US3528156A

US3528156A - Method of manufacturing heated cathode

Info

Publication number: US3528156A
Application number: US680517A
Authority: US
Inventors: August J Kling
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1964-12-07
Filing date: 1967-11-03
Publication date: 1970-09-15
Anticipated expiration: 1987-09-15
Also published as: FR1460995A; BE673331A; US3400294A; NL6515865A

Description

Se t. 15, 1970 A. J. KLING METHOD OF MANUFACTURING HEATED CA'IHODE 7. 1964 2 Sheets-S'neet 1.

Original Filed Dec.

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His Affor gey Sept. 15, 1970 A. J. KLING METHOD OF MANUFACTURING HEATED CATHODE 2 Sheets-Sheet 2 Original Filed Dec. '7. 1964 a m y 8 W 7% m m V S ,V, MU lab Odd/H u A w m United States Patent U.S. Cl. 29-25.17

Int. Cl. H01j 9/00 8 Claims ABSTRACT OF THE DISCLOSURE A low-mass, high-temperature cathode is formed by roughening the underside of the planar metal surface by depositing powdered metal particles on such underside and sintering. A ceramic slurry is deposited on the sintered metal particles to provide interlocking engagement with the metal particles. A heating coil is at least partially embedded in the ceramic slurry which is fired and sintered to form a quick-heating rigid structure, the interlocking engagement between the ceramic and the sintered metal particles preventing separation of the sintered slurry and the embedded heater coil from the planar metal surface during operation. Metal powder or particles may also be sintered to the upper surface of the cathode to provide interlocking anchorages for electron emitting materials.

This application is a division of my application Ser. No. 418,591, filed Dec. 7, 1964, now U.S. Pat. No. 3,400,294 which application is a continuation-in-part of my application Ser. No. 247,171, filed Dec. 26, 1962, now abandoned, entitled Heated Cathode and Method of Manufacture and assigned to the assignee of the present application.

This invention relates to a method of manufacturing heated cathodes for electric discharge devices, and particularly to the manufacture of miniature cathodes capable of reaching a high operating temperature in a short period of time.

Present day electronic equipment is frequently used in apparatus which is subject to great shocks and vibration, requiring stability in the amplifying devices in the electronic equipment. One of the particular problems encountered in such conditions is the separation of the heater from the cathode in the amplifying devices. Such present day electronic equipment, to an increasing extent, include greatly miniaturized components which, despite small size, must exhibit complete reliability. Moreover, the amplifying devices employed in such equipment should desirably attain immediately operable condition from a cold start, that is, without any substantial warmup period after initial turn-on while retaining their reliability. The slow heating or warm-up period characteristic of many vacuum tubes does not recommend their use in equipment called upon for immediate service.

The slow warm-up time, on the order of 20 seconds for usual vacuum tube types, is attributable to the time required for the vacuum tube filament to raise the temperature of the cathode to its electron-emitting operating temperature. Most vacuum tubes and other electric discharge devices employ radiation heater coils mechanically inserted inside the cathode structure and electrically insulated therefrom. The heat in general must radiate from the coil to the back of the cathode, causing a temperature increase on the forward or emitting surface of the cathode.

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In addition to the time lag, a disadvantage relating to the conventional heated cathode arrangement resides in the usual temperature differential between the heater coil and the emitting cathode during operation. This temperature differential is especially serious when miniature high efficiency discharge devices are operated at quite high temperatures. For example, in order to achieve a cathode temperature on the order of 750 to 875 C., the ordinary heater must attain a temperature as great as 1300 C. In many tubes, after some period of operation, internal chemical reaction initiated by the high heater temperature, causes gaseous evolution from the heater area causing intolerable operating conditions within the tube, including an impaired vacuum.

It is therefore an object of the present invention to provide a method of manufacturing an improved heated cathode for electron discharge devices which requires a minimum of time for attaining cathode operating temperatures after turn-on, while retaining complete reliability.

