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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 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

Kinetic Monte Carlo Simulation of Oxygen Diffusion in Ytterbium Disilicate

Ytterbium disilicate is of interest as a potential environmental barrier coating for aerospace applications, notably for use in next generation jet turbine engines. In such applications, the transport of oxygen and water vapor through these coatings to the ceramic substrate is undesirable if high temperature oxidation is to be avoided. In an effort to understand the diffusion process in these materials, we have performed kinetic Monte Carlo simulations of vacancy-mediated and interstitial oxygen diffusion in Ytterbium disilicate. Oxygen vacancy and interstitial site energies, vacancy and interstitial formation energies, and migration barrier energies were computed using Density Functional Theory. We have found that, in the case of vacancy-mediated diffusion, many potential diffusion paths involve large barrier energies, but some paths have barrier energies smaller than one electron volt. However, computed vacancy formation energies suggest that the intrinsic vacancy concentration is small. In the case of interstitial diffusion, migration barrier energies are typically around one electron volt, but the interstitial defect formation energies are positive, with the result that the disilicate is unlikely to exhibit experience significant oxygen permeability except at very high temperature.

Coatings↗
DOE OSTI2024

Thermodynamic origin of nonvolatility in resistive memory

Electronic switches based on the migration of high-density point defects, or memristors, are poised to revolutionize post-digital electronics. Despite significant research, key mechanisms for filament formation and oxygen transport remain unresolved, hindering our ability to predict and design device properties. For example, experiments have achieved 10 orders of magnitude longer retention times than predicted by current models. Here, using electrical measurements, scanning probe microscopy, and first-principles calculations on tantalum oxide memristors, we reveal that the formation and stability of conductive filaments crucially depend on the thermodynamic stability of the amorphous oxygen-rich and oxygen-poor compounds, which undergo composition phase separation. Including the previously neglected effects of this amorphous phase separation reconciles unexplained discrepancies in retention and enables predictive design of key performance indicators such as retention stability. Furthermore, this result emphasizes non-ideal thermodynamic interactions as key design criteria in post-digital devices with defect densities substantially exceeding those of today’s covalent semiconductors.

36 MATERIALS SCIENCE↗
NASA NTRS1996

Mechanism of unassisted ion transport across membrane bilayers

To establish how charged species move from water to the nonpolar membrane interior and to determine the energetic and structural effects accompanying this process, we performed molecular dynamics simulations of the transport of Na+ and Cl- across a lipid bilayer located between two water lamellae. The total length of molecular dynamics trajectories generated for each ion was 10 ns. Our simulations demonstrate that permeation of ions into the membrane is accompanied by the formation of deep, asymmetric thinning defects in the bilayer, whereby polar lipid head groups and water penetrate the nonpolar membrane interior. Once the ion crosses the midplane of the bilayer the deformation "switches sides"; the initial defect slowly relaxes, and a defect forms in the outgoing side of the bilayer. As a result, the ion remains well solvated during the process; the total number of oxygen atoms from water and lipid head groups in the first solvation shell remains constant. A similar membrane deformation is formed when the ion is instantaneously inserted into the interior of the bilayer. The formation of defects considerably lowers the free energy barrier to transfer of the ion across the bilayer and, consequently, increases the permeabilities of the membrane to ions, compared to the rigid, planar structure, by approximately 14 orders of magnitude. Our results have implications for drug delivery using liposomes and peptide insertion into membranes.

NASA Discipline Exobiology↗
DOE OSTI2023

Understanding the Mechanism of Light and Elevated Temperature Induced Degradation of p-type Silicon Solar Cells (Final Report)

