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

In long lengths of Bi-2212 tapes and wires, gases such as CO2 rind H2O 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 degrees C in 100% O-2 (VA+835), eliminates H2O and reduces CO2 that evolves during heating. To also counteract porosity, an overpressure heat treatment was utilized. Air was compressed to 5 atm total pressure (P-total) while keeping pO(2) fixed at I atm. Thickness variations were measured for a number of variables: with and without VA+835; P-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.