Marin E. Kordesch
The Condensed Matter & Surface Sciences Colloquium Series presents Martin E. Kordesch of Ohio University on “Thermionic Cathodes-Tales from the Trailing Edge of Science.” on Thursday, Sept. 3, at 4:10 p.m. in Walter 245.
Abstract: Thermionic emission from barium on platinum foil was studied with the very first electron microscopes in 1932. The electron emission microscope at Ohio University has been used to observe elemental composition information using a unique method that deposits known materials in shapes recognizable in the image and with a location index. Observing the evolution of the shapes, combined with the real-time in situ electron emission mapping can reveal or confirm the origins of work function reduction on metals. The image can show several types of surface under identical conditions. Because of the location index, ex situ examination and comparison are also possible.
The theory of work function reduction by alkali metal adsorption on simple metals has been used to describe the work function variation of thermionic cathodes covered with barium and other additives. The validity of this model is not questioned, even though many claims for cathode operation directly contradict the dipole model. Even modern ab-initio calculations struggle to find stable cathode surface structures. Thick oxide–semiconductor models are also proposed, often with a wide range of “thick” and “semiconductor”.
The chemical composition, the diffusion behavior and electron emissivity for model cathode using Ba, BaO, Sc2O3, and ThO2 will be presented and discussed relative to the dipole model, semiconductor models and possible alternatives.
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