Method for manufacturing an impregnated cathode

Abstract: A method for manufacturing an impregnated cathode having a cathode pellet in which a pore portion of a sintered body of porous metal is impregnated with electron emitting material, comprising the steps of: placing said sintered body of porous metal and said electron emitting material in a container for impregnation in such a manner that said electron emitting material contacts with an entire surface of said sintered body of porous metal when said electron emitting materials are melted; and impregnating the pore portion of said sintered body of porous metal with said electron emitting material.

Description: The present invention relates to a method for manufacturing an impregnated cathode used for an electron tube. An impregnated cathode has a basic structure in which pores of a sintered body of porous metal (pellet) are impregnated with an electron emitting material. A method for manufacturing an impregnated cathode comprises the steps of: press molding powder of a high melting point metal such as tungsten, etc.; then sintering the press molded product to form a reducing substrate having a proper porosity; and then impregnating the pores of the substrate with molten electron emitting material comprising BaO, CaO and Al 2 O 3 as the main components. Thus, a cathode pellet is obtained. This cathode pellet is impregnated with emitting material in an amount corresponding to the volume of the sintered body and the porosity, i.e. the volume of pores. The principle of operation of the cathode pellet will be explained below. When the cathode pellet is subjected to a high temperature activation, BaO is reduced by the pellet to generate free Ba. This free Ba thermally diffuses in pores and reaches the surface of the pellet. Then, the free Ba thermally diffuses on the surface of the pellet, to thus form a Ba monoatomic layer on the surface of the pellet. At this time, a monoatomic layer spreads to cover an area corresponding to the difference between an amount of Ba evaporated from the monolayer, which is dependent upon the temperature of the pellet, and an amount of Ba supplied from the inside of the pellet. This Ba monoatomic layer reduces the effective work function that is involved in an electron emission from 4 to 5 eV of the metal itself constituting the pellet to about 2 eV. Consequently, excellent thermionic emission is provided. If little Ba is supplied from the inside of the pellet at the time of the operation, a necessary and sufficient area of Ba monoatomic layer cannot be formed, causing a deficiency of emission. Moreover, there arise some problems, for example, the activation takes a long time, etc. On the contrary, if too much Ba is supplied, Ba evaporated from the surface of the pellet is increased, so that the BaO impregnated in the pellet is consumed in a short time and in turn the lifetime is shortened. Furthermore, the evaporated Ba is deposited on a counter electrode, causing unnecessary electron emission, etc. The most important point of the operation of the impregnated cathode is to form a necessary and sufficient Ba monoatomic layer in an early stage and to keep it for a long time. The factors for forming a Ba monoatomic layer include: the amount of impregnated BaO; the reducing rate of the impregnated BaO being reduced by the pellet; the thermal diffusion velocity of free Ba in pores; and the surface thermal diffusion rate of Ba on an electron emitting face. The design parameters for controlling the operations are: the amount of impregnation of electron emitting material; the porosity of the pellet and the spatial distribution of pores; and the cleanness of the electron emitting face, more specifically, an absence of extra electron emitting material attached to the electron emitting face. The most important thing for mass production is to control these parameters with high precision and with less variation. Publication of Japanese Patent Application (Tokkai Sho) No. 58-87735 discloses a manufacturing method in which compressed electron emitting materials placed on the upper surfaces of the individual pellets are melted and impregnated in order to ensure the amount of impregnation of the electron emitting material . Furthermore, Publication of Japanese Patent Application (Tokkai Hei) No. 6-103885 discloses a method of mass production in which the amount of the impregnated electron emitting materials is kept stable by classifying metal raw material powder of the pellet and controlling the porosity of the pellet. Furthermore, a mechanical method using a brush, a metalclad needle, etc., a polishing method by means of cutting, etc., and ultrasonic cleaning in water, etc. have been conventionally suggested. Furthermore, Publication of Japanese Patent Application (Tokkai Sho) No. 50-103967 discloses a method in which a pellet is provided on the specific jigs one by one and then washed by ultrasonic cleaning in clean water. However, the above prepared conventional impregnated cathodes have the following problems. (1) In order to manufacture the impregnated cathode having a two-layer structure, it is necessary to use two different kinds of raw material powders or to carry out press molding twice. Consequently, the production process is complicated. (2) In the method in which a pellet is treated one by one or the raw material powder is classified, the productivity is poor and mass production is difficult. (3) The method of mechanically removing extra electron emitting materials by using a brush, metallic needle, etc., is difficult to carry out. Furthermore, a treatment is necessary for each pellet, so that mass production is difficult. (4) The manufacturing process in which the sintered pellets are provided on the specific jig one by one is complicated. It takes not less than 1 hour to perfectly remove extra electron emitting materials by way of only the ultrasonic cleaning method. Consequently mass production is difficult. It is the object of the present invention to solve the above mentioned conventional problems and to provide a method of manufacturing