THE ELECTRONIC AND STERIC PROPERTIES OF IMPREGNATED THERMIONIC CATHODES: TOWARD THE DEVELOPMENT OF A SCIENTIFIC BASIS FOR 'AB INITIO' CATHODE ARCHITECTURE.

摘要

Present day thermionic emitters employ the work function lowering mechanism of electropositive monolayers adsorbed onto the surface of a metallic conductor to achieve high current densities, in excess of 2 Amps./cm;Specifically, the study has led to the development of cathodes that exhibit thermionic emission in excellent agreement with the theoretical emission equations. In addition, the fabrication sequence results in reproducible cathode structures, activation procedures which require lower temperatures, and, from the practical point of view, actual devices that exhibit comparable emission densities at temperatures 300 degrees Kelvin below conventionally produced cathodes, i.e., zero field emission density of 1.5 Amps/cm;Mossbauer Spectroscopy was used to investigate the emissive layer bonding structure. This revealed that enhanced thermionic emission was concomitant with the presence of a low-spin state 5-d transition metal ion. Such compounds are analogous to the hexagonal barium titanates, and establish a surface template that facilitates the dispersal of barium. With this information, a procedure was developed that would ultimately lead to the desired structure. This method, based on the principles of heterogeneous supported catalysis, and experimentally verified by differential thermal analysis, revealed that cathode impregnant thermochemistry, if properly controlled, could result in the formation of an optimal active site for thermionic emission.