Method for manufacturing impregnated cathodes

Abstract: A method for manufacturing an impregnated cathode wherein an impregnated pellet is fixedly fitted in a cathode cup. The method comprises the step of disposing electron emitting materials together with a porous pellet in the cathode cup and impregnating the electron emitting materials in the porous pellet to produce the impregnated pellet. The cathode cup is constituted by alloying an oxidative metal or alloy, such as silicon (Si), nickel (Ni) or chromium (Cr), which tends to react oxidatively with the electron emitting materials, in a high heat-resistant metal. In the impregnation process, a bonding of the impregnated pellet to the cathode cup can be achieved by an oxidation reaction between the electron emitting materials in the impregnated pellet and the oxidative material in the cathode cup, without any expensive brazing metals or alloys. As a result, it is possible to reduce the manufacturing cost and the total manufacturing processes.

Description: 1. Filed of the Invention The present invention relates to a method for manufacturing an impregnated cathode wherein an impregnated pellet is fixedly fitted in a cathode cup, and more particularly to a method for manufacturing an impregnated cathode wherein upon a process of impregnating an electron emitting material in a porous pellet to produce an impregnated pellet, fixing of the impregnated pellet to a cathode cup is achieved by an oxidation reaction between the electron emitting material and an oxidative material of the cathode cup. Methods of manufacturing cathodes are disclosed in GB-A- 1 264 087 and Patent Abstracts of Japan, unexamined applications, E field, vol. 12, no 486, December 19, 1988 page 3 E 695 (SONY CORPORATION) 2. Description of the Prior Art Generally, impregnated cathodes have been used in oscilloscopes which require high current density. Recently, they have been also applied to electron tubes which are used in televisions, since the electron tubes require high resolution and large screen in televisions. Referring to FIG. 1, there is shown an example of general impregnated cathode constructions. As shown in the drawing, the cathode comprises a cylindrical cathode cup 2 closed at its lower end and made of a high-resistant material, such as molybdenum (Mo). An impregnated pellet 1 is fixedly fitted in the cathode cup 2. The impregnated pellet 1 is made by impregnating an electron emitting material in a porous pellet of a heat-resistant metal such as tungsten (W). The cathode also comprises a cylindrical cathode sleeve 3 made of a high heat-resistant material such as molybdenum (Mo). The cathode sleeve 3 receives the cathode cup 2 in its upper end. Within the cathode sleeve 3, a heater 4 adapted to heat the cathode is disposed at the lower portion of cathode Sleeve 3. The impregnated cathode with the above-mentioned construction is disposed in position within an electron gun of an electron tube. In operation, as a drive power is applied to the heater 4 disposed in the cathode sleeve 3, the heater 4 generates heat. According to the heating of heater 4, heat is accumulated in the cathode sleeve 3 and then transferred to the cathode cup 2. The transferred heat to the cathode cup 2 is then transmitted to the impregnated pellet 1, so that the impregnated pellet 1 emits electrons, by virtue of the transmitted heat. In manufacturing such a general impregnated cathode, the electron emitting material is conventionally prepared by mixing BaO and CaO obtained by discomposing BaCO₃ and CaCO₃ at high temperature, with Al₂O₃. Such type of electron emitting material is melted and impregnated in pores of a porous pellet under a predetermined impregnation atmosphere, so as to form the impregnated pellet 1. As the impregnation atmosphere, a vacuum or inert gas atmosphere maintained at a temperature of about 1,600°C is used. After completing the preparation of impregnated pellet 1, a process for fixedly fitting the impregnated pellet 1 in the cathode cup 2 is performed. As the process, there has been used a method comprising the steps of providing a metal material 5, which is an alloy of molybdenum (Mo) and ruthenium (Ru) or a brazing metal, between the inner closed bottom surface of cathode cup 2 and the impregnated pellet 1 fitting into the cathode cup 2, and then carrying out a brazing at a high temperature. After completing the fitting, the cathode cup 2 is fixedly fitted in the upper end of cathode sleeve 3 such that its outer peripheral surface is in tight contact with the inner peripheral surface of the upper end of cathode sleeve 3. Thereafter, the heater 4 is inserted into the lower portion of cathode sleeve 3. Thus, the above-mentioned cathode construction is obtained However, this conventional method, wherein a brazing at high temperature is carried out to bond the impregnated pellet 1 to the cathode cup 2 under the condition that the metal material 5 is filled between the impregnated pellet 1 and cathode cup 2, has a disadvantage of an increase in manufacturing cost, since the material 5 which is a brazing metal or alloy is expensive. SUMMARY OF THE INVENTION Therefore, an object of the invention is to provide a methode for manufacturing an impregnated cathode which is capable of reducing the manufacturing cost. Another object of the