Synthesis of Dense MgB₂ Superconductor via In Situ and Ex Situ Spark Plasma Sintering Method

In this study, high-density magnesium diboride (MgB₂) bulk superconductors were synthesized by spark plasma sintering (SPS) under pressure to improve the field dependence of the critical current density (J_c-B) in MgB₂ bulk superconductors. We investigated the relationship between sintering conditions (temperature and time) and J_c-B using two methods, ex situ (sintering MgB₂ synthesized powder) and in situ (reaction sintering of Mg and B powder), respectively. As a result, we found that higher density with suppressed particle growth and suppression of the formation of coarse particles of MgB₄ and MgO were found to be effective in improving the J_c-B characteristics. In the ex situ method, the degradation of MgB₂ due to pyrolysis was more severe at temperatures higher than 850 °C. The sample that underwent SPS treatment for a short time at 850 °C showed higher density and less impurity phase in the bulk, which improved the J_c-B properties. In addition, the in situ method showed very minimal impurity with a corresponding improvement in density and J_c-B characteristics for the sample optimized at 750 °C. Microstructural characterization and flux pinning (f_P) analysis revealed the possibility of refined MgO inclusions and MgB₄ phase as new pinning centers, which greatly contributed to the J_c-B properties. The contributions of the sintering conditions on f_P for both synthesis methods were analyzed.