Literature survey of some surface and subsurface processes affecting materials in thermionic diodes

Abstract

I Supplementary Notes AbstractThis report is a literature survey of surface and subsurface chemical and physical processes affecting materials in the thermionic diode. The processes considered were the evaporation and condensation of substrate materials, the bulk diffusion of metallic substrate materials, reactions between insulators and metals, and the surface self-diffusion and reconstruction of clean surfaces, faceting, and other surface reconstruction caused by contaminants or additives, solution, and oxidation. Author(s1) 18. Key Words (Suggested by SUMMARYThis report is a literature survey of surface and subsurface chemical and physical processes affecting materials in the ther mionic diode. The processes considered were the evaporation and condensation of substrate materials, the bulk diffusion of metallic substrate materials, the reactions between insulators and metals, the surface selfdiffusion and reconstruction of clean surfaces, faceting and other surface reconstruction caused by contaminants o r additives, solution, and oxidation. INTRODUC TIONThe thermionic diode is a collection of interfaces o r spaces between materials of unlike composition. Because of these inhomogeneities, processes result that can be lumped under the general heading of surface chemistry. There a r e interfaces between the solid conductors, insulators o r containment walls and the gas o r vacuum phases. There a r e contacts between insulators and metallic conductors. Also, there a r e contacts between unlike metals.Figure 1 summarizes these contacts and some of the resulting physico-chemical p r oc e s s e s which a r e surveyed herein. F o r the purpose of discussionin this report, two broad subdivisions of topics a r e made in the processes indicated in figure 1. In category I a r e placed those interfacial p r o c e s s e s that can be expected in all diodes including the highly idealized vacuum diode: evaporation and condenstation of substrate materials, bulk diffusion of metallic substrate materials, reactions between insulators and metals, and s u rface self -diffusion and reconstruction of clean surfaces.In practice, no t r u e vacuum diode can be constructed. T o the foregoing processes must be added those surface processes resulting from inevitable gaseous contaminants.The inability ' EQ c r e a t e perfect laboratory vacua gives r i s e to atmospheric contaminants, In addiliora, construction materials evolve gaseous contaminants either because of chernieal decomposition, as in the ease of insulator compounds, o r because ol impurr"ces.Although some gas phase contaminants a r e unavoidably present, additional conkminants may be deliberately introd~aced to affect the properties of the surface or the gas phase. Cesium, of course, i s the most familiar example of such a n additive. Cesium, barium, and other metallic additives will not be considered here since little chemical effect on substrate materials has been reported for them. However, the use of such electronegative additives as oxygen and fluorine in conjunction...