Effect of high-energy ball milling time on superconducting properties of MgB₂with low purity boron powder

Abstract

MgB2 bulks and tapes were fabricated using ball-milled precursor powders. The selected B powders have the β-rhombohedral crystalline structure as a main phase, with a small amount of MgO impurity. Milling time was tested from 0.5 to 120 h to get an optimized value. It is found that ball milling can facilitate MgB2 phase formation, refine the MgB2 grains and increase the defect density. Moreover, the active area, Bc2 and flux pinning are improved, which is of benefit to the enhancement of Jc performance. The 80 h milled MgB2 bulk sample has a Jc value of 2.8 × 104 A cm−2 at 5 K and 6 T, 41 times larger than that of the 0.5 h sample. At the same time, the transport Jc of the 80 h MgB2 tape reaches 2.3 × 103 A cm−2 at 4.2 K and 9 T, 32 times larger than that of the 0.5 h one. Due to the increase of impurities and the release of lattice deformation and strain, the Jc was decreased for the sample milled up to 120 h.