Nb-1 Wt Research Articles

Very high upper critical fields and enhanced critical current densities in Nb3Sn superconductors based on Nb–Ta–Zr alloys and internal oxidation

The inhibition of Nb3Sn grain growth in the presence of ZrO2 nanoparticles appears to be one of the most promising method for pushing the critical current densities of Nb3Sn superconducting wires to levels that meet the requirements set for the Future Circular Collider. We have investigated the effect of ZrO2 nanoparticles formed by the internal oxidation of Zr on the superconducting properties and microstructure of Nb3Sn formed from Nb-1 wt%Zr, Nb-7.5 wt%Ta, Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr alloys. A monofilamentary wire configuration was used, with a 0.22 mm outer diameter Nb-alloy tube containing a core of powdered metal oxide (SnO2, CuO or MoO3) as oxygen source and successive deposits of Cu, Sn and Cu on the outer surface. As determined from inductive measurements, the layer critical current densities of the samples based on Nb alloys with internally oxidized Zr were superior to those based on Nb-7.5 wt%Ta. The samples based on Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr showed higher critical current densities at high magnetic fields (above 10–15 T), and upper critical fields exceeding 28.5 T at 4.2 K (99% normal state resistivity criterion). A record value of 29.2 T of the upper critical field at 4.2 K was obtained on samples based on Nb-7.5 wt%Ta-2 wt%Zr. Hypotheses are proposed and discussed for explaining this unexpected increase of the upper critical field, by considering the possible effects of non-oxidized Zr on the superconducting properties of Nb3Sn and of the oxidized Zr on the formation and microchemistry of Nb3Sn. Regardless of sample type the Nb3Sn grains observed in our samples have an aspect ratio of 1.5–1.7. When compared in the short axis direction, the mean distance between grain boundary intercepts (lineal intercept method) is ∼40% smaller in the samples with internally oxidized Zr than in the reference samples based on Nb-7.5 wt%Ta. In the long axis direction the reduction is of 20%–30%.

Effect of strain on evolution of dynamic recrystallization in Nb-1 wt%Zr-0.1 wt%C alloy at 1500 and 1600 °C

Uniaxial compression tests were carried out on Nb-1wt%Zr-0.1wt%C alloy at temperature of 1500 and 1600°C and strain rate of 0.1s−1 to study the evolution of dynamic recrystallization with strain. Electron back scatter diffraction was used to quantify the microstructural evolution. Nb-1Zr-0.1C alloy showed a necklace structure at a strain of 0.9 when deformed at 1500°C and at strain of 0.6 when deformed at 1600°C, both at strain rate of 0.1s−1. This suggested the occurrence of dynamic recrystallization. At 1500°C and strain of 0.9 the local average misorientation and the grain orientation spread was low confirming the presence of dynamic recrystallization at this deformation condition. At both 1500 and 1600°C and all measured strains the recrystallized grains had a strong fiber component of 〈001〉.

High temperature deformation behavior of Nb–1 wt.%Zr alloy

The hot deformation behavior of Nb–1wt.%Zr alloy was studied using uniaxial compression tests carried out in vacuum to a true strain of 0.6 in the temperature range of 900 to 1700°C and the strain rate range of 3×10−3 to 10s−1. The optimum regime of hot workability of Nb–1Zr alloy was determined from the strain rate sensitivity (m) contour plots. A high m of about 0.2 was obtained in the temperature and strain rate range of 1200–1500°C and 10−3 to 10−1s−1 and 1600–1700°C and 10−1 to 1s−1. Microstructure of the deformed samples showed features of dynamic recrystallization within the high strain rate sensitivity domain. Compared to the study on Nb–1Zr–0.1C alloy, Nb–1Zr showed a lower flow stress and an optimum hot working domain at lower temperatures. In the 1500 to 1700°C range the apparent activation energy of deformation for Nb–1Zr was 259kJmol−1, the stress exponent 5, and the activation volume about 200 to 700 b3.

Fabrication and characterization of a-oriented TbFeO3/Nb-doped SrTiO3 heterostructure

The a-axis oriented TbFeO 3, (TFO) thin film was epitaxially grown on (001) Nb-1 wt.%-doped SrTiO 3 (Nb–STO) single crystal substrate to construct a heterostructure. The heterostructure exhibited a good rectifying behavior over the temperature range of 25–300 K, and the rectification ratio was continuously enhanced with increasing temperature. In positive bias direction, transport mechanism of the heterostructure showed a crossover from the Schottky-emission-like to a space-charge-limited type at 120 K, the temperature at which the TFO shows a magnetic ordering, while in the negative bias direction it was solely dominated by the space-charge-limited mechanism. The result indicates that coupling between electrical and magnetic orders can influence the rectifying behavior of the heterostructure. The observation hints the possibility to tune rectifying properties of a heterostructure via influencing magnetic properties of the magnetic layer.

