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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Lithium-excess cathode material and co-precipitation formation method

A lithium-excess cathode material according to Li1+xNiaMnbCocModO2−y (0<x<0.3, 0≤a≤1, 0≤b≤1, 0≤c≤1, 0≤d≤0.2, 0≤y≤0.25) in the form of secondary spherical microparticles formed from primary spherical nanoparticles. The primary nanoparticles can in the range of ˜130 nm to 170 nm and the secondary in the range of ˜2-3 μm. A method of formation includes mixing a carbonates or hydroxides solution into a mixed solution of transition metal (M) ions with predetermined stoichiometry under stirring, and aging resulting transition metal carbonates or hydroxides at a predetermined temperature for period of time to produce primary nanoparticles of a predetermined size. A gas-solid interface reaction to uniformly creating oxygen vacancies without affecting structural integrity of Li-excess layered oxides is also provided.

Meng, Ying Shirley↗
DOE OSTI2021

Catalyst coating of a perovskite film and particles exsoluted from the perovskite film

A hybrid catalyst coating composed of a conformal thin film with exsoluted PrOx nano-particles. The conformal PNM thin film can be a perovskite composition of PrNi0.5Mn0.5O3 (PNM). The PrOx nano-particles dramatically enhance the oxygen reduction reaction kinetics via a high concentration of oxygen vacancies while the thin PNM film effectively suppresses strontium segregation from the cathode of an intermediate-temperature solid oxide fuel cell.

Liu, Meilin↗
DOE OSTI2021

Segregation resistant perovskite oxides with surface modification

A method and a composition to stabilize the surface cation chemistry of the perovskite or related oxides, and thus, to minimize or completely avoid the detrimental segregation and phase separation of dopant cations at the surface can include modifying the surface with more oxidizable metal cations and/or more oxidizable metal oxides, thereby reducing the oxygen vacancy concentration at the very surface.

Yildiz, Bilge↗
DOE OSTI2022

Assessing the Role of Interfacial and Metal Sites in Pt/TiO2-Catalyzed Acetic Acid Hydrodeoxygenation

Thermochemical conversion of biomass to produce drop-in quality biofuels typically involves hydrodeoxygenation (HDO) steps following catalytic fast pyrolysis (CFP) to remove excess oxygen and create a more stable bio-oil product. HDO involves co-feeding the CFP vapor-phase product and H2 gas over a bi-functional catalyst. Reducible metal oxide-supported noble-metal catalysts (e.g., Pt/TiO2) are promising materials for HDO chemistry, with recent work aiming to elucidate the role of various Pt/TiO2 actives sites (i.e., Pt-metal, TiO2-support, Pt-TiO2-interfacial sites) in the competing desired deoxygenation and undesired decarboxylation/decarbonylation reactions for important classes of CFP vapor model compounds. Carboxylic acids are one important class of bio-derived compounds that has not been studied extensively for HDO on Pt/TiO2, particularly at the atomic level. Past experimental work evaluating Pt/C and Pt/TiO2 catalysts for acetic acid HDO (AA-HDO) demonstrated selectivity toward C-C and C-O bond-dissociation products, respectively. This work utilizes atomic-scale modeling to discern the role of Pt-metal and Pt-TiO2-interfacial sites in promoting key C-C bond-breaking, C-O bond-breaking, and (de)hydrogenation steps in AA-HDO. Using Pt(111) and Pt-TiO2-interface surface models, adsorption and reaction energetics calculated by density functional theory provide fundamental insights into the role of interfacial sites and oxygen vacancies in promoting desired deoxygenation pathways over undesired decarboxylation/decarbonylation pathways.

