Microstructure and current-voltage characteristics of Nb3Sn-composites

Nb3Sn based superconductors are considered state of the art for high field applications. The present work focuses optimization of commercial Nb3Sn conductors with respect to their current carrying capabilities in high magnetic fields. For that purpose differently heat treated Nb3Sn conductors of binary, ternary and quaternary type were investigated. In that context it was essential to come to a more detailed understanding of how microstructural properties are connected to the shape of the current-voltage-curve. In extension of previous approaches, the microstructure was quantified by the statistical distribution of grain boundary density rather than average grain size only. For the current-voltage-curve the model of distribution of local critical currents ic was used. In order to overcome the drawbacks of that model in practical use a new numerical approach for parameter estimation was developed. This tool permitted reliable calculation of ic-distribution data out of current-voltage-curves. (orig.)