JPH06150811A - Method for manufacturing impregnated cathode and electron tube - Google Patents

Abstract

(57) [Summary] [Object] To provide a Sc-coated impregnated cathode and an electron tube using the same, which are not deteriorated in electron emission characteristics due to oxidation of the coating film in the cathode encapsulation process. [Structure] A protective film layer containing barium and oxygen is formed on the upper surface of a coating film of a Sc-coated impregnated cathode after preheating the cathode, and the protective film layer containing barium and oxygen is further sealed in an electron tube and vacuum exhausted. It is reheated later. [Effect] The protective film layer protects the coating film by heating at 400 ° C. in the atmosphere during the encapsulation process. Then, the protective film disappears if the cathode is heated from 1050 ° C. to 1250 ° C. after the electron tube is evacuated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impregnated cathode suitable for electron tubes such as cathode ray tubes and image pickup tubes, and more particularly to an impregnated cathode having a surface coating film layer containing scandium for exhibiting good electron emission characteristics. The present invention relates to an impregnated cathode having a thermal oxidation protection film for a coating film, a method for manufacturing the same, and a method for manufacturing an electron tube using the same.

[0002]

2. Description of the Related Art An impregnated cathode is a cathode capable of high current density operation, and is an essential cathode for achieving high output of electron tubes, especially for cathode ray tubes to achieve high brightness and high definition.

The impregnated cathode is basically composed of a heat-resistant porous substrate made of tungsten, mainly BaO, and Al ₂
O ₃ or CaO is heat-melted in a hydrogen atmosphere and impregnated. This melt impregnated material is commonly referred to as the impregnant.

During operation, the impregnated cathode is constantly heated to about 1000 ° C., and the heat-resistant porous substrate and the impregnating agent react with each other to release barium. Barium is supplied to the surface of the cathode by diffusion, and is adsorbed on the surface of the cathode, that is, the tungsten substrate together with oxygen supplied from the atmosphere inside the cathode or the atmosphere inside the electron tube. Thus, when the adsorption layer composed of barium and oxygen is formed on the metal surface, the work function of the metal surface is substantially lowered, and the electron emission becomes easy. This is the principle of operation of the impregnated cathode, and a consideration for this is, for example, Journal of Physics D: Applied Physics, Volume 15 (1982).
Year) 1519 to 1529 (J.Phys. D: App
I. Phys., 15 (1982) 1519-1529). The impregnated cathode has a low electric resistance because the electron emission portion is basically a metal. Therefore, the impregnated cathode does not undergo cathode deterioration due to decomposition of the cathode material itself due to Joule heat generation, unlike an oxide cathode having an electrically high resistance using an alkaline earth metal carbonate as a raw material. Therefore,
The impregnated cathode can operate at a higher current density than an oxide cathode.

However, while the impregnated cathode can operate at high current density, it needs to be operated at a higher temperature than the operating temperature of about 750 ° C. for oxide cathodes. In the case of the impregnated cathode having the above-mentioned basic structure, heating to about 1100 ° C. is required to obtain a current density of 10 A / cm ² .

Therefore, the impregnated cathode has been improved for the purpose of reducing the operating temperature. For example,
An impregnated cathode in which osmium is coated on the surface of the impregnated cathode having the above-mentioned basic structure has been developed, and its operating temperature is about 10
It dropped to 00 ° C. Furthermore, the impregnated cathode having a thin film containing scandium oxide or scandium tungstate mainly containing tungsten is capable of the above-mentioned current density operation at 900 ° C. Hereinafter, these will be referred to as an Os-coated impregnated cathode and a Sc-coated impregnated cathode, respectively. Regarding these, IEE Proceeding, No. 12
Volume 8, Part 1, Number 1 (1981), pages 19 to 32 (IEE Proc., 128, Pt1, No. 1 (1981)
19-32) and Japanese Journal of Applied Physics, Vol. 27, No. 8
(1988) pp. 1411 to 1414 (Jpn. J.
Appl. Phys. 28, No. 8 (1988) 1411-141.
4).

