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Results for “Oxide Phase Formation”

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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Pd-promoted reduction and restructuring of an In 2 O 3 -based catalyst for CO 2 hydrogenation at room temperature

An unconventional reaction mechanism in an In 2 O 3 /Pd(1 1 1) inverse model catalyst for the CO 2 hydrogenation reaction has been uncovered: In 2 O 3 is partially reduced at room temperature in a reaction atmosphere as a result of its direct contact with Pd(1 1 1), which is an efficient H 2 splitter. The reduction induces changes in surface free energy, leading to a dynamical restructuring at the In 2 O 3 /Pd(1 1 1) interface via formation of InO x and outward diffusion of Pd, as revealed by ambient pressure X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and density functional theory simulations. This dynamical restructuring eventually promotes the growth of 2D InPd y O x nanodomains as the catalytically active phase and the exclusive formation of methanol upon hydrogenation of CO 2 at room temperature. A comparable high selectivity toward CH 3 OH was found in more realistic bulk catalytic systems (2 wt% Pd/In 2 O 3 catalyst and commercial CZA catalyst). Scanning tunneling microscopy under ultrahigh vacuum and ambient pressure reaction atmospheres further reveals the structural dynamics at the InO x /Pd(1 1 1) interface, where we follow in situ the evolution of the InO x particles on Pd(1 1 1) and the mobility of the InPd y O x nanodomains in a CO 2 + H 2 environment. The present findings of the formation of a mixed oxide phase in a dynamically restructuring metal/reducible-oxide interface indicate further implications for other heterogeneous catalytic systems beyond the present CO 2 hydrogenation example and highlight the importance of in situ investigations.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The influence of cooling rate on condensation of iron, aluminum, and uranium oxide nanoparticles

Fundamental observations of particle size distributions are needed to develop models that predict the fate and transport of radioactive materials in the atmosphere following a nuclear incident. The extent of material transport is influenced by the time scales of particle formation processes (e.g., condensation, coagulation). In this study, we investigated the influence of cooling time scales on size distributions of uranium, aluminum, and iron oxide particles that are synthesized separately under identical run conditions inside the controlled environment of an argon plasma flow reactor. Two distinct temperature distributions are imposed along the flow reactor by varying the argon flow rate downstream of the plasma torch. The vaporized reactants of uranium, aluminum, and iron are cooled from about 5000K to 1000K before they are collected on silicon wafers for ex situ scanning electron microscope analysis. The microscope images show that the sizes of the largest aluminum and iron oxide particles heavily depend on the cooling time scales, whereas significant size variation with cooling rate is not observed for uranium oxide particles. In addition, the size distribution of aluminum oxide particles exhibits the broadest range among all three metal oxides studied. We performed simulations of particle size distributions using a kinetic model that couples gas phase oxidation chemistry with particle formation processes, including nucleation, condensation, and coagulation. The model results demonstrate the strong sensitivity of particle size distribution to different cooling histories (i.e., temperature vs residence time) along the flow reactor. In conclusion, the kinetic model also helps identify directions for future research to improve the predictions.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Kinetics and Thermodynamics of Sr Permeation in CeO 2 -Based Barrier Layers for Solid-Oxide Electrolyzer Cells

Solid-oxide electrolyzer cells (SOECs) convert steam to hydrogen efficiently at high temperatures. However, during operation, the diffusion of cations or impurities through the cells due to electrode degradation can cause unwanted secondary phases to form, which may degrade device performance. Here, in this study, we use atomistic and mesoscale simulations coupled with experimental analysis to study the diffusion of Sr through the Gd-doped CeO 2 (GDC) barrier layer used to protect the yttria-stabilized zirconia (YSZ) electrolyte in SOECs. From our atomistic calculations, we find Sr diffusion to be negligibly slow in bulk GDC; however, surface diffusion is much more favorable. Subsequent mesoscale simulations show that Sr diffusion is activated when the porosity of GDC exceeds ∼10% and significantly exceeds diffusion in bulk and grain boundary regions. We also find that SrO-based species can accumulate at GDC surfaces; however, SrO aggregation and coarsening will be limited by the large lattice mismatch between GDC and SrO. Energy-dispersive X-ray spectroscopy (EDS) and electron diffraction confirm that Sr can accumulate within GDC pores and form disperse Sr-containing secondary phases. Altogether, Sr diffusion in dense GDC is unlikely to give rise to thick SrO layers, which would severely limit device performance. The formation of Sr-containing secondary phases can largely be avoided by restricting the porosity of the GDC layer as much as possible.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Surface oxidation and crystallization of FeNi-Based soft magnetic nanocrystalline and amorphous nanocomposite alloys

