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

摘要

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 $^{circ}{rm C}$ /84 h) and for an optimized heat treatment (625 $^{circ}{rm C}$ /320 h). For the first time, we show that the wire reacted at 625 $^{circ}{rm C}$ /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 ${rm B}_{{rm c}2}$ .