The effects of heating conditions on the critical current properties and microstructures of undoped bulk MgB2 was systematically studied. A strong correlation was observed between JC and the microstructure. The network structure, with an excellent inter-grain connectivity of MgB2 grains, contributed to high JC values under low magnetic fields, and a small grain size of MgB2 enhanced the grain-boundary flux-pinning. Long-term heating at low temperatures below the melting point of Mg was discovered to be most effective for synthesis of bulk MgB2 having a strongly connected MgB2 network structure with small grains. Samples heated at 550C for 1200h exhibited a JC of 4.02 x 105A/cm2 at 20K in self-field, while high-temperature and long time heating caused significant grain growth and resulted in a low JC.

Effects of Sintering Conditions on Critical Current Properties and Microstructures of MgB2 Bulks. A.Yamamoto, J.Shimoyama, S.Ueda, Y.Katsura, I.Iwayama, S.Horii, K.Kishio: Physica C, 2005, 426-431[2], 1220-4