In this work, we present characterization of surface oxidation and crystallization behavior of Metal Amorphous Nanocomposite (MANC) (Fe 70 Ni 30 ) 80 Nb 4 Si 2 B 14 alloys via complementary x-ray diffraction, electron microscopy, and x-ray photoelectron spectroscopy techniques. An adherent amorphous oxide of roughly 15–20 nm in thickness is observed on the wheel side of the ribbon, enriched in Fe relative to Ni and with significant content of Si and Nb as would be expected based upon a comparison of the Gibbs free energy of formation of the stable oxide phases. The amorphous character of the oxide is likely to result from the significant Si-content coupled with the amorphous and nanocrystalline character of the underlying metallic layer. Higher density of FeNi-enriched nanocrystals is also observed within ~100 nm of the wheel-side surface, presumably due to reduction in local glass former concentration. A rough, crystalline copper oxide layer is also present and forms through contact between the copper wheel and the solidifying melt during the rapid solidification process. On the air side of the ribbon, a crystalline oxide layer of ~15–20 nm is formed as well as intergranular oxidation that penetrates along the grain boundaries between highly textured surface crystallites of ~50–100 nm in size. The crystalline nature of the oxide on these surfaces is asserted to result from the lack of significant Si-content and the pre-existing surface crystallites that form during the rapid solidification process which promote a crystalline oxide surface layer. Furthermore, these results shed new insights into the details of surface oxidation and can help to inform processing and alloy composition adjustments which seek to tailor surface properties.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Correlating Local Structure with Electrochemical Activity in L i2 MnO 3

Li 2 MnO 3 is of interest as one component of the composite lithium-rich oxides, which are under development for high capacity, high voltage cathodes in lithium ion batteries. Despite such practical importance, the mechanism of electrochemical activity in Li 2 MnO 3 is contested in the literature, as are the effects of long-term electrochemical cycling. Here, Raman spectroscopy and mapping are used to follow the chemical and structural changes that occur in Li 2 MnO 3 . Both conventional slurry electrodes and thin films are studied as a function of the state of charge (voltage) and cycle number. Thin films have similar electrochemical properties as electrodes prepared from slurries, but allow for spectroscopic investigations on uniform samples without carbon additives. Spectral changes correlate well with electrochemical activity and support a mechanism whereby capacity is lost upon extended cycling due to the formation of new manganese oxide phases. Raman mapping of both thin film and slurry electrodes charged to different voltages reveals significant variation in the local structure. Poor conductivity and slow kinetics associated with a two-phase reaction mechanism contribute to the heterogeneity.

25 ENERGY STORAGE↗
DOE OSTI2021

Stable and selective catalysts for propane dehydrogenation operating at thermodynamic limit

Hitting the limits on propene synthesis The greater abundance of propane from shale gas has spurred efforts to use it as a propylene feedstock. Direct dehydrogenation catalysts consisting of platinum–tin alloy nanoparticles supported on alumina often must run with hydrogen dilution to avoid carbon buildup and excess tin to avoid alloy segregation. Motagamwala et al. report that platinum–tin nanoparticles interact more weakly with a silica support and the metals thus do not segregate. The use of undiluted reactants allowed the reaction to run near the thermodynamically limit of about 67% conversion with a selectivity to propylene of more than 99%. This catalyst also does not build up carbon and could run up to 30 hours without deactivation. Science , abg7894, this issue p. 217

Science & Technology - Other Topics↗
NASA NTRS2005

Ultrasonic Guided-Wave-Scan System Used to Characterize C-Enhanced Silicon Carbide Composite During Creep- Rupture Tests

Ceramic matrix composites (CMCs) are being developed for advanced aerospace propulsion applications in order to save weight, improve reuse capability, and increase performance. However, mechanical and environmental loads applied to CMCs can cause degradation in the form of discrete flaws and distributed microdamage that can play a significant role in reducing desirable physical properties. Categories of microdamage include fiber/matrix debonding (interface failure), matrix microcracking, fiber fracture and buckling, oxidation, and second-phase formation. Distributed microdamage in CMCs has proven difficult to characterize nondestructively because of the complex microstructure and macrostructure of these materials, and a recent study regarding the durability of a ceramic matrix composite discussed the requirement for improved nondestructive evaluation (NDE) methods for monitoring degradation in these materials.

