JPS6237842A - Indirectly-heated cathode heater - Google Patents

Abstract

PURPOSE:To enhance the withstanding properties against voltage and thermal stress, by coating the surface of a tungsten coil with a sintered alumina material in which high-purity spherical alumina grains having two prescribed different diameters are contained at a prescribed ratio. CONSTITUTION:100 weight parts of high-purity alumina grains, which are made spherical by a hydrothermal crystallization method or the like and have a mean diameter of 1-5mum, and weight parts of high-purity alumina grains, which are made spherical by the method or the like and have a mean diameter equal to 1/10-1/20 of that of the former, are mixed together. A sintered alumina material in which the alumina grains are contained at a filling ratio of 50-70% is manufactured to coat a tungsten coil. Even if the sintered alumina material is repeatedly heated to a temperature of 1,600 deg.C or more, the thermal stress thereof is reduced and the high-purity alumina grains are unlikely to excessively diffuse. Therefore, the generation of a crack in the sintered alumina material at the time of heating thereof is suppressed. The withstanding properties against voltage and thermal stress are thus enhanced.

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to an indirectly heated cathode heater (for example, for a cathode ray tube), and particularly to an indirectly heated cathode heater (for example, for a cathode ray tube), in which an improved alumina sintered coating on the surface of a tungsten coil constituting the heater T is used. The present invention relates to type cathode heaters.

[Background of the invention]

Conventional heaters for the indirectly heated cathodes of cathode ray tubes, for example, are usually made entirely of tungsten wire, and the surface is coated with a sintered coating material mainly composed of alumina for electrical insulation. In order to improve performance, it is necessary to heat the cathode to a maximum of about 1000°C or higher, and therefore the tungsten heater itself needs to be heated to 1600°C to 1800°C. However, this causes a thermal stress load on the surface layer of the heater, which causes cracks to occur and propagate in the alumina sintered covering material, causing the alumina layer to peel off, causing contact between the heater and the heater cover, and causing insulation. The phenomenon of causing defects is rare. Known example (% opening 57-433
37, 55-41694'), a method is adopted in which each part of the heater coil is coated with alumina particles having different particle sizes and particle size distributions to alleviate thermal stress.
It has been disclosed to use a mixture of 0 and 900 mesh alumina, but since the particle size of alumina is too large and crushed grains are used, it is difficult to control the filling rate within an appropriate range. There was a problem in that the withstand voltage could not be further improved.

[Purpose of the invention]

Therefore, an object of the present invention is to provide an indirectly heated cathode heater in which the dielectric strength characteristics and thermal stress load characteristics of the AlZ sintered body as an insulating material covering the surface of the indirectly heated cathode heater are improved. .

[Summary of the invention]

In order to achieve the above object, the present invention is characterized by using high-purity spherical aluminum particles on the surface of the tungsten coil of an indirect cathode heater, and controlling the filling rate of the alumina particles in the range of 50 to 70 particles. At the same time, the particles were coated with a sintered alumina coating to ensure uniform particle gaps.

[Embodiments of the invention]

FIG. 1 schematically shows an image observed by a scanning electron microscope of a cross section of an alumina coating layer (before sintering) on the surface of an indirectly heated cathode heater according to an embodiment of the present invention.

In this example, the alumina constituting the alumina coating layer was made of high-purity alkylene particles with two types of average particle sizes: (1) average particle size of 2.0 μm s, and (2) average particle size of 0.2 μm. . The particles (1) and (2) were each approximately spherical. The ratio of the particles is 100 by weight of the particles of (1) and 10 of the particles of (2).
Assuming that the alumina particles of 1) are packed in a dense hexagonal shape, it is equal to the filling rate when the alumina particles of (2) with a small average particle size are filled into the voids. The reason for this combination is that the filling rate when coating the tungsten surface with alumina is in the range of 50 to 70 inches as described later. In order to obtain such a result, the alumina particles in (1) and (2) above are thoroughly mixed and dispersed, and alumina particles are applied to the surface of a tungsten wire using an electrophoretic method using an alcoholic solution. was coated. the result,
As shown in FIG. 1, Al2 particles 2 with an average particle size of 0.2 μm entered the gaps between the alumina particles 1 with an average particle size of 2.0 μm, resulting in a friendly arrangement.

The alumina-coated fully sintered friend formed as described above. The number of cracks and the crack length appearing on the surface of this alumina sintered body are as follows: (1) average grain size, 2. θ
μ・Moshikke (2) When comparing the case where alumina particles with an average particle size of 0.2β・ were used alone, the results were as shown in Figure 2. The vertical axis shows the results of microscopic observation of the alumina sintered coating layer surface on the surface of an indirectly heated cathode heater obtained by sintering at 1600°C for 1 hour in a hydrogen stream. The average value of the observed crack lengths, and the horizontal axis also shows the average number of cracks.

