arXiv subjects

Dane Morgan

Publications and source records attributed to Dane Morgan.

121 records · Page 7Linked to original sources

arXiv2016

Electron Emission Energy Barriers and Stability of $Sc_2O_3$ with Adsorbed Ba and Ba-O

In this study we employ Density Functional Theory (DFT) methods to investigate the surface energy barrier for electron emission (surface barrier) and thermodynamic stability of Ba and Ba-O species adsorption under conditions of high temperature (approximately 1200 K) and low pressure (approximately $10^{-10}$ Torr) on the low index surfaces of bixbyite $Sc_2O_3$. The role of Ba in lowering the cathode surface barrier is investigated via adsorption of atomic Ba and Ba-O dimers, where the highest simulated dimer coverage corresponds to a single monolayer film of rocksalt BaO. The change of the surface barrier of a semiconductor due to adsorption of surface species is decomposed into two parts: a surface dipole component and doping component. The lowest surface barrier with atomic Ba on $Sc_2O_3$ was found to be 2.12 eV and 2.04 eV for the (011) and (111) surfaces at 3 and 1 Ba atoms per surface unit cell (0.250 and 0.083 Ba per surface O), respectively. The lowest surface barrier for Ba-O on $Sc_2O_3$ was found to be 1.21 eV on (011) for a 7 Ba-O dimer-per-unit-cell coverage (0.583 dimers per surface O). Generally, we found that Ba in its atomic form on $Sc_2O_3$ surfaces is not stable relative to bulk BaO, while Ba-O dimer coverages between 3 to 7 Ba-O dimers per (011) surface unit cell (0.250 to 0.583 dimers per surface O) produce stable structures relative to bulk BaO. Ba-O dimer adsorption on $Sc_2O_3$ (111) surfaces was found to be unstable versus BaO over the full range of coverages studied. Investigation of combined n-type doping and surface dipole modification showed that their effects interact to yield a reduction less than the two contributions would yield separately.

cond-mat.mtrl-sci↗

arXiv2016

Lithium transport through Lithium-ion battery cathode coatings

The surface coating of cathodes using insulator films has proven to be a promising method for high-voltage cathode stabilization in Li-ion batteries. However, there is still substantial uncertainty about how these films function, specifically with regard to important coating design principles such as lithium solubility and transport through the films. This study uses Density Functional Theory to examine the diffusivity of interstitial lithium in crystalline α-$AlF_3$, α-$Al_2O_3$, m-$ZrO_2$, c-MgO, and α-quartz $SiO_2$, which provide benchmark cases for further understanding of insulator coatings in general. In addition, we propose an Ohmic electrolyte model to predict resistivities and overpotential contributions under battery operating conditions. For the crystalline materials considered we predict that Li+ diffuses quite slowly, with a migration barrier larger than 0.9 eV in all crystalline materials except α-quartz $SiO_2$, which is predicted to have a migration barrier of 0.276 eV along <001>. These results suggest that the stable crystalline forms of these insulator materials, except for oriented α-quartz $SiO_2$, are not practical for conformal cathode coatings. Amorphous $Al_2O_3$ and $AlF_3$ have higher Li+ diffusivities than their crystalline counterparts. Our predicted amorphous $Al_2O_3$ resistivity (1789 MΩm) is near the top of the range of fitted resistivities extracted from previous experiments on nominal $Al_2O_3$ coatings (7.8 to 913 MΩm) while our predicted amorphous $AlF_3$ resistivity (114 MΩm) is close to the middle of the range. These comparisons support our framework for modeling and understanding the impact on overpotential of conformal coatings in terms of their fundamental thermodynamic and kinetic properties, and support that these materials can provide practical conformal coatings in their amorphous form.

cond-mat.mtrl-sci↗

arXiv2016

Intrinsic defects and conduction characteristics of $Sc_2O_3$ in thermionic cathode systems

