Fabrication and superconducting properties of internal Mg diffusion processed MgB 2 wires using MgB 4 precursors

Monofilament MgB 2 /Nb/Monel wires were fabricated using three different MgB 4 precursors by an internal Mg diffusion (IMD) process. The wire geometry and heat-treatment conditions were optimized in order to improve the critical current density ( J c ) of the MgB 2 wire. The influences of the quality of MgB 4 powders, such as the particle size and MgO impurity, on the microstructure and superconducting properties of the wires were discussed. Although there were small amounts of voids, unreacted MgB 4 particles and MgO impurity existed in the superconducting layers, and the transport layer J c of the wire with the MgB 4 precursor reached 3.0 × 10 4 A cm −2 at 4.2 K and 10 T, which was comparable to that of IMD-processed wires fabricated using boron precursors. Both the non-barrier J c and engineering J c of MgB 2 wire made using a MgB 4 precursor were enhanced due to the improved grain connectivity and the enlarged fill factor.