Formation Mechanism of a Monoatomic Order Surface Layer on a Sc-Type Impregnated Cathode

Abstract

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 Sc 2 W 3 O 12 (about 5–10 atomic percent) to produce a (W-Sc 2 W 3 O 12 ) 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 Sc 2 W 3 O 12 and Ba atoms supplied from the substrate 4) as Sc 2 W 3 O 12 +3Ba=3BaWO 4 +2Sc. This monoatomic layer formation improves the electron emission current density measured at 850°C b (brightness temperature) about threefold compared to that of conventional Os coated impregnated cathodes.