US2401737A - Phototube and method of manufacture - Google Patents

Description

3mm@ Us 94@ R, JAMES E? PHOTOTUBE vAND METHOD OF MANUFACTURE Filed March 14, 1942 AT-[DRNEY Patented June 11, 1946 PHoToTUE AND METHOD F MANUFACTURE Robert B. Janes, Verona, N, J., assigner to Radio Corporation of America., a corporation of Dela- Application March 14, 1942, Serial No. 434,681

(Cl. Z50-165) 12 Claims. 1

My invention relates to photoelectric tubes and their method of manufacture and particularly to tubes incorporating coatings of elements, such as antimony, arsenic or bismuth sensitized with an alkali metal.

This application is an improvement over my copending original application, Serial No. 342,199, led June 25, 1940. In my said application I disclosed methods for manufacturing phototubes having alkali metal sensitized cathodes formed by a Istep of evaporating and condensing a quantity of antimony, arsenic or bismuth on a cathode foundation. While the phototubes therein described oier great advantages over tubes of the pror.`art, the` mechanical construction renders the tubes somewhat fragile and subject to structural failure in certain applications where great ruggedness is necessary. From a time prior to filing of my said original application, I have made many phototubes in attempts to obtain a more rugged structure suitable for use in applications wherein the tubes are subjected to high Avalues of acceleration both longitudinally ofthe tube and when the tube is subjected to circumferential acceleration, such as a whirling motion. Thus, certain applications require a highdegree of ruggedness to withstand such accelerations which are several thousands times the force of gravity. While various structural modifications are possible with conventional phototubes utilizing an alkali metal sensitized oxygen treated silvercathode, such features are diiiicult to incorporate in tubes of the antimony, arsenic and bismuth types because of the instability of the antimony cathode during tube assembly and processing. `Furthermore, I have found that any oxidation of such a cathode during processing, such as while sealing the cathode within an envelope, is exceedingly detrimental to the cathode.

It is an object of my invention to provide a rugged phototube of the antimony coated cath- 0de type. Y It is a further object to provide a phototubeof the type described which will `with-- stand high forces of acceleration without distortion or mechanica1 failure of the parts or variation inthe operating characteristics of the tube. Itis afurther object to provide an improved method of manufacturing a rugged phototube of the antimony, arsenic and bismuth cathode type; and it is still a further object to provide a method of manufacture wherein deleterious effects upon an antimony coated cathode may be minimized or, entirely eliminated. In accordance with my invention I provide a phototube having two end closure caps preferably of metal sealed to opposite of glass wherein one of the caps supports an anti-V mony, arsenic or bismuth coated cathode foundationfacing the opposite cap or is itself lthe foundation for such a coating, the other or opposite cap supporting a source of alkali metal. for activating` the cathode coating. Further in accordance with my invention I provide a method of manufacturinga phototube of the above type wherein one closure cap is sealed to the tubular member prior to the formation of the condensed coating on the cathode foundation while protecting the tubular member from condensation of the coating material, such as antimony, followed by sealing of the other closure cap to the member without damaging the coating and the photoactivation of the coating. These and other objects, features and advantages of my invention will become apparent when taken with the following description and the accompanying drawing.` wherein- Figure 1 is a view in longitudinal section of a rugged phototube made in accordance with my invention;

Figure 2 is a cross section of the tube of Figure l taken along the lines 2-2; and

Figure 3 shows an apparatus suitable for performing one of the principal operations in the manufacture of my phototube. t

In the following description I will refer specifically Yto antimony coatings and,`it will be appreciated thateither arsenic or bismuth may be substituted for the antimony without departing from the scope of my invention.

