Effects of sintering conditions on critical current properties and microstructures of MgB2 bulks

The effects of heating conditions on critical current properties and microstructures of undoped MgB2 bulks were systematically studied. Strong correlation was observed between Jc and microstructures. The network structure with an excellent inter-grain connectivity of MgB2 grains contributed to high-Jc under low magnetic fields, and small grain size of MgB2 enhanced the grain boundary flux pinning. Long time heating at low temperatures below the melting point of magnesium was discovered to be most effective for synthesis of MgB2 bulks having strongly connected MgB2 network structure with small grains. The sample heated at 550 °C for 1200 h recorded a high-Jc of 4.02 × 105 A cm−2 at 20 K in self-field, while high-temperature and long time heating brought a significant grain growth which resulted in low Jc.