Tian H.

Paper	Title	Page
TUP18	Recent XPS Studies of the Effect of Processing on NB SRF Surfaces	158
	H. Tian, B. Xiao, M. J. Kelley JLab & College of William and Mary C. E. Reece JLab A. DeMasi, L. Pipe, K. E. Smith Boston University
	XPS studies have consistently shown that Nb surfaces for SRF chiefly comprise of a few nm of Nb2O5 on top of Nb metal, with minor amounts of Nb sub-oxides. Nb samples after BCP/EP treatment with post-baking at the various conditions have been examined by using synchrotron based XPS. Despite the confounding influence of surface roughness, certain outcomes are clear. Lower-valence Nb species are always and only associated with the metal/oxide interface, but evidence for an explicit layer structure or discrete phases is lacking. Post-baking without air exposure shows decreased oxide layer thickness and increased contribution from lower valence species, but spectra obtained after subsequent air exposure cannot be distinguished from those obtained prior to baking, though the SRF performance improvement remains.
WE104	Novel Characterization of the Electropolishing of Niobium with Sulfuric and Hydrofluoric Acid Mixtures	370
	H. Tian, M. J. Kelley TJNAF and College of William and Mary S. G. Corcoran Virginia Tech C. E. Reece TJNAF
	Niobium surfaces are commonly electropolished in an effort to obtain optimally smooth surfaces for high-field SRF cavity applications. We report the first use of controlled electrochemical analysis techniques to characterize electropolishing of Nb in a sulfuric and hydrofluoric acid electrolyte. Through the use of a reference electrode we are able to clearly distinguish the anode, cathode polarization potentials as well as the electrolyte voltage drop that sum to the applied power supply voltage. We then separate the temperature and HF concentration dependence of each. We also report the first use of Electrochemical Impedance Spectroscopy (EIS) on this system. EIS results are consistent with a presence of a compact salt film at the Nb/electrolyte interface that is responsible for the limiting current. Microscopic understanding of the basic Nb EP mechanism is expected to provide an appropriate foundation with which to optimize the preparation of high-field niobium cavity surfaces. The implication of EIS for monitoring Nb surface during electropolishing shows this technology could be potentially used as a source of on-line feedback.
	Slides(PDF)
WEP04	Surface roughness characterization of niobium subjected to incremental BCP and EP processing steps	438
	H. Tian, M. J. Kelley TJNAF/College of William and Mary G. Ribeill North Carolina State University C. E. Reece TJNAF
	The surface of niobium samples polished under incremental Buffered Chemical Polish (BCP) and Electro- Polishing (EP) have been characterized through Atomic Force Microscopy (AFM) and stylus profilometry across a range of length of scales. The results were analyzed using Power Density Spectral (PSD) technique to determine roughness and characteristic dimensions. This study has shown that the PSD method is a valuable tool that provides quantitative information about surface roughness at different length scales.