US3463978A - Monolithic electrode for electron tubes - Google Patents

Description

6, 1969 P. F. VARADI 3,463,978

MONOLITHIC ELECTRODE FOR ELECTRON TUBES Filed Dec. 22, 1966 //V VENTOI? PETER E VARAD/ United States Patent 3,463,978 MONOLITHIC ELEgTRODE FOR ELECTRON UBES Peter F. Varadi, Stamford, Conn, assignor to The Machlett Laboratories, Incorporated, Springdale, 'COIJIL, a corporation of Connecticut Filed Dec. 22, 1966, Ser. No. 603,861 Int. Cl. H0lj 1/88, 19/12 US. Cl. 313-250 1 Claim ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention pertains to electrode structures for electron discharge devices and has particular reference to novel monolithic electrode structures wherein both the cathode and the grid electrodes of a vacuum tube are supported in fixed immovable relation upon a single insulated substrate. In the prior art, it has been common to provide a vacuum tube With electrodes in the form of separate spaced elements which are supported at their ends by the tube envelope or by other suitable supporting structure. The active electrode elements in such devices are generally relatively closely spaced and when the tubes are operated the resultant heat causes bowing or other deformation which often results in shorting between electrodes. Other causes of shorting and breaking of such electrodes are mechanical shocks and vibration which are especially damaging when the electrodes are thin and fragile. It has also been found that in tubes exposed to magnetic fields, one or more of the electrodes are often moved by the influencing field.

Attempts have been made to overcome these disadvantages but have not been succssful. For example, cathode strips have been mounted on ceramic substrates, but no really efiective provision has been made for restraining grid elements from movement into contact with the cathode strips under severe environmental conditions. In other cases, cathode heaters have been embedded within heat conducting materials and electrically insulated therefrom so that heat from the heaters will be transmitted through the encompassing material to an overlying electron-emissive element. This again does not solve the problem of preventing possible shorting between grid and cathode electrodes.

In vacuum tubes of the triode types, or types embodying a larger number of electrodes, it is desirable and necessary that the grid electrode elements be constantly maintained at a predetermined distance from other electrodes.

SUMMARY OF THE INVENTION In accordance with the present invention, all of the advantages of the prior art are achieved and the abovementioned disadvantages of the prior art are overcome by a structure wherein grid elements and cathode elements are both mounted upon a single preshaped ceramic substrate and are fixed thereto throughout their entire effective lengths whereby movements of the elements are impossible so that inter-electrode spacings are constantly maintained under all types of operating conditions. The substrate is a monolithic block of ceramic having a num- Patented Aug. 26, 1969 ber of cathode-receiving areas spaced apart by raised land areas. On each cathode-receiving area is a strip of electron-emissive material which is bonded to the substrate by a strip of metallization. The metallized strips are connected at their ends to a source of electrical potential whereby they may be heated to provide heater elements for the emissible strips. The metallized strips may be tungsten, or metal combinations such as molybdenummanganese, molybdenum-titanium, nickel-titanium, or other suitable metal or combination of metals. The metallizing strips may be painted or otherwise deposited on the body and sintered thereto at a temperature well above the operating and activating temperatures of the emissive material.

The electron-emissive material may be any selected wellknown material such as the commonly used combination of barium-strontium-calcium carbonate and may be painted or otherwise deposited over the metallizing heater strips. Alternatively, the cathode may be in the form of phormat or impregnated material. When tungsten is being used, or other metallizing material which is compatible Within the electron-emissive material, the emissive strips may be applied directly over the sintered metallizing strips. However, in cases where incompatible metallizing materials are used, an intermediate layer of suitable barrier material should be employed such as, for example, nickel, platinum or tungsten. Such barrier or isolating layers should also be sintered before the emissive layer is applied.

The various layers may be applied as solutions containing the desired active ingredients, and during sintering the binder or solvent is evaporated to leave the desired residue. However, the layers preferably are prepared selfsupporting tapes, sheets or films comprising the active ingredients supported within a binder such as nitrocellulose or methacrylate. The tape is precut to the desired shape and size and applied to the substrate by a suitable solvent or, sometimes, by pressure alone. Then, during sintering, the binder is driven off. Such tapes or films are common and well known, and therefore, need not be more fully described here.