Abnormal rectifying behavior of In/SrTiO 3/SrTiO 3 :Nb capacitor

In/SrTiO 3/SrTiO 3:Nb capacitors have been fabricated by depositing SrTiO 3 thin film on Nb-1 wt%-doped SrTiO 3 substrate and using In as the top electrode. The current–voltage characteristics were measured and backward rectifying characteristics are found below room temperature. The interesting observation is that at constant negative bias the leakage current increases with decreasing temperature below 150 K. Analysis shows that the abnormal increasing current was determined by the tunneling process through the SrTiO 3/SrTiO 3:Nb junction. This work demonstrates that tunneling diode may be realized in homologous perovskite oxides and engineered by distortions and defects.

Anelastic Relaxation due to Interstitial Solutes Diffusion in Nb and Nb-1 wt% Zr

Metals and alloys containing solute atoms dissolved interstitially often show anelastic behavior due to a process know as stress-induced ordering. The application of mechanical spectroscopy measurements to diffusion studies in body-centered cubic metals has been extensively used in the last decades. However the kind of preferential occupation of interstitial solutes in bodycentered cubic metals is still controversial. The anelastic properties of the Nb and Nb-1 wt% Zr polycrystalline alloys were determined by internal friction and oscillation frequency measurements using a torsion pendulum inverted performed between 300K and 650K, operating in a frequency oscillation in the hertz bandwidth. The interstitial diffusion coefficients of oxygen and nitrogen in Nb and Nb-1 wt% Zr samples were determined at two distinct conditions: (a) for low concentration of oxygen and (b) for high concentration of oxygen.

Influence of Oxygen on the Damping Properties of Nb-Zr Alloys

Nb-1 wt% Zr polycrystalline and single crystalline samples were submitted to annealing, at pressure of 2.5 x 10-8 Torr and subsequently annealed in an oxygen atmosphere for three hours, in a temperature of 1170 K in oxygen at partial pressure of 5 x 10-5 Torr. Internal friction measurements as a function of temperature were performed between 300 K and 650 K, using a torsion pendulum inverted Kê-type operating in a frequency oscillation in the hertz bandwidth. Three internal friction peaks, which were assigned as Nb-O, Zr-O and Nb-N peaks were observed in oxidized single crystal sample and the relaxation parameters of the three peaks were obtained.

Strain-assisted tunneling current through TbMnO3∕Nb-1 wt %-doped SrTiO3 p–n junctions

Simple oxide heterostructures have been fabricated by growing multiferroic TbMnO3 thin film on Nb-1 wt %-doped SrTiO3 substrate. In addition to the beneficial rectifying characteristics in a temperature range from 350 to 30 K, the intriguing observation is that at constant reverse bias the current increases with decreasing temperature below 275 K. By analyzing the band diagram, the anomalous increasing current with decreasing temperature was ascribed to the strain-assisted tunneling current through TbMnO3∕Nb-doped p–n junctions.

Fabrication and characterization of all-perovskite oxide p–n junctions based on La 1− xSr xMnO 3 and Nb-1wt% doped SrTiO 3

All-perovskite oxide p–n junctions have been fabricated by pulsed laser deposition. Semiconducting p-type La 1− x Sr x MnO 3 (LSMO) and n-type Nb-1 wt% doped SrTiO 3 (NSTO) were used. Thin films of LSMO were epitaxially grown on (1 0 0) NSTO single crystal substrate at 650 °C and under an ambient oxygen pressure of 100 mTorr. Heteroepitaxial relationship of (1 0 0) LSMO||(1 0 0) NSTO has been obtained. Good electrical rectifying characteristics have been observed at room temperature. LSMO is a well known colossal magnetoresistive material with a Curie temperature T c at around room temperature. The I– V characteristics of the p-LSMO/n-NSTO junction were studied under the temperature range of 77–700 K and an applied magnetic field of up to 1 T.

Effect of fiber volume fraction on the fracture behavior of Nb-1 wt pct Zr/218W composites at elevated temperatures

Nb-1 wt pct Zr/218W long-fiber composite monotapes, nominally containing 0 to 70 vol pct of 218 tungsten fibers, were fabricated by arc spraying the Nb-1 pct Zr matrix onto the tungsten fibers. The monotapes were consolidated by hot pressing and hot isostatic pressing techniques. Tensile tests conducted between 1400 and 1600 K, under engineering strain rates varying between 1.5×10−5 and 1.5×10−3 s−1, demonstrated that composites containing 70 vol pct of fibers had the highest strength-to-density ratio. Microstructural observations of specimens tested at 1400 K revealed that composites containing less than 50 vol pct of fibers showed extensive matrix cavitation, fiber-matrix debonding, and necking of the fibers. Above 50 vol pct, the composite matrix was less prone to cavitation, with an increasing tendency toward shear deformation of the fibers as the fiber volume fraction increased. No fiber damage was observed at 1400 K away from the fractured end, but significant fiber damage was observed at higher temperatures. A phenomenological model is presented to rationalize these observations.

