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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 1,045 records · Page 58
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Molybdenum Contacts to MoS 2 Field‐Effect Transistors: Schottky Barrier Extraction, Electrical Transport, and Low‐Frequency Noise

MoS 2 has great interest for nanoscale electronic devices, including transistors and sensors. Defect‐free structure, clean interface with the gate dielectric, and metals with suitable metal–semiconductor junctions are key for reliable devices. Molybdenum is shown as an excellent electrode for MoS 2 field‐effect transistors (FETs), but the correlation among device current, field‐effect mobility, and the low‐frequency noise (LFN) with number of atomic layers is not studied. Furthermore, the impact of Mo/MoS 2 Schottky barrier on electronic injection and scattering in the device needs to be understood. Herein, by studying the FETs with channels of different numbers of atomic‐layer thicknesses, it is shown that a molybdenum metal contact to MoS 2 provides reliable current injection with low Schottky barrier height, low contact resistance, low electron–phonon scattering, and negligible impurity scattering. The mobility and LFN are optimal at a channel thickness of ≈12 atomic layers, which is consistent with the “mobility maximum window” in MoS 2 transistors with prior studies using other contact metals.

Kwon, Jiseok↗
DOE OSTI2018

Strain Tuning in Complex Oxide Epitaxial Films Using an Ultrathin Strontium Aluminate Buffer Layer

A reliable method to apply biaxial strain over a wide range of values with minimal dislocation generation is critical for the study of strain dependent physical properties in oxide thin films and heterostructures. In this study, we systematically controlled the strain state in a perovskite manganite thin film by as much as 1% using a new ultrathin strain-releasing buffer layer Sr 3 Al 2 O 6 , and observed signatures of accompanying magnetic and metal–insulator transitions. The near-zero strain state is achieved within five nanometers of buffer layer thickness, substantially thinner than any oxide epitaxial buffer layers that can continuously tune the film strain states. Furthermore, the majority of misfit dislocations were confined to the Sr 3 Al 2 O 6 layer, structurally decoupling defects in the film from the substrate.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Potential benefit of amorphization in the retention of gaseous species in irradiated pyrochlores

Understanding the structure-property relationship for materials destined for irradiation extremes is a key step in developing materials with reliable, long-term performance. One crucial relationship is the ability of a material to withstand or accommodate amorphization, as this dictates its potential use as a nuclear waste form. Pyrochlores are one such class of materials for consideration as waste forms and there has been significant work examining how both the crystal structure and chemistry impacts amorphization resistance, leading to the important conclusion that the amorphization resistance of pyrochlores (A 2 B 2 O 7 ) is very sensitive to the nature of the B cations. For example, pyrochlores with B=Ti amorphize much more readily than B=Zr compounds. However, there are still questions regarding how these types of materials respond to prolonged or high-dose irradiation conditions. Gd 2 Ti 2 O 7 (GTO) and Gd 2 Zr 2 O 7 (GZO) pyrochlores were implanted with 400 keV Kr++ ions at room temperature to calculated peak damages of 119 and 135 displacements per atom (dpa), respectively. As expected, GTO amorphized completely under irradiation. However, discrete bubbles of Kr coalesced within the amorphous matrix without micro-cracking or spallation. In contrast, GZO transforms to a disordered fluorite structure under irradiation with no indications of localized amorphization. But, the accumulation of Kr within the host material leads to sub-grain structures, extended defects, and the development of micro-cracks. Thus, while GTO readily amorphizes even under low dose irradiations, the resistance of the amorphous GTO matrix to micro cracking and gas release, even in the presence of large bubble formation, suggests an enhanced propensity to retain gaseous species. Finally, consideration of long-term dose accumulation effects in nuclear waste forms would suggest reconsideration of amorphization processes in pyrochlores and related materials as a potential beneficial effect for immobilization and long-term storage of actinide materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Corrosion-resistant polyurethane coatings from structure-homogenized biorefinery lignin through fractionation and oxypropylation

Using lignin for polyurethane (PU) synthesis is challenging because of its heterogeneous structure, i.e., broad molecule weight (MW) distribution and diverse hydroxyl (OH) functionality. This complexity influences the crosslinking reaction between lignin and isocyanate and affects the quality of PU products. Here, we propose a two-step strategy for lignin structure homogenization by lowering its MW and uniforming OH functionality (step 2). Step 1 involves a bioethanol fractionation process, yielding fractionated lignin with one order lower MW than the raw lignin while exhibiting better processability for the subsequent process. Step 2 is performed by an oxyalkylation process using fractionated lignin that converts phenolic and carboxylic OHs to aliphatic OHs. As a result, we obtained structure-homogenized lignin with good compatibility and crosslinking with hexamethylene diisocyanate for PU coating synthesis. Moreover, PU coatings prepared from structure-homogenized lignin display a defect-free microscopic structure and show better corrosion resistance than coatings fabricated from its raw lignin alternative. Overall, the structure homogenization process is a beneficial way of using lignin as biopolyol in PU synthesis toward the application-oriented design of corrosion-resistant lignin-based PU coatings.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Phase degradation in B x Ga 1–x N films grown at low temperature by metalorganic vapor phase epitaxy

