Microstructural Characteristics of Ball-Milled Self-Sintered Ex Situ MgB2 Bulks

The ex situ method yields MgB
₂
samples with high packing factor. However, the critical current density of MgB
₂
bulks and wires fabricated using the ex situ method (ex situ MgB
₂
) requires improvement for practical use because of insufficient connectivity. In this study, we evaluated the microstructure of self-sintered ex situ MgB
₂
bulks fabricated from laboratory-made ball-milled powder. We observed that green compacts of the ball-milled bulks contain fine MgB
₂
grain aggregates and a decreased gap length between MgB
₂
grain aggregates compared with those prepared without ball-milling. Therefore, the number of connections between the MgB
₂
grain aggregates and current path increased. However, the width of the connections between MgB
₂
grain aggregates remained narrow, suggesting the importance of controlling the initial microstructure of green compacts, such as the size of MgB
₂
grain aggregates and packing factor, by optimizing milling and pressing conditions.