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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Vertical nanoscale strain-induced electronic localization in epitaxial La2/3Sr1/3MnO3 films with ZrO2 nanopillar inclusions

Abstract Unusual electrical transport properties associated with weak or strong localization are sometimes found in disordered electronic materials. Here, we report experimental observation of a crossover of electronic behavior from weak localization to enhanced weak localization due to the spatial influence of disorder induced by ZrO 2 nanopillars in (La 2/3 Sr 1/3 MnO 3 ) 1− x :(ZrO 2 ) x ( x = 0, 0.2, and 0.3) nanocomposite films. The spatial strain regions, identified by scanning transmission electron microscopy and high-resolution x-ray diffraction, induce a coexistence of two-dimentional (2D) and three-dimentional (3D) localization and switches to typical 2D localization with increasing density of ZrO 2 pillars due to length scale confinement, which interestingly accords with enhancing vertically interfacial strain. Based on the excellent agreement of our experimental results with one-parameter scaling theory of localization, the enhanced weak localization exists in metal range close to the fixed point. These films provide a tunable experimental model for studying localization in particular the transition regime by appropriate choice of the second epitaxial phase. Graphical Abstract

36 MATERIALS SCIENCE↗
DOE OSTI2021

Preliminary results from Low Pressure Steam Oxidation Testing of ALD ZrN and ZrO2 Coating Deposited over UCN Fuel Kernels

Steam oxidation testing was used to investigate as-developed 100 nm ALD ZrN and 1000 nm ZrO 2 coatings deposited over UC1-xNx fuel kernels. The results of these tests were compared against oxidation of uncoated kernels. This work was performed to support development of coatings which both provide high temperature Zr metal diffusion barrier and also provide resistance against oxidation, especially against high temperature steam and/or air. This work was performed in collaboration with Oak Ridge National Laboratory (ORNL), using ORNL-provided samples. The oxidation tests yielded valuable information, including relative differences in performance of different coating materials and coating thicknesses.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Rare Earth and Transition Metal Containing Glasses

Transition metal (TM) and rare earth (RE) ions have been incorporated into many glass systems such as silicate, phosphate, and borosilicate-based oxide glasses, as well as in halide and chalcogenide glasses, that find applications ranging from optical, photonic, and magnetic devices, solid-state battery, to nuclear waste disposal. Understanding the structural role of RE and TM in these glasses can help to develop glass compositions for targeted applications with either high-optical emission efficiency, electrical conductivity, or chemical durability. In this chapter, we first provide a general introduction of the applications and structural features of RE and TM in glasses, then the critical aspects of molecular dynamics (MD) simulations of these glasses such as interatomic potentials, structural analysis tools to study RE and TM ions in glasses and their clustering behaviors, Quantitative Structure–Property Analysis (QSPR), diffusion and dynamic property calculations, and electronic structure calculations to understand electronic defects such as charge trapping and radiation effects are introduced. Three representative case studies are presented: the first one is on MD simulations of erbium- and europium-doped silica and silicate glasses, as well as cerium doped aluminophosphate glasses, that revealed the effect of glass composition on RE ion local structure and clustering behavior. Electronic structure calculations of cerium-doped glass show how the existence of multioxidation states help to mediate radiation-induced damages caused by excited electron–hole pairs was also discussed. The second one focuses on alkali vanadophosphate glasses where the existence of two vanadium oxidation states help to provide electronic conduction in the glasses while alkali ions provide ionic conduction. MD simulations were used to understand vanadium environments and other structural aspects in the phosphate glasses, as well as the ionic transport behaviors of alkali ions. The third case study is on zirconium-containing borosilicate and aluminosilicate glasses that find wide applications in nuclear waste disposal. MD simulations help to provide structural details of zirconium ions that are validated by diffraction and EXAFS spectra. The structural information was used to interpret changes of mechanical properties and chemical durability by using QSPR and other analyses-based MD-generated structure models.

Du, Jincheng↗
DOE OSTI2020

Area-selective atomic layer deposition enabled by competitive adsorption

In this work, we investigate the atomic layer deposition (ALD) of ZrO2 thin films on Cu and SiO2 substrates, using Zr[N(C2H5CH3)]4 as the thin-film precursor, and H2O or O2 as the coreactants. Here, we introduce 3-hexyne as a coadsorbate molecule during the thin-film precursor half-cycle and examine its effect on the selectivity of growth. We find that 3-hexyne strongly inhibits growth on Cu, while having essentially no effect on the growth on SiO2. Calculations using dispersion-inclusive density functional theory verify that 3-hexyne undergoes sp → sp2 rehybridization on Cu, which results in strong chemisorption on the metal surface, while only binding weakly to SiO2 via nonbonded van der Waals/dispersion interactions. After 10 cycles of ALD using 3-hexyne as the coadsorbate, we observed the deposition of ∼1.5 nm of ZrO2 on SiO2. On a Cu substrate, we only detected <0.15 nm of ZrO2 after the same number of cycles of ALD. At this point in the process, we find evidence of the formation of cuprous oxide (Cu2O) from in situ x-ray photoelectron spectroscopy and a significant increase in the roughness of the Cu substrate. We conclude that both factors likely contribute to the loss of selectivity due to the formation of sites (e.g., Cu2O) that bind 3-hexyne less strongly and/or an increase in the density of highly reactive sites (e.g., steps, kinks) that promote dissociative chemisorption of the thin-film precursor.