an impregnated cathode, which is excellent in initial electron emitting performance, lifetime property, and insulating property and which is suitable for mass production by continuously increasing the porosity of the sintered body of porous metal as the distance in the depth direction from the electron emitting face is increased. In the method for manufacturing an impregnated cathode having a cathode pellet in which a pore portion of a sintered body of porous metal is impregnated with electron emitting material comprises the steps of placing the sintered body of porous metal and the electron emitting material in a container for impregnation in such a manner that the electron emitting material contacts the entire surface of the sintered body of porous metal when the electron emitting material are melted, and impregnating the pore portion of the sintered body of porous metal with the electron emitting material. With the above mentioned impregnated cathode, deficiency of impregnation can be prevented. Consequently, stable impregnation can be obtained. It is preferable in the above method for manufacturing an impregnated cathode that electron emitting material is filled in the container for impregnation in such a manner that the depth of the electron emitting material is uniform, and the sintered body of porous metal is located at the middle portion in the direction of the depth of the electron emitting material or located at the top of the electron emitting material. It is further preferable in the afore-said method that the weight of the electron emitting material to be filled in the container for impregnation is in the range of 10 to 100 times as much as the impregnatable weight of the sintered body of porous metal in the container for impregnation. Herein, impregnatable weight means the total effective weight of emitting material that is carried by the porous sintered bodies, or something similar. By the above mentioned method for manufacturing an impregnated cathode, the variation of the amount of impregnation can be reduced. It is further preferable in the afore-said method that extra electron emitting materials are removed by shaking a container in which an impregnated cathode pellet and alumina ball are placed and washing by ultrasonic cleaning in water. By the above mentioned method for manufacturing an impregnated cathode, extra electron emitting materials can be removed while inhibiting the fracture rate of the pellet and the variation of the amount of impregnation can be reduced. Fig. 1 is a conceptual view of a cross section of an impregnated cathode of one embodiment of the present invention. Fig. 2 (A) is a graph showing the relationship between the location of the pellets at the time of impregnation and the amount of impregnation to the pellet of an impregnated cathode of one embodiment of the present invention. Fig. 2 (B) shows each location of the pellets in the container for impregnation. Fig. 3 is a graph showing the relationship between the shaking time and the amount of impregnation to the pellet of an impregnated cathode of one embodiment of the present invention and a comparative Example. Hereinafter, the present invention will be explained with reference to the drawings. Fig. 1 is a conceptual view of a cross section of an impregnated cathode pellet of the present invention. The pellet is a compressed sintered body of metal raw material powder 1. The pellet has pores in it, and the pores are filled with electron emitting materials 2. Arrow 4 illustrates the direction of the electron emission. Porosity is continuously increased along the direction from an electron emitting face 3 to the side opposite to the electron emitting face (the direction expressed by arrow 5). Moreover, the surface roughness A (maximum height) of the electron emitting face 3 is maintained in the range of 5 to 20 µm. The invention refers to a method for locating pellets on the containers for impregnation. In the method, the pellets are located in such a manner that the entire surface of the pellet contacts with the electron emitting materials at the times of impregnation. In this embodiment, the following experiments were carried out. The filling amount of the electron emitting materials was set to 3000 times, which is the preferable range shown in Embodiment 9. The impregnation was conducted in the following 4 kinds of pellet locations; a to d. Fig. 2 (B) shows the location relationship of a container for impregnation 20, pellets 21 and electron emitting material 22, respectively in a case of a to d. In a, 100 pellets were set in the same level in one stage on the bottom of the container for impregnation, and electron emitting material is filled on the pellets. In this location, the cylindrical bottom face of the pellets contact with the container for impregnation. In b, 50 pellets per stage were set in two stages on the bottom of the container for impregnation, and electron emitting material is filled on the pellets. In this location, the cylindrical upper face of the pellet of the first stage contacts with the cylindrical bottom face of the pellet of the second stage. The cylindrical bottom face of the pellet of the first stage contacts with the bottom area of the container. In c, electron emitting material is filled in the container for impregnation in a half amount by making the depth constant, then 100 pellets are set in the same level in one stage on the electron emitting material, and then the rest of the electron emitting material is uniformly filled by making the depth constant. In this location, the entire surface of the pellet contacts with the electron emitting materials. In d, whole amount of electron emitting materials is placed in the container for impregnation by making the depth constant and 100 pellets are set in the same level in one stage. In this location, the cylindrical upper face of the pellet contacts with space. Fig. 2 (A) shows the relationship between the above mentioned locations and the amount of impregnation to the pellet. The horizontal axes a to d correspond to the above mentioned