Oxide cathode

Abstract: The initial emission and the lifetime of oxide cathodes comprising, for example, BaO and SrO in their emissive layer are considerably improved by adding small quantities of rare earth metals (10-500 ppm).

Description: The invention relates to a cathode having a supporting body substantially comprising nickel and being coated with a layer of electron-emissive material containing not only alkaline earth metal oxide as a principal component containing at least barium a rare earth metal. The invention also relates to a an electron beam tube provided with a cathode according to the invention. Such cathodes are described, for example, in EP-A-0,210,805. The emission of such cathodes is based on the release of barium from barium oxide. In addition to the barium oxide the electron-emissive material usually comprises strontium oxide and sometimes calcium oxide. Improved electron emission properties are obtained by the addition of rare earth metals. EP-A-0,210,805 teaches that the addition of rare earth metals should be at least 0.05 % by weight to obtain at least some improvement of the emission. The actual emission is mainly ensured by small areas (so-called "sites") having the lowest effective electron work function, which sites are spread over the electron-emissive material. In practice sites having a slightly higher work function will hardly contribute to the electron current generated by the cathode. For a high effective electron emission it is therefore favourable to choose the number of sites having a minimal work function and the distribution of the sites over the emissive layer as optimally as possible. It is one of the objects of the invention to realize such an optimum distribution in a cathode of the type mentioned in the opening paragraph with minimal additions of rare earth metals. It is another object of the invention to provide such a cathode which is well resistant to various manufacturing steps when it is being built into an electron tube, and which has a long lifetime. A cathode according to the invention is therefore characterized in that the number of rare earth metal atoms in the electron-emissive material as a fraction of the number of alkaline earth metal atoms is 10-500 ppm and in that the rare earth metal atoms are distributed substantially uniformly over at least the upper part of the layer of emissive material. The layer of electron-emissive material may be obtained by decomposition of a co-precipitated alkaline earth metal-rare earth metal compound. In this respect it is to be noted that rare earth metals are not only understood to mean the metals of the lanthanides but also the metals yttrium and scandium. In this connection "distributed substantially uniformly" is understood to mean that each one of the separate particles of alkaline earth metal oxides in the layer of emissive material comprises rare earth metal atoms. It is further to be noted that the provision of, for example, cerium in an emissive layer by means of co-precipitation is known per se from JP-A-74/12758. However, much larger quantities are concerned than in the present invention, viz. quantities within the range mentioned in EP-A-0,210,805. A carbonate is preferably used for the alkaline earth metal-rare earth metal compound, but, for example, oxalates or formiates are alternatively possible. The invention is based, inter alia, on the recognition that the uniform distribution of the rare earth metals leads to a uniform distribution of the number of sites. It is found that better cathode properties (higher emission, longer lifetime, etc.) are obtained when using smaller quantities of yttrium, seandium or one of the lanthanides than in cathodes without additions. Notably, additions of yttrium or europium yield good results. Said lifetime improvement need not always become manifest in a reduced rate of the decrease of the emission, but may also become manifest in a less repid decrease of other properties which are important for the lifetime, such as, for example, the cut-off voltage. For example, a cathode according to the invention may have a decrease of the emission which is comparable to or even larger than that of a cathode in accordance with EP-A-0,210,805 with 2.5% by weight of Y₂O₃ in the emissive layer, but it may have further lifetime properties which are so much better that it is to be preferred for use in an electron tube. Moreover, said advantages of a better resistance to processing and use of fewer rare earth metals remain valid. The invention will now be described in greater detail with reference to an embodiment and the drawing in which Fig. 1 is a diagrammatic cross-sectional view of a cathode according to the invention. The cathode 1 in Fig. 1 has a cylindrical nichrome cathode shaft 3 provided with a cap 7 in this embodiment. The cap 7 substantially comprises nickel and may comprise reducing means such as, for example, silicon, magnesium, manganese, aluminium and tungsten. The cathode shaft 3 accommodates a helical filament 4 which comprises a metal helically wound core 5 and an electrically insulating aluminium oxide layer 6. An approximately 70 µm thick layer of emissive material 2 is present on the cap 7, which layer comprises, for example, a mixture of barium oxide, strontium oxide and a rare earth metal obtained by providing and subsequently decomposing a co-precipitated barium strontium-rare earth carbonate, or a mixture of barium oxide, strontium oxide, calcium oxide and a rare earth oxide. A carbonate comprising 60 ppm of yttrium (as a fraction of the number of alkaline earth metal atoms) was obtained by dissolving 20.1 kg of barium nitrate and 16.5 kg of strontium nitrate in 160 ml of water and by heating this mixture to 88°C, together with 16.4 ml of an yttrium nitrate solution which comprised 50 mg of yttrium/litre. An aqueous solution comprising 18 kg of sodium carbonate was subsequently added thereto at a rate of 1.1 litre/minute so that a completely co-precipitated barium strontium yttrium carbonate was obtained. The carbonate thus obtained was subsequently filtered, washed and dried. The desired suspension was obtained by adding 2 litres of a binder solution (diethyl carbonate to which a small quantity of binder material (cellulose nitrate) is added) to 1.1 kg of the co-precipitated carbonate. Similarly, suspensions were prepared which comprised 300 ppm of europium. Cathodes having an emissive layer of this type of carbonates comprising 60 ppm of yttrium atoms 300 ppm of europium atoms, respectively, (as a fraction of the alkaline earth metal atoms) were mounted in a cathode ray tube. After this standard mounting and activation of the cathodes in the tube, with the carbonates decomposing to oxides, the cathode ray tubes were operated for 2000 hours at a filament voltage of 7 Volts, which is comparable with approximately 10,000 real operating hours. Before and after this life test emission measurements were performed at a filament voltage of 7 Volts after 30 seconds of conveying current at a cathode load of 2.2 A /cm² (referred to as Δi k,30 measurement). The decrease in emission current was 2% when yttrium was added and approximately 5% when europium was added, while this decrease was 24% in the case without any additions. Moreover, the initial emission in all cases was found to be approximately 3% higher than in cathodes without any additions. Also other properties such as, for example, the resistance to gases and a thermal treatment of the tube were found to be considerably better. The above-mentioned values of Δi k (decrease of emission current) are stated in Table I, as well as the decrease of the cut-off voltage (ΔV k ) and the slump (i.e. a measure of the emission current decrease). The Table also states the values for a cathode in accordance with EP-A-0,210,805 with 2.5% by weight of Y₂O₃ and for a cathode without additions. It is apparent from the Table that in all respects the cathode with 60 ppm of Y atoms has better lifetime properties than the cathode with 2.5% by weight of Y₂O₃ and is by far better than a cathode without additions. Although the cathode with 300 ppm of Eu has a slightly poorer lifetime behaviour, it has all the advantages of a better resistance and less use of material (rare earth metals). Similar lifetime tests as described hereinbefore were performed in other types of cathode ray tubes with cathodes in accordance with the invention in which 10 ppm of Eu, 60 ppm of Eu, 20 ppm of Y, 60 ppm of Y and 500 ppm of Y had been added to the emissive layer. The results are stated in Table II. It is apparent from Table II that for small quantities (10-20 ppm) of rare earth metal atoms the emission decreases to a greater extent than in a cathode with 60 ppm of Y, but notably ΔV k is much lower (under identical circumstances). Similar remarks as for the cathode with 300 ppm of Eu in Table 1 apply to the cathode with 500 ppm of Y. In the latter series of tests one cathode was also tested which had an emissive layer consisting of a 40 µm thick layer without additions while on top of it a 20 µm thick layer was provided to which 60 ppm of Y atoms had been added in a uniformly distributed manner. The comparable values of Δi k were 10%, 2% and 1.8%, respectively, so that also in this case notably the low decrease of the cut-off voltage leads to a long lifetime. The invention is of course not limited to the embodiments shown, but several variations are possible. For example, the cathode may be designed in various manners (cylindrical, concave, convex, etc.) and there are various methods of providing the electron-emissive layer. This layer with the uniform distribution of the rare earth metals can also be obtained by depositing Ba-Sr-carbonade particles in a solution comprising yttrium (for example, an acetyl acetate) and by subsequent drying, with yttrium being left on each particle. By filtering, washing and drying an emissive material can then be obtained again with the powder thus obtained.