It is another object of the present invention to provide new and improved methods for manufacturing heated cathodes in which the heater is firmly and rigidly attached to the cathode so that it does not separate from the cathode when subjected to extreme shocks and vibrations.

It is another object of the present invention to provide an improved heated cathode for electric discharge devices wherein the heating element operates at high temperatures quite close to those of the cathode emitting surface.

Another object of this invention is to provide an im proved electrically insulated heated cathode which is more efficient because heat transfer to the cathode surface is quite direct and effective.

Briefly stated, in accordance with an embodiment of the present invention, an electric discharge device metal cathode including the shape of a fiat metal disc has attached to the under surface thereof sintered metal particles and a ceramic refractory material in interlocking engagement with the sintered particles to provide an intimate bonded relation between the ceramic refractory material and the cathode metal disc. The ceramic material contains a heater element or coil which is desirably incrementally folded and integrally bonded. with the ceramic material at the same time the ceramic is fired whereby a close and heat-transmissive but electrically insulating bond exists between the heater element and the cathode surface. The heater element desirably comprises a low heat mass wire helix.

According to a particular embodiment of the present invention, a slurry of ceramic material is applied directly to the back of a metal planar cathode structure or disc in adherent contact therewith. This slurry forms a layer which is quite thin, for example. on the order of about one to five mils in thickness. The slurry is dried or desirably fired in place at a high temperature to sinter the ceramic. Now a second layer of ceramic slurry is applied, and a heater element is at least partially embedded therein. The ceramic material is adherent to the heater element. Now the combination is fired to a high temperature to fire the ceramic in place for sintering the last ceramic layer and establishing an intimate and strong heat conducting integral bond between the heater element and the cathode, but a bond which is electrically insulating.

In accordance with one embodiment of the present invention wherein the heater element is not completely embedded in ceramic, the heater element, which may comprise a coil helix or fluted ribbon, is completely embedded at regular reentrant increments along the element and thereby firmly secured to the cathode disc. This form of the invention allows for differential expansion between the heater element and the ceramic. This problem of differential expansion is particularly troublesome in the case of the fiat cathode, inasumch as heat expansion forces during temperature cycling tend to pull a heater element away from a flat surface more than would be the case with an enclosed cylindrical surface or the like.

The heated cathode as constructed in accordance with the present invention is frequently quite miniature, and the planar cathode is disc-shaped at the end of a thin cathode cup. The planar portion may have a diameter on the order of inch and a thickness of about one mil. The cathode and the heater element have low heat mass for aiding rapid heating of the heater element and the cathode surface. Heating to a final temperature of approximately 800 C. typically requires from one to two seconds. Also, the heater temperature is found to be very close to that of the emitting surface. Moreover the heater element itself is preferably covered with at least a thin layer of ceramic material and no emission of objectionable gases result during operation. Depending upon the type of heater element employed, the element may be either completely or only partially embedded in the ceramic material.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. 1 is a cross-sectional view of a miniature cathode which is partially complete, and further illustrating a first step according to one embodiment of the present invention,

FIG. 2 is a plan view of the underside of a miniature cathode of FIG. 1 illustrating a second manufacturing step in accordance with an embodiment of the present invention,

FIG. 3 is a cross-sectional side view of the FIG. 2 cathode taken at 33,

FIG. 4 is a view of an end connection for the heater element in FIG. 2,

FIG. 5 illustrates the FIG. 2 cathode completed and with heater connections in place,

FIG. 5a is an enlarged view of a cross-section of the cathode of FIG. 5,

FIG. 6 is a cross-sectional view taken at 66 in FIG. 5,

FIG. 7 is a cross-sectional view of a cathode in accordance with another embodiment of the present invention,

FIG. 8 is a cross-sectional view of a high temperature ceramic discharge device embodying the FIG. 7 cathode,

FIG. 9 is a view of another embodiment of the heated cathode according to the present invention,

FIG. 10 is a cross-sectional view taken at 10-10 in FIG. 9,

FIG. 11 is a view of another embodiment of the heated cathode in accordance with the present invention, and

FIG. 12 is a cross-sectional view of yet another em bodiment according to the present invention.

