US2727178A - Thoriated cathode - Google Patents

Description

Dec. 13, 1955 H. J. DAILEY ETAL THORIATED CATHODE Filed March 25, 1952 INVENTORS //.J". .DH/EY C'. H. Scl/L/N. 5y/jy ATTORNE United States Patent O THORIATED CATHODE Hampton J. Dailey, Verona, and Carl H. Scullin, Florham Park, N. J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvanla Application March z5, 1952, serial No. 278,465

7 Claims. (c1. 313-341) This invention relates to thoriated cathodes, and particularly to a directly heated thoriated tungsten cathode for use with high voltage electron discharge devices.

Directly heated cathodes of the filament type have been used in electron tubes for many years, and are usually fabricated, either as wire or ribbon, from an intermixture of tungsten and thorium and usually with some carbon also added to the mixture. ln fabrication, heating of the cathode changes a portion of the tungsten to tungsten carbide by virtue of the presence of the carbon. The percentage of the lament cross section which is carburized is usually no greater than as greater proportion results in the filament becoming excessively brittle. These prior art thoriated tungsten filaments have been found to have a life span of electron emission which ends when the available carbon has disappeared. They are also limited in that it is difficult to get a uniform distribution of carbide throughout the filament.

indirectly heated cathodes or" cylindrical type with an internal heater have been employed and in general are fabricated in a manner similar to the filamentary cathodes as described above. This type of cathode also has the limitation of a relatively short life span limited by the using up of the available tungsten carbide. Likewise an even -distribution of carbide is often diicult to attain.

According to the present invention we propose an even distribution of carbon from, for all practical purposes, an inexhaustible reservoir, to supply an adequate amount at all times to maintain the carbon content of the cathode.

A more specific but highly important objective we accomplish is to maintain an even distribution of the carbon to the cathode.

in relation to the prior constructions noted above, a distinct purpose of our invention is to avoid detrimental elects of differential of expansions caused by variations in temperature.

Other objects of the invention, which include simplicity of construction and operation, will become apparent to persons skilled in the art to which the invention appertains as the description progresses, both by direct recitation thereof and by implication from the context.

Referring to the accompanying -drawing in which like numerals of reference indicate similar parts throughout the several views:

Fig. l is a longitudinal central section of an electron discharge device having our improved cathode therein yand likewise shown in longitudinal central section.

Fig. 2 is a cross-section on line lI-H of Fig. l; and

Fig. 3 is a longitudinal central section of a-modified construction of cathode.

ln the specic embodiment of the invention illustrated in said drawing, and referring initially to Figures l and 2 thereof, the reference numeral 10 designates in general an evacuated envelope. The selected construction 'of envelope here shown includes a domed metallic anode 11 as part thereof, the open end of the anode having a vacuum-tight seal with a dished annular deck 12 therearound. Vacuum-tight seal is made between the peripheral margin of said deck with an end edge of a glass cylindrical ring 13 coaxial to the anode and extending partly above and partly below the level of the open edge of said anode. The other or lower edge of said ring 13 is sealed to a anged metallic body ring 14 coaxial thereto and having its other end sealed to a glass header 15. A tubular metallic outer lead 16 is sealed through said header 15 at the middle thereof, and inside and protruding at the lower end of said outer lead is an inner lead 17 of cupped formation with the open end of the cup directed inwardly of the outer lead and with said inner lead sealed, as with glass bead 18, to said outer lead. Thus a completely sealed envelope is obtained which may be evacuated as usual with this type of electron discharge device.

