Reduction of Gas Bubbles and Improved Critical Current Density in Bi-2212 Round Wire by Swaging

Bi-2212 round wire is made by the powder-in-tube technique. An unavoidable property of powder-in-tube conductors is that there is about 30% void space in the as-drawn wire. We have recently shown that the gas present in the as-drawn Bi-2212 wire agglomerates into large bubbles and that they are presently the most deleterious current-limiting mechanism. By densifying short 2212 wires before reaction through cold isostatic pressing, the void space was almost removed and the gas bubble density was reduced significantly, resulting in a doubled engineering critical current density (J_E) of 810 A/mm² at 5 T, 4.2 K. Here we report on densifying Bi-2212 wire by swaging, which increased J_E (4.2 K, 5 T) from 486 A/mm² for as-drawn wire to 808 A/mm² for swaged wire. This result further confirms that enhancing the filament packing density is of great importance for making major J_E improvements in this round-wire magnet conductor.