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Electropolishing of Niobium in Sulfuric Acid-Methanol Electrolytes: Development of Hydrofluoric Acid-Free Electrolytes

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dc.contributor.authorZhao, Xinen
dc.contributor.committeechairCorcoran, Sean G.en
dc.contributor.committeecochairKelley, Michael J.en
dc.contributor.committeememberEdwards, Marc A.en
dc.contributor.committeememberAning, Alexander O.en
dc.contributor.committeememberReynolds, William T. Jr.en
dc.contributor.departmentMaterials Science and Engineeringen
dc.date.accessioned2014-03-14T20:14:42Zen
dc.date.adate2009-08-11en
dc.date.available2014-03-14T20:14:42Zen
dc.date.issued2009-07-16en
dc.date.rdate2010-10-22en
dc.date.sdate2009-08-03en
dc.description.abstractNiobium (Nb) has the highest superconducting transition temperature (9.2 K) of the pure metals, which makes it the most used material for the construction of superconducting radio frequency (SRF) accelerators. The performance of the accelerator is critically dependent upon the quality of Nb surface. Electropolishing (EP) in hydrofluoric acid (HF)-containing electrolytes is the currently accepted treatment process. The presence of HF is necessary for the removal of the passive oxide surface film formed in aqueous electrolytes. But HF is hazardous and must be contained without human exposure and eliminated in an environmentally appropriate manner. In the present dissertation project, HF-Free EP of Nb was performed in sulfuric acid-methanol electrolytes. Sulfuric concentrations of 0.1 M, 0.5 M, 1 M, 2 M, and 3 M were used. Cyclic voltammetry and potential hold experiments were performed in cells of both two-electrode and three-electrode setups to evaluate the electrochemical process. The influence of electrolyte concentration, temperature, and EP duration was investigated. At room temperature, both the corrosion rate and the surface quality obtained were comparable to those currently obtained with HF-based processing. With decreasing temperature, the mean current level decreased and the surface quality improved substantially. For a desired average material removal of 100 μM, nanometer scale surface roughness was obtained under multiple conditions. Mechanism of EP was also investigated by electrochemical impedance spectroscopy (EIS). The EIS diagram indicates the presence of a compact film during EP at mass transport controlled limiting current and a film-free surface during EP at ohmic controlled current. Transfer from a film-free surface to an anodic film precipitation with decreasing temperature was also observed. Microsmoothing is only achieved under mass transport control. Nb⁵⁺ ions are determined to be the mass transport limiting species.en
dc.description.degreePh. D.en
dc.identifier.otheretd-08032009-170244en
dc.identifier.sourceurlhttp://scholar.lib.vt.edu/theses/available/etd-08032009-170244/en
dc.identifier.urihttp://hdl.handle.net/10919/28507en
dc.publisherVirginia Techen
dc.relation.haspartDissertation_Xin_Zhao.pdfen
dc.rightsIn Copyrighten
dc.rights.urihttp://rightsstatements.org/vocab/InC/1.0/en
dc.subjectsulfuric acid-methanolen
dc.subjectniobiumen
dc.subjectelectropolishingen
dc.subjecthydrofluoric acid-freeen
dc.titleElectropolishing of Niobium in Sulfuric Acid-Methanol Electrolytes: Development of Hydrofluoric Acid-Free Electrolytesen
dc.typeDissertationen
thesis.degree.disciplineMaterials Science and Engineeringen
thesis.degree.grantorVirginia Polytechnic Institute and State Universityen
thesis.degree.leveldoctoralen
thesis.degree.namePh. D.en

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