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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Oxygen self-diffusion in ThO 2 under pressure: Connecting point defect parameters with bulk properties

ThO 2 is a candidate material for use in nuclear fuel applications and as such it is important to investigate its materials properties over a range of temperatures and pressures. In the present study molecular dynamics calculations are used to calculate elastic and expansivity data. These are used in the framework of a thermodynamic model, the cBΩ model, to calculate the oxygen self-diffusion coefficient in ThO 2 over a range of pressures (–10–10 GPa) and temperatures (300–1900 K). As a result, increasing the hydrostatic pressure leads to a significant reduction in oxygen self-diffusion. Conversely, negative hydrostatic pressure significantly enhances oxygen self-diffusion.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Influence of point defects on the electronic and topological properties of monolayer WTe 2

In some topological insulators such as monolayers of WTe 2 or graphene with spin-orbit coupling, band inversion originates from chemical bonding and space group symmetry, in contrast to materials such as Bi 2 Se 3 , where the band inversion derives from relativistic effects in the atoms. In the former, band inversion is susceptible to changes of the chemical environment, e.g., by defects, while the latter are less affected by defects due to the larger energy scale associated with atomic relativistic effects. Motivated by recent experiments, we study the effect of Te vacancies and Te adatoms on the electronic properties of WTe 2 . We find that the Te vacancies have a formation energy of 2.21 eV, while the formation energy of the Te adatoms is much lower with 0.72 eV. Here, the vacancies strongly influence the band structure and we present evidence that band inversion is already reversed at the nominal composition of WTe 1.97 . In contrast, we show that the adatoms do not change the electronic structure in the vicinity of the Fermi level and thus the topological properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Point defect induced incommensurate dipole moments in the K Ca 2 Nb 3 O 10 Dion-Jacobson layered perovskite

Local structural distortions due to isolated atomic defects and defect complexes strongly affect the macroscopic properties of oxide ceramics. While the characterization of local defect structures is more common in simple AB O 3 perovskites, unambiguous determination of the same in layered perovskites is more difficult due to their complex crystal structures. In this study, we combined x-ray pair distribution function and density functional theory calculations to characterize the structure of cation-oxygen divacancy pairs in a Dion-Jacobson (D-J) layered perovskite. Our results indicate that local incommensurate dipole moments with polarization density in the range of ~ 0.1–17μC/cm 2 are created due to divacancy-induced structural distortions in the D-J phase K Ca 2 Nb 3 O 10 . This is comparable with defect dipole moments observed in well-known perovskite ferroelectrics. The current results imply that controlling the atomic defects can potentially lead to significant control of dielectric properties in D-J layered perovskites.

3-dimensional systems↗
DOE OSTI2015

Point defect stability in a semicoherent metallic interface

Here, we introduce a comprehensive density functional theory (DFT) -based study of different aspects of one vacancy and He impurity atom behavior at semicoherent interfaces between the low-solubility transition metals Cu and Nb. Such interfaces have not been previously modeled using DFT. A thorough analysis of the stability and mobility of the two types of defects at the interfaces and neighboring internal layers has been performed and the results have been compared to the equivalent cases in the pure metallic matrices. The different behavior of fcc and bcc metals on both sides of the interface has been specifically assessed. The modeling effort undertaken is the initial attempt to study the stability and defect energetics of noncoherent Cu/Nb interfaces from first principles, in order to assess their potential use in radiation-resistant materials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Thermal conductivity of InN with point defects from first principles

We present ab initio calculations of thermal conductivity of InN with vacancies and substitutional defects using a full solution of the Peierls-Boltzmann transport equation. Our parameter-free calculations are in good agreement with experimental measurements demonstrating the predictive power of this approach. Phonon-defect scattering rates are computed from a Green's function methodology that is nonperturbative and includes interatomic force constant variance induced near the defects. Restricting calculations to first-order perturbation approaches can overestimate optic phonon scattering rates by nearly three orders of magnitude. On the other hand, neglecting the force variance weakens the scattering rates by about an order of magnitude, mostly in the low-frequency region below 2 THz. Here, this work elucidates important properties of phonon-defect scattering in thermal transport and demonstrates the predictive power of the coupling of Peierls-Boltzmann transport, Green's function methods, and density functional theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Fundamental Resolution of Difficulties in the Theory of Charged Point Defects in Semiconductors

