JP2003234058A - Heater for cathode and its manufacturing method, electron gun and cathode-ray tube equipped with the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the reduction of darkening degree of a darkened layer formed on a heater for heating a cathode. <P>SOLUTION: In the heater 1 for heating the cathode, the surface of a coiled metal wire 5 is coated with an insulation layer 3 composed of alumina film, the surface of the insulation layer 3 coated with the darkened layer 4 composed of tungusten and alumina. The surface of the darkened layer 4 is then coated with an oxidation-resistant protection layer 6 composed of alumina film, wherein an alumina sol is adhered to the surface of the darkened layer 4, and heat- sintered to remove its structural water, resulting in the oxidation-resistant protection layer 6 which suppresses the reduction of darkening degree of the darkened layer 4 by preventing oxidation of tungusten constituting the darkened layer 4. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heater for heating a cathode for heating a cathode used in an electron tube, a method for manufacturing the heater, an electron gun equipped with this heater, and a cathode ray tube. Specifically, by covering the blackening layer formed on the surface of the heater with an oxidation-resistant protective layer or by covering the particles constituting the blackening layer with an oxidation-resistant protective layer, oxidation of the blackening layer components is prevented, It is intended to suppress a decrease in the blackening degree of the oxide layer.

[0002]

2. Description of the Related Art In an electron tube such as a cathode ray tube, an indirectly heated cathode is generally used to radiate electrons. Therefore, as described in Japanese Patent Laid-Open No. 10-289645, the cathode heating is used. A heater is provided. In order to improve the performance of cathode ray tubes in recent years, it is necessary to increase the current density of the cathode, and the operating temperature of the heater tends to be set high.

Now, when the heater temperature rises, the dielectric strength of the insulating layer coated on the heater for insulation from the cathode decreases, and finally the insulating function is lost, and the function of the electron gun equipped with the cathode and this The probability of losing the function of the cathode ray tube provided with the electron gun is increased.

For this reason, a blackened layer mainly made of tungsten and alumina (aluminum oxide) is formed on the insulating layer of the heater for the purpose of improving the thermal efficiency and reducing the operating temperature. This technique is a useful element for increasing the reliability of not only a cathode called an impregnated cathode, which requires a high heater operating temperature, but also all indirectly heated cathodes, and it is now widely used.

FIG. 7 shows the above-mentioned Japanese Patent Laid-Open No. 10-289645.
7 is a flowchart showing a conventional method for manufacturing a heater shown in Japanese Patent Publication No. In the coil winding step of step 1, the metal wire is formed into a coil shape. In the insulating layer coating step of step 2, a film made of alumina is formed on the surface of the metal wire by an electrodeposition method or the like. In the blackened layer coating step of step 3, a mixed liquid of mainly tungsten and alumina is applied. In the sintering step of Step 5, the heater was heated and sintered to fix the insulating layer and the blackened layer, thus completing the heater.

[0006]

However, the tungsten forming the blackened layer easily reacts with oxygen in the air due to heat, so that the oxidation of the tungsten may proceed rapidly depending on the conditions for forming the cathode ray tube.

That is, when an electron gun equipped with a cathode having a heater is incorporated in a glass tube constituting a cathode ray tube, a glass member called a stem portion provided in the electron gun and a portion called a neck portion of the glass tube. Weld the glass. At this time, if welding is performed in air, the oxidation of tungsten may rapidly proceed.

During operation of the heater, the temperature of the heater becomes high under high vacuum, so that tungsten oxide evaporates and diffuses (vapor pressure higher than that of tungsten) to cause a decrease in blackening degree (blackness). When the degree of blackening of the blackened layer is lowered, the heater efficiency is lowered and the core temperature is increased. In addition, it is conceivable that when the heater is stopped, tungsten oxide is deposited on the components inside the cathode ray tube, so that the insulation characteristics are deteriorated.

On the other hand, under the conditions for producing the cathode ray tube, by substituting the atmosphere during welding with nitrogen, it becomes possible to suppress the oxidation of the tungsten forming the blackened layer to some extent. However, it is required for the heater body to enhance the oxidation resistance of the blackening layer and suppress the decrease in the blackening degree under any conditions.

The present invention has been made to solve the above problems, and is a cathode heating heater capable of suppressing a decrease in the blackening degree of a blackening layer, a method for manufacturing the same, and an electronic device equipped with this heater. It is intended to provide a gun and a cathode ray tube.

