Electron gun cathode with a metal layer having a recess

Details Electron gun cathode with a metal layer having a recess Electron gun cathode with a metal layer having a recess

1999-04-23

2001-07-03

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

Electrode and shield structures

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

C313S3460DC, C313S270000

Reexamination Certificate

active

06255764

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cathode for an electron gun used in a cathode ray tube, and more particularly, to a cathode for an electron gun for increasing its life cycle under a high current density load by ensuring a steady diffusion path of reducing component served for generating free radical barium.
2. Description of the Prior Art
A cathode ray tube is a device for forming an image by excitation light emission of a fluorescent material of a screen by landing an electron emitted from an electron gun and accelerated by high voltage on the fluorescent material.
FIG. 8
is a general structural diagram of a cathode for an electron gun in a cathode ray tube.
In
FIG. 8
, the cathode comprises a heater
4
in a sleeve
2
, a cap-formed base metal
6
composed of nickel Ni as a main component and a small amount of reducing component such as silicon Si and magnesium Mg on the upper side of sleeve
2
, and an electron emitting layer
8
mainly composed of alkaline earth metal oxide containing at least barium on the cap-formed base metal
6
.
In such a cathode, the metal oxide and the reducing component react to each other by heat from the heater to generate free radical barium, and thereafter thermion is emitted by using free radical barium.
An electron emission capacity of the cathode for the electron gun is depended on a supply amount of free radical barium contained in the metal oxide.
However, since the cathode ray tube has a tendency of high brightness and long-life cycle recently, a cathode which can supply free radical barium for a long time in high current density is required.
A cathode restraining free radical barium from evaporating by adding both of lanthanum La compound and magnesium compound Mg or La—Mg mixed compound to the electron emitting layer containing alkaline earth metal oxide is disclosed.
However, in the conventional cathode, an intermediate layer
10
is generated in a boundary between the base metal
6
and electron emitting layer
8
by reaction as shown in
FIG. 9
, and it results to shorten the life of the cathode under high current density load of 2~3 A/cm
2
.
The intermediate layer
10
is generated by reaction of barium oxide pyrolized from barium carbonate and silicon or magnesium.
BaO+Mg→Mg+Ba↑  [Reaction formula 1]
4BaO+Si→Ba
2
SiO
4
+2Ba↑  [Reaction formula 2]
Free radical barium generated by the reaction formula 1 or 2 is served to emit electron. However, MgO or Ba
2
SiO
4
is additionally generated by the same reaction formulas to generate the intermediate layer
10
in the boundary between the base metal
6
and the electron emitting layer
8
.
Such an intermediate layer
10
interferes the reaction for generating free radical barium requiring the reducing component by obstructing diffusion of the reducing component contained in the base metal
6
, to shorten the life of the cathode. In addition, since the intermediate layer
10
has a high resistance, it limits the current density possible to emit the electron by interfering flow of the electron emitting current.
In another aspect, a cathode for an electron gun comprising a metal layer mainly composed of tungsten of which the reducing degree is same as or smaller than silicon or magnesium and larger than nickel between the base metal and the electron emitting layer, and the electron emitting layer containing rare earth metal oxide to decompose the compound generated from the reaction, and thereby enabling the reducing component in the metal layer to serve to generate free radical barium is disclosed in Japanese patent laid-open No. 91-257735.
However, the cathode described above is stable at the beginning but its life cycle is suddenly deteriorated with the lapse of time since the additional reaction compound is generated when free radical barium is generated.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a cathode for an electron gun that 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 cathode for an electron gun for increasing its life cycle under a high current density load by ensuring a steady diffusion path of reducing component of a base metal used for good generation of free radical barium.
Another object of the present invention is to provide a cathode for an electron gun for preventing the shortening of its life cycle due to the loss of reducing component by obstructing the backward diffusion of reducing component contained in the base metal.
To achieve the above objectives, the present invention provides a cathode for an electron gun comprising:
a base metal composed of nickel and at least one kind of reducing component,
a metal layer having a recess to enlarge an overall surface area of the metal layer, the metal layer being disposed on the base metal ; and
an electron emitting layer containing alkaline earth metal oxide including at least barium on the metal layer.
The metal layer is formed by spreading a material selected from the group consisting of nickel, tungsten, nickel-zirconium, zirconium-tungsten and nickel-tungsten to have a recess in its center using a mask and heating it or by adhering a powder selected from the group consisting of nickel, tungsten, nickel-zirconium, zirconium-tungsten or nickel tungsten thereon to have a particle size smaller than that of the base metal.
According to another aspect, the present invention also provides a cathode for an electron gun further comprising a second metal layer made of at least one material selected from the group consisting of nickel, tungsten, tantalum and molybdenum on a lower side of the base metal. The same layer is formed by spreading or coating.
According to the present invention, since the metal layer has a particle size smaller than that of the base metal and particularly the diffusion area of reducing component is enlarged by a recess formed in the metal layer, the metal layer effectively disperses a reactant generated by the reaction of BaO and Si or Mg to prevent the generation of the intermediate layer having high resistance and to ensure a steady diffusion path of the reducing component.
In result, the reaction for generating free radical barium requiring the reducing component can be continued to increase life cycle of the cathode under high current density load of 2~3 A/cm
2
.
Further, according to the present invention, since a second metal layer is formed on the lower side of the base metal to obstruct the backward diffusion and loss of reducing component, much more reducing component reacts to electron emitting material to increase life cycle of the cathode.
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: 4893052 (1990-01-01), Tanabe et al.
patent: 5698937 (1997-12-01), Ju et al.
patent: 6091189 (2000-07-01), Shinjo et al.

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