Fabrication of Double-Sheathed Bi₂Sr₂CaCu₂O_x Multifilamentary Wires for Solenoidal Coil

Abstract

Double-sheated Bi₂Sr₂CaCu₂O_x (Bi-2212) multifilamentary wires have been prepared in order to fabricate an oxide superconducting solenoidal coil for insertion in ultrahigh magnetic fields. The multifilamentary wires have double sheaths which are composed of the outer Ag-alloy layer and the inner Ag layer surrounding the Bi-2212 filament. The octagonal wire which was deformed by a groove roller shows a 0.2% proof strength over 200 MPa at 20°C, and the same critical current density as the Ag-sheathed Bi-2212 multifilamentary wire in which the Ag-alloy is replaced by Ag. By adjusting the total pressure and the oxygen partial pressure during the heat treatment in the mixture atmosphere of O₂ and N₂ gages to 0.6 and 0.24 MPa, respectively, a critical current density of 4.1×10⁴ A/cm² has been realized in the Bi-2212 filament at 4.2 K and 0 T. Meanwhile, the double-sheathed Bi-2212 multifilamentary round wire which was deformed by die-drawing shows a critical current density of 7.9×10⁴ A/cm² under the same condition. A small solenoidal coil fabricated employing the double-sheated octagonal wire has stably generated magnetic flux densities up to 0.18 T in a backup one of 21 T at 4.2 K.