Stoichiometry of Bulk Nb1−βSnβ Superconductors Synthesised by Arc Melting

We present an alternative process for production of binary Nb1−βSnβ 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 Nb1−βSnβ with varying stoichiometry between 0.18 < β < 0.25 depending on annealing time and temperature. We show, through magnetization measurements of these Nb1−βSnβ bulks, that annealing of arc-melted samples at 900 °C for 3 hours significantly enhances Jc values compared with arc-melted Nb1−βSnβ samples without annealing. Microstructural analyses show that optimum grain size and orientation is achieved by sintering and annealing at lower temperatures (i.e. 720 oC and 900 oC, respectively) with short annealing times (i.e. < 10 hours). Processing at higher temperatures and for longer times enhances grain growth and fewer pinning centres. This process creates effective pinning centres that deliver a Jc = 0.61×105 A/cm2 at 10 K, compared to Jc = 0.34×105 A/cm2 for Nb1−βSnβ subjected to a longer annealing time at a higher temperature and Jc = 775 A/cm2 for an arc melted sample without post-annealing. We suggest that further work addressing post-treatment annealing times between 3 hr < tpost < 60 hr at temperatures between 900 oC and 1000 oC will offer opportunity to control stoichiometric and microstructural imperfections in bulk Nb1−βSnβ materials.