Analysis of impurities in high purity niobium: surface VS bulk

Surface and interface segregation is a well known phenomenon occurring in ultra pure metals. So when one has to deal with properties more specifically correlated with surface or interface properties, bulk analysis is no more relevant. Surface analysis can provide more relevant data and give some hints about other interfaces (grain boundary) behavior. In this paper, some examples will be given of analysis conducted on niobium dedicated to the fabrication of RF superconducting cavities. In this case, only a small portion of the surface (50-100 nm) is involved in the dissipation phenomena. But although we use high purity material, one shall see that the surface includes a lot of contaminants that might influence the superconducting properties of the material. Preferential diffusion/segregation at grain boundary or near the surface are phenomena leading to large inhomogeneity inside the material, which cannot be considered as ultra pure in very localized places, even if a global analysis gives 'good' results. (author)