Characterizations of A15 phase composition and T_c for Internal-Sn Nb₃Sn strands

Four sets of monoelementary (ME) and two kinds of multifilamentary (MF) internal-Sn Nb₃Sn superconducting strands were designed and fabricated, in which various component ratios, different composite configurations, and some third-element additions were arranged. All strands were submitted to a first heat treatment (HT) of 210°C/50 h + 340°C/25 h for CuSn mixing, followed by the A15 phase formation HT. The four ME strands were reacted at 675 °C, 700 °C, and 725 °C for 100 and 200 h, respectively, and the two MF strands at 650 °C, 675 °C, 700 °C, and 725 °C for 128 and 200 h, respectively. The analysis of the reacted strands comprised the A15 phase composition distribution by means of X-ray energy-dispersive spectroscopy and the critical temperature T_c by means of superconducting quantum interference device magnetization measurements. The obtained results indicate that, for sufficiently reacted internal-Sn Nb₃Sn strands, the final A15 phase composition and T_c are determined by the diffusion and solid reaction mechanism of the A15 phase formation. In particular, the onset T_c values and the average Sn content in a grain do not depend on the reaction temperature, the local compositions in the strand, the composite configuration arrangement, and the third-element addition.