High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering

Abstract

In situ MgB2 superconducting samples were prepared by using the spark plasma sintering method. The density of the obtained bulks was up to 95% of the theoretical value predicted for the material. The structural and microstructural characterizations of the samples were investigated using X-ray diffraction and SEM and correlated to their superconducting properties, in particular their critical current densities, Jc, which was measured at 20 K. Extremely high critical current densities of up to 6.75 × 105 A/cm2 in the self-field and above 104 A/cm2 at 4 T were measured at 20 K, indicating that vortex pinning is very strong. This property is mainly attributed to the sample density and MgB2 nanograins in connection to the presence of MgO precipitates and areas rich in boron.

Overview of spark plasma synthesis and sintering of MgB2 superconductor

“…The distribution of the MgO phase in samples D2 and D4 is similar (figures 9(B) and (D)), which means that nano MgO particles aggregate as plates or strips form and are scattered in the MgB 2 matrix. As mentioned in previous work [8], boron-enriched areas are visible in dark gray zones of D4 (figure 9(D)), which are supposed to consist in amorphous boron unreacted at the low sintering temperature applied. Moreover, figure 9(H) displays an evident enhancement in grain connectivity and an abundance of grain boundaries in the D4 sample.…”

“…The specific tungsten carbide (WC) die used here allows to apply sintering pressures up to 1 GPa. A detailed description of experimental conditions was reported elsewhere [8]. The density of the samples was measured using the Archimedes method with ethanol.…”

“…The additional process step at 1000 • C was used to assess the densification mechanism. From another point of view, it is possible to enhance grain connectivity and control the growth of grains with the Mg + 2B mixture by increasing the pressure and decreasing the temperature applied during SPS [8]. Therefore, a sample (D4) was processed at 750 • C under 300 MPa using a specific tungsten carbide (WC) mold with this precursor.…”

“…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.…”

“…This method is beneficial as it saves processing time and controls grain growth. In previous work, we reported new SPS sintering conditions for MgB 2 bulk fabrication that resulted in exceptional properties [8]. In this study, our aim was to further understand the SPS sintering mechanism of MgB 2 materials using different sintering formulations to prepare high-density samples.…”