Stoichiometry of Bulk NbSn Superconductors Synthesised by Arc Melting.

We present an alternative process for production of binary NbSn superconducting phases using pre- and post-treatment of arc-melted Nb + Sn ingots. This process combines sequential sintering, arc melting, and annealing procedures that provide dense, bulk samples of NbSn with varying stoichiometry between 0.18 < β < 0.25 depending on annealing time and temperature. We show, through magnetization measurements of these NbSn bulks, that annealing of arc-melted samples at 900 °C for 3 h significantly enhances values compared with arc-melted NbSn samples without annealing. Microstructural analyses show that optimum grain size and orientation are achieved by sintering and annealing at lower temperatures (i.e., 720 °C and 900 °C, respectively) with short annealing times (i.e., <10 h). Processing at higher temperatures and for longer times enhances grain growth and results in fewer pinning centres. The optimum process creates effective pinning centres that deliver a = 6.16 × 10 A/cm at 10 K (and ~0.2 T), compared with = 3.4 × 10 A/cm for NbSn subjected to a longer annealing time at a higher temperature and = 775 A/cm for an arc-melted sample without post-annealing. We suggest that further work addressing post-treatment annealing times between 3 h < t < 60 h at temperatures between 900 °C and 1000 °C will provide the opportunity to control stoichiometric and microstructural imperfections in bulk NbSn materials.