Influence of Particle Size of Mg Powder on the Microstructure and Critical Currents of In Situ Powder-in-tube Processed MgB2 Wires

We fabricated in situ powder-in-tube(PIT) MgB2 wires using three kinds of Mg powders with particle size of similar to 45 mu m, similar to 150 mu m and 212 similar to 600 mu m. Mg particles were elongated to filamentary structure in the wires during cold drawing process. Especially, long Mg filamentary structure was obtained for large Mg particle size of 212 similar to 600 mm. Critical current density, J(c), increased with increasing Mg particle size for 1 mm diameter wires. This is due to the development of filamentary structure of high density MgB2 superconducting layer along the wires. This MgB2 structure is similar to that of the internal Mg diffusion (IMD) processed MgB2 wires. However, J(c) of the wires fabricated with 212 similar to 600 mu m Mg particle size decreased and the scattering of J(c) increased with decreasing wire diameter, while the J(c) of the wires with similar to 45 mu m Mg particle was almost independent of the wire diameter. The cross sectional area reduction of the Mg particles during the wire drawing is smaller than that of the wire. When using large size Mg particle, the number of Mg filaments in the wire cross section is small. These two facts statistically lead to the larger scattering of Mg areal fraction in the wire cross section with proceeding of wire drawing process, resulting in smaller volume fraction of MgB2 in the wire and lower J(c) with larger scattering along the wire. SiC nano powder addition is effective in increasing J(c) for all Mg particle sizes.