Evaluation of quenching-induced lattice strain and superconducting properties in un-doped and glycine-doped MgB₂ bulks

Bulk MgB₂ samples with or without glycine doping were sintered at 800 °C followed by furnace cooling and quenching, respectively. The strain analysis and the microstructure observation revealed that the un-doped and glycine-doped MgB₂ showed contrary response to the used quenching treatment, in terms of the crystallinity, the lattice parameters, and the superconducting properties. Accordingly, the critical current density of the quenched MgB₂ is enhanced with respect to the furnace-cooled one, due to the pinning dislocations and the well-connected MgB₂ net induced by the reserved strain. As for the glycine-doped samples, the impurity particles, which served as effective pinning centers in the furnace-cooled sample, segregated at the grain boundary under the driving force of the residual strain, and are destructive to the critical current density of the quenched one.