Referring to FIG. 1, illustrating a first basic portion of an embodiment of a cathode in accordance with the present invention, there is depicted a thin, low heat mass metal cathode cup having a planar end member or portion 1 and a cylindrical portion 2. The cylindrical portion :acts as a support for stiffening the planar portion 1 and for accurately positioning the outside surface 3 of the planar portion in very close spaced electron emitting relation to an anode or other electrode. The cathode may be formed of various of the more refractory cathode base metals, e.g., molybdenum, tungsten, or rhenium, although platinum, hafnium, rhodium, or niobium are also suitable. Also alloys of these metals may be employed. A thin roughened layer isdesirably provided inside the flat end portion of the cup for subsequent adherence of a ceramic material, as hereinafter more fully described. If the cathode metal is molybdenum or the like, it is preferred to provide a tungsten inter-surface between oxide emitting material, which may be applied to surface 3, and the molybdenum cathode metal. In the illustrated embodiment, the cathode surface may be disc-shaped and quite small, having a diameter on the order of about inch and a thickness on the order of about one mil to obtain a low heat mass while the cylindrical portion may be on the order of 0.03 inch deep. Surface 3 will be customarily coated with barium-calcium-strontium carbonate or a similar material for enhancing electron emission from a heated cathode.

In accordance with the present invention, a ceramic material is tightly adhered in intimate relation to the planar undersurface of the cathode. In order to provide strong adherence, the cathode undersurface is first desirably roughened with an adherent powdered metal forming a part of this surface and which provides interlocking anchorages for the ceramic material. In an exemplary procedure according to the present invention, metal powder is first temporarily attached or applied to the cathode undersurface and then the combination is brought to a high temperature whereupon the powder solders, sinters, or welds to the surface providing a suitably rough base in the form of a porous mass of metal particles rigidly attached to the undersurface for the intimate bonding of ceramic material thereto. When this roughening procedure is followed, the cathode is preferably constructed from one of the more refractory materials, e.g., tungsten, molybdenum, or rhenium, so it can be raised to a high temperature. Hafnium, niobium, rhodium and platinum are also usable.

A number of metal powders and combinations and mixtures thereof are suitable for roughening purposes. A basic powder is usually one of the refractory materials such as molybdenum, tungsten or rhenium; molybdenum or tungsten being preferred. This basic powder will provide the interlocking anchorages for the ceramic material. Mixed therewith is a soldering metal powder such as platinum or a semi-reactive metal powder such as niobium, vanadium, zirconium, or tantalum. The last four are conveniently employed in the form of a hydride. Powder size should be rather fine, e.g., in the range of about two to ten microns.

Ten to 100 percent of the soldering or semi-reactive metal powder is mixed with 0 to percent of the basic refractory powder metal. When platinum powder or the like is used, about 40 percent by weight thereof makes a very suitable mixture, while 20 to 30 percent by weight of the semi-reactive materials is more suitable. A very thin layer of the mixture is first temporarily adhered to the underside of the cathode as by dusting it upon a sticky film applied to the cathode. An appropriate adhesive film for this purpose is polybutylene which is later volatilized during heat treatment. After the metal powder particles are applied in a thin layer, the cathode is raised to a temperature of from about 1700 to 2100 C., preferably a vacuum enclosure. The temperature should, of course, be below the melting temperature of the cathode base. Temperatures in this range as exceed the melting temperature of platinum, vanadium, and zirconium, and particularly the platinum, will tend to solder the base powder constituent onto the cathode undersurface. In the case of materials which do not melt, the adherent action is more accurately described as sintering; niobium is a particularly good example of the latter type. It is understood the foregoing mixtures are given by way of example. The various metal powders mentioned above, either singly or in combination, may be adhered to the metal cathode so as to form a part thereof. The cathode undersurface is now suitably roughened for the application and adherence of ceramic material thereto. The front or emitting surface of the cathode is desirably similarly roughened for better adherence of the emitting material to the cathode, which is applied thereafter. The powder in this case is preferably tungsten, molybdenum, rhenium, platinum, rhodium or mixtures thereof and the surface upon which the powder is attached is preferably tungsten or rhenium.

