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XLI. Thermionic emission and electrical conductivity of oxide cathodes

1930; Taylor & Francis; Volume: 9; Issue: 57 Linguagem: Inglês

10.1080/14786443008565017

1941-5990

A.L. Reimann, R. Murgoci,

Plasma Applications and Diagnostics

Abstract The following observations were made:— 1. (1) The electrical conductivity, c, of a “formed” alkaline earth oxide varies with the temperature, T, according to a law of the form where α and β are constants.2. (2) During the process of forming, both the thermionic emission and the conductivity grow similarly, and after its completion both are “poisoned” similarly by exposure of the oxide to (a) oxygen, (b) a discharge in carbon monoxide, and (c) a discharge in hydrogen. Complete recovery of the formed condition by re-forming is possible only a few times in succession after poisonings by (a) and (b), but any nmnber of times after poisoning by (c).3. (3) At current densities comparable with those in the coatings of oxide cathodes from which saturated thermionie space current is being taken, the current conducted through an oxide powder between two electrodes, embedded therein, also saturates.On the basis of these observations, together with the results of related work by other investigators, we have formulated the following theory of the action of oxide cathodes :—4. (1) The coating conducts the space current, which, of necessity, passes through it, eleetrolytically. Practically only the metallic ions are mobile, the oxygen ions playing no active part in the electrolysis.5. (2) The whole surface of each crystal of oxide of a formed cathode is covered with a mobile monatomic layer of alkaline earth metal. The passage of space current is accompanied by a continual circulation of this metal, which diffuses outward along the surfaces of the crystals and inward through the crystals in the form of electrolytic ions.6. (3) At the usual operating temperatures of oxide cathodes the average life of alkaline earth metal particles on the emitting surface is of the order of 10-3 second, and the rate of flow of this metal over the surface of an idealized independent unit of barium circulation in the form of a cube of side ι would be of the order of 300 ι per second.7. (4) The coating is probably in very imperfect contact with the core metal, so that the space current passes from core metal to coating mainly in the form of thermionically emitted electrons. Sufficiently copious electron emission of the core metal at the tow temperatures of operation of these cathodes would be made possible by a contanfination of its surface with adsorbed barium or with barium and oxygen.Explanations are suggested for8. (1) the eventual “life-failure” of oxide cathodes,9. (2) the observed phenomena relating to poisoning,10. (3) the considerable variation in the published values of the thermionie constants of oxide cathodes.