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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Biokinetic models for Group VB elements

This paper reviews biokinetic data for the Group VB elements vanadium, niobium, and tantalum, and presents biokinetic models describing their systemic behaviour. The model for systemic niobium in adults was developed earlier and described in Publication 134 of the International Commission on Radiological Protection. The model for niobium is used as a starting point for the development of models for vanadium and tantalum. Published biokinetic data for vanadium, including comparisons with niobium, indicate that the initial distribution of vanadium is broadly similar to that of niobium but that vanadium is less firmly fixed in most tissues and is excreted more rapidly than niobium. Biokinetic data for tantalum are more limited but suggest that its systemic behaviour closely resembles that of niobium at early times after administration. The model for niobium is proposed for application to tantalum in view of the suggested biological similarities of tantalum and niobium, their generally strong coherence in nature due to similar ionic radii and identical valence states, and the difficulties in developing parameter values directly from available data for tantalum. In conclusion, the proposed model for vanadium relies largely on vanadium-specific information and varies considerably from the model for niobium.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2018

Kohn Anomaly and Phase Stability in Group VB Transition Metals

In the periodic table, only a few pure metals exhibit lattice or magnetic instabilities associated with Fermi surface nesting, the classical examples being α-U and Cr. Whereas α-U displays a strong Kohn anomaly in the phonon spectrum that ultimately leads to the formation of charge density waves (CDWs), Cr is known for its nesting-induced spin density waves (SDWs). Recently, it has become clear that a pronounced Kohn anomaly and the corresponding softening in the elastic constants is also the key factor that controls structural transformations and mechanical properties in compressed group VB metals—materials with relatively high superconducting critical temperatures. This article reviews the current understanding of the structural and mechanical behavior of these metals under pressure with an introduction to the concept of the Kohn anomaly and how it is related to the important concept of Peierls instability. We review both experimental and theoretical results showing different manifestations of the Kohn anomaly in the transverse acoustic phonon mode TA (ξ00) in V, Nb, and Ta. Specifically, in V the anomaly triggers a structural transition to a rhombohedral phase, whereas in Nb and Ta it leads to an anomalous reduction in yield strength.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Predicting two-dimensional topological phases in Janus materials by substitutional doping in transition metal dichalcogenide monolayers

Abstract Ultrathin Janus two-dimensional (2D) materials are attracting intense interest currently. Substitutional doping of 2D transition metal dichalcogenides (TMDs) is of importance for tuning and possible enhancement of their electronic, physical and chemical properties toward industrial applications. Using systematic first-principles computations, we propose a class of Janus 2D materials based on the monolayers MX 2 (M = V, Nb, Ta, Tc, or Re; X = S, Se, or Te) with halogen (F, Cl, Br, or I) or pnictogen (N, P, As, Sb, or Bi) substitution. Nontrivial phases are obtained on pnictogen substitution of group VB (V, Nb, or Ta), whereas for group VIIB (Tc or Re), the nontrivial phases are obtained for halogen substitution. Orbital analysis shows that the nontrivial phase is driven by the splitting of M- d yz and M- d xz orbitals. Our study demonstrates that the Janus 2D materials have the tunability and suitability for synthesis under various conditions.

36 MATERIALS SCIENCE↗
NASA NTRS1988

High temperature regenerative H.sub.2 S sorbents

Efficient, regenerable sorbents for removal of H.sub.2 S from high temperature gas streams comprise porous, high surface area particles. A first class of sorbents comprise a thin film of binary oxides that form a eutectic at the temperature of the gas stream coated onto a porous, high surface area refractory support. The binary oxides are a mixture of a Group VB or VIB metal oxide with a Group IB, IIB or VIII metal oxide such as a film of V-Zn-O, V-Cu-O, Cu-Mo-O, Zn-Mo-O or Fe-Mo-O coated on an alumina support. A second class of sorbents consist of particles of unsupported mixed oxides in the form of highly dispersed solid solutions of solid compounds characterized by small crystallite size, high porosity and relatively high surface area. The mixed oxide sorbents contain one Group IB, IIB or VIIB metal oxide such as copper, zinc or manganese and one or more oxides of Groups IIIA, VIB or VII such as aluminum, iron or molybdenum. The presence of iron or aluminum maintains the Group IB, IIB or VIIB metal in its oxidized state. Presence of molybdenum results in eutectic formation at sulfidation temperature and improves the efficiency of the sorbent.

