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

Self-sintered ex-situ MgB2 polycrystalline bulks have experienced a two-step sintering process, initially at 900 °C for 0–20 min and then at 650 °C for 1 h. MgB2 was decomposed to MgB4 and Mg at 900 °C and composed again at 650 °C from MgB4 and Mg. The reversible reaction promotes the material migration, and thus eliminates pores and enhances the connectivity between the grains. The critical current density (Jc) is significantly improved due to both improved grain connectivity and the additive pinning centers such as MgB4 and new-born boundaries. This two-step sintering process can be a promising method to fabricate high-performance ex-situ MgB2 bulks and wires. •The Jc for ex-situ MgB2 bulks is improved by a “MgB2-MgB4” reversible reaction.•The reversible reaction eliminates pores and thus promotes grain connectivity.•New-born grain boundaries and MgB4 can serve as additive pinning centers.