Some experiments on the role of oxygen and surface reactions for tungsten and scandate thermionic emitters

It is shown that the state of oxidation of the tungsten before activation on an S-type cathode is irrelevant to the eventual emission level. The reason for this is that during activation the original surface oxygen is partly removed by desorption, while the remaining tungsten oxide is reduced by means of atomic barium produced by the reaction between impregnant and tungsten. This can also be demonstrated by exposing an unimpregnated heated tungsten matrix to an external atomic barium flux: the surface oxygen disappears. In addition, experiments showed the transport of oxygen to the surface. The possibility for the formation of stable complexes by means of surface reactions is shown by exposing a heated (1125°C_b) matrix with a W+Sc₂O₃ top layer to an external barium flow. This may be related to the surface compound formation on activation of a cathode made by pressing and sintering a mixture of W, Re₂₄Sc₅, and Ba-Ca-aluminate