Flytani-Stephanopoulos, Maria↗
NASA NTRS1993

Hot gas, regenerative, supported H.sub.2 S sorbents

Efficient, regenerable sorbents for removal of H.sub.2 S from moderately high temperature (usually 200.degree. C.-550.degree.C.) gas streams comprise a porous, high surface area aluminosilicate support, suitably a zeolite, and most preferably a sodium deficient zeolite containing 1 to 20 weight percent of binary metal oxides. The binary oxides are a mixture of a Group VB or VIB metal oxide with a Group IB, IIB or VIII metal oxide such as V-Zn-O, V-Cu-O, Cu-Mo-O, Zn-Mo-O or Fe-Mo-O contained in the support. The sorbent effectively removes H.sub.2 S from the host gas stream in high efficiency and can be repetitively regenerated at least 10 times without loss of activity.

Voecks, Gerald E.↗
DOE OSTI2024

Formation mechanism of two-dimensional hexagonal silica on SiO 2 /Si substrate

In this study, owing to their remarkable electronic properties, silica ultrathin films have been utilized as an insulating layer in nanoelectronics systems. Silica films have been epitaxially grown on different substrates using various synthesis methods. Among all fabrication approaches, chemical vapor deposition has long been an advanced method for synthesizing two-dimensional (2D) materials due to its ability to ensure precise stacking control and minimize contamination between layers. This study harnessed the potential of CVD to atomically fabricate thin layered 2D silica on a SiO 2 /Si substrate. Significantly, a unique combination of multiple transition metals and salt as the catalysts aided the formation of 2D silica for the first time. Salt is a crucial catalyst in promoting the evaporation of high-melting-point metal catalysts, resulting in hexagonal nucleation sites on the SiO 2 /Si wafer. By meticulously controlling growth parameters, a distinctive hexagonal structure was obtained. Correspondingly, this work delves into the growth mechanism of 2D silica, as evidenced by experiments involving salt alone and individual transition metals. Group VB transition metals played a prominent role in achieving the hexagonal structure compared to their group IVB counterparts. This research offers insight into the formation and growth mechanism of 2D silica, expanding the understanding of silica nanostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Revisiting point defect thermodynamics in group IVB and VB transition metal carbides

We present a comprehensive re-examination of point defect thermodynamics in group IVB and VB transition metal carbides (TMCs) with the rocksalt structure using a combination of density functional theory (DFT) calculations and a statistical mechanical Wagner-Schottky model within the canonical ensemble. The most stable configurations of point defects were discovered using basin-hopping global optimization, driven by either a machine learning interatomic potential (MLIP) or DFT. A key finding is the identification of previously unreported dicarbon antisites—a C–C dimer occupying a metal site—as the structural (constitutional) defects on the carbon-rich side of stoichiometry in all group IVB and VB TMCs except TaC. Furthermore, dicarbon antisite-containing thermal defect complexes, such as quadruple and interbranch defects, can dominate in TMCs under specific stoichiometric and temperature conditions. In conclusion, by incorporating dicarbon antisites into the defect landscape, this work provides a revised understanding of the thermodynamics of point defects in TMCs.

Carbides↗
DOE OSTI2022

A bi-layer barrier design for 122-type iron-based superconducting wires and tapes

Iron-based superconducting wires and tapes hold great promise for high-field magnet applications. A promising design for 122-type wires and tapes based on the powder-in-tube method is using silver and copper double-layer sheaths. For this design a heat treatment temperature below ~ 779 °C is required to prevent Ag-Cu liquid formation. However, this may be below the optimal heat treatment temperature for the critical current density, and still cannot prevent Ag-Cu interdiffusion occurring in the solid state. In this work we propose adding a niobium or tantalum or vanadium (or their alloys) barrier layer between the Ag and Cu to solve the Ag-Cu interdiffusion issue, given that the group-VB metals (vanadium, niobium, tantalum) are relatively inert to both Ag and Cu. To investigate the effectiveness of this design, BaFe 1.84 Co 0.16 As 2 wires and tapes with Ag/Cu and Ag/Ta/Cu sheaths, as well as Ba 0.6 K 0.4 Fe 2 As 2 wires and tapes with Ag/Cu and Ag/Nb/Cu sheaths, were fabricated. It was found that both the Ta and Nb layers kept integral after wire drawing, but after a large flat-rolling reduction the Ta layer broke while the Nb layer kept integral. In the tapes with Ag/Cu sheaths (without the Ta or Nb layer) Cu diffused through the Ag layer and into the powder cores during 740 °C heat treatment, while in the tapes with Ag/Nb/Cu sheaths the Nb layer effectively blocked Ag-Cu interdiffusion even at 900 °C. Further, this work demonstrates that Ta is a suitable barrier material for 122-type wires, while Nb is suitable for both wires and tapes. In this design using Ag/Nb (or Ta)/Cu sheaths, we can regard the outer Cu as the conductor matrix while the Ag and Nb (or Ta) serve as two layers of barriers that suppress reactions between the components. Thus, we call this design a “bi-layer barrier” design for 122-type wires and tapes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Anomalous elastic behavior of tantalum at high pressures: Experimental and theoretical studies

