Scandium function in “scandate” thermionic cathodes: A microspot synchrotron radiation x-ray photoelectron spectroscopy study of co-adsorbed Ba-Sc-O on W(100)

Abstract

The coadsorption of barium, scandium, and oxygen on W(100) was investigated with microspot synchrotron radiation x-ray photoelectron spectroscopy ( μ-SRXPS) in the emission microscope at the electron-spectro-microscopy beam line at the National Synchrotron Light Source II. The Ba-Sc-O-W(100) system is a model for “scandate” thermionic electron emitters. The barium 4d, tungsten 4f, and scandium 3p (Sc3p) levels were observed after metal deposition and adsorption of oxygen. The initial deposits of metallic scandium and barium were confirmed with μ-SRXPS. After oxygen adsorption, the Sc3p peaks shifted to higher binding energy (by >4 eV), indicating the formation of Sc2O3 or a related compound. After heating to 800 °C, the barium 4d peaks were no longer observed. The shifted Sc3p peak was also absent. The results indicate that scandium acts as a cleaning agent to remove oxygen from the tungsten surface. The oxidized scandium is removed by reaction with barium and desorption below 800 °C. The temperature of desorption indicates that scandium does not remain on the surface at a temperature where a contribution to the reduction of the surface barrier to electron emission could be expected.