Stoichiometry of Bulk Nb1−βSnβ Superconductors Synthesised by Arc Melting

TL;DR

Researchers developed a new method to make superconducting materials with controlled composition and improved performance by adjusting the melting and heating steps.

Contribution

A novel process combining sintering, arc melting, and annealing to control the stoichiometry and microstructure of Nb1−βSnβ superconductors.

Findings

Annealing at 900 °C for 3 h significantly increases Jc values in Nb1−βSnβ superconductors.

Optimum grain size and orientation are achieved with sintering at 720 °C and annealing at 900 °C for short durations.

Longer and higher-temperature processing reduces pinning centers and lowers Jc values.

Abstract

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 h significantly enhances Jc values compared with arc-melted Nb1−βSnβ 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 Jc = 6.16 × 104 A/cm2 at 10 K (and ~0.2 T), compared with Jc = 3.4 × 104 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 h < tpost < 60 h at temperatures between 900 °C and 1000 °C will provide the opportunity to control stoichiometric and microstructural imperfections in bulk Nb1−βSnβ materials.

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