[0011]

In order to solve the above-mentioned problems, the heater for cathode heating according to the present invention is a heater for cathode heating in which at least the surface of a metal wire that generates heat when energized is covered with at least a blackening layer. And an oxidation resistant protective layer for covering the oxidization layer.

In the cathode heating heater according to the present invention, by covering the blackened layer with the oxidation-resistant protective layer, the blackened layer does not come into contact with air, moisture, etc., and therefore, the components constituting the blackened layer Can prevent the oxidation of. As a result, it is possible to suppress a decrease in the degree of blackening of the blackened layer. The oxidation resistant protective layer is preferably a film made of alumina, for example.

The heater for cathode heating according to the present invention is a heater for cathode heating in which at least the surface of a metal wire that generates heat when energized is covered with a blackening layer. In the blackening layer, the particles forming the blackening layer are acid resistant. It is covered with a chemical protection layer.

In the cathode heating heater according to the present invention, the particles constituting the blackening layer are covered with the oxidation-resistant protective layer, so that the components constituting the blackening layer do not come into contact with air, moisture or the like. Therefore, it is possible to prevent the components constituting the blackened layer from being oxidized. As a result, it is possible to suppress a decrease in the degree of blackening of the blackened layer. The oxidation resistant protective layer is preferably a film made of alumina, for example.

The method for producing a heater for cathode heating according to the present invention is the method for producing a heater for cathode heating in which at least the surface of a metal wire that generates heat when energized is covered with a blackening layer. By heat-sintering, the oxidation-resistant protective layer made of an alumina film is formed.

In the method of manufacturing the heater for cathode heating according to the present invention, the structural water is dehydrated by heating and sintering the alumina sol attached to the surface of the blackening layer, and the surface of the blackening layer is coated with the alumina film. It

Since the particles of the alumina sol are ultrafine particles, when the blackening layer is coated by, for example, the electrodeposition method or the dipping method, a very dense film having a thickness of several tens nm is formed on the surface of the blackening layer. It is possible to form the control. This makes it possible to avoid contact between the constituent components of the blackening layer and moisture / air when forming the oxidation resistant protective layer using alumina sol, and because of its thinness, blackening of the blackening layer Degradation can be minimized.

A method for manufacturing a heater for cathode heating according to the present invention is a method for manufacturing a heater for cathode heating in which at least a surface of a metal wire which generates heat when energized is covered with a blackening layer, and which contains a component constituting a blackening layer. By adding an alumina sol solution to the suspension, and adhering the mixed solution containing the alumina sol solution to a metal wire and sintering it by heating, an oxidation-resistant protective layer made of an alumina film is formed on the particles constituting the blackening layer. The formed blackening layer is formed.

In the method of manufacturing the heater for cathode heating according to the present invention, when the alumina sol solution is added to the suspension containing the components forming the blackening layer, the alumina sol adheres to the particles forming the blackening layer. Then, by heating and sintering the alumina sol, the structural water is dehydrated, a blackening layer is formed on the surface of the metal wire, and the particles forming the blackening layer are covered with the alumina film.

This makes it possible to avoid contact between the particles forming the blackening layer and moisture / air, and because of its thinness, the deterioration of the blackening degree of the blackening layer can be minimized. it can.

The electron gun according to the present invention is an electron gun having a cathode for a cathode heating heater in which at least the surface of a metal wire which generates heat when energized is covered with a blackening layer. It is covered with an oxidation resistant protective layer.

In the electron gun according to the present invention, since the blackening layer of the heater for heating the cathode is covered with the oxidation resistant protective layer,
It is possible to prevent the components constituting the blackening layer from being oxidized and to prevent the blackening degree of the blackening layer from decreasing. As a result, the efficiency of the heater does not decrease and the temperature rise of the heater can be suppressed.

The cathode ray tube according to the present invention is a cathode ray tube in which an electron gun equipped with a cathode heating heater in which at least the surface of a metal wire which generates heat when energized is covered with a blackening layer is incorporated. Indicates that the blackened layer is covered with an oxidation resistant protective layer.

