Low power impregnated cathode of cathode-ray tube

Abstract: A low power impregnated cathode consists of a pellet (30), a cup (12), an inner sleeve (10), a tap (3), and an outer sleeve (20), the diameter (D) of the pellet is less than at least one and half times of the thickness (H) of the pellet; The outer diameter of the bottom part (24) of the outer sleeve (20) is larger than an outer diameter of the top part (22) thereof.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an impregnated cathode of a cathode-ray tube (hereinafter, referred to as CRT), and more particularly to a low power impregnated cathode of the CRT. Discussion of the Related Art In general, in a wide CRT requesting high definition and high luminance, an impregnated cathode capable of emitting high current beam for a long time has been used. Referring to Fig. 1, there will be explained the structure of the impregnated cathode according to the prior art. A pellet 1 is installed in a cup 2, the pellet 1 being used for emitting electron. The cup 2 is coupled with an inner sleave 4 by laser welding. The inner sleeve 4 is coupled with an outer sleeve 5 by a tab 3. A cathode support (not shown) is positioned near to the outer sleeve 5. Further, a heater 6 is mounted in the internal part of the inner sleeve 4, i.e., in the bottom part of the pellet 1. In the process of the manufacture of the pellet 1, metal powder is made of tungsten powder cr made by mixing the tungsten powder with rare earth metal such as Os, Ir, and Ru. The metal powder becomes a porous metal having the porosity of 20-30% by sintering. Then, the pellet 1 is made by impregnating the porous metal with electron emission material. The electron emission material consists of BaO, CaO, and Al 2 O 3 at the mole ratio of 4:1:1, 5:3:2, or 3:1:1. Further, materials such as Ir, Cs, Ru, Os-Ru, W-Ir, and W-Os is sputtered at Ar gas atmosphere and then the surface 7 of the pellet 1 is coated with the sputtering material of the thickness of 150µm, so that the pellet 1 is operated at the cathode temperature of 950-1000°C b (brightness temperature). Meanwhile, there are two kinds of the impregnated cathodes, i.e., a normal impregnated cathode having heater current of 680mA, and a low power impregnated cathode having heater current of 320mA. In the normal impregnated cathode, the pellet 1 is designed to have the thickness H of 0.5mm and the diameter D of 1.5mm. In order to implement the low power impregnated cathode, the size of all the cathode components like the pellet 1 should be reduced. Accordingly, according to the prior art, the diameter D of the pellet 1 is reduced to 1.0mm and its thickness H thereof is maintained at 0.5mm. However, there are provided the following problems in the low power impregnated electrode according to the prior art. First, the amount of electron emission material to be impregnated in the pellet is reduced, since the size of the pellet is reduced in order to lower the heater current. Accordingly, the heater current is reduced and also the electron emission capability is reduced. Second, since the cathode consists of the pellet, cup. and inner sleeve, and the cup having the pellet is connected with the inner sleeve by welding, a lot of components are used. Further, the pellet might be damaged because of the welding of the cup and the inner sleeve. Third, the loss due to heat conduction can be increased because the contact surface between the outer sleeve and the cathode support is wide. Lastly, the diameter of the outer sleeve of the low power impregnated cathode is smaller than that of the outer sleeve of the normal impregnated cathode, so that the components like the cathode support of the normal impregnated cathode should be changed. EP 0 436 360 A2 discloses a cathode for an electron gun comprising a reservoir for thermoelectron emissive material and a sleeve which has an outward flange at the top thereof and which receives the reservoir within its upper portion. A heat shielding tube has an inward flange at the top thereof which corresponds to and overlaps with the flange of the sleeve and is welded thereto. The heat shielding tube is fixed to and supported by a holder which, at its lower end, is of greater diameter than the heat shielding tube. JP 60-62034 A discloses an impregnated cathode which is accepted in a bore formed within a heat resistant body, to allow a smaller sized cathode to be used in a conventional heater and sleeve arrangement while also reducing the amount of barium evaporation. SUMMARY OF THE INVENTION Therefore, it would be desirable to provide a low power impregnated cathode capable of maintaining its life time and its electron emission capability at the same level as the normal impregnated cathode. It would also be desirable to provide a low power impregnated cathode capable of reducing the number of the components and simplifying the assembling. Accordingly, the invention provides an impregnated cathode as set out in claim 1. Preferably, in the impregnated cathode described above, a concave part where the pellet is installed may be formed in an end of the inner sleeve so that the cup and the inner sleeve can be implemented in a one-body. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: Fig. 1 is a perspective view illustrating an impregnated cathode according to the prior art; Fig. 2 is a cross sectional view illustrating a low power impregnated cathode embodying the present invention; Fig. 3 is a graph illustrating current density for cathode temperature obtained by comparing the normal impregnated cathode according to the prior art with the low power impregnated cathode embodying the present invention; and Fig. 4 is a graph illustrating cathode current for the time obtained by comparing the normal impregnated cathode according to the prior art with the low power impregnated cathode embodying the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Fig. 2 is a cross sectional view illustrating a low power impregnated cathode embodying present invention. A pellet 30 for emitting electron, is directly coupled with an inner sleeve 10 by welding, without having a cup of the prior art. The inner sleeve 10 is coupled with an outer sleeve 20 by a tab 3. A cathode support 8 is positioned near to the outer sleeve 20. Further, a heater 6 is mounted in the inner sleeve 10, i.e., the bottom, part of the pellet 30. Of course, the pellet 30 can be mounted by using the cup and the inner sleeve, like the prior art, but, it is desirable that the cup and inner sleeve are implemented in a one-body so as to reduce the number of the components and to simplify the assembling. There will be explained hereinafter examples of a construction element embodying the present invention. The inner sleeve 10 is a cylindrical shape in which one end is blocked. In one end of the inner sleeve 10 a concave part 12 is formed. That is, the bottom part of the pellet 30 is mounted in the concave part 12 of the inner sleeve 10 by a laser welding. Further, the outer sleeve 20 is designed to have the larger diameter in the bottom part than in the top part, so that the low power impregnated cathode can be implemented without changing the conventional cathode support 8 and the heat loss to the outside can be reduced accordingly. In such a structure, in order to reduce the consumption power of the heater, the bulk of the pellet should be reduced. In this case, if the diameter of the pellet 30 is reduced and the thickness thereof is constant, the electron emission material to be impregnated is reduced and the electron emission capability goes down accordingly. Thus the diameter of the pallet is reduced in order to lower the heater power and at the same time the thickness of the pellet is increased in order to obtain the electron emission capability in the same level of the normal impregnated cathode, thereby it is possible to reduce the heater power and also the electron emission capability and the life time is the same as the normal impregnated cathode according to the prior art. According to a preferred embodiment, the diameter D of the pellet 30 is less than at least one and half times as much as the thickness H thereof. In a preferred embodiment of the present invention, the heater current is designed to be below 340mA under the heater voltage of 6.3V(cathode temperature =990°C b ) Further, in a preferred embodiment of the present invention, the diameter D of the pellet 30 is designed to be less than 1.2mm and the thickness H thereof is designed to be more than 0.8mm. Further, the pellet 30 is made of more than one kind of metal powder such as W, Mo, Ta, Os, and Ir. Also, the pellet is impregnated by the electron emission material, which is made by mixing BaO, CaO, and Al 2 O 3 at the mole ratio of 4:1:1, 5:3:2, or 3:1:1. Moreover, the surface 32 of the pellet 30 is intented to be coated with one kind of metal amcng Ir, Os, Ru, Re, Mo-Os, Ir-Ta, and W-Re. On the other hand, the outer sleeve 20 is made of Ta or Kovar(Cr·Co·Fe alloy system). The outer diameter of the top part 22 of the outer sleeve 20 is more than 1.8mm and chat of the bottom part 24 thereof is less than 3.2mm. Figs 3 and 4 are graphs illustrating current density for cathode temperature and cathode current of the time obtained by comparing the normal impregnated cathode according to the prior art and the low power impregnated cathode of the type described. Referring to Fig. 3, it is known that a low impregnated cathode embodying the present invention has the electron emission capability in the same level as the normal impregnated cathode according to the prior art and also the cathode is operated under the low power. Referring to Fig. 4, it is also known that cathode current of more than 90% can be maintained even for the time up to 10,000 hours. As mentioned above, it is possible to reduce the consumption power while the electron emission capability of the type described is maintained at the same level as the conventional normal impregnated cathode. Further, the outer diameter of the bottom part of the outer sleeve is larger than that of the top part thereof and only its bottom part is rhus contacted thereto, so that it is possible to design the low power heater since the heat loss to the outside can be reduced. Also, it is possible to install a low power impregnated cathode embodying the present invention without changing the conventional cathode support. Moreover, the number of the components and the assembling steps can be reduced because the conventional cup and the inner sleeve can be implemented in a one-body. In the present invention, the stable electron emission can be made. The low power impregnated cathode of the present invention can be applied to a wide Brawn tube, a high definition tube (HDT), and a wide color display tube(CDT).

