Enhancement of synthesis efficiency and critical current density in glycine-doped MgB₂ bulks by two-step sintering

Glycine (Gly)-doped and Cu-and-Gly-co-doped MgB₂ samples were two-step synthesized by a short 800 °C period followed by sintering at 600 °C for 2 h. In view of the depression of oxidation and the significant reduction of the synthesis time, the sintering efficiency was improved due to the instantaneous high-temperature process, in contrast with the one-step sintered samples at 800 °C. The critical current density was examined to be 5.1 × 10³ and 8.4 × 10³ A cm^{− 2} at 20 K and 3 T for the Gly-doped and the co-doped samples, respectively. Such enhancement, with respect to the one-step sintered un-doped sample, is attributed to the fine MgO (~20 nm) and Mg₂Cu (~10 nm) pinning centers. The MgO and Mg₂Cu in the co-doped sample precipitated at the grain boundary and within the grains, respectively. Besides, the residual Mg phase in the Gly-doped sample remained on the surface of the MgB₂ grains, which proved the screw-dislocation growth of the MgB₂ grains, i.e. the grains extended from inner center to outer zone at low temperature.