Fabrication and Superconducting Properties of Highly Dense ${\rm MgB}_{2}$ Bulk Using a Two-Step Sintering Method

We investigated on microstructure and superconducting properties of high-density MgB₂ superconductors synthesized by a heat-treatment method combining a short period of sintering at 1100degC with a following annealing at a low temperature. The two-step sintering method was found to achieve a coexistence of a high density and broadened XRD full width at half maximum (FWHM) of MgB₂ peak in the samples. The onset of diamagnetism at 5 T was approximately 2 K higher than those of the samples heat-treated by single-step sintering at high or low temperatures. Different second-step annealing temperatures were examined in this work. Microstructural analysis indicates that a lower second-step annealing temperature led to further suppression of the large grain growth in the MgB₂ samples, which is beneficial to the pinning property of the samples. The energy loss peaks of ac susceptibility measurement in fields confirm that by decreasing the second-step sintering temperature flux pinning of MgB₂ in high fields was enhanced.