In situ MgB2 round wires with improved properties

For technical application of high current MgB2 wires, a multifilamentary composite with small filaments and good thermal stabilization is required to avoid damage from excessive heat generation in the resistive regime above J(c). With reduced filament size, however, a degradation of J(c) due to a more and more inhomogeneous microstructure, sausaging effects and a reaction layer between filament and sheath, usually increases dramatically. The influence of filament size on Jc was investigated in the light of these aspects, developing very thin steel reinforced wires for a satellite application in the ASTRO-E2 XRS instrument (NASA), applying the 'in situ route', which starts with Mg + B powder mixtures. Compared to wires processed at temperatures above 900 degreesC, a phase formation heat treatment at 640 degreesC significantly reduces the reaction layer between filament and sheath to about 1-2 mum and favours the nucleation of small MgB2 grains in the nanometre scale. This leads to the best transport currents achieved so far in undoped round wires made from commercial powders. Low n-values indicate an improved intrafilamentary current sharing during the I-c transition. However, at low fields and very high currents the E (I) characteristics indicate a still thermally driven transition with excessive heat generation and irreversible damage of the wire.