Electron tube cathode

Abstract: An electron tube cathode which comprises, in order, a base (11) containing nickel as the main ingredient and further containing a reducing agent, a metal layer (14) containing tungsten and/or molybdenum and an emissive material layer (15) containing an alkali earth metal oxide as the main ingredient and 0.01 to 25 wt% of a rare earth metal oxide, said alkali earth metal oxide comprising barium oxide, has enhanced life characteristics especially during operation at a high current density, such as not less than 2A/cm².

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to the improvement of an electron tube cathode which is used for a TV cathode ray tube or the like. Description of the Related Art Fig. 3 shows an electron tube cathode which is used for a TV cathode ray tube or an image pick-up tube such as that described in, for example, Japanese Patent Publication No. 5417/1989. In Fig. 3, the reference numeral 1 represents a base composed of nickel as the main ingredient and further containing a trace amount of reducing element such as silicon (Si) and magnesium (Mg), 2 a cathode sleeve composed of nichrome or the like, 5 an emissive material layer which is formed on the upper surface of the base 1 and composed of alkaline earth metal oxides as the main ingredients and 0.1 to 20 wt% of a rare earth metal oxide such as scandium oxide, the alkali earth metal oxides containing at least barium oxide and further strontium and/or calcium oxide, and 3 a heater disposed in the base 1 for heating the cathode so as to emit thermions from the emissive material layer 5. A method of forming the emissive material layer 5 on the base 1 in an electron tube cathode having the above-described structure will now be explained. Barium carbonate, strontium carbonate, calcium carbonate and a predetermined amount of scandium oxide are first mixed together with a binder and a solvent to prepare a suspension. The suspension is sprayed onto the base 1 to a thickness of about 800 µm and thereafter heated by the heater 3 during the cathode ray tube evacuating process. At this time, the carbonates of the alkali earth metals are converted into alkali earth metal oxides. Thereafter, a part of the alkali earth metal oxides are reduced and activated so as to have semiconductivity. Thus, the emissive material layer 5 composed of a mixture of the alkali earth metal oxides and a rare earth metal oxide is formed on the base 1. A part of the alkali earth metal oxides are reacted in the following manner in the activating process. The reducing elements such as silicon and magnesium which are contained in the base 1 move to the interface between the alkali earth metal oxides and the base 1 by diffusion and react with the alkali earth metal oxides. For example, if the alkali earth metal oxide is assumed to be barium oxide (BaO), the reducing elements react in accordance with the following reaction formulas: BaO + 1/2Si = Ba + 1/2Ba₂SiO₄ BaO + MgO = Ba + MgO As a result of these reactions, a part of the alkali earth metal oxides which are formed on the base 1 are reduced to be an oxygen deficient semiconductor, thereby facilitating electron emission. If the emissive material layer contains no rare earth metal oxide, the operation is possible at a temperature of 700 to 800°C and a current density of 0.5 to 0.8 A/cm². If the emissive material layer contains a rare earth metal oxide, the operation is possible at a current density of 1.32 to 2.64 A/cm². Since the electron emission capability of an oxide cathode generally depends on the excess Ba content existing in the oxide, if no rare earth metal oxide is contained, the supply of excess Ba sufficient for the operation at a high current is not procured and the current density which enables the operation is low. In this case, excess Ba is not supplied sufficiently because the by-products of the above reactions such as magnesium oxide (MgO) and barium silicate (Ba₂SiO₂) are concentrated on the grain boundary of the nickel of the base 1 or the interface between the base 1 and the emissive material layer 5 to form what is called an intermediate layer, so that the rates of the reactions represented by the formulas (1) and (2) are controlled by the diffusion rates of the magnesium and silicon in the intermediate layer. On the other hand, if the emissive material layer contains a rear earth metal oxide, for example, scandium oxide (Sc₂O₃), a part of reducing agent which diffuses and moves in the base 1 during the operation of the cathode reacts with scandium oxide (Sc₂O₃) in accordance with the reaction formula (3) in the interface between the base 1 and the emissive material layer 5, thereby producing a small amount of scandium in the form of a metal, and a part of the metal scandium dissolved in the nickel in the base 1 in the form of a solid and a part thereof exists in the interface. 