Enhancement of critical current density by a “MgB₂-MgB₄” reversible reaction in self-sintered ex-situ MgB₂bulks

Self-sintered ex-situ MgB₂polycrystalline bulks have experienced a two-step sintering process, initially at 900 °C for 0–20 min and then at 650 °C for 1 h. MgB₂was decomposed to MgB₄and Mg at 900 °C and composed again at 650 °C from MgB₄and Mg. The reversible reaction promotes the material migration, and thus eliminates pores and enhances the connectivity between the grains. The critical current density (J_c) is significantly improved due to both improved grain connectivity and the additive pinning centers such as MgB₄and new-born boundaries. This two-step sintering process can be a promising method to fabricate high-performance ex-situ MgB₂bulks and wires.