US2830215A

US2830215A - Getter structure for electron tube

Info

Publication number: US2830215A
Application number: US541285A
Authority: US
Inventors: Merrald B Shrader
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1955-10-18
Filing date: 1955-10-18
Publication date: 1958-04-08
Anticipated expiration: 1975-04-08

Description

April 1958 M. B. SHRADER 2,830,215

GETTER STRUCTURE FOR ELECTRON TUBE Filed Oct. 18, 1955 IN V EN TOR.

v MEF/PALD 5.SHRAO/7 TTORNEY United States Patent GETTER STRUCTURE FUR ELECTRON TUBE Merrald B. Shrader, Mount Joy, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application October 18, 1955, Serial No. 541,285

3 Claims. (Cl. 313-176) This invention relates to the art of fabricating electron tubes and to methods of and means for gettering electron tubes. In particular, this invention relates to elec tron tubes having improved sorption getter structures adapted to co-operate with self-supporting, hollow cathode structures to provide continuous gettering action and to methods for assembling such getter structures.

The presence of certain gases within the evacuated envelope of an electron tube will tend to produce a number of deleterious effects, including the occurrence of ionization and poisoning of the cathode, which will impair the operating characteristics and life of the tube. It is difficult and impractical to remove all of the undesirable gases from within the envelope by vacuum pumping operations. Furthermore, the electrode structures enclosed by the envelope often absorb or adsorb the undesirable gases or contain impurities capable of breaking down under the influence of heat to form undesirable gases. When the tube is operated, such electrode structures Will be heated and the gases retained by sorption driven oif or the impurities broken down thus releasing undesirable gases within the evacuated envelope.

In order to prevent the deleterious effects of such undesirable gases it is common practice to place a material within the evacuated envelope which Will either chemically combine with such gases to form inert and harmless compounds therewith or will take up and hold such gases by a sorption process. Such materials are generically called getter materials. Zirconium and tantalum are examples of the sorption type of getter material which will retain occluded gases even at high temperatures.

For best operation the sorption type of getter material should be located centrally within the evacuated envelope and should present as much area as possible for contact with the undesirable gases. Further, the gettering action of the material will be increased if the material is heated. In fact, it has been found that the gettering action of sorption type getter material with respect to different gases varies with the amount of heating thereof. In other words at one temperature the material will effectively occlude one gas but not others, Whereas at a higher temperature the material will be more effective with respect to other gases but less effective with respect to the first gas.

Accordingly, it is an object of my invention to provide an improved electron tube having an improved sorption type getter structure.

Itv is a further object of my invention to provide an improved sorption type getter structure for an electron tube, different portions of which may be heated to different temperatures during operation of the tube.

It is another object of my invention to provide an improved sorption type getter structure in which the getter material presents a maximum of area for contact with undesirable gases and is centrally located within the envelope of the electron tube.

In electron tubes designed for high-power output it is desirable to use tungsten or thoriated tungsten directly heated cathodes because of their ruggedness and long life characteristics. Such cathodes may take the form of a self-supporting bird cage or basket structure of tungsten filaments. The hollow central portion of such a structure provides an ideal location for the sorption type of getter structure since it is centrally located within the evacuated envelope of the tube and since the getter structure may be heated by radiation from the cathode filaments.

However, the cathode structure is ordinarily subjected to a flashing process when assembled to relieve strains which may exist in such structureand to set the crystal growth of the tungsten material. In the case of ordinary tungsten such flashing process comprises heating the struc ture to an elevated temperature in an atmosphere of hydrogen to inhibit oxidation. In the case of thoriated tungsten a carburizing process is combined with the above described flashing operation, through the use of benzolated hydrogen in place of pure hydrogen, for example. The hydrocarbon gas or vapor is broken down by the heat, depositing free carbon on the thoriated tungsten which reacts with the tungsten to form a tungsten carbide surface. It has been found that the rate of evaporation of thorium from the carburized tungsten surface is less than that from an uncarburized tungsten surface and that the grain structure of tungsten carbide facilitates the diffusion of thorium to the surface, both of which effects are advantageous to the operation of the cathode.

If sorption types of getter material were subjected to such flashing or carburizing processes their effectiveness would be greatly impaired. They would occlude hydrogen from the hydrogen atmosphere used insuch processes and would tend to deteriorate or become saturated and incapable of occluding other gases or more hydrogen.

For this reason it is necessary that the getter structure be inserted into the cathode structure after the fabrication and processing of the cathode structure.

Accordingly, it is yet another object of my invention to provide a sorption type getter structure particularly adapted for use within a self-supporting hollow cathode structure.

It is still another object of my invention to provide a sorption type getter structure which may be assembled within a self-supporting bird-cage type of cathode structure after such cathode structure has been completely processed.