Further in accordance with the process according to the present invention, a slurry of ceramic material 4 is applied to the underside of the planar portion of the cathode cup to a thickness of approximately two mils. A uniform coating of thickness just sufiicient to provide good electrical insulation is desired. This ceramic material is one which securely adheres to the cathode metal, but one which will provide good heat conduction characteristics. In the instance of one example, such ceramic material comprised a ceramic frit including 2 /2 percent CaO (calcia) and approximately 95 percent A1 (alumina). This mix was prepared by thoroughly blending and grinding the 95 percent of A1 0 with a finely-ground and fused composition and comprising the remaining percent, this latter composition being 48 weight percent CaO and 52 weight percent A1 0 The whole blend is incorporated in a homogeneous slurry with a mixture of 15 mgs. per ml. of nitrocellulose in butyl carbitol, about one gram of ceramic powder to 0.25 mil of the vehicle giving a satisfactory working consistency.

A1 0 or CaO can be used individually if desired. Other suitable ceramic materials which have adherent properties and good heat conductivity may be employed either singly or in a mixture. Such materials include beryllia, lanthana, yttria, hafnia, magnesia and the rare earth oxides. It is desirable the ceramic have expansion characteristics similar to the cathode metal.

After application of the slurry to the metal cathode member, the cathode is heated in air to dry the ceramic slurry. It is then desirably fired in vacuum or a suitable atmosphere at approximately 1700 C. (or higher for the more refractory materials) for about ten minutes, to sinter the ceramic material in place in the cathode member, forming a tightly adherent bond with the cathode member.

As illustrated in FIGS. 2 and 3, a second slurry layer of ceramic material 5, which may have the same constituency as the first, is next applied over the first layer of the ceramic material. If layer 4 has been fired, it is desirable to saturate the sintered ceramic layer 4 with the same solvent as used in the slurry, just before application of the second layer, inasmuch as the first layer is usually quite porous. Application of the solvent assures freeflowing and good blending-of the second coat of slurry. The second slurry layer of ceramic material 5 may be deposited to a depth of approximately two to five mils in the case here described.

A second slurry coating thickness of less than ten mils is preferred. A thicker coating tends to shrink during sintering and distorts the overall structure. Shrinkage close to the planar portion 3 of the cathode is prevented because of the close adherence of the ceramic to the cathode.

A low heat mass filamentary heater element 6 is deposited into the slurry so that it rests upon layer 4. This filament is a small high resistance conductor and may comprise a tungsten, molybdenum, rhenium, or in some cases platinum or rhodium, wire. Alloys of these metals are also suitable as heater elements. In one particular instance the wire was quite small being approximately 0.00068 inch in diameter enfolded into a helix, wound at 360 turns per inch on a 0.0027 inch mandrel. The enfolded helical configuration having a long wire length results in considerable conduction of heat from the heater element to the surrounding ceramic and thus to the oathode to which the ceramic is intimately bonded. Wire may be employed up to about 0.001 inch or 0.002 inch wire diameter. Above this size, a small ribbon, also formed into a helix is preferred. The helix is desirably formed into the configuration illustrated, that is, with one end approximately centrally located of the cathode cups underside and thence outward in a circular or coiled configuration passing relatively closer to the outside periphery of the cathode cup. The configuration may be primarily circular or spiral. It is appreciated the heater element may be disposed in other configurations as hereinafter set forth and may be either embedded or partially embedded in the ceramic slurry. However, it is important in the case of a helical coil heater, for example, that each incremental turn or element of the coil, where it is reentrant towards the cathode, be at least partially embedded in the slurry in order to result in tight bonding of the heater element in the ceramic material on the underside of the cathode. The helix turns then give excellent bonding-in contact with the ceramic on the flat underside of the cathode.

