Process to densify Bi2Sr2CaCu2Ox round wire with overpressure before coil winding and final overpressure heat treatment.

Abstract

Overpressure (OP) processing of wind-and-react Bi₂Sr₂CaCu₂O_x (2212) round wire compresses the wire to almost full density, decreasing its diameter by about 4 % without change in wire length and substantially raising its J _c . However, such shrinkage can degrade coil winding pack density and magnetic field homogeneity. To address this issue, we here present an overpressure predensification (OP-PD) heat treatment process performed before melting the 2212, which greatly reduces wire diameter shrinkage during the full OP heat treatment (OP-HT). We found that about 80 % of the total wire diameter shrinkage occurs during the 50 atm OP-PD before melting. We successfully wound such pre-densified 1.2 mm diameter wires onto coil mandrels as small as 10 mm diameter for Ag-Mg-sheathed wire and 5 mm for Ag-sheathed wire, even though such small diameters impose plastic strains up to 12% on the conductor. A further ~20% shrinkage occurred during a standard OP-HT. No 2212 leakage was observed for coil diameters as small as 20 mm for Ag-Mg-sheathed wire and 10 mm for Ag-sheathed wire, and no J _c degradation was observed on straight samples and 30 mm diameter coils.