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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Donor doping of corundum (Al x Ga 1- x ) 2 O 3

We report corundum (Al x Ga 1-x ) 2 O 3 alloys have been proposed as a candidate ultrawide-bandgap oxide for a number of applications, but doping is unexplored. We examine the prospects for n-type doping with H, Si, Ge, Sn, Hf, Zr, and Ta in corundum (Al x Ga 1-x ) 2 O 3 alloys using first-principles calculations. All of the dopants are shallow donors in corundum Ga 2 O 3 . In the (Al x Ga 1-x ) 2 O 3 alloy, they transition from shallow to deep donors at Al compositions that are unique to each donor. Si and Hf remain shallow donors up to the highest Al contents in corundum (Al x Ga 1-x ) 2 O 3 alloys and are still shallow even as the (Al x Ga 1-x ) 2 O 3 bandgap exceeds 6.5 eV. Finally, we address the detrimental role of cation vacancies as compensating deep acceptors and suggest that doping in a hydrogen-rich environment under cation-rich conditions can be used to overcome this problem.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Reduction kinetics of hercynite redox materials for solar thermochemical water splitting

Solar thermochemical water splitting (STWS) using a hercynite redox cycle is a promising technology for producing renewable H 2 . Here, the reduction kinetics of hercynite (FeAl 2 O 4 ) is evaluated using thermogravimetric and XRD analyses. The results indicate that as-prepared hercynite materials undergo reduction via two different reaction mechanisms. The reaction first proceeds by a nucleation and growth mechanism (AE1), followed by a third-order kinetic model (F3). XRD analyses show the occurrence of superstoichiometric oxygen in the spinel structure of FeAl 2 O 4+δ in the second reaction mechanism, which indicates the formation of cationic vacancies. TGA and XRD analyses confirm that hercynite maintains its spinel structure when the materials are thermally reduced and oxidized with steam. Differential scanning calorimetry (DSC) performed on FeAl 2 O 4 particles shows two heats of reaction in agreement with the two observed mechanisms. This is the first kinetics study to report on the thermal reduction of hercynite.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Atomistically-informed modeling of point defect clustering and evolution in irradiated ThO 2

A cluster dynamics (CD) model has been developed to investigate the nucleation and growth of point defect clusters, i.e., interstitial prismatic loops and nanoscale and sub-nanoscale voids, in ThO 2 during irradiation by energetic particles. The model considers cluster off-stoichiometry due to the asymmetry of point defect generation on the O and Th sublattices under irradiation, as well as the point defect diffusivities and the defect binding energies to clusters. The energies were established using detailed molecular dynamics simulations considering the statistical variability of cluster configuration. A high-order adaptive time-integration has been used to solve the model. The predicted loop density and their average size is in good agreement with reported experimental observations for proton irradiated ThO 2 at 600°C. In conclusion, the model did not predict void evolution due to the sluggish kinetics of cation vacancies, explaining the absence of voids in proton irradiated ThO 2 (and other oxides) at relatively low temperatures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Influence of TiO 2 on the densification behaviour of Yb 2 O 3

The effect of temperature and heating rate on the densification of ytterbia (Yb 2 O 3 ), with and without titania (TiO 2 ) doping was investigated. It is shown that up to a certain doping level, titania doping enhances the densification behaviour of ytterbia. The effect of titania doping on crystal structure confirms that titania is substitutionally incorporated in ytterbia up to the solubility limit, which corresponds well with the densification results. The increased densification rate of titania-doped ytterbia is attributed to the formation of cation vacancy and lattice distortion. Using constant heating rate experiments, the activation energy for densification has been calculated and it is shown that in the intermediate density range (60% to 85%), the activation energy is independent of the density. Furthermore, titania doping increases the activation energy for densification.

