Phenomena in Oxide Coated Filaments II. Origin of Enhanced Emission

Abstract

Various theories have been advanced regarding the mechanism of emission of electrons from oxide coated filaments. These theories postulate that; (1) the active layer is (a) at the outer oxide surface, (b) at the core-oxide interface; (2) the thermionic electrons come from (a) the adsorbed barium, (b) the oxide just underneath the adsorbed layer; (3) the current is carried through the oxide coating by (a) an entirely electrolytic process, (b) thermionic electrons which come from the core, diffuse through the pores of the coating and form a space charge therein, (c) electrolytic conduction through the oxide crystals and thermionic conduction between crystals, (d) electronic conduction, a small portion being carried by ions. A number of experiments were designed to test these various hypotheses. These experiments show that; (1) When barium is brought to the outer surface of the oxide, either by electrolysis or evaporation from an external source, the emission increases at first, passes through a maximum and then decreases. This change in activity is similar to that for barium on tungsten. (2) When oxygen is brought to the surface of the oxide of an activated filament, the activity decreases rapidly at first and then more slowly. (3) In these two respects, a filament with a core made of an alloy called "Konel" and consisting of nickel, cobalt, iron and titanium acts just like filaments with other cores. (4) When the oxide was stripped from a Konel core filament, the activity decreased by a factor of 6000. (5) The emission-limited current is independent of the area of the core provided that the area of the outer oxide surface remains constant. (6) The conductivity of the oxide varies with the time of sending current through the oxide. (7) The conduction current in the oxide obeys Ohm's law and does not saturate even though its value is hundreds of times larger than the saturated emission. (8) The oxide acquires a positive potential with respect to the core regardless of whether the space current is limited by space charge or by emission. This potential varies linearly with the space current drawn to the plate and is of the order of a few tenths of a volt. (9) The emission for the optimum amount of barium on the oxide surface depends upon the previous treatment of the oxide. From these results we conclude that; (1) The active layer is at the outer oxide surface. The activity depends on the concentration of barium and oxygen on this surface and also upon the amount of metallic barium dispersed through the oxide. The core material does not directly affect the emission but it does greatly affect the ease with which free barium is produced by heat treatment or electrolysis. (2) The thermionic electrons originate in the oxide just underneath the adsorbed barium. (3) Most of the current through the oxide is conducted by electrons, a small portion being carried by barium and oxygen ions.