Cathode in cathode ray tube

Details Cathode in cathode ray tube Cathode in cathode ray tube

2002-04-30

2004-02-03

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Electrode and shield structures

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

C313S270000, C313S337000

Reexamination Certificate

active

06686682

ABSTRACT:

This application claims the benefit of the Korean Application No. P2002-00419 filed on Jan. 4, 2001, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cathode in a cathode ray tube having a high current density and a long endurance.
2. Discussion of the Related Art
Generally, a cathode ray tube, as shown in
FIG. 1
, includes a panel
1
to which a florescent film is attached, a shadow mask
4
coupled with an inner face of the panel
1
, and a funnel
2
having a neck pipe
3
backwardly like a funnel. An electron gun
5
having a cathode
10
inside is in the neck pipe
3
so as to form electron beams by concentrating hot electrons irradiated from the cathode
10
. The electron beams are controlled by a magnetic field of a deflection yoke
6
attached outside a neck part and color selection is carried out by the shadow mask
4
so as to collide with a predetermined spot of the fluorescent film to make a fluorescent material emit lights. Hence, an image is displayed by the cathode ray tube.
Moreover, the cathode
10
, as shown in
FIG. 2
, includes an emission layer
12
, a base metal
14
, a heater
16
, a sleeve
19
, and a holder
18
.
In this case, an electron emission material of the emission layer
12
is one of BaO, SrO, CaO and the like, which is hygroscopic to react with water aggressively so as to be changed into Ba(OH)
2
, Sr(OH)
2
, Ca(OH)
2
, or the like. Such a hydroxide keeps on absorbing crystallization water so as to reduce porosity required for hot electron emission.
Substantially, a method of changing alkaline earth metal carbonate such as Ba(OH)
2
, Sr(OH)
2
, Ca(OH)
2
, or the like into oxide instead of the hygroscopic material is used for the fabrication of the cathode. A method of fabricating a cathode in CRT according to a related art is explained as follows centering on the emission layer.
First, alkaline earth metal carbonate such as CaCO3, SrCO3, CaCO3, or the like is spin-coated on the base metal
14
containing a small quantity of a reducer such as Mg, Si, Al, W and the like, and then activated by heating at about 900~1000° C.
Carbonate is dissolved into oxide and carbon dioxide by the above activation process as shown in the following Chemical Equation 1. In this case, carbon dioxide is removed by pumping or adsorption by a getter.
 (Ba, Sr, Ca)CO
2
→(Ba, Sr, Ca)CO
3
  [Chemical Equation 1]
After the activation process, an aging process is carried out by heating at a high temperature between about 800~1050° C. as well as applying a suitable electric field for stable electron emission.
The aging process is carried out for the formation of free Ba on a cathode surface and the provision of a stable and optimal electron emission environment, whereby BaO is reduced by a small quantity of the reducer such as Mg, Si, Al, W, or the like in the base metal so as to form free Ba.
Chemical Equation 2 shows an example of chemical reaction between BaO and Mg as the reducer.
BaO+Mg→Ba+MgO  [Chemical Equation 2]
In the aging process, BaO can be dissolved into Ba and O directly by electrolysis, which in shown in Chemical Equation 3.
BaO→Ba+O  [Chemical Equation 3]
The cathode in CRT is fabricated through the activation and aging processes. Oxygen(O) formed in the aging process is removed in vacuum due to evaporation at the cathode surface and ion impact, whereby excessive barium(Ba) exists in the cathode so as to be free Ba. Thus, the remaining free Ba is a positive charge to generate electrons so as to become a generating source of the emission electrons relatively.
The formation process of the emission electrons is explained in detail as follows.
In defect reaction, free Ba has a meaning equivalent to oxygen vacancy. Namely, the formation of free Ba is accompanied with that of oxygen vacancy, whereby electrons are generated. Specifically, oxygen generates free electrons enabling to be emitted by the following chemical equation of vacancy forming reaction.
O
X
o
→
1/2
O
2
(
g
)+
V
. .
0
+2
e
1
  [Chemical Equation 4]
The above equation is called a “defect reaction”, which is used in discussion of electrochemical equilibrium in a solid constructed with ion bonds like a ceramic material. IN this case, a representation of the defect type and electrical property like the right notation is the “Kroger-Vink notation”, in which upper and lower subscripts mean the electrical property and the defect type, respectively.
As known by the above equation, if oxygen(O
X
o
), which should be at an oxygen site, is removed by vacuum or reaction in the above aging process(O
2
(g)), oxygen vacancy(V
. .
0
) is formed to be electrically positive. Hence, electron(e
1
) is formed to make an electrical equilibrium so as to correspond to oxygen vacancy (V
. .
0
). Therefore, the more oxygen is removed, the more electrons are formed. In this case, it is a matter of course that the supply source of electrons is free BA having the electrons substantially.
Yet, in the method of fabricating the cathode in CRT according to the related art, byproducts of high resistance like magnesium oxide as well as Ba are formed by the chemical reaction between BaO and reducer in the aging process, whereby a middle layer is formed at an interface between the emission layer and base metal. Such byproducts grow during operation endurance to be a reason for the generation of Joule heat, thereby evaporating free Ba from the emission layer.
Moreover, the cathode is operated at a high temperature, about 1000° C., whereby sintering between particles progresses gradually to make the particles coarse. Therefore, electro-conductivity of the emission layer and the pore conductivity of electrons are reduced, thereby degrading the endurance.
Furthermore, when the cathode operates at a high temperature, Ba or BaO may evaporate as well as loss of degradation may occur, whereby free Ba becomes extinct with ease.
In order to overcome the above problems or disadvantages, a method of fabricating a cathode by adding a specific additive to an emission layer has been proposed.
U.S. Pat. No. 5,075,589 discloses a method enabling to improve an electron emission characteristic by adding a micro particles such as Y
2
O
3
, Sc
2
O
3
, or rare-earth metal oxide(ex. Eu
2
O
3
) to an mission layer containing BaO and SrO.
And, Korean Patent No. 97-51633 discloses a cathode including an emission layer, of which main elements are an activation metal containing at least one of Mg, Si, Zr, Mn, W, and Th, its oxide, and BaO, containing at last one of SrO, CaO, ScO, and aluminum oxide.
Unfortunately, the above-disclosed methods are still inadequate in prohibiting sintering and evaporation of free Ba.
Thus, the degradation of endurance of the cathode depends on the generation and extinction of free Ba. Hence, required is a method enabling to control the mechanism of the generation and extinction of free Ba as well as prohibit middle layer and the sintering of particles.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a cathode in a cathode ray tube that substantially obviates one or more problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a cathode in a cathode ray tube enabling to prevent the degradation of endurance of the cathode by carrying out the generation and extinction of free Ba stably.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these objects and other advantages and in accordance with the purpo

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