Suh, Taewon (ORCID:0000000163823811)↗
DOE OSTI2021

Understanding Selectivity in CO2 Hydrogenation to Methanol for MoP Nanoparticle Catalysts Using In Situ Techniques

Molybdenum phosphide (MoP) catalyzes the hydrogenation of CO, CO2, and their mixtures to methanol, and it is investigated as a high-activity catalyst that overcomes deactivation issues (e.g., formate poisoning) faced by conventional transition metal catalysts. MoP as a new catalyst for hydrogenating CO2 to methanol is particularly appealing for the use of CO2 as chemical feedstock. Herein, we use a colloidal synthesis technique that connects the presence of MoP to the formation of methanol from CO2, regardless of the support being used. By conducting a systematic support study, we see that zirconia (ZrO2) has the striking ability to shift the selectivity towards methanol by increasing the rate of methanol conversion by two orders of magnitude compared to other supports, at a CO2 conversion of 1.4% and methanol selectivity of 55.4%. In situ X-ray Absorption Spectroscopy (XAS) and in situ X-ray Diffraction (XRD) indicate that under reaction conditions the catalyst is pure MoP in a partially crystalline phase. Results from Diffuse Reflectance Infrared Fourier Transform Spectroscopy coupled with Temperature Programmed Surface Reaction (DRIFTS-TPSR) point towards a highly reactive monodentate formate intermediate stabilized by the strong interaction of MoP and ZrO2. This study definitively shows that the presence of a MoP phase leads to methanol formation from CO2, regardless of support and that the formate intermediate on MoP governs methanol formation rate.

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

Kinetics and Reactor Modeling of VFA Ketonization for Sustainable Aviation Fuel Production

Ketonization of volatile fatty acids (VFAs) produced via arrested methanogenesis of wet waste represents the only unit operation of wet waste upgrading to sustainable (net-zero or negative life cycle CO2 emissions) aviation fuel not currently in industrial practice. Ketone product yields of close to 100% have been obtained during gas-phase reactions of VFAs over oxide catalysts at the laboratory scale, but design of an industrial ketonization reactor requires understanding of the impacts of reactant and product partial pressures, deactivation, and heat and mass transport phenomena on observed ketonization rates. Our work leverages rigorous kinetic analysis of ketonization processes to inform reactor scale-up efforts through packed-bed reactor modeling. We first present results of a kinetic study of ketonization of a model VFA, hexanoic acid, over an industrial ZrO2 catalyst performed in a packed-bed microreactor in conditions free of significant heat or mass transfer gradients. Major findings of the analysis include: (i) hexanoic acid saturates all catalyst active sites at relatively low partial pressure (~10 kPa) and (ii) ketonization products 6-undecanone, water, and CO2 inhibit reaction rates. Kinetic data are used to fit a rate expression quantifying the functional dependence of ketonization rate on partial pressures of VFA reactants and ketone, water, and CO2 products. A packed-bed reactor model describing vapor-phase hexanoic acid ketonization over ZrO2 extrudate catalysts is developed based on the kinetic model. The effects of (i) bed- and pellet-scale mass- and heat-transfer limitations and (ii) axial pressure drop guide our development of recommendations for optimal sizing, temperature, and influent flow composition of an industrial-scale ketonization reactor. The quantitative understanding of VFA ketonization developed in this study represents an advance toward derisking the VFA ketonization step of wet waste upgrading to sustainable aviation fuel.

biojet↗
DOE OSTI2022

Synthesis of americium trichloride via chlorination of americium oxide using zirconium tetrachloride in LiCl-KCl molten salt

To better inform electrorefining operations of used nuclear fuel, a fundamental understanding of the electrochemical properties of each actinide in the molten salt electrolyte is needed. Access to actinide chlorides is thus required to support measurement of these properties. In this work, we synthesized americium trichloride through a novel pathway comprising chlorination of Am2O3 with ZrCl4 in a LiCl-KCl molten salt at 500 degrees C. The formation of AmCl3 was confirmed by cyclic voltammetry and gamma spectroscopy. This chlorination method does not produce mixed hazardous-radioactive waste and the byproduct, ZrO2, is not electrochemically active and does not affect the actinide electrodeposition reaction.

actinide chloride synthesis↗
DOE OSTI2023

Determining the tensile strength of fuel surrogate TRISO-coated particle buffer, IPyC, and buffer-IPyC interlayer regions

