Effects of Variable Tin Content on the Properties of A15 Superconducting Niobium-3-Tin

Although 2011 marks the 50th anniversary of Nb3Sn as the first high field superconductor and Nb3Sn is the presently most widely used high field superconductor, there is no detailed quantitative understanding of how its properties, particularly the upper critical field (Hc2), varies with composition. Since all practical wire forms of Nb3Sn contain the full range of A15 phase compositions, generally thought to range from ~18-25 at% Sn, this uncertainty of Hc2 compromises their understanding. To provide this understanding and to address some recent uncertainties about the binary phase diagram, we have carefully fabricated samples of various compositions, evaluating their properties by multiple techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), superconducting quantum interference device (SQUID), specific heat and Magneto-optical technique (MO) and reversible magnetization which are sensitive to average properties and compared them to the more often applied percolative measurements such as the transport measurements of Hc2. We found that it was much easier to make homogeneous samples from reaction sintering of high Sn mixtures of Nb and Sn powders such as the nominal 25 at% and 27 at% Sn samples. Lower Sn samples could not be made homogeneous, thus forcing us to arc melt such samples. We found that Tc and Hc2 of our homogenous Sn samples fell into 2 classes, Sn rich with 23.33~24.9 at% Sn (Tc~18K and Hc2 ~29T) and Sn-poor with 16.62~18.21 at% Sn (Tc ~6K and Hc2 ~8T). Multiple characterizations indicate that our samples, though not yet completely homogeneous, are almost certainly more homogeneous than samples in the literature. An unexpected and important result is that samples exhibited identical upper critical field Hc2(0.3 K) ~ 29± 0.2 T with or without undergoing the cubic-to-tetragonal transition, a result in strong contrast to widely used multiple-source data compilations that show a strong depression of Hc2(0K) from 29 T to 21.4 T in the tetragonal state. In agreement with the literature, we did find that the highest Hc2 occurs for slightly dirtier Nb3Sn. Our results are largely consistent with the phase diagram of Charlesworth et al. for A15 phase compositions but with the correction that the line compound at the right hand side of Nb3Sn region is NbSn2 not Nb6Sn5 is preferred and verified by our homogenous bulk binary Nb3Sn samples. We found that the lattice parameters of Sn-rich Nb3Sn are identical over the Sn range 23.33~24.9 at% Sn. We have established the route to fabricate samples of the highest homogeneity and the techniques to evaluate their structure, homogeneity, and superconducting properties. The fabrication route and characterization technique will provide a basis for future ternary and quaternary Nb3Sn investigation. Furthermore, our homogenous binary bulk Nb3Sn results offer new and reliable benchmarks for Nb3Sn strand comparisons.