Fabrication of seven-core multi-filamentary MgB2 wires with high critical current density by an internal Mg diffusion process

We found that the reaction between a Mg core and a B powder layer in an internal Mg diffusion (IMD)-processed multi-filamentary wire can proceed rapidly even at a furnace temperature lower than the melting point of Mg (650 degrees C), resulting in the formation of a reacted layer with a fine composite structure and, hence, excellent in-field critical current properties. The multi-filamentary wire is composed of an outermost Cu-Ni sheath and seven filaments with a Ta sheath, a Mg core, and B + SiC powder filled in the space between the Ta sheath and the Mg core. Heat treatment at 645 degrees C for 1 h produced a reacted layer with dense composite structure along the inner wall of the Ta sheath and a hole at the center of each core. This reaction probably initiated from the heat generation at the B/Mg interface, resulting in a temperature rise of the Mg core and the occurrence of liquid Mg infiltration. The J(c) value at 4.2 K for the reacted layer exceeds 10(5) cm(-2) at 9 T, which is the highest reported so far for MgB2 wire, including powder-in-tube (PIT)-processed wires. These results indicate that the IMD process can compete in terms of practical wire fabrication with the conventional PIT process.