SIMS and TEM Analysis of Niobium Bicrystals

Interstitial impurities such as C, N, O and H on the Nb surface play a key role in the efficiency of superconducting radio frequency (SRF) cavities for particle accelerators. Thus, it is important to understand the behaviour of these interstitial impurities with respect to grain boundaries. The large single crystal grains in large grain Nb make it possible to select bicrystal samples that have a well defined grain boundary. In this work, Dynamic SIMS was used to analyze two Nb bicrystal samples, one of them heat treated and the other chemically treated by buffered chemical polishing (control). H levels were found to be higher for the control sample and a difference in the H intensity and sputtering rate was also observed across the grain boundary for both samples. Transmission Electron Microscopy (TEM) was used to study the bicrystal interface and the surface oxide layer. The interface showed no discontinuity and the oxide layer was uniform across the grain boundary for both samples. TOF-SIMS imaging was also performed to analyze the distribution of the impurities across the grain boundary in both samples. C was observed to be segregated along the grain boundary for the heat treated sample, while H and O showed a difference in signal intensity across the grain boundary. Crystal orientation appears to have an important role in the observed sputtering rate and impurity ion signal differences, both across the grain boundary and between samples.