Phase stability and grain growth in an Ag/Bi-2223 composite conductor prepared using fine-grained Bi-2223 as a precursor

We have investigated the stability and microstructural transformability of the Bi-2223 phase in a silver-sheathed monofilament composite tape fabricated using fine grained Bi/sub 1.7/Pb/sub 0.3/Sr/sub 1.9/Ca/sub 2.0/Cu/sub 3.0/O/sub y/ (Bi-2223) as the precursor powder. The fully formed Bi-2223 precursor was prepared using established procedures. The purpose of this study was to explore the prospects for growing textured, large-grain-size Bi-2223 from the fine-grained precursor by process parameter perturbations. These perturbations included thermal ramp up variations, programmed heat treatment temperature and oxygen pressure fluctuations, and parameter manipulations during cool-down. Our results show that the types of heat treatments used in conventional oxide-powder-in-tube (OPIT) processing do not facilitate Bi-2223 grain growth when the precursor powder is preconverted Bi-2223. We also observed that the Bi-2223 partially decomposed during conventional thermal ramp-up in 0.075 atm O/sub 2/, but that this decomposition can be inhibited by ramping up in a reduced oxygen pressure. A pathway was found for back-reacting the fine-grained Bi-2223 (to Bi-2212, Bi-2201 and nonsuperconducting secondary phases), then reforming large-grained Bi-2223 in a colony microstructure having some distinct differences from that produced during conventional OPIT processing.