CATHODE FOR ELECTRON TUBE

Abstract: The cathode is of an indirect heated type having im­ proved electron emission characteristics. The cathode comprises: a base (1) containing Ni as a major element; and a layer (2) of an electron-emissive subst­ ance formed on the base, this layer comprising not only alka­ line earth method oxide as a principal component containing at least Ba, but also rare earth metal oxide e.g. an oxide of Sc and/or Y of 0.1 to 20 wt.% of rare earth metal e.g. Sc and/or Y of 0.05 to 15 wt.%.

Description: This invention relates to a cathode for an electron tube such as a cathode-ray tube of a TV set and particularly to an improvement in electron emission characteristics of the cathode. Description of the Prior Art Fig. 1 is a schematic sectional view illustrating a structure of a cathode for use in a cathode-ray tube (CRT) or an image pickup tube for a TV system. In a conventional cathode, a layer 2 of an electron-emissive substance made of an alkaline earth metal oxide containing at least BaO and further containing SrO and/or CaO is formed on a cylindrical base 1 essentially composed of Ni and containing a small amount of a reducing agent such as Si or Mg. A heater 3 is provided inside the base 1 and the electron-emissive layer 2 is heated by the heater 3 to emit thermal electrons. Such a conventional cathode is manufactured by a process as described below. First, a suspension of a carbonate of an alkaline earth metal (Ba, Sr, Ca, etc.) is sprayed on the base 1 and the applied suspension is heated by the heater 3 in a dynamic vacuum. As a result, the alkaline earth metal carbonate is converted to an oxide. Then, the alkaline earth metal oxide is partially reduced at a high temperature of 900 to 1000°C so that it is activated to have a semiconductive property, whereby an electron-emissive layer 2 made of an alkaline earth metal oxide is formed on the base 1. In the above described activation process, a reducing element such as Si or Mg contained in the base 1 diffuses to move toward the interface between the alkaline earth metal oxide layer and the base 1, and then reacts with the alkaline earth metal oxide. For example, if the alkaline earth metal oxide is barium oxide (BaO), the reaction is expressed by the following formula (1) or (2). BaO + 1/2Si = Ba + 1/2SiO₂ (1) BaO + Mg = Ba + MgO (2) Thus, the alkaline earth metal oxide layer 2 formed on the base 1 is partially reduced to become a semiconductor of an oxygen vacancy type. Consequently, an emission current of 0.5 to 0.8 A/cm² is obtained under the normal condition at an operation temperature of 700 to 800°C. However, in the cathode thus formed, a current density higher than 0.5 to 0.8 A/cm² can not be obtained for the following reasons. As a result of the partial reduction of the alkaline earth metal oxide, an intermediate layer of an oxide or a composite oxide such as Si0₂, Mg0 or Ba0.Si0₂ is formed in the interface region between the base 1 and the alkaline earth metal layer 2 as is obvious from the formulas (1) and (2), so that the current is limited by a high resistance of the intermediate layer. In addition, it is believed that the intermediate layer serves to prevent the reducing element in the base 1 from diffusing into the electron-emissive layer 2 so that a sufficient amount of Ba may not be generated. The above mentioned problem of oxide barrier formation has been recognised for many years. Reference to this problem is made at page 2 lines 44 to 59 of German Patent DE-C-976 106 which was granted in February 1963. Although this problem has been avoided by using reducing agents other than silicon and/or magnesium no solution to this problem has been made available to the public hitherto. The present invention is intended as a solution to the problem of oxide barrier formation just discussed. In accordance with the present invention there is provided an oxide coated cathode for an electrode tube comprising: a metal base essentially composed of nickel and including either one or both silicon and magnesium reducing agents; and a coating of electron emissive alkaline earth metal oxide, which oxide is coated on said base, and is essentially composed of barium oxide; wherein said oxide-coated cathode is characterised by: incorporation of a rare earth metal additive, containing at least one of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, holmium, dysprosium, erbium or thulium, in said metal base, or said coating, or as an interlayer located between said metal base and said coating, as follows: as 0.01 to 0.5 wt.% rare earth metal in said base; or as 0.05 to 15 wt.% rare earth metal in said coating; or as 0.1 to 20 wt.% rare earth metal oxide in said coating; or as a partially reduced rare earth metal oxide in said coating, this being produced from 0.1 to 20 wt.% rare earth metal oxide and being of enhanced reactivity; as an interlayer of rare earth metal of thickness not greater than 6 µm; or as an interlayer of rare earth metal oxide of thickness not greater than 10 µm; whereby in operative use said rare earth metal additive shall be active to inhibit or reduce formation and accumulation of oxide of said one or both silicon and magnesium or composite oxides of these, or composite oxides of these including barium oxide at an interface between said coating and said metal base, or if said interlayer is incorporated, in said interlayer. Accordingly it has been found that the incorporation of rare earth metal additives is particularly beneficial and affords a solution to the problem of oxide barrier formation. The advantages of incorporating rare earth metal additives as aforesaid are many fold. Rare earth metal additive when present in the electron emissive alkaline earth metal oxide coating contributes to the liberation of free barium. This alone contributes to an enhancement and maintenance of emission current. Furthermore, the rare earth metal additive is active to inhibit or reduce the formation and accumulation of barrier oxide and thus enables diffusion of the reducing agent silicon and/or magnesium to be sustained during operation. Accordingly the emission current versus time characteristic of the oxide coated cathode is significantly improved and also it is better adapted for use at high current densities i.e. current densities in excess of 0.8 A/cm². Further preferred features of the invention are defined in the appended claims. EP-A-0204477 being a prior art according to Article 54(3) EPC for DE, FR and NL discloses an oxide coated cathode for an electron tube having a base essentially composed of nickel and including at least one of silicon and magnesium as a reducing agent. The base is coated with a coating essentially composed of barium oxide but which also may include strontium and calcium alkaline earth metal oxide constituents. A rare earth metal additive, scandium, is incorporated as 0.1 to 20 wt.