Degradation of superconducting properties in MgB2 by formation of the MgB4 phase

Abstract

Superconducting MgB 2 polycrystalline samples have been fabricated under two different conditions in order to determine the effect of MgB 4 phase. A series of samples was placed in an α-alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an α-alumina boot without any lid and fired under similar conditions. For the first series of samples, we have found pure MgB 2 phase formation and a narrow transition width at 0.4 K. For the second series of samples, significant amount of MgB 4 phase were formed and the T zero was decreased to 27 K. For both the group of samples magnetization hysteresis loops obtained at various temperature range and applied field up to 2 T. The best J cmag for the first series of samples was 1.9 x 10 5 A/cm 2 at 10 K and 0 T, and for the second series of samples was 0.7 x 10 4 A/cm 2 at 10 K and 0 T.