In the cathode ray tube according to the present invention, the blackening layer of the heater for heating the cathode is covered with the oxidation-resistant protective layer, so that the components constituting the blackening layer are prevented from being oxidized and the blackening layer is blackened. It is possible to suppress a decrease in degree. As a result, the efficiency of the heater does not decrease and the temperature rise of the heater can be suppressed, so that the reliability of the cathode ray tube can be improved.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a heater for heating a cathode of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory view of a heater for heating a cathode according to the present embodiment, and FIG.
Is a cross-sectional view of the cathode 2 provided with the heater 1, FIG.
FIG. 3 is an enlarged cross-sectional view of a portion A of FIG. Generally, an indirectly heated cathode 2 is used in an electron tube such as a cathode ray tube which will be described later. Therefore, a heater 1 for heating the cathode 2 is used.
Is provided.

In the heater 1 of this embodiment, the blackening layer 4 is formed by further covering the metal wire 5 covered with the insulating layer 3 and the blackening layer 4 with the oxidation-resistant protective layer 6. It prevents the oxidation of the component (tungsten). Note that FIG.
Cathode 2 shown in is a so-called impregnated cathode among the cathodes used in the cathode ray tube.

That is, the cathode 2 is obtained by inserting the heater 1 made of a coiled metal wire 5 into a sleeve 7 made of a cylindrical metal member, and a thermoelectron emitting material is provided on the top of the sleeve 7. A disk 8 impregnated with is provided.

With the above construction, the cathode 2 is heated by energizing the heater 1 to heat the sleeve 7, and the disk 8 impregnated with the thermionic emission material is kept at a high temperature, so that the cathode 2 has an electron emission function. Is obtained.

The metal wire 5 is made of, for example, a rhenium-tungsten alloy and has a double coil shape. Insulation layer 3
In order to prevent a leakage current from flowing between the metal wire 5 and the sleeve 7, an inorganic insulator such as an alumina film is formed on the surface of the metal wire 5.

The blackening layer 4 enhances the thermal efficiency of the heater 1,
Moreover, in order to reduce the operating temperature, a film made of, for example, tungsten and alumina is formed on the surface of the insulating layer 3. The oxidation resistant protective layer 6 is formed by forming, for example, a film of alumina on the surface of the blackened layer 4 in order to enhance the oxidation resistance of the blackened layer 4.

The oxidation-resistant protective layer 6 not only prevents contact of the blackening layer 4 with moisture and oxygen that cause oxidation of the tungsten constituting the blackening layer 4, but also lowers the efficiency of the heater 1. It must be dense and transparent or black as it must be. In this embodiment, alumina sol is used as a coating agent for forming a film that satisfies this condition.

Alumina sol is a colloidal solution of alumina hydrate (Al ₂ O ₃ .nH ₂ O) in terms of composition. Since the particles are ultrafine particles, the heater 1 is coated by, for example, the electrodeposition method or the dipping method. In this case, it is possible to form a very dense film on the surface of the blackened layer 4 with a thickness of several tens of nm.

The film obtained here is 1500-500.
By heating and sintering at about 1700 ° C., the structural water is dehydrated to form an α-Al ₂ O ₃ film. When the oxidation-resistant protective layer 6 is formed on the heater 1 on which the blackened layer 4 is thus formed by using alumina sol, it is possible to avoid contact between the tungsten forming the blackened layer 4 and moisture / air. This is possible, and because of its thinness, the decrease in the degree of blackening of the blackening layer 4 can be minimized.

FIG. 2 is a flow chart showing an embodiment of the method for manufacturing the heater 1 described above. The heater 1 manufacturing method according to the present embodiment includes a coil winding step of step 1,
It comprises an insulating layer coating step of step 2, a blackened layer coating step of step 3, an alumina sol coating step of step 4, and a sintering step of step 5. The details of each step will be described below.

In the coil winding process of step 1, the metal wire 5
Is molded into a double coil shape as shown in FIG. In the insulating layer coating step of Step 2, a film made of alumina to be the insulating layer 3 is formed on the surface of the coiled metal wire 5 by an electrodeposition method or the like. In the blackened layer coating step of step 3, a suspension containing a mixture of tungsten and alumina to form the blackened layer 4 is applied onto the insulating layer 3.

In the alumina sol coating step of step 4,
For example, the alumina sol layer is formed by the dipping method. That is, the metal wire 5 coated with the blackening layer 4 is dipped in an alumina sol solution placed in a tank (not shown) to form an alumina sol layer serving as the oxidation resistant protection layer 6 on the surface of the blackening layer 4. In the sintering step of Step 5, the metal wire 5 formed up to the alumina sol layer is sintered in a reducing atmosphere at 1500 to 1700 ° C.