% oxide in the coating and in use serves to reduce the rate of decrease in emission current occurring during the lifetime of the cathode. This effect is attributed to the formation of a thermally unstable composite oxide of barium and scandium: Ba₃Sc₄0₉, which decomposes to liberate free barium. This is in addition to that free barium liberated by silicon or magnesium diffusing from the nickel base and this then enhances the electron emission. Although it is acknowledged that both silicon and magnesium have in general a tendency to form and accumulate as an oxide barrier between the base and the coating, it is not suggested that the rare earth metal species scandium is active to inhibit and/or reduce the formation and accumulation of this barrier. Effective and prolonged operation at a high current density, i.e. a density in excess of 0.8 A/cm², specifically 2A/cm², is disclosed. The incorporation of rare earth metal additives in cathodes other than those comprising a base essentially composed of nickel and including either silicon and/or magnesium reducing agents have been well documented hitherto. Incidentally, in a cathode disclosed in Japanese Patent Laying-Open Gazette No. 20941/1984, the thickness of the base 1 is made thin to obtain a rapid response rate in reaction in the cathode and for the purposes of preventing exhaustion of the reducing agent during the lifetime of the cathode and preventing lowering of the strength of the base 1, lanthanum is contained in a dispersed manner in the base 1 in the form of La N i₅ and La₂0₃. A cathode formed by pressing powder of mixture of W and Ba₃ Sc₄ O₉ is disclosed by A. van Oostrom et al. in Applications of Surface Science 2 (1979), pp. 173-186. German Patent Laying-Open DE-A-2626700 discloses an electron-emissive substance for high-pressure discharge lamp where an alkaline earth metal oxide such as BaO is mixed with an oxide of W or Mo and a rare earth metal oxide. British Patent No. GB-A-1592502 discloses an electron-emissive substance for a discharge lamp in which BeO and Y₂O₃ are added to Ba 2-x Sr x CaWO₆ (x = 0 - 0.5). Patent DE-C-477232 (Telefunken) discloses a cathode with a nickel wire to which is added a rare earth metal such as yttrium or scandium. It does not disclose the presence of an alkaline earth metal oxide (principally containing barium oxide) nor the presence of a reducing agent. Patent US-A-1794298 (Just) discloses a cathode which is of the impregnated type rather than a coated oxide cathode. In its composition it has a base including nickel, but there is no teaching of the inclusion of a reducing agent. This composition may include calcium, strontium or barium oxide (alkaline earth metal oxides) in the amount of e.g. 3% and a rare earth metal oxide of not more than 1% but exemplified as 0.003%. As will be apparent hereafter from the test data for enhanced current density operation, such a low proportion of rare earth metal oxide and the critical absence of a reducing agent do not permit the attainment of enhanced current density operation as afforded by cathodes according to embodiments hereinafter described. German Patent Specification DE-C-880181 discloses the incorporation of the rare earth metals lanthanum or cerium in the nickel base of an oxide coated cathode. This is used in substitition for carbon in the production of passive nickel. Only sufficient amount of rare earth metal is added to reduce any nickel oxide that is formed during heat treatment of the nickel during processing. German Patent Specification DE-C-976106 concerns the manufacture of cathode grade nickel. It discloses the addition of rare earth metal during processing and is directed to provide improvement in cold forming. It is said that reducing agent additives such as silicon should be avoided since these have a tendency during device operation to form an oxide barrier between the oxide coating and the nickel base. Japanese Kokai JP-A-535011 discloses a pressed oxide cathode and not an oxide coated cathode as in the present invention. The pressing is composed of nickel powder, alkaline earth metal oxides and reducing metals such as tungsten, molybdenum, tantalum, boron, aluminium, silicon, titanium, zirconium or manganese and rare earth metals such as cerium. Japanese Kokai JP-A-59138033 discloses an oxide cathode structure in which the base metal contains zirconium or hafnium reducing agents. O.1 to 2 wt.% lanthanum or yttrium is added during manufacture as a controlling agent to prevent or reduce crystal grain growth such as might arise during high temperature processing. From the description of embodiments of the above defined invention which follows and in particular the test results exhibited it will be readily apparent that greatly enhanced current density operation over an extended lifetime is afforded by embodiments of the present invention in contrast to the performance of prior art cathodes. Furthermore, an analysis giving reasons for this enhanced performance is set forth. In the accompanying drawings: Fig.1 is a schematic sectional view illustrating a cathode for an electron tube. Fig.2A is a graph showing the relation between the life test period and the emission current under the normal condition after the test in an embodiment of the present invention and Fig.2B is a graph showing the relation between the current density during the life test and the emission current under the normal condition after the test. Figs.3A and 3B are graphs showing results of chemical analyses by EPMA as to the interface region between the base and the electron-emissive layer after a long period of the life test in a conventional cathode and a cathode of an embodiment, respectively. Fig. 4A is a graph showing the relation between the life test period and the emission current after the test in another embodiment of the present invention and Fig. 4B is a graph showing the relation between the current density during the life test and the emission current after the test. Fig. 5A is a graph showing the relation between