It was recalled that, for the practical application of high-current MgB2 wires, a multifilamentary composite with small filaments and good thermal stabilization was required, in order to avoid damage from excessive heat generation in the resistive regime above JC. However, with reduced filament size, there was a degradation of JC due to an increasingly inhomogeneous microstructure, so-called sausage effects and a reaction layer that formed between the filament and sheath. The influence of filament size upon JC was investigated by using an in situ route which began with Mg+B powder mixtures. As compared to wires processed at temperatures above 900C, a phase formation heat treatment at 640C significantly reduced the reaction layer between filament and sheath, to some 1 to 2µm, and favoured the nucleation of small MgB2 grains at the nm-scale. This led to the best transport currents then achieved so far in undoped round wires made from commercial powders. Low n-values indicated an improved intrafilamentary current-sharing during the IC transition. However, at low fields and very high currents, the E(I) characteristics still indicated a thermally driven transition with excessive heat generation and irreversible damage to the wire.

In situ MgB2 Round Wires with Improved Properties. W.Goldacker, S.I.Schlachter, B.Obst, M.Eisterer: Superconductor Science and Technology, 2004, 17, S490-5