Effect of fiber volume fraction on the fracture behavior of Nb-1 Wt pct Zr/218W composites at elevated temperatures

Effect of fiber volume fraction on the fracture behavior of Nb-1 Wt pct Zr/218W composites at elevated temperatures

Enhanced thermal emittance of space radiators by ion-discharge chamber texturing

A discharge chamber of a 30 cm argon ion source was successfully used to texture potential space radiator materials for the purpose of obtaining values of thermal emittance greater than 0.85 at 700 and 900 K. Some samples were also treated in acid before texturing. To evaluate the durability of the textured materials to atomic oxygen, samples were exposed to a plasma asher, a device that produced atomic oxygen. The spectral emittance between 2.0 and 15 μm was measured before and after the textured materials were exposed to the plasma asher. The results indicate that copper with extremely high values of emittance after texturing (0.978 and 0.983) at 700 K and 900 K respectively did not change its values of emittance after ashing, whereas the emittance of stainless steel fell below 0.85 after ashing. These data, scanning electron photomicrographs and the results of texturing and ashing titanium and Nb-1 wt.%Zr are presented in this paper.

The Oxidation Kinetics of Some Zirconium Alloys in Flowing Carbon Dioxide at High Temperatures

The oxidation kinetics of three zirconium alloys (Zr-2.2 wt% Hf, Zr-2.5 wt% Nb, and Zr-3 wt% Nb-1 wt% Sn) have been measured in flowing carbon dioxide in the temperature range from 873 to 1173 K to 120 ks (2000 min). At all oxidation temperatures, Zr-2.5 Nb and Zr-3 Nb-1 Sn showed a transition to rapid linear kinetics after initial parabolic oxidation. The Zr-2.2 Hf showed this transition at temperatures in the range from 973 to 1173 K; at 873 K, no transition was observed within the oxidation times reported. The Zr-2.2 Hf showed the smallest weight gains, followed in order by Zr-2.5 Nb and Zr-3 Nb-1 Sn. Increased oxidation rates and shorter times-to-rate-transition of Zr-2.2 Nb and Zr-1 Sn as compared with Zr-2.2 Hf can be attributed to the presence of niobium, tin, and hafnium in the alloys. This is considered in terms of the Nomura-Akutsu model, according to which hafnium should delay the rate transition, while niobium and tin lead to shorter times-to-rate-transition. The scale on Zr-2.2 Hf was identified as monoclinic zirconia, while the tetragonal phase, 6ZrO/sub 2/.Nb/sub 2/O/sub 5/, was contained in the monoclinic zirconia scales on both other alloys.

AC loss and DC critical current densities of Nb<inf>3</inf>Sn tapes by the solid state diffusion process

The effects of metallurgical processing on 60 Hz ac losses and dc critical currents in Nb 3 Sn tapes fabricated by the solid state diffusion technique were investigated. An addition of Al to the Cu-Sn alloy for the matrix resulted in large reduction in the ac losses of Nb 3 Sn tapes, but the highest linear critical current densities were observed in Nb 3 Sn tapes produced with a Nb-1 wt% Zr core in a Cu-13 wt% Sn matrix. Values of the losses and the critical currents in these tapes can meet the present requirements for the ac superconducting power cables.

Alternating field losses in Nb<inf>3</inf>Sn multifilamentary superconductor

Transverse alternating field losses at 4.2K have been measured from 0.5 Hz to 10 kHz in a Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn multifilamentary superconductor in bias fields to 5 Tesla. The 0.020 inch diameter sample was prepared by heat treating a Cu, Nb-1 wt % Zr, CuSn composite at 700°C for 20 hours to form Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn on the inside surface of the annular filaments. Metallurgical studies have been made to determine the Sn distribution and to estimate the thickness of the Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn layer. The I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> -H curve and resistive and inductive transition curves are presented. The losses are analyzed with respect to the present loss theories using the conductor characteristics measured and excellent agreement between experiment and theory is achieved.

The effect of heat treatment on the tensile strength and hardness of a quenched ternary zirconium-base alloy (Zr-0.5 wt.% Nb-1 wt.% Cr)

The response of a ternary zirconium-base alloy, Zr-0.5 wt.% Nb-1 wt.% Cr, to hardening by thermal treatment has been investigated. A marked increase in the strength of the alloy was observed when it transformed to a martensitic structure after rapid cooling from the high temperature phase field, α + β or β. The structure of the as-quenched martensite was found to be hexagonal close-packed and it was supersaturated with respect to the solute elements (Cr and Nb). Tempering of this structure in the temperature range 400°–700°C lowered the hardness, an appreciable decrease being observed on tempering above 500°C. Factors responsible for the high strength of the as-quenched martensite are suggested.