By the way, in the manufacturing process in which the cathode is enclosed together with other electrodes in the electron tube of the glass tube,
It is heated at a temperature of about 400 ° C. or higher in the air atmosphere. This is an unavoidable process in electron tube manufacturing of glass tubes.

As a result, in the Os-coated impregnated cathode, the osmium-coated film is oxidized to produce harmful and volatile OsO ₄ , and the electron emission characteristics are deteriorated. For this reason, an Os / Ru-coated impregnated cathode that uses an alloy film with lutemium instead of osmium alone as a coating film is generally used.

Similarly, in the Sc-covered impregnated cathode, the electron emission characteristic is deteriorated by the encapsulation process. The cause of this is not clear, but it is presumed that as a result of the oxidation of tungsten in the coating film, tungsten oxide reacts with barium supplied from the inside of the cathode to form BaWO ₄ , which does not contribute to electron emission. . By the way, the impregnated cathode is preheated in a vacuum vessel at 900 ° C. or higher before encapsulation in order to suppress the gas release in the cathode ray tube or to shorten the cathode activation time in the cathode ray tube manufacturing process. I have something to do. However, the Sc-coated impregnated cathode that has been subjected to the preheating treatment has a significantly deteriorated electron emission characteristic due to oxidation of the coating film in the encapsulation process. In order to prevent the coating film of the cathode from being oxidized, the encapsulation step may be performed in a nitrogen atmosphere, but this is not suitable for mass production because it causes an increase in manufacturing cost.

[0010]

As described above, the osmium film and the tungsten film containing scandium, which are coated for the purpose of lowering the operating temperature of the impregnated cathode, are oxidized in the step of enclosing in the electron tube, This causes deterioration of electron emission characteristics. In the case of the Os coating type, this can be prevented by adding ruthenium to the osmium film. However, in the case of the Sc-covered type, there has been no countermeasure until now, except for encapsulation in a nitrogen atmosphere to suppress oxidation. Moreover, encapsulation in a nitrogen atmosphere increases the manufacturing cost and is not suitable for mass production.

It is an object of the present invention to prevent oxidation of a coating film in an encapsulation process in the manufacture of an electron tube using a Sc coating type impregnated cathode. Another object of the present invention is to provide a Sc-coated impregnated cathode having excellent oxidation resistance and an electron tube using the same.

[0012]

The above technical problems can be achieved by the following means. That is, the Sc-coated impregnated cathode of the present invention is characterized by forming a protective film containing barium and oxygen on the surface of the coating film after preheating at 900 ° C. or higher. Here, the protective film containing barium and oxygen can be easily formed by a general film forming method such as a sputtering method or a vapor deposition method.

[0013]

As described above, the cathode is heated in the atmosphere at a temperature of about 400 ° C. or higher in the encapsulation which is a manufacturing process of the electron tube. At this time, since the protective film containing barium and oxygen is almost an oxide, the damage due to oxidation does not occur and the original oxidation of the surface of the coating film is prevented. After encapsulation, the electron tube is evacuated and heated at about 1150 ° C. to activate the cathode. In this step, the protective film containing barium and oxygen is evaporated, and the original coating film containing Sc appears on the cathode surface.

By the way, it is necessary that the protective film uniformly covers the entire surface of the cathode to sufficiently prevent the coating film under the coating film from being oxidized. Furthermore, it is necessary that the cathode easily disappears in a short period when the cathode is activated. For this, 20 nm or more, 20
It must be 0 nm or less.

Therefore, by forming a protective film on the surface of the Sc-coated impregnated cathode, it is possible to prevent the coating film from being oxidized in the manufacturing process of the electron tube.

[0016]

【Example】

<Example 1> FIG. 1 is a block diagram showing an example of the production of the impregnated cathode of the present invention. In the figure, the part surrounded by the broken line is different from the conventional one.

