Superconducting Properties of Diffusion-Processed Multifilamentary ${\rm MgB}_{2}$ Wires

We fabricated mono-, seven-, and 19-filament MgB₂ 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 MgB₂ and some impurity phases such as MgB₄. The thickness of the reacted MgB₂ layer increased rapidly with increasing heat-treatment temperature and/or heat treatment time. The 10 mol% SiC monofilamentary wires had a critical current density Jc (calculated for the reacted layer) of over 10⁵A/cm² at 4.2 K and 10 T in the early stage of heat treatment at 600-640 °C, despite the critical current Ic being as low as ~ 10 A. At this stage, only the B area near the Mg core reacted with Mg to form a thin MgB₂ layer; a large amount of B remained unreacted. The Vickers hardness of the reacted MgB₂ 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 MgB₂ layer has a much higher density than the PIT-processed wire. Excellent Jc values with high Ic 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 MgB₂. The seven- and 19-filament wires had Jc values of 0.7-1 × 10⁵ A/cm² at 4.2 K and 10 T and 1.3 × 10⁵ A/cm² at 20 K and 3 T. These high Jc values are attributable to the high-density MgB₂ layer produced by the diffusion method.