20 FOSSIL-FUELED POWER PLANTS↗
DOE OSTI2023

Complex defect chemistry of hydrothermally-synthesized Nb-substituted β$'$-LiVOPO 4

Lithium vanadyl phosphate (LiVOPO 4 ) is a next-generation multielectron battery cathode that can intercalate up to two Li-ions per V-ion through the redox couples of V 4+ /V 3+ and V 5+ /V 4+ . However, its rate capacity is undermined by the sluggish Li-ion diffusion in the high-voltage region (4 V for V 5+ /V 4+ redox). Nb substitution was used to expand the crystal lattice to facilitate Li-ion diffusion. Here, in this work, Nb substitution was achieved via hydrothermal synthesis, which resulted in a new, lower symmetry β'-LiVOPO 4 phase with preferential Nb occupation of one of the two V sites. This phase presents complex defect chemistries, including cation vacancies and hydrogen interstitials, characterized by a combination of X-ray and neutron diffraction, elemental and thermogravimetric analyses, X-ray absorption spectroscopy, and magnetic susceptibility measurements. The Nb-substituted samples demonstrated improved capacity retention and rate capabilities in the high-voltage region, albeit an enlarged voltage hysteresis related to a partial V 4+ /V 3+ redox reaction, as evidenced by ex situ X-ray absorption spectroscopy and pair distribution function analysis. This work highlights the importance of understanding the complex defect chemistry and its consequence on electrochemistry in polyanionic intercalation compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2020

The role of chemical potential in compensation control in Si:AlGaN

Reduction in compensation in Si-doped Al-rich AlGaN is demonstrated via chemical potential control (CPC). The chemical potentials and the resulting formation energies of carbon on the nitrogen site (C N ) and cation vacancy complex with Si (V III + nSi III ) were related to growth variables through a thermodynamic supersaturation model, which quantitatively predicted the incorporation of C N and the generation of the V III + nSi III complex. The compensation “knee” behavior, i.e., decreasing conductivity with increasing Si incorporation beyond a certain concentration, was successfully controlled. The maximum free carrier concentration was improved by impeding the formation of V III + nSIi III complexes under III-richer conditions, while the impurity compensation by C N was reduced by making the growth environment N-richer. The results of Hall effect measurement and photoluminescence agreed well with quantitative theoretical predictions of the CPC model. Based on the developed model, the highest conductivity of 160 Ω -1 cm -1 with free carrier concentration of 3 × 10 19 cm -3 in Al 0.7 Ga 0.3 N ever reported was achieved on single crystal AlN substrates. The demonstrated predictive power of the CPC model should greatly reduce the empirical analysis or iterative experimentation that would otherwise be necessary.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Ultrawide bandgap semiconductor heterojunction p–n diodes with distributed polarization-doped p-type AlGaN layers on bulk AlN substrates

Ultrawide bandgap heterojunction p–n diodes with polarization-induced AlGaN p-type layers are demonstrated using plasma-assisted molecular beam epitaxy on bulk AlN substrates. Current–voltage characteristics show a turn-on voltage of Vbi≈5.5 V, a minimum room temperature ideality factor of η≈1.63, and more than 12 orders of current modulation at room temperature. A stable current operation of the ultrawide bandgap semiconductor diode is measured up to a temperature of 300 °C. The one-sided n+–p heterojunction diode design enables a direct measurement of the spatial distribution of polarization-induced mobile hole density in the graded AlGaN layer from the capacitance–voltage profile. The measured average mobile hole density is p∼5.7×1017 cm−3, in close agreement with what is theoretically expected from distributed polarization doping. Light emission peaked at 260 nm (4.78 eV) observed in electroluminescence corresponds to interband radiative recombination in the n+ AlGaN layer. A much weaker deep-level emission band observed at 3.4 eV is attributed to cation-vacancy and silicon complexes in the heavily Si-doped AlGaN layer. These results demonstrate that distributed polarization doping enables ultrawide bandgap semiconductor heterojunction p–n diodes that have wide applications ranging from power electronics to deep-ultraviolet photonics. These devices can operate at high temperatures and in harsh environments.