Embedding of only a portion of each turn, or incremental bonding allows differential expansion between the turns and between the flat heater element and the ceramic as necessary. This can be particularly desirable in the case of the flat cathode because the flat construction does not aid in retaining the heater element in place. That is, the heater element is in general not surrounded or mechanically supported other than through the ceramic layer tightly adhered to the cathode. It is desirable to first coat the filament turns individually with some ceramic slurry material as by dipping it in a larger quantity there of before placing it in the cathode cup as shown.

Complete embedding of the heater element (i.e., when the entire form of the helix is out of sight beneath the surface of the ceramic as illustrated in FIGS. 3 and 6) provides increased mechanical protection of the heater element. Total embedding can also be of advantage in heat shielding the heater element, that is in preventing undue escape of heat by radiation from the back of the filament. But embedding to too great a depth, e.g., cover ing the heater element too deeply can unnecessarily increase the total heat mass without producing attendant advantage. At least partial embedding is necessary to adequately conduct heat to the cathode surface from the bonded heater element as hereinafter explained.

FIG. 5a is an enlarged cross-section of a portion of the cathode of FIG. 5 and shows the metal powder particles 3a sintered or bonded to the surface of disc 3 as described previously and in interlocking relation with the grains of ceramic material 4. This figure also shows one of the turns of the heater element 6 embedded in the ceramic ayer.

Next the layer of slurry 5 is partially dried and

end conductors

7 and 8, which may be platinum, rhodium or platinum-rhodium alloy wires or stranded leads of fine tungsten, molybdenum, or rhenium, for example, are connected to the ends of the filamentary element, as illustrated in FIGS. 5 and 6. An end conductor prior to mounting is shown in FIG. 4 and is preferably formed with a fiat loop at the connecting extremity for attaching to the heater element, and with the remainder of the conductor extending perpendicularly away from the flat loop. To secure the wire loop to the heater element the loop end is first dipped into a cold solder paste (actually a slurry of principally fine metallic powder) to pick up a small ball of paste. The paste may conveniently comprise a mixture of 325 fine mesh platinum powder in the case of the platinum-rhodium wires and 6-8 weight percent of the CaO-AI O slurry composition, mentioned above, as finely ground and blended in butyl carbitol containing about 50 mg. nitrocellulose per ml. of solvent. After thus containing a small ball of such paste on the loop end of the conductor, the loop end is set over the end of the heater element, being careful to make a blend contact with the surrounding ceramic slurry but avoiding force which might dislodge the coil. In the case of tungsten, rhenium, or molybdenum wires, the powder is preferably the same metal as the wires.

When the leads are thus attached, the unit is dried and then fired at about 1700 C. (or higher for the more refractory materials) in vacuum or suitable atmosphere, for about ten minutes, firing the ceramic in place and keeping integral the sintering of ceramic to ceramic and the ceramic to the filamentary conductor. At the same time the metal powder employed for connecting the end conductors to the filament is sintered and bonded by the same heat treatment.

It is important in the construction of the heated cathode in accordance with the present invention that each turn of the heater element, which in the example comprises a wire helix, be cooled by the heat mass or heat sink comprising the bonded metal ceramic body. Should a few increments or turns of the filament not be bonded in their ceramic environment, the temperature thereof would rise during operation causing the resistance of that portion of the filament to increase. Further increase in resistance results in further heating and eventual burnout of such unbonded section. In cases where the heater element is not completely covered by the bonding for it, the intervals or increments of bonding are regularly controlled and are small enough to avoid hot spots on the heater element. It is understood the filamentary conductor need not be enfolded in specifically a helical shape, but other enfolded heater conductors having a long total conductor length may be substituted as hereinafter described.

