Thermal and electron-beam irradiation effects on the surfaces of niobium for RF cavity production

Results of a study of thermal and electron-beam irradiation effects on the surfaces of Nb are presented. The samples were prepared by chemical etching methods used in superconducting rf cavity production. For such prepared samples, a layer of Nb2O5 covered the surface with some Nb2O interfacing the Nb metal. Some hydrocarbon species were also found on the surface. Oxidation in air at room temperature will be discussed. Heating the samples at temperatures from 100 - 300 degC caused reduction of the surface oxide. The reduction rate increased with temperature. Near 100 degC, some Nb2O5 reduced and some NbOx (x < 2.5) formed. Near 170 degC, only a little Nb2O5 remained after > 70 h. Substantial amounts of NbOx (1 ≤ x < 2.5) formed. Heating at these temperatures also caused hydrocarbon decomposition, leaving graphitic carbon on the surface, but had no effect on Nb2O at the interface. At temperatures > 210 degC, Nb2O5 disappeared. Only Nb2O remained on the surface that exhibited a metallic character. In addition, more than half of the surface carbon reacted with Nb and formed metal carbide. Reaction kinetics will be discussed. Under 1-keV electron beam irradiation, surface damage occurred. Changes in the secondary electron yield (SEY) were observed and identified to be due to two causes: surface chemical changes and defect creation in the oxide. There was a slope change in the SEY-vs-dose relationship that led to two cross-section values of 4.8 x 10-18 cm2 and 2.3 x 10-19 cm2 for the electron/matter interaction. The larger value primarily results from changes in surface chemistry and the smaller one from defect creation. (author)