US2501089A - Thermionic electron emitter - Google Patents

Description

March 1950 M. A. POMERANTZ THERMIONIC ELECTRON EMITTER Filed Nov. 29, 1945 Patented Mar. 21, 1950 THERMIONIC ELECTRON EMITTER Martin A. Pomerantz, Philadelphia, Pa.

Application November 29, 1945, Serial No. 631,668

9 Glaims.

This invention relates to electron emission structures for electron discharge devices, and more particularly to thermionic emitting cathodes.

It is well recognized that the electron emitter or cathode of an electron discharge device, such as a vacuum tube, is a very important part or element of the device, and consequently considerable effort has been directed in the past to development and improvement of electron emitters or cathodes. Such effort has been further stimulated by the rapid and ever-increasing adaptations or uses of electron discharge devices in various fields.

The thermionic emitting cathodes most Widely used in the past fall into three distinct categories or classes, as follows:

(1) Pure metal, such as tungsten filaments,

(2) So-called thoriated tungsten filaments in which free thorium diffused to the tungsten surface is the active agent, and

(3) Oxide coated cathodes comprising various alkaline earth oxide mixtures coated onto a metallic base, either in the form of a filament or a sleeve.

Cathodes falling within class (1) employing a pure refractory metal, have been limited in use due to the fact that refractory metals, such as tungsten, are relatively poor emitters. Consequently, resort has been had to the other types of cathodes employing a combination of a refractory metal and a substance or material having good emission properties.

While the cathodes of classes (2) and (3) above have been generally satisfactory from the standpoint of emission, they have other shortcomings. Such cathodes require activation procedures, and they are easily poisoned as a consequence of various conditions encountered in tube operation. They cannot generally be exposed to air after activation without suffering deleterious effects. Moreover, their maximum available emission density is limited.

In the past there have been attempts to coat substances such as thorium oxide onto base metals, but such attempts have been unsatisfactory because of the dimculty of insuring adhesion between the coating and the base metal. Consequently, thoria coated cathodes were discarded as being commercially impracticable.

It has also been proposed heretofore to form electrodes for luminous electric discharge tubes of a sintered mixture comprising refractory metal and one or more non-metallic substances of high electronic emissivity, e. g. tungsten and a mixture of barium oxide and calcium oxide, the proportion of the metal being greater than the proportion of non-metallic substances. Such a structure, while perhaps suitable for luminous discharge tubes, is not suitable for other types of discharge devices such as vacuum tubes, because it is not capable of producing as high values of emission densities as are often required under conditions of high vacuum.

The principal object of the present invention is to provide a new and improved electron emitting structure or cathode which will overcome the objections and shortcomings of prior cathode structures, as mentioned above.

A more specific object of the invention is to provide a novel cathode structure which requires no activation procedure and. is not susceptible to poisoning, and which is characterized by high electron emission, ability to withstand various types of punishment such as sparking and bombardment, and ability to withstand higher than normal operating temperatures due to back bombardment or other cause.

Another object of the invention is to provide a rugged cathode structure havin longer life than prior cathodes.

A further object of the invention is to provide a cathode structure which is adapted to fulfill the needs of various types of electron tubes, and which is particularly adapted for use in magnetrons.

Still another object of the invention is to pro vide a cathode structure formed of high-melting-point materials.

In accordance with the present invention, there is provided a thermionic emitter comprising a rigid or self-supporting body composed principally of sintered, refractory, non-metallic, electron-emitting material. Preferably, the said body is formed of sintered thorium oxide, but it may be composed of other substances having similar properties, or it may be composed of a mixture of substances. In certain embodiments in which the electron-emitting material is in engagement with a heater element, the invention makes use of the fact that at operating temperatures the electrical conductivity of said material is on the one hand sufiiciently low compared with the conductivity of the heater element to prevent shortcircuiting of the latter, and on the other hand sufiiciently high to pass the electron emission current even up to current densities heretofore unattainable with any other type of thermionic cathode. Thus, the material comprising the cathode, as utilized according to the invention, may

3 simultaneously constitute both the electron emitter and the insulator for the heater wire.

The invention may be fully understood by reference to the accompanying drawing, in which Fig. l is a somewhat enlarged elevational view of a thermionic cathode structure constructed according to the invention;

Fig. 2 is a greatly enlarged sectional view of the cathode structure;

Fig. 3 is a fragmentary perspective view showing an end portion of the heater element employed;

Fig. 4 is a view, similar to Fig. l of a modified form of the cathode structure;

Fig. 5 is a greatly enlarged sectional view of the device shown in Fig. 4;

Fig. 6 is an enlarged sectional view of a further embodiment of the invention;

Fig. '7 is an enlarged sectional view of a still further embodiment of the invention; and

Fig. 8 is an enlarged sectional view of another embodiment of the invention.