The resulting low heat mass cathode constructed in accordance with the foregoing example, requires about two seconds to heat to 90 percent of full equilibrium temperature and the power requirement is about one watt for emitting temperatures of 850 C. With the size of the cathode given in the example, having a diameter of approximately 0.1 inch, this is about 14 watts per square centimeter of emitting surface at the temperature of 850 C. The rapid heating is attributed to the intimate bonding of cathode and the enfolded heater element, and to their low heat mass. The cup shape and size and heater element size and configuration have been found to be very eflicient in fast heating service compared to other configurations which have been employed in miniature tubes having close spaced elements. It is of course understood the present invention is not restricted to any particular size of cathode, but is also applicable to larger cathodes, for example.

Remarkably little temperature gradient exists between the heater element and cathode. The heater element runs as little as to C. above the temperature of the cathode base at 850 C. in the example. In one instance, a cathode has been operated at 1360 C. with a filamentary temperature of 1440 C. Platinum cathode bases have even been melted without burning out the filamentary element and molybdenum bases have been operated above 1600 C. This operation has been attained inside vacuum tubes and the like while retaining very good vacuum, that is, without the generation of undesirable gases at the heated filament.

Because of the close coupling between the temperature of the heater element and the temperature of the cathode in the bonded arrangement, it becomes possible to approximately judge the actual cathode temperature by the resistance of the heater element. The volt-ampere characteristic of the heater element, therefore, is a measure of the cathode temperature and becomes sensitive to heat energy also received or given up by the cathode in relation to other sources, e.g., ion bomdardment, electron cooling, etc.

There is found to be extremely low leakage current between cathode and filament despite their close proximity. The ceramic provides substantially complete insulation for the filamentary element as well as physical support and heat conduction therefrom so that breakage, burnout, or shorting out of the filamentary element is very infrequent. The ceramic itself exhibits no deleterious effects with respect to cathode operation. The overall member exhibits considerable structural strength despite its small size. Effectively percent bonding between the cathode and heating element is effectively achieved and reliability is excellent.

FIG. 7 illustrates a heated cathode in accordance with another embodiment of the present invention. This cathode is substantially the same as the cathode thus far described especially as regards like elements referred to with like reference numerals and is constructed in substantially the same manner. In this embodiment the cathode planar section 1 is formed of molybdenum being provided with a tungsten sub-layer 3 for application of further emissive material. The planar portion 1 is again disc-shaped but has a varying thickness with diameter. An outer portion 9 is quite thin, having a thickness on the order of inch, while central portion 10 extends farther into the cathode cup area providing a cylindrical expansion and ledge around which a high resistance, e.g., tungsten, heater element 6 is disposed. Helical heater element 6 is here partially embedded in ceramic material 11 which may be the aforementioned combination of CaO and A1 0 Each turn of the helix is also preferably precoated with the same ceramic material.

In completing the heated cathode of FIG. 7, a first slurry layer of ceramic material may be deposited around cylindrical extension at 10 and then fired, after which a second layer of ceramic material is employed to partially embed the heater element. Alternatively, one slurry of ceramic material may be adhered around the cylindrical extension 10 and the heater element positioned therein for firing in one step. In this case it may be convenient to deposit one layer of slurry which is then dried. Another layer of slurry, having a binder, not compatible with the first layers binder, is then deposited and the heater element partially embedded therein. It is understood that end connections are also applied as described in connection with the previous embodiment.

The shape of the heated cathode of FIG. 7 provides certain advantages in that heat is conduced from heater element 6 towards the metal cathode in two directions, namely, towards surface 3 and towards cylindrical extension 10. However, the addition of cathode metal at extension 10 increases the heat mass such that the additional conduction achieved by partially surrounding the heater element does not all contribute to faster heating.

