Effects of a compaction method for powder compacts on the critical current density of MgB2 bulk superconductors

In this study, the effects of the compaction method for (Mg+2B) powders on the apparent density and superconducting properties of MgB2 bulk superconductor were investigated. The raw powders used in this study were nano-sized boron (B) and spherical magnesium (Mg). A batch of a powder mixture of (Mg+2B) was put in a steel mold and uniaxially pressed at 1 ton or 3 tons into pellets. Another batch of the powder mixture was uniaxially pressed at 1 ton and then pressed isostatically at 1800 kg/cm2 in the water chamber. All pellets were heat-treated at 650℃ for 1 h in flowing argon gas for the formation of MgB2. The apparent density of powder compacts pressed at 3 ton was higher than that at 1 ton. The cold isostatic pressing (CIP) in a water chamber allowed further increase of the apparent density of powder compacts, which influenced the pellet density of the final products (MgB2). The compaction methods (uniaxial pressing and CIP) did not affect the formation of MgB2 and superconducting critical temperature (Tc) of MgB2, but affected the critical current density (Jc) of MgB2 significantly. The sample with the high apparent density showed high Jc at 5 K and 20 K at applied magnetic fields (0-5 T).