Rapid Nb₃Sn film growth by sputtering Nb on hot bronze

Nb deposited by magnetron sputtering onto hot Cu-15 wt.%Sn bronze substrates at temperatures above 700 °C achieved Nb₃Sn film growth at a rate of 33 nm min⁻¹, which was an order of magnitude faster than that achieved for deposition of Nb on bronze at low temperature followed by in situ post reaction at the same high temperatures. Tin content in the Nb₃Sn films made on hot bronze was ∼26.3%, which is significantly higher than ∼24.5% obtained by post-reaction as well as for typical bulk reactions between Nb and α-bronze. The Nb₃Sn lattice parameter was consistent with measured tin content and predicted elastic strain for both routes. Critical temperatures of 14 K–16 K, instead of 18 K, were consistent with elastic strain due to coefficient of thermal expansion mismatch between the Nb₃Sn and bronze substrate and, for the hot-bronze samples, stress related to the growth mechanism. Films were fully coalesced and had surface roughness values <20 nm over a 100 μm² scan. Grain structure of the Nb₃Sn films produced by Nb sputtering on hot bronze resembles zone 2 in the Thornton structure-zone diagram, in contrast to the equiaxed grain structure reminiscent of microstructure observed in reacted Nb₃Sn wires exhibited by the post-reaction route. (letter)