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We have investigated Nb3Sn film growth via bronze route by magnetron sputtering of Nb films on to bronze substrates for potential application in SRF cavities. Two main routes were followed: 1) deposition of 500 nm equivalent Nb onto hot (650 °C to 775 °C) bronze substrates, where Nb3Sn formed during the Nb deposition; 2) deposition of Nb onto bronze substrates at lower temperature (200 °C) with a follow up reaction heat treatment (650 °C – 775 °C). Control of oxidation is paramount, since oxidation at the Nb-bronze interface blocks the Nb-Sn diffusion reaction, in addition to consumption of the intended coating by niobium oxides. Residual impurities P and S have deleterious effects similar to oxidation. Cu-15%wt Sn and Cu-15%wt Sn – 0.3%wt Ti bronzes with zero P were used as the substrates. Nb3Sn coatings obtained by both routes show surface roughness < 15 nm within regions defined by the bronze grain structure. Critical temperature of 14–16.5 K is obtained for films with 22 - 26% Sn, indicating a loss of ~2 K due to elastic compression of the Nb3Sn since thermal contraction is dominated by the bronze. Samples made via route 1 showed relatively large columnar grains with the length of the film thickness while samples made using the route 2 showed smaller grains. Both samples did not show any cracks in the Nb3Sn film even after a thermal cycle from room temperature to 4.2 K. Management of thermal expansion mismatches is a key challenge going forward, and adaptations using more complicated methods are ongoing.
This work is supported by the U.S. Department of Energy, Office of Science, Office of High Energy Physics under Award No. DE-SC 0018379. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida.