By this heating and sintering, the structural water of the alumina sol layer is dehydrated, and the oxidation resistant protective layer 6 made of a film of α-Al ₂ O ₃ is obtained on the blackening layer 4. As a result, the heater 1 in which the blackened layer 4 is covered with the oxidation resistant protection layer 6 is completed.

Here, in the above-mentioned alumina sol coating step, the alumina sol layer was formed by the dipping method, but since the alumina sol particles are positively charged, the coating by the electrodeposition method is also possible. That is, although not shown, the metal wire 5 covered with the blackening layer 4 and the electrode paired with the metal wire 5 are put in an alumina sol solution, and the metal wire 5 and the counter electrode are connected to a power source to obtain the blackening layer. The metal wire 5 coated with 4 can be coated with an alumina sol layer.

Next, another embodiment of the method for manufacturing the heater 1 will be described. In this embodiment, the blackened layer coating step of step 3 and the alumina sol coating step of step 4 shown in FIG. 2 are integrated into one process.

That is, the alumina sol solution is added to the suspension of tungsten and alumina used in the blackening layer coating step, and the alumina sol is adsorbed by the tungsten powder in the suspension. Then, the blackened layer 4 is formed on the insulating layer 3 using a suspension obtained by adsorbing alumina sol on tungsten powder. In this blackened layer 4, the alumina sol layer is formed on the tungsten particles that are the constituents of the blackened layer 4, so that the blackened layer coating and the alumina sol coating are performed simultaneously.

Then, the structural water of the alumina sol layer is dehydrated by carrying out the sintering process of step 5, and the oxidation resistant protective layer 6 made of a film of α-Al ₂ O ₃ is formed on the components constituting the blackening layer 4. can get. This makes it possible to avoid contact between the tungsten forming the blackening layer 4 and moisture / air,
Moreover, because of its thinness, it is possible to minimize the decrease in the degree of blackening of the blackening layer 4.

FIG. 3 is an explanatory view showing a schematic structure of an electron gun in which the heater 1 of this embodiment is incorporated. The electron gun 10 is incorporated in a color cathode ray tube.
The electron gun 10 for a color cathode ray tube is provided with three cathodes 11 which are the cathodes 2 in which the heater 1 shown in FIG. 1 is incorporated. In the electron gun 10 called in-line type, these cathodes 11 are arranged in a line.

The cathode 11 is, as described in FIG. 1,
Electrons are emitted by energizing the heater 1. A plurality of electrodes G1 to G5 are provided to accelerate and focus the electrons. The number of electrodes is not limited to this.

Now, the electron gun 10 is provided with a cathode 13 and a terminal 13 for connecting the electrodes G1 to G5 to the outside, and a stem portion 12 used for integrating the electron gun 10 with a cathode ray tube described later. Is provided. The stem portion 12 is made of glass and is provided near the cathode 11.

FIG. 4 is an explanatory view of a cathode ray tube in which the heater 1 of this embodiment is incorporated, and FIG. 4 (a) is a cathode ray tube 20.
4B shows an example of the manufacturing process of the cathode ray tube 20. The cathode ray tube 20 is a glass outer tube 21 in which the electron gun 10 shown in FIG. The outer tube 21 has a neck portion 21a into which the electron gun 10 is inserted, and a panel portion 21b having a surface for displaying a screen.
And a funnel portion 21c connecting the neck portion 21a and the panel portion 21b. A deflection yoke 22 is provided on the outer side of the outer tube 21 near the joint between the neck portion 21a and the funnel portion 21c. Further, in the case of a color cathode ray tube, a color selection mechanism (not shown) is provided inside the panel portion 21b.

When incorporating the electron gun 10 into the outer tube 21,
As shown in FIG. 4B, the electron gun 10 is attached to the neck portion 21a.
Insert the stem portion 12 and the neck portion 21a into the burner 2
3 by glass welding, stem portion 12 and neck portion 21a
As one. Then, the air inside the outer tube 21 is exhausted to create a vacuum inside.

