MgB_2 superconductors with abnormally improved J_c sintered after autoxidation of milled original powders

摘要

An autoxidation treatment of short-time milled original powders was introduced for the synthesis of MgB_2 superconductors and the critical current density J_c of the sintered MgB_2 bulks was measured. It is unusually found that the undoped MgB_2 bulks sintered with those autoxided milled original powder exhibit abnormally excellent J_c (above 1 × 10~4 A cm~(-2) even at 3.5 T, 20 K). Combined with the investigation of sintering process, it was found that the autoxidation treatment of the milled powders affects the subsequent sintering process dramatically and finally leads to the formation of MgB_2 nanocrystalline with lots of dislocation and self-generated MgO nanoinclusions embedded in them. This unique microstructure brought up a significant improvement of J_c at high fields. Besides, the formation mechanism of this unique microstructure during the sintering process was also discussed in detail. It suggested that the MgO preformed by the reaction between Mg and B_2O_3 in the interface between Mg particles and B particles might serve as nuclei for the heterogeneous nucleation of the MgB_2 phase and finally be included in the MgB_2 grains as they grew up. The present method provides possible windows for the development of practical MgB_2 superconductors without adopting expensive nanodopants.