Rigidly secured in said cupped inner lead 17 is a metallic rod 19 which extends coaxially toward the open end of the anode, and has a connector 20 on its upper end extending into the anode where it is provided with a perforated ange 21 to the periphery of which is secured a hollow carbon domed cylinder 22 smaller than but having a contour proportionate to the shape of the anode. This carbon domed cylinder 22 is coaxial to the anode and spaced therefrom an appropriate distance to permit inclusion of a similarly dome-shaped cathode 23 and grid 24 coaxially between the carbon cylinder and anode. Said cathode is an imperforate hollow dome of thoriated tungsten whereas the grid is composed of a plurality of wires in bird-cage assembly. The lower margin of the cathode is supported by brackets 25 secured thereto and extending to and secured to the inwardly projecting end of the outer coaxial lead 16, said brackets also affording electrical connection from the said lead to the cathode. At the peak of the carbon and cathode domes is interposed a conductive lug 26 by which longitudinal and coaxial spacing of the domes in general is maintained in addition to the electrical connection afforded at that particular area of the two domes.

The lower ends of the wires constituting the grid are secured to a collar 27 which flares outwardly downwardly and has both supporting and electrical connection with body ring 14. Thus it will be seen that all external lead-ln connections for the carbon cylinder, cathode, and grid are coaxial. It should be understood that whereas for convenience of description the term dome is herein ena.- ployed, it is not to be construed as a restriction, it being contemplated that cylinders, each with a closed end of any configuration, may be employed and not necessarily one having a bulbous contour.

Heating current for the Cathode of Fig. l is applied to the outer and inner coaxial leads 16 and 17 so the current ows in by way of one lead, for instance outer lead 16, and out by way of the other lead, as 17. Assuming this stated direction of current flow, the current passes up through the cathode cylinder to the lug 25 and then down through the carbon cylinder, heating both cylinders. However, if desired, only one cylinder may be directly heated by current ow therein, such modiiication being indicated in Fig. 3.

The structure shown in Fig. 3 comprises a domed cathode 23 as before. The carbon cylinder 22a, however, has a perforated insulator cap 28 at its end next the dome of the cathode, and that cap is also mounted on a coaxial rod 19a therein which in this instance extends to the cathode end or dome. Said rod in this instance, therefore, makes electrical connection to the upper end of the cathode but has no direct relectrical connection with the carbon cylinder. The cap 28 maintains proper spacing of the carbon cylinder coaxially in the cathode at their upper ends. Lower end spacing is maintained by a basal ange 29 on the carbon cylinder making peripheral engagement with the inside ot the cathode. Current flow in this construction will be introduced into the device by the coaxial terminals as shownv and described withFig: l, and then flows the length of the cathode for heating purposes. Current does not ow in theV carbon cylinder of Fig; 2 and heatingthereofI is obtained by ra iiationtheretoI from the cathode.

In both modifications heating current mayV be supplied in any suitable manner, a, transformerV 30 forthat purpose having been indicated in Figure l specifically andequally, applicable in Figure 3. Likewise, in both modifications the cathode is fabricated from appropriate mixture of' tungsten and thorium and up to 30% carbon. It is believed the carbon, to extent available, forms tungsten carbide. In use, thorium is vaporized from'thecathode surface and in ord-er to free the thorium it is believed each atom of thorium evaporated. But s ince't'he physi-A cal characteristics of tungsten with more than 30% of where W is grams per square centimeter per second and T is degrees of temperature Kelvin;

econd, the number-n of atoms in a given welght of material may be expressed by the known formula carbon are prohibitive, a reservoir of carbon is provided" by the carbon cylinder to supply the added carbon neces- A sary to replace that which isused up by the reaction described.

It has been found that at the following listed temperatures, atoms of thorium per second per square centimeter evaporate from the tungsten surface as listed opposite the several temperatures in the following tabulation:

Th Atoms vaporized carbon can be thus supplied and furthermore can obtain the necessary vaporization of the carbon from the carbon cylinder with a heating of the carbon cylinder to av temperature no greater than the heating of the cathode in use. Thus, the presence of the carbon cylinder provides a means for replacing car'oon consumed in operation of the thoriated tungsten cathode, extending the life of the cathode indefinitely. The vaporization o'tthe carbon ontov the cathode obtains a constant, active and evenly distributed supply of carbon which is continuously converted to tungsten carbide in the cathode. In the construction of Fig. l, the carbon cylinder or reservoir is both enclosed within the heated cathode andv is itself heated by current owing through it against the inherent resistance of the carbon. In the construction ofy Fig. 2, the carbon cylinder or reservoir is heated` by radiationv from the enclosing cathode so that the temperature of the reservoir cylinder may substantially equal, but not exceed, the cathode temperature.