In accordance with my invention I have found that a tube of the type described in my said original application is subject to injury of the photocathode during tube manufacture of it any time during the formation of the antimony coating, this coating should become overheated, especially when such overheating occurs in the presence of a normal oxygen-containingatmosphere. Thus, while tubes of this type offer many advantages over those of the prior art, particularly in ease of manufacture wherein a cathode foundation is provided with an antimony, arsenic or bismuth coating in a raried atmosphere, subjected to the normal atmosphere and then sealed within an envelope; such a procedure when applied directly to the manufacture of an exceedingly rugged tube produced poor photosensitivity. I have found that a minimum distance must be maintained between the lcathode foundation and the envelope seal when following my prior teaching of sealing an antimony, arsenic or bismuth coated. cathode in an envelope and that while the construction shown in my said application is satisfactory for the relatively large tubes shown therein, additional precautions must be followed and the sealing steps performed in a definite sequence to provide a rugged tube of the type shown in Figure 1. Referring to- Figure 1 wherein I show a phototube made in accordance with my invention, the tube comprises principally two end closure caps I and 2 sealed to opposite ends of a tubular insulato-r, such as a short length of glassy tubing 3.

Preferably one of the closure caps is aperture'd' as shown at 4 and is provided witha glass tubulation 5 sealed around the aperture. 'Iheo-pposite closure cap 2 is non-apertured and supports a source 6 of alkali metal, such as a caesium compound reducible`by heating to caesium. As indicated above the closure cap I may serve as a foundation for the cathode coating although Iv prefer to attach a metal disk I to the cap I as shown. Further in accordance with my invention, I providefa coating B" on the foundation 'I or on the inner surface of the closure cap I of antimony, arsenic or bismuth which is at a predetermined minimum distance D from the seal between the opposite end cap 2 and the tubing 3.

More particularly and in accordance with my invention' I have found that it is not only desir'able but necessary tofprovide the coating 8 of antimony either on a foundation, such as the metal disk 1, or directly on the inner surface of the cap Lat a denite time during the manufacturingV steps to prevent injury to this coating'during subsequent steps of the 'manufacturing. process. For example, if theV coating 8 is subjected to relatively high temperature, such as occurs when the closure cap I is sealed to the glasstubing, permanent injury of the coating 8'occurs. Thus; as pointed out in my said copending application, the antimony cathode coating should not be oxidized 4and I believe that my sequence of processing steps avoids or substantially eliminates material oxidation of the'antimony coating on the foundation. Furthermore,V

the closure cap I' must be subjected to certain processing to provide good sealing propertiesat the time the seal is made to the glass tubing 3. Therefore, in accordance with my invention I so treattheclosure cap prior to the application ofthe coating 8 such that a good seal may be made followed .immediately by making the seal prior tothe application of the coating 8, and further in accordance with my invention I deposit the coating 8 of antimony 'by evaporation and condensation steps without providing a coating of antimony on the glass tubing 3.

The closure caps I and 2 are preferably of material such as chrome-iron having a coefficient ofl expansion substantially matched with that of the glass of the tubing 3L A chromeiron which I have found` satisfactory for use with lime glass contains asV its principal constituents.-

' Per cent Chromium f 26-30 Nickel 0.5 Silicon 0.5 Manganese 0.5 Iron 'Balance Following formation of the closure cap metal to aV satisfactory form one cap' for each tube is aperturedl as shown at 4 in the drawing, and where the use; cfa foundation memberother than the cap is desired,.the` diskl 1 is attached to the cap I, such as by welding. The material of the disk metal such as arsenic or bismuth which may bey I is preferably nickel although a similarchromeiron may be used and the welding operation should be done with a minimum of heating to avoid puncture of the metal of the cap I, inasmuch as the cap forms a closure for the completed phototube following the welding of the disk 'I to the cap I. I then oxidize the cap by heating it in moist hydrogen to provide a coating of oxide which is conducive to the manufacture of a good seal to the glass of the tubing 3. Such oxidation by heating in moist hydrogen is well known in the art and is described by Hull,