In FIG. 8 there is illustrated a cross-section of the miniature high temperature discharge device employing the heated cathode of FIG. 7. In this device, the cathode 13 is supported in a 0.0005 inch thick fernico type alloy cylinder 14 which is in turn attached to cathode connecting ring 15. It is noted that cylinder 14 closes off the underside of the cathode from the interelectrode or electron discharge region of the device. Opposite the cathode emitting surface is disposed an anode 16 formed of titanium, while a titanium grid 17 supported on a titanium connecting ring 18 is interposed between the cathode and the anode. The anode, cathode and grid are insulated from one another by means of fosterite insulating cylinders 19, while fosterite base 20 closes off the underside of the cathode. The cathode heater element 21 is provided with conducting leads 22 for supplying current thereto. A tube of this general type is set forth and claimed in the patent to James E. Beggs, 2,981,897, assigned to the assignee of the present invention. It is understood the other cathodes described herein are applicable to substantially this same discharge device construction.

FIGS. 9 and 10 illustrate respectively the underside plan view and cross-section of a further heated cathode in accordance with the present invention, which is again substantially the same regarding like elements indicated by like reference numerals. In this embodiment the heater element 23 is a tungsten tape or ribbon, less than five mils in thickness, disposed edgewise against the ceramic member 4 and at least partially embedded in the second layer of ceramic material 5. The tungsten ribbon heater element is disposed in the general shape of a spiral connected to conductor 7 at its center, with conductor 8 forming the outer terminal. The spiral ribbon configuration has the advantage of high heater current capacity in a small area with a maximum conductive relation between the cathode and the heater element for the size of the heater element. However, because the heater configuration is not enfolded, it is not as advantageous as regards minimum strain and therefore maximum strength of adherence to the cathode.

FIG. 11 illustrates another embodiment of the heated cathode in accordance with the present invention and is similar in general construction to the embodiment of FIGS. 9 and 10. The distinguishing feature as apparent from the drawing in this embodiment is an accordion fluted or folded spiral ribbon 24, which may be formed of tungsten less than mils in thickness, in place of spiral ribbon 23. The heater element is again at least partially embedded in a ceramic layer to effectively bond the heater element to the cathode body. This embodiment has an advantage in that a greater length of heater element is bonded in heat conductive relation to the cathode body, with the same high heater current capacity. Moreover, differential expansion is facilitated in a radial direction. However in regard to strength of bond to the cathode, this embodiment is not as advantageous as those embodiments disclosed herein, illustrating heater elements which are reentrant toward the cathode. This embodiment has essentially the same cross-section as depicted in FIG. 10.

Another embodiment is illustrated in cross-section in FIG. 12. In this instance a ribbon 24 has its flat side disposed toward the ceramic material but is fluted in accordion fashion so that fluted increments 25 therealong arereentrant in the ceramic material. This incremental bonding allows more freedom of expansion between the heater element and the ceramic. This incremental bonding principle, in addition to being applicable to a wire helix and a fluted ribbon, is also applicable to an embodiment which may employ ribbon having axial twist, with at least incremental portions therealong embedded in the ceramic material.

From the foregoing it is apparent that in accordance with the present invention, a heater element is integrally and incrementally bonded in place in intimate and secure contact with the flat cathode member by means of a fritted ceramic material which is sintered and which at least partially covers the heater element. The materials of the cathode member, heater, associated leads, and bonding frit are chosen to provide for firing at temperature high enough to achieve adhesion and sintered strength which will maintain the integral relation during high temperature service. These materials, proportions, configurations and sintering conditions are directed as above indicated to the achievement of an intimate and lasting, high thermal conductivity, stable bond between the heater element and cathode which will, during long and intermittent service, withstand the stresses of transient heating and cooling and unbalances in thermal expansion which may exist.

