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 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.

High current density Sc2O3-W matrix dispenser cathode

“…The calculation denotes that the theoretical results match with the experimental data tested in both Pierce electron gun and in a closed-spaced diode when the Wright and Wood's semiconductor model combined with the Schottky effect is applied with a doping level of 10 15 electrons/cm 3 . Therefore, from this analysis, we conclude that emission from Sc 2 O 3 -W matrix dispenser cathodes (and other Scandate cathodes) is related to a semiconductor structure instead of a monolayer model as suggested by Hasker et al [5] and Yamamoto et al [17]. This proposal was substantiated by applying the Longo-Vaughan (L-V) relations to the Scandate cathodes.…”

“…Since the Ba-type cathode has a mono-atomic layer surface structure, it is clear that both theories can be used for describing such a monolayer emission model. Hasker and Yamamoto [5,17] used the monolayer model to explain the emission property of the scandate cathode. However, we find that the active substance layer formed on the cathode surface is not a monolayer.…”

A review on scandia doped tungsten matrix scandate cathode

“…The performances of the SD and SG are similar to that of the top-layer cathode prepared by Gärtner et al [14]. But Vancil et al [10,41] and Yamamoto et al [42] found that the cathode prepared with the powder by LS method also could provide a highemission property.…”

Near-Surface Material Phases and Microstructure of Scandate Cathodes

“…Sc has a stronger affinity for O than does Ba, which means that Ba cannot reduce Sc 2 O 3 to Sc directly [ 9 ]. In a cathode study by Yamamoto et al [ 35 ], it was proposed that metallic Sc can be produced via a chemical reaction between Sc 2 W 3 O 12 and Ba, as described in Equation (1), which was based on their work on (W–Sc 2 O 3 )-coated [ 42 ] and (W–Sc 2 W 3 O 12 )-coated impregnated cathodes [ 35 ]. Sc 2 W 3 O 12 + 3Ba = 3BaWO 4 + 2Sc …”

“…This scandate cathode variant is a powder metallurgy (P/M) porous tungsten plug fabricated with nanosized scandia-doped tungsten powder and impregnated with barium calcium aluminate (in a specific molar ratio) prior to activation at the proper temperature. In order to enable commercialization and application of scandate cathodes, however, several issues must first be addressed, including emission uniformity, poor reproducibility, and an incomplete understanding of the mechanisms that govern emission [ 7 , 16 , 19 , 27 , 32 , 33 , 34 , 35 ].…”