A novel micro tensile sample fabrication technique for determining the tensile strength of the buffer, IPyC, and buffer -IPyC interlayer regions of surrogate (ZrO2) TRISO fuel particle layers was refined and implemented. Copper micro tensile samples served as baseline materials to verify the methods used. Tensile tests performed in this study, while limited in number, were analyzed using standard and Weibull statistics. As expected, the buffer layer was weakest, with an average ultimate tensile strength of 138.70 MPa, and the IPyC layer samples, were strongest, with an average ultimate tensile strength of 189.74 MPa. In the buffer -IPyC interface samples, all breaks occurred in the buffer region, though the average ultimate tensile strength of the samples, 159.80 MPa, was between the pure buffer and IPyC samples. These results suggest the interlayer region has unique properties, perhaps associated with pyrocarbon infiltration into the buffer layer during particle coating. All interlayer samples fractured within the buffer side; however, the stress strain behavior of some of these samples resembled the behavior of the IPyC layer samples. Here, the buffer and IPyC layer strengths had a normal distribution under Weibull analysis, while the interlayer region had a Rayleigh distribution. Further testing is needed to clarify both the standard and Weibull statistical results

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2025

Thermally Stable Ruthenium Contact for Robust p‑Type Tellurium Transistors

Tellurium (Te) is attractive for p-channel transistors due to its high hole mobility. Despite having a low thermal budget suitable for back-end-of-line (BEOL) monolithic integration, the practical realization of Te transistors is hindered by its thermal stability. In this work, we investigate thermal stability for Te thin films grown via scalable thermal evaporation. Our findings identify ruthenium as a more thermally stable contact for p-type Te transistors, capable of withstanding temperatures up to 250 °C. Ruthenium exhibits significantly lower diffusivity in Te compared to other contact metals commonly used such as nickel and palladium. Using the transfer-length method, we measured a contact resistance of 1.25 kΩ·μm at the ruthenium-tellurium interface. Additionally, the incorporation of high-κ ZrO2 encapsulation not only suppresses the sublimation of the Te channel at elevated temperatures but also serves as the gate dielectric in top-gate devices operating at 1 V, achieving an on/off current ratio of 105.

Rahman, I K M Reaz↗
DOE OSTI2022

Olefin metathesis over supported MoO x catalysts: influence of the oxide support

Here, a series of supported MoO x catalysts on different oxide supports (Al2O3, TiO2, ZrO2, SiO2) were synthesized and investigated for propylene metathesis, characterized with in situ spectroscopies (DRIFTS, Raman, UV-vis) and chemically probed with propylene-TPSR-MS, propylene-TPSR-IR, and ethylene/2-butene titration. Under dehydrated conditions at monolayer coverage or maximum surface dispersion, the surface MoO x sites are present as a mixture of isolated di-oxo (O=) 2 Mo(–O–Al) 2 and oligomeric mono-oxo O=Mo(–O–Al) 4/5 sites on Al 2 O 3 , primarily oligomeric mono-oxo O=Mo(–O–Ti) 4/5 on TiO 2 , isolated di-oxo (O=) 2 Mo(–O–Zr) 2 and oligomeric mono-oxo O=Mo(–O–Zr) 4/5 on ZrO 2 , and isolated di-oxo (O=) 2 Mo(–O–Si) 2 on SiO 2 . The bridged (S 2 -OH) and tri-coordinated (S 3 -OH) anchoring surface hydroxyls of the oxide supports with strong support cation electronegativity control the activation and number of active surface MoO x sites at low temperatures (<100 °C). The isolated anchoring surface hydroxyls (S-OH) of the oxide supports with strong support cation electronegativity control the activation and number of active surface MoO x sites at high temperatures (>350 °C). Olefin metathesis by the more redox active supported MoO x /TiO 2 and MoO x /ZrO 2 catalysts is retarded by the formation of stable surface acetone and acetate species that block olefin adsorption. The oxide supports are potent ligands that tune the activation and surface chemistry of the surface MoOx sites for olefin metathesis. This is the first time that the influence of oxide supports on the activation and surface chemistry of supported MoO x sites has been systematically examined.

02 PETROLEUM↗
DOE OSTI2023

Final-stage densification kinetics of direct current–sinteredZrB2

Final-stage sintering was analyzed for nominally phase pure zirconium diboride synthesized by borothermal reduction of high-purity ZrO2. Analysis was conducted on ZrB2 ceramics with relative densities greater than 90% using the Nabarro–Herring stress–directed vacancy diffusion model. Temperatures of 1900°C or above and an applied uniaxial pressure of 50 MPa were required to fully densify ZrB2 ceramics by direct current sintering. Ram travel data were collected and used to determine the relative density of the specimens during sintering. Specimens sintered between 1900 and 2100°C achieved relative densities greater than 97%, whereas specimens sintered below 1900°C failed to reach the final stage of sintering. The average grain size ranged from 1.0 to 14.7 μm. The activation energy was calculated from the slope of an Arrhenius plot that used the Kalish equation. The activation energy was 162 ± 34 kJ/mol, which is consistent with the activation energy for dislocation movement in ZrB2. The diffusion coefficients for dislocation motion that controls densification were 5.1 × 10−6 cm2/s at 1900°C and 5.1 × 10−5 cm2/s at 2100°C, as calculated from activation energy and average grain sizes. This study provides evidence that the dominant mechanism for final-stage sintering of ZrB2 ceramics is dislocation motion.

36 MATERIALS SCIENCE↗