Recent experimental observations indicate that bulk $Sc_2O_3$ (~200 nm thick), an insulator at room temperature and pressure, must act as a good electronic conductor during thermionic cathode operation, leading to the observed high emitted current densities and overall superior emission properties over conventional thermionic emitters which do not contain $Sc_2O_3$. Here, we employ ab initio methods using both semilocal and hybrid functionals to calculate the intrinsic defect energetics of Sc and O vacancies and interstitials and their effects on the electronic properties of $Sc_2O_3$ in an effort to explain the good conduction of $Sc_2O_3$ observed in experiment. The defect energetics were used in an equilibrium defect model to calculate the concentrations of defects and their compensating electron and hole concentrations at equilibrium. Overall, our results indicate that the conductivity of $Sc_2O_3$ solely due to the presence of intrinsic defects in the cathode operating environment is unlikely to be high enough to maintain the magnitude of emitted current densities obtained from experiment, and that presence of impurities are necessary to raise the conductivity of $Sc_2O_3$ to a high enough value to explain the current densities observed in experiment. The necessary minimum impurity concentration to impart sufficient electronic conduction is very small (approximately $7.5x10^{-3}$ ppm) and is probably present in all experiments.

cond-mat.mtrl-sci↗

arXiv2016

Oxygen Point Defect Chemistry in Ruddlesden-Popper Oxides (La$_{1-x}$Sr$_{x}$)$_{2}$MO$_{4{\pm}δ}$ (M = Co, Ni, Cu)

Stability of oxygen point defects in Ruddlesden-Popper oxides (La$_{1-x}$Sr$_{x}$)$_{2}$MO$_{4{\pm}δ}$ (M = Co, Ni, Cu) is studied with density functional theory calculations to determine their stable sites, charge states, and energetics as functions of Sr content ($x$), transition metal (M) and defect concentration (δ). We demonstrate that the dominant O point defects can change between oxide interstitials, peroxide interstitials, and vacancies. Generally, increasing $x$ and atomic number of M stabilizes peroxide over oxide interstitials, as well as vacancies over both peroxide and oxide interstitials; increasing δ destabilizes both oxide interstitials and vacancies, but affects little peroxide interstitials. We also demonstrate that the O 2$p$-band center is a powerful descriptor for these materials and correlates linearly with the formation energy of all the defects. The trends of formation energy versus $x$, M and δ and the correlation with O 2$p$-band center are explained in terms of oxidation chemistry and electronic structure.

cond-mat.mtrl-sci↗

arXiv2016

Thermal Diffusion Boron Doping of Single-Crystal Diamond

With the best overall electronic and thermal properties, single crystal diamond (SCD) is the extreme wide bandgap material that is expected to revolutionize power electronics and radio-frequency electronics in the future. However, turning SCD into useful semiconductors requires overcoming doping challenges, as conventional substitutional doping techniques, such as thermal diffusion and ion implantation, are not easily applicable to SCD. Here we report a simple and easily accessible doping strategy demonstrating that electrically activated, substitutional doping in SCD without inducing graphitization transition or lattice damage can be readily realized with thermal diffusion at relatively low temperatures by using heavily doped Si nanomembranes as a unique dopant carrying medium. Atomistic simulations elucidate a vacancy exchange boron doping mechanism that occur at the bonded interface between Si and diamond. We further demonstrate selectively doped high voltage diodes and half-wave rectifier circuits using such doped SCD. Our new doping strategy has established a reachable path toward using SCDs for future high voltage power conversion systems and for other novel diamond based electronic devices. The novel doping mechanism may find its critical use in other wide bandgap semiconductors.

cond-mat.mtrl-sci↗

arXiv2016

High-Throughput ab-initio Dilute Solute Diffusion Database

We demonstrate automated generation of diffusion databases from high-throughput density functional theory (DFT) calculations. A total of more than 230 dilute solute diffusion systems in Mg, Al, Cu, Ni, Pd, and Pt host lattices have been determined using multi-frequency diffusion models. We apply a correction method for solute diffusion in alloys using experimental and simulated values of host self-diffusivity. We find good agreement with experimental solute diffusion data, obtaining a weighted activation barrier RMS error of 0.176 eV when excluding magnetic solutes in non-magnetic alloys. The compiled database is the largest collection of consistently calculated ab-initio solute diffusion data in the world.

