Correlation between the overall T-c distribution in Bronze Route Nb3Sn wires and the Sn gradients in the filaments

From specific heat measurements of Ta + Ti alloyed Bronze Route multifilamentary Nb3Sn wires, a wide, nearly symmetric overall distribution of T-c can be determined. This is apparently in contradiction with the almost linear decrease of the Sn content towards the centre of the A15 filaments, which suggests a rather sharp drop of the T-c distribution at the higher T-c side. In order to find an explanation to this problem, high resolution Auger spectroscopy measurements have been performed in filament bundles located at various positions of the wire cross section. It was found that the degree of reaction as well as the Sn gradient in each filament show substantial differences depending on its location, either in an innermost or outermost bundle. A model was established, assuming three different filament groups: 1) outer bundles: fully reacted filaments, almost constant Sn content, 2) intermediate bundles: partially reacted, shallow Sn gradient, and 3) inner bundles: partially reacted, steep Sn gradient. Summing together the contribution of the three groups, a total Sn distribution for the wire was obtained. Based on the relationship between T-c and the Sn content in alloyed Nb3Sn, a T-c distribution can be found which corresponds to that one measured by specific heat.