BIOMASS FUELS,INORGANIC, ORGANIC, PHYSICAL, AND AN↗
DOE OSTI2022

Ce:YAG/A12O3 composites for laser-excited solid-state white lighting

A method for fabricating a composite useful in a white light emitting device, includes mixing a phosphor and a filler to form a mixture; sintering the mixture (e.g., using spark plasma sintering) to form a composite; and annealing the composite to reduce oxygen vacancies and improve optical properties of the composite. Also disclosed is a white light emitting device including a laser diode or light emitting diode optically pumping the phosphor in the composite to produce white light. The composite fabricated using the method (and having. e.g., at most 50% phosphor by weight) can (1) reduce an operating temperature of the phosphor by 55 degrees, (2) increase an external quantum efficiency (e.g., by at least 15%) of the white light emitting device, and (3) result in color points and quality of the white light that is equal to or improved, as compared to without the filler.

Cozzan, Clayton J.↗
DOE OSTI2022

Thermally sensitive ionic redox transistor

A thermally sensitive ionic redox transistor comprises a solid channel, a solid reservoir layer, and a solid electrolyte layer disposed between the channel and the reservoir layer. A conductance of the channel is varied by changing the concentration of ions such as oxygen vacancies in the channel layer. Ionic conductivity of the gate, electrolyte, and channel layers increase with increasing temperature. Ion or vacancy transport between the channel and the reservoir layer across the electrolyte layer occurs in response to applying a voltage between the channel and the reservoir layer when the device is heated to an elevated temperature. When the device is cooled below the elevated temperature, the ions are trapped in one or more of the layers because the materials lose their ionic conductivity. A state of the redox transistor can be read by measuring the conductance of the channel.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2023

Magnetic Fluctuations in Niobium Pentoxide

Using a spin-polarized muon beam we were able to capture magnetic dynamics in an amorphous niobium pentoxide thin film. Muons are used to probe internal magnetic fields produced by defects. Magnetic fluctuations could be described by the dynamical Kubo-Toyabe model considering a time-dependent local magnetic field. We state that observed fluctuations result from the correlated motion of electron spins. We expect that oxygen vacancies play a significant role in these films and lead to a complex magnetic field distribution which is non-stationary. The characteristic average rate of magnetic field change is on the order of 100~MHz. The observed dynamics may provide insight into potential noise sources in Nb-based superconducting devices, while also highlighting the limitations imposed by amorphous oxides.

Krasnikova, Y.↗
DOE OSTI2024

Systems and methods for dynamically reconfigurable artificial synapses and neurons with tunable activation functions

An ionic redox transistor comprises a solid channel, a solid reservoir layer, and a solid electrolyte layer disposed between the channel and the reservoir layer. The channel exhibits a substantially linear current-voltage relationship in a first range of voltages, and a nonlinear current-voltage relationship in a second range of voltages that is greater than the first range of voltages. One or both of the substantially linear current-voltage relationship or the nonlinear current-voltage relationship of the channel is varied by changing the concentration of ions such as oxygen vacancies in the channel. Ion or vacancy transport between the channel and the reservoir layer across the electrolyte layer occurs in response to applying a voltage between the channel and the reservoir layer. Subject to the first range of voltages, the channel can function as a synapse device. Subject to the second range of voltages, the channel can function as a neuron device.

Talin, Albert Alec↗
DOE OSTI2023

Guided anisotropic oxygen transport in vacancy ordered oxides

Abstract Anisotropic and efficient transport of ions under external stimuli governs the operation and failure mechanisms of energy-conversion systems and microelectronics devices. However, fundamental understanding of ion hopping processes is impeded by the lack of atomically precise materials and probes that allow for the monitoring and control at the appropriate time- and length- scales. In this work, using in-situ transmission electron microscopy, we directly show that oxygen ion migration in vacancy ordered, semiconducting SrFeO 2.5 epitaxial thin films can be guided to proceed through two distinctly different diffusion pathways, each resulting in different polymorphs of SrFeO 2.75 with different ground electronic properties before reaching a fully oxidized, metallic SrFeO 3 phase. The diffusion steps and reaction intermediates are revealed by means of ab-initio calculations. The principles of controlling oxygen diffusion pathways and reaction intermediates demonstrated here may advance the rational design of structurally ordered oxides for tailored applications and provide insights for developing devices with multiple states of regulation.

