The processing and properties of Ag-sheathed Bi-based superconducting tapes prepared using the powder-in-tube technique have been reviewed. Powder processing and heat treatment, among various other factors, were found to be critical to the Jc of Ag/Bi-based superconducting wires. Reactive, homogeneous, carbonate-free and incompletely reacted powders, produced by the solution route are desirable. Liquid phase sintering, which is essential to achieve intimate grain connectivity and high grain alignment, can be achieved using a short period of melt or by annealing at temperatures near the melting point of the composites. An Ag sheath has been proved to be sufficient for achieving the flexibility and stability of these tapes. The interfacial layer between Ag and oxides shows an improved strength and flexibility. The thermally activated flux creep determines the Jc at high temperature. Weak links have been largely eliminated in high-quality tapes at high temperatures but the weak-link behaviour becomes pronounced at low temperatures. The dispersed 2212 phase and defects in the Ag/2223 tapes may act as potential pinning sites. Multiprobe measurements show no accumulative grain boundary resistance for long tapes. Fabrication of long tapes shows a better reproducibility than that for the short tapes.

https://iopscience.iop.org/article/10.1088/0953-2048/6/5/001/pdf

Ag-sheathed Bi(Pb)SrCaCuO superconducting tapes

We calculate the transport critical-current density in a granular superconductor in magnetic fields below about 5 x 10/sup -3/ T. The field dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions. Various model calculations are fitted to transport critical-current data on bulk Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub delta/ ceramic superconductors, whose average grain size somewhat exceeds 10 ..mu..m. The results yield an average junction cross-sectional area (thickness x length) of 4--6 ..mu..m/sup 2/. If the junctions are at the grain boundaries, a London penetration depth of about 150--300 nm is inferred, consistent with other estimates. We conclude that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries. The parameters of the fit are not consistent with Josephson junctions at twinning boundaries.

Josephson Junction Model of Critical Current in Granular Y1Ba2Cu3O7-δ

The rf residual surface resistance Rres is a figure of merit for superconducting microwave devices and for the homogeneity of superconducting surfaces. In granular superconductors not only high Rres values but also strong field dependencies Rres(T, ω, Hrf) at rather small fields Hrf ≤ 1 Oe are observed accompanied by penetration depth changes λres(T, ω, Hrf). These observations are explained by insulating and weakly conducting weak links causing additional degrees of freedom as strip line modes and fluxoids. This causes additional interaction mechanisms with rf fields:

rf Residual losses, surface impedance, and granularity in superconducting cuprates.

Effect of deoxygenation on the weak-link behavior of YBa2Cu3O7−δ superconductors

Large critical currents with a relatively weak magnetic field dependence are obtained in thick films of TlBa2Ca2Cu3Oz. Transport measurements indicate Jc ≳105 A/cm2 at 77 K, zero field, and Jc≳104 at 60 K in a 2 T field applied along the c‐axis. The observed behavior is attributed to a large degree of uniaxial alignment of platelike grains, and to superior intragranular flux pinning. These results are consistent with recent theories concerning the nature of vortices in highly anisotropic (layered) superconductors and ‘‘brick wall’’models of intergranular current transport.

Transport critical currents in spray pyrolyzed films of TlBa2Ca2Cu3Oz on polycrystalline zirconia substrates

A double step characteristic is observed at 76 K in the transport critical current as a function of magnetic field (10 -4 T to 10 T) in bulk sintered Y-, Bi- and Tl-based high- T c superconducting materials. The low-field, step-like drop in the critical current density J c commences at magnetic fields B between about 0.3 and 2 mT. This is followed by a plateau region of relatively constant critical current extending from about 30 to 300 mT, and then a second drop at fields between about 0.3 and 10 T. These features occur for all three superconductor materials and are interpreted respectively as a self-field/weak-link regime, a remnant percolation path regime and a flux-flow/upper-critical-field regime. The sharpness of the transition of the voltage-current ( V-I characteristic, represented by the transition parameter n (i.e., V ∝ I n ), has a similar double-step shape as a function of magnetic field directly corresponding to the features of the J c ( B ) characteristic.

Double-step behavior of critical current vs. magnetic field in Y-, Bi- and Tl-based bulk high-Tc superconductors

Abstract A double step characteristic is observed at 76 K in the transport critical current as a function of magnetic field (10 -4 T to 10 T) in bulk sintered Y-, Bi- and Tl-based high- T c superconducting materials. The low-field, step-like drop in the critical current density J c commences at magnetic fields B between about 0.3 and 2 mT. This is followed by a plateau region of relatively constant critical current extending from about 30 to 300 mT, and then a second drop at fields between about 0.3 and 10 T. These features occur for all three superconductor materials and are interpreted respectively as a self-field/weak-link regime, a remnant percolation path regime and a flux-flow/upper-critical-field regime. The sharpness of the transition of the voltage-current ( V-I characteristic, represented by the transition parameter n (i.e., V ∝ I n ), has a similar double-step shape as a function of magnetic field directly corresponding to the features of the J c ( B ) characteristic.

K. Togano

Papers

Double-step behavior of critical current vs. magnetic field in Y-, Bi- and Tl-based bulk high-Tc superconductors

Double-Step Behavior of Critical Current vs. Magnetic Field in Y-