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 conditions in FIG. 11 and FIG. 12 are that the voltage of the heater to heat the cathode is operated at a 10% increased rate with respect to the ordinary use condition to accelerate the change with the passage of time, the so-called examination results under the accelerated conditions.
"a", "b", "c" in FIG. 11 and FIG. 12 denote the results when the alkaline-earth metal carbonate crystalline particles of the spherical form having an average diameter of 0.7 .mu.m, the dendritic form having an average length of 5 .mu.m, and the bar-like form having an average length of 7 .mu.m illustrated in FIG. 8, FIG. 9, FIG. 10 respectively are used as the material. The length of the dendritic crystals is the length between the edge of the trunk to the farthest edge of the branch on the opposite side.
From these FIGS., the tendency that one having a comparatively small cut-off drift amount does not have good emission characteristic and one having comparatively good emission characteristic has a large cut-off drift amount can be read. Thus it can be learned that by merely selecting the above mentioned shape of the crystalline particles the improvement of both the cut-off drift and the emission characteristic at the same time is difficult.
The object of the present invention is to solve the problem in the above mentioned conventional example to provide a cathode for electron tube improved both in the cut-off drift and in the emission characteristic of the cathode for electron tube.