US2788460A - Electrodes for electron discharge devices and methods of making same - Google Patents

Description

April 9, 1957 TANTA l. UM Z/PC ON/ UM CARBIDE V. J. DE SANTIS ET AL ELECTRODES FOR ELECTRON DISCHARGE DEVICES AND METHODS OF MAKING SAME Filed May 23., 1951 NON-EMfiS/VE CARBON- PLAT/[WIN (TANTALUM) cum/q 3 4(z/RcoN/1/M CARBIDE) uolv-Enlaslvs CARBON-PLATINUM COATING INVENTORS VINCENT 1/- DESANTIS FRED L- HUNTER 13%, fi ATTORNEY ate ELECTRODES FOR ELECTRQN DISCHARGE DE- VICE AND METHODS OF MAKING SAME.

Application May 23, 1951, Serial No. 227,882

Claims. (Cl. 313-107) This invention relates to electrodes for electron dis charge devices, and more particularly to electrodes having thereon a coating to maintain them non-emissive.

Electron discharge devices usually comprise a cathode as a source of electrons, an anode or collector to which the electrons move from the cathode, and, in the majority of types, one or more other electrodes so located with relation to the electron stream that the stream that the stream may be controlled, for example, by variation of the potential of the electrode relative to the anode or cathode. Hereinafter, for convenience in describing the subjectmatter of this invention, reference will be made 10* grid electrodes which function as control electrodes in devicesof this type, but it is to be understood that the principles of the invention are applicable to the fabrication of other types of electrodes for use in electron-discharge devices in which the electrode is to be substantially non-emissive. By the term non-emissive as herein employed is meant substantial freedom from primary and secondary electron emission.

It has been proposed heretofore to coat the grid conductors either before or after electrode fabrication with a layer of emission inhibiting material, such as carbon or a metal of the platinum group. Carbon is one of the most satisfactory non-emissive materials at elevated temperatures because of its excellent radiation quality and further because it combines with thorium vapors distilled upon the grid during tube operation to form thorium carbide which also is non-emissive. Platinum is useful as a non-emissive coating since it alloys with the thorium vapors thus also rendering deposited thorium non-emissive.

The use of a carbon layer on electrodes for the purpose of inhibiting emission has heretofore presented the problem of carbon erosion or migration during tubeoperation. Carbon subjected to heavy electron bombardment tends to migrate to adjacent areas which are subject to less bombardment. This migration, in time, exposes the underlying material of the electrode in those areas normally exposed to heavy electron bombardment. This results in the occurrence of electron emission from the exposed material as well as the provision of areas onto which the thorium may become deposited and remain active as additional sources for electron emission.

One of the objects of this invention is to provide a stable, non-erosive, non-emissive coating for electrodes.

A further object of the invention is to provide electrodes having high radiation, non-emissive and non-erosive qualities.

Still another object is to provide a method of making a coating of non-emissive material on electrodes which is stable and will not erode or migrate under electron bombardment.

The improved electrode according to the present invention may comprise any one of a large number of core materials. For example, Where the electrode is to be used as a grid, the base metal may comprise any one of the class consisting of tantalum, molybdenum, zirconium,

columbium, tungsten and hafnium. If desired, the base metal core may be provided with a barrier layer especially where the electrode is to be used in power amplifier tubes operating at elevated temperatures for prolonged periods. The barrier layer may comprise a single carbide or a double carbide, such as carbides of the base metal and a second metal applied thereto, or it may comprise a carbide of an inter-metallic compound of two metals. Examples of barrier layers and the methods of forming the barrier layers on core materials are disclosed in the copending applications of De Santis, Hunter, Majkrzak, Serial No. 72,403, filed January 24, 1949; now U. S. 2,681,876 and Serial No. 72,404, filed January 24, 1949, now U. S. 2,552,535 and De Santis Hunter Serial No. 225,896, filed; May 11, 1951, now Patent- No. 2,711,980. Regardless of whether or not the base metal is provided with a barrier layer, a stable layer of non-emissive material according to the present invention may be applied thereto.