Referring first to Figs. 1 to 3, the cathode structure comprises a rigid or self-supporting body I composed of sintered, ceramic-like refractory, non-metallic, electron-emitting material, and a heater element 2 formed of a highly refractory metal, such as tungsten. Preferably, the electron-emitting body I is formed of sintered thorium oxide, but the invention contemplates the use of any non-metallic substance havin the properties of thorium oxide, i. e. high electron emission, ability to withstand high temperatures, and capability of being fired to form a solid se1fsupporting body. In the embodiment of Figs. 1 to 3, the electron-emitting body l is in the form of a cylindrical sleeve, and the heater element 2 is embedded therein, Such construction is desirable in that it provides a single cathode and heater unit.

At the ends of the sleeve 5 are metallic end members 3 and 4 which are preferably formed of molybdenum. These members are formed so as to seat within the ends of the sleeve I, as shown, and they are brazed to the end turns of the helical heating coil by means of flat rings and 6 of platinum or other high melting point metal. Preferably, the end turns of th heater wire are flattened to increase the area of engagement with the flat rings.

The end member 3 has a central aperture therein, the purpose of which will appear presently. Member 3 also has a threaded recess which is adapted to receive a hollow threaded terminal element 8. The latter is adapted to receive a lead wire 9, one end of which may be inserted within the terminal element 3, and may be soldered or otherwise secured thereto.

The other end member 4 has a central threaded aperture which is adapted to receive a hollow threaded terminal element If]. The latter is adapted. to receive one end of a lead wire H which is suitably secured to the terminal element It, and which extends axially within the sleeve i and through the aperture 1. An insulating plug or bushin i2 is adapted to seat within the aperture l and serves to support the lead wire H and to maintain it in spaced relation to the wall of aperture. The bushing l2 may be formed of the same material as the cathode proper.

Thus the lead wires 9 and H are connected to the respective ends of the heater element 2 through the metallic end members 3 and 4. It will be noted that the structure illustrated enables electrical connection to both ends of the 4 helical heating element from one end of the structure.

In addition to serving as electrical connectors, the end members 3 and 4 impart greater rigidity and strength to the structure as a whole. This is highly desirable in many instances.

In Figs. 4 and 5, there is illustrated another form of the novel cathode structure which is particularly adapted for use in a magnetron. In this instance there is provided an electronernitting sleeve 53 similar to the sleeve I of the first-described embodiment, and there is also provided a helical heater element [4 embedded Within the sleeve. At the nds of the sleeve there are provided metallic end members I5 and I6 which are brazed to the end turns of the heater element, as previously described. The end members are formed to provide bosses l1 and I8 within which plugs l9 and 20 are adapted to be inserted. The latter elements serve as terminal elements, and they are suitably secured to the end members by brazing or welding. The said el ments are provided with transverse threaded apertures and 22, into which suitable connector elements may be inserted to effect electrical connection of lead wires.

In magnetrons, it is customary to employ socalled hats in association with the cathode, and such elements are shown at 23 and 2 3. These elements are in the form of apertured metal discs having turned edges. They are securely held by the bosses I1 and i8 and the associated plugs l9 and 20, as illustrated.

In Fig. 6 there is illustrated a further embodiment of the invention in which the cathode structure is of extremely simple form, and. the heater element is independent of the electronemitting body. Thus there is provided a sleevelike electron-emitting body 25 of the same character as the electron-emitting sleeves of the previously described embodiments. The body 25 is closed at one end. as shown at 26, except for an aperture through which the lead wire 2! may extend. A helical heater element 28 is disposed within the hollow body 25 and has one end secured to the lead wire 2'! by brazing or otherwise. The other end of the heating element is similarly secured to a lead wire 29. which extends through an apertured plug 30 seated within the end of the body 25. The plug 30 may be formed of thoria or etal, or it may be formed of any other suitable material. The helical turns comprising the heater element 28 may be in spaced relation to the inner wall of the body 25, as illustrated, or they may be in direct engagement with said wall.

In Fig. '7 there is shown another simple embodiment of the invention in which the cathode functions essentially as a directly heated cathode. In this instance, there is provided a simple electron-emitting sleeve 3| of the same character as previously described, and the conductivity of the sleeve is increased by an inner sleeve or coating 32. The latter may be formed by evaporating metal on the inner wall of sleeve 3 l, or it may take the form of a sleeve fabricated of refractory metal such as molybdenum or tantalum. The lead conductors 33 and 34 extend into the ends of the sleeve into engagement with the metalizecl inner surface thereof. The heating current flows from one lead wire through the metalized sleeve to the other lead wire.

