Surface Nb oxide variation with surface treatment

On exposure to air niobium forms a native pentoxide layer on its surface. This material is itself a benign dielectric. Just what happens in the bulk-to-pentoxide interface is of considerable interest because it bears on the ultimate SRF performance that may be attained. Suboxides of niobium, which may well be present in this transition, are normal conducting at temperatures of interest. The crystalline texture of the niobium at the interface may facilitate magnetic flux penetration at less than the theoretically anticipated critical field strength.

JLab in collaboration with the College of William and Mary, Boston University, and others, is examining the influence of flow conditions and thermal processing steps on the resultant nature of the surface roughness and oxides.

• In the standard etching process for SRF niobium cavities (BCP), solution flows in one end and out the other. The maximum flow rate occurs at the cavity irises. We subjected batches of identical samples to BCP with different flow rates, then measured surface roughness by stylus profilometry and surface composition by the XPS. A surprising correlation was found. Its implications have yet to be sorted out.
• We also chemically polished polycrystalline and single crystal Nb coupons, then measured surface roughness by stylus profilometry and surface composition by the variable photon energy XPS, which was carried out at beamline X1B at the National Synchrotron Light Source (NSLS). Such measurements are aimed at discerning depth profile information via the increase of electron escape depth with energy. This would avoid surface topology complications to occur when interpreting angle-resolved XPS of representative niobium surfaces.

This work is largely the effort of a PhD student from the College of William and Mary, Hui Tian.