US3454816A - Indirectly heated dispenser cathode for electric discharge tube - Google Patents

Description

July 8, 1969 3,454,815

INDIRECTLY HEATED DISPENSER CATHODE FOR ELECTRIC DISCHARGE TUBE H. HOFFMANN ETAL Filed Aug. 3, 1967 INVENTORS HE K5EZ7 Manama 1v firzn/z/M BY TOR-NEYS United States Patent US. Cl. 313346 5 Claims ABSTRACT OF THE DISCLOSURE A dispenser cathode along the lines of a metal capillary cathode, in which the porous carrier disk for the emission material, in operation with emission promoting material, is so supplied from a supply disposed therebehind that the emission-promoting material migrates through the pores thereof to the surface forming the emitting area and there ditfuses, in which the porous carrier disk for the emission material comprises a porous carbon disk as the base structure, the surface of which serves the migration process and is provided Wtih a metal coating of tungsten and/or a light or heavy platinum metal, and provided with an intermediate layer of a material having a high thermal stability, such as a carbide of at least one metal selected from the group consisting of molybdenum, tungsten, tantalum, zirconium and titanium, and the process of making the same utilizing zirconium carbide for the intermediate layer, in which the highly pure porous carbon disk is immersed in a liquid zirconium compound, for example a 4% solution zirconylacetylacetonate, decomposing the separated zirconium salt in a dry hydrogen atmosphere for about minutes at 1200 C. subsequently spraying the disk with a zirconium suspension, and thereafter annealing the disk in a vacuum at about 1800 C.

The invention relates to a dispenser cathode in the general form of a metal capillary cathode in which the porous carrier disk for the emission material, in operation with emission-promoting material such as barium, is so supplied by a donor disposed therebehind that the emission-promoting material migrates through the pores thereof to the portion of the surface forming the emitting area and there difiuses.

It is of particular importance not only for such cathodes, in which the porous carrier disk for the emission material covers a replenishing vessel so tightly that the pores represent the only fine openings, but also for such cathodes in which the donor delivering the emissionpromoting materials is a second metal capillary cathode disposed at a distance therebehind without the presence, at the sides, of a particularly tight closure between the two.

In many cases it is necessary in electric discharge tubes that the emitting area, particularly the porous carrier disk for emission material of the cathode have a specific profile, e.g. the form of a hollow cup. Technically, however, there are some difficulties connected with the pressing of thin carrier disks of emission material from a metal powder, e.g. tungsten or rhenium, and subsequently making them sufficiently stable through a sintering process, as mechanical tensions occur in such portions, usually leading to a high waste. This is particularly true as the cathode area takes on larger dimensions. Moreover, tungsten has a very high specific weight so that considerable accelerative forces occur on such porous tungsten disks of electric discharge tubes in very strongly accelerated and rapidly moving systems.

Consequently, it has already been suggested to manufacture the porous carrier disk for the emission material from a carbon body for use in an electron gun, in which such emission disk is heated through electronic bombardment froma second dispenser cathode disposed therebehind and supplied With an emission-promoting material. However, carbon does not have pronounced migration properties with respect to barium and, furthermore, exhibits only a very high electron affinity. A known measure which has been successfully applied to porous tungsten disks in metal capillary cathodes, namely to reduce in particular, the electron aflinity by coating with a metal of the platinum group, particular osmium, fails as far as the technical production is concerned, when directly applied to porous carbon bodies, due to insuflicient moistening possibilities on the surface of the carbon body, so that practicaly no cohesive metal coatings can be obtained.

The problem underlying the invention therefore resides previously in the elimination of the difficulties above described.

According to the invetnion, this is achieved by means of a dispenser cathode as initially described, wherein the porous carrier disk for emission material comprises a porous carbon disk as the base structure, whose surface (surface portions) serving the migration process has a metal coating of tungsten and/or a light or heavy platinum metal, and as an inter-carrier an intermediate layer of a material with high thermal stability.

As high temperatures are required in the activation, the quality of high thermal stability must be stressed, as a result of which only a relatively small number of materials are suitable for use as such an intermediate layer.

In a particularly advantageous development of such a capillary-film cathode (CFC), the intermediate layer applied to the carbon body comprises the carbide of at least one of the metals molybdenum, tungsten, tantalum, zirconium or titanium. Depending on the metal components, the percentage of carbon across the cross section of the intermediate layer can amount from a maximum value, corresponding to stoichiometric composition of the carbide, to zero at the outer surface. Above all, the primary advantage of the intermediate layer described resides in the fact that it is well moistened by each of the metals such as tungsten and platinum metal, so that a particularly cohesive dense metal layer is formed. This is of particular importance, because barium, as an emission-promoting material, itself forms a carbide and, therefore, can react with many of the metal carbides at the occurring high temperatures.