Light- and elevated-temperature-induced degradation (LeTID) was first discovered in multicrystalline Si (mc-Si) solar cells and was initially attributed to metal impurities. Later, LeTID was reported in Czochralski (Cz) and float-zone (FZ) Si, and is considered as an important efficiency loss mechanism in p-type passivated emitter rear contact (p-PERC) Cz Si solar cells. LeTID causes ~10% relative and permeant efficiency losses in these cells in warmer climate regions where the module temperature is > 50 °C. Unlike light-induced degradation (LID), which is also observed in p-PERC cells, LeTID is slower and takes weeks to months in the field to saturate. Another difference compared to LID is that regeneration in LeTID proceeds very slowly, and field regeneration could take > 25 years — essentially the life of the module. Unlike B-O defects that are responsible for LID, neither B nor O impurities are directly involved in LeTID. LeTID appears to be unique to p-type Si, and is also observed in Ga-doped Si. Currently, most experimental evidence relates LeTID to the injection of hydrogen present in the dielectric surface passivation layers, such as SiN x and Al 2 O 3 , into the monocrystalline Si (c-Si) bulk during the fast-firing step. The involvement of hydrogen is further strengthened by controlled studies that show that increasing the amount of hydrogen in the dielectric during fast-firing increases the degree of LeTID. Similar to LID, a regeneration process has been discovered for LeTID. Regeneration of LeTID defects occurs when samples are exposed to 2–4 Suns illumination at elevated temperatures of 140–220 °C for 2–15 hr. Given the slower kinetics of LeTID and sample regeneration compared to LID, this poses a challenge for the manufacturing and field reliability of p-PERC cells, which will be the leading photovoltaic technologies over the next decade. Therefore, there is a need to understand LeTID and develop strategies to mitigate this effect. The defect responsible for LeTID has been extensively studied with over 100 publications, but direct spectroscopic evidence of this defect’s structure is lacking. Without an atomistic understanding of the LeTID defect, it is difficult to assess the long-term efficacy of the current industrial mitigation strategies. This, in turn, has implications on energy production for tens of gigawatts of these cells that will be deployed yearly worldwide. Using electron paramagnetic resonance, we identified a defect associated with LeTID with a g-value of 2.006, which we attribute to an Si dangling bond in an extended defect such as a vacancy agglomerate with H possibly within or in close vicinity. These vacancy agglomerates are likely created during the firing process, during which time H atoms are also injected into the bulk from the hydrogenated SiN x dielectric layer. Our atomistic-level insight shows that the LeTID defect can be mitigated by targeted intrinsic defect engineering of the c-Si material through a slower pull rate of the Cz ingot or 1000 °C oxygen ambient processing of the Si wafer to reduce the vacancy concentration. This project was a collaborative effort between the Colorado School of Mines and the National Renewable Energy Laboratory.

14 SOLAR ENERGY↗
DOE OSTI2020

CeTi 2 O 6 —A Promising Oxide for Solar Thermochemical Hydrogen Production

A large entropy of reduction is crucial in achieving materials capable of high-efficiency solar thermochemical hydrogen (STCH) production through two-step thermochemical water splitting cycles. We have recently demonstrated that the onsite electronic entropy of reduction attains an extreme value of 4.26 kB at 1500 K in Ce 4+ → Ce 3+ redox reactions, which explains the high performance and uniqueness of CeO 2 as an archetypal STCH material. However, ceria requires high temperatures (T > 1500 °C) to achieve a reasonable reduction extent because of its large reduction enthalpy, which is a major obstacle in practical applications. Therefore, new materials with a large entropy of reduction and lower reduction enthalpy are required. Here, we perform a systematic screening to search for Ce 4+ -based oxides which possess thermodynamics superior to CeO 2 for STCH production. We first search the Inorganic Crystal Structure Database (ICSD) and literature for Ce 4+ -based oxides and subsequently use density functional theory to compute their reduction enthalpies (i.e., oxygen vacancy formation energies). Here, we find that CeTi 2 O 6 with the brannerite structure is the most promising candidate for STCH because it possesses three essential characteristics of an STCH material: (i) a smaller reduction enthalpy compared to ceria yet large enough to split water, (ii) a high thermal stability, as reported experimentally, and (iii) a large entropy of reduction associated with Ce 4+ → Ce 3+ redox. Our proposed design strategy suggests that further exploration of Ce 4+ oxides for STCH production is warranted.

