J. E. Ostenson

The effect of composition and magnetic heat treatment on the magnetostriction of TbxDy1−xFey twinned single crystals

Twinned single crystals of the giant magnetostrictive intermetallic compound TbxDy1−xFey with a 〈112〉 axial alignment have been prepared by float‐zone solidification with Y varying from 1.90 to 1.98 and X being 0.32 or 0.28. The chemical compositions of the crystals were carefully analyzed. A very sharp jump in the magnetostrictive strain λ sometimes occurs in these alloys as the applied field H is increased to values of around 280 Oe under a compressive stress of 6.9 MPa. This large jump in λ is shown to depend strongly on the values of X and Y, and possible reasons for this result are discussed. A magnetic heat treatment has been developed which involves cooling the samples through the Curie temperature under a field applied transverse to the crystal axis. It is shown that this treatment is very effective at increasing the magnetostrictive properties at low applied compressive stresses for certain values of X and Y.

Superconducting properties of in situ prepared Nb‐Cu‐Sn alloys

Alloys of Cu–20 wt% Nb have been prepared by a chill casting technique. After drawing to fine wire, Nb3Sn was formed by plating and diffusion of Sn. The critical‐current properties and the resistance to degradation of Jc upon mechanical deformation both compare favorably to commercial Nb3Sn tape.

Directional solidification and heat treatment of terfenol-D magnetostrictive materials

Directional solidification techniques have been developed to produce grain-aligned rods of the highly magnetostrictive material Terfenol-D, TbxDy1-xFey The magnetostrictive properties have been compared to those of single-crystal, bicrystal, and polycrystal rod prepared by a float zone melting (ZM) technique. It was found that the properties of the grain-aligned material were comparable to three-and four-grain ZM material and the magnetostriction for both materials was less than the ZM single crystal and the better bicrystal rods by around 20 pct. It was demonstrated that a heat treatment consisting of a 1-hour hold at 950 °C was very effective at improving the magnetostrictive properties of all of the materials. Experiments have demonstrated that, to be effective, the heat-treatment temperature must be above somewhere between 850 °C and 900 °C. It is postulated that the mechanism for the improved properties involves a relief or redistribution of residual stresses produced by localized melting near second-phase particles at the predicted eutectic temperature of around 875 °C in the Terfenol-D material.

Preparation of superconducting Bi‐Sr‐Ca‐Cu‐O fibers

Procedures have been developed for the preparation of long slender fibers of Bi‐Sr‐Ca‐Cu‐O using a gas jet to shape and freeze the material. The fibers emerge in amorphous form and are converted to the Bi2Sr2Ca1Cu2O x phase by subsequent heat treatment. Magnetization and scanning electron microscopy studies show the material to be a very fine grained bulk superconductor with intragranular critical currents of 7×106 A/cm2 at 5 K and 1.8 T.

Synthesis, structures and properties of cubic R3In and R3InZ phases (R = Y, La; Z = B, C, N, O): The effect of interstitial Z on the superconductivity of La3In

Abstract Samples of La3In, Y3InCx and La3InZx for 0.3 ⩽ x ⩽ 1.5 , Z = C and O, La3InBx for 0.7 ⩽ x ⩽ 1.5, and La3InN were synthesized in welded Ta containers by powder sintering, fusion and annealing techniques. Y3In and La3Ga could not be obtained. All of the R3InZx examples were single phase between x ≈ 0.3–0.5 and x ≈ 1.0 and had the inverse perovskite structure (stuffed Cu3Au). Single crystal refinements for Y3InC and La3InN confirmed the type and stoichiometry (Pm¯3m, a = 4.9023(7)A˚, 5.102(1)A˚, and R(F)/Rw = 1.1/1.3%, 2.2/2.4% respectively). Magnetic susceptibilities measured for both normal and superconducting states showed Tc ≈ 10K and 104χRT ≈ 4 emu mol−1 were retained through the B and O series, with some enhancement of density and supercurrent shielding for B and a reduction of both for O. The absence of superconductivity (greater than 2 K) for Y3InC, La3InC and La3InN parallels their 104χRT values of less than or equal to 2 emu mol−1; the temperature dependencies of χ for these were also notably less than for the superconductors. These last phases are also more brittle. Extended Hu¨ckel band calculations indicated appreciable In and B (La In and La B) contributions at EF for La3InB. Thereafter, the La Z bonding states rapidly fall in energy as does the amount of La In bonding at EF, the last returning somewhat with the electron-richest oxygen interstitial. The decrease in Tc with increasing x in La3InCx parallels a decrease in densities-of-states at EF.

