Considerations on critical currents and stability of MgB2 wires made by different preparation routes

MgB2 wires and tapes are attractive for the application in persistent mode coils, due to the absence of inter-granular weak links, sharp critical current transitions with high n-values and the chance to prepare superconducting contacts. High current densities are achieved for a row of different conductor concepts. A common bottleneck in most conductors is the still non-sufficient thermal stabilisation at high transport currents which leads to early quenches and limits the achievable transport currents. The evaluation of n-values from E(I) transitions can therefore be influenced by thermal effects from the dissipated energy, especially in the high current regime at lower fields or in self field. The microstructure of MgB2, i.e. the phase purity and grain size distribution, differs significantly for in situ and ex situ preparation routes. It depends on the precursor constitution, heat treatments and the conductor geometry and influences percolation path and flux pinning and the characteristics of the E(I) transitions. We present E(I) data, measured at different background fields for MgB2 wires from different preparation routes and extract information about the thermal instability of the conductors. The results are correlated and discussed with the specific features of the MgB2 microstructure in the different conductors. (C) 2003 Published by Elsevier B.V.