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

Mixed excitonic nature in water-oxidized BiVO 4 surfaces with defects

BiVO 4 is a promising photocatalyst for efficient water oxidation, with surface reactivity determined by the structure of active catalytic sites. Surface oxidation in the presence of oxygen vacancies induces electron localization, suggesting an atomistic route to improve the charge transfer efficiency within the catalytic cycle. In this paper, we study the effect of oxygen vacancies on the electronic and optical properties at BiVO 4 surfaces upon water oxidation. We use density functional theory and many-body perturbation theory to explore the change in the electronic and quasiparticle energy levels and to evaluate the electron-hole coupling as a function of the underlying structure. Further we show that while the presence of defects alters the atomic structure and largely modifies the wave-function nature, leading to defect-localized states at the quasiparticle gap region, the optical excitations remain largely unchanged due to the substantial hybridization of defect and nondefect electron-hole transitions. Our findings suggest that defect-induced surface oxidation supports improved electron transport, both through bound and tunable electronic states and via a mixed nature of the optical transitions, expected to reduce electron-hole defect trapping.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Molecular dynamics study of grain boundaries as defect sinks under irradiation in LiAlO 2 and LiAl 5 O 8

Lithium aluminate ceramics, LiAlO 2 and LiAl 5 O 8 , show promise in nuclear environments due to their excellent radiation tolerance. Molecular dynamics simulations investigate grain boundaries (GB) and their role in defect evolution. Results reveal that GBs act as efficient defect sinks, with Li and Al atoms exhibiting distinct behaviors during displacement cascades. Tritium migration in LiAlO 2 is also studied, showing rapid diffusion and stable configurations with oxygen, corroborated by ab initio simulations from the literature. The calculated tritium diffusion coefficient of 1.33 × 10 - ¹⁴ m²/s aligns with the literature, validating the model. LiAl 5 O 8 demonstrates superior defect healing compared to LiAlO 2 , attributed to enhanced atomic transfer between grains and GBs. These findings reveal key insights into defect dynamics, providing essential insights for their application in tritium-producing burnable absorber rods (TPBARs).

36 MATERIALS SCIENCE↗
DOE OSTI2023

Degradation of Ta 2 O 5 / SiO 2 dielectric cavity mirrors in ultra-high vacuum

In order for optical cavities to enable strong light-matter interactions for quantum metrology, networking, and scalability in quantum computing systems, their mirrors must have minimal losses. However, high-finesse dielectric cavity mirrors can degrade in ultra-high vacuum (UHV), increasing the challenges of upgrading to cavity-coupled quantum systems. We observe the optical degradation of high-finesse dielectric optical cavity mirrors after high-temperature UHV bake in the form of a substantial increase in surface roughness. We provide an explanation of the degradation through atomic force microscopy (AFM), X-ray fluorescence (XRF), selective wet etching, and optical measurements. We find the degradation is explained by oxygen reduction in Ta 2 O 5 followed by growth of tantalum sub-oxide defects with height to width aspect ratios near ten. We discuss the dependence of mirror loss on surface roughness and finally give recommendations to avoid degradation to allow for quick adoption of cavity-coupled systems.

Optics↗
NASA NTRS2009

NASA Engineering and Safety Center Technical Bulletin No. 09-05: Self-Contained Oxygen Generator: Effects of Organic Contamination, Briquette Fracturing, Pressure Containment, and Variations in Chemical Composition

A Self-Contained Oxygen Generator (SCOG) produces breathable oxygen (O 2 ) by using chemically-reactive briquettes that begin producing O 2 when mechanically ignited. Contamination with organic material of, or manufacturing defects in, the briquettes have been shown to lead to conditions that can cause an explosive failure

Self Contained Oxygen Generator↗
DOE OSTI2021

Synthesis and ion-irradiation tolerance of the Dy2TiO5 polymorphs

In developing improvements in nuclear fuels, work has been conducted in replacing boron with lanthanides to act as neutron absorbent materials. To further this development for the first time the three major polymorphs of Dy2TiO5 have been fabricated as bulk, single-phase materials in a systematic study. Crystal structure refinement has been carried out from powder x-ray diffraction data, with further structural detail attained via electron diffraction. A cubic phase, defect-pyrochlore, Fd-3m space group, was found with cell parameter a = 10.2996 (1) angstrom and oxygen x48f positional parameter = 0.331 (1). Dy2TiO5 was also fabricated as a bulk, single-phase compound with hexagonal symmetry, P6(3)/mmc space group, with cell parameters a = b = 3.6307(2), c = 11.8963(8) angstrom. The radiation response for each of the polymorphs has been investigated in-situ via 1 MeV Kr - ion-irradiation and transmission electron microscopy characterisation at the IVEM-TANDEM facility, Argonne National Laboratory. Critical temperatures, T-c, for maintaining crystallinity during irradiation were determined and showed Dy2TiO5 with either orthorhombic or cubic symmetry to perform the best, with the lowest T-c values of 711 and 761 K respectively when compared with the hexagonal form. These relatively low T-c values may be attributed to different characteristics for each phase; high anti-site defect formation energy for the orthorhombic symmetry, and the ability to accommodate disorder for the cubic symmetry.

