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

Abstract

The time evolution of the phases present in the ceramic cores of silver-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox (Ag/Bi-2223) multifilament superconducting tapes heat-treated at selected oxygen partial pressure/temperature (pO2/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 pO2/T set points investigated (21.0% O2/835 °C, 7.5% O2/825 °C and 4.0% O2/815 °C) revealed characteristic patterns of recurring maxima and minima in the time evolution of the NSPs. (Ca, Sr)14Cu24O41 was found to be the stable phase at 21.0% O2/835 °C, possibly evolving as a co-product of the Bi-2223 formation reaction, while (Ca, Sr)2CuO3 and CuO were stable at 4.0% O2/815 °C, 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 21.0% O2/835 °C, while the best conversion to Bi-2223, together with the least amount of NSP particles >0.5 μm in major dimension, was produced by the 7.5% O2/825 °C 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 pO2/T set points.