Physics↗
DOE OSTI2021

Antiphase boundary migration as a diffusion mechanism in a P3 sodium layered oxide

The electrode materials of Na-ion batteries must exhibit fast ion transport, a property that is often negatively impacted when mobile cations order. Many layered Na (and K) intercalation compounds adopt the P3 crystal structure, which tends to stabilize cation-vacancy orderings over a wide range of Na concentrations. The ordered phases have been predicted to consist of domains that are periodically separated by antiphase boundaries (APBs) to accommodate variations in Na concentration. The mechanisms and rates with which Na diffuses within such ordered phases of P3 are not understood. We have conducted a first-principles study of Na diffusion in the P3 structure, using Na x CoO 2 as a model system. We find that most simple nearest-neighbor hops in Na-vacancy ordered phases in P3 result in unstable configurations and are therefore forbidden. We identify an alternative mechanism of Na transport that is mediated by APB migration via the formation and expansion of kinks along domain boundaries. The limiting kinetic barriers for this mechanism are quite low (0.03 to 0.30 eV). Vacancy defects are not required, and furthermore have formation energies that are higher than those of the APB kink defects. Furthermore, our results suggest that APB migration is a fundamental diffusion mechanism in the P3 structure.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Reducing Dark Conductivity of Cesium Tin Halide Perovskites with Donor Doping

The inorganic tin halide perovskites (such as Cs Sn Br 3 and Cs Sn I 3 ) exhibit high hole concentrations, which have been attributed to the presence of intrinsic defects such as cation vacancies. Despite their promise as photovoltaic materials, this high intrinsic conductivity limits deployment. Using hybrid density functional theory, we examine the impact of these intrinsic defects on the electronic properties of Cs Sn Br 3 and Cs Sn I 3 . We find that tin and cesium vacancies have low formation energies, especially under Sn -poor conditions, in agreement with prior work, and that the presence of these native acceptor defects can lead to high hole concentrations. However, effective donor doping of these systems can be obtained via Sc or Y incorporation. These impurities substitute on the Sn site, where they act as single donors. By examining the formation energy of these dopants versus the native defects, we show that Sc or Y doping should lead to a strong compensation, reducing hole concentrations under Sn -rich conditions. Published by the American Physical Society 2025

Lyons, John L. (ORCID:0000000180233055)↗
DOE OSTI2021

New experimental approach to understanding the chemical reactivity of oxide surfaces

Metal oxides have been an attractive option for a range of applications, including hydrogen sensors, microelectronics, and catalysis, due to their reactivity and tunability. The properties of metal oxides can vary greatly on their precise surface structure; however, few surface science techniques can achieve atomistic-level determinations of surface structure, and fewer yet can do so for insulator surfaces. Low energy ion beam analysis offers a potential insulator-compatible solution to characterizing the surface structure of metal oxides. As a feasibility study, we apply low energy ion beam analysis to investigate the surface structure of a magnetite single crystal, Fe 3 O 4 (100). We obtain multi-angle maps using both forward-scattering low energy ion scattering (LEIS) and backscattering impact-collision ion scattering spectroscopy (ICISS). Both sets of experimental maps have intensity patterns that reflect the symmetries of the Fe 3 O 4 (100) surface structure. However, analytical interpretation of these intensity patterns to extract details of the surface structure is significantly more complex than previous LEIS and ICISS structural studies of one-component metal crystals, which had far more symmetries to exploit. To gain further insight into the surface structure, we model our experimental measurements with ion-trajectory tracing simulations using molecular dynamics. Our simulations provide a qualitative indication that our experimental measurements agree better with a subsurface cation vacancy model than a distorted bulk model.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Local Atomic Configuration in Pristine and A-Site Doped Silver Niobate Perovskite Antiferroelectrics

Antiferroelectrics have attracted increasing research interests in recent years due to both their great potential in energy storage applications and intriguing structural characteristics. However, the links between the electrical properties and structural characteristics of distorted perovskite antiferroelectrics are yet to be fully deciphered. Here, we adopt local-structure methods to elucidate the nanoscale atomic structure of AgNbO3-based antiferroelectrics and their structural evolution upon La doping. The local structural features including interatomic distance distributions and atomic displacements have been analyzed using neutron small-box pair distribution function (PDF) refinement in conjunction with large-box Reverse Monte Carlo modelling. Our results highlight the correlation of cation displacements in AgNbO 3 and its disruption by the incorporation of La, apparently in corroboration with the observed anomalous dielectric properties. Spatial ordering of cation vacancies is observed in La-doped AgNbO 3 samples, which coordinates with oxygen octahedral tilting to relieve lattice strain. These results provide renewed insights into the atomic structure and antiferroelectric phase instabilities of AgNbO 3 and relevant perovskite materials, further lending versatile opportunities for enhancing their functionalities.

