Phase Formation, Composition and rm T rm c Distribution of Binary and Ta-Alloyed rm Nb 3 rm Sn Wires Produced by Various Techniques

Low temperature calorimetry was used to determine the distribution of the superconducting transition temperature ( rm T rm c ) in binary and Ta-doped wires fabricated by the Bronze route and by the Powder-In-Tube (PIT) method. From this analysis we were able to discern the effects of Sn and Ta compositions on the distribution of the superconducting parameters rm T rm c and rm B rm c 2 in the different samples. The influence of the heat treatment conditions on the superconducting properties was investigated for the PIT wires. In particular we determined the field dependence of the rm T rm c distribution for a commonly used reaction schedule (675 [compfn] rm C /84 h) and for an optimized heat treatment (625 [compfn] rm C /320 h). For the first time, we show that the wire reacted at 625 [compfn] rm C /320 h exhibits two separated contributions in the rm T rm c distribution directly related to the grain morphology of the A15 layer: a narrow peak determined by the large grains, with a lower rm B rm c 2 , and a broad peak due to the fine grains, with a higher rm B rm c 2 .