1/2Sc₂O₃ + 3/2Mg = Sc + 3/2MgO It is considered that since the metal scandium produced by the reaction represented by the formula (3) has an action of decomposing the intermediate layer which has been formed on the base 1 or on the grain boundary of the nickel of the base 1 in accordance with the formula (4), the supply of excess Ba is improved and the operation is possible at a higher current density than in the case of containing no rare earth metal oxide. 1/2Ba₂SiO₄ + 4/3Sc = Ba + 1/2Si + 2/3Sc₂O₃ Japanese Patent Laid-Open No. 91358/1977 discloses a technique of producing a direct-heated cathode by preparing a base of an Ni alloy which contains a high-melting metal such as W and Mo for increasing the mechanical strength and a reducing agent such as Aℓ, Si and Zr and coating the surface of the base on which an emissive material layer is formed with a layer of an alloy such as Ni-W and Ni-Mo. Japanese Patent Laid-Open No. 75128/1990 discloses a a cathode composed of a nickel base metal, an oxide layer of an alkali earth metal containing barium oxide and formed on the nickel base metal and a metal layer containing scandium and at least one selected from the group consisting of platinum, iridium and rhodium and formed between the nickel base metal and the oxide layer. In the electron tube cathodes having the above-described structures, although the rare earth metal oxide improves the supply of excess Ba, the excess Ba supplying rate is controlled by the diffusion rate of the reducing agent in the nickel of the base and the life characteristics of the cathode are greatly deteriorated in the operation at a high current density such as not less than 2A/cm². The technique disclosed in Japanese Patent Laid-Open No. 91358/1977 is aimed at ameliorating the thermal deformation of the base, which is the intrinsic problem of a direct-heated cathode for emitting thermions from the emissive material layer by utilizing the heat of the base itself which is heated by the application of a current, by coating the base with a layer of an alloy such as Ni-W and Ni-Mo. This technique does not enable the operation at a high current density. In the cathode disclosed in Japanese Patent Laid-Open No. 75128/1990, since the metal layer on the base is composed of a metal having smaller reducibility than tungsten or molybdenum, it has almost no barium oxide reducing effect for enabling the operation at a high current density. Document DE-B-1 120 605 discloses a cathode compound of a Nickel base metal, an emissive layer of alkali earth metal oxides and a layer of tungsten between the base and the missive layer. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an electron tube cathode with the life characteristics in the operation at a high current density enhanced by forming a metal layer containing at least one selected from the group consisting of tungsten and molybdenum on a base containing at least one reducing agent, and forming an emissive material layer containing an alkali earth metal oxide as the main ingredient and 0.01 to 25 wt% of a rare earth metal oxide, the alkali earth metal oxide containing at least barium oxide, on the metal layer. In the present invention, since not only the reducing agent in the base but also the metal layer formed on the base contributes to the supply of excess Ba and the metal layer also contributes to the production of a rare earth metal which stably has an intermediate layer decomposing effect in the interface, the life characteristics of the cathode especially in the operation at a high current density such as not less than 2A/cm² are greatly enhanced. The above and other objects, features and advantages of the present invention will become clear from the following description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the structure of an embodiment of an electron tube cathode according to the present invention; Fig. 2 is a graph showing the life characteristics of the embodiment shown in Fig. 1 at a current density of 2A/cm² in comparison with those of a conventional electron tube cathode; and Fig. 3 shows the structure of an embodiment of a conventional electron tube cathode DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be explained hereinunder with reference to Fig. 1. In Fig. 1, the reference numeral 14 represents a metal layer containing at least one selected from the group consisting of W and Mo and formed on the upper surface of a base 11, and 15 an emissive material layer which is formed on the metal layer 14 and contains an alkali earth metal