Using metalorganic vapor phase epitaxy, a comprehensive study of B x Ga 1-x N growth on GaN and AlN templates is described. BGaN growth at high-temperature and high-pressure results in rough surfaces and poor boron incorporation efficiency, while growth at low-temperature and low-pressure (750–900 °C and 20 Torr) using nitrogen carrier gas results in improved surface morphology and boron incorporation up to ~7.4% as determined by nuclear reaction analysis. However, further structural analysis by transmission electron microscopy and x-ray pole figures points to severe degradation of the high boron composition films, into a twinned cubic structure with a high density of stacking faults and little or no room temperature photoluminescence emission. Films with <1% triethylboron (TEB) flow show more intense, narrower x-ray diffraction peaks, near-band-edge photoluminescence emission at ~362 nm, and primarily wurtzite-phase structure in the x-ray pole figures. For films with >1% TEB flow, the crystal structure becomes dominated by the cubic phase. As a result, only when the TEB flow is zero (pure GaN), does the cubic phase entirely disappear from the x-ray pole figure, suggesting that under these growth conditions even very low boron compositions lead to mixed crystalline phases.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Glassy correlated state induced by disorder in the frustrated antiferromagnet Tb 2 Zr 2 O 7

Here, we study the low-temperature thermomagnetic properties of the polycrystalline material Tb 2 Zr 2 O 7 through ac susceptibility and specific heat measurements. This zirconate displays a defect-fluorite structure in which the magnetic Tb 3+ and nonmagnetic Zr 4+ cations sit randomly on the same metal sublattice. No long-range magnetic order is found down to 100 mK, although dominant antiferromagnetic interactions are observed and the spins remain dynamic down to the lowest temperatures investigated. We observed a frequency-dependent peak around 2.5 K which is well described by many models of a canonical spin-glass transition. In-field specific heat measurements and the recovered entropy of the system R ln(4) suggest a two doublets ground state separated by 7 K. Comparisons to the pyrochlores Tb 2 Ti 2 O 7 and Tb 2 Hf 2 O 7 reinforce the collective spin-glass-type behavior and the opening for discussions of an exotic Coulomb phase in this material.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The defect chemistry of U O 2 ± x from atomistic simulations

Control of the defect chemistry in UO 2±x is important for manipulating nuclear fuel properties and fuel performance. For example, the uranium vacancy concentration is critical for fission gas release and sintering, while all oxygen and uranium defects are known to strongly influence thermal conductivity. Here the point defect concentrations in thermal equilibrium are predicted using defect energies from density functional theory (DFT) and vibrational entropies calculated using empirical potentials. Electrons and holes have been treated in a similar fashion to other charged defects allowing for structural relaxation around the localized electronic defects. Predictions are made for the defect concentrations and non-stoichiometry of UO 2±x as a function of oxygen partial pressure and temperature. If vibrational entropy is omitted, oxygen interstitials are predicted to be the dominant mechanism of excess oxygen accommodation over only a small temperature range (1265 K to 1350 K), in contrast to experimental observation. Conversely, if vibrational entropy is included oxygen interstitials dominate from 1165 K to 1680 K (Busker potential) or from 1275 K to 1630 K (CRG potential). Below these temperature ranges excess oxygen is predicted to be accommodated by uranium vacancies, while above them the system is hypo-stoichiometric with oxygen deficiency accommodated by oxygen vacancies. Our results are discussed in the context of oxygen clustering, formation of U 4 O 9 , and issues for fuel behavior. In particular, the variation of the uranium vacancy concentrations as a function of temperature and oxygen partial pressure will underpin future studies into fission gas diffusivity and broaden the understanding of UO 2±x sintering.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2015

Growth and luminescent properties of scintillators based on the single crystalline films of Lu{sub 3−x}Gd{sub x}Al{sub 5}O{sub 12}:Ce garnet