Between the cathode-receiving areas of the substrate body are raised land areas upon which are located grid elements. The land areas are raised above the cathode surfaces a predetermined distance which establishes the inter-electrode spacing between the cathode and the grid elements. To provide the grid structure, the land areas are first metallized with one of the metallizing materials mentioned above, the metallization is sintered, and a grid element is added by applying a material such as tungsten, molybdenum, or other high temperature metal stri or wire to this metallization. Such grid materials may also be initially formed as paint solutions or thin films. For example, a thin film or sheet of grid material supported in a suitable binder may be made and cut into the shapes of the metallized land areas with cutout portions in the areas of the cathode elements. This shaped film is brought into overlying relation with the substrate, with longitudinally extending areas being aligned with the metallized land areas. Then, the film is welded to the metallization by application of heat. However, in some cases it is satisfactory to lay the grid directly onto the land areas without the intermediate metallization. The grid elements thus are fixed in predetermined relation to the cathode elements and are likewise immovable.

As a further improvement, the grid elements may be made of or coated with a thin layer of tantalum, platinum, gold, or other high work function, high emissivity material to provide low grid emission during operation of the device. Additionally, a blocking layer may be provided when necessary to prevent contamination of the grid material by the metallization.

The land areas can be fabricated in many ways so as to prevent great amounts of heat from transferring to the grid elements such as, for example, by undercutting the substrate beneath the land areas or by providing a, series of small spaced land areas having bridging elements for supporting the grid elements.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a horizontal sectional view through an electrode structure embodying one form of the invention;

FIG. 2 is a fragmentary perspective view of the electrode structure shown in FIG. 1;

FIG. 3 is a fragmentary perspective view of a modified form of the grid electrode;

FIG. 4 is an enlarged sectional view of a second modified form of the grid electrode; and

FIG. 5 is an enlarged sectional View of a further modification of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more particularly to the drawings wherein like characters of reference designate like parts throughout the several views, the electrode structure disclosed in FIG. 1 comprises a substrate having thereon cathode elements 12 and grid elements 14 and supporting an anode 16 in overlying relation to the cathode and grid electrodes.

The substrate 10 is made of insulating material, for example a ceramic such as alumina, and actually may have any selected desirable configuration uch as disclike, cylindrical, annular, or other shape. The substrate 10 is depicted in FIGS. 1 and 2 as a flat rectangular discwafer or plate having a series of alternately disposed longitudinally extending grooves 18 and lands 20 on both of the opposing broad surfaces thereof. The grooves and lands may be substantially equally spaced and of predetermined widths in accordance with device requirements. The grooves are of uniform depth, and the bottoms thereof lie at a predetermined distance from the surfaces of the lands in accordance with the desired interelectrode spacing between cathode and grid electrodes. The grooves 18 are adapted to retain the individual cathode elements 12, and the grid elements 14 are located on the surfaces of the lands 20.

The lands 20 are formed in the structure of FIG. 2 merely by machining the grooves 18 in the surfaces of the substrate 10, thereby forming between each pair of grooves a longitudinally extending uniformly wide, flattopped elongated mesa or land area 20. However, the grid-supporting structure may take other forms if desired, as will be described hereinafter.

The bottom surface of each groove 18 and the exposed fiat surface of each land 20 is metallized by superimposing thereon a layer of metallic particles such as tungsten, or combination of metals such as moly-manganese, moly" titanium, nickel-titanium, or moly-manganese silicon dioxide, or other selected material which can be readily adhered to the ceramic material and to which the cathode strips 12 may be bonded, as will be described, whereby the cathode strips are firmly and fixedly aflixed to the rigid supporting substrate. The metallization material should also be capable of efficiently functioning as a heater for the cathode strips when the ends thereof are connected to a suitable external source of potential (not shown).

The metallizing heater strips 22 in the grooves 18 and the similar metallizing strips 24 on the lands 20 may be sintered by heating to a temperature of about 1500 C., in the case of tungsten, for example, at which time the hinder or solvent which holds the metallic particles is driven off.

At this point in the process, the electron-emissive elements 12 are placed on the heater strips 22, and this may be accomplished by spraying, screening, evaporating, flame spraying, or painting a suspension of known emissive materials over the surfaces of the heater strips. Instead of such a deposited or painted solution, the emissive elements may comprise a thin film or sheet wherein emissive materials such as a combination of bariumstrontium-calciurn carbonate are supported in a dried binder such as a suitable volatile cellulosic material. Films of this type are conventional and well known and greater detail thereof is believed unnecessary here. Such films are preferred over painted solutions because they are of more uniform thickness and the emissive materials are generally more uniformly distributed throughout the binder material. The film may be applied to the surfaces of the heater strips by heat and pressure or by use of a solvent or a suitable adhesive.

Strips of the desired cathode material, either paint or film, are deposited upon the heater strips after which they are heated to drive off the binder material and convert the carbonate to oxide which is securely bonded to the ceramic surface by the underlying metallization.

