Effects of sintering atmosphere on the superconductivity of MgB2

Pure MgB2 bulk samples were sintered in different atmospheres: high purity Ar, high purity Ar with 0.01% O2, high purity Ar with 0.03% O2 and high purity Ar with 5% H2. The superconductivity and microstructure of these samples were studied. It was found that in the range of this study the impurity gas in Ar had strong effects on aspects of the superconductivity such as critical current density, Jc, the irreversibility field, Hirr, and the upper critical field Hc2. When the samples were sintered in Ar with O2 impurity, their low field Jc decreased as the O2 content increased. The high field Jc, in contrast, was improved as the concentration of the O2 impurity increased. For the sample sintered in Ar with H2 impurity, its Jc is significantly higher than that of the sample sintered in high purity Ar. The smaller grain size and crystallinity degradation caused by the impure atmosphere is proposed to be the reason for the Jc improvement in both the O2 and the H2 added to the Ar gas sintering cases.