Burger and Navis in an article entitled, Grlass-` `to-metal seals,`part2, Journal of Applied Physics, volume 12, No. 9, September 1941, pages 698 t070'7. For the metal composition given above heating of the caps in moisthydrogen for 6 minutes at a temperature of 1150 C. is suiiicient to provide a suitable oxide coating which appears to be chromium oxide. Following the oxidation step I prefer to seal the tubulation 5 to the oxidized cap I around the aperture 4 so that following complete assembly, the tube may be evacuated and further processed in accordance with my prior teaching. Preferably the tubulation 5 is of lea'd glass which has the property of absorbing excess alkali metal during tube processing. Simultaneously or sequentially with the sealing of thejtubulation 5, I seal the periphery of the cap I to the glass of the tubing 3 to provide 4anhermetic seal and prevent .leakage of air within the tube during and following evacuation. Following the sealing steps I clean the inner surface of the cap I, if it istobe used as the cathode foundation, or clean the upper surface of the disk 'I to provide a clean surface on which to form the antimony 8. This cleaning operation may be performed by bufling or by electrolytic cleaning wherein the cap I is madel the anode in an electrolytic bath, a portion of thiemetaljthereof or of the disk 1 being deplated Y such as the disk 'I or the inner surface of the cap I, y' on which the antimony is to be deposited. Ther shield l0 is preferably supported within a chamber or bell jar II capable of being evacuated. I also support opposite the disk 'Iand the cap I a source of antimony to be vaporized and condensed upon the cathode foundation. Such a source of antimony may comprise a refractory metal Coil I2 supporting a quantity `of antimony I3 or other vaporized by passing an electric current from a battery or other current source through the connecting leads I4. Following the insertion of the glass tubing 3 over the shield I0 I evacuate the bell jar II to a high vacuum, the residual air pressure preferably being less than 0.5' micron, and I then evaporate a sufficient quantity of antimony from the source I3 to provide the coating 8 of the desired thickness. I have ,found that a thickness corresponding to 0.1 to 0.16 .mil-V ligram per square centimeter is sufficient to pro.-

vide good photosensitivitywhen such a coating is treated with alkali metal. The rate of Y,Condensation of antimony to form thecoating 8 is preferably less than 0.2 milligramV per sguare centimeter per minute, since I have found that 5. a higher rate of condensation may be detrimental and form a non-uniform coating. The entire bell jar assembly with the exception of the coil I2 and the heated antimony source I3 is preferably maintained at room temperature during the evaporation and condensation steps, except for such slight heating of the disk 1 which may occur by the radiation from the coil I2. Obviously while I have shown a form of bell jar II suitable for coating only one foundation at a time, a plurality of shields, such as the shield ID, may be located around a centrally disposed source of antimony so that a number of cathodes may be processed simultaneously. Following the formation of theantimony coating 8, I remove the glass tubing 3 from the shield I0 and close the open end of the glass tubing 3, with the cap 2 to which is attached a source of alkali metal 6 which may be subsequently vaporized and condensed on the antimony coating 8. Obviously the cap 2 may be oxidized in a manner similar to that of the cap I prior to the sealing operation and preferably prior to the attachment of the alkali metal source 6 thereto. I have found that in tubes made in accordance with my invention, the sealing of the cap 2 to the glass tubing 3 does not have any detrimental effects upon the antimony coating Biprovided the distance D, in Figure 1, is at least inches and the cathode foundation is cooled during the sealing operation. In accordance with this teaching of my `invention I maintain heat conductive means in contact with the closure cap I during the time the opposite cap 2 is sealed to the glass tubing 3. Referring again to Figure l. I have shown a relatively large metal member I which is maintained in metallic contact with the cathode foundation during the sealing step. The mass ofthe member I5 may be suiiicient` to ab- .sorb the heat from the cap I or disk 'I although the member I5 may be artificially cooled. The temperature of the cathode should be not higher and preferably lessthan 150 C.during this sealing` operation, higher temperature being detrimental. Satisfactory cooling is shown by the absence of any color change of the coating 8, excessive temperaturesresulting in a color change to a light yellow or red. It should be noted that` the construction of my tube shown4 in Figure 1 is particularly adapted to this method of cathode cooling, In addition the member I5 may be used to push the glass tubing 3 into contact with the cap 2 during the sealing operation thereby working the glass slightly andcausing it to seal to the chrome-iron to better advantage.

