The more bubbles there are then the lower the critical temperature of the superconducting material
Most studies of Bi2Sr2CaCu2Ox (Bi2212) show that the critical current density Jc 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 Jc. 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 Jc values shows an inverse relation between Jc 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 Jc. 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 Jc of Bi2212 round wires.
In summary, we identified agglomerated porosity (bubbles) as a major current-blocker in Ag-sheathed, PIT multifilamentary Bi2212 round wire. By studying how microstructure develops through processing, we revealed that bubbles of agglomerated porosity appear in the wire only upon melting of the Bi2212 powder. The consequences are major: the Bi2212 filaments become subdivided into discrete segments, greatly reducing their long-range connectivity, though Bi2212 grains can partially fill the bubbles on cooling back into the solid state. We conclude that densifying Bi2212 wires is a key approach to improving Bi2212 Jc, where in fact the critical current density is controlled more by poor connectivity than by flux pinning.