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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Mobility assessment of the BCC and carbide phases in the C-Nb, C-U and Nb-U systems

Uranium carbides with refractory metal additions are considered for Gen IV nuclear reactors and nuclear thermal propulsion as fuels for their high-temperature and corrosion resistant properties. Understanding kinetic effects that dictate microstructural evolution during fabrication and operating conditions is essential to advance technological development of these fuels. This work presents the development of an atomic mobility database for C-Nb-U systems based off available experimental data supported with ab-initio methods. The mobility assessments and uncertainty quantification (using Markov chain Monte Carlo) were conducted in the Kawin software. Carbon diffusion is considered dominant, as metal diffusion is much slower, with niobium diffusion being even slower and rate limiting than uranium metal. We provide a comprehensive and self-consistent thermo-kinetic database that is validated by diffusion couple simulations through Kawin. In conclusion, this enables prediction of microstructural and phase evolution critical for the development and lifetime assessment of next generation nuclear fuels.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Effect of substrate temperature on the growth of Nb 3 Sn film on Nb by multilayer sputtering

Here, we report on the fabrication of niobium tin (Nb 3 Sn) films by multilayer sequential sputtering on niobium at substrate temperatures ranging from room temperature to 250 °C. The multilayers were then annealed inside a separate vacuum furnace at 950 °C for 3 h. The material properties of the films were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy, and transmission electron microscopy. The superconducting properties of the films were studied by four-point probe resistivity measurements from room temperature to below the superconducting critical temperature T c . The highest film T c was 17.76 K, obtained when the multilayers were deposited at room temperature. When the deposition temperature was raised to 250 °C, a significant reduction in voids was achieved while the film's T c was 17.58 K.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of substrate temperature on the growth of Nb$_3$Sn film on Nb by multilayer sputtering

Nb3Sn films were fabricated by multilayer sequential sputtering on Nb substrates at substrate temperatures ranging from room temperature to 250 °C. The film material properties were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy, and transmission electron microscopy. The films’ superconducting properties were studied by four-point probe resistivity measurements from room temperature to below the superconducting critical temperature Tc. The highest Tc was17.76 K, when the multilayers were deposited at room temperature. A superconducting Nb3Sn thin film with a smoother surface morphology but a lower Tc of 17.58 K was obtained on the film deposited at a substrate temperature of 250 °C.

Sayeed, Md. Nizam↗
DOE OSTI2020

Materials Data on Nb by Materials Project

Nb is beta Plutonium-like structured and crystallizes in the triclinic P1 space group. The structure is three-dimensional. there are twelve inequivalent Nb sites. In the first Nb site, Nb is bonded in a 1-coordinate geometry to thirteen Nb atoms. There are a spread of Nb–Nb bond distances ranging from 2.66–3.45 Å. In the second Nb site, Nb is bonded in a 1-coordinate geometry to twelve Nb atoms. There are a spread of Nb–Nb bond distances ranging from 2.66–3.31 Å. In the third Nb site, Nb is bonded in a 1-coordinate geometry to fourteen Nb atoms. There are a spread of Nb–Nb bond distances ranging from 2.68–3.44 Å. In the fourth Nb site, Nb is bonded in a 1-coordinate geometry to fourteen Nb atoms. There are a spread of Nb–Nb bond distances ranging from 2.96–3.35 Å. In the fifth Nb site, Nb is bonded in a 1-coordinate geometry to thirteen Nb atoms. There are a spread of Nb–Nb bond distances ranging from 2.64–3.38 Å. In the sixth Nb site, Nb is bonded in a 1-coordinate geometry to eleven Nb atoms. There are a spread of Nb–Nb bond distances ranging from 2.88–3.16 Å. In the seventh Nb site, Nb is bonded in a 1-coordinate geometry to thirteen Nb atoms. There are a spread of Nb–Nb bond distances ranging from 2.71–3.12 Å. In the eighth Nb site, Nb is bonded to twelve Nb atoms to form a mixture of corner and face-sharing NbNb12 cuboctahedra. There are a spread of Nb–Nb bond distances ranging from 2.68–3.14 Å. In the ninth Nb site, Nb is bonded to twelve Nb atoms to form a mixture of corner and face-sharing NbNb12 cuboctahedra. There are a spread of Nb–Nb bond distances ranging from 2.82–2.92 Å. In the tenth Nb site, Nb is bonded in a 1-coordinate geometry to twelve Nb atoms. There are one shorter (2.88 Å) and one longer (3.14 Å) Nb–Nb bond lengths. In the eleventh Nb site, Nb is bonded in a 10-coordinate geometry to eleven Nb atoms. The Nb–Nb bond length is 2.68 Å. In the twelfth Nb site, Nb is bonded in a 10-coordinate geometry to eleven Nb atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2024

The nontrivial effects of annealing on superconducting properties of Nb single crystals

The effect of annealing on the superconducting properties of niobium single crystals was studied using optical, magnetic, and scanning tunneling microscopy (STM) methods. Pieces of the same crystal boule were studied before and after the annealing at 800 ${^\circ}\textrm{C}$, 1400 ${^\circ}\textrm{C}$, and near the melting point of niobium (2477 ${^\circ}\textrm{C}$). The initial samples had a high hydrogen content and low-temperature imaging revealed large hydrides (hundreds of micrometers) appearing below 190 K. The formation of these large precipitates is already completely suppressed by annealing at 800 ${^\circ}\textrm{C}$. However, the overall superconducting properties of the annealed samples did not improve and, in fact, worsened. In particular, the superconducting transition temperature decreased, the upper critical field increased, and the pinning strength increased. In the STM study, the sample was annealed initially at 400 ${^\circ}\textrm{C}$, measured, annealed at 1700 ${^\circ}\textrm{C}$, and measured again. The STM revealed a ‘dirty’ superconducting gap with a significant spatial variation in tunneling conductance after annealing at 400 ${^\circ}\textrm{C}$. The clean gap was recovered after annealing at 1700 ${^\circ}\textrm{C}$. This is likely due to oxygen redistribution near the surface, which is always covered by oxide layers in as-grown crystals. Our results indicate that vacuum annealing at least up to 1400 ${^\circ}\textrm{C}$, while removing a large percentage of hydrogen, introduces additional nanosized defects, likely hydride precipitates, that act as efficient pair-breaking and pinning centers. The dimensionless scattering rate is estimated to have increased from $\Gamma = 0.2$ to about $\Gamma = 0.4$ after annealing at 1400 ${^\circ}\textrm{C}$. These results on single crystals differ drastically from those obtained in polycrystalline bulk niobium (i.e. cut from superconducting radio-frequency cavities), where annealing is known to have a significant positive effect that is attributed to the improvement of the crystalline structure masking the more subtle influence of the hydrides.

43 PARTICLE ACCELERATORS↗