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.
Highlights
Since 1953, intermetallic MgB2 with a hexagonal structure has been known [1]
The X-ray diffraction (XRD) show a small amount of MgO in the starting powder, which was included for comparison
The X-ray diffraction patterns of all the samples show that the highest intensity Bragg peaks are representing the MgB2 phase as major
Summary
Since 1953, intermetallic MgB2 with a hexagonal structure has been known [1]. Its superconducting critical temperature (Tc), up to 39 K, was observed in 2001 due to the presence of two distinct superconducting gaps [2]. The inexpensive cost, strong mechanical properties [3,4], and long coherence length of this material make it ideal for a variety of applications [5]. It has a greater upper critical field than standard NbTi and Nb3Sn superconductors. It has fewer anisotropic effects and no weak links as compared to superconducting cuprates. The grain boundaries do not act as weak links that inhibit superconducting current; this makes it possible to fabricate superconducting bulk and wires in crystalline forms [5,6]
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