necessary for a range of technologies, including efficient and cost-effective solid oxide fuel cells, gas separation membranes, oxygen sensors, chemical looping devices, and memristors. Strain is often proposed as a method to enhance the performance of oxygen transport materials, but the magnitude of its effect and its underlying mechanisms are not well-understood, particularly in the widely-used perovskite-structured oxygen conductors. This work reports on an ab initio prediction of strain effects on migration energetics for nine perovskite systems of the form LaBO3, where B = [Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga]. Biaxial strain, as might be easily produced in epitaxial systems, is predicted to lead to approximately linear changes in migration energy. We find that tensile biaxial strain reduces the oxygen vacancy migration barrier across the systems studied by an average of 66 meV per percent strain for a single selected hop, with a low of 36 and a high of 89 meV decrease in migration barrier per percent strain across all systems. The estimated range for the change in migration barrier within each system is approximately 25 meV per percent strain when considering all hops. These results suggest that strain can significantly impact transport in these materials, e.g., a 2% tensile strain can increase the diffusion coefficient by about three orders of magnitude at 300 K (one order of magnitude at 500 °C or 773 K) for one of the most strain-responsive materials calculated here (LaCrO3). We show that a simple elasticity model, which assumes only dilative or compressive strain in a cubic environment and a fixed migration volume, can qualitatively but not quantitatively model the strain dependence of the migration energy, suggesting that factors not captured by continuum elasticity play a significant role in the strain response.

cond-mat.mtrl-sci↗

arXiv2016

Ag diffusion in SiC high-energy grain boundaries: kinetic Monte Carlo study with first-principle calculations

The diffusion of silver (Ag) impurities in high energy grain boundaries (HEGBs) of cubic (3C) silicon carbide (SiC) is studied using an ab initio based kinetic Monte Carlo (kMC) model. This study assesses the hypothesis that the HEGB diffusion is responsible for Ag release in Tristructural-Isotropic fuel particles, and provides a specific example to increase understanding of impurity diffusion in highly disordered grain boundaries. The HEGB environment was modeled by an amorphous SiC. The structure and stability of Ag defects were calculated using density functional theory code. The defect energetics suggested that the fastest diffusion takes place via an interstitial mechanism in a-SiC. The formation energy of Ag interstitials and the kinetic resolved activation energies between them were well approximated with Gaussian distributions that were then sampled in the kMC. The diffusion of Ag was simulated with the effective medium model using kMC. At 1200-1600C, Ag in a HEGB is predicted to exhibit an Arrhenius type diffusion and with a diffusion prefactor and effective activation energy of (2.73+-1.09)*10-10 m2s-1 and 2.79+-0.18 eV, respectively. The comparison between HEGB results to other theoretical studies suggested not only that GB diffusion is predominant over bulk diffusion, but also that the HEGB is one of fastest grain boundary paths for Ag diffusion in SiC. The Ag diffusion coefficient in the HEGB shows a good agreement with ion-implantation measurements, but is 2-3 orders of magnitude lower than the diffusion coefficients extracted from integral release measurements. The discrepancy between GB diffusion and integral release measurements suggests that other contributions are responsible for the fast release of Ag in some experiments and we propose that these contributions may arise from radiation enhanced diffusion.

cond-mat.mtrl-sci↗

arXiv2016

Origin of Fe3+ in Fe-containing, Al-free Mantle Silicate Perovskite

We have studied the ferrous (Fe2+) and ferric (Fe3+) iron concentrations in Al-free Fe containing Mg-silicate perovskite (Mg-Pv) at pressure (P), temperature (T), and oxygen fugacity (fO2) conditions related to the lower mantle using a thermodynamic model based on ab-initio calculations. We consider the oxidation reaction and the charge disproportionation reaction, both of which can produce Fe3+ in Mg-Pv. The model shows qualitatively good agreement with available experimental data on Fe3+/ΣFe (ΣFe = total Fe in system), spin transitions, and equations of state. We predict that under lower-mantle conditions Fe3+/ΣFe determined by the charge disproportionation is estimated to be 0.01-0.07 in Al-free Mg-Pv, suggesting that low Al Mg-Pv in the uppermost pyrolitic mantle (where majoritic garnet contains most of the Al) and in the harzburgitic heterogeneities throughout the lower mantle contains very little Fe3+. We find that the volume reduction by the spin transition of the B-site Fe3+ leads to a minimum Fe3+/ΣFe in Mg-Pv at mid-mantle pressures. The model shows that configurational entropy is a key driving force to create Fe3+ and therefore Fe3+ content is highly temperature sensitive. The temperature sensitivity may lead to a maximum Fe3+/ΣFe in Mg-Pv in warm regions at the core-mantle boundary region, such as Large Low Shear Velocity Provinces (LLSVPs), potentially altering the physical (e.g., bulk modulus) and transport (e.g., thermal and electrical conductivities) properties of the heterogeneities.

astro-ph.EP↗

arXiv2016

Understanding and controlling the work function of perovskite oxides using Density Functional Theory

Perovskite oxides containing transition metals are promising materials in a wide range of electronic and electrochemical applications. However, neither their work function values nor an understanding of their work function physics have been established. Here, we predict the work function trends of a series of perovskite ($ABO_3$ formula) materials using Density Functional Theory, and show that the work functions of (001)-terminated AO- and $BO_2$-oriented surfaces can be described using concepts of electronic band filling, bond hybridization, and surface dipoles. The calculated range of AO ($BO_2$) work functions are 1.60-3.57 eV (2.99-6.87 eV). We find an approximately linear correlation ($R^2$ between 0.77-0.86, depending on surface termination) between work function and position of the oxygen 2p band center, which correlation enables both understanding and rapid prediction of work function trends. Furthermore, we identify $SrVO_3$ as a stable, low work function, highly conductive material. Undoped (Ba-doped) $SrVO_3$ has an intrinsically low AO-terminated work function of 1.86 eV (1.07 eV). These properties make $SrVO_3$ a promising candidate material for a new electron emission cathode for application in high power microwave devices, and as a potential electron emissive material for thermionic energy conversion technologies.

cond-mat.mtrl-sci↗

arXiv2016

Origins of Large Voltage Hysteresis in High Energy-Density Metal Fluoride Lithium-Ion Battery Conversion Electrodes

Metal fluoride and oxides can store multiple lithium-ions through conversion chemistry to enable high energy-density lithium-ion batteries. However, their practical applications have been hindered by an unusually large voltage hysteresis between charge and discharge voltage-profiles and the consequent low energy efficiency (< 80%). The physical origins of such hysteresis are rarely studied and poorly understood. Here we employ in situ X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), density-functional-theory (DFT) calculations, and galvanostatic intermittent titration technique (GITT) to first correlate the voltage profile of iron fluoride ($FeF_3$), a representative conversion electrode material, with evolution and spatial distribution of intermediate phases in the electrode. The results reveal that, contrary to conventional belief, the phase evolution in the electrode is symmetrical during discharge and charge. However, the spatial evolution of the electrochemically active phases, which is controlled by reaction kinetics, is different. We further propose that the voltage hysteresis in the $FeF_3$ electrode is kinetic in nature. It is the result of Ohmic voltage drop, reaction overpotential, and different spatial distributions of electrochemically-active phases (i.e. compositional inhomogeneity). Therefore, the large hysteresis can be expected to be mitigated by rational design and optimization of material microstructure and electrode architecture to improve the energy efficiency of lithium-ion batteries based on conversion chemistry.

cond-mat.mtrl-sci↗