36 MATERIALS SCIENCE↗
DOE OSTI2024

MOCVD growth of Si-doped α-(AlGa) 2 O 3 on m-plane α-Al 2 O 3 substrates

We reported the growth of (AlGa) 2 O 3 layers on ($10\overline{10}$) α-Al 2 O 3 substrates using cold-wall metalorganic chemical vapor deposition, and the electrical characterization of Si-doped (AlGa) 2 O 3 layers. In the Ga 2 O 3 growth, the α phase was dominant at low growth temperature, achieving the growth rate of 2.4 μm h –1 at 650 °C. Sheet resistance and electrical conductivity of the Ga 2 O 3 layers with a Si concentration of 3 × 10 20 cm –3 were 1 × 10 4 Ω/square and 8.3 S cm –1 , respectively, at the measurement temperature of 500 °C. The Al composition in the (AlGa) 2 O 3 layers was controlled from 0% to 74%. In our initial attempts, we obtained electrically conductive α-(AlGa) 2 O 3 layers by Si doping (2 × 10 –9 S cm –1 in the sample with an Al composition of 56%). Finally, hybrid functional calculations suggest the conductivities are limited by compensation of Si through cation vacancy complexes, and not by the significant amounts of co-incorporated C and N that are predicted to be electrically passivated by hydrogen.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Determination of the vacancy distribution over Al cation sites in γ-Al 2 O 3

Although gamma-alumina (γ-Al 2 O 3 ) is an extensively used material with wide-ranging applications due to its inherently high surface area and acidity, its atomic structure is still not fully understood. γ-Al 2 O 3 is described as having a spinel-like structure, where the O sublattice has a face-centered cubic (FCC) arrangement and Al cations are placed in the spinel tetrahedral and octahedral interstitial sites. Achieving the correct stoichiometry of Al 2 O 3 , however, requires the introduction of Al vacancies into some of the interstitial sites. Despite the importance of accurately describing the structure of γ-Al 2 O 3 , the distribution of vacancies between tetrahedral and octahedral sites remains unclear, in part because of the usually poor crystalline quality of γ-Al 2 O 3 that has often been used in previous studies. To determine the actual cation distribution in γ-Al 2 O 3 , single-crystalline γ-Al 2 O 3 was investigated using a correlative approach of experimental and simulated selected-area electron diffraction (SAED) and high-resolution electron energy-loss spectroscopy (EELS). Comparison of the reflection intensities in single-crystal SAED to simulated SAED from models with varied vacancy distributions revealed that vacancies exist primarily on tetrahedral sites, contrary to the placement of vacancies on octahedral sites proposed in several common models. Comparison of EELS spectra—acquired with the highest energy resolution reported so far for γ-Al 2 O 3 —with ab initio multiple scattering EELS simulations confirmed the distribution of vacancies on tetrahedral sites. These results enable more accurate modeling of γ-Al 2 O 3 to better predict its properties in existing and future applications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Atomically Revealing Bulk Point Defect Dynamics in Hydrogen‐Driven γ‐Fe 2 O 3 → Fe 3 O 4 → FeO Transformation

Understanding how point defects in the bulk govern redox transformations is essential for advancing hydrogen-based metal production and designing high-performance oxide materials. This study reveals the atomic-scale mechanisms driving hydrogen-induced reduction of γ-Fe 2 O 3 to Fe 3 O 4 , focusing on how bulk vacancy dynamics dictate structural evolution and reaction kinetics. A key finding is the pronounced contrast in defect behavior between the two oxides: in γ-Fe 2 O 3 , intrinsic Fe vacancies promote oxygen vacancy clustering, destabilizing the local lattice and driving nanopore formation. In contrast, Fe 3 O 4 exhibits a higher oxygen vacancy formation energy and lacks intrinsic Fe vacancies, suppressing vacancy aggregation and maintaining a dense, pore-free structure. This divergence governs distinct reduction pathways—γ-Fe 2 O 3 undergoes an interface-reaction-limited transformation confined to the γ-Fe 2 O 3 /Fe 3 O 4 boundary, while Fe 3 O 4 supports a uniform increase in oxygen vacancy concentration, enabling bulk-phase reduction to lower-oxide FeO. Integrated in situ electron microscopy and density functional theory modeling uncover a vacancy-mediated mechanism, where synergistic cation-anion vacancy dynamics steer microstructure evolution and phase progression. These insights highlight the critical role of vacancy dynamics in controlling oxide reactivity and offer a pathway toward vacancy engineering to enhance reduction kinetics in hydrogen metallurgy and to tailor porosity, reactivity, and structural resilience in oxide-based catalysts and energy materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Ab Initio Insights to Metal Passivation: Diffusion and Defect Formation in Amorphous Zirconia and Alumina