We wish to make it definitely clearl thatthe4 structures shown will develop carbon vaporization from the reservoir at a rate, under the temperature conditions inlposed,` to furnish the carbon to the cathode atleast as fast as required and at the rate' which will form, two molecules oiv carbide for each atom of thorium evaporated from the cathode.

Assuming the. cathode employed is to run atA 2000 K., it was shown hereinabove that 3.06)(109 carbon atoms will be required per second at this iigure being substituted for n in Formula 3 enables a determination to be derived that the temperature T to which the carbon must be heated for the minimum requirement of carbon atoms inthe filament, is l772 K. A similar solution for a cathode at 2100 K. gives the temperaturey required for thecarbon as 1,860o K. Consequently the structures disclosed, wherein the carbon cylinder and the cathode are bothy heated to substantially the same temperatur@ a supply is obtained which is somewhat betterV than the minimum requirement of carbon atoms to replace carbon atoms lost from the cathode by the reaction permitting the release and evaporation of thorium atoms. As cathodes are generally operated at about 2050 K. the two solutions are satisfying for this interveningV temperature. Solutions with respect to other cathode temperatures given hereinabove also show adequate supply of carbon atoms to maintain desired presence of carbon in the cathode operating at those temperatures,lbut the mathematics will not be further elaborated herein.

We claim:

1. A thoriated tungsten cathode with a carbon reservoir comprising a self-sustaining cylinder of carbon proximate to said thoriated tungsten cathode, said cylinder in operation being heated to a temperature high enough to vaporize carbon at a sufficient rate to replace the carbon used by and in addition replace the carbon vaporized from, the thoriatedA tungsten.

2. A thoriated tungsten cathode essentially enclosingv a carbon section with a space separating the same throughout the major portions thereof.

3. A thoriated tungsten cathode essentially enclosing a carbon` sectionV with contact at one endV only of the carbon section and said cathode for effecting heating of the cathode by passing current through the tungsten sections only.

4. A thoriated tungsten cathode` essentially enclosing av carbon section with Contact at; one end only of the carbon section and said cathode for effecting heating of the cathodeV by passing current through the tungsten sections only. and the carbon section heated only by absorption.

5. A- thoriated tungsten cathode essentially enclosing a carbon section withY said cathode and said section connected in series' whereby heating is accomplished by passing, current through the carbon and tungsten Sections.

6. An electron emitter of the character described comprising a hollow cylindrical cathode of thoriated tungsten having a carbonized emissive surface, and a cylindrical. carbon body in proximity to said cylinder at the side.: thereof: opposite. from saidcarbonized surface,

saidcarbonbody comprising` a reservoir for carbon which in: normalY operation of thel cathode is heated and supplies vaporized carbon to the thoated tungsten of the supplies vaporized carbon to the thoriated tungsten of cathode to replace the carbon vaporized from said carthe cathode to replace the carbon vaporized from said bonized surface. carbonized surface.

7. An electron emitter of the charactcer described comprising a hollow cylindrical cathode of thoriated 5 References Cited inthe me 0f hls Patent tungsten having an exterior carbonized surface, and a UNITED STATES PATENTS carbon cylinder within said cathode in proximity to and of substantially equal length as said cylindrical cathode, "1g/11e said carbon cylinder comprising a reservoir for carbon 2,497,111 Williams Feb. 14, 1950 which is normal operation of the cathode is heated and l0

Claims (1)

1. 2. A THORIATED TUNGSTEN CATHODE ESSENTIALLY ENCLOSING A CARBON SECTION WITH A SPACE SEPARATING THE SAME THROGHOUT THE MAJOR PORTIONS THEREOF.

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