An explanation of how split melt processing can enhance the critical current density of Bi2212 round wires based on examination of bubble size and density formed in the melt

The recent discovery that gas bubbles formed in the melt state are a major current-limiting mechanism in Bi₂Sr₂CaCu₂O_x (Bi2212) round wires has prompted explicit examination of the bubble density in split melt processed samples which, under optimized 1 bar processing conditions, can exhibit significant (30–50%) enhancement of critical current density, J_c. By examining quenched and furnace-cooled samples from different points in the split melt processing (SMP), we found that the bubble size correlates well to the J_c. Compared with standard processed samples, the bubble size is smaller in SMP samples which are cooled directly to room temperature by an intermediate cooling from the first melt before being reheated to the second melt. Bubble size and density observations suggest that J_c can only be increased when bubble growth in the second melt is prevented by very tight control of the reheat temperature. Smaller bubble size is favorable for J_c because filament connectivity is determined by the effectiveness of bubble bridging by Bi2212 grain growth on cooling from the second melt. Because SMP appears to allow higher J_c by shrinking bubble size rather than by diminishing the bubble volume fraction, we conclude that SMP is unlikely to offer benefits to newer processes like over-pressure processing which raise J_c much more significantly by full Bi2212 densification and bubble elimination. (paper)