Significant improvement in the critical current density of MgB₂ bulks in situ sintered at low temperature by excess Mg addition

MgB₂ bulks with excess Mg addition were rapidly synthesized by sintering at low temperature in present work. It is found that even after ball milling treatment of original powders, the reaction between Mg and B during subsequent low temperature sintering process was uncompleted within 5 h and there is still some residual Mg. On the other hand, the presence of residual Mg can make the sintering microstructure more homogeneous and dense, and also reduce lattice defects and mechanical strains. All the factors are contributed to the improvement of the grain connectivity in the samples with excess Mg addition sintered at low temperature compared to the reference MgB₂ sample sintered at high temperature. Hence, J_c of these prepared samples is enhanced significantly across the whole measured fields. Especially, at 20 K, 2 T, the value of J_c in the 5 h-sintered MgB₂ bulk with Mg addition is above 1 × 10⁵ A cm^-2. The technique developed in present work is an effective and low-cost way to further enhance J_c in the MgB₂ superconductors without using expensive nanometer-size dopants.