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Results for “oxygen stoichiometry”

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Spark plasma sintering of fuel meats for U 3 O 8 based dispersion fuels

Research and test reactors often use dispersion-type fuel due to its increased thermal conductivity and burn-up capabilities compared to conventional fuel. Al-U 3 O 8 (aluminumtriuranium octaoxide) dispersion fuels have several advantages over their competitors, such as higher service temperature and better stability of oxygen stoichiometry. However, the two-step fabrication of dispersion fuel causes undesirable porosity in cold-pressed fuel meats that is preserved in co-extruded fuel plates. To combat this, spark plasma sintering (SPS) was used for the fabrication of Al-15, 20, and 30 vol% U 3 O 8 and 8 and 12 vol% Mo-U 3 O 8 fuel meats for the Al-U 3 O 8 time. The in situ SPS data was used to construct and validate Master Sintering Curves (MSCs) with accuracies in Al fuels at 0.02 g/cm 3 , and Mo fuels at 0.07 and 0.17 g/cm 3 . The as-sintered fuel meats were characterised using x-ray diffraction (XRD) and scanning electron microscopy (SEM) to understand chemical and physical changes following the SPS process. The pellets exhibited very high relative densities, the U 3 O 8 was observed to undergo reduction to UO 2 .

Aluminium↗
DOE OSTI2024

Signatures of ambient pressure superconductivity in thin film La 3 Ni 2 O 7

Recently, the bilayer nickelate La 3 Ni 2 O 7 has been discovered as a new superconductor with transition temperature T c near 80 K under high pressure. Despite extensive theoretical and experimental work to understand the nature of its superconductivity, the requirement of extreme pressure restricts the use of many experimental probes and limits its application potential. Here, in this work, we present signatures of superconductivity in La 3 Ni 2 O 7 thin films at ambient pressure, facilitated by the application of epitaxial compressive strain. The onset T c varies approximately from 26 K to 42 K, with higher T c values correlating with smaller in-plane lattice constants. We observed the co-existence of other Ruddlesden-Popper phases within the films and dependence of transport behavior with ozone annealing, suggesting that the observed low zero resistance T c of around 2 K can be attributed to stacking defects, grain boundaries, and oxygen stoichiometry. This finding initiates numerous opportunities to stabilize and study superconductivity in bilayer nickelates at ambient pressure, and to facilitate the broad understanding of the ever-growing number of high temperature and unconventional superconductors in the transition metal oxides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Synthesis and characterization of amorphous Fe2.75Dy-oxide thin films demonstrating room-temperature semiconductor, magnetism, and optical transparency

Recently, amorphous/disordered oxide thin films made from Fe and lanthanides like Dy and Tb have been reported to have a rich set of magnetic, optical, and electronic properties, as well as room-temperature magneto-electric coupling with multiferroics [A. Malasi et al., Sci. Rep. 5, 18157 (2015); H. Taz et al., Sci. Rep. 6, 27869 (2016); and H. Taz et al., Sci. Rep. 10, 1–10 (2020)]. Here, we report the synthesis and detailed characterization of Fe2.75Dy-oxide thin films prepared on various substrates using electron beam co-evaporation. The structure, chemistry, electric, magnetic, and optical properties were studied for the as-prepared and annealed (373 K, in air, 1 h) films of thickness 40 nm. High resolution transmission electron microscopy and electron diffraction study showed that the films were amorphous in both the as-prepared and annealed states. The electron energy-loss spectroscopy studies quantified that metal oxygen stoichiometry changed from Fe2.75Dy-O1.5 to Fe2.75Dy-O1.7 upon annealing. Synchrotron-based x-ray absorption spectroscopy investigation confirmed that the as-prepared films were highly disordered with predominantly metallic Fe and Dy states that became slightly oxidized with annealing in air. The as-prepared amorphous films demonstrated significantly high value of ordinary (∼10 cm2/V s) and anomalous (∼102 cm2/V s) Hall mobility and high electrical conductivity of ∼103 S/cm at room temperature. The cryogenic magnetic property measurement shows two-step magnetization below 200 K, suggesting exchange-spring magnetic interaction. The nature of the field cooled and zero-field cooled curves suggested a spin-glass like transition between 78 K and 80 K, with a characteristic broad peak. The Tauc plot analysis from optical transmission spectra confirms the existence of an optical bandgap of ∼2.42 eV that increased slightly to ∼2.48 eV upon annealing. This rich set of transport, optical, and magnetic properties in these thin films is very exciting and points to potential applicability in low-cost multifunctional devices requiring a combination of transparent, semiconducting, and magnetic responses, such as in spintronics.

