JPH1027538A - Impregnated cathode and cathode-ray tube using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the force and the total amount by which metal scandium is supplied to the surface of an impregnated cathode of basic constitution to enhance an electron emission characteristic and reliability by providing an intermediate layer between the surface of the cathode and the metal scandium. SOLUTION: An Sc-covered impregnated cathode is formed by first forming an intermediate layer 1 composed mainly of a high-melting-point metal, such as tungsten or molybdenum, over the upper surface of a basic impregnated cathode 4. A cover film 2 composed chiefly of metal scandium is formed over the upper surface of the intermediate layer 1, and an outermost surface layer 3 composed chiefly of a high-melting-point metal such as tungsten or molybdenum is formed as the adsorbent surface of a monolayer. In this case, the intermediate layer 1 of the high-melting-point metal is at least one kind of metal selected from a group consisting of osmium, iridium, rhenium, and platinum, in addition to tungsten and molybdenum. Preferably the film thickness of the intermediate layer 1 is 50 to 1nm.

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 an electron tube such as a cathode ray tube and an image pickup tube, and more particularly to an impregnated cathode having a surface coating film layer containing scandium for exhibiting good electron emission characteristics. .

[0002]

2. Description of the Related Art An impregnated cathode is a cathode capable of operating at a high current density, and is an indispensable cathode for increasing the output of an electron tube, particularly for a cathode ray tube for achieving higher brightness and higher definition.

The basic structure of the impregnated cathode is heat porous substrate made of tungsten, mainly of BaO, other Al ₂
O ₃ and CaO are heated and melted in a hydrogen atmosphere or vacuum,
The impregnated cathode. This melt impregnated material is generally referred to as an impregnant. The impregnated cathode is always in operation
Heated to about 00 ° C., the heat-resistant porous substrate reacts with the impregnating agent 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 inside of the cathode or the inside of the electron tube. As described above, when the adsorption layer (monomolecular layer) composed of barium and oxygen is formed on the metal surface, the work function of the metal surface is substantially reduced, and electron emission becomes easy. This is the operating principle of the impregnated cathode, and detailed considerations are given in, for example, Journal of Physics D: Applied Physics, 15 (1982), pp. 1519 to 152.
Page 9 (J. Phys. D: Appl. Phys., 15 (1982) pp1519-152
9).

[0004] The impregnated cathode has a small electrical resistance because the electron-emitting portion is basically a metal. Therefore, the impregnated cathode does not suffer from cathode degradation due to decomposition of the cathode material itself due to Joule heat, such as an electrically high resistance oxide cathode made of alkaline earth metal carbonate. Therefore, the impregnated cathode can operate at a higher current density than the oxide cathode.

[0005] However, while the impregnated cathode can operate at a high current density, it must be operated at a higher temperature than the operating temperature of the oxide cathode of about 750 ° C. In the case of the impregnated cathode having the above-described basic configuration, it is necessary to heat the cathode to about 1100 ° C. in order to obtain a current density of 10 A / cm ² .

[0006] For this reason, the impregnation type cathode has been improved for the purpose of lowering the operating temperature. For example,
An impregnated cathode in which the surface of the impregnated cathode having the above-described basic configuration is coated with osmium has been developed, and its operating temperature is about 10%.
The temperature dropped to 00 ° C. In this regard, IEE Proc., 128, Pt1, No. 1 (1981) pp.
19-32. Further, the impregnated cathode coated with a thin film containing scandium together with a high melting point metal such as tungsten or molybdenum can perform the above-described current density operation at 900 ° C. In the following, this will be described as a Sc-coated impregnated cathode, but in this regard, U.S. Pat. No. 4,626,470
No. 8 and the Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.) 27 , No. 8 (1988)
pp1411-1414.

When the coating film of the Sc coating type impregnated cathode is selected from tungsten as the high melting point metal of the coating film, a tungsten metal and scandium metal, or a metal and these oxides, etc. are formed by a sputtering film forming method. A film can be formed using the configured sputter target. Then, when the cathode is heated to about 1150 ° C. for activation, barium is supplied from the tungsten porous portion to the surface of the coating film, scandium is supplied from the coating film, and a monomolecular layer is formed from these and oxygen. It is formed. As a result, it is considered that the work function is lowered and good electron emission is enabled.
These are described in Jpn. J. Appl. Phys. 28 , No. 3 (1989) pp.
490-494. That is, in order to exhibit good electron emission characteristics of the Sc-coated impregnated cathode, it is necessary to supply scandium to the surface from the coating film together with barium to form a monomolecular layer.

