Near-net-shape fabrication of continuous Ag-clad Bi-based superconductors
We have developed a near-net-shape process for Ag-clad Bi-2212 superconductors. This alternative to the powder-in-tube process offers the advantages of nearly continuous processing, minimization of processing steps, reasonable ability to control the Bi-2212/Ag ratio, and early development of favorable texture of the Bi-2212 grains. The powder is rolled into an Ag channel, a cap is applied, and the assembly is lightly rolled. To minimize distortion during heat treatment, it is especially important that the Bi-2212 powder be nearly free of volatile species such as carbon. A vacuum-calcination process was developed to control CO/sub 2/ evolution during heating. Superconducting properties are discussed.
Transport critical currents of Bi(2223) tapes at 77 K under mechanical stress
The transport critical current, j/sub c/, depends on the mechanical stress applied during or before the j/sub c/-determination. Two experiments with different aims were performed : a) bending of the tape tests the resistance against winding (coils, power cables); b) axial strain tests give information on the effect of forces acting on the conductor during operation. Bending of Bi(2223)/Ag tapes from a radius r=/spl infin/ to r=1 cm as well as an axial strain of about 0.2% decreases j/sub c/ by some 80%. For bent as well as for elongated tapes, we find an almost unaffected field dependence of j/sub c/, which is only on a lower j/sub c/ level due to crack formation, i.e. a reduction of the percolation path. The strong j/sub c/ reduction can be avoided by reinforcement of the sheath material by dispersion hardening with 2% Mg. The j/sub c/ decrease after bending in these improved tapes is only some 20%, the maximum axial strain tolerance about 0.8%. The yield of the reinforced tapes is 300-350 MPa, which is more than 5 times the value for Ag sheathed tapes.
Structures and Superconducting Properties of Tl-Base High-Tc Oxides Synthesized by a Diffusion Process
The diffusion reaction between a high melting point substrate and a low melting point coating layer may produce denser and more uniform high-Tc oxide phases than those prepared by a conventional powder sintering process. Meanwhile, the T1-Ba-CaCu-0 (TBCC0) system is an attractive one due to its highest transition temperature Tc among superconducting oxides.1 In this paper, superconducting properties and structures of the TBCCO high-Tc phase formed by the diffusion reaction will be reported.