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 Bi-2223 precu rsor was prepared using established procedures. The purpose of this study w as to explore the prospects for growing textured, large-grain-size Bi-2223 from the fine-grained precursor by process parameter perturbations. These p erturbations 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 i n conventional oxide-powder-in-tube (OPIT) processing do not facilitate Bi- 2223 grain growth when the precursor powder is preconverted Bi-2223. We als o observed that the Bi-2223 partially decomposed during conventional therma l ramp-up in 0.075 atm O-2, but that this decomposition can be inhibited by ramping up In a reduced oxygen pressure. A pathway was found for back reac ting the fine-grained Bi-2223 (to Bi-2212, Bi-2201 and nonsuperconducting s econdary phases), then reforming large-grained Bi-2223 in a colony microstr ucture having some distinct differences from that produced during conventio nal OPIT OPIT processing.