When assembling the cathode ray tube 20, the stem portion 12 is provided in the vicinity of the cathode 11 in which the heater 1 is incorporated, so that the stem portion 12 and the neck portion 21a are formed.
During welding, the ambient temperature of the heater 1 becomes high. The relationship between the temperature rise of the heater 1 at the time of welding, the degree of blackening of the blackening layer 4 and the temperature of the core wire when the heater 1 is energized and operated is manufactured by the manufacturing method shown in FIG. The heater 1 was compared with a conventional heater having no oxidation-resistant protective layer.

FIG. 5 is a graph showing the correlation between the temperature around the heater 1 during glass welding in air and the degree of blackening of the heater 1 after the actual operation. FIG. 6 is a graph showing the correlation between the temperature around the heater 1 during glass welding in air and the core temperature during operation.

In FIGS. 5 and 6, the heater 1 manufactured by the manufacturing method shown in FIG. 2 is referred to as a product of the present invention. In addition, a conventional heater having no oxidation-resistant protective layer for comparison is referred to as a conventional product.

During welding of the stem portion 12 and the neck portion 21a, the maximum temperature around the heater 1 reaches 450 to 550 ° C. The welding time is constant. In the conventional heater, when the ambient temperature exceeds 450 ° C., the degree of blackening is extremely lowered, and the core temperature is increased accordingly.

On the other hand, the heater 1 of the present embodiment
Since the blackened layer 4 is covered with the oxidation-resistant protective layer 6 to have high oxidation resistance, the blackening degree is hardly reduced even when the ambient temperature during welding is high.

Further, in the heater 1 of the present embodiment, since the decrease in blackening degree is suppressed, the core wire temperature during operation becomes lower than that of the conventional heater regardless of the ambient temperature during welding. As described above, since the core wire temperature of the heater 1 is lowered, the recrystallization of the metal wire 5 which leads to the embrittlement of the metal wire 5 can be suppressed, and thus the heater 1 is less likely to be broken.
The life of the heater 1 is extended. Therefore, high reliability is ensured for the electron gun in which the heater 1 is incorporated and the cathode ray tube in which the electron gun is incorporated.

Since the oxidation resistance of the heater 1 is improved as described above, the dependency on the manufacturing conditions such as the welding temperature and the welding atmosphere is eliminated, so that the replacement of nitrogen in the welding atmosphere becomes unnecessary and the cost can be reduced. It will be possible.

Further, by preventing the oxidation of tungsten,
Since it is possible to prevent the tungsten component from evaporating during the operation of the heater 1, the tungsten oxide is not deposited on the components inside the cathode ray tube when the operation is stopped, and the insulation characteristics are improved.

Although the blackening layer 4 is made of tungsten and alumina in the present embodiment, the blackening layer 4 is made of other materials such as tungsten carbide, aluminum nitride, hafnium oxide, etc. in the present invention. It is also applicable to the heater 1 configured as described above.

When the oxidation-resistant protective layer 6 is formed by using alumina sol, the insulating layer 3 and the blackening layer 4 are reinforced by the glue effect of the alumina sol, and the life of the heater can be further extended.

[0057]

As described above, the heater for heating a cathode according to the present invention has a blackened layer formed on the surface of a metal wire that generates heat when energized, and is covered with an oxidation resistant protective layer.
As a result, the blackened layer does not come into contact with air, moisture, etc., so that it is possible to prevent the components constituting the blackened layer from being oxidized and prevent the blackening degree of the blackened layer from decreasing.

Further, the cathode heating heater according to the present invention is one in which the particles constituting the blackening layer formed on the surface of the metal wire which generates heat when energized are covered with an oxidation resistant protective layer.
As a result, the constituents of the blackening layer do not come into contact with air, moisture, etc., so that the constituents of the blackening layer can be prevented from being oxidized and the reduction in the blackening degree of the blackening layer can be suppressed.

The method for manufacturing a heater for heating a cathode according to the present invention is carried out by depositing an alumina sol on the surface of a blackening layer formed on the surface of a metal wire that generates heat when energized and heating and sintering it to obtain an acid resistant film made of an alumina film. To form a chemical protection layer.

As a result, a very dense alumina film having a thickness of several tens of nm can be formed on the surface of the blackening layer, the components constituting the blackening layer are prevented from being oxidized, and the blackening layer is covered. It is possible to minimize the decrease in the blackening degree due to.

In the method for manufacturing the heater for cathode heating according to the present invention, when the surface of the metal wire that generates heat when energized is coated with the blackening layer, the suspension containing the components forming the blackening layer is added to the alumina sol. By adding a solution, adhering the mixed solution containing this alumina sol solution to a metal wire and sintering it by heating, an oxidation resistant protective layer made of an alumina film was formed on the particles constituting the blackening layer. It forms a layer.