the life test period and the emission current in a further embodiment of the present invention and Fig. 5B is a graph showing the relation between the current density during the life test and the emission current. Fig. 6A is a graph showing the relation between the life test period and the emission current in a still further embodiment of the present invention and Fig. 6B is a graph showing the relation between the current density during the life test and the emission current. Fig. 7 is an enlarged fragmentary sectional view schematically illustrating a cathode according to a still further embodiment of the present invention. Fig. 8 is a sectional view illustrating a cathode according to a still further embodiment of the present invention. Fig. 9 is a sectional view illustrating a cathode according to a still further embodiment of the present invention. Fig.10 is a graph showing the relation between the thickness of a rare earth metal oxide layer in a cathode of the embodiment in Fig.9 and the emission current after the life test. Fig.11 is a graph showing the relation between the thickness of a rare earth metal layer in a cathode of the embodiment in Fig.9 and the emission current after the life test. Fig.12 is a graph showing the relation between the rare earth metal content in the base of a cathode and the emission current after the life test according to a still further embodiment of the present invention. So that this invention shall be better understood, embodiments thereof will now be described and particular reference will be made to the drawings. The description that follows is given by way of example only. In a cathode according to an embodiment of the present invention, a layer 2 of an electron-emissive substance formed on a base 1 comprises an alkaline earth metal oxide as a principal component containing at least BaO and additionally containing SrO and/or CaO in certain circumstances. This layer 2 of the electron-emissive substance further contains a rare earth metal oxide of Sc or Y in 0.1 to 20 wt.%. The above described cathode can be manufactured by the below described process. First, scandium oxide powder or ytrium oxide powder is mixed in a ternary carbonate containing Ba, Sr and Ca, by an amount corresponding to a desired wt.% (to be obtained after the above stated ternary carbonate has been all converted to oxide). Then, nitrocellulose lacquer and butyl acetate are added to the mixture thus obtained so that a suspension is prepared. This suspension is applied to the base 1 containing Ni as a major element by a spray method so that the applied suspension has a thickness of approximately 80 µm. After that, the carbonate is decomposed to oxide, in the same manner as in the prior art, and the oxide is partially reduced so that the electron-emissive layer 2 on the base 1 is activated. In the above described manner, cathodes provided with electron-emissive layers 2 containing Sc₂O₃ or Y₂O₃ in various wt.% were prepared. Then, diode vacuum tubes using those cathodes were prepared and they were subjected to life tests using various constant current densities so that changes in the emission current under the normal condition after the tests were examined. Fig. 2A shows the emission current in a cathode containing Sc₂O₃ in 5 wt.%, a cathode containing Y₂O₃ in 12 wt.% and a conventional cathode not containing any rare earth metal oxide, respectively, after the life test using a constant current density (2.05 A/cm²) 3.1 times as large as the operation current density 0.66 A/cm² of a conventional cathode for CRT under the normal condition. The vertical axis in Fig. 2A represents the ratio of the emission current under the normal condition after the life test to the initial emission current under the normal condition. With the cathodes according to this embodiment, an initial emission current of 1 to 2 A/cm² can be obtained under the normal condition at the operation temperature of 700 to 800°C. As is obvious from this figure, the cathodes containing rare earth metal oxides have characteristics that the emission current after the life test with the high current density is less lowered as compared with the conventional cathode. Fig. 2B shows the ratio of the emission current under the normal condition after the life tests of 6000 hr to the initial emission current under the normal condition, as the result of the life tests conducted using a constant current density of 0.66 A/cm² and constant current densities of twice, 3.1 times and 4 times that value with respect to the cathodes provided with electron-emissive layers 2 containing Sc₂O₃ or Y₂O₃ in various wt.%. As can be seen from Fig. 2B, Sc₂O₃ or Y₂O₃ more than 0.1 wt.% has an effect in preventing lowering of the emission current under the normal condition after the life test with the high current density. Though not shown in Fig. 2B, this effect was found up to the concentration of 20 wt.% of Sc₂O₃ or Y₂O₃. However, if the concentration of Sc₂O₃ or Y₂O₃ exceeds 20 wt.%, it becomes difficult to obtain a stable emission current unless a further aging process for a long period is applied after the manufacturing process. Therefore, the content of a rare earth metal oxide in the electron-emissive layer 2 is preferably in the range from 0.1 to 20 wt.% and more preferably in the range from 0.3 to 15 wt.%. It is believed that the good electron emission characteristics of the cathodes according to the above described embodiment are obtained from the following reasons. (1) The powder of Sc₂O₃ or Y₂O₃ mixed in the electron-emissive layer 2 reacts with the alkaline earth metal oxide, e.g., BaO and forms a composite oxide Ba₃Sc₄O₉ or Ba₃Y₄O₉. This composite oxide dispersed in the electron-emissive layer 2 tends to thermally decompose and produce free Ba at the operation temperature of the cathode. Although the formation of free Ba in the conventional cathode completely depends on the reducing process caused by a small amount of the reducing element Si or Mg in the base 1, the thermal decomposition of the composite oxide produces additional free Ba in this embodiment. Therefore, there exists a sufficient amount of free Ba in the cathode of this embodiment, even though the reducing process is limited by the intermediate layer as described previously. (2) Some of the composite oxide also set the Sc element or Y element free and produce metallic Sc or Y dispersed in the electron-emissive layer 2. This metallic Sc or Y increases electric conductivity of the electron-emissive layer 2, compensating for the resistance of the intermediate layer. In order to precisely examine the effect of the rare earth metal oxide contained in the electron-emissive layer 2, the cathode containing Sc₂O₃ in 5 wt.