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

Abstract

Most studies of Bi2Sr2CaCu2Ox (Bi2212) show that the critical current densityJc is limited by the connectivity of the filaments, but what determines the connectivity isstill elusive. Here we report on the role played by filament porosity in limitingJc. By a microstructural investigation of wires quenched from the melt state, we find thatporosity in the unreacted wire agglomerates into bubbles that segment the Bi2212 melt withinthe filaments into discrete sections. These bubbles do not disappear during subsequentprocessing because they are only partially filled by Bi2212 grains as the Bi2212 forms oncooling. Correlating the microstructure of quenched wires to their final, fully processedJc values shows an inverserelation between Jc and bubble density. Bubbles are variable between conductors and perhaps fromsample to sample, but they occur frequently and almost completely fill thefilament diameter, so they exert a strongly variable but always negative effect onJc. Bubbles reduce the continuous Bi2212 path within each filament andforce supercurrent to flow through Bi2212 grains that span the bubbles orthrough a thin Bi2212 layer at the interface between the bubble and the Agmatrix. Eliminating bubbles appears to be a promising new path to raise theJc of Bi2212 round wires.