Burnable poison↗
DOE OSTI2022

Interactions of Water with Pristine and Defective MoS 2

Molybdenum disulfide (MoS 2 ) is a lamellar solid lubricant often used in aerospace applications because of its extremely low friction coefficient (~0.01) in inert environments. The lubrication performance of MoS 2 is significantly impaired by exposure to even small amounts of water and oxygen, and the mechanisms behind this remain poorly understood. Here we use density functional theory calculations to study the binding of water on MoS 2 sheets with and without defects. In general, we find that pristine MoS 2 is slightly hydrophilic but that defects greatly increase the binding affinity for water. Intercalated water disrupts the crystal structure of bulk MoS 2 due to the limited space between lamellae (~3.4 Å), and this leads to generally unfavorable adsorption, except in the cases where water molecules are located on the sites of sulfur vacancies. We also find that water adsorption is more favorable directly below a surface layer of MoS 2 compared to in the bulk.

36 MATERIALS SCIENCE↗
DOE OSTI2023

First-Principles Investigation of Near-Surface Divacancies in Silicon Carbide

The realization of quantum sensors using spin defects in semiconductors requires a thorough understanding of the physical properties of the defects in the proximity of surfaces. We report a study of the divacancy (V Si V C ) in 3C-SiC, a promising material for quantum applications, as a function of surface reconstruction and termination with -H, -OH, -F and oxygen groups. Here we show that a V Si V C close to hydrogen-terminated (2 x 1) surfaces is a robust spin-defect with a triplet ground state and no surface states in the band gap and with small variations of many of its physical properties relative to the bulk, including the zero-phonon line and zero-field splitting. However, the Debye-Waller factor decreases in the vicinity of the surface and our calculations indicate it may be improved by strain-engineering. Overall our results show that the V Si V C close to SiC surfaces is a promising spin defect for quantum applications, similar to its bulk counterpart.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Complete Phase Transformation of Ir Nanowire Network into Defect-Rich Oxide Catalyst for High-Performance PEM Water Electrolysis

Iridium-based catalysts remain the most reliable option for the oxygen evolution reaction (OER) in proton exchange membrane water electrolyzers (PEMWEs). However, their high cost and limited performance represent critical barriers to the commercialization of this green hydrogen production technology. Herein, we report the creation of a metallic Ir nanowire network (IrNWN), which exhibits superior OER performance through its in-situ transition into an oxide structure with high intrinsic activity. At a low loading of 0.25 mgIr/cm2 in PEMWEs, IrNWN achieved a current density of 3.13 A/cm2 at a cell voltage of 1.8 V, outperforming the commercial Ir-based catalyst and surpassing the Department of Energy (DOE) 2026 technical target. Moreover, the high activity of IrNWN was maintained for 900 hours in a durability test at 2 A/cm2, showing a low degradation rate of 0.042 mV/hour. Structural analysis of the electrochemically oxidized IrNWN revealed the presence of mixed Ir oxidation states and a high density of surface terminal oxygen groups (μ1-O), which contributed to a reduced energy barrier for the rate-determining O-O coupling step.

oxygen evolution reaction↗
DOE OSTI2023

The vacuolar iron transporter mediates iron detoxification in Toxoplasma gondii

Iron is essential to cells as a cofactor in enzymes of respiration and replication, however without correct storage, iron leads to the formation of dangerous oxygen radicals. In yeast and plants, iron is transported into a membrane-bound vacuole by the vacuolar iron transporter (VIT). This transporter is conserved in the apicomplexan family of obligate intracellular parasites, including in Toxoplasma gondii . Here, we assess the role of VIT and iron storage in T. gondii . By deleting VIT, we find a slight growth defect in vitro, and iron hypersensitivity, confirming its essential role in parasite iron detoxification, which can be rescued by scavenging of oxygen radicals. We show VIT expression is regulated by iron at transcript and protein levels, and by altering VIT localization. In the absence of VIT, T. gondii responds by altering expression of iron metabolism genes and by increasing antioxidant protein catalase activity. We also show that iron detoxification has an important role both in parasite survival within macrophages and in virulence in a mouse model. Together, by demonstrating a critical role for VIT during iron detoxification in T. gondii , we reveal the importance of iron storage in the parasite and provide the first insight into the machinery involved.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Resistance of Boron Nitride Nanotubes to Radiation-Induced Oxidation

