The impact of Cu/Sn precursor tin content on the properties and growth mechanism of Nb3Sn thin films via bronze methods

This study systematically investigates the effect of Cu/Sn precursor tin content on the growth mechanism and superconducting properties of Nb3Sn coatings synthesized using the electrochemical and thermal synthesis (ETS) bronze methods. The ETS process entails annealing Cu/Sn coatings at 210 °C and 400 °C to form bronze alloy precursors, which are then annealed at 700 °C to form Nb3Sn coatings on Nb surfaces. To evaluate the effect of tin content, Cu/Sn coatings with varying tin concentrations were coated onto Nb substrates. The results highlight the critical role of tin content in determining the characteristics of Nb3Sn. Specifically, a high tin content (>20 at.%) leads to a decrease in critical temperature (Tc) and an increase in transition width (ΔTc), accompanied by the formation of insoluble niobium oxide impurities. Conversely, precursors with a moderate tin content (15-20 at.%) are found to be optimal for Nb3Sn coating applications in superconducting radio frequency (SRF) technology. Furthermore, this study delves into the one-dimensional diffusion process that governs the growth of Nb3Sn coatings during high-temperature heat treatment. By calculating the growth exponent and diffusion coefficient of the Nb3Sn coatings, we demonstrate that the growth mechanism of Nb3Sn encompasses both grain boundary diffusion and interface reaction. Overall, the experimental findings and growth kinetics presented in this study establish a foundation for future research and applications aimed at advancing Cu-based Nb3Sn thin film SRF (TFSRF) cavities.