Electric lamp and discharge devices – Electrode and shield structures – Cathodes containing and/or coated with electron emissive...
Patent
1996-10-18
1999-09-28
Patel, Nimeshkumar D.
Electric lamp and discharge devices
Electrode and shield structures
Cathodes containing and/or coated with electron emissive...
313409, 313411, 313337, H01J 114, H01J 1906, H01J 120, H01K 104
Patent
active
059593954
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to cathodes for electron tubes used for a cathode ray tube (CRT), etc., and relates in particular to improvement of the emitter thereof.
BACKGROUND ART
Conventionally, cathodes for electron tubes, which comprise a base mainly comprising nickel and including a reducing element such as silicon and magnesium coated with alkaline-earth metal carbonate crystalline particles and thermally decomposed in a vacuum to generate an emitter mainly comprising an alkaline-earth metal oxide, have been used broadly.
Scanning electron microscope images illustrating the shapes of representative alkaline-earth metal carbonate crystalline particles used for an emitter of cathodes conventionally used for electron tubes are shown in FIG. 8-FIG. 10. Various shapes of the alkaline-earth metal carbonate crystalline particles are known such as spherical represented by FIG. 8, dendritic represented by FIG. 9, and bar-like represented by FIG. 10. In coating these on the cathode base, an aggregate of crystalline particles having the same shape, namely, only spherical particles or only dendritic particles (JP-A-3-280322) has been used. The "same shape" herein denotes the shape of crystalline particles obtained under the same synthetic conditions, and thus strictly speaking, individual crystalline particles may have slight variations in size or shape, but the shape of one kind by a geometric classification is suggested.
When the above mentioned emitter mainly comprising an alkaline-earth metal oxide produced by coating the cathode base with an alkaline-earth metal carbonate and thermally decomposing in a vacuum is used as a cathode for a CRT, since the emitter is maintained at a temperature around 700.degree. C. in a usual CRT operation state, a problem occurs in that the entire emitter gradually has thermal shrinkage as time passes. The thermal shrinkage triggers the gradual drift of the cut-off voltage to cut off the emission (hereinafter called cut-off drift). The amount of the cut-off drift (hereinafter called cut-off drift amount) varies depending upon the shape of the crystalline particles of the above mentioned alkaline-earth metal carbonate; and the cut-off drift amount is smaller in the dendritic than in the bar-like, and smaller in the spherical than in the dendritic. However, on the other hand, the emission characteristic varies depending upon the above mentioned shape; and the emission characteristic is better in the dendritic than in the spherical, and better in the bar-like than in the dendritic.
An example of the emitter mainly comprising an alkaline-earth metal oxide generated by using a cathode base mainly comprising nickel and including 0.1 weight % of magnesium and 0.05 weight % of aluminum with respect to the base weight as the reducing elements, and using an alkaline-earth metal carbonate containing barium and strontium in the composition ratio (molar ratio) of 1:1 as the above mentioned alkaline-earth metal component, and further adding 3 weight % of scandium oxide as the rare earth metal oxide into the alkaline-earth metal carbonate so as to improve the emission characteristic, coating the above mentioned base with the composition at a thickness of approximately 50 .mu.m, and thermally decomposing in a vacuum (a high vacuum of 10.sup.-6 Torr or less herein) at about 930.degree. C. is shown in FIG. 11 regarding the state of the cut-off drift with respect to the operation time, and shown in FIG. 12 regarding the saturation current remaining ratio, an indicator of the emission characteristics when used as the cathode of a CRT. The saturation current remaining ratio is the normalized value of the saturation current with respect to the operation time based on the initial value of the saturation current as 1 (the ratio of the saturation current with respect to the operation time in the case of setting the initial value of the saturation current as 1), and it can be said that the larger the saturation current remaining ratio, the better the emission characteristic. The operation
Hayashida Yoshiki
Kubo Masayuki
Sakurai Hiroshi
Yamashita Katsuyuki
Haynes Mack
Matsushita Electronics Corporation
Patel Nimeshkumar D.
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