Key Morphological and Chemical Features of Anodized Nb Relevant for Sn Nucleation in Nb3Sn Growth

Abstract

The performance of superconducting radiofrequency (SRF) cavities is governed by material properties within the RF penetration depth. Nb3Sn is a promising next-generation SRF material, capable of Q-factors of 1010 at 4.2 K, yet its performance remains below theoretical predictions due in part to surface inhomogeneities within the ∼100 nm RF penetration depth. Preanodization of Nb enhances Sn nucleation and improves Nb3Sn stoichiometry, but the factors responsible for this improvement remain poorly understood. Using in situ X-ray photoelectron spectroscopy and atomic force microscopy, we investigate the chemical and morphological evolution of anodized polycrystalline Nb while annealing to 500 °C, representative of Nb3Sn growth conditions. The anodized surface is ∼10 times rougher than the control and exhibits isotropic bumps and pores that persist after annealing. Kinetic modeling of the thermal evolution predicts that the anodic Nb2O5 surface is more thermally stable than native Nb2O5 during the Nb3Sn growth process. Na contamination is also observed on the anodized surface and persists after annealing. Complementary DFT calculations predict favorable oxidation reactions and greater Sn stability relative to SnO and SnO2 on Nb2O5 compared with NbO. Together, these results suggest that the persistent Nb2O5 layer and rough, porous morphology may promote Sn nucleation and stabilization on anodized Nb, providing insight for optimizing anodization and vapor-diffusion growth of Nb3Sn coatings for advanced SRF cavities.