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

Gallium vacancy formation in oxygen annealed β-Ga 2 O 3

Here, the formation and character of gallium vacancies (V Ga ) and their complexes in near surface and bulk regions of single crystal β-Ga 2 O 3 were explored using unintentionally doped single crystals grown by the Czochralski method. As-grown and O 2 annealed (up to 1550 °C) samples were investigated using positron annihilation spectroscopy (PAS) to study the top 0.05–6 μm, and also current–voltage measurements and infrared (IR) spectroscopy, with hydrogenated samples to probe V Ga , to study the bulk. After annealing in O 2 > 1000 °C, the β-Ga 2 O 3 resistivity begins increasing, up to ~10 9 Ω cm for 1550 °C treatment, with the top 0.5 mm being many orders of magnitude more resistive. PAS measurements of the top 6 μm (S values) and very near surface 200 nm (diffusion length, L) indicate differential behavior as a function of peak annealing temperature. At least four temperature regimes of behavior are described. V Ga are present in the bulk after growth, but considerable changes occur upon annealing at a temperature ≈1000 °C, where L and S decrease simultaneously, suggesting an increasing defect concentration (L) but a decreasing defect volume (S). Annealing at a temperature ≈1400 °C increases S again, showing an increasing volume concentration of V Ga , with IR absorption showing a large signature of V Ga -2H, indicative of increased V Ga formation that was not present when annealing at a temperature ≈1000 °C. These results suggest that defect changes from annealing in oxygen are depth dependent, and that V Ga configuration may not be the same near the oxygen-exposed surface of the sample and in the bulk.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS2011

Kinetic Monte Carlo Simulation of Oxygen and Cation Diffusion in Yttria-Stabilized Zirconia

Yttria-stabilized zirconia (YSZ) is of interest to the aerospace community, notably for its application as a thermal barrier coating for turbine engine components. In such an application, diffusion of both oxygen ions and cations is of concern. Oxygen diffusion can lead to deterioration of a coated part, and often necessitates an environmental barrier coating. Cation diffusion in YSZ is much slower than oxygen diffusion. However, such diffusion is a mechanism by which creep takes place, potentially affecting the mechanical integrity and phase stability of the coating. In other applications, the high oxygen diffusivity of YSZ is useful, and makes the material of interest for use as a solid-state electrolyte in fuel cells. The kinetic Monte Carlo (kMC) method offers a number of advantages compared with the more widely known molecular dynamics simulation method. In particular, kMC is much more efficient for the study of processes, such as diffusion, that involve infrequent events. We describe the results of kinetic Monte Carlo computer simulations of oxygen and cation diffusion in YSZ. Using diffusive energy barriers from ab initio calculations and from the literature, we present results on the temperature dependence of oxygen and cation diffusivity, and on the dependence of the diffusivities on yttria concentration and oxygen sublattice vacancy concentration. We also present results of the effect on diffusivity of oxygen vacancies in the vicinity of the barrier cations that determine the oxygen diffusion energy barriers.

Good, Brian↗
DOE OSTI2025

Vacancy-Dependent Diffusion Mechanism in Oxygen-Defective SrFeO 3 Perovskite Materials: First-Principles Density Functional Theory and Experimental Approach