The new stable non-emissive coating material of this invention comprises a mixture of carbon particles, plat inum and a suitable binder, which may be applied to electrodes by dipping, spraying or by electrophoresis. The ratio of carbon to platinum is preferably in the order of 1 mole to 0.1 mole. This mixture isapplied to form a layer which is of a dense, substantially uniform thickmess. The coated electrode is then vacuum fired at an elevated temperature of about 1700" C. for a period of one minute. This causes the platinum-carbon mixture to sinter and form a hard homogeneous sleeve of carbon particles bonded together by platinum. The ratio of 1 mole of carbon to 0.1 mole of platinum may be varied, but when the ratio is increased it decreases the radiation quality thereby resulting in a higher operating temperature. When the ratio is decreased the bond of the platinum on the carbon particles is reduced. It is found that the ratio of l to 0.1 gives very satisfactory results in that the resulting carbon-platinum layer is exceptionally stable under elevated temperatures and heavy electron bombardment.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a cross-section of an electrode greatly enlarged and exaggerated to indicate thickness of the barrier layers and the layer of emission inhibiting material;

Fig. 2 is an enlarged cross-sectional view of a fragmentary part of the carbon-platinum coating prior to the fusing operation;

Fig. 3 is an enlarged cross-sectional view of the coating indicated in Fig. 2 after the fusing operation; and

Fig. 4 is an enlarged cross-section of an electrode core coated directly with a layer of non-emissive material.

Referring to the drawings, an electrode core 1 is repre sented in the form of a wire of one of the metals of the class consisting of tantalum, molybdenum, zirconium, columbium, tungsten and hafnium. In the particular example shown, the core material is tantalum. A stable layer of emission inhibiting material 2 may be applied directly to the core material 1 as indicated in Fig. 4, or one or more intervening barrier layers 3 and 4, Figs. 1, 2 and 3, may be provided where high operating temperatures are contemplated to prevent inter-action between the material of the non-emissive layer 2 and the base metal of the core 1. For low temperature operation up to about 1000 C. there is a minimum of inter-action between the non-emissive materials and the base metal and, therefore, a barrier layer may be omitted. For operating temperatures exceeding 1000" C. to about 1700" C., it is preferred to have a barrier layer of greater density and stability to prevent this inter-action. The inter-action between the non-emissive material and the core material is undesirable for the reasons that the contamination of the non-emissive material by a small amount of core material increases electron emission by a relatively large amount and the core material becomes brittle by absorption of the non-emissive material whether carbon or a metal of the platinum group, with the accompanying loss of the non-emissive material from the outer surface of the electrode. Loss of nonemissive material from the coating usually results in electron emission.

The core material may be provided with a carbide layer at the outer surface thereof by a carburizing operation, that is, the application of a coating of carbon and the subjection of the coated electrode to a vacuum firing operation to a temperature preferably exceeding 1400 C. Such a single carbide barrier will function as a barrier to migration of non-emissive material to the core metal during operating temperatures up to approximately 1300 C. As the operating temperature exceeds about 1300" C. some emission occurs. For barrier protection at elevated temperatures exceeding 1300 C. a more stable barrier than the single carbide is therefore required. Such stable bar rier layers are disclosed in the copending patents above referred to. For example, a double carbide or a carbide of an inter-metallic compound of two metals is provided which is found to be very stable at elevated temperatures even above 2000 C. This double or inter-metallic carbide barrier may be formed by either the oxide process disclosed in U. S. Patents 2,552,535 and 2,681,876, or by the hydride process disclosed in the copending application, Serial No. 225,896. In either case the process is similar and may be described briefly as follows. A layer ofzirconium or other refractory oxide or hydride, as the case may be, is applied to the base metal which, for example, may be tantalum. The coated base metal is then vacuum fired to decompose the oxide or hydride. Where the oxide or hydride is of zirconium, for example, the decomposing operation leaves a coating of zirconium on the base metal. The decomposing operation is also believed to result in the formation at least partially of a compound of the base metal and the coating material. To complete the barrier layer a layer of carbon is next applied and the electrode vacuum fired at an elevated temperature ranging from about 1700 C. to 2000 C. more or less, for a period of about 25 minutes. This high temperature carburization converts the outer strata of the base metal, which may be combined in a compound with the zirconium, to an intermetallic carbide which is believed to be a carbide of an inter-metallic compound of the two metals and may be represented by the formula T azZrCa. In additiomthere may be a carbide of the coating material, that is, zirconium carbide. This resulting barrier has proved to be a very effective barrier at extremely high temperatures to the migration of electron emission inhibiting material such as carbon and platinum.

