Microstructure in High-Density ${\rm MgB}_{2}$ Wires Prepared by an Internal Mg Diffusion Method

Several reaction-induced diffusion processes to fabricate high-density MgB₂ materials are developed, and the critical current density (J_c) has been notably enhanced. In this study, microstructure in high-density MgB₂ wires fabricated by an internal Mg diffusion (IMD) process has been investigated. The inner reacted region of the wire heat-treated at 640°C for 1 h shows dense polycrystalline MgB₂ of 20-200 nm in grain sizes. Fine MgO and Mg₂Si particles of 10-30 nm in sizes are dispersed in this region. On the other hand, the outer region near the Ta sheath is composed of unreacted B and SiC powders, fine MgO particles and small voids. Sizes of voids in the IMD MgB₂ wire are small compared with the PIT MgB₂ wire. Oxidation of Mg in the IMD process forms fine dispersion of MgO which may be effective for flux pinning.