Effect of the premixing of MgB2 powder on microstructures and electromagnetic properties in PIT-processed MgB2 wires

Nowadays, in situ- and ex situ-processed MgB2 superconducting wires are commercially available. A premix process is a hybrid of the in situ and ex situ processes, in which a mixture of magnesium, boron, and MgB2 powders is used as a precursor. Hence, the premix process should cause less trouble in the manufacturing of practical wires. In this study, to clarify the potential regarding the critical current density of the premix process, the mono-filamentary MgB2 wires are prepared at premix ratios, xp, of 0.25, 0.50, and 0.75 by drawings with a large total area reduction ratio of 99.92%, and their electromagnetic properties and microstructures are investigated. Here, xp is defined as the mass ratio of MgB2 to the total mixture; that is, the composition of the mixture is Mg: B: MgB2 = 1 − xp: 2(1 − xp): xp on a molar basis. Although a high MgB2 packing factor is obtained by the premix process, it does not lead to an increase in electrical connectivity of MgB2 filaments because of the poor sinterability of the premixed MgB2 powder. On the other hand, strong electron scattering, which is introduced to the premixed MgB2 particles in the pulverisation process, leads to the enhancement of the upper critical field in the low temperature region. As a result, the critical current density in a low temperature and high magnetic field region is improved in wires prepared at xp = 0.50.