Distribution of Tc from calorimetry and the determination of Sn gradients in bronze route Nb3Sn wires with an internal and external Ti source

We have investigated the influence of two different Ti doping methods on the superconducting properties of bronze route Nb3Sn multifilamentary wires. The first doping method introduces NbTi rods into NbTa filaments embedded in a Cu15.4Sn bronze matrix ('internal' Ti). The second method uses Cu15.5Sn0.25Ti bronze as an external source of Ti ('external' Ti). The wires prepared by 'external' Ti doping were found to exhibit the highest critical current values although the conductor configurations were identical. In order to understand the origin of the observed differences in transport properties, the grain morphology and the local composition profile of the A15 layer have been studied by means of systematic TEM/EDX measurements, which revealed a Sn gradient within the filaments. In addition, specific heat measurements have also been carried out for determining the T-c distribution in the whole wire volume, using a data deconvolution at the superconducting transition. A T-c distribution over a wide range of temperatures was found, corresponding to the Sn gradient revealed by TEM/EDX. Combining TEM/EDX analysis and calorimetry, it was possible to explain the differences between the two Ti doping methods studied in this work. As supplied from an internal source (cores), a higher overall Ti concentration was obtained, pushing the total Ta+Ti alloying content beyond the value of optimal doping corresponding to a maximum of Tc and B-c2.