A defect’s formation energy is a key theoretical quantity that allows the calculation of equilibrium defect concentrations in solids and aids in the identification of defects that control the properties of materials and device performance, efficiency, and reliability. The theory of formation energies is rigorous only for neutral defects, yet the Coulomb potentials of charged defects require additional ad hoc numerical procedures. In this report, we invoke statistical mechanics to derive a revised theory of charged-defect formation energies, which eliminates the need for ad hoc numerical procedures. Calculations become straightforward and transparent. We introduce calculations demonstrating the significance of the revised theory for defect formation energies and thermodynamic transition levels.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Oxygen point defect accumulation in single-phase U O 2 + x

UO 2.07 was characterized using neutron total scattering in order to elucidate defect morphology in the low oxygen-to-metal regime (x < 0.125 for UO 2+x ). Data were collected at temperatures (600 and 1000 °C) coinciding with the single-phase UO 2+x region of the established phase diagram, and results were compared with data of stoichiometric UO 2 collected at near-identical temperatures. Experimental data were modeled and interpreted using a holistic approach employing a combination of analyses that characterized multiple spatial length scales. Preferential modeling of long-range atomic arrangements with Rietveld refinement suggests the existence of primarily monointerstitials in UO 2.07 , whereas preferential modeling of short-range atomic structures with small-box pair distribution function (PDF) refinement indicates the presence of defect clusters in UO 2.07 . Simultaneous modeling of multiple length scales using complementary reverse Monte Carlo (RMC) and molecular dynamics (MD) methods confirms that excess oxygen atoms in UO 2.07 exist as small defects, such as monointerstitials and di-interstitials. RMC and MD results agree with diffraction analysis but differ significantly from small-box PDF refinements, which may be related to a lack of intermediate- and long-range structural information gained from the small-box PDF refinement procedure. Employing a combination of analysis methods with varying length-scale sensitivities enabled more accurate assessment of the UO 2+x defect structure. Furthermore, our findings provide experimental support for previously predicted di-interstitial defect morphologies in UO 2+x that highly influence the accurate prediction of bulk physiochemical properties of UO 2+x , such as oxygen diffusivity.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2024

Point defects in CdTe and CdSeTe alloy: a first principles investigation with DFT+U

CdTe and its alloy CdSeTe are widely used in optoelectronic devices, such as radiation detectors and solar cells, due to their superior electrical properties. However, the formation of defects and defect complexes in these materials can significantly affect their performance. As a result, understanding the defect formation and recombination processes in CdTe and CdSeTe alloy is of great importance. In recent years, density functional theory (DFT) calculations have emerged as a powerful tool for investigating the properties of defects in semiconductors. In this paper, we use DFT+U calculations to comprehensively study the properties of intrinsic defects as well as extrinsic defects induced by commonly used dopants, such as Cu and group V elements, in CdTe and CdSeTe alloy. This work provides insights into the effects of these defects on the electrical and optical properties of the material.

14 SOLAR ENERGY↗
DOE OSTI2018

Impact of point defects on electrochromism in WO3

Using hybrid density functional theory, we investigate the influence on electronic structure of common defects and impurities in tungsten oxide (WO3). As an easily reducible perovskite with the A-site atom missing, high concentrations of foreign dopants and oxygen deficiencies are possible. Our calculations show that both oxygen vacancies and alkali dopants are shallow donors, and we explore the physical origins for this behavior. In particular, we examine whether oxygen vacancies can give rise to localized states or small polarons. Our results show that in crystalline material no such charge localization occurs. We discuss how these results impact electrical conductivity and optical properties.

Peelaers, Hartwin↗