The Role of the Core Metal in Oxide Coated Filaments

Abstract

Temperature-power relations have been determined for numbers of oxide coated filaments. Part of these filaments had cores of platinum-10% iridium and others had cores of "Konel," an alloy of nickel, cobalt, iron and titanium. Oxide coated Konel is a much better radiator than oxide coated platinum-iridium which causes a considerable difference in their temperature -input power characteristics. N vertheless, emission measurements show that oxide coated Konel filaments yield higher electron emissions than oxide coated platinum-iridium under the same conditions of filament power. Experimental proof is also given that these filaments need no activation other than the decomposition of the alkaline earth carbonates to oxides in vacuo. The enormous difference shown between the emission characteristics of these two types of oxide filament necessitates the conclusion that the core metal has a definite function other than simply a mechanical support for the alkaline earth oxides.Suggested mechanism of thermionic emission from oxide coated filaments. In order to account for this difference, a modification is suggested in existing ideas concerning the mechanism of emission from this type of cathode. This modification consists in assuming that the source of emission is the composite layer formed by occlusion of alkaline earth metal on the surface of the core and that the electrons emitted diffuse through the interstices in the oxide coating into the vacuous space. Argument is presented to show that this explanation will also account for other peculiarities in the behavior of oxide cathodes. (1) The decay of emission during life may be explained by a slow sintering of the coating and a consequent closing of these pores. (2) De-activation of the filament by over-heating may be attributed to the same cause. (3) Non-saturation may be due to a pseudo-space charge formed by occlusion of electrons on the surface of the coating particles.