% and the conventional cathode after the life test of 6000 hrs as shown in Fig. 2A were analyzed by using an electron probe micro analyzer (EPMA). Fig. 3A shows the results of the analysis in the interface region between the base 1 and the electron-emissive layer 2 of the conventional cathode. As is obvious from Fig. 3A, the reducing agents Si and Mg are segregated in the vicinity of the interface between the base 1 containing Ni as a major element and the electron-emissive layer 2. In the segregated state, a peak of Si and that of Mg are observed at a position of approximately 5 µm from the interface toward the base 1 and at a position of approximately 3 to 5 µm from the interface toward the electron-emissive layer 2, respectively. The largest peak of Si is observed at a position of approximately 13 µm from the interface toward the electron-emissive layer 2. Though not shown, peaks of Ba were observed at the same positions as the peak positions of Mg and Si in the electron-emissive layer. Since these peak positions of Si, Mg and Ba are almost coincident to the peak positions of oxygen, these elements are considered to exist as oxides or composite oxides. More specifically, in a conventional cathode, layers of SiO₂, MgO and a composite oxide thereof are formed in the grain boundary in the base 1 near the interface during the life test with the high current density and layers of oxides BaO, MgO and SiO₂ and composite oxides thereof are formed in the electron-emissive layer 2 at locations near the interface. The layer of SiO₂·MgO and the layer of BaO·SiO₂ suppress diffusion of the reducing agents Si and Mg from the base 1 into the electron-emissive layer 2 and also suppress flow of electric current because of high resistance of those layers. On the other side, Fig. 3B shows results of the analysis of the cathode containing Sc₂O₃ according to this embodiment. Referring to Fig. 3B, the elements Si and Mg are dispersed uniformly in each of the base region and the electron-emissive region and such high peaks as shown in Fig. 3A are not observed. This is supposed to be because the rare earth metal oxide prevents oxidation of the interfacial layer of the base 1 when the alkaline earth metal carbonate is decomposed to oxide or when dissociation reaction occurs in BaO or the like during the operation of the cathode. For example, when Sc₂O₃ is selected as a rare earth metal oxide, reaction as indicated below is considered to occur in the interface region. BaCO₃ → BaO + CO₂ (3) Ni + 1/2CO₂ → NiO + 1/2C (4) Sc₂O₃ + 3CO₂ → Sc₂ (CO₃)₃ (5) BaO → Ba + O (6) Ni + O → NiO (7) 3Ba + 3O + Sc₂O₃ → Ba₃Sc₄O₉ (8) More specifically stated, when Sc₂O₃ is not contained in the electron-emissive layer, BaCO₃ in that layer reacts with Ni in the base according to the formulas (3), (4), (6) and (7) whereby an oxide layer of NiO is formed in the interfacial layer of the base 1. On the other hand, if Sc₂O₃ is contained in the electron-emissive layer 1, Sc₂O₃ reacts preferentially with BaCO₃ or BaO according to the formulas (3), (5), (6) and (8) and accordingly there is not formed any oxide layer of NiO on the surface of the base 1. Since the base 1 contains Si and Mg as reducing agents, layers of SiO₂ and MgO are formed in the vicinity of the interface if Sc₂O₃ is not contained in the electron-emissive layer. Accordingly, diffusion of the reducing agents Si and Mg into the electron-emissive layer 2 is limited by the oxide layers of SiO₂ and MgO and the reactions represented by the formulas (1) and (2) occur only in the vicinity of those oxide layers. As a result, oxide layers of SiO₂ and MgO are formed preferentially in the vicinity of the interface particularly during the life test with the high current density and diffusion of Si and Mg into the electron-emissive layer is further limited, and thus the emission current under the normal condition is extremely lowered. In a cathode according to this embodiment, the rare earth metal oxide in the electron-emissive layer 2 suppress oxidation of Ni, Si and Mg to prevent formation of an oxide film in the interface region and in consequence the reducing elements Si and Mg easily diffuse deep into the electron-emissive layer 2. Accordingly, the reactions represented by the formulas (1) and (2) occur more homogeneously within the electron-emissive layer 2. In addition, since the rare earth metal oxide suitably controls diffusion rate of the reducing elements in the electron-emissive layer, the emission characteristics of the cathode can be maintained stably and in good condition even after the life test with the high current density for a long period. However, a cathode containing a rare earth metal oxide of less than 0.1 wt.% can not achieve satisfactorily the effect of suppressing formation of the oxide layers of SiO₂ and MgO in the vicinity of the interface and as a result the emission characteristics can not be improved sufficiently. To the contrary, a rare earth metal oxide of more than 20 wt.% suppresses excessively diffusion of the reducing elements in the electron-emissive layer 2 and the emission characteristics can not be improved sufficiently either. On the other hand, in a cathode containing a rare earth metal oxide of 0.2 to 20 wt.%, the rare earth metal dissolved into the base 1 was observed. In addition, separation of the electron-emissive layer 2 from the base 1 never occurred after the life test for 6000 hrs (with a current density of 2.05 A/cm²). As for the conventional cathodes, separation of the electron-emissive layer 2 was observed with frequency of 30 %. Although a cathode using Sc₂O₃ and/or Y₂O₃ as the rare earth metal oxide(s) was described in the above embodiment, the same effect can also be obtained if rare earth metal oxides containing La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, etc. are used. Such oxides as Sc₂O₃, Y₂O₃ and Ce₂O₃ are particularly preferred. According to another embodiment of the present invention, 0.1 to 20 wt.