oxide as the main ingredient and 0.01 to 25 wt% of a rare earth metal oxide such as scandium oxide and yttrium oxide. The alkali earth metal oxide of the emissive material layer 15 contains at least barium oxide and further strontium oxide and/or calcium oxide. A method of forming the metal layer 14 on the base 11 in an electron tube cathode having the above-described structure will now be explained. The Ni base 11 containing a small amount of Si and Mg is first welded to a cathode sleeve 12, and the base portion of the cathode is disposed in, for example, an electron beam depositing device so as to deposit W by heating by the electron beam in a vacuum atmosphere of about 10⁻⁵ to 10⁻⁸ Torr. Thereafter, the base portion of the cathode is heat treated at 800 to 1,000°C in, for example, a hydrogen atmosphere in order to remove the impurities such as oxygen remaining in the interior or on the surface of the metal layer 14 and to sinter or recrystallize the metal layer 14 or to diffuse the metal layer 14 in the base 11. On the cathode base with the metal layer 14 formed thereon in this way, the emissive material layer 15 is formed in the same way as in the related art. Fig. 2 is a graph showing the life characteristics of the electron tube cathode of this embodiment mounted on an ordinary cathode ray tube for a television set, which is completed through an ordinary evacuating process and operated at a current density of 2A/cm², in comparison with the life characteristics of a conventional electron tube cathode. In this embodiment, a W film of 0.2 µm thick was formed as the metal layer 14 and heat treated at 1.000°C. As the emissive material 15, alkali earth metal oxides containing 3 wt% of scandium oxide were used both in this embodiment and in the conventional example. As is obvious from Fig. 2, the deterioration of emission in the life characteristics is much less than that in the conventional example. The excellent characteristic of the electron tube cathode of this embodiment is ascribed to the following fact. Since the metal layer 14 of this embodiment is formed as a thin layer, the metal layer 14 distributes only on the Ni grains of the base 11 during operation, and since the grain boundary of Ni is exposed to the side of the emission material layer 15 on the upper surface of the base 11, the reducing agent in the base 11 is not influenced by the metal layer 14 and supplies excess Ba on the basis of the formulas (1) and (2). In addition, W of the metal layer 14 contributes to the supply of excess Ba by the reduction of the emissive material layer 15 in accordance with the following formula: 2BaO + 1/3W = Ba + 1/3Ba₃WO₆ Furthermore, since W is distributed on and in the Ni grains of the base 11, the reaction with the scandium oxide in the emissive material layer 15 is comparatively easily carried out in spite of the smaller reducibility of W than those of Si and Mg which are the reducing agents in the base 11, and also contributes the production of Sc having an intermediate layer decomposing effect. As a result of examining the distribution of W on the surface of the base metal and in the direction of the depth of the base metal immediately after aging by an Auger analyzing apparatus, it was observed that W had diffused approximately uniformly in the direction of the depth of the base metal. In other words, since W diffuses approximately uniformly in the Ni grains during the heat treatment and the operation of the cathode, the effect of forming the W layer is manifested while maintaining the reducing effects of the reducing agents Si and Mg which diffuse on the grain boundary in the Ni base. In this embodiment, the metal layer 14 is composed of W. The metal layer 14 preferably contains at least one selected from the group consisting of W and Mo. The reason for this is as follows. Since Mo has similar properties to those of W although the reducibility is slightly smaller than W, and forms an intermetallic compound with Ni like W, Mo diffuses in the Ni grains during the heat treatment of the base or during the operation of the cathode, thereby forming a uniform Ni-Mo layer and producing a similar effect to that of W. The composition of the metal layer 14 depends on the structure of the reducing agent in the base 11, and at least one is selected from the group consisting of W and Mo. It is also possible to add Ni, Pt, Ir, Rh or the like to at least one selected from the group consisting of W and Mo for the metal layer 14. The thickness of the metal layer 14 is preferably not more than 2.0 µm. Especially, if it is not more than 0.8 µm, the life characteristics in the operation at a high current density are greatly enhanced. This is because if the metal layer 