invention is to provide a method for manufacturing an impregnated cathode which is capable of reducing the total manufacturing processes, as compared with the prior art. In one aspect, the present invention provides a method for manufacturing an impregnated cathode, comprising the steps of: disposing a first electron emitting material with a predetermined thickness and then a porous pellet on the inner bottom surface of a cathode cup containing an oxidative material; applying a predetermined pressure downwardly to the upper portion of the porous pellet, to impregnate the first electron emitting material in the porous pellet and at the same time to fix the porous pellet to the cathode cup; disposing a second electron emitting material with a predetermined thickness on the upper portion of porous pellet; and impregnating the second electron emitting material in the porous pellet in a predetermined impregnation atmosphere and at the same time fixing the porous pellet to the cathode cup. In another aspect, the present invention also provides a method for manufacturing an impregnated cathode, comprising the steps of: disposing a first electron emitting material with a predetermined thickness, a porous pellet and a second electron emitting material with a predetermined thickness, in turn, on the inner bottom surface of a cathode cup; and applying a predetermined pressure downwardly to the second electron emitting material, to impregnate both the first electron emitting material and the second electron emitting material in the porous pellet and at the same time fixedly bond the porous pellet to the cathode cup. In accordance with the present invention, the cathode cup is made of a high heat-resistant metal alloy which is obtained by alloying an oxidative metal or alloy, such as silicon (Si), nickel (Ni) or chromium (Cr), which tends to react oxidatively with the electron emitting materials, in a high heat-resistant metal such as moloybdenum (Mo) or tantalum (Ta). BRIEF DESCRIPTION OF THE DRAWINGS Other objects and aspects of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which: FIG. 1 is a sectional view of a general impregnated cathode construction; and FIGs. 2A to 2D are schematic views for explaining a method for manufacturing an impregnated cathode, wherein FIG. 2A shows a first impregnation step, FIG. 2B the result obtained by the first Impregnation, FIG. 2C a second impregnation step, and FIG. 2D the result obtained by the second impregnation. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGs. 2A to 2D, there is shown a method for manufacturing an impregnated cathode in accordance with an embodiment of the present invention. In accordance with the method of the present invention, a first electron emitting material 11 is first disposed on the inner bottom surface of a cathode cup 20 containing an oxidative material, as shown in FIG. 2A. On the first electron emitting material 11, a porous pellet 30 is disposed. Thereafter, an impregnation process is performed by applying a predetermined pressure P downwardly to the upper portion of the porous pellet 30 in a vacuum or inert gas atmosphere maintained at a temperature of about 1,600°C. By the impregnation process, the first electron emitting material is melted and impregnated in the porous pellet 30. At the same time, the first electron emitting material reacts oxidatively with the oxidative material contained in the cathode cup 20, producing a bonding layer 13 therebetween, so that the porous pellet 30 is fixedly bonded to the cathode cup 20, by virtue of the bonding layer 13. At this state wherein the porous pellet 30 is fixedly bonded to the cathode cup 20, the electron emitting material has been impregnated only in the lower portion of porous pellet 30. In order to also impregnate the upper portion of porous pellet 30, a second electron emitting material 12 is disposed on the porous pellet 30 and an impregnation process is performed in a vacuum or inert gas atmosphere maintained at a high temperature of about 1,600°C, as shown in FIG. 2C. As a result, an impregnated pellet 31 is obtained from the porous pellet 30 which is totally impregnated with electron emitting materials 11 and 12, as shown in FIG. 2D. The bonding layer 13 is produced by an oxidation reaction of the electron emitting materials 11 and 12 in the impregnated pellet 31 and the oxidable material in cathode cup 20 and serves to bond the impregnated pellet 31 and the cathode cup 20. The first electron emitting material 11 is of a composite oxide such as BaO, CaO, or Al₂O₃. As the electron emitting material 11, a sintered product is used which is cut to have a proper thickness. On the other hand, the cathode cup 20 is constituted by alloying an oxidative metal or alloy, such as silicon (Si), nickel (Ni) or chromium (Cr), which tends to react oxidatively with the electron emitting materials, in a high-resistant metal such as molybdenum (Mo) or tantalum (Ta). The electron emitting materials are likely to react oxidatively with the oxidative material of the cathode cup 20, thereby producing the bonding layer 13. For example, in case where silicon (Si) is used as the oxidative material of cathode cup 20, the following typical oxidation reaction is expected between the electron materials of impregnated pellet 31 and the oxidative material of