The role of oxygen in the low work function surface complex of barium on oxygen on tungsten (substrate)

Abstract

The barium on oxygen on tungsten (Ba-O-W) surface complex is responsible for the high thermionic emission properties of the barium impregnated tungsten cathode used in high frequency microwave tubes. The investigation reported here is a continuation of a study of the physical and chemical properties of the Ba-O-W surface, and in particular, of the role of surface oxygen. Desorption characteristics of a barium monolayer on an oxygenated tungsten substrate were measured at elevated temperatures as a function of time and initial oxygen concentration using a computerized scanning Auger microprobe. The oxygen concentration was varied by exposing the clean tungsten surface to oxygen levels ranging from 1.4 to 126 L. Barium desorption measurements were made at elevated temperatures, and the surface concentrations of barium, oxygen and tungsten were monitored with time. The results showed that the most stable Ba-O-W surface complex has an oxygen concentration of approximately 5×10 14 atoms/cm 2. This surface has the lowest work function and shows least barium desorption at a given temperature; the major barium evaporation loss mechanism is given by the surface chemical reaction Ba-O-W→Ba↑+(O-W). At higher oxygen concentrations the Ba-O-W complex is less stable, has a higher work function and, at elevated temperatures, barium desorbs more rapidly and in two different modes. The first is similar to that described above and the second is by means of the surface reaction Ba-O-W→ BaO↑+W. These results were confirmed by experimental line of sight measurements made with a mass spectrometer.