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

Abstract

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 (Jc-B) in MgB2 bulk superconductors. We investigated the relationship between sintering conditions (temperature and time) and Jc-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 Jc-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 Jc-B properties. In addition, the in situ method showed very minimal impurity with a corresponding improvement in density and Jc-B characteristics for the sample optimized at 750 °C. Microstructural characterization and flux pinning (fP) analysis revealed the possibility of refined MgO inclusions and MgB4 phase as new pinning centers, which greatly contributed to the Jc-B properties. The contributions of the sintering conditions on fP for both synthesis methods were analyzed.

Overview of spark plasma synthesis and sintering of MgB2 superconductor

“…To address these issues, various techniques have been developed to enhance the effective pinning sites in MgB 2 , such as irradiation, chemical doping or substitution, precursor size refinement, and other methods [2,3]. Additionally, highly dense bulk MgB 2 samples can be elaborated through infiltration and growth process [4,5], hot pressing or hot isostatic pressing sintering [6,7] and spark plasma sintering (SPS) [8][9][10]. SPS is an unconventional rapid consolidation method that allows for the preparation of dense samples with complex shapes and/or large sizes.…”

Review on high-performance bulk MgB2 superconductors

“…In the 'in-situ' method, precursor materials are mixed in a stoichiometric ratio and heat-treated in an inert environment to react into the MgB 2 bulk of a desired form. Conversely, the 'ex-situ' approach utilizes pre-produced MgB 2 powders to shape the desired forms [39]. 'Ex-situ' procedures offer the advantage of producing homogeneous materials with dense inner cores due to the use of already reacted MgB 2 powders.…”

“…Additionally, the sintering productivity is low, and doping during the procedure is ineffective compared to 'in-situ' methods. Previous studies have shown that critical current density (J c ) in MgB 2 can be enhanced by various means, such as reducing grain size, carbon doping, incorporating additives, and introducing MgO nanoparticles in moderate quantities [36,38,39]. Increasing the density of grain boundaries by refining the precursors through high-energy planetary ball milling serves as a source of pinning centres, resulting in an increased J c [40,41].…”

Very strong levitation force and stability achieved with a large MgB2 superconductor disc

“…These characteristics make MgB 2 highly advantageous for a wide range of applications including electric engines for planes, ships and spaceships as well as for the magnetic levitation of MAGLEV trains. The frequently used techniques to prepare MgB 2 bulks involve the spark plasma sintering (SPS) [14], hot pressing or isostatic pressing (HP/HIP) [15] and infiltration and growth process (IG) [16]. Perini et al [16][17][18] have successfully fabricated a large size MgB 2 square sample with an edge length of 117 mm and a thickness of 10 mm using reactive liquid Mg infiltration.…”