JPH0654633B2 - Indirect heating type cathode heater - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an indirectly heated cathode heater (for example, for a cathode ray tube), and more particularly, an indirectly heated cathode having an improved alumina sintered coating on the surface of a tungsten coil constituting the heater. Regarding the heater.

BACKGROUND OF THE INVENTION Conventional heaters for indirectly heated cathodes of cathode ray tubes, for example, are usually formed by using a tungsten wire as a heater and applying a sintered coating material containing alumina as a main component for electrical insulation. However, in order to improve the performance of the cathode ray tube, it is necessary to heat the cathode up to about 1000 ° C. or higher, and therefore the tungsten heater itself needs to be heated to 1600 ° C. to 1800 ° C. However, due to this, a thermal stress load is generated on the heater surface layer, cracks are generated and propagate in the alumina that is the sintered coating material, and as a result, the alumina layer peels off and the heater and the heater cover come into contact with each other, resulting in poor insulation. There was a phenomenon that caused. Known example (JP-A-57-43)
337, 55-41694), a method of relaxing thermal stress by coating alumina particles having different particle sizes and particle size distributions on each part of the coil of the heater is adopted, and a mixture of 500 mesh and 900 mesh alumina is adopted. Although it is disclosed that the particle size of alumina is too large and crushed particles are used, it cannot be controlled within an appropriate filling rate range, and the withstand voltage cannot be improved differently. There was a problem.

[Object of the Invention]

Therefore, it is an object of the present invention to provide an indirectly heated cathode heater in which an alumina sintered body as an insulating material for covering the surface of the indirectly heated cathode heater has improved withstand voltage characteristics and heat stress load characteristics.

[Outline of Invention]

The indirectly heated cathode heater according to the present invention is characterized in that in order to achieve the above object, spherical high-purity alumina particles, which are small particles having an average particle size of 1 μm or more and 5 μm or less,
Filling ratio 5 made from a mixture of two kinds of particles with fine particles having an average particle size of 1/10 to 1/20 of the particle size of the small particles 5
The surface of the tungsten coil is coated with 0% to 70% of alumina sintered body.

Example of Invention

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

In the present embodiment, the alumina constituting the alumina coating layer has (1) an average particle size of 2.0 μm, and (2) an average particle size of 0.2 μm.
High-purity alumina particles having different average particle sizes were used. (1)
Each of the particles in (2) and (2) had a substantially spherical shape. The mixing ratio of the particles was such that the particles of (1) were 100 and the particles of (2) were 10 by weight. The above blending ratio is based on the assumption that the alumina particles having a large average particle size (1) are packed in a close-packed hexagonal shape, and when the alumina particles having a small average particle size (2) are packed into the voids, the packing is performed. Equal to the rate. The reason for using such a composition is that the filling rate when coating the surface of the tungsten with alumina is 50 to 50% as described later.
It is in the range of 70%. Then, in order to obtain such a result, the above-mentioned (1) and (2) alumina particles were sufficiently mixed and dispersed, and an alcohol solution was used to disperse the alumina particles on the surface of the tungsten wire by electrophoresis. Was coated. As a result, as shown in FIG. 1, the average particle size is 2.0 μm.
An array was obtained in which alumina particles 2 having an average particle diameter of 0.2 μm entered the mutual gaps of the alumina particles 1.

The alumina coating formed as described above was sintered. The number of cracks and the crack length appearing on the surface of the alumina sintered body are the same as in the case of this example, and (1) the average particle size of 2.0 μm or (2) the alumina particles of the average particle size of 0.2 μm are used alone. When compared with, it was as shown in FIG.

That is, FIG. 2 shows the results of microscopic observation of the surface of the alumina sintered coating layer on the surface of the indirectly heated cathode heater obtained by sintering the alumina coating at 1600 ° C. for 1 hour in a hydrogen stream. The vertical axis represents the average value of crack lengths observed in a 100 μm square visual field, and the horizontal axis represents the average number of cracks. Curve A in the figure is a sintered body obtained by coating and sintering only alumina particles having an average particle size of 2.0 μm, and curve B is 0.2 μm.
The curve C is obtained by coating and sintering mixed particles having a mixing ratio of particles of 2.0 μm and 0.2 μm of 10: 1 according to the present embodiment. The results are for the sintered body.

According to this example, the number of cracks per unit area is larger than that of the sintered body obtained by sintering and coating alumina particles having an average particle size of 0.2 μm, but the crack length is short as a whole, which is advantageous. Is recognized.

