US2210674A - Cathode for electronic tubes - Google Patents

Description

Aug. 6, 1940. D. D. KNOWLES CATHODE FOR ELECTRONIC TUBES Filed Sept. 16, 1937 WITNESSES:

1 INVENTOR Dewey 0. Know/es.

Patented Aug. 6, 1940 UNITED srAr-ss 2,210,674. CATHODE ELECTRONIC TUBES Dewey D. Knowles,,Wilkinsburg, Pa yassignor to Westinghouse Electric & Manufacturing Company; East Pittsburgh, Pa., a corporation of Pennsylvania Application September 16, 1937, Serial No. 164,147

' Claims. (01. 25.0-27.5)

My invention relates to electrical discharge devices, and in particular to electron-emissive electrodes for such devices. It is particularly adapted to devices of the above mentioned'type 5 which embody a control electrode for regulating the discharge passing from the electron-emissive electrode, and especially to such devices as contain agaseous atmosphere which plays a substantial part in the transport of electric current therein.

= In electrical discharge devices of the type in which at least one electrode is artificially heated to a temperature at which its surface readily emits electrons, considerable difliculty has been found in providing a type of electrode which will not deteriorate substantially in the course of protracted operation. This. is particularly true where a gaseous atmosphere of substantial pressure is present in the tube. A common type of electron-emissive cathode for high-vacuum tubes is onein whicha metallic core adapted to be electrically heated is coated with oxides of barium and/or strontium which readily emit electrons upon being heated to a temperature of the order of 900 C. It has been found, particularly where a-substantial gaseous atmosphere is present, that the oxide coatings tend to break or, possibly because of the severe bombardment of gaseous ions which the electrical discharge subjects them to. Thetendency of the discharge, where substantial pressures of gas are present, to concentrate in a cathode spot of restricted area probably plays a part in this disintegrating tendency also. a 1 One object of my invention is accordingly to disclose several ways of constructing thermi onically-emissive cathodes which I have found to be great improvements over the above described oxide-coated cathodes and other cathodes of the prior art with which I am acquainted.

Another object of my invention is to disclose several types of cathodes which make it possible to maintain a considerable gaseous atmosphere in the immediate vicinity of the electron-emissive surface, but at the same time, to maintain amuch smaller pressure of gas in certain other portions of the path of the discharge emanating at the said cathode. T

I Another object of my invention is to disclose an electrical discharge tube in which the control electrode is particularly effective in regulating the discharge from the cathode to any cooperative'electrode. 1 i i I Still another object of my invention is to disclose anelectrical discharge tube in which the voltage drop through the tube characterizinga given ourrent between .the electrodes is considerably smaller than that characterizing the flow of currentof'the same. magnitude between electrodes separated .by the same distance, but of the type characteristic of the prior art.

.Another object of .my invention is .to disclose an electrical discharge tube in which much larger instantaneous currents may flow from the thermionically-emissive electrode without injury to the latter than was the case in tubes utilizing thermionically-emissive electrodes of the prior art. v p

Still another object of my invention is to disclose a form of thermionically-emissive' electrode in which the rate of disappearance of emissive material from the cathode surface is materially less than that occurring in the case of the same material operating at the same temperature in cathodes of the prior art.

Still another object of' my invention is to "disclose a form of cathode or electrical discharge devices in which theloss of a given amountof material from the cathode surface will have a.

less injurious effect than wasthe case in-cathodes of the prior-art.

A further object of my invention is to disclose a method of making a thermionically-emissive cathode which is convenient, economical, and readily adapted to quantity production.

With the foregoing objects and principles in mind, my invention will best be understood by reference to the following description, taken in conjunction with the accompanying drawing in which:

Figure 1 illustrates a longitudinal mid-section of an electrical discharge tube embodying my invention; s

Fig. 2 illustrates one particular form in which a cathode embodying the principles of my invention may be constructed, and

Fig. 3 is a drawing useful in describing the method of constructing the cathode shown in Fig. 2.

