Superconducting Properties of Diffusion-Processed Multifilamentary ${\rm MgB}_{2}$  Wires
We fabricated mono-, seven-, and 19-filament MgB<sub>2</sub> composite 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 MgB<sub>2</sub> and some impurity phases such as MgB<sub>4</sub>. The thickness of the reacted MgB<sub>2</sub> layer increased rapidly with increasing heat-treatment temperature and/or heat treatment time. The 10 mol% SiC monofilamentary wires had a critical current density <i>Jc</i> (calculated for the reacted layer) of over 10<sup>5</sup>A/cm<sup>2</sup> at 4.2 K and 10 T in the early stage of heat treatment at 600-640 °C, despite the critical current <i>Ic</i> being as low as ~ 10 A. At this stage, only the B area near the Mg core reacted with Mg to form a thin MgB<sub>2</sub> layer; a large amount of B remained unreacted. The Vickers hardness of the reacted MgB<sub>2</sub> layer 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 MgB<sub>2</sub> layer has a much higher density than the PIT-processed wire. Excellent <i>Jc</i> values with high <i>Ic</i> values 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 MgB<sub>2</sub>. The seven- and 19-filament wires had <i>Jc</i> values of 0.7-1 × 10<sup>5</sup> A/cm<sup>2</sup> at 4.2 K and 10 T and 1.3 × 10<sup>5</sup> A/cm<sup>2</sup> at 20 K and 3 T. These high <i>Jc</i> values are attributable to the high-density MgB<sub>2</sub> layer produced by the diffusion method.