Critical Current Densities in AgCu Alloy-Sheathed Bi-2212 Superconducting Wires in Round Shape

Abstract

The critical current densities of round AgCu alloy-sheathed Bi-2212 wires have been studied for superconducting magnet use. The wires have been fabricated by the powder-in-tube method, using co-precipitated powders with a nominal composition of 2 : 2 : 0.64 : 1.64 (Bi : Sr : Ca : Cu). In these wires each core has been designed to consist of a twofold tube of the oxide layer to increase the interfacial area of the Ag-sheath and the oxide core. The critical current densities per overall cross-sectional area of the wires (J_c) have been increased by increasing the number of cores to 37. Sheathing Ag/Bi-2212 wires with the AgCu-alloy tube as the outermost sheath has enhanced both the mechanical strength and J_c values compared with Ag/Ag/Bi-2212 wires where the sheath is made of pure Ag tubes only. J_c values exceeding 2.6×10⁸ A/m² at 4.2 K and 10 T have been obtained for AgCu/Ag/Bi-2212 wires heat-treated in air. Heat treatments under oxygen gas flow have caused deleterious effects on J_c values. The micro-structural analyses of the smoothness of the Ag-sheath/oxide-core interface and the packing density of the oxide core have indicated that there is much room for improving weak links within the wires.