Metal passivation refers to the formation of protective oxide films on metals, which shield them from further corrosion and oxidation, playing a crucial role in maintaining their stability. The mesoscopic Point Defect Model has successfully predicted passivity as a steady state process where oxide growth from oxygen vacancies at the metal/film interface competes with oxide dissolution at the film/environment interface. In this work, informed by the Point Defect Model parameters, we use first-principles calculations to calculate defect formation and atomic diffusion in amorphous materials and correlate these descriptors with the behavior and growth of the oxide film. Focusing on amorphous zirconia and alumina, we demonstrate that defect formation energies exhibit significant variability in amorphous systems. In alumina, vacancies dominate, with cation and anion vacancies occurring at comparable concentrations. Diffusion calculations for stoichiometric amorphous alumina and zirconia, as well as oxygen-deficient zirconia, reveal faster diffusion in the oxygen-deficient case, highlighting the impact of defects on transport. Comparison of calculated self-diffusion coefficients for the dominant defect species with experimentally measured oxide thicknesses shows a clear correlation, suggesting that first-principles-derived diffusivity information can serve as a key descriptor for surface passivation film growth.

Farnell, Mackinzie S. [University of California, B↗
DOE OSTI2020

First Principles Modeling of Tritium Trapping in γ-LiAlO 2 Nanovoids

Experimental STEM and APT investigations both indicate the presence of nanovoids in γ-LiAlO 2 pellets. While those nanovoids can affect the diffusion of tritium in pellets, first principles simulations have been conducted to evaluate the potential for tritium trapping by single and double cation and anion vacancies. It was found that tritium trapping allows to recover a significant amount of the formation energy cost of cation vacancy, while the trapping of tritium by anion vacancies further increase the energy cost of forming a vacancy. The simulations also indicated the potential for tritium to be trapped in various forms by forming bonds such as Al—T, O—T or T 2 . These results raise further questions regarding the speciation and dynamics of tritium trapped in nanovoids.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2026

Deciphering competing elementary steps to correlate electrocatalyst chemical state with activity

The overpotential in multielectron transfer heterogeneous electrocatalysis fundamentally arises from thermodynamic and kinetic disparities among elementary steps; however, deciphering coupled and competing steps has long remained a challenge. Here, we establish an electrochemical deconvolution paradigm based on key processes in electrocatalytic reactions, such as charge accumulation, electron/proton transfer, and intermediate evolution, to resolve competing elementary steps. Taking the oxygen evolution reaction as a prototypical reaction, we design a model catalyst featuring a precise isolated cation-anion vacancy pair and track the previously elusive electrochemical behavior of lattice oxygen by disentangling interference from adsorbed oxygen intermediates. Mechanistically, the lattice oxygen oxidation pathway originates from the spontaneous, nonelectrochemical deprotonation of replenished water molecules coordinated to unsaturated cation sites. Alternating current techniques further reveal that although lattice oxygen oxidation requires a higher potential than metal oxidation, it exhibits faster kinetics, providing insight into its superior catalytic activity. These findings establish a direct experimental correlation between the initial chemical state and the catalytic activity and prove surface-confined lattice oxygen cycling. Furthermore, expanding conventional potential-current analysis into a multidimensional framework enables disentanglement of thermodynamic and kinetic contributions of key elementary steps, thereby guiding the rational optimization of various complex multielectron transfer reactions.

OER↗