cathode cup 20: Ba₂SiO₄ produced by the above reaction forms the bonding layer 13 and functions to bond strongly the impregnated pellet 31 and the cathode cup 20. After completing the fixing of impregnated pellet 31 to the cathode cup 20, a cathode sleeve 3 is fitted around the cathode cup 20. Within the cathode sleeve 3, a heater 4 is disposed. Thus, a cathode construction in accordance with the embodiment of the present invention is obtained. In accordance with another embodiment of the present invention, there is also provided a method for manufacturing an impregnated cathode which is modified from the above-mentioned method. This method comprises the steps of disposing the first electron emitting material 11, the porous pellet 30 and the second electron emitting material 12, in turn, on the inner bottom surface of cathode cup 20, applying a predetermined pressure downwardly to the second electron emitting material 12, to impregnate both the first electron emitting material 11 and the second electron emitting material 12 in the porous pellet 30 and at the same time to fixedly bond the porous pellet to the cathode cup. Similarly to the first embodiment, the cathode cup 20 is made of a high heat-resistant metal alloy which is obtained by alloying an oxidative metal or alloy, such as silicon (Si), nickel (Ni) or chromium (Cr), which tends to react oxidatively with the electron emitting materials, in a high heat-resistant metal such as molybdenum (Mo) or tantalum (Ta). As the impregnation atmosphere, a vacuum or inert gas atmosphere maintained at a temperature of about 1,600°C is used. Since the impregnation of electron emitting materials 11 and 12 in the porous pellet 30 is achieved by a single impregnation step of impregnating both the first electron emitting material 11, and the second electron emitting material 12 in the porous pellet 30, to form the impregnated pellet 31 and provide the bonding between the impregnated pellet 31 and the cathode cup 20 in accordance with the second embodiment, one impregnation step can be eliminated, as compared with the first embodiment. As apparent from the above description, the present invention provides a method for manufacturing an impregnated cathode wherein the bonding of the impregnated pellet to the cathode cup can be achieved by an oxidation reaction between the electron emitting materials in the impregnated pellet and the oxidative material in the cathode cup, without any expensive brazing metals or alloys. As a result, it is possible to reduce the manufacturing cost. The bonding between the impregnated pellet and the cathode cup is accomplished in the impregnation process, thereby reducing the total manufacturing processes.

Claims: A method for manufacturing an impregnated cathode, comprising the steps of: disposing a first electron emitting material (11) with a predetermined thickness and then a porous pellet (30) on an inner bottom surface of a cathode cup (20) containing an oxidative material; applying a predetermined pressure downwardly to the upper portion of the porous pellet (30) under a predetermined impregration atmosphere, to impregnate the first electron emitting material (11) in the porous pellet (30) and at the same time to fix the porous pellet (30) to the cathode cup (20); disposing a second electron emitting material (12) with a predetermined thickness on the upper portion of porous pellet (30); and impregnating the second electron emitting material (12) in the porous pellet (30) under a predetermined impregnation atmosphere and at same time further fixing the porous pellet (30) to the cathode cup (20). The methode of claim 1, wherein the cathode cup (20) is constituted by alloying an oxidative metal or alloy, such as silicon (Si), nickel (Ni) or chromium(Cr), which tends to react oxidatively with the electron emitting materials (11,12) in the impregnation step, in a high-resistant metal. The method of claim 1, wherein the impregnation atmosphere is a vacuum or inert gas atmosphere maintained at a temperature of about 1,600°C. A method for manufacturing an impregnated cathode, comprising the steps of: disposing a first electron emitting material (11) with a predetermined thickness, a porous pellet (30) and a second electron emitting material (12) with a predetermined thickness, in turn, on an inner bottom surface of a cathode cup (20); and applying a predetermined pressure downwardly to the second electron emitting material (12) under a predetermined impregnation atmosphere, to impregnate both the first electron emitting material (11) and the second electron emitting material (12) in the porous pellet (30) and at the same time to fixedly bond the porous pellet (30) to the cathode cup (20). The method of claim 4, wherein the cathode cup (20) is constituted by alloying an oxidative metal or alloy, such as silicon (Si), nickel (Ni) or chromium (Cr), which tends to react oxidatively with the electron emitting materials (11,12) in the impregnation step, in a high heat-resistant metal. The method of claim 4, wherein the impregnation atmosphere is a vacuum or inert gas atmosphere maintained at a termperature of about 1,600°C.

Inventors: LEE KYUNG SANG, LEE, KYUNG SANG
Applicants: GOLD STAR CO, GOLDSTAR CO. LTD.
Application year: 1992
Main classification: H01J9
IPC codes: G01R13, H01J9, H01J1, B22F3