Cathode with an electron emitting layer for a cathode ray tube

Details Cathode with an electron emitting layer for a cathode ray tube Cathode with an electron emitting layer for a cathode ray tube

1998-10-08

2001-05-15

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

Electrode and shield structures

Cathodes containing and/or coated with electron emissive...

C313S3460DC, C313S337000

Reexamination Certificate

active

06232708

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATION
This application is based on application No. 97-54446 filed in Korean Industrial Property Office on Oct. 23, 1997, the content of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a cathode for a cathode ray tube (CRT) and, more particularly, to a cathode which is used in a color CRT to display high-definition color picture images
(b) Description of the Related Art
Generally, in the CRTs, the cathode is designed to emit electrons for exciting phosphors coated on a faceplate panel. For the electron emission purpose, the cathode is provided with electron-emitting materials and, during operation, the electron-emitting materials are activated to produce the phosphor-exciting electrons.
FIG. 3
is a schematic cross-sectional view showing a conventional CRT cathode. As shown in
FIG. 3
, the cathode includes a base
31
having a cylindrical shape and an opening portion, and an electron-emitting material layer
32
coated on a closed top portion of the base
31
to emit thermal electrons. The base
31
contains nickel as its main component together with a small amount of reducing elements such as silicon, magnesium, etc. The electron-emitting material layer
32
is formed with alkali-earth metal oxides comprising barium, strontium and calcium.
The cathode further includes a heater
33
placed in the opening portion of the base
31
to heat it up to a predetermined temperature, and a control grid
34
positioned on a front portion of the electron-emitting material layer
32
to focus streams of the electrons emitted therefrom. A tungsten wire coated with alumina is commonly used as the heater
33
.
In the meantime, the coating operation of the electron-emitting material layer
32
can be described as shown below.
First, a carbonate suspension containing alkali-earth metals such as barium, strontium, calcium, etc. is applied onto the top portion of the base
31
and heated by the heater
33
in a vacuum state. Then, the alkali-earth metal carbonates are changed into alkali-earth metal oxides. Subsequently, the alkali-earth metal oxides are again heated and aged so that some of the oxides are reduced through reacting with silicon, magnesium, etc. contained in the base
31
to be thereby endowed with a semiconducting property. In this way, the coating of the electron-emitting material layer
32
can be completed.
When the aforementioned electron-emitting material layer
32
is heated up to 800~900° C. in normal conditions, it emits electrons with a current density of 0.5~0.8 A/cm
2
. However, since high-definition televisions have recently been developed, the need for a fine stream of electrons with a high current density is created and, hence, it is required to heighten the current density of the electrons up to 1~3 A/cm
2
. But, in case the conventional CRT cathode is employed for that purpose, the electron emission capacity of the cathode abruptly deteriorates so that it would be soon have to be discarded and replaced with a new one.
Therefore, the conventional CRT cathode has been currently improved and adapted to emit electrons with a current density of 1~3 A/cm
2
.
For example, Japanese Patent Laid-open No. Sho61-269828 discloses a technique of mixing the carbonate suspension of alkali-earth metals such as barium, strontium, calcium, etc. with an oxidizing scandium.
Furthermore, Japanese Patent Laid-open No. Hei2-33822 discloses a technique of attaching rare-earth metals or their compounds on the electron-emitting material layer. In the technique, the attachment is performed through applying the rare-earth metals or their compounds onto a cathode-side surface of the control grid and making the electrons emitted from the cathode to collide against the control grid. In this way, the electrons are scattered toward the electron-emitting material layer and attached thereon.
However, the experimental tests with respect to the CRT cathode manufactured in accordance with the technique disclosed in Japanese Patent Laid-open No. Sho61-269828 exhibited that the electron-emitting material layer
32
was slightly separated from the base
31
, creating an unstable electron emission.
Furthermore, the experimental tests with respect to the CRT cathode manufactured in accordance with the technique disclosed in Japanese Patent Laid-open No. Hei2-33822 exhibited that the distance between the electron-emitting material layer
32
and the control grid
34
was extremely short compared to a diameter of the electron guide hole in the control grid
34
. As a result, the scattered rare-earth metal oxides were not attached on the center portion of the electron-emitting material layer
32
opposite the electron guide hole. Thus, as a high current density is investigated with the cathode, the electron emission capacity is liable to deteriorate.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a CRT cathode which substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a CRT cathode which is not deteriorated in electron emission capacity even when electrons are emitted therefrom with a current density of 1~3 A/cm
2
.
Another object of the present invention is to provide a CRT cathode which improves the technique suggested in Japanese Patent Laid-open No. Hei2-33822 such that the rare-earth metals or their compounds can be attached on the center portion of the electron-emitting material layer to thereby heighten the current density of the electrons.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To accomplish these and other advantages, the CRT cathode includes a base having a closed top portion and containing nickel as its main component, an electron-emitting material layer coated on the top portion of the base and containing alkali-earth metal oxides as its main component, rare-earth metals or their compounds attached on a surface of the electron-emitting material layer, and a heater placed under the top portion of the base to heat it. The CRT cathode further includes a second electron-emitting material layer coated on the rare-earth metals or their compounds and containing alkali-earth metal oxides as its main component.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.


REFERENCES:
patent: 5808404 (1998-09-01), Koizumi et al.
patent: 5881355 (1999-03-01), Sugimura et al.
patent: 61-269828 (1986-11-01), None
patent: 2-33822 (1990-02-01), None

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