Understanding oxygen diffusion at the atomic scale in SrFeO 3−δ perovskites is crucial for developing oxygen storage materials with optimal performance. Such materials are required to have high stability, corrosion resistance, and acceptable oxygen storage capacity at moderate operating temperatures and pressures. Here, in this study, we used first-principles density functional theory and thermogravimetric analysis to study the vacancy-dependent oxygen diffusion in oxygen-deficient SrFeO 3−δ (δ = 0, 0.065, 0.125, 0.25, 0.5) perovskites. The electronic structures, including the partial- and spin-resolved density of states, for different SrFeO 3−δ phases were calculated and compared with available experimental and theoretical results. By mapping the migration pathways, we investigated diffusion mechanisms and calculated the energy barriers for oxygen diffusion in cubic, orthorhombic, and brownmillerite phases of SrFeO 3−δ perovskites. Using the calculated energy barriers, we deduced the diffusion time scales and diffusion coefficients within SrFeO 3−δ . A diffusion coefficient on the order of 10 –8 m 2 /s was obtained for SrFeO 2.875 . We experimentally investigated the roles of temperature and oxygen partial pressures on the redox kinetics and deduced the kinetics rate and diffusion density, which agreed well with the calculated values for the density of diffusing oxygen vacancy in the lattice. Our results showed that the energy barrier tends to reduce at higher oxygen concentrations. Our results serve as an important guideline for designing oxygen storage materials with optimal redox kinetics.

chemical looping with oxygen uncoupling (CLOU)↗
NASA NTRS2023

Oxidation of Silicon Particles Suspended in Mullite Matrix With Dual-Mode Vacancy Diffusion of Oxygen

Mullite bond coats filled with silicon particles have recently been proposed for the next generation of environmental barrier coatings (EBCs). The sacrificial oxidation of the silicon particles eliminates some of the environmental durability issues associated with traditional EBC designs. To investigate this novel design approach, a mathematical formulation and resulting numerical method have been developed for simulating the oxidation of silicon particles embedded in a mullite matrix. The focus is on the oxidation of the silicon in a dry oxygen environment where the oxidant is presumably oxygen. Oxygen transport from the exterior environment through the mullite is assumed to occur via two lattice defect diffusion mechanisms. As a result, the rate of oxygen transport is dictated by the self-diffusivity of the lattice defects. The numerical method is implemented using the COMSOL Multiphysics® program (COMSOL, Inc.). The method is applied to simulate the oxidation of silicon particles in a mullite pellet in recent oxidation experiments. Numerical results compare favorably with the oxidation behavior observed in the oxidation experiments. The numerical method is also used to investigate the effects of various features of the mullite-silicon particle system on the oxidation behavior.

silicon oxidation↗
DOE OSTI2020

Substituted SrFeO 3 as robust oxygen sorbents for thermochemical air separation: correlating redox performance with compositional and structural properties

Thermochemical air separation via cyclic redox reactions of oxide-based oxygen sorbents has the potential to achieve high energy efficiency. Although a number of promising sorbents have been investigated, further improvements in sorbent performance through a fundamental understanding of the structure–performance relationships are highly desirable. In this study, we systematically investigated the effects of A and B site dopants on the oxygen uptake/release properties (i.e., vacancy formation energy, reduction enthalpy, oxygen release temperature, and oxygen capacity) of the SrFeO 3 family of perovskites as oxygen sorbents. A monotonic correlation between DFT calculated oxygen vacancy formation energy and oxygen release temperature demonstrates the effectiveness of DFT for guiding sorbent selection. Combining vacancy formation energy with stability analysis, dopants such as Ba and Mn have been identified for tuning the redox property of SrFeO 3 sorbents, and increasing the oxygen capacity for temperature and pressure swings when compared to undoped SrFeO 3 . The Mn doped sample proved to be highly stable, with less than a 3% decrease in capacity over 1000 cycles. Although the dynamic nature of the redox process makes it difficult to use a single vacancy formation energy as the descriptor, a systematic approach was developed to correlate the oxygen storage capacities with the sorbents’ compositional properties and vacancy formation energies. The combination of DFT calculations with experimental studies from this study provides a potentially effective strategy for developing improved sorbents for thermochemical air separation.