% rare earth metal oxide powder is subjected to a heat treatment in a reducing atmosphere before it is mixed with an alkaline earth metal oxide. This heat treatment may be perfomed in a gas containing hydrogen at a temperature of 800 o C or more, preferably 1000 o C or more, for a period of 10 minutes or more. This heat treatment causes partial reduction of the rare earth metal oxide thereby to enhance the reactive property of the rare earth metal oxide. Fig. 4A shows, in the same manner as in Fig. 2A, the emission current after the life test with 2.05 A/cm² with regard to cathodes according to this embodiment. The lowering of the emission current in Fig. 4A is suppressed a little further than that in Fig. 2A. Fig. 4B shows, in the same manner as in Fig. 2B, the emission current of cathodes according to this embodiment after the life tests of 6000 hrs using various high current densities. The decrease of the emission current in Fig. 4B is suppressed a little more than that in Fig. 2B. According to a further embodiment of the present invention, a rare earth metal oxide is contained in the electron-emissive layer in the form of a composite oxide of Ba₃Sc₄O₉ or Ba₃Y₄O₉. Fig. 5A shows, in the same manner as in Fig. 2A, the emission current after the life test with 2.05 A/cm² with regard to cathodes according to this embodiment. Fig. 5B shows, in the same manner as in Fig. 2B, the emission current after the life test with various high current densities with regard to cathodes according to this embodiment. Although a cathode containing Ba₃Sc₄O₉ or Ba₃Y₄O₉ was shown in this embodiment, other composite oxides such as BaSc₂O₄, BaY₂O₄, Sr₃Sc₄O₉, Ca₃Sc₄O₉ and Ba₃Ce₄O₉ containing alkaline earth metals and rare earth metals can also be used effectively. According to a still further embodiment of the present invention, the electron-emissive layer 2 contains not only a rare earth metal oxide of 0.1 to 20 wt.% but also powder of 10 wt.% or less comprising at least one of Ni and Co. Ni and/or Co powder serves to provide a better conductivity for the electron-emissive layer 2 and to improve the adhesive property of this layer 2 to the base. Table I indicates the emission current under the normal condition as to cathodes according to this embodiment after the life test of 6000 hrs using a high current density (2.6 A/cm²) 4 times as large as 0.66 A/cm². Sample Content in electron-emissive layer (wt.%) Normalized emission-current after life test (%) Sc₂O₃ Ni 0 - - 32 1 0.05 0.1 38 2 0.1 0.1 60 3 0.5 0.1 70 4 5 0.1 83 5 10 0.1 85 6 20 0.1 61 7 25 0.1 40 8 5 0.05 78 9 5 1 85 10 5 5 87 11 5 10 65 12 5 13 45 In this table, sample 0 is a conventional cathode in which the electron-emissive layer comprises a ternary alkaline earth metal oxide of (Ba, Sr, Ca) 0. Samples 1 through 12 contain Sc₂O₃ and Ni in addition to the ternary alkaline earth metal oxide. As is clear from this table, there is less deterioration in the emission current after the life test with the high current density in the cathodes containing Sc₂O₃ and Ni as compared with the conventional cathode. Particularly, Sc₂O₃ of 0.1 to 20 wt.% and Ni of less than 10 wt.% are preferred for improvement of the emission characteristics of the cathode. If the content of Ni exceeds 10 wt.%, sintering occurs between the Ni powder and the alkaline earth metal oxide powder to cause unfavorable influence on the surface of the electron-emissive layer, resulting in deterioration of the electron emission characteristics. Although the electron-emissive layer containing Ni was described in this embodiment, an electron-emissive layer containing Co can also be used effectively. According to a still further embodiment of the present invention, the electron-emissive layer 2 contains not only scandium oxide of 0.1 to 20 wt.% but also a reducing metal of 1 wt.% or less. Table II shows, in the same manner as Table I, the emission current after the life test with the high current density as to cathodes containing Fe as a reducing element. Sample Content in electron-emissive layer (wt.%) Normalized emission-current after life test (%) Sc₂O₃ Fe 0 - - 32 1 0.05 0.1 38 2 0.1 0.1 60 3 0.5 0.1 68 4 5 0.1 80 5 10 0.1 82 6 20 0.1 60 7 25 0.1 37 8 5 0.007 75 9 5 0.01 75 10 5 0.05 78 11 5 0.3 80 12 5 0.5 75 13 5 1.0 60 14 5 1.3 43 The reducing element Fe assists the rare earth metal oxide in suppressing formation of oxide layers of SiO₂ and MgO in the interfacial layer of the base 1. The content of Fe is preferably 1 wt.% or less. If it exceeds 1 wt.%, the alkaline earth metal oxide is reduced excessively and Ba is produced in an excessive amount, causing the lifetime of the cathode to be decreased. Although Fe was described as the reducing metal in this embodiment, such metals as Ti, Zr, Hf, V, Nb, Ta, Si Al, Cu, Zn, Cr, Mo and W may also be used. Although only Sc rare earth metal oxide has been discussed above, any of the rare earth metal oxides containing La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er or Tm may be substituted. According to a still further embodiment of the present invention, the electron-emissive layer 2 contains as a major element an alkaline earth metal oxide containing at least Ba and also contains a rare earth metal of 0.05 to 15 wt.%. Fig. 6A shows, in the same manner as in Fig. 2A, the emission current after the life test with the current density of 2.05 A/cm² as to cathodes according to this embodiment. As can be seen from this figure, lowering of the emission current in the cathodes of this embodiment is much suppressed as compared with the conventional cathode. Fig. 6B shows, in the same manner as in Fig. 2B, the emission current after the life tests of 6000 hrs with various high current densities as to cathodes according to this embodiment. As can be seen from this figure, a rare earth metal of more than 0.05 wt.% contributes effectively to an improvement of the emission characteristics. However, if the rare earth metal exceeds 15 wt.%, it becomes difficult to obtain a stable emission current unless aging for a long period is applied, and such procedure is not preferred from a practical point of view. Therefore, the content of the rare earth metal oxide in the electron-emissive layer 2 is preferably in the range from 0.1 to 15 wt.% and more preferably in the range from 0.2 to 7 wt.%. Although the cathode containing Sc or Y was shown in this embodiment, La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er or Tm may also be used. Fig. 7 is an enlarged fragmentary sectional view schematically illustrating a cathode according to a still further embodiment of the present invention. In this embodiment, the electron-emissive layer 2 comprises a first layer 2a formed on the base 1 and a second layer 2b formed on the first layer 2a. The first layer 2a contains not only alkaline earth metal oxide powder 21 but also rare earth metal oxide powder 22 of 0. 