As a result, an oxidation resistant protective layer for protecting the blackened layer can be formed in the step of forming the blackened layer, so that a decrease in the blackening degree of the blackened layer can be suppressed without increasing the number of manufacturing steps. You can create a heater.

The electron gun according to the present invention is an electron gun provided with a cathode heating heater in which the surface of a metal wire that generates heat when energized is covered with a blackening layer. It is covered with a chemical protection layer. Further, the cathode ray tube according to the present invention is provided with the electron gun in which the blackening layer of the heater for heating the cathode is coated with the oxidation resistant protection layer as described above.

As a result, it is possible to prevent the constituents of the blackened layer of the heater from being oxidized and to prevent the deterioration of the blackened degree of the blackened layer. Therefore, the efficiency of the heater does not decrease, the temperature rise of the heater can be suppressed, and the reliability of the cathode ray tube can be improved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a heater for heating a cathode of the present embodiment.

FIG. 2 is a flowchart showing an embodiment of a heater manufacturing method.

FIG. 3 is an explanatory diagram showing a schematic configuration of an electron gun incorporating the heater of the present embodiment.

FIG. 4 is an explanatory diagram of a cathode ray tube incorporating the heater according to the present embodiment.

FIG. 5 is a graph showing the correlation between the temperature around the heater during glass welding in air and the degree of blackening of the heater after actual operation.

FIG. 6 is a graph showing the correlation between the temperature around the heater during glass welding in air and the core wire temperature during operation.

FIG. 7 is a flowchart showing a conventional heater manufacturing method.

[Explanation of symbols]

1 ... Heater, 2 ... Cathode, 3 ... Insulating layer, 4
... Blackening layer, 5 ... Metal wire, 6 ... Oxidation-resistant protective layer, 7 ... Sleeve, 8 ... Disk, 10 ...
Electron gun, 11 ... Cathode, G1 to G8 electrodes, 12 ...
..Stem part, 20 ... Cathode ray tube, 21a ... Neck part

Claims (9)

[Claims] 1. A cathode heating heater in which a surface of a metal wire that generates heat when energized is covered with at least a blackening layer, characterized by comprising an oxidation-resistant protective layer that covers the blackening layer. . 2. The heater for heating a cathode according to claim 1, wherein the oxidation-resistant protective layer is an alumina film. 3. A cathode heating heater in which at least a surface of a metal wire which generates heat when energized is covered with a blackening layer, wherein the blackening layer has particles constituting the blackening layer covered with an oxidation-resistant protective layer. A heater for heating a cathode, which is characterized by being a thing. 4. The heater for heating a cathode according to claim 3, wherein the oxidation-resistant protective layer is an alumina film. 5. A method for manufacturing a heater for a cathode heating, wherein at least a surface of a metal wire that generates heat when energized is covered with a blackening layer, wherein a sol of alumina is adhered to the surface of the blackening layer and is sintered by heating. A method for manufacturing a heater for heating a cathode, comprising forming an oxidation resistant protective layer comprising 6. The method for manufacturing a heater for cathode heating according to claim 5, wherein alumina sol is attached to the surface of the blackened layer by an immersion method or an electrodeposition method. 7. A method for producing a heater for cathode heating, wherein at least a surface of a metal wire that generates heat when energized is covered with a blackening layer, wherein an alumina sol solution is added to a suspension containing components that form the blackening layer, By adhering the mixed solution to which the alumina sol solution is added to the metal wire and heating and sintering, a blackening layer in which an oxidation resistant protective layer made of an alumina film is formed on particles constituting the blackening layer is formed. A method for manufacturing a heater for heating a cathode, comprising: 8. An electron gun having a cathode for a cathode heating heater in which at least a surface of a metal wire that generates heat when energized is covered with a blackening layer, wherein the heater for cathode heating has an oxidation-resistant protective layer as the blackening layer. An electron gun characterized by being coated with. 9. A cathode ray tube in which an electron gun having a cathode heating heater in which at least a surface of a metal wire that generates heat when energized is coated with a blackening layer is incorporated in the cathode ray heater, A cathode ray tube, characterized in that the layer is covered with an oxidation-resistant protective layer.