Phase formation of MgB2 superconducting materials fabricated by spark plasma sintering

Much research on MgB2 has been carried out because MgB2 has a higher transition temperature (Tc) of 39 K than that of other metallic superconductors and because the bulk form of MgB2 has exhibited high current density. In this study, Mg powder of less than 10 μm and B powder of less than 3 μm with equivalent MgB2 composition were mixed simply under argon atmosphere. In order to consider the effect of a pinning element on the superconducting properties, activated carbon of 5 at.% was added to mixed powders. The MgB2 bulk was fabricated with mixed powders in graphite molds at the various temperatures by spark plasma sintering. The formation of the MgB2 phase was confirmed with Differential Thermal Analysis (DTA) at 550 °C. The relative density of sintered MgB2 was 97 %, which increased as the sintering temperature increased. The sintering proceeded initially in the solid state and then by liquid phase sintering with increasing temperature without abnormal grain growth. In the Physical Property Measurement System (PPMS) result, the Tc was about 37 K in the carbon-added sintered sample. The 300 nm size MgB2 grains of hexagonal shape were formed after spark plasma sintering, but the MgB4 phase did not produce precise Tc.