Elastic wave velocities of polycrystalline tantalum have been measured up to 13.6 GPa at room temperature using ultrasonic interferometry technique in a multi-anvil apparatus. The bulk and shear moduli, as well as their pressure derivatives are obtained using Eulerian 3rd order finite strain equations, yielding K S0 = 193.9(29) GPa, G 0 = 69.3(10) GPa, K S0' = 3.18(5) and G 0' = 0.88(1). Here the Steinberg-Guinan yield stress model is examined using the current shear modulus and its pressure derivative, where a slightly lower A 0 = 0.0127 GPa –1 compared to previous studies is obtained. First-principles calculations on the elasticity of tantalum have also been performed up to 200 GPa. Anomalous softening of the elastic shear constant C 44 and shear wave velocity V S is observed between 80– 180 GPa, whereas the anisotropy shows a softening-stiffening behavior with increasing pressure and reaches a minimum at around 150 GPa. Such anomalies might commonly originate from the nesting of Fermi surface for Group VB metals.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Effects of transition metal carbide dispersoids on helium bubble formation in dispersion-strengthened tungsten

Abstract The formation of helium bubbles and subsequent property degradation poses a significant challenge to tungsten as a plasma-facing material in future long-pulse plasma-burning fusion reactors. In this study, we investigated helium bubble formation in dispersion-strengthened tungsten doped with transition metal carbides, including TaC, ZrC, and TiC. Of the three dispersoids, TaC exhibited the highest resistance to helium bubble formation, possibly due to the low vacancy mobility in the Group VB metal carbide and oxide phases. Under identical irradiation conditions, large helium bubbles formed at grain boundaries in tungsten, while no bubbles were observed at the interfaces between the carbide dispersoid and tungsten matrix. Moreover, our results showed the interfaces could suppress helium bubble formation in the nearby tungsten matrix, suggesting that the interfaces are more effective in trapping helium as tiny clusters. Our research provided new insights into optimizing the microstructure of dispersion-strengthened tungsten alloys to enhance their performance.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Superconductivity Observed in Tantalum Polyhydride at High Pressure

We report experimental discovery of tantalum polyhydride superconductor. It was synthesized under high-pressure and high-temperature conditions using diamond anvil cell combined with in situ high-pressure laser heating techniques. The superconductivity was investigated via resistance measurements at pressures. The highest superconducting transition temperature T c was found to be ∼ 30 K at 197 GPa in the sample that was synthesized at the same pressure with ∼ 2000 K heating. The transitions are shifted to low temperature upon applying magnetic fields that support the superconductivity nature. The upper critical field at zero temperature μ 0 H c2 (0) of the superconducting phase is estimated to be ∼ 20 T that corresponds to Ginzburg–Landau coherent length ∼ 40 Å. Our results suggest that the superconductivity may arise from I 4 ¯ 3 d phase of TaH 3 . It is, for the first time to our best knowledge, experimental realization of superconducting hydrides for the VB group of transition metals.

Physics↗
NASA NTRS1989

Alkali metal-refractory metal biphase electrode for AMTEC

An electrode having increased output with slower degradation is formed of a film applied to a beta-alumina solid electrolyte (BASE). The film comprises a refractory first metal M.sup.1 such as a platinum group metal, suitably platinum or rhodium, capable of forming a liquid or a strong surface adsorption phase with sodium at the operating temperature of an alkali metal thermoelectric converter (AMTEC) and a second refractory metal insoluble in sodium or the NaM.sup.1 liquid phase such as a Group IVB, VB or VIB metal, suitably tungsten, molybdenum, tantalum or niobium. The liquid phase or surface film provides fast transport through the electrode while the insoluble refractory metal provides a structural matrix for the electrode during operation. A trilayer structure that is stable and not subject to deadhesion comprises a first, thin layer of tungsten, an intermediate co-deposited layer of tungsten-platinum and a thin surface layer of platinum.

Williams, Roger M.↗
NASA NTRS1988

Analysis of microalloy precipitate reversion in steels

The influence of the ferrite to austenite allotropic transformation on the stability of MXn precipitates in an iron matrix is studied. In the MX phase, M is a group IVb or Vb transition metal, such as niobium, titanium, or vanadium. X is carbon or nitrogen and n is in the range of 0.75-1.0. The application of the present model to the case of vanadium carbide reversion in a microalloyed steel is discussed.