Hence, particularly advantageous is the selection of zirconium carbide for the intermediate layer, as zirconium carbide is thermally extremely stable and is not attacked by barium under any circumstances.

'On the other hand, the dense metal layer requires that the porous cathode body be well and reliably covered, particularly on the portion forming the pores, and thus in the pores themselves, with the initial material selected for the carbide formation. For molybdenum and tungsten, the soluble oxygen compounds, in particular such as molybdate and tungstate, have stood the test. For the production of zirconium carbide, weak solutions of organic materials, such as zirconylacetylacetonate, are especially suitable.

In the production of a zirconium carbide intermediate layer on a carbon disk as intercarrier for a metal layer, the highly pure, porous carbon disk involved is initially immersed in a liquid zirconium compound, e.g. in a 4% solution of zirconylacetylacetate and subsequently the separated zirconium salt is thermally decomposed in a dry hydrogen atmosphere for about 10 minutes at 1200 C. Nevertheless, the surface layer thus created is not completely continuous as yet. Therefore, to obtain a really continuous zirconium surface layer, zirconium, in the form of, a zirconium suspension, e.g. with butylacetate as a suspension medium, is additionally sprinkled on such carbon disk. Subsequently, the definitive decomposition is effected through annealing in a vacuum at about 1 800" C.

Further details are explained by means of the illustrative embodiment in the drawing which presents merely a diagrammatical representation in longitudinal cross section of an electron gun with two successively disposed dispenser cathodes, of which at least the main cathode is constructed along the lines of a metal-film cathode. Parts which are not absolutely necessary for understanding the invention have been eliminated therefrom or unmarked.

Reference numeral 1 designates a porous carbon disk having a relatively large surface forming a carrier disk for emission material, essentially representing the main cathode or ray cathode, e.g. of an electron gun of an HF power tube. It does not have its own supply and replenishing vessel but is disposed as a working electrode at the end of an auxiliary discharge space for the production of heat by a cathode bombardment. Hence, the foil supporting cylinder 2, therefor is so connected, over an insulating piece 3 with the corresponding foil 4 of the auxiliary cathode =6, that a closed chamber is formed between the t-wo emission- material carrier disks 1 and 5.

To reduce the electron afiinity on the emitting area formed by the carbon disk 1, for the purpose of lowering the operating temperature, the carbon disk is provided on its surface, particularly on the portion serving the migration mechanism of the barium, with a metal coating 12 of tungsten and/or a light or heavy platinum metal, and as an intercarrier with an intermediate layer 11 of a material with high thermal stability, e.g. a carbide of the metals molybdenum, tungsten, tantalum, titanium or particularly of zirconium. While the metal coating 12 is not applied to the rear face of the disk, a carbide lining which, for example, might be formed there during production need not absolutely be avoided, because it does not interfere at such point with the operating mechanism.

Changes may be made within the scope and spirit of the appended claims which define What is believed to be new and desired to have protected by Letters Patent.

We claim:

1. In a dispenser cathode along the lines of a metal capillary cathode, in which a porous carrier disk for the emission material, in operation with emission-promoting material, is so supplied from a supply disposed therebehind that the emission-promoting material migrates through the pores thereof to the surface forming the emitting area and there diffuses, the combination of a porous carrier disk for the emission material having a porous carbon disk as the base structure, the surface of which serves the migration process and is provided with a coating of metal selected from a group consisting of tungsten or platinum metal, and provided with an intermediate layer therebetween of a material having high thermal stability.

2. A dispenser cathode according to claim 1, wherein the intermediate layer comprises a carbide of at least one metal selected from the group consisting of molybdenum, tungsten, tantalum, zirconium and titanium.

3. A dispenser cathode according to claim 1, wherein the intermediate layer comprises zirconium carbide.

4. A process for the production of a cathode structure having a base structure in the form of a carbon disk upon which is disposed a coating of metal selected from a group consisting of tungsten or light or heavy platinum metal, and having an intermediate layer disposed therebetween of zirconium carbide, comprising the steps of immersing a disk of highly pure porous carbon in a liquid zirconium compound, decomposing the separated zirconium salt in a dry 'hydrogen atmosphere for' about 10 minutes at 1200 C. subsequently spraying the disk with a zirconium suspension, and thereafter annealing the disk in a vacuum at about 1800 C.

5. A process according to claim 4, wherein said zirconium compound comprises zi-rconylacetylacetonate.

References Cited UNITED STATES PATENTS 2,925,514 2/1960 Lemmens et a1 313-346 3,010,046 11/1961 Dailey et a1 313-346 3,403,282 9/1968 Katz 313338 X JOHN W. HUCKERT, Primary Examiner.

J. R. SHEWMAKER, Assistant Examiner.

US. Cl. X.R.

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