Fibrous structure and critical current density of MgB2 superconducting wire

A novel MgB2 superconducting wire has been fabricated by the in situ powder-in-tube method with fine B powders (0.25 μm) and coarse Mg ones (150 μm). The core of this wire has a fibrous structure, which consists of a lot of MgB2 filaments. The critical current density Jc has been found to increase with decreasing a wire size, and Jc of the 1 mm wire is 1.4 × 104 A/cm2 at a temperature of 20 K in a field of 4 T. In addition, it has been revealed from an experiment with a MgB2 tape made by flat-rolling the wire that filament surfaces have a substantial contribution to flux pinning.