Doubled critical current density in Bi-2212 round wires by reduction of the residual bubble density

We have recently shown that the gas present in the only similar to 70% dense filaments of as-drawn Bi-2212 wire agglomerates into large bubbles that fill the entire filament diameter during the melt phase of the heat treatment. Once formed, these bubbles never disappear, although they can be bridged by 2212 grains formed on cooling. In order to test the effect of these bubbles on the critical current I-c, we increased the density of the filaments after drawing using 2 GPa of cold isostatic pressure, finding that the bubble density and size were greatly reduced and that I-c could be at least doubled. We conclude that enhancement of the filament packing density is of great importance for making major I-c improvements in this very useful, round superconducting wire.