Improved critical current properties of MgB₂bulks by controlling microstructures

Abstract

Effects of controlled grain size on the critical current properties of in-situ fabricated polycrystalline MgB2 bulks were systematically investigated. Grain sizes of MgB2 bulks were controlled by three different methods: changing the heating conditions, such as temperature and holding time, changing the molar ratio between Mg and B, and starting from B powders with different grain sizes. MgB2 bulks with smaller grain sizes were obtained when heating at low temperatures or for short time at high temperatures starting from fine B powder. Small MgB2 grains were also found in the bulks starting from Mg-deficient composition using fine B powder. Through the careful analyses on the grain size of MgB2 and critical current properties, we found that MgB2 bulks with smaller grain sizes exhibit higher irreversibility fields and larger maxima of pinning force density. Our present results supported the scheme that grain boundaries are the predominant pinning centres in polycrystalline MgB2 bulks.