Enhancement of Critical Current Properties of MgB$_{2}$ Bulks by Controlling Microstructure and Crystallinity

Critical current properties of MgB$_{2}$ bulks with various grain size, crystallinity and bulk density were systematically studied. Samples with low crystallinity, which was achieved by the low temperature reaction and/or carbon substitution for boron, exhibited high $J_{c}$ in fields and high irreversibility fields, reflecting enhanced $H_{c2}$. In addition, a strong relationship was found between pinning force density and grain size controlled by changing starting particle size of boron, heating conditions and molar ratio between magnesium and boron. The highly dense MgB$_{2 }$bulks prepared by a diffusion method showed excellent $J_{c}$ characteristics particularly in low fields. The carbon substitution for the dense bulk was effective for enhancement of $J_{c}$ under high fields. Essential pinning potential of the MgB$_{2 }$was also evaluated using the dense bulks. Based on these results, a guiding principle to improve critical current properties of practical MgB$_{2}$ conductors will be discussed from various viewpoints.