1 to 20 wt.% containing Sc. The second layer 2b contains only alkaline earth metal oxide powder 21. Usually, each of the first and second layers 2a and 2b is formed to be approximately 40 µm in thickness. The cathode of this embodiment has a particularly stable initial electron-emission characteristic of 1 to 2 A/cm² under the normal condition at the operation temperature of 700 to 800°C. Although only Sc rare earth metal oxide has been discused above, any of the rare earth metal oxides containing La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er or Tm may be substituted. Fig. 8 shows a cathode according to a still further embodiment of the present invention. In this embodiment, a sintered Ni powder layer 4 is formed on the surface of the base 1, and the electron-emissive layer 2 containing not only an alkaline earth metal oxide but also a rare earth metal oxide of 0.1 to 20 wt.% is formed on the sintered powder layer 4. The sintered Ni powder layer is formed in the following manner. Ni metal powder having a grain size of 3 to 5 µm is mixed with nitrocellulose lacquer and butyl acetate so that a suspension is prepared. This suspension is applied to the base 1 by a spray method so that the applied suspension has a thickness of approximately 30 µm. Then, the applied suspension is subjected to a heat treatment in an atmosphere of hydrogen at 1000°C for 10 minutes so that it is sintered. The sintered Ni powder layer 4 is porous and thus a part of the electron-emissive layer 2 applied thereon penetrates the sintered layer 4 to be in direct contact with the base 1. Even if the above described intermediate layer of SiO₂, MgO or the like is formed in the region of contact with the base 1, lowering of the conductivity due to the formation of the intermediate layer can be prevented because a considerably large part of the electron-emissive layer 2 contacts the sintered layer 4. The thickness of the sintered Ni powder layer 4 is chosen as 10 to 50 µm. A sintered layer of less than 10 µm is not effective because the intermediate layer of oxide might be formed on the side of the electron-emissive layer, exceeding the sintered layer. On the contrary, if the thickness exceeds 50 µm, the alkaline earth metal oxide can not be sufficiently penetrated into the sintered layer 4 and thus does not sufficiently come in contact with the base 1 containing the reducing element and, as a result, activation of the electron-emissive layer 2 can not be made in a satisfactory manner. Fig. 9 shows a cathode according to a still further embodiment of the present invention. In this embodiment, a rare earth metal oxide layer 5a or a rare earth metal layer 5b is provided between the base 1 and the electron-emissive layer 2 made of an alkaline earth metal oxide. The rare earth metal oxide layer 5a or the rare earth metal layer 5b is formed by an electron beam evaporation method or a sputtering method prior to formation of the electron-emissive layer 2. In the above described cathode, the rare earth metal dissolves from the layer 5a or 5b into the base 1. Accordingly, even if oxygen produced by dissociation of BaO or other similar phenomenon is diffused into the base 1, segregation of SiO₂ and MgO in the interfacial region of the base 1 is suppressed because the rare earth metal dissolved in the base 1 reacts with the oxygen to form a rare earth metal oxide. In addition, the rare earth metal dissolved into the base 1 serves to strengthen the adhesion between the layer 5a or 5b and the base 1 and to prevent embrittlement of the base 1 containing Ni as a major element. Fig. 10 shows the emission current after the life test of 6000 hr with the current density of 2.05 A/cm² with regard to cathodes provided with the rare earth metal oxide layer 5a of Sc₂O₃ or Y₂O₃ having various values of thickness. As is clear from this figure, the cathode having the rare earth metal oxide layer of less than 10 µm in thickness shows an extremely excellent characteristic in prevention of lowering of the emission current as compared with a conventional cathode. However, if the thickness of the rare earth metal oxide layer exceeds 10 µm, the reducing elements Si and Mg can not be diffused sufficiently from the base 1 into the electron-emissive layer 2 and separation of the rare earth metal oxide layer 5a from the base 1 may occur during the life test with the high current density. Fig. 11 shows, in the same manner as Fig. 10, the emission current with regard to cathodes provided with the rare earth metal layer 5b containing Sa or Y having various values of thickness. As is clear from this figure, the cathode having the rare earth metal layer of less than 6 µm shows much less deterioration in the emission current as compared with a conventional cathode. However, if the thickness of the rare earth metal layer exceeds 6 µm, the reducing elements Si and Mg can not be diffused sufficiently from the base 1 into the electron-emissive layer 2, causing the emission current to be considerably decreased. Although the oxide layer 5a or the metal layer 5b containing Sc or Y was described in the embodiment in Fig. 9, an oxide or a metal containing at least one of the metals La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er and Tm may also be used. In a cathode according to a still further embodiment of the present invention, a rare earth metal of 0.01 to 0.5 wt.% is contained in the base 1. An electron-emissive layer 2 made of an alkaline earth metal oxide containing at least Ba is formed directly on this base 1. Fig. 12 shows the relation between the rare earth metal content of Sc and/or Y in the base of the cathode according to this embodiment and the emission current after the life test of 6000 hrs with the current density of 2.05 A/cm². As is clear from this figure, the cathode having the base 1 containing rare earth metal of 0.01 to 0.5 wt.% shows a by far smaller degree of lowering of the emission current compared with a conventional cathode. If the rare earth metal concentration is less than 0.01 wt.%, it can not serve to sufficiently suppress formation of oxide layers of SiO₂ and MgO in the interfacial layer of the base 1. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the present invention being limited only by the scope of the appended claims.