It is a still further object of this invention to provide a sorption type getter structure which is inexpensive, simple to fabricate, and highly effective.

Briefly, this invention comprises a plurality of getter parts of sorptio-n type getter material mounted along a support rod and extending outwardly from adjacent thereto. The getter parts have apertures therethrough and through which the suport rod passes. According to one embodiment of this invention, the getter parts may comprise spaced members and spacer rings may be interposed between the members. According to one feature of this invention the support rod and the getter parts thereon may extend axially along a major portion of the interior of a self-supporting hollow cathode structure. According to another feature of this invention the center rod may be placed within a bird-cage type cathode structure during the fabrication thereof and the getter parts may be mounted on the center rod after the fabrication and processing of the cathode structure.

The invention will be better understood from the following detailed description when read' in conjunction with the appended single sheet of drawing wherein:

Fig. 1 is a view in elevation and partially in section of an electron tube embodying this invention.

Fig. 2 is a cross-sectional view taken along line 2-2 in Figure l.

Referring to the drawing, an electron tube adapted for high-power operation may comprise a base 12 through which two massive lead-ins, 14 and 15, are insulatingly sealed in spaced relation to each other. The first lead-in 14 is longer than the second leadin 15 and a first conductive filament supporting plate 16 is eccentrically and conductively mounted on the end of the first lead-in 14. A second conductive filament supporting plate 17 is eccentrically and conductively mounted on the end of the second lead-in 15 adjacent the first plate 16. The eccentricity in mounting of the two plates 16 and 17 causes them. to overlap each other in spaced relation between the two lead-

ins

14 and 15.

A plurality of filament connector rods 18 arranged in cylindrical array are mounted at one end thereof on the supporting plates 16 and 17, adjacent rods of such array being mounted alternately, one on one plate and the next on the other plate. This is accomplished by providing the first plate 16 with plurality of holes (not shown) therethrough equal in number and corresponding in array to that of the rods 18. Alternate rods 18 of such array pass through alternate ones of such holes in insulated relation to the edges thereof and are conductively mounted in holes (not shown) provided in the second support plate 17. The remaining rods 18 are conductively mounted in and terminate at the remaining holes in the first plate 16.

A plurality of rigid cathode filaments 20 are each conductively connected to the free end of one of such rods 18 to form a cylindrical array of filaments 20. The free ends of such filaments 20 are bent inwardly of the array of filaments 20 and brought together and conductively connected to an annulus or core 22 located at the center of such array. Thus, a self-supporting birdcage type directly-heated cathode is formed. The heating current passes through the first lead-in 14, the first support plate 16, up alternate ones of the connector rods 18 and filaments 20 to the core 22 and back down the other filaments 20 and connector rods 18 to the second support plate 17 and lead-in 15. The filaments may be composed of tungsten or thoriated tungsten which materials are rugged enough to be self-supporting and yet display good thermal electron emissivity.

A cylindrical control grid structure surrounds the above-described cathode structure and is supportedon the base 12. The control grid structure may comprise an annular support member or collar 24 upon which is mounted one end of a cylindrical array of support or side rods 26, such cylindrical array of side rods being of larger diameter than the diameter of the array of filaments 20. A plurality of turns of wire 28 are wound about the outer surface of the cylindrical array of side rods 26 and are conductively mounted thereon. A top cap 30 may be mounted on the other end of the cylindrical array of side rods 26 to provide mechanical rigidity in the control grid structure.

The above described structure forms a cathode-control grid sub-assembly which extends into a cup-shaped external anode 32. The anode 32 is insulatingly and hermetically sealed to the base 12 through an annular instructure 40 is located within the bird-cage type cathode structure, above described. According to the embodiment shown, the getter structure 40 comprises a center support rod 42 mounted axially within the array of cathode filaments 20. The center support rod 42 does not extend the full length of the cathode structure thus leaving a space between the free end of the support rod 42 and the core 22 at the end of the cathode structure. The center support rod may be conveniently supported on a pair of heat shields 44 which are insulatingly supported on the connecting rods 18 to protect the base from heat radiated by the filaments 20. The heat shields may be mounted on the connector rods 18 by means of ceramic washers 46 around the rods and engaging the edges of holes in the heat shields 44. The support rod 42 may be mounted in holes centrally located in such heat shields 44 thus centering the support rod 42 with respect to the cathode structure.

