Microstructures and critical currents of single- and multi-filamentaryMgB2 superconducting wires fabricated by an internal Mg diffusion process

Abstract

A single-filament wire and 7- and 19-filament wires ofMgB2 superconductor were fabricated by an internal Mg diffusion (IMD) process. The wire issheathed by a Cu–Ni alloy and each filament is composed of an outermost Ta, an intermediateB + SiC powder layer and an Mg core at the center. Despite the large total area reduction, the crosssections of all wires show uniform deformation of the composite. During the subsequentheat treatment, a reacted layer with a dense composite structure composed of aMgB2 matrix and fine particles is formed by Mg liquid infiltration and the reaction with theB + SiC powder. For all wires,the highest transport Ic was obtained at furnace temperatures of 640–645 °C, which is just below the melting point of Mg. In the single-filament wire, a fairly large amount ofB + SiC remains outside the reacted layer, while the residualB + SiC is much reduced in the multi-filamentary wires, resulting in higherIc, than that of the single-filament wire. However, theJc, estimatedfor the reacted layer is not so different between the wires. When the heat treatment temperature exceeds650 °C, theIc value rapidly decreases, although the volume fraction of theMgB2 detected continues to increase. It is observed that the thickness of the reactedlayer formed at higher temperatures becomes significantly inhomogeneous,which is thought to be responsible for the deterioration of transportIc values. Thehighest Jc(layer) estimated for the reacted layer is as high as9.9 × 104 A cm − 2 at 4.2 K and10 T and 3.3 × 105 A cm − 2 at 20 K and 1 T achieved for the multi-filamentary wires. TheJc(core) estimated for the area including the hole and remnant B is about1/3 ofthe Jc(layer). From good workability of the composite and excellentJc values, it is expected that the IMD process can compete in terms of practical wirefabrication with the conventional powder-in-tube (PIT) process.