Referring particularly to Figure 1, a vacuum type container I, which may be of glass or other suitable material, has supported in its interior in ways well known in the art, an anode 2 and a control electrode 3, both of which may be of conventional form; An electrode adapted to emit electrons thermionically is shown at land comprises a heater 5 supplied with electric current through leads 6 and a crater-like electron-emitting element 1 of a type more fully illustrated in Fig. 2. The heater 5 may, if desired, be embedded in electrically non-conducting refractory coating 8, and

the latter may be surrounded by metallic sleeves 9 and II, thelatter supporting the cathode from the envelope I. For many purposes it will be desirable to provide a quantity of a material I2, such as mercury in suflioient quantity so that it reaches just to, or a little above, the lower face of the body 7. The material I2 may, if desired, constitute the connection from the lower end of heater 5 to the lower lead 6.

Referring particularly to Figs. 2 and 3, the member I is formed by winding up into a spiral a tapered metallic ribbon coated with electronemissive material, such as barium-oxide and strontium oxide. As illustrated in Fig. 3, the narrow end of the ribbon may be attached to a mandrel I3 and the ribbon thereby wound, by revolving the latter, to form a spiral structure such as appears in Fig. 2. It will be desirable for some purposes to leave the mandrel I3 in place to form a lead by which current may be introduced into the innerend of the spiral, the cur.- rent being taken out again through a second lead attached to the outer. :end of the ribbon. Electron-emissive coatings which are fair electrical insulators are well known, and when these are used the current flows along the ribbon of the spiral from itsinner toits outer end. It may be desirable in certain instances to let the turns of the spiral spring slightly apart after winding it .so. that spacings exist between the successive turns when the spiral ,is in position in the discharge tube. Such spacing might even be insured by superposing a spacing-layer of metal or some suitable othermaterial upon the ribbon at the time it is wound. into the spiral, this auxiliary spacer being removed after the winding operation.

For many purposes, however, it will be desirable to remove the mandrel I3 after the completion of the winding operation, and in the cathode illustrated in Fig. 1 the mandrel has been .removed. The spiral is then positioned by suitably supporting its outer turn from the tube wall, for example, by the in-leading wire 6.

Instead of forming the electroneemissive material I from an oxide coated ribbon as described above,it maybe formed by molding a mixture of nickel or other metallic particles and electronemissive material suchas a mixture of barium oxide and strontium oxide, said mixture being molded in the form of a cylinder with a depressed ,crater in its center in much the same shape as the crosssectioned portion ofFig. 2. Pearcy Patent 1,981,245 described in detail the method of molding such a mixture.

Upon the connection of such a tube as is shown in Fig. 1 into a conventional radio circuit, the flow of heating current-through heater 5 will cause the surfaces of the emissive oxides to freely emit electrons, and current flow will start between the anode 2 and cathode 4. The gaseous atmosphere, particularly in the interior of the crater of cathode 1, and in any spaces which may exist between successive turns of the spiral will become readily ionized. The same is true of the oxide particles and small cavities between the particles of the sintered matter disclosed in the Pearcy patent should the latter be employed instead of the ribbon spiral of Fig. 2.

V The external coating ofthe cylinder I I may be made of ,a good heat reflecting. material or some other structure ofiering resistance to the outflow of heat may be employed to minimize flow of heat from thecathode 4 to its surroundings.

which only one of the Where a readily vaporizable material such as mercury is made to contact thelower face of the cathode 4, it will be freely vaporized, particularly in the interior of the element II and provide a gaseous atmosphere of substantial pressure within and immediately above the crater. On the other hand, it is possible to so design the tube I by principles well known in the art in such a way that the'vapor will be condensed not far above the cathode 4, and a continual up-flow of vapor through the crater and condensation and down-flow outside the walls of the cathode 4 will occur. In this case the vapor pressure in the neighborhood of the control electrode 3 may be very much lower than that within and immediately above the element 1. The same is true of the vapor pressure between the control-electrode 3 and the anode 2. As a result of this pressure distribution, the effectiveness in controlling the discharge of the grid 3 is greatly increased over what it would be if the dense vapor like that in the crater I. filled the entire interior of the tube I.