cond-mat.mtrl-sci↗

arXiv2016

Applicability of DFT + U to U metal and U-Zr alloy

In the Letter [J. Nucl. Mater. 444, 356 (2014)] and Comment [Phys Rev B 90, 157101 (2014)], Soderlind et al. argue that 1) DFT based on GGA already models U metal and U-Zr alloy accurately, and 2) DFT + U models them worse than DFT according to results they calculate or select from our recent study [Phys. Rev. B 88, 235128 (2013)]. Here we demonstrate in response to 1) that previously neglected and more recent experimental data indicate that DFT, even when implemented in all-electron methods, does not model the bulk modulus and elastic constants of αU very accurately. Furthermore, Soderlind et al.'s claim that deficiency exists in our PAW calculation is unfounded and hence our results, including those that show DFT results compare unfavorably with experimental/computational references, are valid. We also demonstrate in response to 2) that Soderlind et al.'s arguments are unsound for three reasons. First, they focus on just the BCC phases γU and γ(U,Zr)--which at the ab initio modeling temperature of 0 K are unstable and hence difficult to model and benchmar--as primary subjects of examination; however it is results for the stable phases mostly neglected by Soderlind et al. that are the primary evidence of our argument. Second, they make unfounded generalization of DFT + U results at selected U eff values to argue that DFT + U in general gives wrong or unsatisfactory results. Third, some key points in Soderlind et al.'s criticisms of DFT + U are not well supported, including the claims that for γ(U,Zr) DFT + U at Ueff = 1.24 eV gives positive deviations from linear dependence of composition that are unprecedentedly large and partially negative enthalpies of mixing that are inconsistent with the existence of miscibility gap in the experimental phase diagram. We therefore maintain our conclusion that DFT + U can be of value for modeling U and U-Zr.

cond-mat.mtrl-sci↗

arXiv2013

AFLOW: An automatic framework for high-throughput materials discovery

Recent advances in computational materials science present novel opportunities for structure discovery and optimization, including uncovering of unsuspected compounds and metastable structures, electronic structure, surface, and nano-particle properties. The practical realization of these opportunities requires systematic generation and classification of the relevant computational data by high-throughput methods. In this paper we present Aflow (Automatic Flow), a software framework for high-throughput calculation of crystal structure properties of alloys, intermetallics and inorganic compounds. The Aflow software is available for the scientific community on the website of the materials research consortium, aflowlib.org. Its geometric and electronic structure analysis and manipulation tools are additionally available for online operation at the same website. The combination of automatic methods and user online interfaces provide a powerful tool for efficient quantum computational materials discovery and characterization.

cond-mat.mtrl-sci↗

arXiv2005

Coarse-graining protein energetics in sequence variables

We show that cluster expansions (CE), previously used to model solid-state materials with binary or ternary configurational disorder, can be extended to the protein design problem. We present a generalized CE framework, in which properties such as energy can be unambiguously expanded in the amino-acid sequence space. The CE coarse grains over nonsequence degrees of freedom (e.g., side-chain conformations) and thereby simplifies the problem of designing proteins, or predicting the compatibility of a sequence with a given structure, by many orders of magnitude. The CE is physically transparent, and can be evaluated through linear regression on the energies of training sequences. We show, as example, that good prediction accuracy is obtained with up to pairwise interactions for a coiled-coil backbone, and that triplet interactions are important in the energetics of a more globular zinc-finger backbone.

