Influence of nanocrystalline boron precursor powder on superconductivity in MgB ₂ bulk

In this report, high-purity nanocrystalline boron powders processed by ball-milling were used as the precursor powders to fabricate MgB ₂ superconductor. The transport properties and the critical current density in the samples made from ball-milled boron powders and as-supplied boron powders were investigated. It was found that the ball-milled boron powders led to a significant enhancement of the critical current density in MgB ₂ sintered at 650 °C. The reason can be attributed to the small MgB ₂ grain size caused by the ball-milled boron precursor powders. The resistivity of the samples made from the ball-milled boron powder was lower than that of the sample from as-supplied boron powder. As the sintering temperature increased, both resistivity and upper critical field decreased in the samples using the ball-milled boron powders as a precursor. Poor connectivity and large strain are responsible for the high resistivity.