Effects of overpressure processing on porosity in Ag-sheathed Bi-2212 multifilamentary tapes with various geometries

Abstract

In long lengths of Bi-2212 tapes and wires, gases such as CO/sub 2/ and H/sub 2/O can cause bubbling of the Ag-sheath and porosity in the superconducting core. Applying a pre-processing step, which consists of vacuum annealing and annealing at 835/spl deg/C in 100% O/sub 2/ (VA+835), eliminates H/sub 2/O and reduces CO/sub 2/ that evolves during heating. To also counteract porosity, an overpressure heat treatment was utilized. Air was compressed to 5 atm total pressure (P/sub total/) while keeping pO/sub 2/ fixed at 1 atm. Thickness variations were measured for a number of variables: with and without VA+835; P/sub total/ 1 or 5 atm; and sample geometry such as round wires and rectangular tapes with different silver wall thickness. Light micrographs and SEM images show thin-walled monocore and multifilamentary tapes bubble unless they are pre-processed. The size and shape of pores in thick and thin multifilaments changes with overpressure processing. Microstructures with smaller and more uniformly distributed pores correlate with higher critical current values.