As another embodiment of the present invention, the average particle size is 4.5 μm particles /
Particles having an average particle size of 0.6 μm were mixed at a mixing ratio of 10/1 to prepare an alumina sintered body in the same manner as described above, and particles having an average particle size of 1.5 μm / particles having an average particle size of 0.1 μm were mixed at a ratio of 10/1.
Then, an alumina sintered body was similarly prepared by mixing. All the particles used are spherical high-purity alumina particles. Test the same as above for these alumina sintered coatings,
As a result of the same arrangement as in FIG. 2, FIG. 3 was obtained. The contents of FIG. 2 are also shown in FIG. The effectiveness of the present invention is also confirmed by this figure.

The purity of the high-purity alumina used is
Na ₂ O <0.01%, SiO ₂ <0.01%, Fe ₂ O ₃ <0.001%, and other impurity component concentrations were below the trace. The analysis results of the alumina coating made using the same alumina show that after firing, Na ₂ O is almost trace, SiO ₂ = 0.02%, Fe ₂ O ₃ = 0.005.
%, MgO = 0.008%. Here, MgO is based on Mg (NO ₃ ) ₂ added to the electrophoretic solution as an electrolyte component.

In the present invention, high purity alumina is desired as shown in FIG. In Fig. 4, attention is paid to SiO ₂ as an impurity, and Al ₂ O
₃ shows the relationship between the ratio of SiO ₂ mixed with 100 parts and the heater breakage rate after 5000 hours of the heat cycle test.

The spherical alumina particles can be manufactured by the "hydrothermal crystallization method". "Hydrothermal crystallization method" means H ₂ under high temperature and high pressure.
In this method, an amorphous substance such as sol-gel is crystallized in the presence of O to obtain a crystalline substance. The characteristics of the particles produced by this method are that they are fine particles, that they are well-developed crystalline, that the width of the particle size distribution is narrow, that there is little distortion, that they can be produced at relatively low temperatures.

FIG. 5 shows the relationship between the alumina film thickness and the alumina filling rate of the insulating alumina sintered body, and FIG. 6 shows the relationship between the withstand voltage and the alumina film thickness. The data of each point shown by each symbol in these figures is as shown in the following table.

From these figures, if the alumina filling rate is 50% or less, the withstand voltage will not be sufficiently high, and conversely, if the alumina particle filling rate reaches 70% or more, cracks will occur during heating of the heater, and the withstand voltage will also increase. It can be seen that it will decrease. Therefore, 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 within the range of 50 to 70%. Incidentally, in the case of the mixed coating of crushed alumina particles of 900 mesh (average particle size 3.5 μm) and 500 mesh (average particle size 6 μm) found in the above-mentioned known example, the alumina filling rate does not reach 50% as shown in the figure, The withstand voltage did not reach a satisfactory value.

〔The invention's effect〕

According to the present invention, a conventional alumina sintered body (for example, 500
Compared to a sintered body composed of crushed alumina particles having a mesh size and a 900 mesh particle size), the alumina filling rate is in an appropriate range, which means that thermal stress load against thermal expansion when repeatedly heated to 1600 ° C or higher. , And by using high-purity alumina particles, it is difficult for excessive mutual diffusion between particles to proceed, and it is possible to suppress the crack length and number that occur when repeatedly heated. As a result, the withstand voltage of the indirectly heated cathode heater can be further improved as a total effect.

[Brief description of drawings]

FIG. 1 is a schematic vertical cross-sectional view showing the coating state of high-purity alumina particles coated on the surface of a tungsten coil according to an embodiment of the present invention, and FIGS. 2 and 3 show the crack length and the number of cracks of an alumina sintered body. Fig. 4 is a diagram of experimental results showing the relationship, Fig. 4 is an experimental diagram showing the relationship between the content of impurities SiO ₂ and the heater breakage rate, and Fig. 5 is a relationship between the alumina film thickness of the alumina sintered body and the alumina filling rate. FIG. 6 is a diagram of the experimental results and FIG. 6 is a diagram of the experimental results showing the relationship between the alumina film thickness and the 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

Claims (1)

[Claims] 1. High-purity alumina particles having a spherical shape, and small particles having an average particle size of 1 μm or more and 5 μm or less,
Filling ratio 5 made from a mixture of two kinds of particles with fine particles having an average particle size of 1/10 to 1/20 of the particle size of the small particles 5
An indirectly heated cathode heater characterized in that the surface of a tungsten coil is covered with an alumina sintered body of 0% or more and 70% or less.