The impregnated cathode has a basic structure in which tungsten powder is pressed and sintered, and then impregnated with an impregnating agent. This is called a basic type impregnated cathode. In this embodiment, as the impregnating agent, a mixture of BaO, CaO and Al ₂ O ₃ in a ratio of 4: 1: 1 was used. Then, this impregnating agent was heated and melted at 1900 ° C. in a hydrogen atmosphere to impregnate a tungsten sintered body, that is, a porous body. The size and shape of the basic type impregnated cathode used in this example are 1.2 mm in diameter and 0.4 mm in thickness.
Is a cylindrical pellet of.

The Sc-covered impregnated cathode was produced by coating the upper surface of the basic impregnated cathode with a film containing W as a main component and containing Sc by a sputter deposition method. At this time, a composite target of tungsten and Sc ₂ W ₃ O ₁₂ was used as the sputtering target. The composition of the coating film is 0.10 in terms of Sc / W element ratio.
To 0.15 and the film thickness was 300 nm. The protective film containing barium and oxygen was formed by a sputtering film forming method. At this time, barium carbonate was used as the sputter target. The formed protective film is presumed to be mainly BaO, but BaO ₂ , BaCO ₃ or Ba,
It is possible that each element of C and O is mixed. Further, after film formation, Ba (OH) ₂ may be generated depending on the storage atmosphere. The formation of this protective film must be performed after the preheating treatment of the cathode. In this example, as preheating treatment, heating (activation) was performed at 1150 ° C. for 4 hours in a vacuum container, and then the protective film containing barium and oxygen was formed. This was heated in air at 400 ° C. for 5 minutes in order to confirm the effect of preventing the deterioration of the electron emission characteristics in the encapsulation process. Then, reactivation by heating at 1150 ° C. was performed again in a vacuum container, and the emission current and its heating change were measured.

FIG. 2 shows the electron emission characteristics of the impregnated cathode of this example in comparison with the conventional example having no protective film. The horizontal axis is the reheating (reactivating) time at 1150 ° C. in the vacuum vessel, and the vertical axis is the cathode emission current amount at 900 ° C. However, the emission current amount is standardized and expressed with the value after preheating being 1.0. The measurement was performed by disposing the anode at a distance of 7 mm from the cathode and applying a voltage of 4 kV to the anode. The results of the protective film thickness of 10 nm to 300 nm are shown in comparison with the conventional example 1 without the protective film.

It can be seen from FIG. 2 that the cathodes 3, 4, and 5 having the protective film of 20 nm to 120 nm have the emission current amount restored to the value before the atmospheric oxidation if they are reheated in vacuum after the atmospheric oxidation. Here, of the cathode 5 having a protective film thickness of 120 nm,
When the cathode surface after reheating was observed with SEM and EDX, a thin film of Ba or its oxide did not remain, and the original surface of the impregnated cathode was exposed. In the case of the cathode 2 having a protective film thickness of less than 20 nm, the protective film cannot cover the surface of the cathode, and the coating film containing scandium is oxidized and the deterioration of electron emission cannot be recovered. Further, in the cathode 6 having a protective film of 200 nm and the cathode 7 having a thickness of 300 nm, it is considered that the evaporation of the protective film takes time and the original deterioration of the cathode begins before the emission current is recovered.

<Embodiment 2> In Embodiment 1, preheating is performed.
Combines gas discharge process from cathode and cathode activation process 1
Although performed at 150 ° C., heating at 900 ° C. is sufficient if this is only for outgassing treatment. At this time, 1
The rise of the emission current when reheated at 150 ° C. is shown in FIG. The horizontal axis is the reheating (activation) time at 1150 ° C. in the vacuum vessel, and the vertical axis is the cathode emission current amount at 900 ° C. However, the maximum value of the emission current of the cathode 9 which is not subjected to the above-mentioned simulated encapsulation test is standardized and expressed as 1.0. The measurement was performed by disposing the anode at a distance of 7 mm from the cathode and applying a voltage of 4 kV to the anode. Protective film thickness 10nm, 200nm, 300nm cathode 11,12,
The results of No. 13 are shown in comparison with Conventional Example 10 without a protective film.