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

High performance, electroforming-free, thin film memristors using ionic Na 0.5 Bi 0.5 TiO 3

Here, in ionically conducting Na 0.5 Bi 0.5 TiO 3 (NBT), we explore the link between growth parameters, stoichiometry and resistive switching behavior and show NBT to be a highly tunable system. We show that the combination of oxygen ionic vacancies and low-level electronic conduction is important for controlling Schottky barrier interfacial switching. We achieve a large ON/OFF ratio for high resistance/low resistance ( R HRS / R LRS ), enabled by an almost constant R HRS of ~10 9 Ω, and composition-tunable R LRS value modulated by growth temperature. R HRS / R LRS ratios of up to 10 4 and pronounced resistive switching at low voltages (SET voltage of <1.2 V without high-voltage electroforming), strong endurance (no change in resistance states after several 10 3 cycles), uniformity, stable switching and fast switching speed are achieved. Of particular interest is that the best performance is achieved at the lowest growth temperature studied (600 °C), which is opposite to the case of most other perovskite oxides for memristors, where higher growth temperatures are required for optimum performance. This is understood based on the oxygen vacancy control of interfacial switching in NBT, whereas a range of other mechanisms (including filamentary switching) occur in other perovskites. The study of NBT has enabled us to determine key parameters for achieving high performance memristors.

36 MATERIALS SCIENCE↗
NASA NTRS2020

On the Oxidation of the Silicon Bond Coat in Environmental Barrier Coatings

Deal and Grove’s formulation is applied to model the oxidation of the silicon bond coat in environmental barrier coatings. The approach is similar to the author’s previous study, except now the oxidant is assumed to flow through the coating as oxygen ions via vacancies on the oxygen sublattice. The use of an “effective permeability” of the oxidant in the coating is required under the vacancy diffusion assumption. It is found that the linear-parabolic growth equation from Deal and Grove, 2 xox + Axox = Bt , is still applicable for the oxidation of the silicon bond coat, but the equation for the parameter A must be modified to account for the coating. The expression for B is the same as that obtained by Deal and Grove and thus is unaffected by the presence of a coating. The modified expression for A is a linear function of the coating thickness and includes the ratio of the oxidant permeability in the oxide to its effective permeability in the coating

Roy M Sullivan↗
NASA NTRS2021

On the Oxidation of the Silicon Bond Coat in Environmental Barrier Coatings

Deal and Grove’s formulation is applied to model the oxidation of the silicon bond coat in environmental barrier coatings. The approach is similar to the author’s previous study, except now the oxidant is assumed to flow through the coating as oxygen ions via vacancies on the oxygen sublattice. The use of an “effective permeability” of the oxidant in the coating is required under the vacancy diffusion assumption. It is found that the linear-parabolic growth equation from Deal and Grove, , is still applicable for the oxidation of the silicon bond coat, but the equation for the parameter A must be modified to account for the coating. The expression for B is the same as that obtained by Deal and Grove and thus is unaffected by the presence of a coating. The modified expression for A is a linear function of the coating thickness and includes the ratio of the oxidant permeability in the oxide to its effective permeability in the coating.

silicon oxidation, ionic diffusion, permeation, li↗
NASA NTRS1984

The effects of lithium counterdoping on radiation damage and annealing in n(+)p silicon solar cells

Boron-doped silicon n(+)p solar cells were counterdoped with lithium by ion implantation and the resultant n(+)p cells irradiated by 1 MeV electrons. Performance parameters were determined as a function of fluence and a deep level transient spectroscopy (DLTS) study was conducted. The lithium counterdoped cells exhibited significantly increased radiation resistance when compared to boron doped control cells. Isochronal annealing studies of cell performance indicate that significant annealing occurs at 100 C. Isochronal annealing of the deep level defects showed a correlation between a single defect at E sub v + 0.43 eV and the annealing behavior of short circuit current in the counterdoped cells. The annealing behavior was controlled by dissociation and recombination of this defect. The DLTS studies showed that counterdoping with lithium eliminated three deep level defects and resulted in three new defects. The increased radiation resistance of the counterdoped cells is due to the interaction of lithium with oxygen, single vacancies and divacancies. The lithium-oxygen interaction is the most effective in contributing to the increased radiation resistance.

Weinberg, I.↗