Synthesis of superconducting Nb₃Sn coatings on Nb substrates

Abstract

In the present work the electrochemical and thermal syntheses of superconducting Nb 3 Sn films are investigated. The Nb 3 Sn phase is obtained by electrodeposition of Sn layers and Cu intermediate layers onto Nb substrates followed by high temperature diffusion in inert atmosphere. Electrodeposition was performed from aqueous solutions at current densities in the 20-50 mA cm −2 range and at temperatures between 40°C and 50°C. Subsequent thermal treatments were realized to obtain the Nb 3 Sn superconductive phase. Glow discharge optical emission spectrometry demonstrated that after thermal treatment interdiffusion of Nb and Sn occurred across a thickness of about 13 μm. Scanning electron microscopy allowed accurately measuring the thickness of the Nb 3 Sn phase, whose average for the various types of film samples was between 5.7 and 8.0 μm. X-ray diffraction patterns confirmed the presence of a cubic Nb 3 Sn phase (A15 structure) having (210) preferred orientation. The maximum obtained T c was 17.68 K and the B c20 ranged between 22.5 and 23.8 T. With the procedure described in the present paper, coating complex shapes cost-effectively becomes possible, which is typical of electrochemical techniques. Furthermore, this approach can be implemented in classical wire processes such as 'jelly roll' or 'rod in tube', or directly used for producing superconducting surfaces. The potential of this method for superconducting radiofrequency structures is also outlined.

Kinetically induced low-temperature synthesis of Nb3Sn thin films

“…2 Among the reported thin film deposition methods are tin evaporation, [3][4][5] chemical vapor deposition, 6 sputtering, [7][8][9][10][11] electron beam evaporation, 12 bronze processing, 13 and electrodeposition. 14 However, the tin diffusion process is the only process used so far for the production of cavities. A key challenge is to control the small stoichiometry window (24%-26% tin content 15 ) needed for best performance.…”

“…Therefore, the difference in diffusivity of niobium and tin results in concentration gradients after the annealing step. For most annealing approaches, phase pure Nb 3 Sn is only formed above 930 C. 14 For electrochemical approaches in combination with annealing, the lowest formation temperature of superconducting Nb 3 Sn was reported to be 630 C, requiring additional Cu as a flux medium. 17 However, Cu interdiffusion at this temperature results in a severe decrease of performance, and only a considerable further reduction of growth temperature (as reported here) would allow the combination of Nb 3 Sn with Cu.…”

Development and Performance of the First Nb3Sn Thin-Film Cavity via Bronze Process

“…[5] A devoted global effort in developing Cu cavity structures coated with Nb3Sn would make the International Linear Collider or other major factories more affordable and more likely to be built. [6] When the SRF community was looking for technologies that could synthesize Nb3Sn thin-film cavities at lower temperature, a number of candidates came onto the horizon, including bronze process, [7] chemical vapor deposition, [8] and magnetron sputtering. [5,[9][10][11] Com-pared with its competitors, the bronze process is simpler, cheaper, better controllable, and easier for mass production.…”

“…The first high-quality Nb3Sn thin films via bronze process were fabricated by Barzi's team in 2015. [7] Based on this method, the SRF community designed a twostep strategy toward the final target, Nb3Sn on copper cavity: first, high-quality Nb/Cu cavity fabrication (see Refs. [12][13][14]) and then low-temperature Nb3Sn syntheses on Nb substrates.…”

“…Nb3Sn thin film was synthesized after vacuum annealing at 700 ∘ C for 24 h. The heating rate of the cavity is about 0.5 ∘ C per minute, with natural cooling after annealing. Usually, a Nb3Sn film prepared by the bronze process has a bronze impurity layer on the surface, [7] which needs complete removal to avoid affecting the SRF properties of the Nb3Sn-film cavity. To obtain a high-quality Nb3Sn surface, we removed the surface residue bronze layer by electropolishing (similar to the samples in Ref.…”