Enhanced Critical Current Density in Bulk MgB₂

The object of this investigation is to improve the flux pinning and critical current density (Jc) of bulk MgB₂ materials for daily applications including superconducting permanent magnets. We prepared two sets of bulk MgB₂ materials employing commercial high-purity powders of Mg metal and carbon-coated amorphous B using a single-step solid-state reaction process. The first set of samples was produced by a single-step solid-state reaction at various temperatures between 794 °C and 806 °C in steps of 3 °C in pure argon atmosphere to optimize the sintering temperature. Further, the second set of samples was produced from Mg-rich MgB₂ material adding 1.5 wt% carbon-coated amorphous B powder to it, combined with 4 wt% of silver. X-ray diffraction analysis showed that samples were single-phase MgB₂ with minor trace of impurities. In all samples, sharp superconducting transition temperatures were observed around 38 K, which was decreased around 1 K in MgB₂ material produced with 1.5 wt% of carbon encapsulated boron. The critical current densities in Mg-rich MgB₂ material with 4 wt% of Ag were lower than in silver-free MgB ₂ bulks. The sample with 1.5 wt% of carbon-coated B exhibited the highest Jc of 460 kA/cm² at 20 K and self-field. Our results demonstrate a strong correlation between the microstructure achieved and the resulting pinning performance.