Roth, Don J.↗
DOE OSTI2015

Influence of fuel on phase formation of ZnFe{sub 2}O{sub 4} prepared by self-propagated combustion route

Zinc iron oxide (ZnFe{sub 2}O{sub 4}) nanoparticles were prepared by self-propagated combustion route.The fuel plays a major role on the formation of structure and particle size. Here three different fuels like alanine, glycine and proline were used to synthesis the zinc iron oxide nanoparticle. Influence of combustion nature through the fuel, the phase formation, particle size, band gap and surface morphology has been modified. The prepared powder was characterized by powder X-ray diffraction method (PXRD), field-emission scanning electron microscopy (FE-SEM) and the composition of the material was analysed by X-ray energy dispersive spectroscopy (EDAX).The elemental mapping was confirmed the uniform distribution of Zn, Fe and O elements in the prepared material of ZnFe{sub 2}O{sub 4}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effects of Sodium and Tungsten Promoters on Mg 6 MnO 8 -Based Core–Shell Redox Catalysts for Chemical Looping—Oxidative Dehydrogenation of Ethane

The present study investigates the effect of sodium and tungsten promoters on Mg 6 MnO 8 -based redox catalysts in a chemical looping oxidative dehydrogenation (CL-ODH) scheme. CL-ODH has the potential to significantly lower energy consumption and CO 2 /NO x emissions for ethylene production compared with conventional steam cracking. Sodium tungstate (Na 2 WO 4 ) was previously shown to be an effective promoter for Mg 6 MnO 8 -based redox catalysts. Overall, the CL-ODH reaction proceeds via parallel gas-phase cracking of ethane and selective combustion of H 2 on the surface of the Na 2 WO 4 -promoted redox catalyst. Reaction testing indicates that both Na and W are necessary to form Na 2 WO 4 and to achieve high ethylene selectivity. A Na:W ratio lower than 2:1 lead to significant formation of additional mixed tungsten oxide phases and decreases ethylene selectivity. Further characterizations based on low-energy ion scattering (LEIS) and differential scanning calorimetry (DSC) indicate that the NaW promoter forms a molten shell around the Mg6MnO8 redox catalyst. Methanol TPSR and in situ DRIFTS experiments indicate that the promoter significantly suppresses the number of basic sites on Mg 6 MnO 8 . 18 O– 16 O exchange experiments reveal that the promoter decreases the rate of oxygen exchange. O 2 cofeed studies indicate that below the melting temperature of Na 2 WO 4 , H 2 and CO conversions are both inhibited, but above the melting temperature, H 2 combustion significantly increased while CO combustion is still inhibited. On the basis of extensive characterizations, it was determined that H 2 is primarily combusted at the gas–Na 2 WO 4 molten shell interface via redox reactions of the tungsten salt, likely between the WO 4 2– (tungstate) and WO 3 – (tungsten bronze).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Effect of Thermal Oxidation on the Structure, Surface Texturing, and Microstructure Evolution in Nanocrystalline Ga-O-N Films

An extensive examination of the nanoscale, crystallographic growth dynamics of the system, which is impacted by the thermal energy given to the GaN, is carried out to derive a deeper understanding of the growth kinetics, morphology and microstructure evolution, chemical bonding, and optical properties of Ga-O-N films. Thermal annealing of GaN films is performed in the temperature range of 900–1200 °C. Crystal structure, phase formation, chemical composition, surface morphology, and microstructure evolution of Ga-O-N films are investigated as a function of temperature. Increasing temperature induces surface oxidation, which results in the formation of stable β-Ga2O3 phase in the GaN matrix, where the overall film composition evolves from nitride (GaN) to oxynitride (Ga-O-N). While GaN surfaces are smooth, planar, and featureless, oxidation induced granular-to-rod shaped morphology evolution is seen with increasing temperature to 1200 °C. The considerable texturing and stability of the nanocrystalline Ga-O-N on Si substrates can be attributed to the surface and interface driven modification because of thermal treatment. Corroborating with structure and chemical changes, Raman spectroscopic analyses also indicate that the chemical bonding evolution progresses from fully Ga-N bonds to Ga-O-N. While the GaN oxidation process starts with the formation of β-Ga 2 O 3 at an annealing temperature of 1000 °C, higher annealing temperatures induce structural distortion with the potential formation of Ga-O-N bonds. The structure-phase-chemical composition correlation, which will be useful for nanocrystalline materials for selective optoelectronic applications, is established in Ga-O-N films made by thermal treatment of GaN.

