Initial growth of tin on niobium for vapor diffusion coating of Nb₃Sn

Abstract

Nb3Sn has the potential to achieve superior performance in terms of quality factor, accelerating gradient and operating temperature (4.2 K vs 2 K) resulting in significant reduction in both capital and operating costs compared to traditional niobium SRF accelerator cavities. Tin vapor diffusion coating of Nb3Sn on niobium appears to be a simple, yet most efficient technique so far to fabricate such cavities. Here, cavity interior surface coatings are obtained by a two step process: "nucleation" followed by "deposition". The first step is normally accomplished with Sn/SnCl2 at a constant low temperature (~500 °C) for several hours. To elucidate the role of this step, we systematically studied the niobium surface nucleated under varying process conditions. The surfaces obtained in typical tin/tin chloride processes were characterized using SEM/EDS, AFM, XPS, SAM and TEM. Examination of the surfaces nucleated under the "standard" conditions revealed not only tin particles, but also tin film on the surfaces resembling the surface obtained by Stranski-Krastanov growth mode. All the nucleation attempted with SnCl2 yielded better uniformity of Nb3Sn coating compared to coating obtained without nucleation, which often included random patchy regions with irregular grain structure. Even though the variation of nucleation parameters was able to produce different surfaces following nucleation, no evidence was found for any significant impact on the final coating. 6

Electron backscatter diffraction of Nb3Sn coated niobium: Revealing structure as a function of depth

“…Several examples include coatings grown on niobium which was anodized prior to the coating. For indepth discussion of the Nb3Sn vapor deposition setup, process, and anodization refer to reference [11].…”

“…[27,28] Other studies involving anodization and the nucleation process have shown mixed results. [11,29] Here we use EBSD and analysis by NIH ImageJ software to help gain insight into the structural differences between Nb3Sn grown on anodized niobium surfaces versus that grown on NP niobium surfaces. Where θF is the Feret angle and θG is the orientation angle of the grain with respect to the x-axis.…”

Properties of Nb3Sn films fabricated by magnetron sputtering from a single target

“…Nb 3 Sn is of interest as a coating for superconducting radiofrequency (SRF) cavities due to its higher critical temperature T c of ~18.3 K and superheating field H sh of ~400 mT [1]. Nb 3 Sn cavities have the potential to achieve high quality factor Q 0 when operated at 4 K and can replace the bulk Nb cavities that are operated at 2 K [2,3]. The higher operating temperature of Nb 3 Sn cavities compared to Nb can significantly reduce the operating cost.…”

Kinetically induced low-temperature synthesis of Nb3Sn thin films

“…Among the different intermetallic phases of the Nb-Sn system, only the A15-type Nb 3 Sn is superconducting above 10 K. However, thin film synthesis of high quality Nb 3 Sn is still a huge challenge. 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.…”

“…Previously reported approaches potentially lead to inhomogeneous or graded distributions of Nb and Sn and, furthermore, to the formation of non-superconducting phases limiting the desired performance since they all require annealing. 4,17,18 Tin evaporation is the first and most advanced process for actual production of cavities. Sputtering approaches are significant as an alternative route of coating copper cavities.…”