As shown in FIG. 3, when preheating was performed at 900 ° C. for 1 hour, in the cathode 10 without the protective film,
The emission test deteriorates the emission current to 80% or less of the original value. On the other hand, a cathode 12 having a protective film coated to 200 nm
In, the emission current almost equal to that of the cathode 9 not subjected to the enclosure test was obtained. The protective film thickness of 200 n shown in FIG.
Sufficient emission current was not obtained with the cathode 6 of m, but this is because the cathode activation was not performed by preheating in this embodiment. In other words, in this example, the reheating time at which the cathode originally starts to deteriorate is relatively longer than that in Example 1, and it can be estimated that the protective film has evaporated before the deterioration.

Therefore, the upper limit of the protective film thickness depends on the preheating temperature and time. However, even in the present embodiment, the protective film is 3
At the cathode 13 of 00 nm, the original deterioration of the cathode begins before the protective film evaporates and a sufficient rise of the emission current is obtained. Further, with the cathode 11 having a protective film thickness of 10 nm, the protective film cannot cover the coating film, and a sufficient emission current cannot be obtained. Further, although not shown in FIG.
The emission current of the cathode having a protective film of less than nm is 2
An emission current equal to or higher than that of the cathode 12 of 00 nm is obtained. Therefore, the protective film thickness is approximately 20 nm to 200
nm is suitable.

In Examples 1 and 2, as the impregnating agent, a mixture of BaO, CaO, and Al ₂ O ₃ in a ratio of 4: 1: 1 was used, which is 5: 3: 2 or another composition. It is easily inferred that the same effect will be exhibited even with the above. further,
Although the sputtering film formation method was used as the method for forming the protective film in this embodiment, it is obvious that this may be another film formation method such as a vapor deposition method. Further, in Examples 1 and 2, reheating is 1
Although it was performed at 150 ° C., the reheating temperature is determined in consideration of the time for the protective film to disappear. The higher the reheating temperature, the faster it disappears. However, heating above 1250 ° C. is not preferable because the original coating film containing Sc is significantly deteriorated. Also, 1
If the temperature is lower than 050 ° C, it takes a long time for the protective film to disappear, which is not practical.

<Embodiment 3> FIG. 4 shows the structure of a cathode ray tube as an embodiment of an electron tube on which the impregnated cathode of the present invention is mounted. In the cathode ray tube, the cathode 15 heated by the heater 14
The electrons emitted from are focused on the fluorescent screen 20 by an electron gun composed of several electrodes. The electron gun is composed of an electron extracting portion formed of a first electrode 16 and a second electrode 17 arranged next to the front surface of the cathode, and a main lens portion that mainly focuses an electron beam. The main lens portion 18 is generally 2 to 4
It consists of a sheet of electrodes. The electron beam is scanned by the deflection coil 19 over the entire fluorescent screen. By the way, in the manufacturing process of a cathode ray tube, the cathode is incorporated into an electron gun and then attached to a current introducing terminal to which a metal terminal is fixedly arranged by glass. Then, this is integrated with a glass tube having a fluorescent screen by using a gas burner. In this encapsulation process,
The cathode is heated to about 400 ° C. or more in air for a few minutes. At this time, in the conventional Sc-coated impregnated cathode, the coating film is oxidized and the electron emission characteristics inherent in the cathode cannot be exhibited. The deterioration of electron emission due to the oxidation of the coating film is particularly remarkable in the cathode subjected to the preheating treatment. In order to exhibit the original characteristics, it is necessary to perform a special encapsulation method such as encapsulation in a nitrogen atmosphere. However, after impregnating the impregnated cathode of the present invention in an electron tube by a usual method and evacuating,
Sufficient electron emission characteristics are exhibited by reheating at 050 to 1250 ° C.