While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects; and I, therefore, intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United. States is:

1. The method of manufacturing a heated cathode for an electric discharge device comprising the steps of: adhering metal powder particles to the underside of an electric discharge device metal cathode for roughening the undersurface thereof, securely adhering a tfirst thin layer of ceramic slurry to the roughened underside of the electric discharge device cathode, adhering a second layer of ceramic slurry to the first ceramic layer, at least partially embedding a heater element having an extended length and reentrant increments in the last mentioned slurry, and firing the cathode to sinter the slurry thereby providing interlocking engagement of the ceramic to the metal particles and an intimate ceramic layer bond between the heater element and the cathode.

2. The method according to claim 1 wherein the metal cathode is formed of refractory material selected from the group consisting of tungsten, molybdenum and rhenium, wherein the metal powder particles are selected from the group consisting of molybdenum, tungsten, rhenium, platinum, niobium, vanadium, zirconium, tantalum, and mixtures thereof, and wherein the ceramic slurry comprises at least one ceramic material selected from the group consisting of calcia, alumina, lanthana, beryllia, magnesia, yttria, and hafnia.

3. The method of manufacturing a heated cathode for an electric discharge device according to claim 1 with the additional steps of: adhering metal powder particles to the surface of the electric discharge device cathode for roughening the surface thereof, and securely adhering a layer of electron emitting material to the roughened sur face, wherein interlocking anchorages for the electron emitting material are provided by the roughened surface.

4. The method according to claim 3 wherein the metal powder particles adhered to the surface of the cathode are selected from the group consisting of tungsten, molybdenum, rhenium, platinum, and rhodium, and wherein the surface is formed of a material selected from the group consisting of tungsten and rhenium.

5. A method of manufacturing a heated cathode for an electric discharge device comprising the steps of: at taching particles of a metal powder to the underside of a planar disc-shaped cathode metal member, securely adhering a ceramic slurry to said underside by interlocking engagement with the particles, enfolding a heater element of extended length to provide it with reentrant increments, at least partially embedding the heater element having an extended length and reentrant increments in the slurry, and firing the cathode to sinter the slurry providing intimate ceramic layer bonding between the heater element and the cathode.

6. The method of manufacturing a heated cathode for an electric discharge device comprising the steps of:

adhering metal powder particles to the underside of the electric discharge device flat metal cathode for toughening the surface thereof,

adhering a layer of ceramic slurry on the roughened underside of the cathode,

at least partially embedding a heater element having an extended enfolded length in the slurry,

at least partially drying the slurry,

disposing a solder paste at each end of the heater element,

adhering end connections thereto, and

firing the cathode to sinter the slurry layer and the solder paste to thereby intimately bond the heater element to the cathode and the end connections to the heater element.

7. The method according to claim 6 'wherein the end connections are formed of a material selected from the group consisting of tungsten, molybdenum, rhenium, platinum, rhodium, and their alloys, and wherein the solder paste includes material selected from the group consisting of the same metals.

8. A method of manufacturing a heated cathode for electric discharge device comprising the steps of adhering a first thin layer of ceramic slurry on the flat underside of an electric discharge device cathode, firing the cathode including the slurry to sinter the slurry, adhering a second layer of ceramic slurry to the first layer, at least partially embedding a heater element having an extended enfolded length in the last mentioned slurry, partially 1 1 drying the last mentioned slurry, disposing a solder paste at each end of the heater element and also adhering end connections thereto, and firing the cathode to sinter the slurry layer and the solder paste thereby intimately bonding the heater element to the cathode and the end connectiOns to the heater element.

References Cited 12 FOREIGN PATENTS 9/1959 Great Britain. 6/ 196 2 Great Britain.

5 JOHN F. CAMPBELL, Primary Examiner V. A. DI PA-LMA, Assistant Examiner UNITED STATES PATENTS U.S. c1. X.R. 2,577,239 12/1951 Eitel et a1. 313-340 10 313-270, 337, 340 3,041,209 6/ 1962. Beggs. 3,307,241 3/1967 Crapuchettes 29-2517