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Microfilamentary composites of continuous Nb fibers in Cu matrices were tested for superconducting transport properties after heat treatments at 200–900 °C. The critical current density at applied magnetic field was reduced dramatically as the heat treatment temperature was increased. The scaling relations describing the critical current as a function of applied field shifted from conventional (1−h)2 scaling to higher‐order relations as the heat treatment history was varied. Critical temperature transitions were abnormally broad in the fine‐filament composites examined. The upper critical fields and transition temperatures approached bulk values after severe heat treatment.

A systematic study of the effects of cold work and precipitation on the critical current density of an Nb-27at.%Zr alloy is reported. Heavy cold working, leading to a reduction of more than 99% in the area by swaging and wire drawing increases Jc in a transverse magnetic field of 4 T by a factor of about 13 relative to Jc for the recrystallized alloy. Precipitation of β-Zr, in combination with cold work, further enhances Jc. The highest level of Jc results when the precipitation heat treatment is given at an intermediate stage of working. Such a process in this study gives a value of Jc at H = 4 T, that is more than five times higher than Jc in the most heavily cold-worked specimen. Optical metallographic examination reveals that deformation twinning takes place in this alloy and that precipitation of β-Zr occurs preferentially at deformation twin boundaries.

Microfilamentary superconducting composites of Nb fibers in Cu matrices prepared by the stack and draw method were tested for tensile critical‐current performance at 4.2 K. The superconducting critical‐current densities increased exponentially under the influence of an applied mechanical strain until the onset of Nb fiber plastic deformation. In the elastic range, the critical‐current densities conformed to log10 Jc =m (strain)+b. In several tests the critical current was increased by more than an order of magnitude by the applied strain. This behavior is consistent with an increase in the upper critical field of the Nb fibers by the applied stress.

Niobium‐thorium eutectic alloys having fine acicular microstructures were produced by fast cooling from a vacuum melt. Although the solidified material was normal, continuity between the superconducting niobium‐rich phase, which was essentially pure niobium, was attained by plastic deformation at room temperature. The resulting wire was tested for critical current at 4.2°K, in transverse magnetic fields up to 82.5 kG; at the highest field, critical current densities of slightly more than 104 A per square centimeter were observed. The critical current density was independent of applied field from 20 kG to the highest field used; the level of critical current density depended on diameter in a manner which suggested dependence on cold work. It was concluded that the cold work had reduced the thickness of the needles of niobium below the superconducting penetration depth, and had brought them sufficiently close together to allow the superconducting correlation to interconnect the niobium, in the manner sugges...

Direct measurements of the magnetic field distribution in the gap between half‐ellipsoids of niobium in plastically deformed and annealed states were made using a bismuth thin‐film magnetoresistance probe. An empirical equation of the form r−r0=C0B+C1 (eC2B−1) for the field distribution in the deformed sample was found. For the annealed sample, a surface discontinuity of the field was observed, which is a boundary condition for the equilibrium magnetization. Flux jumping was observed when the field was not increased slowly. The distribution in the annealed specimen had reproducible irregularities.

Resistive measurements of the critical current density Jc in transverse magnetic fields were made on single crystals of niobium, which had been subjected to systematic thermal, mechanical, and chemical treatments in order to produce a wide range of substructural densities, and variations in the oxygen content. The introduction of plastic deformation into niobium crystals produced a peak in Jc at applied magnetic fields of strength Hc2, the upper critical field. The peaks in Jc were removed by annealing the crystals in the recovery range. The critical current density was increased both below and above Hc2 by producing a dislocation substructure in the crystals. Small additions of oxygen also caused Jc to increase above and below Hc2. The upper critical field, however, was relatively insensitive to structure, varying only through the structure sensitivity of the bulk GLAG parameters.

The magnetic behavior of composites of very fine (less than 1000 A) niobium wires in copper matrices has been measured at temperatures below 10°K. Three Meissner effects were observed, two of which were apparently due to strong proximity effects in the copper. The temperature dependence of the magnetization curve and measurements of the real and imaginary components of zero‐field ac susceptibility agree. The geometry and properties of the composite, which are known in some detail, are used together with current models for proximity effects to develop an adequate explanation of the data.

Nb‐oxide‐In tunneling junctions were constructed on the as‐grown surfaces of Nb single crystals at controlled orientations. The energy gap was observed to be anisotropic for crystals with resistivity ratios of 178 or greater. The lowest gap, 2.84 mV, was observed for the 〈100〉 direction. Local gap maxima were observed at the 〈311〉 and 〈111〉 directions, where the gap was about 3.19 mV. No anisotropy was observed in crystals having resistivity ratios of 65, presumably due to impurity scattering. Some coincidence of the energy gap anisotropy with main features of the proposed group V transition metal Fermi surface is noted.

