Oxidation state of zirconium and tungsten on a Schottky electron source at high temperature

Abstract We investigated the zirconium-oxide-coated-tungsten (100) surface, which is widely used as the electron-emitting surface in Schottky electron sources. To extract electrons, the emitter is heated to approximately 1800 K. When the surface reaches a temperature greater than 1400 K, the work function of the (100) surface is selectively lowered by the coating of zirconium and oxygen, and surface diffusion of the atoms is rapid on the tungsten surface. The mechanism behind this process, however, is not clear because it is difficult to analyze the surface at high temperatures. In this study, we resolved this issue by improving our sample heating system and investigated the zirconium and oxygen-coated tungsten (100) surface using X-ray photoemission spectroscopy at 1500 K. The stable heating system helped us acquire highly reliable spectra, enabling us to elucidate the detailed bonding state of the atoms on the surface. We also found that a large amount of oxygen is present on the surface even at high operating temperatures. Moreover, zirconium and tungsten are partially oxidized, and the stoichiometry of these oxides is distinctly different from that of the stable oxide within the bulk.