Claims: A cathode (1) having a supporting body (7) substantially comprising nickel and being coated with a layer (2) of electron-emissive material containing not only alkaline earth metal oxide as a principal component containing at least barium but also rare earth metal, characterized in that the number of rare earth metal atoms in the electron-emissive material as a fraction of the number of alkaline earth metal atoms is 10-500 ppm and in that the rare earth metal atoms are distributed substantially uniformly over at least the upper part of the layer of emissive material. A cathode as claimed in Claim 1, characterized in that the rare earth metal atoms are distributed substantially uniformly over the layer (2) of emissive material. A cathode (1) having a supporting body (7) substantially comprising nickel and a layer (2) of electron-emissive material provided on the body, the layer containing alkaline earth metal oxide particles as a principle component, the alkaline earth metal oxide containing at least barium oxide, but also a rare earth metal component, characterized in that the rare earth metal component is present in such a concentration that the number of rare earth metal atoms in the electron-emissive material, as a fraction of the number of alkaline earth metal atoms in the electron-emissive material is 10-500 ppm and in that at least the alkaline earth metal oxide particles in the upper part of the layer of electron-emissive material comprise the rare earth metal atoms whereby the rare earth metal atoms are distributed substantially uniformly. A cathode as claimed in Claim 1, 2 or 3 characterized in that the layer of electron-emissive material comprises europium atoms or yttrium atoms. A cathode as claimed in Claim 1, 2, 3 or 4, characterized in that the electron-emissive material mainly comprises barium oxide and strontium oxide. A cathode as claimed in any one of Claims 1 to 5, characterized in that the supporting body comprises reducing means. An electron beam tube provided with a cathode as claimed in any one of Claims 1 to 6.

Inventors: DERKS PETRUS JACOBUS ANTONIUS, DERKS, PETRUS JACOBUS ANTONIUS MARIA
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1991
Main classification: H01J1
IPC codes: H01J1