Prasad Koirala, Krishna (ORCID:000000029137917X)↗
DOE OSTI2021

Surface structure of mass-selected niobium oxide nanoclusters on Au(111)

The structures formed by the deposition of mass-selected niobium oxide clusters, Nb 3 O y ( y = 5, 6, 7), onto Au(111) were studied by scanning tunneling microscopy. The as-deposited Nb 3 O 7 clusters assemble into large dendritic structures that grow on the terraces as well as extend from the top and bottom of step edges. The Nb 3 O 6 cluster also forms dendritic assemblies but they are generally much smaller in size. The assemblies are composed of smaller discrete structures (<1 nm) which are likely to be single clusters. Furthermore, the dendritic assemblies for both the Nb 3 O 7 and Nb 3 O 6 clusters have fractal dimensions of about 1.7 which is very close to that expected for simple diffusion limited aggregation. Annealing the Nb 3 O 7,6 /Au(111) surfaces up to 550 K results in changes in assembly sizes and increases in heights, while heating to 700 results in the disruption of the assemblies into smaller structures. By contrast, the as-deposited Nb 3 O 5 /Au(111) surface at RT exhibits compact cluster structures which become 3D nanoparticles when annealed above 550 K. Differences in the observed surface structures and thermal stability are attributed to differences in metal-oxygen stoichiometry which can influence cluster binding energies, mobility and inter-cluster interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1988

Structural chemistry of Au(III)-substituted Ba2YCu3O(7-delta)

A series of gold-substituted perovskite superconductors Ba2Y(Cu/1-x/Aux)3O(7-delta)(x = 0-0.1) was synthesized. For x = 0.1, there was no change in the a and b lattice parameters (a = 3.826 A and b = 3.889 A), but a 0.06 A c-axis expansion to 11.75 A was observed. Substituted gold was found to be trivalent by X-ray photoelectron spectroscopy. Replacing Cu(1) in the copper oxide chain with a slight reordering of oxygen is consistent with c-axis expansion. The formal charge of the site remains trivalent, while remaining Cu in the chains is reduced to Cu(I), resulting in an oxygen stoichiometry of less than 7. Finally, no large effect on Tc is observed (Tc = 89 K for x = 0.10), in contrast to the effect of a number of other metal ion dopants. These results are discussed relative to the chemistry of Au(III) and to the use of the metal in structures containing gold and ceramic superconductors.

Hepp, A. F.↗
NASA NTRS1990

Study of local structure and magnetism in high-T(sub c) copper oxide superconductors

The muon spin rotation (MUSR) study of local magnetism of Sr-doped La2CuO4 is reviewed. Emphasis is placed on magnetic order as detected by local and bulk probes with local atomic environments studied by x ray absorption fine structure (XAFS). Correlations between the MUSR study of local magnetic ordering and the bulk magnetization study are presented along with a discussion of the dependence upon oxygen stoichiometry. Results are presented for both superconducting phases and magnetic phases. Recent data which reveals the existence of local magnetic ordering in the hydrogen-doped YBa2Cu3O7 system are also discussed.

