Time evolution of phase composition and microstructure in the Ag/Bi-2223 composite superconductor heat-treated at specific pO_2/temperature set points

摘要

The time evolution of the phases present in the ceramic cores of silver-sheathed (Bi, Pb)_2Sr_2Ca_2Cu_3O_x (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 (21.0 percent O_2/835 deg C, 7.5 percent O_2/825 deg C and 4.0 percent O_2/815 deg C) revealed characteristic patterns of recurring maxima and minima in the time evolution of the NSPs. (Ca, Sr)_(14)Cu_(24)O_(41) was found to be the stable phase at 21.0 percent O_2/835 deg C, possibly evolving as a co-product of the Bi-2223 formation reaction, while (Ca, Sr)_2CuO_3 and CuO were stable at 4.0 percent O_2/815 deg 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 percent O_2/835 deg C, while the best conversion to Bi-2223, together with the least amount of NSP particles >0.5 mu m in major dimension, was produced by the 7.5 percent O_2/825 deg 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 pO_2/T set points.