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An autoxidation treatment of short-time milled original powders was introduced for the synthesis of MgB2 superconductors and the critical current density Jc of the sintered MgB2 bulks was measured. It is unusually found that the undoped MgB2 bulks sintered with those autoxided milled original powder exhibit abnormally excellent Jc (above 1×104 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 MgB2 nanocrystalline with lots of dislocation and self-generated MgO nanoinclusions embedded in them. This unique microstructure brought up a significant improvement of Jc 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 B2O3 in the interface between Mg particles and B particles might serve as nuclei for the heterogeneous nucleation of the MgB2 phase and finally be included in the MgB2 grains as they grew up. The present method provides possible windows for the development of practical MgB2 superconductors without adopting expensive nanodopants.

Combined with the thermal analysis and phase identification, it is found that during the low-temperature sintering process of the SiC-doped MgB2 samples, the Cu addition can improve both the reaction between Mg and B and the reaction between Mg and SiC and thus accelerate the formation of the MgB2 phase with effective C substitution for B. Accordingly, we successfully synthesize MgB2 bulks with excellent critical current density (Jc) by proper amount of Cu and SiC multidoping sintered at 575 °C for only 5 h.

MgB2 samples with 3 at. % Cu addition were synthesized by means of in situ sintering at different temperatures (525, 575, and 850 °C). Based on thermal analysis, phase identification, and microstructural observation, it is first found that the Cu addition mainly concentrates at the edge of the void in the form of Mg2Cu and thus does not degrade the grain coupling of MgB2 phase in the low-temperature sintered sample. As sintering temperature rises to 850 °C, the MgB2 grains grow larger and some Cu addition may form MgCu2 nanoinclusions distributing in the MgB2 grains through the reaction, 2Mg2Cu(L)+6B=3MgB2+MgCu2. As a result, the mechanism of enhancement in the Jc of the Cu-doped MgB2 samples varies distinctly from the grain-boundary pinning to the possible MgCu2 nanoinclusions pinning as the sintering temperature increases from low temperature to high-temperature.