If the heater strips 22 are made of tungsten or other material which is compatible with the emissive material, the emissive strips 12 may be applied directly upon the heater strips. However, if the heater strips 22 or the underlying ceramic material are deleterious or damaging to the cathode material, then an intermediate blocking layer 26 (FIG. 5) is interposed between the strips 12 and 22. Such layer 26 can be provided by depositing a suitable barrier material such as nickel, platinum, iridium, or tungsten, for example, on the heater strips 22 before the emissive material 12 is deposited. The blocking layer can be an evaporated layer, a paint solution or a film and may be made to adhere to the strips 22 by heating as described in connection with the forming of the emissive elements 12.

The grid elements 14, like the cathode elements 12, may be painted onto the sintered metallization strips 24 or may be formed as a film comprising the selected grid material, such as tungsten or molybdenum or other high temperature metal, in particle form suspended Within and uniformly distributed throughout a dried binder such as nitrocellulose or methacrylate. The painted solution or film is placed upon the metallized strips 24 and made to adhere thereto by application of sufficient heat to drive off the binder or suspension material and bond the particles of grid material to the sintered metallization strips whereby there is formed a grid comprising a number of grid elements 14 located in predetermined spaced relation to the cathode elements 12 and retained immovably in position whereby the inter-electrode spacing is constant and unvarying. The metallization strips 24 may themselves be the grid elements, if desired, and in such cases the outer strips 14 can, if utilized, be barrier layers which suppress electron emission.

A blocking layer 28, similar to blocking layer 26, may be disposed between layers 14 and 24 where the metallizing material is incompatable with the grid material, as shown in FIG. 5.

To complete a triode structure, metal anode plates 16 are disposed on opposite sides of the grid-cathode structure in spaced overlying relation thereto. To form a unitary structure, the substrate 10 is provided along each edge with a longtiudinally extending flange portion 30 (FIGS. 1 and 2) upon which the respective anode plates 16 are mounted so as to span the underlying grid and cathode electrodes. The height of the flange portions 30 determines the anode-grid inter-electrode spacing. To secure the anode plates securely in place, the adjacent end surfaces of the flange portions 30- are provided with metallized strips 32 by which the anode plates 16, preferably copper, are bonded to the ceramic substrate material as by the use of an intermediate layer 33 of Kovar or other material which satisfactorily compensates for differences in thermal expansion characteristics.

In some electron tubes the heat conentrated in the substrate may be damaging to the grid elements 14.

Therefore, means may be provided for preventing excessive amounts of the heat from reaching the grid elements. As shown in FIG. 3, one means comprises forming the raised grid-supporting land areas as a number of relatively short pedestals 34, each pedestal being provided with metallized layers 36 on the top surface, and a grid element 38 being laid in spanning relation across the pedestals and connected thereto through the metallized layers 36, as by soldering, welding or brazing. In such a device, in order to prevent movement of the freely suspended portions of the grid elements 38 and consequent change in inter-electrode spacing, the grid element is preferably formed as a ribbon or wire of suitable thickness and rigidity. As an alternative, the pedestals may be made shorter, greater in number, and spaced closer together so that intervening spaces are too small to permit undesirable movement of the bridging portions of the grid elements.

Another means of preventing excessive heat from being conducted through the ceramic substrate to the grid elements 14 is illustrated in FIG. 4. In this structure, the side walls of the longitudinally extending land areas 20 are undercut so that actually the land areas are connected to the main body portion of the substrate 10 by narrow elongated ceramic portions 40 which restrict the passage of heat to the land areas 20.

It will be readily apparent that other means may also be provided for preventing excessive heat from reaching the grid elements while still restraining the grid and cathode elements from undesirable movements. It will also be apparent that other modifications and changes in the device shown and described may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claim.

I claim:

1. An electrode structure for electron discharge devices, comprising a body of rigid electrically insulating material, alternately disposed raised and depressed land areas on a surface of said body, heater elements in the form of electrically conductive metallic strips mounted on said depressed land areas of the body and immovably fixed thereto throughout the active lengths thereof, grid elements mounted on said raised land areas and immovably fixed thereto, and strips of electron-emissive material mounted upon said heater elements, said raised land areas comprising a plurality of spaced longitudinally aligned supports, and the grid elements being mounted on said aligned supports in spanning relation to the spaces therebetween.

References Cited UNITED STATES PATENTS 3,066,236 11/1962. Sandbank 313-250 3,119,041 6/ 1964 Harris 313293 2,899,590 8/1959 Sorg 313-250 3,374,385 3/ 1968 -Lattimer 313337 JOHN W. HUCKERT, Primary Examiner M. EDLOW, Assistant Examiner US. Cl. X.R.

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