Claims: An impregnated cathode comprising a pellet (30), a cup (12), an inner sleeve (10), a tab (3), and an outer sleeve (20), the pellet (30) being installed in the cup (12) at an end of the inner sleeve (10), and the inner sleeve (10) being coupled with the outer sleeve (20) by the tab (3), characterized in that said impregnated cathode is a low power cathode having a heater current below 340 mA, the diameter of said pellet (30) is less than one and a half times the thickness of said pellet (30), and the outer diameter of the top part (22) of said outer sleeve is more than 1.8mm and less than the outer diameter of the bottom part (24) thereof which is less than 3.2mm. An impregnated cathode as claimed in claim 1, wherein a concave part where said pellet (30) is installed is formed in an end of said inner sleeve (10), thereby said cup (12) and said inner sleeve (10) is implemented in a one-body. An impregnated cathode as claimed in claim 1, wherein said diameter of said pellet (30) is less than 1.2mm and said thickness thereof is more than 0.8mm. A cathode-ray tube incorporating the cathode as claimed in any of claims 1 to 3.

Inventors: LEE JOHN HWAN, LEE, JOHN HWAN
Applicants: LG ELECTRONICS INC, LG ELECTRONICS INC.
Application year: 1997
Main classification: H01J1
IPC codes: H01J29, H01J1