In order to supply scandium to the cathode surface from the coating film containing the scandium oxide film, the scandium oxide must be reduced to generate free scandium. Reduction of the oxide is performed by reaction with tungsten or barium. If scandium is formed as a metal instead of an oxide, a reduction reaction is not required, and improvement in characteristics and reliability of the cathode can be expected. The problem here is the reaction between barium oxide and scandium on the surface of the impregnated cathode having the basic structure. Compounds such as Ba ₂ Sc ₂ O ₅ , Ba ₃ Sc ₄ O ₉ , and Ba ₁ Sc ₂ O ₄ are produced by the reaction between the _two . This compound hardly releases scandium necessary for forming a monolayer. Therefore, when these compounds are generated, the effect of metallically forming scandium on the coating film is lost. Also, the effective scandium supply is reduced, resulting in a shorter life than originally expected.

[0009]

As described above, this is an Sc-coated impregnated cathode capable of operating the impregnated cathode at a low temperature. In order to improve electron emission characteristics and reliability, the coating film is metallic. When a film is formed, scandium reacts with barium oxide and becomes a compound that hardly generates free scandium, so that there is a problem that a sufficient effect cannot be obtained.

An object of the present invention is to solve this problem with an Sc-coated impregnated cathode, and to provide a coating film configuration for that purpose.

[0011]

The above technical problem can be solved by providing an intermediate layer between the surface of the impregnated cathode having the basic structure and metal scandium, and forming a barrier layer for preventing this reaction. . For example, in a Sc-coated type impregnated cathode using metal scandium as a surface coating film, a high melting point metal such as tungsten or molybdenum having a thickness of 50 nm to 1 μm is used as an intermediate layer between the cathode surface and the surface coating film. Provide.

By providing the intermediate layer between the cathode surface and the surface coating film made of metal scandium, the reaction between barium oxide and metal scandium can be prevented. As a result, the characteristics of the metal scandium film that does not require a reduction reaction to generate free scandium can be brought out, so that the electron emission characteristics are good and a sufficient life can be achieved.

[0013]

FIG. 1 shows an embodiment of the structure of an impregnated cathode of the present invention, and FIG. 2 schematically shows a conventional example of the structure. FIG.
As shown in (1), in the conventional Sc-coated impregnated cathode, a lower coating film 20 mainly composed of scandium metal is formed on the upper surface of the basic impregnated cathode 4 which is a basic structure. Then, an outermost surface layer 30 mainly composed of a high melting point metal such as tungsten or molybdenum is formed as an adsorption surface of the monomolecular layer.

On the other hand, in the Sc-coated impregnated cathode of the present invention, first, an intermediate layer 1 mainly composed of a refractory metal such as tungsten or molybdenum is formed on the upper surface of the basic impregnated cathode. A coating film 2 mainly composed of scandium metal is formed on the upper surface, and an outermost surface layer 3 mainly composed of a high melting point metal such as tungsten or molybdenum is formed as an adsorption surface of a monomolecular layer.

In this embodiment, the intermediate layers 1 and 2 having a thickness of 100 nm are used.
The coating film 2 having a thickness of 00 nm and the outermost surface layer 3 having a thickness of 700 nm were formed by vapor deposition using electron beam heating, using molybdenum, scandium, and molybdenum as vapor deposition sources, respectively, by continuous vapor deposition. The basic impregnated cathode 4 was obtained by pressing and sintering a tungsten powder and then impregnating with an oxide mainly composed of BaO, that is, an impregnating agent 42. As the impregnating agent,
A mixture of BaO, CaO, and Al ₂ O ₃ in a ratio of 4: 1: 1 was used. Then, the impregnating agent 42 was heated and melted at 1900 ° C. in a hydrogen atmosphere to impregnate the sintered body of tungsten, that is, the porous body 41. The size and shape of the basic impregnated cathode 4 used in the present example were 1.2 mm in diameter and 0.4 mm in thickness.
It is a cylindrical pellet of mm.

FIG. 3 is a diagram showing the dependence of the emission current on the heating time. The impregnated cathode of this embodiment is compared with a conventional cathode. As the comparative cathode, one formed by forming 200 nm of scandium metal as the lower coating film 20 and 700 nm of molybdenum as the outermost surface layer 30 by an evaporation method using electron beam heating was used. The horizontal axis represents the heating time at 1000 ° C. in the vacuum vessel, and the vertical axis represents the amount of emission current at 850 ° C. 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.