As a specific example, the tube illustrated in Fig. 1 might have an overall length of 15 centimeters, a diameter of 8 centimeters and be dimensioned to operate on a 440 volt supply delivering 6 amps. steady load current. The grid-tocathode voltage should then be of the order of 6 volts. The width of the ribbon shown in Fig. 3, which might be of Konal, nickel or the'like, might at its two ends be centimeter and 1 centimeters respectively; its thickness be .01 centimeter; the central hole be centimeter in diameter, and the outside diameter of the spiral be 1 centimeter. The heater 5 might take 11 amps. at 5 volts; the refractory wall 8 be centimeter thick by 1 centimeters high; and the sleeves 9. and ll be of nickel .01 centimeter thick by 2 centimeters high. The control electrode 3 might be of carbonized nickel mesh wires .02 centimeter in diameter and be spaced 2 centimetersfrom the top of cathode 4 and 2 centimeters from the face of anode 2. 4

Were the member 1 made of the sintered mixture mentioned above, its overall dimensions might be the same asthose given above for the spiral type. 1

It is possible, insteadof employing the vaporizable liquid I2, to fill the container I with a permanent gas at a desired pressure, and even in such a gas the crater-like cathode herein described has many advantages. It is also possible to so proportion the amount of the liquid I2 that its upper surface is separated by some distance from thelower face of the cathode 4,,thereby regu lating therelative magnitudes of the vaporization occurring immediately below the center of the crater 1 and that occurring entirely outside the confines of the cathode 4. It is within the purview of my invention to provide fluid cooled surfaces in the region external to and above the periphery of the cathode 4 to assist in regulating the rapidity of condensation of vapor emanating from the crater I. I

While I have described a discharge device in principal electrodes is of the thermionically-emissive type, application of the construction which all of the electrodes or all of the electrodes except those performing a control function, are of this type is within the scope of my invention. The typeof. cathode which I have'describ'ed is particularly useful in tubes such as I have'described which contain a gaseous 1 atmosphere,

above described to tubes in' but employment of cathode structures of the same type in high-vacuum tubes will still be found to possess certain advantages over the prior art.

By reason of the fact that a large proportion of thesurface of the oxide coatings are faced by surfaces similarly coated with oxide, material vaporized at one point may be projected and condensed upon a similar surface at another point. In consequence of this, the total loss of oxide from the cathode crater goes on only very slowly.

Likewise the intense electronic bombardment of I the gaseous atmosphere within the crater results,

in an extremely low cathode-voltage drop and low electrical losses.

Where the liquid 12 is an electrical conductor, as in the case of mercury, a cathode spot may form on the mercury surface itself, particularly when the load current is high. As a result of this the structure which I have described has instantaneous and other current-overload capacities far in excess of arrangements known to the prior art.

In accordance with the patent statutes, I have described one particular embodiment of my invention, but the principles thereof are capable of quite different applications in embodiments which will be apparent to those skilled in the art. I accordingly desire that the following claims be given the broadest construction of which their language is reasonably capable.

I claim as my invention:

1. In combination with a vacuum-tight container enclosing a first main-electrode and control-electrode together with a material which vaporizes readily not'far above room temperature, a second main-electrode comprising a heater and a core comprising a spiral of tapered ribbon in contact with said material.

2. In combination with a vacuum-tight container enclosing a first main-electrode and control-electrode together with a material which vaporizes readily not far above room temperature, a second main-electrode comprising a heater and a core comprising a spiral of tapered ribbon.

3. An electrode adapted to emit electrons freely when heated comprising a spiral of tapered ribbon.

4. An electrode adapted to emit electrons freely when heated comprising a spiral of tapered ribbon coated with thermionically-emissive material.

5. In combination with a vacuum-tight container enclosing the first main electrode and a control-electrode together with a material'which vaporizes readily not far above room temperature, a second main-electrode comprising a heater and a core which comprises a spiral of tapered ribbon coated with thermionically-emissive .material, said spiral beingin good heat exchanging relation with the first said material.

DEWEY. D. KNOWLES.

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