Referring again to Figs. 1, 2 and 3, the stratum 3 is shown to represent the double carbide, that is, tantalumzlrconium carbide and the stratum 4 represents zirconium carbide.

The improved non-emissive layer is formed of a homogeneous mixture of carbon, in the form of micronized graphite, and platinum. When the carbon-platinum mixture is heated in vacuum to a sintering temperature of approximately 1700 C. or to the melting temperature of platinum, namely 1773 C., a solid, dense, homogeneous layer is formed wherein particles of carbon are cemented together by platinum. This dense homogeneous layer of platinum-bonded carbon particles presents a non-emissivc coating which has a high radiation quality and which combines with thorium vapors to render the thorium particles non-emissive. The carbon and platinum are in such quanadded. This mixture is milled for several days in av ceramic jar containing flint pebbles. The milling is complete when the carbon particles are 'micronized and substantially all coated with platinum chloride. The mixture is then heated to about 70 C. to remove the Water, the dehydration being completed at approximately this temperature in partial vacuum. The heating is thereafter increased. to. about 400 C. and maintained at this elevated temperature for about 15 minutes during which the platinum tetrachloride, PtCLi, is at least partially decomposed to platinum dichloride, PtClz, which is insoluble in water and other solvents which may be used in the coating vehicle.

This platinized graphite mixture is thereafter prepared for spraying by adding 180 cc..of binder which is prepared by adding to 940 cc. ofdistilled water at 90 to 100 C.

continuously stirred, 60 grams of 100 cps. methyl cellulose binder. The binder, if desired, may be further diluted with distilled water. The mixture is ball-milled for about 24 hours and to this mixture 6 additional grams of micronized graphite are addedwhile stirring until uniformity is obtained. The ratio of the carbon to the platinum in this ly stable and capable of remaining consistently non-emisfinal mixture should preferably be l mole of carbon to 0.1 mole of platinum. This ratio imparts not only high thermal radiation properties to the grid but also insures the provision of a non-emissive coating which is extremesive.

The platinized graphite mixture thus formed is sprayed onto the surfaces of the electrode. The coating obtained should be smooth, dense and of uniform thickness. The desired minimum thickness corresponds to an increase in weight of 6 mg. per sq. cm. This weight includes the binder. v

The final operation comprises vacuum firing of the electrode to an elevated temperature of about 1700' C. for one minute. The heat is then cut off and the electrode removed after cooling.

From the foregoing it will be understood by those skilled in the art that, while the platinized graphite mixture is described as being prepared for a consistency desired for spraying, the mixture may be prepared differently for different types of application. It will also be understood that while platinum is described as the bonding ingredient other metals of the platinum group may be substituted so long as the ratio of carbon to the metal of the platinum group is made approximately 1 to 0.1, molecular weight. Also, itshould further be understood that this description is made by way of example only and not as a limitation to the scope of our invention, as set forth in the objects thereof and in the accompanying claims.

