Evolution of surface oxides and impurities in high vacuum heat treated Nb: A TEM and TOF-SIMS in-situ study, mechanism and repercussions on SRF cavity applications

Abstract A detailed in-situ study of the transformations occurring in the top nano-layer of a chemically treated Nb sample undergoing thermal cycling was performed using transmission electron microscopy (TEM) and time of flight secondary ion mass spectrometry (TOF-SIMS) techniques, with an objective to understand the effects of high temperature degassing (HTD) treatment of superconducting radio frequency cavity. TEM imaging of the top layer at various temperature provided evidences of the evolution of top oxide layer, while the associated chemistry explored through TOF-SIMS spectra and depth profiles revealed an interesting transformation of the oxide layer in to a carbide-dominated layer between ~200–400 °C. The oxide layer transformation dominated at the vacuum-oxide interface until 250 °C, beyond which, oxygen diffusion and carbon enrichment at the metal-oxide interface and associated reactions transitioned the layer to a thin Nb(OC) mixed layer up to ~1–2 nm from top surface, followed by a uniform carbide layer of ~5 nm thickness. Furthermore, desorption of bulk H was found to be co-related to the oxide layer transition up to 350 °C, beyond which, near surface hydrogen enrichment became the dominating factor. Finally, the proposed mechanisms were applied to understand the surface contamination and nitrogen doping during standard HTD of Nb samples.