Microstructure Observations of Ag and Ag-Alloy Sheathed Bi2223 Tapes

37 filament Bi2223 tapes with different configurations of Ag, AgAu7wt% AgSb0.6wt% and AgMg0.2wt% as the precursor and restack sheaths were fabricated using commercial Bi2223 precursor material and powder-in-tube techniques. Short length samples were heat treated at a temperature in the range of 832degC to 846degC for the first stage (HT 1), followed by a second stage (HT 2) at 825degC for 40 h and slow cooled to 785degC. An intermediate roll pass was performed between the heat treatment stages. The critical current (I_c) of the tapes was measured at 77 K in self-field and in fields up to 1 T. The microstructure of the alloy sheaths was examined using optical microscopy, and the Bi2223 filaments after HT 1 and HT 2 were examined using scanning electron microscopy (SEM). It was observed that the sequence of hardness, tensile strength, and critical bend strains from higher to lower levels is very much related to the grain sizes in the restack sheath. J_c of the tapes in zero field and in applied field was influenced by the phase composition, core density, grain connectivity, grain alignment and the interface between the Bi2223 filament and sheath.