Claims: Claims for the following Contracting States : GB, IT An oxide coated cathode for an electron tube comprising: a metal base essentially composed of nickel and including either one or both silicon and magnesium reducing agents; and a coating of electron emissive alkaline earth metal oxide, which oxide is coated on said base, and is essentially composed of barium oxide; wherein said oxide-coated cathode is characterised by: incorporation of a rare earth metal additive, containing at least one of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, holmium, dysprosium, erbium or thulium, in said metal base, or said coating, or as an interlayer located between said metal base and said coating, as follows: as 0.01 to 0.5 wt.% rare earth metal in said base; or as 0.05 to 15 wt.% rare earth metal in said coating; or as 0.1 to 20 wt.% rare earth metal oxide in said coating; or as a partially reduced rare earth metal oxide in said coating, this being produced from 0.1 to 20 wt.% rare earth metal oxide and being of enhanced reactivity; or as an interlayer of the said rare earth metal of thickness not greater than 6 µm; or as an interlayer of the said rare earth metal oxide of thickness not greater than 10 µm; whereby in operative use said rare earth metal additive shall be active to inhibit or reduce formation and accumulation of oxide of said one or both silicon and magnesium or composite oxides of these, or composite oxides of these including barium oxide, at an interface between said coating and said metal base, or if said interlayer is incorporated, in said interlayer. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated in said coating, and said cathode includes a further coating of an electron emissive alkaline earth metal oxide, which latter oxide is coated on said coating incorporating said rare earth metal additive, is essentially composed of barium oxide, and is, as provided, substantially free of any rare earth metal additive. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated as oxide in said coating, said cathode including a layer of porous sintered nickel interposed between said metal base and said coating, which layer is of thickness 10 to 50 µm and incorporates a portion of said alkine earth metal oxide and said rare earth metal additive. A cathode, as claimed in claim 1, wherein said rare earth metal additive is yttrium or scandium and is incorporated as 0.3 to 15 wt.% oxide in said coating. A cathode, as claimed in claim 1, wherein said rare earth metal additive contains yttrium and scandium incorporated as 0.1 to 20 wt % rare earth metal oxide in said coating. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated as oxide in said coating and is formed with said barium oxide or other alkaline earth metal oxide constituent of said coating as a composite oxide. A cathode as claimed in claim 6 wherein said coating includes at least one of the following composite oxides: Ba₃Sc₄0₉; Ba₃Y₄0₉; Ba Sc₂ 0₄; Ba Y₂ 0₄; Sr₃ Sc₄ 0₉; Ca₃ Sc₄ 0₉; or Ba₃ Ce₄ 0₉. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated as oxide in said coating, said coating also including up to 10 wt.% of a conductive additive being either one of or both nickel and cobalt. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated as oxide in said coating, said coating also including up to 1 wt.% of a reducing metal additive, which reducing metal additive is selected from iron, titanium, zirconium, hafnium, vanadium, niobium, tantalum, aluminium, copper, zinc, chromium, molybdenum and tungsten. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated as 0.2 to 7 wt.% oxide in said coating. A method of producing a coating for the oxide coated cathode as claimed in claim 1 and wherein said rare earth metal additive is incorporated as the partially reduced rare earth metal oxide in said coating, which method inlcudes a step of heating 0.1 to 20 wt.% of rare earth metal oxide powder in a reducing atmosphere containing hydrogen at a temperature of 800 o C or more for a period of 10 minutes or more before mixing it with alkaline earth metal oxide and applying said coating. Claims for the following Contracting States : DE, FR, NL An oxide coated cathode for an electron tube comprising: a metal base essentially composed of nickel and including either one or both silicon and magnesium reducing agents; and a coating of electron emissive alkaline earth metal oxide, which oxide is coated on said base, and is essentially composed of barium oxide; wherein said oxide-coated cathode is characterised by: incorporation of a rare earth metal additive, containing at least one of yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, holmium, dysprosium, erbium or thulium, in said metal base, or said coating, or as an interlayer located between said metal base and said coating, as follows: as 0.01 to 0.5 wt.% rare earth metal in said base; or as 0.05 to 15 wt.% rare earth metal in said coating; or as 0.1 to 20 wt.% rare earth metal oxide in said coating; or as a partially reduced rare earth metal oxide in said coating, this being produced from 0.1 to 20 wt.% of rare earth metal oxide and being of enhanced reactivity; or as an interlayer of the said rare earth metal of thickness not greater than 6 µm; or as an interlayer of the said rare earth metal oxide of thickness not greater than 10 µm; whereby in operative use said rare earth metal additive shall be active to inhibit or reduce formation and accumulation of oxide of said one or both silicon and magnesium or composite oxides of these, or composite oxides of these including barium oxide at an interface between said coating and said metal base, or if said interlayer is incorporated, in said interlayer. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated in said coating, and said cathode includes a further coating of an electron emissive alkaline earth metal oxide, which latter oxide is coated on said coating incorporating said rare earth metal additive, is essentially composed of barium oxide, and is, as provided, substantially free of any rare earth metal additive. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated as oxide in said coating, said cathode including a layer of porous sintered nickel interposed between said metal base and said coating, which layer is of thickness 10 to 50 µm and incorporates a portion of said alkaline earth metal oxide and said rare earth metal additive. A cathode, as claimed in claim 1, wherein said rare earth metal additive is yttrium and is incorporated as 0.3 to 15 wt.