Tomio Yaguchi

Fluctuation-Free Electron Emission from Non-Formed Metal-Insulator-Metal (MIM) Cathodes Fabricated by Low Current Anodic Oxidation

Fluctuation-free electron emission is obtained from MIM (Al-Al2O3-Au) cathodes. The Al2O3 layer is fabricated by anodic oxidation with a reduced electrolysis current density, i.e., a reduced oxidation rate. The slow oxidation process improves the insulating effect of the Al2O3 layer, and enables the MIM cathodes to operate in the non-formed state. The fluctuation-free emission is reproducible when the diode voltage is cut off instantaneously. With a thin Al2O3 layer, the diode voltage reguired for the cathode operation is reduced to values slightly above the work function of the top electrode.

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.

Reducing electron energy dispersion of nonformed metal–insulator–metal electron emitters using the near‐threshold drive method

The energy distribution of electrons emitted from an Al/Al2O3/Au metal–insulator–metal (MIM) electron emitter is measured. The thickness of the insulator is 5.5 nm. The energy distribution becomes narrower as the operating voltage Vd decreases since the low energy tail of the distribution is cut off by the potential barrier of the surface work function φ of the emitter. When the emitter is operated in the nonformed state, ΔE, the full width at half‐maximum of the distribution, is 0.32 eV for Vd=5.0 V, which is slightly above φ of Au (4.7 eV). As Vd increases, the high‐energy tail of the distribution broadens whereas the shape of the low‐energy tail remains unchanged. For a formed MIM emitter, ΔE becomes broader by 0.15–0.2 eV more than ΔE of a nonformed emitter at each Vd; thus, operation in the nonformed state is essential to obtain good monochromaticity. The spatial distribution of the work function in the emitter surface is also measured by the retarding potential method. The variation of φ, which limi...

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...

Field emission from flat metal surfaces covered with Ba atoms

Abstract The work function and micro-roughness of Fe–42Ni alloy under oxidation are measured for various degrees of Ba deposition, in order to understand the field emission process on the Fe–42Ni alloy. It is found that surface roughness under Ba deposition is suppressed by oxidation treatment around 200°C. Surface roughness is enhanced by oxidation at 400°C. Sample oxidation does not suppress work function reduction caused by Ba deposition.

Work Function Measurements of (W-Sc2W3O12)-Coated Impregnated Cathode by Retarding Potential Method Utilizing Titaniated W(100) Field Emitter

The work function of a (W-Sc2W3O12)-coated impregnated cathode, known as a high current density cathode operable at low temperature, is measured for the first time by the retarding potential method utilizing a titaniated W(100) field emitter as an electron source of high current stability and high brightness. A gradual change in the work function of the cathode is depicted during the activation procedure. Activation is completed after 2 h heating at 1150°Cb, when the work function becomes 1.15 (-0.02, +0.112) eV.

In Situ Transmission Electron Microscope Observation of Carbon Nanotubes in Electric Fields

Transmission electron microscope is used to examine the movements of carbon nanotubes in electric fields. Carbon nanotubes lying along the surface of the cathode electrode start to move into alignment with the electric field vector when the field strength reaches 0.5 V/µm and become increasingly well-aligned with the vector as field strength increases. The carbon nanotubes return to their original positions when the electric field strength returns to zero. We also examine the abrupt breakdown of carbon nanotubes when the electric field is maintained at 5.5 V/µm. The corresponding breakdown emission current density is estimated as 3.4×107 A/cm2. The distance between the nearest nanotubes standing to align with the electric field vector is approximately 2 µm. This fact means that emission site density could be increased up to 3×107 points/cm2 (which corresponds to one tube for each 2 µm square).

Absolute work function measurements with the retarding potential method utilizing a field emission electron source

Abstract The absolute work function of a scandate cathode is measured by a newly developed retarding potential method utilizing a titaniated W(100) field electron emission source reference. The electron trajectories in the lens system are computed to determine the conditions of the parallel beam impinging on the sample. The experimental results show a good agreement with the computations. The extremely low work function (∼ 1.2 eV) of the scandate cathode is obtained for the first time with this method. A systematic work function drift is observed during repeated work function measurements, suggesting the possibility of surface atom movement caused by the very low-energy electron beam irradiation of 1 to 2 eV.

Work function and microstructure of a monoatomic order surface layer grown on a (WSc2W3O12)-coated impregnated cathode

Abstract Activation processes of a (WSc2W3O12)-coated impregnated cathode are clarified by analysing the change in coated film structure before and after activation, the change in atom concentration on the cathode surface as well as the work function of the cathode during activation processes, and the activation time required to attain electron emission enhancement. It is found that there are three steps necessary to complete the activation of the cathode; (1) separation of Sc2W3O12 from W in the coated film; (2) production of free Sc atoms as a result of the reaction between Sc2W3O12 and Ba atoms from the substrate cathode; and (3) formation of an ordered monoatomic surface layer composed of Ba, Sc and O. It is also found that the overall activation energy is about 4.6 eV, and the work function after activation is about 1.15 eV.

Possibility of atom movement by very low energy electron beam irradiation

The change in the work function of a (W-Sc2W3O12) coated impregnated cathode is observed during the course of work function measurements by means of the retarding potential method utilizing a titaniated W(100) field emitter electron source. The change in work function is negligibly small at the work function minimum but it increases as the final steady state work function value increases. This paper discusses how low energy electron beam irradiation on the order of 2–3 eV, which reduces the effective coverage of the surface monolayer on the cathode, increases the work function.