Effect of powder pre-annealing on microstructural development inside Bi2223/Ag-sheathed tapes

The fabrication of (Bi,Pb)2Sr2Ca2Cu3O10 (Bi2223)/Ag tapes with appropriately oriented microstructure and capable of carrying high currents at the boiling temperature of nitrogen is of great interest for the development of practical devices. Processing Bi2223/Ag superconducting composite tapes with cross sections of approximately 3 x 0.2 mm2 requires several steps including two or three heat treatments at 830-840 DGC with intermediate mechanical densification. Of these steps, the first heat treatment plays a key role. The powder, which is initially packed into Ag-tubes of a few mm diameter, does not consist of the Bi2223 phase yet. Instead, a structurally related compound (Bi,Pb)2Sr2Ca1Cu2Os (Bi2212) is the predominant phase. The Pb included in the nominal composition can be present in various forms depending on the details of the powder preparation process: (a) in the Ca2PbO4 phase, (b) in the Pb3Sr2.5Bi0.5Ca2CuOy ("3321") phase, (c) incorporated in Bi2212. The powder preparation process determines the phase assembly, which in turn has an influence on the phase and microstucture development inside the tapes during further processing.