Highlights: • Single crystalline films of Lu{sub 3−x}Gd{sub x}Al{sub 5}O{sub 12} garnets at x = 0 ÷ 3.0 were grown by LPE method onto YAG substrates. • Lattice constant of Lu{sub 3−}Gd{sub x}Al{sub 5}O{sub 12}:Ce film and the misfit m between films and YAG substrate changed linearly with increasing of Gd content. • Effective Gd{sup 3+}–Ce{sup 3+} energy transfer occurs in the Lu{sub 3−x}Gd{sub x}Al{sub 5}O{sub 12}:Ce films. • Best scintillation light yield is observed in the Lu{sub 3}Al{sub 5}O{sub 12}:Ce and Lu{sub 2.4}Gd{sub 0.6}Al{sub 5}O{sub 12}:Ce films. • Increase of the Gd content in x = 1.5–2.5 range results in decreasing the scintillation LY of Lu{sub 3−x}Gd{sub x}Al{sub 5}O{sub 12}:Ce films. - Abstract: The work is related to the growth of scintillators based on the single crystalline films (SCF) of Ce{sup 3+} doped Lu{sub 3−}Gd{sub x}Al{sub 5}O{sub 12} mixed rare-earth garnets by Liquid Phase Epitaxy (LPE) method. We have shown, that full set of Lu{sub 3−}Gd{sub x}Al{sub 5}O{sub 12} SCFs with x values ranging from 0 to 3.0 can be successfully crystallized by the LPE method onto Y{sub 3}Al{sub 5}O{sub 12} (YAG) substrates from the melt-solutions based on PbO-B{sub 2}O{sub 3} flux. The absorption, X-ray excited luminescence, photoluminescence, thermoluminescence and light yield measurements, the latter under excitation by α-particles of {sup 239}Pu and {sup 241}Am radioisotopes, were applied for their characterization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Electric-field-assisted-sintering of rare-earth oxide dispersion strengthened Fe-Cr-Mo alloys

Oxide dispersion-strengthened (ODS) alloys are widely recognized for their exceptional high-temperature strength, creep resistance, and radiation tolerance, making them indispensable for advanced nuclear reactors, aerospace, and energy systems. Achieving a fine and stable dispersion of oxide nanoparticles is critical, as these particles act as strong barriers to dislocation motion and effective sinks for irradiation-induced defects, ensuring structural integrity under extreme conditions. Here, in this study, Fe–Cr–Mo-based ODS alloys were fabricated via mechanical alloying and consolidated using electric-field-assisted sintering (EFAS) with additions of Y 2 O 3 , La 2 O 3 , and CeO 2 . EFAS processing produced ultrafine-grained microstructures (average grain size <1 μm) with uniformly distributed oxide clusters (2–4 nm). Atom probe tomography revealed that La 2 O 3 -containing alloys exhibited the highest nanoparticle number density, resulting in superior tensile strength compared to yttria- and ceria-bearing counterparts. The combined effect of grain refinement and rare-earth oxide dispersion significantly enhanced mechanical performance, demonstrating the potential of EFAS for developing high-strength ferritic alloys for demanding environments such as nuclear systems.

36 - MATERIALS SCIENCE↗
DOE OSTI2019

A comprehensive study on the fabrication and characterization of Ti–48Al–2Cr–2Nb preforms manufactured using electron beam melting

This feasibility study examines the additive manufacturing of Ti–48Al–2Cr–2Nb preforms in a quest to make more complex engine components from this promising but hard to machine, brittle material. The preforms are built using the electron beam melting technique; hot isostatic pressing (HIP) was performed as the only post-process. Microstructural characterizations were carried with scanning electron microscopy techniques, X-ray diffraction and X-ray computed tomography while mechanical properties were obtained via tensile testing. We exploited computational thermodynamic modeling (widely known as CALPHAD) approach to investigate the phase stability pertinent to Nb and Cr modified TiAl. Defect-free structures were obtained in the HIPed condition with mechanical behavior comparable to/better than traditionally manufactured counterparts. Equiaxed duplex microstructures were achieved in the as-built condition that were sustained post-HIP. The effect of the build conditions and phase transformation kinetics on the observed microstructures is discussed. Furthermore, the microstructure-mechanical behavior relationship is also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Cooling-Rate-Driven Polymorphism and Vacancy Ordering in Ce 2 MnGe 6 Intergrowth Phases

Intergrowth phases in intermetallic systems provide a compelling framework for investigating structure–property relationships as a function of crystallographic subunit stacking. In this study, we examine the influence of Mn vacancy ordering on subunit stacking and reassign the structure of Ce 2 MnGe 6 to a monoclinic C2/m space group (a = 8.3486(17) Å, b = 8.6181(18) Å, c = 10.778(2) Å, β = 101.17(2)°). Additionally, we report a monoclinic polymorph of the Ln 2 MGe 6 family characterized as a tripled c-axis supercell derivative of monoclinic Ce 2 MnGe 6 , 3×c-Ce 2 MnGe 6 (a = 8.3482(16) Å, b = 8.6179(18) Å, c = 31.813(6) Å, β = 93.763(7)°). This polymorph emerges under rapid cooling conditions during the synthesis and offers insight into the structural relationship between the orthorhombic and monoclinic variants of the Ln 2 MGe 6 phases. Notably, the supercell form of Ce 2 MnGe 6 (3×c-Ce 2 MnGe 6 ) exhibits increased electrical resistivity and suggests enhanced Zintl-like behavior, potentially indicating greater thermodynamic stability relative to the parent monoclinic phase.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Crystal Structure and Atomic Vacancy Optimized Thermoelectric Properties in Gadolinium Selenides

