Microstructures and critical currents of single- and multi-filamentary MgB₂superconducting wires fabricated by an internal Mg diffusion process

Abstract

A single-filament wire and 7- and 19-filament wires of MgB2 superconductor were fabricated by an internal Mg diffusion (IMD) process. The wire is sheathed by a Cu–Ni alloy and each filament is composed of an outermost Ta, an intermediate B + SiC powder layer and an Mg core at the center. Despite the large total area reduction, the cross sections of all wires show uniform deformation of the composite. During the subsequent heat treatment, a reacted layer with a dense composite structure composed of a MgB2 matrix and fine particles is formed by Mg liquid infiltration and the reaction with the B + 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 of B + SiC remains outside the reacted layer, while the residual B + SiC is much reduced in the multi-filamentary wires, resulting in higher Ic, than that of the single-filament wire. However, the Jc, estimated for the reacted layer is not so different between the wires. When the heat treatment temperature exceeds 650 °C, the Ic value rapidly decreases, although the volume fraction of the MgB2 detected continues to increase. It is observed that the thickness of the reacted layer formed at higher temperatures becomes significantly inhomogeneous, which is thought to be responsible for the deterioration of transport Ic values. The highest Jc(layer) estimated for the reacted layer is as high as 9.9 × 104 A cm − 2 at 4.2 K and 10 T and 3.3 × 105 A cm − 2 at 20 K and 1 T achieved for the multi-filamentary wires. The Jc(core) estimated for the area including the hole and remnant B is about 1/3 of the Jc(layer). From good workability of the composite and excellent Jc values, it is expected that the IMD process can compete in terms of practical wire fabrication with the conventional powder-in-tube (PIT) process.