2005 年 40 巻 11 号 p. 460-465
Dense MgB2 bulk superconductors were prepared by a diffusion method in sealed stainless tubes, into which magnesium and boron powders were separately packed. After diffusion reaction at a temperature above 780°C for a long time, dense MgB2 bulks with densities of 2.35˜2.60 g cm-3 formed at the initially boron packed part. The critical current properties of the dense bulks were high, and the highest Jc was more than 0.8 MA cm-2 at 20 K in low magnetic fields. This value is almost threefold higher than that of conventionally processed MgB2 bulks with low densities of ˜1.3 g cm-3. A largely increased effective current path is essential for enhanced Jc characteristics. Doping of boron, carbon and silicon-carbide further improved the critical current properties of the dense MgB2 bulks, particularly under high magnetic fields. This is mainly due to the substitution effect of carbon for the boron site. In addition, starting from premixed MgB2 and boron powders was found to be effective for suppressing the generation of cracks, thereby resulting in the successful fabrication of long and dense MgB2 plates. The newly developed diffusion method can provide homogeneous and dense MgB2 bulks that are suitable for basic research on flux pinning properties including doping effects, as well as for application to large current carrying devices such as current leads.