Rapid Nb₃Sn film growth by sputtering Nb on hot bronze

Abstract

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.

Development of a prototype superconducting radio-frequency cavity for conduction-cooled accelerators

“…This could be due to strain in the film resulting from the differential CTE between Cu and Nb. A better understanding of the effect of strain on the field-dependent surface resistance of Nb 3 Sn, along with the search for alternative materials with high thermal conductivity at 4 K and a CTE close to that of Nb, as well as further engineering design of the cavity outer layer would all be important paths for future R&D. Given the ongoing efforts towards forming high-performance Nb 3 Sn films directly onto a Cu substrate [80][81][82][83], the approach described in this article could be a proven solution to utilize Cu/Nb 3 Sn cavities in a conduction-cooled SRF cryomodule for future accelerators.…”

Method for growing Nb3Sn films at fast speed and low cost

“…Withanage et al [15] proposed a method for the rapid growth of Nb3Sn thin films on heated bronze substrates. Nb was grown via DC magnetron sputtering on bronze substrates (Cu-15 wt.% Sn) heated above 700 • C. Nb reacted in situ with Sn in the substrate, and the growth rate of Nb3Sn thin film was up to 33 nm min −1 .…”