Characterizations of A15 Phase Composition and $T_{c}$ for Internal-Sn $\hbox{Nb}_{3}\hbox{Sn}$ Strands

Abstract

Four sets of monoelementary (ME) and two kinds of multifilamentary (MF) internal-Sn Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn 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 Cu-Sn 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> by means of superconducting quantum interference device magnetization measurements. The obtained results indicate that, for sufficiently reacted internal-Sn Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands, the final A15 phase composition and T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> are determined by the diffusion and solid reaction mechanism of the A15 phase formation. In particular, the onset T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> 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.