We present unprecedented results on the damage thresholds and pathways for boron nitride nanotubes (BNNT) under the influence of energetic electrons in an oxidative gas environment, using an environmental aberration-corrected electron microscope over a range of oxygen pressures. We observe a damage cascade process that resists damage until a higher electron dose, compared with carbon nanotubes, initiating at defect-free BNNT sidewalls and proceeding through the conversion from crystalline nanotubes to amorphous boron nitride (BN), resisting oxidation throughout. We compare with prior results on the oxidation of carbon nanotubes and present a model that attributes the onset of damage in both cases to a physisorbed oxygen layer that reduces the threshold for damage onset. Surprisingly, increased temperatures offer protection against damage, as do electron dose rates that significantly exceed the oxygen dose rates, and our model attributes both effects to a physisorbed oxygen population.

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

Dataset for "Resistance of Boron Nitride Nanotubes to Radiation-Induced Oxidation" as published in The Journal of Physical Chemistry C

We present unprecedented results on the damage thresholds and pathways for boron nitride nanotubes (BNNT) under the influence of energetic electrons in an oxidative gas environment, using an environmental aberration-corrected electron microscope over a range of oxygen pressures. We observe a damage cascade process that resists damage until a higher electron dose, compared with carbon nanotubes, initiating at defect-free BNNT sidewalls and proceeding through the conversion from crystalline nanotubes to amorphous boron nitride (BN), resisting oxidation throughout. We compare with prior results on the oxidation of carbon nanotubes and present a model that attributes the onset of damage in both cases to a physisorbed oxygen layer that reduces the threshold for damage onset. Surprisingly, increased temperatures offer protection against damage, as do electron dose rates that significantly exceed the oxygen dose rates, and our model attributes both effects to a physisorbed oxygen population.

Adsorption↗
NASA NTRS1984

Cell and defect behavior in lithium-counterdoped solar cells

Some n(+)/p cells in which lithium is introduced as a counterdopant, by ion-implantation, into the cell's boron-doped p-region were studied. To determine if the cells radiation resistance could be significantly improved by lithium counterdoping. Defect behavior was related to cell performance using deep level transient spectroscopy. Results indicate a significantly increased radiation resistance for the lithium counterdoped cells when compared to the boron doped 1 ohm-cm control cell. The increased radiation resistance of the lithium counterdoped cells is due to the complexing of lithium with divacancies and boron. It is speculated that complexing with oxygen and single vacancies also contributes to the increased radiation resistance. Counterdoping silicon with lithium results in a different set of defects.

Weinberg, I.↗
DOE OSTI2023

Influence of structural defects on charge density waves in 1T-TaS2

Abstract The influence of intrinsic defects of 1T-TaS 2 on charge density waves (CDWs) is studied using scanning tunneling microscopy and spectroscopy (STM, STS), angle-resolved photoelectron spectroscopy (ARPES), and density functional theory (DFT). We identify several types of structural defects and find that most have a local character limited to a single CDW site, with a single exception which effectively behaves as a dopant, leading to band-bending and affecting multiple neighboring sites. While only one type of defect can be observed by STM topographic imaging, all defects are easily resolved in STS mapping. Our results indicate modulation of the Mott band gap commensurate with the CDW and breaking of the three-fold symmetry of electronic states. DFT calculations (with included Coulomb interactions) are used to investigate the electronic structure, focusing on both sulfur vacancy and oxygen-sulfur substitution. The sulfur vacancy system, characterized with a metallic behavior, is identified as the origin of one of the experimentally observed defects. Additionally, the effect of oxidation of 1T-TaS 2 depends on the substitution site, leading to the heterogeneity of electronic properties.

Chemistry↗