In a device of th character shown in Fig. 7, instead of using a metal coating or sleeve, the conductivity of the cathode member may be increased by mixing a metal powder with the nonmetallic electron-emitting material so that the metal powder is embedded therein, as shown in Fig. 8 wherein the cathode member is designated and the lead conductors are designated 36 and 8?. In such case, however, the amount of metal powder should only be suiiicient to give the desired conductivity, and should not be such as to change the essential character of the oathode member, i. e. its composition principally of the non-metallic electron-emitting material.

While the electron-emitting body is shown in the various illustrated embodiments as being in the form of a hollow sleeve or the like, the said body may take any other desired form. For example, the said body may be in the form of a flat disk with a spiral heating element. Also, the heating element may be outside of the cathode member, instead of being inside thereof or embedded therein, regardless of the shape or form of said member. Furthermore, the heating element need not be helical in form but may take other forms, such as a straight wire.

In constructing the electron-emitting body provided by the invention, the said body may be pre formed with the material in a pliable state, and it may then be fired at a sufiiciently high temperature to form the final product. Where the heater element or metal powder is to be embedded in the electron-emitting body, this may be done during the preforming operation while the material is in a pliable state.

By way of specific example, where thoria is the material employed, the electron-emitting body may be formed in the following manner:

The raw thoria may be fused in an arc furnace, after which the fused material may be broken up and then washed, first with hydrochloric acid and then with water. The material may then be roasted at 900-1000 C. in normal air, after which it may be reground to reduce the size of the particles. It may then be mixed with a saturated thorium chloride solution to form a paste for preforming or molding. The degree of plasticity appropriate to the particular molding requirements may be controlled by using the proper relative proportions of the constituent materials. The electron-emitting body is then molded in accordance with any of the conventional molding methods, being permitted to set until it is sufiiciently hard to retain its shape. After thoroughly drying, the molded body may be fired at a temperature of 1900-2000 C. in an atmosphere of argon.

While it is preferred to form the electronemitting body of thorium oxide, any other substance having similar properties may be employed, as previously stated. Some examples of other substances which might be used are refractory oxides such as magnesia, beryllia, and alumina. In some instances it may be desirable to employ a composition or mixture of one or more refractory substances.

The reason why thoria is preferred is because it is superior to other substances for the purposes of the present invention. It can be operated at higher temperatures, and can produce higher emission densities. Its melting point is higher, and the vapor pressure at operating temperatures is lower.

Although certain specific embodiments of the invention have been illustrated and described, it will be apparent from the foregoing description that various modifications are possible within the scope of the invention as defined in the appended claims.

I claim:

1. A thermionic electron emitter, comprising a sleeve composed of sintered, refractory, non-metallic electron-emitting material, a helical heating coil embedded in said sleeve, a pair of conductive members secured to the ends of said sleeve respectively and electrically connected to the ends of said coil, and means connecting lead wires to said end members.

2. A thermionic electron emitter, comprising a sleeve composed of sintered, refractory. nonmetallic, electron-emitting material, a helical heating coil embedded in said sleeve, a pair of conductive members secured to the ends of said sleeve respectively and electrically connected to the ends of said coil, one of said members having an aperture therein, and a. terminal element extending from the other end member within said sleeve receiving a lead Wire extending through the aperture of the first end member, whereby electrical connection to both ends of said coil may be made from one end of the structure.

3. A thermionic electron-emitting cathode structure; comprising a rigid, self-contained, selfsupporting electron-emitting body composed principally of sintered, refractory, non-metallic, electron-emitting material; and electric heating means supported by said body in intimate relation therewith to heat said body to anemitting temperature.

4. A thermionic electron-emitting cathode structure; comprising a rigid, self-contained, self-supporting electron-emitting body composed principally of sintered thorium oxide; and electric heating means supported by said body in intimate relation therewith to heat said body to an emitting temperature.

5. A thermionic electron-emitting cathode structure according to claim 3, wherein said heating means comprises a heater element embedded in said body.

6. A thermionic electron-emitting cathode structure according to claim 5, wherein said body is of tubular or sleeve-like form.

7. A thermionic electron-emitting cathode structure according to claim 3, wherein said heating means comprises a helical conductor.

8. A thermionic electron-emitting cathode structure according to claim 3, wherein said heating means comprises a metal coating on a surface of said body.-

9. A thermionic electron-emitting cathode structure according to claim 3, wherein said body has metal powder embedded therein to render it conductive to heating current.

MARTIN A. POMERANTZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,618,499 White Feb. 22, 1927 2,007,926 Braselton July 9, 1935 2,00:,932 Ruben July 9, 1935 2,175,345 Gaidies et al Oct. 10, 1939 2,251,045 Gaidies et a1 July 29, 1941 2,352,137 Thurber June 20, 1944

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