The electron emission of a Sc-type impregnated cathode is known to be enhanced by a monoatomic-order surface layer composed of Ba, Sc, and O. This layer reduces the work function much lower than those of conventional impregnated cathodes.1,2) It also makes the cathode resistant to gas contamination.3) In the present work, a basic impregnated cathode is coated with a thin tungsten film containing a certain amount of Sc2W3O12 (about 5–10 atomic percent) to produce a (W-Sc2W3O12) coated cathode.
The supplying mechanism of free Sc atoms, and hence the monoatomic-order layer formation mechanism,4) is clarified by Auger electron analysis and X-ray diffraction analyses. Free Sc atoms are produced through the following surface chemical reaction between Sc2W3O12 and Ba atoms supplied from the substrate4) as
Sc2W3O12+3Ba=3BaWO4+2Sc.
This monoatomic layer formation improves the electron emission current density measured at 850°Cb (brightness temperature) about threefold compared to that of conventional Os coated impregnated cathodes.
A new cathode has been developed which shows similar electron emission characteristics as a previously reported Sc2O3 mixed matrix impregnated cathode (Sc2O3 MM Cathode). Contrary to the Sc2O3 MM cathode, the new cathode is resistive to prolonged heating at high temperatures and to ion bombardment. This has been made possible by applying to a standard impregnated cathode a tungsten thin-film containing about 5 weight percent Sc2O3. The electron-emission property is found to be strongly linked to the surface atom composition as well as to the distribution of surface atoms.
Activity of the base metal is evaluated by identifying reaction products with the oxide layer, measuring Ba vapor pressure from the oxide, measuring of emission current properties, and by analyzing the surface composition of the oxide. Reducing capabilities of Zr, W, and Mo activators in Ni base metal are determined. Their activators are alloyed to the Ni core metal up to the solubility limits. The experimental results are evaluated systematically from the standpoint of base metal activity, together with the thermochemical treatment for the various reactions and the emission mechanism of the oxide coated cathode.
A simple formula, based on the classical impulse approxlmation, is given for the cross section for electron detachment collisions between fast Hions and neutral atoms or molecules T h e utility of the formula IS demonstrated by comparisons of calculated and measured cross sections.
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