Michal, G. M.↗
DOE OSTI2019

Comprehensive Lateral Neck Dissection in Papillary Thyroid Carcinoma may Reduce Lateral Neck Recurrence Rates

Objective: To Identify predictors of recurrent disease following lateral neck dissection (LND) for papillary thyroid carcinoma (PTC). Methods: A retrospective review of patients who underwent first-time LND for PTC at our institution (2000–2015) was performed. Medical records were examined for biopsy or pathologically proven lateral neck recurrence. Differences between the groups with and without recurrence were compared. All LNDs were then classified in to two groups: “comprehensive” (CND), involving levels IIa-Vb at minimum, or “selective”, labelling less extensive dissection (SND). Results: Four hundred nine patients underwent 467 LNDs. Surveillance data were available for 317 patients who underwent 362 LNDs (mean age 45 ± 16; range 18–88). The median follow-up was 64 ± 48 months (range 3–197). Recurrence was detected in 71 lateral necks (20%). The total number of lymph nodes was greater in the group without recurrence compared to those with recurrence (23 vs. 19, p = 0.02). Among patient demographics, radioactive iodine treatment, primary tumor characteristics and characteristics of nodal metastases, only an older patient age (mean 50 vs. 43 years) was associated with lateral neck recurrence (p < .01). CND was performed in 102 lateral necks and SND in 143 necks. There were 12 recurrences recorded in the CND group (12%) vs. 31 in the SND group (22%, p = .04). The majority of recurrences (70%) involved levels included in the original dissection. Conclusions: Younger patients, more extensive dissection and a higher total number of lymph nodes removed are associated with a lower incidence of lateral neck recurrence after LND for papillary thyroid carcinoma.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2020

Materials Data on VB by Materials Project

VB1 crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. V3+ is bonded in a 7-coordinate geometry to seven equivalent B3- atoms. There are a spread of V–B bond distances ranging from 2.24–2.33 Å. B3- is bonded in a 9-coordinate geometry to seven equivalent V3+ and two equivalent B3- atoms. Both B–B bond lengths are 1.79 Å.

36 MATERIALS SCIENCE↗
NASA NTRS1982

Radiation-induced loss of unsaturation in 1,2-polybutadiene

The radiation induced loss of unsaturation and methyl production in 1,2-polybutadiene (VB) was studied using IR spectroscopy. It was found that G(-1,2), which depends on the initial vinyl content, decreased from approximately 550 for VB with 98.5% 1,2 initially, to approximately 270 for VB with 85% 1,2 initially. G(-trans-1,4) ranged from approximately 21 for VB with 14% trans-1,4 to nearly zero for VB with less than 1% trans-1,4 initially. Methyl production was found to equal one methyl group formed for every 4-5 vinyl units consumed in the radiation-cyclized VB, in contrast to one methyl formed for every two vinyls reacted during cationic cyclization to give monocyclic structures. The IR spectra of gamma-irradiated VB were very similar to the spectra of UV-irradiated or thermally-treated VB at the same residual vinyl contents. It is suggested that the radiation-induced cyclization of VB occurs by a nonionic, nonradical 'energy chain' mechanism, which apparently holds for the cyclization of VB, whether induced by gamma-rays, UV radiation, or heat.

Golub, M. A.↗
NASA NTRS1981

Thermal Rearrangements of Polybutadienes with Different Vinyl Contents

A study was made of the loss of double bonds in equibinary (1,4-1,2) polybutadiene (EB) and in polybutadienes with 30% 1,2, 70% 1,4 (FI), and 10% 1,2, 90% 1,4 (DI) double-bond content, when heated in vacuum under nonpyrolitic conditions (temperature range 220-280 C). These polymers were found to undergo second-order loss of 1,2 unsaturation with similar activation energies (E(sub a) = 34.0 +/- 3 kcal/mole), by analogy to the previously reported thermally induced loss of double bonds in 1,2-polybutadiene (VB) (E(sub a) = 33.6 +/- 3 kcal/mole). Moreover, EB and Fl exhibited also second-order loss of 1,4 unsaturation, with E(sub a) ca. 36 and 40 kcal/mole, respectively, while DI showed negligible loss of 1,4 unsaturation below 260 C, in common with cis-1,4-polybutadiene (CB) (with 2% 1,2 double bonds) examined earlier. The loss of 1,2 double bonds in the various polybutadienes with different vinyl contents is accompanied by substantial methyl production, ranging from about one methyl group formed for every 4-5 vinyl units lost in VB, to one methyl for every two vinyls lost in EB,and to almost one methyl for each vinyl lost in DI or CB. Mechanisms are proposed for the thermally induced loss of 1,2 and 1,4 unsaturation in various polybutadienes and for the accompanying methyl production.

Golub, Morton A.↗