physics.bio-ph↗

arXiv2005

The electronic structure and band gap of LiFePO4 and LiMnPO4

Materials with the olivine LixMPO4 structure form an important new class of materials for rechargeable Li batteries. There is significant interest in their electronic properties because of the importance of electronic conductivity in batteries for high rate applications. The density of states of LixMPO4 (x = 0, 1 and M = Fe, Mn) has been determined with the ab initio GGA+U method, appropriate for these correlated electron systems. Computed results are compared with the optical gap of LiFePO4, as measured using UV-Vis-NIR diffuse reflectance spectroscopy. The results obtained from experiment (3.8-4.0 eV) and GGA+U computations (3.7 eV) are in very good agreement. However, standard GGA, without the same level of treatment of electron correlation, is shown to make large errors in predicting the electronic structure. It is argued that olivines are likely to be polaronic conductors with extrinsically determined carrier levels and that their electronic conductivity is therefore not simply related to the band gap.

cond-mat.mtrl-sci↗

arXiv2005

First-principles prediction of redox potentials in transition-metal compounds with LDA+U

First-principles calculations within the Local Density Approximation (LDA) or Generalized Gradient Approximation (GGA), though very successful, are known to underestimate redox potentials, such as those at which lithium intercalates in transition metal compounds. We argue that this inaccuracy is related to the lack of cancellation of electron self-interaction errors in LDA/GGA and can be improved by using the DFT+$U$ method with a self-consistent evaluation of the $U$ parameter. We show that, using this approach, the experimental lithium intercalation voltages of a number of transition metal compounds, including the olivine Li$_{x}$MPO$_{4}$ (M=Mn, Fe Co, Ni), layered Li$_{x}$MO$_{2}$ ($x=$Co, Ni) and spinel-like Li$_{x}$M$_{2}$O$_{4}$ (M=Mn, Co), can be reproduced accurately.

cond-mat.mtrl-sci↗

arXiv2005

Accuracy of ab initio methods in predicting the crystal structures of metals: review of 80 binary alloys

Predicting and characterizing the crystal structure of materials is a key problem in materials research and development. We report the results of ab initio LDA/GGA computations for the following systems: AgAu, AgCd, AgMg, AgMo*, AgNa, AgNb*, AgPd, AgRh*, AgRu*, AgTc*, AgTi, AgY, AgZr, AlSc, AuCd, AuMo*, AuNb, AuPd, AuPt*, AuRh*, AuRu*, AuSc, AuTc*, AuTi, AuY, AuZr, CdMo*, CdNb*, CdPd, CdPt, CdRh, CdRu*, CdTc*, CdTi, CdY, CdZr, CrMg*, MoNb, MoPd, MoPt, MoRh, MoRu, MoTc*, MoTi, MoY*, MoZr, NbPd, NbPt, NbRh, NbRu, NbTc, NbY*, NbZr*, PdPt, PdRh*, PdRu*, PdTc, PdTi, PdY, PdZr, PtRh, PtRu, PtY, PtTc, PtTi, PtZr, RhRu, RhTc, RhTi, RhY, RhZr, RuTi, RuTc, RuY, RuZr, TcTi, TcY, TcZr, TiZr*, YZr* (*= systems in which the ab initio method predicts that no compounds are stable). A detailed comparison to experimental data confirms the high accuracy with which ab initio methods can predict ground states. Keywords: Binary Alloys, Ab initio, Intermetallics, Transition Metals, StructureAluminum, Cadmium, Gold, Magnesium, Molybdenum, Niobium, Palladium, Platinum, Rhodium, Ruthenium, Scandium, Silver, Sodium, Titanium, Technetium, Yttrium, Zirconium.

cond-mat.mtrl-sci↗

arXiv2003

Predicting Crystal Structures with Data Mining of Quantum Calculations

Predicting and characterizing the crystal structure of materials is a key problem in materials research and development. It is typically addressed with highly accurate quantum mechanical computations on a small set of candidate structures, or with empirical rules that have been extracted from a large amount of experimental information, but have limited predictive power. In this letter, we transfer the concept of heuristic rule extraction to a large library of ab-initio calculated information, and demonstrate that this can be developed into a tool for crystal structure prediction.

cond-mat.mtrl-sci↗