% oxide in said coating. A cathode as claimed in claim 1 wherein said at least one rare earth metal additive is incorporated as oxide in said coating and is formed with said barium oxide or other alkaline earth metal oxide constituent of said coating as a composite oxide. A cathode as claimed in claim 5 wherein said coating includes at least one of the following composite oxides: Ba₃ Y₄ 0₉; Ba Y₂ 0₄; or Ba₃ Ce₄ 0₉. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated as oxide in said coating, said coating also including up to 10 wt.% of a conductive additive being either one of or both nickel and cobalt. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated as oxide in said coating, said coating also including up to 1 wt.% of a reducing metal additive, which reducing metal additive is selected from iron, titanium, zirconium, hafnium, vanadium, niobium, tantalum, aluminium, copper, zinc, chromium, molybdenum and tungsten. A cathode as claimed in claim 1 wherein said rare earth metal additive is incorporated as 0.2 to 7 wt.% oxide in said coating. An oxide coated cathode for an electrode tube comprising: a metal base essentially composed of nickel and including either one or both silicon and magnesium reducing agents; and a coating of electron emissive alkaline earth metal oxide, which oxide is coated on said base, and is essentially composed of barium oxide; wherein said oxide-coated cathode is characterised by: incorporation of a rare earth metal additive, containing scandium, in said metal base, or as an interlayer located between said metal base and said coating, as follows: as 0.01 to 0.5 wt.% scandium metal in said base; or as an interlayer of scandium metal of thickness not greater than 6 µm; or as an interlayer of scandium oxide of thickness not greater than 10 µm; whereby in operative use said rare earth metal additive shall be active to inhibit or reduce formation and accumulation of oxide of said one or both silicon and magnesium or composite oxides of these, or composite oxides of these including barium oxide at an interface between said coating and said metal base, or if said interlayer is incorporated, in said interlayer. An oxide coated cathode for an electrode tube comprising: a metal base essentially composed of nickel and including either one or both silicon and magnesium reducing agents; a coating of electron emissive alkaline earth metal oxide, which oxide is coated on said base, is essentially composed of barium oxide, and contains a rare earth metal additive, this being incorporated as either: 0.1 to 20 wt.% scandium oxide; or partially reduced scandium oxide, this being produced from 0.1 to 20 wt.% scandium oxide and being of enhanced reactivity; and a further coating of an electron emissive alkaline earth metal oxide, which latter oxide is coated on said coating incorporating said rare earth metal additive, is essentially composed of barium oxide, and is, as provided, substantially free of any rare earth metal additive. An oxide coated cathode for an electrode tube comprising: a metal base essentially composed of nickel and including either one or both silicon and magnesium reducing agents; a coating of electron emissive alkaline earth metal oxide, which oxide is coated on said base, is essentially composed of barium oxide, and contains a rare earth metal additive, this being incorporated as either: 0.1 to 20 wt.% scandium oxide; or partially reduced scandium oxide, this being produced from 0.1 to 2.0 wt.% scandium oxide and being of enhanced reactivity; and a layer of porous sintered nickel interposed between said metal base and said coating, which layer is of thickness 10 to 50 µm and incorporates a portion of said alkaline earth metal oxide and said rare earth metal additive. An oxide coated cathode for an electrode tube comprising: a metal base essentially composed of nickel and including either one or both silicon and magnesium reducing agents; and a coating of electron emissive alkaline earth metal oxide, which oxide is coated on said base, is essentially composed of barium oxide, and contains 0.1 to 20 wt.% scandium oxide which is formed with said barium oxide or other alkaline earth metal oxide constituent of said coating as at least one of the following composite oxides: Ba Sc₂ 0₄; Sr₃ Sc₄ 0₉; or Ca₃ Sc₄ 0₉. An oxide coated cathode for an electrode tube comprising: a metal base essentially composed of nickel and including either one or both silicon and magnesium reducing agents; a coating of electron emissive alkaline earth metal oxide, which oxide is coated on said base, is essentially composed of barium oxide, and contains a rare earth metal additive, this being incorporated as either: 0.1 to 20 wt.% scandium oxide; or partially reduced scandium oxide, this being produced from 0.1 to 20 wt.% scandium oxide and being of enhanced reactivity; said coating also including up to 10 wt.% of a conductive additive being either one or both nickel and cobalt. An oxide coated cathode for an electrode tube comprising: a metal base essentially composed of nickel and including either one or both silicon and magnesium reducing agents; a coating of electron emissive alkaline earth metal oxide, which oxide is coated on said base, is essentially composed of barium oxide, and contains a rare earth metal additive, this being incorporated as either: 0.1 to 20 wt.% scandium oxide; or partially reduced scandium oxide, this being produced from 0.1 to 20 wt.% scandium oxide and being of enhanced reactivity; said coating also including up to 1 wt.% of a reducing metal additive, which reducing metal additive is selected from iron, titanium, zirconium, hafnium, vanadium, niobium, tantalum, aluminium, copper, zinc, chromium, molybdenum and tungsten. An oxide coated cathode for an electron tube comprising: a metal base essentially composed of nickel and including either one or both silicon and magnesium reducing agents; and a coating of electron emissive alkaline earth metal oxide, which oxide is coated on said base, and is essentially composed of barium oxide; wherein said oxide-coated cathode is characterised by: incorporation of scandium oxide and yttrium oxide rare earth metal additive in said coating and consisting in 0.1 to 20 wt % thereof wherein in operative use said rare earth metal additive shall be active to inhibit or reduce formation and accumulation of oxide of said one or both silicon and magnesium or composite oxides of these, or composite oxides of these including barium oxide at an interface between said coating and said metal base.

Inventors: FUKUYAMA, KEIJI C/O MITSUBISHI DENKI K.K., ISHIDA, MASAKO C/O MITSUBISHI DENKI K.K., KAMATA, TOYOKAZU C/O MITSUBISHI DENKI K.K., SAITO, MASATO C/O MITSUBISHI DENKI K.K., SANO, KINJIRO C/O MITSUBISHI DENKI K.K., WATANABE, KEIJI C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1986
Main classification: H01J1
IPC codes: H01J1