We claim: v 1

-1. A non-emissive electrode for electron discharge devices comprising, in combination, a core of a refractory metal chosen from the group consisting of tantalum,

. molybdenum, zirconium, columbium, tungsten, hafnium and alloys in which at least one of said metals is the predominant component, an emission-inhibiting substantially homogeneous coating upon said core consisting of carbon bonded together with one or more metals of the platinum group, and a barrier layer between said coating and core toprevent interaction between the emission-inhibiting substance and the core metal.

=2. A non-emissive electrode according to claim 1 characterized in that the core of refractory metal is tantalum. 5 .3.-A non-emis'sive electrode according to .claim 1 whercinplatinum isselected as'the metal of the platinum group.;..-.;...

4. A non-emissive electrode according to claim 3 wherein the platinum and carbon comprising emissioninhibiting material are present in a ratio of 0.1 mole platinum to about 1 mole of carbon.

5. A non-emissive electrode according to claim 1 wherein said barrier layer includes the carbide of the core refractory metal and a carbide of a second refractory metal chosen from the class consisting of carbides of zirconium, silicon and titanium.

6. A non-emissive electrode according to claim 5 characterized in that the core metal is tantalum and the second refractory metal is zirconium.

7. In a process for producing a non-emissive electrode consisting in intimate combination of a refractory metal core, a non-emissive material and a carbide barrier layer disposed therebetween, the steps comprising selecting a base of a refractory metal, applying thereto a coating of a hydride of a second refractory metal chosen from the class consisting of silicon hydride, titanium hydride and zirconium hydride, elevating the temperature of the coated core to decompose the hydride and convert it into a sintered refractory coating of said second refractory metal, applying a layer of carbon on said sintered coating, elevating the core temperature to form an intermetallic carbide with said sintered coating thereby producing a barrier layer to prevent migration therethrough, applying a layer of an emission-inhibiting substantially homogeneous coating of carbon bonded together with one or more metals of the platinum group, and vacuum firing the coated electrode at an elevated temperature to bond said materials into a stable homogeneous layer.

8. In the process according to claim 7, wherein the mixture of materials is formed prior to coating the electrode by mixing finely divided graphite with platinum chloride and heating the mixture to at least partially decompose the chloride.

9. In the process according to claim 7, wherein the mixture is made of carbon and platinum in the ratio of 1 mole of carbon to 0.1 mole of platinum.

10. In the process according to claim 9, wherein the vacuum firing operation includes an elevated temperature close to the melting point of platinum.

References Cited in the file of this patent UNITED STATES PATENTS 219,807 Dode Sept. 23, 1879 1,608,317 Hyde Nov. 23, 1926 1,923,406 Wiegand Aug. 22, 1933 2,115,828 Prescott May 3, 1938 2,226,720 Hansell Dec. 31, 1940 2,282,097 Taylor May 5, 1942 2,361,203 Holdaway et al. Oct. 24, 1944 2,497,090 Miller Feb. 14, 1950 2,497,109 Williams Feb. 14, 1950 2,497,111 Williams Feb. 14, 1950

Claims (1)

1. A NON-EMISSIVE ELECTRODE FOR ELECTRON DISCHARGE DEVICES COMPRISING, IN COMBINATION, A CORE OF A REFRACTORY METAL CHOSEN FROM THE GROUP CONSISTING OF TANTALUM, MOLYBDENUM, ZIRCONIUM, COLUMBIUM, TUNGSTEN, HAFNIUM AND ALLOYS IN WHICH AT LEAST ONE OF SAID METALS IS THE PREDOMINANT COMPONENT, AN EMISSION-INHIBITING SUSTANTIALLY HOMOGENEOUS COATING UPON SAID CORE CONSISTING OF CARBON BONDED TOGETHER WITH ONE OR MORE METALS OF THE PLATINUM GROUP, AND A BARIER LAYER BETWEN SAID COATING AND CORE TO PREVENT INTERACTION BETWEEN SAID EMISSSION-INHIBITING SUBSTANCE AND THE CORE METAL.

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