Electromagnetic properties and microstructures of in situ MgB2 wires made from three types of boron powders

Abstract

In powder-in-tube processed MgB2 wires, the choice of boron powder as a starting material crucially affects their performance. In this paper, we prepared in situ MgB2 wires from three types of boron powders in various heat-treatment conditions and investigated their electromagnetic properties and microstructures. Their critical current density, Jc, varied over a wide range from sample to sample. The difference in Jc is understood to be caused by the effect of changes in the electrical connectivity, K, and intrinsic residual resistivity, ρ0. Here, K represents the effective cross-sectional area for current, and ρ0 reflects the degree of the charge carrier scattering caused by lattice defects. It was found that the use of boron powder with a large specific surface area leads to a large degree of lattice defects in MgB2 grains and enhances ρ0, resulting in improving Jc. The boron powder produced by thermal decomposition of B2H6 has a large specific surface area. Hence, this boron powder is the most suitable as a starting material for MgB2. Meanwhile, dry pulverization of low-cost boron powder, which is largely produced by active-metal reduction of B2O3, is also effective to increase its specific surface area without introducing impurities, resulting in the enhancement of Jc in the entire magnetic field region. This finding broadens the choice of boron powder and contributes to realizing superconducting applications with excellent balance between performance and cost.