Atomic-scale analyses of Nb₃Sn on Nb prepared by vapor diffusion for superconducting radiofrequency cavity applications: a correlative study

Abstract

We report on atomic-scale analyses of the microstructure of an Nb 3 Sn coating on Nb, prepared by a vapor diffusion process for superconducting radiofrequency (SRF) cavity applications using transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and first-principles calculations.Epitaxial growth of Nb 3 Sn on a Nb substrate is found and four types of orientation relationships (ORs) at the Nb 3 Sn/Nb interface are identified by electron diffraction or high-resolution scanning transmission electron microscopy (HR-STEM) analyses. Thin Nb 3 Sn grains are observed in regions with a low Sn flux and they have a specific OR: Nb 3 Sn (12 ̅ 0)//Nb (1 ̅ 11) and Nb 3 Sn (002)//Nb (01 ̅ 1). The Nb 3 Sn/Nb interface of thin grains has a large lattice mismatch, 12.3%, between Nb (01 ̅ 1) and Nb 3 Sn (002) and a high density of misfit dislocations as observed by HR-STEM. Based on our microstructural analyses of the thin grains, we conclude that the thin regions are probably a result of a slow interfacial migration with this particular OR. The Sn-deficient regions are seen to form initially at the Nb 3 Sn/Nb interface and remain in the grains due to the slow diffusion of Sn in bulk Nb 3 Sn. The formation of Sn-deficient regions and the effects of interfacial energies on the formation of Sn-deficient regions at different interfaces are estimated by first-principles calculations. The finding of ORs at the Nb 3 Sn/Nb interface provides important information about the formation of imperfections in Nb 3 Sn coatings, such as large thin-regions and Sn-deficient regions, which are critical to the performance of Nb 3 Sn SRF cavities for accelerators.

Grain-boundary structure and segregation in Nb3Sn coatings on Nb for high-performance superconducting radiofrequency cavity applications

“…This result indicates that GB diffusion of Sn so fast that there is not a concentration gradient along the GB [50]. Short-circuit diffusion of Sn along GBs (D gb ) is still faster by more than three orders of magnitude than bulk diffusion in Nb 3 Sn (D b ) at 1100 o C; the estimated diffusion lengths of Sn in bulk Nb 3 Sn and in GBs at 1100 o C per 1 hour (√4 ) are ~200 nm and ~120 μm, respectively [15,51]. The root-mean square bulk diffusion distance of Sn at 1100 o C is larger than the GB diameter (ϭ = 3 nm) but much smaller than the grain diameter (d = 2 μm), implying that GB diffusion is still the primary path of Sn diffusion.…”

“…Several studies [12][13][14][15] have been performed to investigate the structural defects in Nb 3 Sn coatings, such as abnormally thin-grains, compositional variations, and Sn-deficient Nb 3 Sn phases [1,8,12], which may be responsible for the Q 0 -slope and quenching, which is a loss of superconductivity. Orientation relationships between Nb substrates and Nb 3 Sn films were measured and the Nb 3 Sn/Nb heterophase interfaces are demonstrated to play a critical role in the formation of thin-grains and Sn-deficient regions [15]. Another candidate for causing degradation of the superconducting properties of Nb 3 Sn SRF cavities is grain-boundaries (GBs).…”

Vortex nucleation in superconductors within time-dependent Ginzburg-Landau theory in two and three dimensions: Role of surface defects and material inhomogeneities

“…In addition to surface roughness, the alloyed nature of these materials often leads to variations in material parameters, such as Sn concentration, that can have a strong effect on the superconducting properties [38][39][40][41][42][43] . To guide future development and keep pace with experimental advancements, more sophisticated theoretical and computational tools are needed to identify the relevant physics for vortex nucleation and quantify their effect on H sh in real materials.…”

Development and Characterization of Nb3Sn/Al2O3 Superconducting Multilayers for High-performance Radio-Frequency Applications

“…The results of this work show that, despite the obvious non-stoichiometry and inhomogeneity of superconducting properties, grain boundaries, Nb inclusions, and incomplete Al2O3 layers, our multilayers exhibit the quality factors on par of those of cavity-grade bulk Nb at 4K and low RF power. The significant local non-stoichiometry of thick (a few micron) polycrystalline Nb3Sn coatings of Nb cavities 20,27 , as well as Sn depletion at grain boundaries in Nb3Sn [34][35][36][37] have been well documented in the literature. Yet, despite these materials issues which are also characteristic of 1-3 m thick Nb3Sn films used in SRF cavities 38 , our Nb3Sn SIS structures exhibit higher low-field Q values than Nb at T > 6K 20 , consistent with the larger superconducting energy gap Δ Nb3Sn ≈ 2Δ Nb and a lower BCS surface resistance ∝ low-field quality factors of Nb3Sn to the significant non-stoichiometry and materials imperfections but also suggest that the SRF performance of Nb3Sn coatings can be further improved by materials treatments.…”