Characterizations of A15 Phase Composition and Tc for Internal-Sn Nb3Sn Strands
Résumé
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 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 Tc 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 Tc are determined by the diffusion and solid reaction mechanism of the A15 phase formation. In particular, the onset Tc 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.