36 MATERIALS SCIENCE↗
DOE OSTI2020

The effects of vanadium substitution on one-dimensional tunnel structures of cryptomelane: Combined TEM and DFT study

Rational design of energy storage materials requires mechanistic understanding which relies on atomic scale characterization of structure and defects. In this paper we utilize vanadium (V)-substituted one-dimensional cryptomelane as a model material to study structural features critical to electrochemical reversibility of the cathode in lithium-ion batteries. We found the substitution of manganese (Mn) atoms in the tunnel wall with V enhances structural uniformity in the parent KMn 8 O 16 material by inhibiting formation of irregular (2 × 3 and 2 × 1) manganese oxide tunnel structures. Upon lithiation, both transmission electron microscopy (TEM) and density functional theory (DFT) reveal significant structural evolution, eventually resulting in phase separation and formation of highly reduced manganese oxide. During this process, Mn 4+ accepts electrons donated by intercalated Li, while the V 4+ substituent is not electrochemically reduced but acts to stabilize the tunnel framework structure. As a result, the V substituted material shows higher operating voltage and improved electrochemical reversibility. Our results highlight the benefits of V-substitution in cryptomelane, and the significance of integrating characterization with simulation for understanding and developing design principles for lithium ion battery materials.

25 ENERGY STORAGE↗
DOE OSTI2016

The effect of sub-oxide phases on the transparency of tin-doped gallium oxide

There have been a number of studies on the fabrication of Sn-doped gallium oxide (Ga 2 O 3 :Sn) films with both conductive and transparent properties using a variety of deposition methods. However, often, synthesis results in films that are not transparent. In this paper, we examine the mechanisms underlying these results in Ga 2 O 3 :Sn thin films prepared at various growth temperatures, Sn concentrations, and oxygen partial pressures. With X-ray absorption spectroscopy, transmission electron microscopy and energy dispersive spectroscopy, we find that when films are grown under the oxygen deficient conditions there are Ga sub-oxide and SnOx phases in the Ga 2 O 3 :Sn thin film. These Ga sub-oxide phases are only found in non-transparent films, and so we infer that the Ga sub-oxide is responsible for the non-transparency. These observations suggest that to obtain transparent Ga 2 O 3 :Sn, films deposition or subsequent annealing must be carefully controlled in both temperature and oxygen partial pressure to avoid the formation of Ga sub-oxide phases.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Irradiation-induced formation of a spinel phase at the FeCr/MgO interface

Oxide dispersion strengthened ferritic/martensitic alloys have attracted significant attention for their potential uses in future nuclear reactors and storage vessels, as the metal/oxide interfaces act as stable high-strength sinks for point defects while also dispersing helium. Here, in order to unravel the evolution and interplay of interface structure and chemistry upon irradiation in these types of materials, an atomically sharp FeCr/MgO interface was synthesized at 500 °C and separately annealed and irradiated with Ni 3+ ions at 500 °C. After annealing, a slight enrichment of Cr atoms was observed at the interface, but no other structural changes were found. However, under irradiation, sufficient Cr diffuses across the interface into the MgO to form a Cr-enriched transition layer that contains spinel precipitates. First-principles calculations indicate that it is energetically favorable to incorporate Cr, but not Fe, substitutionally into MgO. Furthermore, our results indicate that irradiation can be used to form new phases and complexions at interfaces, which may have different radiation tolerance than the pristine structures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Redistribution of Erbium and Oxygen Recoil Atoms and the Structure of Silicon Thin Surface Layers Formed by High-Dose Argon Implantation through Er and SiO{sub 2} Surface Films

Using analytical high-resolution electron microscopy, the Si structure and the redistribution of Er and O recoil atoms embedded in thin (~10 nm) surface layers by Ar{sup +} implantation with an energy of 250–290 keV and a dose of 1 × 10{sup 16} cm{sup –2} through Er and SiO{sub 2} films, respectively, and subsequent annealing are studied. It is established that Si recrystallization fails at a distance of ~20 nm from the surface, where the erbium concentration of 5 × 10{sup 19} cm{sup –3} critical for failure is achieved at T = 950°C. It disproves the generally accepted model of Er-atom transfer by the recrystallization front into SiO{sub 2} on the surface. Instead, it is shown that the redistribution of O recoil atoms to the initial oxide during annealing for immobile Er atoms provides the formation of surface-inhomogeneous erbium phases in such a way that the oxygen-enriched Er–Si–O phase turns out to be concentrated in the oxide, while the depleted Er–Si phase remains in Si. It explains the partial loss of implanted Er after removal of the oxide together with the Er–Si–O phase. It was shown that the formation of a high density of microtwins (locally up to 10{sup 13} cm{sup –2}) is associated with the formation of Ar bubbles and clusters, which is atypical for (100)–Si recrystallization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Modern insights into the mechanisms of neptunium oxalate decomposition

