Time evolution of phase composition and microstructure in the Ag/Bi-2223 composite superconductor heat-treated at specific pO2/temperature set points

The time evolution of the phases present in the ceramic cores of silver-sheathed (Bi, Pb)(2)Sr2Ca2Cu3Ox (Ag/Bi-2223) multifilament superconducting tapes heat-treated at selected oxygen partial pressure/temperature (pO(2)/T) set points was investigated using scanning electron microscopy and energy dispersive spectroscopy coupled with computer-based image processing methods. The numerical values for individual phase contents and non-superconducting second phase (NSP) size distributions generated in this way were used to quantify the temporal evolution of composition in the variably treated Ag/Bi-2223 tapes. Results for the three pO(2)/T set points investigated (2 1.0% O-2/835 degreesC, 7.5 % O-2/825 degreesC and 4.0% O-2/815 degreesC) revealed characteristic patterns of recurring maxima and minima in the time evolution of the NSPs. (Ca, Sr)(14)Cu24O41 was found to be the stable phase at 2 1.0% O-2/835 degreesC, possibly evolving as a co-product of the Bi-2223 formation reaction, while (Ca, Sr)(2)CuO3 and CuO were stable at 4.0% O-2/815 degreesC, presumably as a consequence of competitive secondary reactions proceeding in parallel with Bi-2223 formation. The best-aligned grains were formed in Ag/Bi-2223 tapes treated at 2 1.0% O-2/835 degreesC, while the best conversion to Bi-2223, together with the least amount of NSP particles >0.5 mum in major dimension, was produced by the 7.5% O-2/825 degreesC treatment. A forward-looking conclusion of the study is that it appears possible to minimize the amount and size of NSP particles during Ag/Bi-2223 heat treatment by using a combinatorial sequence of appropriately timed pO(2)/T set points.