Thermoelectric materials enable the energy conversion of waste heat into electricity, helpful to relieve global energy crisis. Here, we report a systematic investigation on high-temperature thermoelectric gadolinium selenides, cubic Gd 3-x Se 4 (x = 0.16, 0.21 and 0.25) and orthorhombic Gd 2 Se 3-y (y = 0.02, 0.06 and 0.08). High energy synchrotron x-ray diffraction and total scattering have been used to investigate the crystallographic and local structures. Atomic-scale clusters of Gd vacancy in the cubic phase are observed by employing the reverse Monte Carlo simulation. For cubic Gd 3-x Se 4 , adjusting Gd vacancy triggers the effect of multiple conduction bands, confirmed by the increase in effective masses and theoretical calculations. A reasonable peak zT value of 0.27 is achieved at 850 K for Gd 3-x Se 4 (x = 0.16). On the other hand, tuning Se vacancy enables the optimization of electron concentration for the orthorhombic Gd 2 Se 3-y . More significantly, its low deformation potential (Ξ = 12eV) gives rise to enhanced electron mobility and higher peak zT value of 0.54 at 850 K for Gd 2 Se 3-y (y = 0.02). Intriguingly, a higher zT of 1.2 at 1200 K is reasonably predicted by quality factor analysis. Finally, this work extends the scope of high-temperature thermoelectric materials and facilitates the exploration of novel high-temperature thermoelectric materials

36 MATERIALS SCIENCE↗
DOE OSTI2021

Linker Contribution toward Stability of Metal–Organic Frameworks under Ionizing Radiation

Here, the effects of radiation on a series of UiO derivative metal–organic frameworks (MOFs) that contain the same zirconium hexamer node and similar organic linkers, UiO-66, UiO-66–NH 2 , UiO-66–OH, and NU-403, were examined using γ-rays and 5 MeV He ions. UiO-66, UiO-66–NH2, and UiO-66–OH contain aromatic linkers and are significantly more stable to radiation than NU-403. Of these, UiO-66 is the most radiation resistant, displaying crystalline features up to 47 MGy of He-ion irradiation. MOFs containing aromatic linkers functionalized by electron-donating groups, UiO-66–NH 2 and UiO-66–OH, retained crystalline features up to 19 MGy. NU-403 contains aliphatic rings and is the least radiation-resistant MOF studied here. NU-403 exhibits small changes in infrared spectra upon 3 MGy of γ-irradiation and significant damage upon 10 MGy of He-ion irradiation. Diffraction data revealed radiation-induced defect formation. Structural locations of radiation-induced breakdown were interrogated experimentally and via density functional theory. The results indicated changes in the carboxylate (-OCO) of the linker and μ 3 -OH vibrational modes, suggesting that introduction of an aliphatic linker into the MOF renders the connection between the linker and metal node most susceptible to radiation damage. This study reveals that the choice of the linker is crucial in designing a radiation-resistant MOF.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Fundamental Thermodynamic, Kinetic, and Mechanical Properties of Lithium and Its Alloys

Lithium alloying reactions are beneficial in promoting uniform plating and stripping of lithium metal in all-solid-state batteries. First-principles calculations are performed to predict thermodynamic, kinetic, and mechanical properties of lithium and several important Li–M alloys (M = Mg, Ag, Zn, Al, Ga, In, Sn, Sb, and Bi). While the Li–Mg binary system forms a solid solution, most other lithium–metal alloys prefer stoichiometric intermetallic compounds with common local motifs that enable fast Li diffusion. Lithium and Li-rich alloys exhibit an unusually flat energy landscape along paths that connect BCC to close-packed structures like FCC and HCP, with important implications for mechanical properties. Very low migration barriers for Li diffusion that rival those of superion conductors are predicted, both in pure Li and in Li–M intermetallics. However, vacancy concentration, which is crucial for substitutional diffusion, is predicted to be low in metallic Li and most Li–M intermetallics. Compounds such as B32 LiAl and LiGa as well as D0 3 Li 3 Sb and Li 3 Bi exhibit structural vacancies at higher ends of their voltage windows, which together with low migration barriers leads to exceptionally high Li mobilities. In the Li–Mg solid solution, the addition of Mg is found to decrease the vacancy tracer diffusion coefficient by an order of magnitude.

36 MATERIALS SCIENCE↗