Bubble formation within filaments of melt-processed Bi2212 wires and its strongly negative effect on the critical current density

Most studies of Bi2Sr2CaCu2Ox (Bi2212) show that the critical current density J(c) is limited by the connectivity of the filaments, but what determines the connectivity is still elusive. Here we report on the role played by filament porosity in limiting J(c). By a microstructural investigation of wires quenched from the melt state, we find that porosity in the unreacted wire agglomerates into bubbles that segment the Bi2212 melt within the filaments into discrete sections. These bubbles do not disappear during subsequent processing because they are only partially filled by Bi2212 grains as the Bi2212 forms on cooling. Correlating the microstructure of quenched wires to their final, fully processed J(c) values shows an inverse relation between J(c) and bubble density. Bubbles are variable between conductors and perhaps from sample to sample, but they occur frequently and almost completely fill the filament diameter, so they exert a strongly variable but always negative effect on J(c). Bubbles reduce the continuous Bi2212 path within each filament and force supercurrent to flow through Bi2212 grains that span the bubbles or through a thin Bi2212 layer at the interface between the bubble and the Ag matrix. Eliminating bubbles appears to be a promising new path to raise the J(c) of Bi2212 round wires.