Superconductivity of composite of fine niobium wire in copper, measuring critical temperature, transverse magnetic field, current density, etc

The effects of heat treatment and composition on the superconducting transition temperature of Nb3Al were studied in conjunction with an X-ray diffraction investigation of ordering and other structural effects. It was determined that an ordering anneal of 750°C for at least 48 hr achieved a maximum midtransitionT c of 18.5 K in arc-melted samples of Nb3Al. The changes in lattice parameter and order parameter during heat treatment indicate that theβ-tungsten crystal structure becomes more ordered as the maximumT c is approached. Further, theT c of 18.5 K could not be reached in Nb3Al material having nonstoichiometric composition. In such material, there was a direct correlation between increasing percentage of second-phase Nb2Al present and decreasingT c after the maximumT c had been obtained. The information obtained during this investigation led to the development of a simple empirical model to explain the change in theT c of Nb3Al. Also based on this work, a method was evolved for producing conductor material based on the highT c Nb3Al alloy (Appendix I).

The critical current density vs applied transverse magnetic field was measured, for a superconducting niobium single crystal of controlled oxygen content, at successively higher levels of concentration; the oxygen content was increased in increments ranging from 2 to 50 ppm, under conditions of constant substructure, from an initial level of approximately 5 ppm to a final level of 360 ppm. The over‐all critical current density increased considerably on the addition of oxygen, and assuming γ to be independent of oxygen concentration, the upper critical field increases in accordance with the predictions of the GLAG theory for type II superconductivity. The critical current density above the upper critical field, which has been ascribed to nucleation effects at defects or surfaces, was also very sensitive to the oxygen content.

Very dilute doping of granular Al films with Ag, Cu, or Au alters the superconducting tunneling behavior of such films in a manner consistent with the appearance of resonant phonon modes.

The transverse and longitudinal resistive critical fields of niobium (columbium) wires with drawing strains up to eleven were measured. The magnitude and anisotropy of the resistive critical field can both be much greater than the bulk Ginzburg-Landau parameters indicate, even if “internal surface” nucleation of superconductivity is assumed. It is suggested that these effects are due to superconductivity in the dislocation cell walls created by drawing or swaging. A simple model for this effect is offered, and various experiments which test this model are discussed.

To investigate the mechanism for the development of large anisotropic resistive critical fields in deformed niobium, the dependence of the parallel critical fieldH∥ on stress σ was measured in both compression and tension, for severely deformed niobium composites. the magnitude of ∂H∥/∂ σ increased with strain up to a value of 27 times that of annealed niobium. Also, both tension and compression increasedH∥. Both of these discrepancies from the thermodynamic predictions can be ascribed to the presence of double-ended dislocation pileups in the cell walls.

X‐ray pole figures were determined for the bcc and A15 layers in surface‐diffusion‐processed V3Ga superconducting tapes. Both layers were strongly textured, the A15 layer appearing similar (but not identical) to the bcc layer, which was (112)[110]+(100)[011]. An orientation relationship based on (001)bcc∥ (001)A15, [120]bcc∥[110]A15 is suggested.

A Mo0.75Re0.25 alloy endplate was tested on a Nb TE011 mode microwave cavity resonant at 11.2 GHz. An upper bound for the surface resistance of 2.8 μΩ and a lower bound of 102 G for the microwave magnetic breakdown field were established for the alloy plate. Comparison of these results to current requirements for a superconducting linear accelerator demonstrated the potential feasibility of this alloy as a practical alternative to Nb and Pb for accelerator applications. This is the first alloy shown to be a potential alternative to Nb and Pb and arguments are presented which indicate possible practical advantages of Mo0.75Re0.25 over Nb and Pb.

The resistive critical field of severely drawn Nb increases far beyond the predictions of the GLAG theory for a homogeneous type‐II superconductor and becomes anisotropic. Although no model fits the situation exactly, nucleation of superconductivity at dislocation cell walls is a possible explanation.

The dependence of the anisotropy of the resistive critical field of severely deformed Nb on plastic strain and temperature, the anomalously large stress dependence, and the existence of resistive transitions in fields up to 48 kG all confirm the role of the dislocation cell wall in giving rise to critical‐field enhancement in this material.

The critical currents and ductilities of alloys containing 25 at. % Zr; 2, 5, or 10 at.% Ta; and the balance of Nb are determined. The effects of processing variables (such as cold-swaging) on the superconducting properties are noted. Specimens containing 5 at.% Ta are found to have critical currents greater than 10 amp and critical current densities greater than 10/sup 4/ amp/cm/ sup 2/ for magnetic fields in excess of 90 kOe. (T.F.H.)