Budnick, J. I.↗
NASA NTRS1988

Synthesis and structural chemistry of Au(III)-substituted Ba2YCu3O(7-delta)

Gold-substituted superconductors, Ba2Y(Au(x)Cu(1-x))3O(7-delta) (x = 0-0.1) were synthesized. For x = 0.1, there was no change in the a and b lattice parameters (a = 3.826 A and b = 3.889 A) but a 0.06 A c axis expansion to 11.75 A was observed. Substituted gold is found to be trivalent by XPS. Replacing Cu(1) in the copper oxide chain with a slight reordering of oxygen is consistent with c axis expansion. The formal charge of the site remains trivalent; remaining Cu in the chains may be reduced resulting in an oxygen stoichiometry is less than or equal to 7. A small effect on T(sub c)(89 K for x = 0.10) is observed upon gold substitution.

Hepp, A. F.↗
NASA NTRS1991

Studies of local magnetism and local structure in La(2-x)Sr(x)CuO4

The muon spin rotation (MUSR) study of local magnetism of Sr-doped La2CrO4 is reviewed. Emphasis is placed on magnetic order as detected by local and bulk probes with local atomic environments studies by x ray absorption fine structure (XAFS). Correlations between the MUSR study of local magnetic ordering and the bulk magnetization study are presented along with a discussion of the dependence upon oxygen stoichiometry. Results are presented for both superconducting phases and magnetic phases. Recent data which reveals the existence of local magnetic ordering in the hydrogen-doped YBa2Cu3O7 system are also discussed.

Budnick, J. I.↗
DOE OSTI2024

Ba₁₋ₓSrₓFeO₃₋ᵧ as an Improved Oxygen Carrier for Chemical Looping Air Separation: A Computational and Experimental Study

Chemical looping air separation (CLAS) is a promising method to generate pure carbon-dioxide from fuel combustion with a pure oxygen stream, which is produced through the capture, and targeted release, of oxygen from the atmosphere using a solid oxide carrier. The performance of this process depends on the redox characteristics of the oxide carrier. Using experimental oxygen-temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA), the study shows that Ba₀․₇₅Sr₀․₂₅FeO₃ has improved oxygen storage capacity (OSC), oxidation, and reduction kinetics over pristine SrFeO₃ at temperatures ranging from 300-500 °C. The redox energetics computed by using first-principles density functional theory (DFT) calculations also depict the measured trend establishing it as an important descriptor of the measured performance. The Ba₁₋ₓSrₓFeO₃₋ᵧ depicts a Sr-substitution and oxygen stoichiometry induced structural phase transition from hexagonal at low temperatures to pseudo-cubic phase with higher OSC up to T=400 °C. Also, the mechanism leading to this structural phase transition is identified by performing electronic and vibrational structural calculations. This presentation was given on March 8, 2024 at the APS March Meeting in Minneapolis, MN.

Acharya, Shree Ram↗
DOE OSTI2026

Influence of Oxygen Flow and Stoichiometry on Optical Properties and Damage Resistance of Hafnium Oxide Thin Films

Hafnium oxide (HfO 2 ) is predominantly used as a high-index material in multi-layer dielectric coatings for high-peak- and high-average-power lasers, but laser damage often initiates within the HfO 2 layers despite their wide bandgap. Oxygen deficiency during deposition can introduce vacancy-related sub-bandgap states and absorptive defects, lowering damage resistance. This study investigates how oxygen flow during HfO 2 deposition with ion beam sputtering (IBS) affects its stoichiometry, defect formation, and nanosecond laser-induced damage threshold (LIDT) and whether single-layer trends predict multilayer performance. Single layers were deposited at varying oxygen flows, characterized for optical and structural properties, and tested for the LIDT at 1064 nm and 355 nm. Increasing oxygen flow drove the layer toward near-stoichiometric HfO 2 , reduced the refractive index, and altered the density of surface pinhole-like features. The single-layer LIDT at 355 nm increased with oxygen, whereas the 1064 nm LIDT was comparatively less sensitive to oxygen flow, consistent with the wavelength-dependent roles of absorptive precursors and microstructural defects. In contrast, a HfO 2 -based high-reflector (HR) showed a higher LIDT at lower oxygen flow, indicating that the family of damage precursors changes between single layers and multilayers; in stacks, structural properties such as stress, gas entrapment and thermal dissipation may outweigh the isolated absorptive defects found in single layers. These results demonstrate that the optimal oxygen flow condition depends on both LIDT wavelength and film architecture. We identified, for single layers, a 15–35 sccm window for maximizing the 1064 nm LIDT and a high-flow optimum (45 sccm) for the 355 nm LIDT and, for 355 nm HR stacks, a distinct lower-flow regime (~10 sccm).

