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 + H 2 SO 4 <-> HFSO 3 + H 2 O. 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.

Evaluation of time-gated Raman spectroscopy for the determination of nitric, sulfuric and hydrofluoric acid concentrations in pickle liquor

“…However, Raman signals due to HF have been reported earlier although with a different Raman apparatus and laser wavelength. 11 An earlier paper 14 suggests that Raman peaks due to HF might be difficult to observe in a sample matrix containing sulfuric acid. Sulfuric acid might react with hydrofluoric acid producing fluorosulfonic acid dampening the HF Raman peaks.…”

Plasma Electrolyte Polishing of Titanium and Niobium Alloys in Low Concentrated Salt Solution Based Electrolyte

“…For the electrochemical technologies, the indicated materials are difficult to treat and polishing processes require usage of toxic electrolytes. Usually, electrochemical polishing of titanium and niobium alloys is carried out in acid electrolytes consisting of a solution based on toxic hydrofluoric acid (20-25%) with the addition of sulfuric, nitric, and perchloric acids [11,12]. Such solutions are unsafe, the use of hydrofluoric acid throughout the world is strictly regulated.…”