Microstructures and critical currents of single-and multi-filamentary MgB_2 superconducting wires fabricated by an internal Mg diffusion process

摘要

A single-filament wire and 7-and 19-filament wires of MgB_2 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 MgB_2 matrix and fine particles is formed by Mg liquid infiltration and the reaction with the B + SiC powder. For all wires, the highest transport I_c 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 I_c, than that of the single-filament wire. However, the J_c, estimated for the reacted layer is not so different between the wires. When the heat treatment temperature exceeds 650 °C, the I_c value rapidly decreases, although the volume fraction of the MgB_2 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 I_c values. The highest J _c(layer) estimated for the reacted layer is as high as 9.9 × 10~4 A cm~(-2) at 4.2K and 10T and 3.3 × 10~5 A cm~(-2) at 20K and 1T achieved for the multi-filamentary wires. The J_c(core) estimated for the area including the hole and remnant B is about 1/3 of the J_c(layer). From good workability of the composite and excellent J_c values, it is expected that the IMD process can compete in terms of practical wire fabrication with the conventional powder-in-tube (PIT) process.