Preparation of cathode structures for impregnated cathodes

Abstract: Disclosed is a process for preparing a cathode structure for impregnated cathodes having desired dimensions by a simplified process without experiencing machining etc. A high-melting-point metal powder (eg. tungsten) is introduced into a mould, and heated with the heater to effect isostatic hot press moulding to provide a cathode structure for impregnated cathodes having a predetermined shape and dimensions by one operation.

Description: This invention relates to a process for preparing a cathode structure as mentioned in the preamble of claim 1. The cathode is an essential constituent of electric discharge tubes, upon which efficiency and life of the electric discharge tubes are decided. Characteristics required for the cathodes for electric discharge tubes are as follows: (a) high electron emission efficiency; (b) high current density; (c) uniform emission energy; (d) stable operation; (e) long life; (f) sufficient resistance to the vacuum pressure imparted to the electric discharge tube; and (g) no electron emission at any part other than the predetermined electron emitting surface. Impregnated cathodes having the above-described characteristics are known from the prior art. An impregnated cathode is prepared by impregnating a porous high-melting-point metal base such as porous tungsten with an electron emitting material comprising a compound oxide based on barium. The thus prepared impregnated cathode is used usually mounted on a heater sleeve with a heater accommodated therein. During operation of the cathode, the compound oxide impregnated in the porous metal base is heated by the heater and reduced at the activation temperature into free metals which diffuse throughout the surface of the porous metal base and form a single atomic layer. The thus formed single atomic layer is designed to have a greatly reduced work function compared with the tungsten, enabling efficient electron emission. The cathode structure for such impregnated cathodes has conventionally been prepared in the following manner: (a) a high-melting-point metal powder is shaped by powder extrusion method and then sintered; (b) the thus sintered porous high-melting-point metal is infiltrated with an acrylic resin as a lubricant so as to facilitate machining thereof; (c) upon completion of machining into predetermined dimensions of cathode base, the infiltrated acrylic resin is removed; and (d) the cathode base is soldered or welded onto a heater sleeve. As described above, this known method of fabrication of a cathode structure for impregnated cathodes requires considerable time, equipments and materials. According to the above prior art method, an impregnated cathode can be obtained through a long process (molding and sintering → acrylic resin infiltration → machining → acrylic resin removal → bonding), disadvantageously leading to increase in the manufacturing cost and facility cost. Meanwhile, the forming of the porous high-melting-point metal base includes a molding step and a sintering step, thereby increasing the working time. The deformation which occurs during sintering makes it impossible to obtain a desired size of the product, which is another disadvantage. The prior art EP-A-0 409 279 discloses a process for preparing a cathode structure for impregnated cathodes which comprises the pressing of a high melting point mixed metal powder and applying an isostatic pressure to the pressed mixture contained in a sealed vacuum capsule which is filed with boron nitride. In this known process many steps are necessary to obtain a desired size of the product. The prior art US-A-4 767 372 discloses a process for preparing a cathode structure for impregnated cathodes, whereby a high melting point metal powder is directly pressed in the cathode holder. It is therefore object of this invention to provide a method for preparing an improved impregnated type of cathode in which the steps are further simplified and decreased in number for further decrease the fabrication costs and to obtain a desired size of a product and also to avoid deformation during sintering. In accordance with the present invention said object is obtained with a method according to claim 1. Preferred embodiments of the invention are mentioned in the dependent claims. The features and advantages of the present invention will be more clearly appreciated from the following description of preferred embodiments taken in conjunction with the attached drawings in which: Fig. 1 shows in vertical cross section illustrating the process for preparing a cathode structure for impregnated cathodes according to a first embodiment of this invention; Fig. 2 shows a vertical cross section of a cathode structure for impregnated cathodes prepared according to the method illustrated in Fig. 1; Fig. 3 shows in vertical cross section a second embodiment of this invention; and Fig. 4 shows in vertical cross section a cathode structure for impregnated cathodes with a heater sleeve, prepared according to the second embodiment of this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The claimed method will now be described specifically referring to the attached drawings. Fig. 1 shows a process for preparing a cathode structure for impregnated cathodes according to the first embodiment of this invention. In this embodiment, an isostatic hot press molding apparatus is used for preparing a cathode structure for impregnated cathodes. A mold 4 filled with a high-melting-point metal powder 3 such as tungsten is set at the center of a heater 2 disposed in an outer vacuum vessel 1. The inside of the outer vacuum vessel 1 is evacuated by an exhaust pump 5 connected to the outer vacuum vessel 1, followed by degassing of the high-melting-point metal powder 3 by the heater 2. Subsequently, a high-pressure argon gas 6 is introduced to carry out isostatic press molding. The entire surface of the thus prepared cathode structure 7 is covered with a thin film 8 of the high-melting-point metal which was formed during molding to a thickness of 1 to 2 µm, as shown in Fig. 2. A predetermined portion of the thin film 8 intended for the electron emitting surface 9 is removed by wet or dry etching, and the etched surface is impregnated with a barium-based compound oxide to give a prescribed size of impregnate cathode structure. Since the thus obtained impregnated cathode structure is entirely covered on the surface with the thin film 8, excluding the electron emitting surface 9, no emission of electron occurs at the rest of the portions, and thus the present impregnate cathode structure can exhibit excellent electron emission characteristics. Fig. 3 shows in vertical cross section a cathode structure according to the second embodiment of this invention. A heater sleeve 11 is set in a mold 12 together with a high-melting-point metal powder 10, followed by isostatic hot press molding. As shown in Fig. 4, in the thus obtained cathode structure 13 for impregnated cathodes, the cathode base 15 is formed on the heater sleeve 11, neither requiring soldering or weldering for bonding these two members nor machining into prescribed dimensions. Of course, the bonding of the cathode base 15 onto the heater sleeve 11 can more securely be achieved by providing recesses 16 on the heater sleeve 11. The thus obtained cathode structure for impregnated cathodes has desired dimensions since isostatic hot press molding is carried out on the heater sleeve, so that the procedures of machining, infiltration with and removal of an acrylic resin and bonding can be omitted, effectively leading to great reduction in the working time and cost. Besides, since the entire surface of the cathode structure, excluding the electron emitting surface, is covered with a thin film, emission of electrons at the portions other than the electron emitting surface, which causes turbulence in the orbital function, can effectively be prevented. As has been described heretofore, since the cathode structure for impregnated cathodes according to this method is prepared by isostatic hot press molding of a high-melting-point metal on a heater sleeve, it enjoys the following effects: (a) A prescribed size of cathode structure can be molded, requiring no operations including machining, acrylic resin infiltration and removal, and bonding; (b) Emission of electron at the portions other than the electron emitting surface can be prevented, since the electron emitting surface is secured by etching; and (c) Migration of impurities, particularly oxygen, potassium or carbon, can be prevented, since the operation process is simplified and molding is carried out after degassing.