locations a to d. In the location of the pellet in a and b, a few deficiencies in the impregnation occurred. In c and d, the amount of impregnation was excellent. This shows that unless the entire surface of the pellet is covered with electron emission materials, the amount of impregnation is insufficient. Moreover, in a case of d, in the state shown in Fig. 2 (B), the entire surface of the pellet is not covered with electron emitting materials. However, as the electron emitting materials are melted, the pellets sink down in the electron emitting materials due to their weight, the whole surface is naturally covered with electron emitting material. In other words, it is an important condition for stable impregnation that the entire surface of the pellet is covered with electron emitting materials when the electron emitting materials are melted. In a further embodiment the invention refers to a method for removing extra electron emitting materials attached to the pellet at the time of the impregnation. Extra emitting materials are physically removed by means of balls for grinding. In this embodiment, the pellets impregnated under the optimum condition by the method of the first-mentioned embodiment were used. These pellets were placed in the glass container having a volume of 100 cm 3 along with, for example, 10 alumina balls having a diameter of ϕ = mm, and were subjected to shaking for 5 minutes to 1 hour. Then, the pellets were subjected to ultrasonic cleaning in ion exchanged water for 5 minutes, and dried in vacuum. The relationship between the shaking time and the fracture rate of the pellets at this time is shown in the following Table 1. Com. Ex. 1 Com. Ex. 2 Ex. 1 Ex. 2 Ex. 3 Com. Ex. 3 Com. Ex. 4 Shaking time (minute) 0 0 5 15 30 60 120 Ultrasonic cleaning time (minute) 5 60 5 5 5 5 5 Fracture rate (%) 0 0 0 0.2 0.3 1 3 Com. Ex.: Comparative Example Ex.: Example Table 1 shows that in the pellet that was subjected to a shaking for 60 minutes or more (Comparative Example 3 and 4), the fracture rate of the pellets is rapidly increased. Furthermore, the amounts of impregnation to the pellets in Comparative Examples 1 to 4 and Examples 1 to 3 in Table 1 are shown in Fig. 3. Fig. 3 shows that the variation of the amount of impregnation to the pellet is minimum in Example 2 (the shaking time is 15 minutes). Since this variation reflects the attaching level of extra electron emitting materials, the pellet is excellent as this variation is smaller. The variation is small when the shaking time is 60 minutes or more (Comparative Examples 3 and 4), however, the fracture rate of the pellets is increased as mentioned above. According to the results of the Comparative Examples 1 and 2 (no shaking was conducted), the variation per pellet is little decreased even if the cleaning time is increased when only the ultrasonic cleaning is conducted. This shows that effective electron emitting materials in pores, as well as extra electron emitting material, are removed over time. In addition, it is found that this method requires an absolutely long time of treatment. Consequently, it is not suitable for mass production. Moreover, the conditions of the shaking or rolling, etc. freely can be changed by selecting the number of balls, size, volume of container, amount of the pellet to be treated, times, number of vibration frequency and amplitude of shaking, and rolling speed. As mentioned above, in each embodiment, tungsten (W) was used as one example of the material constituting the pellet. However, the material is not limited to this alone, it may be the high melting point metals, for example, osmium (Os), ruthenium (Ru), iridium (Ir), rhenium (Re), tantalum (Ta), molybdenum (Mo), etc., an alloy comprising these metals, or materials based on these metals and comprising a small amount of additives. Furthermore, in the above mentioned embodiments, the mixture comprising barium carbonate (BaCO 3 ), calcium carbonate (CaCO 3 ), aluminum oxide (Al 2 O 3 ) in a mole ration of 4 : 1: 1 was used as one example of electron emitting materials. The electron emitting material is not limited to this alone. The mixture in which the above mole ratio is changed may be used, and these mixtures in which a few amount of additives are dispersed may be used. Furthermore, instead of barium carbonate, barium oxide (BaO) may be used; and instead of calcium carbonate, calcium oxide (CaO) may be used.

Claims: A method for manufacturing an impregnated cathode having a cathode pellet in which a pore portion of a sintered body of porous metal is impregnated with electron emitting material, comprising the steps of: placing said sintered body of porous metal and said electron emitting material in a container for impregnation in such a manner that said electron emitting material contacts the entire surface of said sintered body of porous metal when said electron emitting material is melted; and impregnating the pore portion of said sintered body of porous metal with said electron emitting material. The method for manufacturing an impregnated cathode according to claim 1, wherein electron emitting materials is filled in a container for impregnation in such a manner that the depth of the electron emitting material is uniform, and said sintered body of porous metal is located in the half depth of the filled container or located on the top of said filled container. The method for manufacturing an impregnated cathode according to claim 1 and/or 2, wherein the weight of said electron emitting material to be filled in the container for impregnation is in the range of 10 to 100 times as much as the impregnatable weight of the sintered body of porous metal in the container for impregnation. The method for manufacturing an impregnated cathode according to claim 1 to 3, wherein extra electron emitting materials are removed by shaking a container in which an impregnated cathode pellet and alumina balls are placed and washing by ultrasonic cleaning in water.

Inventors: NAKAGAWA SATORU, NAKAGAWA, SATORU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: H01J1
IPC codes: H01J9