Chemical Analysis of Fluorine in Niobium Electropolishing

Abstract

Surface topography and chemical purity are important factors in niobium superconducting radio-frequency (SRF) cavity performance. Electropolishing (EP) is currently being used to minimize the surface roughness and to remove the damaged surface layer created during cavity manufacturing. This process is not ideal for reasons such as safety and performance consistency, so research and development is ongoing. Furthermore, the EP process specifications have been developed empirically, and a molecular level understanding of the process is not complete. The currently employed polishing solution is composed of sulfuric and hydrofluoric acids, where the fluorine ion is active in polishing the niobium. We used vibrational and nuclear magnetic resonance (NMR) spectroscopies to analyze the standard solution, and show that fluorine is bound and released by the reaction of the acid components in the solution: HF + H2SO4 <-> HFSO3 + H2O. This result implies that new recipes can possibly be developed on the principle of controlled release of fluorine by a chemical reaction, which provides a route to improve the safety and effectiveness of EP. We also show that NMR or Raman spectroscopy can be used to monitor the free fluorine when polishing with the standard EP recipe.