14 has a thickness of not less than 2.0 µm, the diffusion rate of the reducing element in the base 11 in the emissive material layer 15 is controlled by the metal layer 14, thereby making it impossible for the reducing element to supply sufficient Ba. As the rare earth metal oxide, Sc₂O₃, Y₂O₃ or a mixture thereof has a marked effect. When the mixing ratio of the rare earth metal oxide to the alkali earth metal oxides was 0.01 to 9 wt%, the most marked effect was produced. The base with the metal layer 14 formed thereon is preferably heat treated in a vacuum or in a reducing agent at a maximum temperature of 800 to 1,100°C. The heat treatment enables the control of the metal layer 14 so as to be distributed mainly on the Ni grains of the base 11, thereby appropriately maintaining the diffusion of the reducing element in the base 11 in the emissive material layer 15. As the reducing agent, at least one selected from the group essentially consisting of Si, Mg, W, Zr and Aℓ is used, and use of at least one selected from the group consisting of Si and Mg brings about a marked effect. The electron tube cathode of this embodiment is applicable to a cathode ray tube for a TV set or an image pick-up tube. If this electron tube cathode is applied to a cathode ray tube such as projection TV and a large-size TV set and operated at a high current, a high-luminance cathode ray tube is realized. This embodiment is effective especially for enhancing the luminance of a cathode ray tube for a high-definition TV set. If this embodiment is applied to a cathode ray tube for a display monitor at a high current density, in other words, with a smaller current output area than in the related art, a higher-definition cathode ray tube than a conventional one is realized. As described above, according to the present invention, since a metal layer containing at least one selected from the group consisting of tungsten and molybdenum is formed on a base containing at least one reducing agent, and an emissive material layer containing an alkali earth metal oxide as the main ingredient and 0.01 to 25 wt% of a rare earth metal oxide is formed on the metal layer, the alkali earth metal oxide containing at least barium oxide, the operation at a high current density such as not less than 2A/cm², which is difficult in a conventional oxide cathode, is enabled and a high-luminance and high-definition cathode ray tube, which is difficult in the related art, is realized.

Claims: An electron tube cathode which comprises, in order, a base (11) containing nickel as the main ingredient and further containing a reducing agent, a metal layer (14) containing tungsten and/or molybdenum and an emissive material layer (15) containing an alkali earth metal oxide as the main ingredient and 0.01 to 25 wt% of a rare earth metal oxide, said alkali earth metal oxide comprising barium oxide. An electron tube cathode according to claim 1, wherein said reducing agent comprises silicon and/or magnesium. An electron tube cathode according to claim 1 or 2, wherein said metal layer (14) is not more than 2.0 µm thick. An electron tube cathode according to claim 3 wherein said metal layer (14) is not more than 0.8 µm thick. An electron tube cathode according to any one of the preceding claims wherein the base (11) has been heat treated in a vacuum or in a reducing atmosphere at 800 to 1,100°C. An electron tube cathode according to any one of the preceding claims wherein the emissive material layer (15) contains 0.01 to 9 wt% of strontium oxide and/or yttrium oxide. An electron tube cathode according to any one of the preceding claims comprising also a cathode sleeve (12) attached to the base (11) and a heater (13) disposed within said sleeve (12). An electron tube comprising a cathode as claimed in any one of the preceding claims.

Inventors: FUKUYAMA KEIJI C O MITSUBISHI, KAMATA TOYOKAZU C O MITSUBISHI, NAKANISHI HISAO C O MITSUBISHI, OHIRA TAKUYA C O MITSUBISHI DE, SAITO MASATO C O MITSUBISHI DE, SANO KINJIRO C O MITSUBISHI DE, SHINJOU TAKASHI C O MITSUBISHI, SUZUKI RYO C O MITSUBISHI DENK, WATANABE KEIJI C O MITSUBISHI, FUKUYAMA, KEIJI, KAMATA, TOYOKAZU, NAKANISHI, HISAO, OHIRA, TAKUYA, SAITO, MASATO, SANO, KINJIRO, SHINJOU, TAKASHI, SUZUKI, RYO, WATANABE, KEIJI, FUKUYAMA, KEIJI, C/O MITSUBISHI DENKI K. K., KAMATA, TOYOKAZU, C/O MITSUBISHI DENKI K. K., NAKANISHI, HISAO, C/O MITSUBISHI DENKI K. K., OHIRA, TAKUYA, C/O MITSUBISHI DENKI K. K., SAITO, MASATO, C/O MITSUBISHI DENKI K. K., SANO, KINJIRO, C/O MITSUBISHI DENKI K. K., SHINJOU, TAKASHI, C/O MITSUBISHI DENKI K. K., SUZUKI, RYO, C/O MITSUBISHI DENKI K. K., WATANABE, KEIJI, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H01J1
IPC codes: H01J29, H01J1, H01J9