Optics and optical instruments↗
DOE OSTI2021

Impact of Oxygen Non-Stoichiometry on Near-Ambient Temperature Ionic Mobility in Polaronic Mixed-Ionic-Electronic Conducting Thin Films

Enhanced ionic mobility in mixed ionic and electronic conducting solids contributes to improved performance of memristive memory, energy storage and conversion, and catalytic devices. Ionic mobility can be significantly depressed at reduced temperatures, for example, due to defect association and therefore needs to be monitored. Measurements of ionic transport in mixed conductors, however, proves to be difficult due to dominant electronic conductivity. This study examines the impact of different levels of quenched-in oxygen deficiency on the oxygen vacancy mobility near room temperature as measured by a novel dynamic current-voltage analysis. A Pr 0.1 Ce 0.9 O 2-δ film was grown by pulsed laser deposition and subsequently annealed, from 400-600° C, in various oxygen partial pressures to modify its oxygen vacancy concentration while minimizing microstructural growth and cation segregation. To monitor changes in film non-stoichiometry, we leverage the existence of an optical absorption center, related to the oxidation state of Pr ions in Pr0.1Ce0.9O2-δ. The oxygen vacancy migration enthalpy was found to exhibit a small increase from 0.73± 0.04 to 0.79 ± 0.02 eV with increasing oxygen deficiency, while the pre-factor was found to increase by a factor of 135. Here, a nearly 13-fold increase in ionic mobility at 60 °C for increases in oxygen non-stoichiometry from 0.032 ± 0.001 to 0.042± 0.001 was thereby detected. Raman spectroscopy was employed to rule out changes in strain as the primary cause for the significant change in mobility. Several factors potentially contributing to the large pre-factor changes are examined and discussed. Insights into how ionic defect concentration can markedly impact ionic mobility should help in elucidating the origins of variations seen in nanoionic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Predicting Oxygen Off-Stoichiometry and Hydrogen Incorporation in Complex Perovskite Oxides

Chemically and structurally complex solid compounds, including those with significant off-stoichiometry, are rapidly extending new material functionality across a variety of applications. Accelerated development of these compounds requires accurate predictions of material defect properties including effective defect formation energies and equilibrium defect concentrations. Traditional first-principles approaches typically examine dilute defect concentrations and relatively ordered atomic structures to identify the lowest energy defect sites. These approaches are rarely suitable for describing the disorder present in these systems and its influence on defect formation, which can lead to unphysically large predictions for defect concentrations. Here, we demonstrate a new method to accurately predict the temperature and pressure dependence of oxygen vacancy concentrations and proton interstitial concentrations in complex oxides. This method extends standard dilute defect calculations to incorporate atomic and magnetic disorder, employs the ensemble descriptions of defect sites resulting in improved predictions of defect formation energies, and accounts for effects beyond the dilute defect limit. To demonstrate our method, we show that the predicted defect concentrations in perovskites used as ceramic fuel cell cathodes, including Ba0.5Sr0.5Fe0.8Zn0.2O3-d, Ba0.5Sr0.5Co0.8Fe0.2O3-d, and BaCo1-x-y-zFexZryYzO3-d, are in good agreement with experimental values, thereby opening the door for predictive design of complex oxides by these applications.

DFT↗