Curve A in the figure is a sintered body made by coating and sintering only alumina particles with an average particle size of 2.0 μm, and curve B is the same 0.2 μm.
Curve C, which is a sintered body formed by coating and sintering only alumina particles of m, is obtained by coating and sintering mixed particles with a mixture ratio of particles of 2.0 μm and 0.2 μm at a mixing ratio of 'Ito:t' according to this example. These results relate to a sintered body made of゛According to this example, the number of cracks per unit area is more streaky than the sintered body formed by sintering and coating alumina particles with an average particle size of 0.2 μm, but overall the crack length is shorter, Advantages are recognized.

As another example of the present invention, (1) a sintered body was prepared by combining particles with an average particle size of 4.5 in the same manner as described above, and (2) mixing particles with an average particle size of 1.5 at a mixing ratio of 10. An aluminum sintered body was also prepared by mixing the mixture at a ratio of 1/1. The particles used were all spherical high-purity alumina particles. These alumina sintered coatings were tested in the same manner as above, and the results were arranged in the same way as shown in Figure 2.
Figure 3 was obtained. Note that FIG. 3 also shows the contents of FIG. 2. This figure also confirms the effectiveness of the present invention.

In addition, the purity of the high-purity alumina used is Na<o. 0 1%, 8102<0.0 1
'1.

F.20, <0.001%, and the concentration of other impurity components was less than a trace. The analysis results of the aluminum coating made using the same alumina showed that the value after firing was only a trace of NazO, 8102 = 0. 02%, Fe12
, -0,005%, Mg0=0,008chi. Here, MgO is based on Mg (NOIS) added to the electrophoresis solution as an electrolyte component.

In the present invention, high purity alumina is desired as shown in FIG. FIG. 4 focuses on the impurity S tO2 and shows the relationship between the ratio of 5102 mixed to 20.100 parts of At and the heater breakage rate after 5000 hours of heat cycle test.

Note that spherical alumina particles can be produced by a "hydrothermal crystallization method." “Hydrothermal crystallization method” refers to H
This is a method of crystallizing an amorphous material such as Zoldal in the presence of 2O to obtain a crystalline material. The characteristics of the nine particles produced by the same method are: ■ Fine grains, ■ Well-developed crystallinity, ■ Narrow particle size distribution, ■ Little distortion, ■ Can be produced at relatively low temperatures, etc. .

FIG. 5 shows the relationship between the alumina filling rate and the alumina film thickness of a sintered body made of insulating aluminum, and FIG. 6 similarly shows the relationship between the withstand voltage and the alumina film thickness.

The data at each point indicated by each symbol in these figures is as shown in the following table.

From these figures, if the alumina filling rate is less than 50-, the withstand voltage will not be high enough, and conversely, if the alumina particle filling rate reaches 70- or more, cracks will occur during heating with the heater, and the withstand voltage will not be high enough. It can be seen that the voltage decreases. Therefore, it can be seen that in order to maintain a high withstand voltage and suppress the occurrence of cracks, it is necessary to set the filling rate of alumina particles in the range of 50 to 70 inches. By the way, in the case of a mixed coating of crushed alumina particles of 900 mesh (average particle size 3.5 μm) and 500 mesh & C (average particle size 6 μm) seen in the previously mentioned known example, the filling rate of the aluminum can reach 50 mesh as shown in the figure. [Effects of the Invention] According to the present invention, the conventional alumina sintered body (for example, 500
Compared to the sintered body made of crushed aluminized particles with a grain size of 90.0 and 90.0, the filling rate of alumina is within an appropriate range, which means that when repeatedly heated to 1600°C or higher, It acts in the direction of relieving the thermal stress load against the thermal expansion of
It is possible to suppress the length and number of cracks that occur due to repeated heating due to excessive interdiffusion between particles, and the overall effect of these is to lower the withstand voltage of indirect cathode heaters to a level higher than that of conventional cathode heaters. This can be further improved.

[Brief explanation of the drawings] Fig. 1 is a schematic vertical cross-sectional view showing the state of coating of high-purity alumina particles on the surface of a tungsten coil according to an embodiment of the present invention, and Figs. Figure 4 is an experimental diagram showing the relationship between the crack length and number of cracks in the compact. Figure 4 is an experimental diagram showing the relationship between the content of 810□ as an impurity and the heater rupture rate. Figure 5 is the alumina film of the alumina sintered body. FIG. 6 is a diagram of experimental results showing the relationship between thickness and aluminum filling rate, and FIG. 6 is a diagram of experimental results showing the relationship between alumina film thickness and withstand voltage. 1... Alumina particles with an average particle size of 2.0 μm 2... Alumina particles with an average particle size of 0.2 μm 3... Tungsten surface 'aC' = θb (%) Item 1 "-1

Claims (2)

[Claims] (1) An indirectly heated cathode heater characterized in that the surface of a tungsten coil is covered with an alumina sintered body made of spherical high-purity alumina particles with a filling rate of 50% to 70%. (2) The alumina particles have an average particle size of 1 μm or more and 5 μm or less, and 1/10 to 1/10 of the above average particle size.
2. The indirectly heated cathode heater according to claim 1, comprising a mixture of two types of particles: particles having an average particle size of /20.