Casting of dendritic Cu‐Nb alloys for superconducting wire

Consumable electrode‐arc‐casting techniques have been developed for the preparation of large billets of dendritic Cu‐Nb alloys which are suitable for the fabrication of multifilamentary superconducting wire. The dendrite structure is somewhat more coarse than chill cast material but metallographic and chemical analyses show acceptably small radial and longitudinal segregation. The billets can be drawn to wire with no intermediate anneals. Both external diffusion after tin plating and internal diffusion of wire with a tin core can be used to transform the Nb filaments to Nb3Sn. The arc cast wire displays Jc values equivalent to previously reported values on in situ wire at high fields, but somewhat lower values at low fields.

The morphology and grain size of Nb3Sn filaments in in situ prepared multifilamentary Nb3Sn-Cu composite wire

Multifilamentary Nb3Sn-Cu wire was prepared from chill-cast Cu-20 wt% Nb ingots by drawing 12.5 mm ingots to 0.15 mm diameter wire, plating with 9 vol% Sn and reacting at temperatures ranging from 450 to 725° C. The morphology and grain size of the as-cast Nb filaments and the Nb3Sn filaments formed on heat treatment were characterized by TEM and SEM studies, and the Sn diffusion process was evaluated using electron microprobe analysis. The as-drawn Nb filaments had a cross-section typically 50 Å by 2500 Å and were generally convoluted in the wide direction. Tin diffusion produced Nb3Sn filaments with an essentially equi-axed cross-section having diameters of around 800 Å for the 550° reaction temperature and grains extending across the diameter. It was shown that the change in cross-sectional shape is produced largely by coarsening of the thin Nb filaments prior to diffusion of Sn to form the Nb3Sn. Optimum Jc values at both 1 and 8 T were found to occur for diffusion temperatures of 550° C and times of 3 to 6 days where the grain size was 750 to 800 Å.

Fabrication of superconducting Nb3Sn‐Cu composites

A new process has been developed for the fabrication of multifilamentary composites of Nb3Sn in a Cu matrix. Dendritic Cu‐Nb alloys are cast as 2.5‐kg billets and extruded to a tube shape. The core of the tube is filled with a Sn–5 wt% Cu alloy, and the assembly is drawn to 0.015‐cm‐diam wire with no intermediate anneals. Reaction of the wire at 550 °C for two days gives a product suitable for magnets in the 8–14‐T range.

Optimization of Nb3SnCu superconductor wire prepared by the in situ process

Abstract The in situ process for preparing Nb3SnCu superconductor wire produces the Nb3Sn filaments in a bronze matrix from as-cast NbCu ingots. The ingots are drawn to produce a composite of aligned Nb filaments in a Cu matrix and following Sn plating a diffusion anneal converts Nb filaments to Nb3Sn filaments. This work shows that an optimum size Nb filament is required to maximize critical current capacity. If the filaments are too big resistance of the Nb3SnCuNb3Sn junctions limit Jc. while if the filaments are too small coarsening occurs during the diffusion anneal and the junctions so formed limit Jc. It is shown that Jc values for wires of optimum filament size compare quite favorably to bronze processed Nb3SnCu wire at fields as high as 15T.

Strain tolerance of doctor‐blade‐processed Bi2Sr2Ca1Cu2O8−δ tapes

The strain tolerance of doctor blade tapes of the high‐temperature superconductor Bi2Sr2Ca1Cu2O8−δ have been studied to determine the suitability of these materials for magnet conductors. Critical currents that are on the order of 150 000 A/cm2 in zero magnetic field fall by about 30% with 0.1% bending strain and by a factor of 6 with 0.2% bending strain. At 20 T, critical currents fall with strain by approximately the same factor as at 4.2 K, but the critical currents fall with field somewhat faster. These bending strain results have a character quite different from Nb3Sn bronze process material in that there is essentially no prestress from the Cu jacket and there seems to be only a very small reversible range where the sample is within the elastic limit.