New model for the role of oxygen in the emitting surface of the impregnated tungsten cathode

Based on the analysis and synthesis of recent experimental results—in particular, on the role of oxygen in the surface of a tungsten substrate—previous suggestions are examined and disproved. Furthermore, a new model for the role of oxygen in the thermionic emitting surface of an impregnated tungsten cathode is proposed. The model assumes that, at oxygen concentrations of approximately 5 × 1014 atoms cm−2 or less, oxygen atoms are incorporated into the outermost layer of the tungsten substrate; this accompanies the surface roughening that lowers the work function, enhances the barium-substrate bond, and forms a compacted Ba-(W,O) atomic configuration that is most stable and has the lowest work function. At higher oxygen concentrations, the excess oxygen lies on a surface site between the barium adsorbate and the tungsten substrate, forming a less stable Ba-O-(W,O) configuration that increases the work function and poisons the emitting surface. This model can effectively explain all the observed experimental results.