According to the embodiment shown in the drawing,

sorption type getter material (e. g. zirconium or tantalum) in the form of outwardly extending members, shown as circular plates or discs 48, having holes therethrough are mounted in spaced relation along the support rod 42. Spacer rings 50 may be interposed between the getter plates 48, the support rod passing through such rings 50 and through the holes in getter plates. The first spacer ring 50 may butt against the upper one of the heat shields 44 and the getter plates 48 and rings 50 alternated along the support rod 42 to a point adjacent the free end thereof where a fastening means such as a retainer ring 52 crimped or heli-arc welded to the sup port rod 42 is used to retain the rings 50 and plates 48 on the rod 42. The spacer rings 50 may be composed of sorption type getter material, or of metal, or of other suitable material. When an electron tube embodying the structure described above is operated, heat radiated from the filaments 20 will raise the temperature of the getter plates 48, thus increasing their efficiency in occluding undesired gases. In addition, the getter plates 48 which are located adjacent the heat shields 44, although heated, will not reach the temperature of the getter plates 48 located centrally within the filament structure. This is largely due to shielding of the lower getter plates 48 by the upper getter plates 48 and to the heat conduction of adjacent massive metallic parts such as the filament lead-ins, etc. It has been found that the lowest plate may only attain a temperature of 400 centigrade when the upper plates attain temperatures as high as 1200 centigrade. Since it has been found that some gases (e. g. hydrogen) are most efiiciently occluded by sorption type getter materials (e. g. zirconium) at temperatures in the neighborhood of 400 C., whereas other gases (e. g. oxygen) are more efliciently occluded thereby at temperatures above 1000 C., the above-described structure results in highly efficient occlusion of both gases. Furthermore, the use of spaced, thin plates 48 of getter material provides the maximum amount of contact area between the getter material and the gases to be occluded for a minimum volume of space occupied since both surfaces of the plates 48 are available for the occlusion of gases. Since the circular getter plates 48 and central rod 50 are symmetrical about the central axis of the cylindrical array of filaments 20, there is uniform heattransfer from the filaments to the getter structure during operation of the tube.

In the embodiment shown, the getter parts are in the form of round circular plates or discs each having a hole through the center thereof and a downwardly turned flange 54 at its outer edge. However, it should be understood that this invention is not limited by this geometry of the getter parts. For example, the getter parts may comprise spaced symmetrical members of other shapes such as cones, etc., which have centrally located holes therethrough.

Similarly, it should be understood that this invention is applicable to other types of tubes than the triode herein described. For example, this invention is applicable to diodes, tetrodes, magnetrons, etc., and may even be used in certain electron tubes in which certain gases are purposely retained or injected within the evacuated envelope thereof.

In the fabrication of an electron tube embodying this invention the cathode structure may be completely assembled with the center rod 42 included therein. The filaments 20 of the completed structure may then be subjected to flashing or carburizing processes as hereinabove described. The getter parts or members may then be placed on the support rod 42 by slipping them between the filaments 20 and inserting the support rod 42 in the holes therethrough. According to the embodiment shown, the getter plates 48 may be turned edgewise to facilitate their passage between the filaments 20 and the spacer rings may be small enough to fit easily between such filaments 20. The control grid structure, and other structures such as the anode and additional grid structures may then be telescoped over the cathode structure and permanently aifixed with respect thereto.

It will be seen that an electron tube having an improved getter structure is herein provided. Since the getter parts may be stamped out of stock material and since the fabrication of the structure is simple, the structure is inexpensive as well as highly efiicient. Furthermore, the method of fabrication herein provided allows the cathode structure to be processed for maximum efliciency without impairing the gettering action of getter structure, although the two are intimately located in the final tube structure.

What is claimed is:

1. An electron tube comprising an envelope, a selfsupporting hollow cathode structure within said envelope, and a getterstructure within said hollow cathode structure and extending along at least half of the axial length thereof, said getter structure comprising a support rod extending axially within said cathode structure and a plurality of spaced centrally-apertured circular getter parts of sorption type getter material coaxially mounted on said rod and extending outwardly therefrom.

2. An electron tube comprising an envelope, a hollow cathode structure within said envelope comprising a plurality of elongated filaments, and a getter structure within said hollow cathode structure and extending along a major portion of the axial length thereof, said getter structure comprising a support rod extending axially within said cathode structure and a plurality of spaced centrallyapertured circular getter parts of sorption type getter material coaxially mounted on said rod and extending outwardly therefrom.

3. An electron tube comprising an envelope, a self-supporting, hollow, bird-cage type cathode within said envelope, said cathode comprising a plurality of directly heated filaments joined together at one end thereof, a getter structure within said cathode structure and extending along a major portion of the axial length thereof, said getter structure comprising a centrally located support rod extending axially within said cathode to a point spaced from said one end, a plurality of discs of sorption type getter material having centrally located holes therethrough coaxially mounted on said support rod with said rod extending through said holes, a plurality of spacer rings, one of said rings being interposed between adjacent ones of said discs, said rod also passing through said rings, and means at each end of said rod retaining said discs and said rings thereon.

References Cited in the file of this patent UNITED STATES PATENTS 1,859,043 Maartens May 17, 1932 2,117,735 Lester May 17, 1938 2,303,174 Rogers et al. Nov. 24, 1942