Superconducting Properties of Diffusion-Processed Multifilamentary MgB₂ Wires

We fabricated mono-, seven-, and 19-filament MgB2composite wires by applying an internal Mg diffusion (IMD) process to Mg cores surrounded with B or B+SiC powder. A Ta or Nb tube and a Cu-Ni tube were used as the inner and outer sheaths, respectively. The wires were heated at 600-700 °C for 0.25-10 h. During the heat treatment, Mg diffused into the B layer and reacted with B to form MgB2and some impurity phases such as MgB4. The thickness of the reacted MgB2layer increased rapidly with increasing heat-treatment temperature and/or heat treatment time. The 10 mol% SiC monofilamentary wires had a critical current densityJc(calculated for the reacted layer) of over 105A/cm2at 4.2 K and 10 T in the early stage of heat treatment at 600-640 °C, despite the critical currentIcbeing as low as ~ 10 A. At this stage, only the B area near the Mg core reacted with Mg to form a thin MgB2layer; a large amount of B remained unreacted. The Vickers hardness of the reacted MgB2layer in the IMD-processed wires is about 1300, which is much higher than that in powder-in-tube (PIT) processed wires. This suggests that the MgB2layer has a much higher density than the PIT-processed wire. ExcellentJcvalues with highIcvalues can be obtained for multifilamentary wires when they were heated at ~ 640°C for 1 h. In this case, the B layer reacts almost completely with Mg to form MgB2. The seven- and 19-filament wires hadJcvalues of 0.7-1 × 105A/cm2at 4.2 K and 10 T and 1.3 × 105A/cm2at 20 K and 3 T. These highJcvalues are attributable to the high-density MgB2layer produced by the diffusion method.