Claims: A method for preparing an impregnated cathode structure (7) using high-melting-point metal powder and isostatic pressure treatment, characterized in that filling said metal powder (3; 10) in an heated isostatic hot pressure moldung apparatus (4; 12) disposed in an outer vacuum vessel (1); evacuating the inside of said vacuum vessel (1) for degassing said metal powder (3) by heating; introducting high-pressure argon gas (6) in said vessel to carry out isostatic press molding, thereby obtaining said cathode structure (7) which is covered with a thin film (8) of said high-melting-point metal of a thickness of 1 to 2 µm formed during molding; removing a predetermined portion of said thin film (8) defining the electron emitting surface of said cathode by wet or dry etching; and impregnating said etched surface with an electron emitting material, thereby obtaining a prescribed size of impregnated cathode structure. The method according to claim 1, wherein a heater sleeve (11) is set in said hot pressure molding apparatus (12) together with said metal powder (10) for forming the base (15) of said cathode on said heater sleeve (11). The method according to claim 2, wherein recesses (16) are formed on the heater sleeve achieving more security for the bonding of the cathode base (15) onto said heater sleeve (11). The method according to one of the above-mentioned claims, wherein said high-melting-point metal is tungsten and said electron emitting material is a barium based compound oxide.

Inventors: AKIYAMA YASUHIRO, AKIYAMA, YASUHIRO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H01J9
IPC codes: B22F5, H01J9