Sadanori Taguchi

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

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.

Electron emission properties and surface atom behavior of an impregnated cathode coated with tungsten thin film containing Sc2O3

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.

Impregnated cathode coated with tungsten thin film containing Sc2O3

An impregnated cathode of a novel structure is proposed, fabricated, and evaluated. A thin tungsten film 100–400 nm in thickness containing various amounts of Sc2O3 is coated on a standard impregnated cathode composed of a porous tungsten body in which electron emissive materials are impregnated. The electron emission property measured with a diode configuration is found to be dependent on Sc2O3 content and surface atom distribution. Surface atom distribution is depicted by means of Auger electron spectroscopy. For high electron emission enhancement it is necessary for Sc2O3 content to be 2.5–6.5 wt. % and for a layer of the order of a monolayer in thickness composed of Ba, Sc, and O to develop on the cathode surface.

Some fundamental properties of Sc2O3 mixed matrix impregnated cathodes

Abstract Some fundamental properties of Sc 2 O 3 mixed matrix impregnated cathodes are studied by means of electron emission measurements and surface analysis is performed by Auger electron spectroscopy. The powders of W and Sc 2 O 3 are mixed and sintered in vacuum to form a porous body in which Ba-Ca aluminate (4BaO·CaO·Al 2 O 3 0 is impregnated. The saturation current density is 10 A/cm 2 at 850–900°C (brightness temperature) for 1–13 wt% Sc 2 O 3 mixed matrix cathodes. This high current density is due to low work function patches distributed over the cathode surface, which is composed of Ba, Sc and O in a certain ratio.

Investigation of Sc 2 O 3 mixed-matrix Ba—Ca aluminate-impregnated cathodes

The Sc<inf>2</inf>O<inf>3</inf>mixed-matrix Ba-Ca aluminate-impregated cathodes are fabricated and electron emission property as well as Ba evaporation rate is evaluated. The saturation current density J<inf>s</inf>= 10 A/cm²can be obtained at 840°C of brightness temperature for Sc<inf>2</inf>O<inf>3</inf>content of 1-13 wt %. Emission anomaly is observed at low applied electric field strength, which is caused by the effective electric field reduction due to patch field. From surface analysis by Auger electron spectroscopy, the patch field is found to be due to imhomogeneous distribution of a surface layer composed of Ba, Sc, and O.

Study of metal film coating on Sc2O3 mixed matrix impregnated cathodes

Abstract Electron emission and surface properties of the Sc 2 O 3 mixed matrix impregnated cathode coated with Ir, Os, Pt and Mo are studied. The cathode is composed of porous body of a metal matrix (5 wt% Sc 2 O 3 in W) and an impregnant of 4BaO·CaO·Al 2 3 , on which metal films of about 500 nm are evaporated. Sc atoms do not appear on the film surface even after prolonged heating at 1150°C, and the coated cathodes do not show the emission anomaly usually seen for Sc 2 O 3 mixed matrix cathodes. The Mo film, however, alloys easily with W in a short heat treatment time and the Sc atoms are distributed nonuniformly over the surface. The Mo coated cathode does show the emission anomaly. A simple coating with Sc film on the ordinary W matrix cathode reveals neither the surface properties nor the emission properties of Sc 2 O 3 mixed matrix cathodes.

Emission life and surface analysis of barium-impregnated thermionic cathodes

Emission life of small Ba‐impregnated thermionic cathodes suitable for use in TV tubes is investigated systematically. Cathodes are covered with an Os layer to enhance the emissivity. They have an unusual emission life in that the emission current decreases gradually with time and then drops abruptly. The early gradual decrease in emission at high cathode temperatures is caused by the phase change of the Os layer to OsW2. A linear relationship is found to exist between the logarithm of the lifetime and the reciprocal of the cathode temperature. This life end is believed to happen after the end of the reduction reaction between BaO and the porous W body and after ceasing replenishment of the low work function Ba+‐O− monolayer on the metal surface. A formula for the emission lifetime is derived from the assumption that the rate determining step is the Ba diffusion inside the pellet. Emission lifetime τ is shown to be given by τ=Ad2 exp(E/kT), where A is a constant, d is the pellet thickness, and E is the ac...

Nonuniform emission distribution on a scandate impregnated cathode

Abstract The effect of barium supply into a (W–Sc2W3O12)-coating film on the emission distribution of a scandate cathode has been investigated. Scandium concentration on the cathode surface was increased by activation heating. This increase was coincident with the rise in emission current by heating. However, when the film was coated on the unimpregnated tungsten pellet, the scandium concentration did not increase. These results indicate that the barium supply into the film is necessary for liberating scandium atoms from the scandium tungstate in the film and for forming a monolayer of Ba–Sc–O on the surface. We measured the heating-induced change in the scandium content of the film. The result showed that the scandium content decreased only in the part of the film that covered the impregnated areas. No decrease was observed in the film that covered tungsten metal. The liberation of scandium is considered to take place only in the film to which barium atoms are mainly supplied. This nonuniform scandium liberation in the film will cause nonuniformity in the Ba–Sc–O monolayer on the cathode surface, which in turn causes emission nonuniformity.

Electron emission enhancement of a (W−Sc2O3)coated impregnated cathode by oxidation of the coated thin film

A monoatomic order surface layer composed of Ba, Sc and O reduces the work function and, thus, enhances electron emission of a (W-Sc2O3)-coated impregnated cathode surface. The dependency of the electron emission property of the new impregnated cathode on the coated film property is evaluated by intentionally oxidizing the coated film during the film deposition process. The degree of oxidation is monitored by measuring the resistivity ratio (room temperature/liquid nitrogen temperature) of the coated film. It is found that the electron emission is enhanced when the resistivity ratio is in the range of 0.8 to 0.9. This leads to a novel electron emission enhancement model: Electron emission enhancement is caused by free Sc atoms produced by the reaction of Ba atoms with Sc2W3O12 which is formed as the result of the oxidation process and heat treatment followed.

Application of an impregnated cathode coated with W-Sc2O3 to a high current density electron gun

Abstract The electron emission properties of a previously proposed [1,2] W-Sc2O3-coated impregnated cathode are evaluated in a high current density electron gun. It is found that the cathode can be operated at a temperature 100–150°C lower than that of Os-coated impregnated cathodes usually operated at 1000°C for a beam average current density of ∼5 A/cm2. The superiority of this cathode increases when used in high resolution, high brightness electron guns where the electron extracting field is high at the cathode surface.