As shown in FIG. 3, in the conventional cathode having the coating film structure not including the intermediate layer 1, the emission current rises up to about 2 hours, but after that, the emission current drops significantly. this is,
This is because the reaction between the barium oxide and the metal scandium proceeds on the surface of the basic impregnated cathode 4 and hinders the supply of free scandium necessary for forming a monolayer. On the other hand, the impregnated cathode of this example shows good electron emission characteristics in about 3 hours, and no decrease in emission current is observed by 10 hours. Further, the intermediate layer 1 needs to be in a uniform film range in order to have a partition effect on the impregnating agent,
Effectively, 50 nm or more is required. Barium, which is a component of the monomolecular layer, needs to be supplied from a finely formed hole, so that a film thickness exceeding 1 μm is not preferable.

In this embodiment, BaO, C
A mixture of aO and Al ₂ O _{3 in a} ratio of 4: 1: 1 was used.
However, the impregnant composition is essentially irrelevant to the effect of the present invention. It is clear that the same effect can be obtained with 5: 3: 2 and other compositions. Further, molybdenum was used as a component of the coating film of the present embodiment in addition to scandium metal. However, the intermediate layer 1 only needs to have a work function lowered by single molecule adsorption, and other high-melting metals such as tungsten can be used in the same manner as molybdenum.

The outermost surface layer 3 is preferably made of a material capable of functioning as a partition after heating the cathode, and is preferably a high melting point metal such as osmium, iridium, rhenium and platinum. Although the metal scandium coating film 2 is formed by a vapor deposition method, other film forming means such as a CVD method may be used. In addition, although the intermediate layer 1 and the outermost surface layer 3 are formed by the electron beam evaporation method, other film forming means may be used similarly.

Although the embodiment has been described with reference to the impregnated cathode, the impregnated cathode of the present invention is used for a cathode ray tube, and its schematic configuration is shown in FIG. 61 is a panel, 62 is a funnel, 63 is a neck, 64 is a phosphor screen (screen), 65 is a shadow mask, 66 is a magnetic shield, 67 is a deflection yoke, 68 is a purity adjustment magnet, 69 is a center beam static convergence adjustment magnet. , 610 is a side beam static convergence adjusting magnet, and 611 is an electron gun.
The impregnated cathode of the present invention is provided at the end of the electron gun 611, and the generated electron beam 612 is focused on the phosphor screen 64. It is clear that the cathode ray tube using the impregnated cathode of the present invention has higher reliability against the thermal factors from the cathode than the cathode ray tube using the conventional impregnated cathode. Accordingly, the present invention includes a cathode ray tube using the impregnated cathode of the present invention.

[0021]

According to the impregnated cathode having the structure of the coating film of the present invention, the supply power and the total supply of scandium to the cathode surface can be improved. As a result, electron emission characteristics and reliability are improved.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of an Sc-coated impregnated cathode according to one embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view of a conventional Sc-coated impregnated cathode.

FIG. 3 is a characteristic diagram showing the heating time dependence of the emission current of the Sc-coated impregnated cathodes of the present invention and the conventional example.

FIG. 4 is a cross-sectional view of a cathode ray tube for explaining a use mode of the impregnated cathode of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Intermediate layer, 2 ... Coating film, 3 ... Outermost surface layer, 4 ... Basic impregnation type cathode.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Matsu ▲ Saki ▼ Naoko 1-280 Higashi-Koigakubo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory of Hitachi, Ltd. Central Research Laboratory, Hitachi, Ltd.

Claims (5)

[Claims] 1. A cathode having a coating film containing at least metal scandium on a surface of a so-called impregnated cathode in which a porous tungsten substrate is impregnated with an oxide containing barium, wherein a cathode is provided between the surface of the impregnated cathode and the metal scandium. An impregnated cathode, wherein an intermediate layer of a high melting point metal is provided thereon. 2. The impregnated cathode according to claim 1, wherein the intermediate layer of the refractory metal is at least one metal selected from the group consisting of tungsten, molybdenum, osmium, iridium, rhenium and platinum. 3. An impregnated cathode according to claim 1, wherein said intermediate layer has a thickness of 50 nm to 1 μm. 4. The impregnated type wherein the coating film according to claim 1 has a multilayer structure comprising a coating film mainly composed of scandium metal and a coating film of a high melting point metal and has a multilayer structure of a plurality of layers. cathode. 5. A cathode ray tube using the impregnated cathode according to claim 1, 2, 3, or 4.