[0026]

According to the method for producing an impregnated cathode of the present invention, it is possible to avoid the oxidation of the coating film of the Sc-coated impregnated cathode which has been preheated in advance in the electron tube manufacturing process, and to prevent electron emission. Can be prevented from deteriorating. At this time, if the thickness of the protective film is 20 nm or more and 200 nm or less, deterioration of electron emission characteristics can be prevented. Further, in the electron tube according to the manufacturing method of the present invention, in the manufacturing process, encapsulation in the nitrogen atmosphere, which is essential in the Sc-covered impregnated cathode, is not required.

[Brief description of drawings]

FIG. 1 is a block diagram illustrating a manufacturing process in an embodiment of an impregnated cathode of the present invention.

FIG. 2 is an explanatory diagram of a protective film thickness dependency of an emission current of a cathode which is preheated at 1150 ° C. and heated in the atmosphere.

FIG. 3 is an explanatory diagram of a protective film thickness dependency of an emission current of a cathode which is preheated at 900 ° C. and heated in the atmosphere.

FIG. 4 is a cross-sectional view illustrating a Braun tube manufactured by the method for manufacturing an electron tube according to the present invention.

[Explanation of symbols]

1 ... Emission current in cathode without protective film, 2 ... Protective film 1
0 nm emission current at the cathode, 3 ... Emission current at the protection film 20 nm cathode, 4 ... Emission current at the protection film 50 nm cathode, 5 ... Emission current at the protection film 120 nm cathode, 6 ... Emission current at the protection film 200 nm cathode , 7 ... Emission current at cathode of protective film 300 nm, 8
… Emission current before pre-encapsulation test after preheating at 1150 ° C., 9 ... Emission current in cathode without encapsulation test, 10 ... Emission current in cathode without protective film, 11 ...
Emission current at cathode of protective film 10 nm, 12 ... Emission current at cathode of protective film 200 nm, 13 ... Protective film 30
Emission current at 0 nm cathode, 14 ... Heater, 15 ...
Cathode, 16 ... First electrode, 17 ... Second electrode, 18 ... Main lens part, 19 ... Deflection coil, 20 ... Phosphor screen.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Emiko Yamada 1-280 Higashi Koikekubo, Kokubunji City, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (72) Tadashi Narusei, 1-280 Higashi Koikekubo, Kokubunji City, Tokyo Hitachi Ltd. (72) Inventor Sadanori Taguchi, 3300 Hayano, Mobara-shi, Chiba Hitachi, Ltd. Mobara factory

Claims (6)

[Claims] 1. An electron-emitting surface of an impregnated cathode in which a porous tungsten substrate is impregnated with an oxide containing barium, and a coating containing tungsten and at least one of scandium, scandium oxide, and scandium tungstate. A coating film, or a coating-type impregnated cathode in which a coating film further containing tungsten oxide is formed on the coating film, after preheating the coating-type impregnated cathode in a vacuum, further on the upper surface of the coating film, A method of manufacturing an impregnated cathode, comprising forming a protective film containing at least barium and oxygen. 2. The method for producing an impregnated cathode according to claim 1, wherein the protective film containing barium and oxygen contains one or more selected from BaO, BaO ₂ , Ba (OH) ₂ and BaCO ₃ . 3. The method for producing an impregnated cathode according to claim 1, wherein the protective film layer has a thickness of 20 nm or more and 200 nm or less. 4. The method for producing an impregnated cathode according to claim 1, 2, or 3, wherein the preheating temperature is 900 ° C. or higher. 5. The method of manufacturing an electron tube according to claim 1, 2, 3 or 4, wherein the electrode containing the impregnated cathode is sealed in a glass tube and evacuated, and then reheated. 6. The reheating temperature according to claim 5, wherein the reheating temperature is 1.
A method of manufacturing an electron tube having a temperature of 050 ° C or higher and 1250 ° C or lower.