Neptunium oxalate (Np(C 2 O 4 ) 2 ·6H 2 O) is a historically relevant solid phase used in nuclear processing as a precursor for neptunium dioxide (NpO 2 ). Although Np oxalate has been synthesized and used for NpO 2 production for decades, the thermal decomposition mechanism of this phase remains poorly understood and has not been evaluated in over 30 years. Conflicting reports in historical literature suggest either a direct conversion from anhydrous oxalate to NpO 2 or a decomposition that includes the formation of Np carbonate or oxidized Np intermediate phases. In this work, we reexamine the decomposition pathway of Np(C 2 O 4 ) 2 ·6H 2 O using thermal analysis coupled with evolved gas analysis and temperature-dependent Raman spectroscopy to elucidate decomposition mechanisms and intermediate phases using modern analytical techniques. Thermal analysis revealed a three-stage decomposition process, including dehydration below 200 °C, oxalate breakdown between 170 and 370 °C, and NpO 2 formation by 500 °C. However, an unidentified plateau in the thermal data was observed during measurements. Raman spectroscopy confirmed the stages of decomposition, and in the analysis of potential intermediate phases, no carbonate phases or Np 2 O 5 were identified. Raman data suggest that residual oxalate or nonstoichiometric oxide are present during decomposition before pure NpO 2 is formed. These findings clarify aspects of the Np oxalate decomposition mechanism and address longstanding discrepancies in the literature, with a specific focus on Np-specific materials chemistry.

Lawson, Kathryn M. [Oak Ridge National Laboratory ↗
DOE OSTI2022

Near-infrared spectra and molar absorption coefficients of trivalent lanthanides dissolved in molten LiCl–KCl eutectic

Determining the concentration of the dissolved lanthanide species in LiCl–KCl eutectic salt is important to the development of pyrochemical reprocessing of used nuclear fuel. In this process, lanthanide fission products are found dissolved in the electrorefiner electrolyte in their trivalent oxidation state. The presence of dissolved trivalent lanthanides increases the liquidus temperature of the electrolyte mixture and can lead to the formation of insoluble oxide or oxychloride phases and must therefore be continuously monitored and controlled during the operation. Absorbance spectroscopy is a promising method for continuous measurement of the concentration of lanthanides and other elements dissolved in the electrolyte. The absorption of light by elements is linearly proportional to the concentration of the element for relatively dilute solutions according to the Beer-Lambert law. Although measurement of the absorption of ultraviolet and visible range light by lanthanides in LiCl–KCl eutectic molten salt have been explored previously, near infrared (NIR) absorption spectroscopy has received far less attention. It may, however, provide a better analytical signal when insoluble phases are present due to less Raleigh scattering compared to shorter wavelength radiation. Additionally, it may allow for concentration determination for certain elements using NIR absorption features where UV and visible range features are overlapping with features from other species. In this study, we report the UV–Vis–NIR spectra of the trivalent lanthanide chlorides of neodymium, samarium, and dysprosium in LiCl–KCl eutectic. Molar absorption coefficients are reported for analytically useful absorption maxima, with a focus on the molar absorption coefficients for NIR absorption maxima which have not been reported previously. Additionally, we observe a NIR-range absorption band of Nd3+ which was previously predicted but never experimentally observed. Here, we compare the calculated crystal field levels to the newly observed absorbance band and find them to be in good agreement with previous predictions.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Linearity optimization of atomic layer deposited ZrO{sub 2} metal-insulator-metal capacitors by inserting interfacial Zr-doped chromia layers

In order to improve the electrical behaviour of metal-insulator-metal capacitors with ZrO{sub 2} insulator grown by Atomic Layer Deposition, the influence of the insertion of interfacial Cr layers between Pt electrodes and the zirconia is investigated. An improvement of the α-voltage coefficient of capacitance as low as 567 ppm/V{sup 2} is achieved for a single layer of Cr while maintaining a high capacitance density of 10.7 fF/μm{sup 2} and a leakage current of less than 1.2 × 10{sup −8} A/cm{sup 2} at +1 V. The role of the interface is discussed by means of X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy showing the formation of Zr stabilized chromia oxide phase with a dielectric constant of 16.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