It will be noted that in accordance with my teaching contained in my original application, the foundation bearing the antimony coating 8 is removed from the bell jar and is subjected to normal atmospheric conditions. I have found that the relative humidity of the atmosphere to which the coated cathodes are subjected should fbe low and preferably less than 80 per cent relative humidity at 25 C. which corresponds to a' water vapor concentration in the atmosphere equivalent to `8 grains of water vapor per cubic foot of air. The satisfactory results obtained areY believed to be due to the fact that antimony does not oxidize when subjected to normal atmospheric conditions at room temperature as I have been unable to find any trace of antimony oxide on the antimony coating following such treatment. However, cathodes subjected to normal atmospheric conditions appear to give better results than cathodes not so treated.

6. In accordance with my original applicatlonI do not oxidize the antimonycoating duringthe tube processing s0 that following the sealing off the cap 2 to the glass tube, I immediately 'exhaust the tube through the tubulation 5 and bake at a temperature of 275 to 310 C; to remove any occluded gases within the tube. Following the baking which may be from 15 minutes to 1/2` hour, I vaporize a quantity of alkali metal, such as caesium, from the source 6. I have found that the temperature of the antimony coating 8 during this caesium vaporization is not critical and may vary from room temperatureV up to the baking temperature of the tube.- Following the vaporization of the alkali metal, the tube: is baked preferably at a temperature of 160 C. for 30 minutes and during this baking I believe that the alkali metal combines with the antimony coating 8 to provide a photocathode having a very high sensitivity to light. Following the baking the tubulation may be tipped off,v and the tube removed from the pump and aged in a manner described in my said original application to stabilize the photoelectric emission from the cathode.

In operation the cap I serves as a cathode connection and is operated at a negative potential esl with respect to the cap 2 serving as the anode. Electrons liberated from the coating 8 in electrical contact withthe cap I are drawn to the anode cap 2, the electrons being substantially proportional to the light incident upon the 'coat-1 ing. The disclosed structure is ideally adapted to i multi-directional use since the cathode coating is exposed from all sides facing'the anode. VFurthermore, inasmuch as thevcathode coating facesV the anode directly,1is symmetrical therewith, and at a uniform distance therefrom, the effectsA of wall charges described in detail in my copending application, Serial No. 425,730, filed January 6, 1942, are minimized so that electron collection does not vary substantially with variation in' anode potential.

While I have described my invention with particular reference to a phototube having a glass envelope closed at opposite ends with a metal cap and the preferred method of manufacturing such a tube, it will be appreciated that various modifications may be made both in the structurer and the method of manufacture Without departing from the spirit of Vmy invention or the scope thereof as set forth in the appended claims.`

I claim: Y

l. AA `phototube comprising a envelope, a metal closure at -each end of and forming the end walls of said envelope, a coating of an element which when sensitized with alkali metal is photosensitive facing and removed from one of said closures anddirectly adjacent the other of said closures, and a source of alkali metal supported by said one closure to sensitize said coating.

2. A phototube comprising a tubular glass envelope, a metal cap sealed to each end 'of said envelope, a coating of an element which when sensitized with an alkali metal is lpholtosensitive supported by one of said metal caps :in a position directly facing and exposed to the other of said caps and an alkali metal deposited on said coating.

3 A phototube comprising a tubular envelope of transparent insulating material, an individual metal cap sealed to and closing each of the opposite ends of said envelope, a coating of an element selected from the group consisttubular glass 7n ingrof. the elements antimony, arsenic, and bismut'lron` one of said caps and a quantity of alkali metal'deposited von said coating to render said coating electron emissive "41.:A phototube comprising a 'cylindrical glasszenvelope'of vpredetermined length, a metal closure. "cap hermetically sealed to each end of said envelope, acoating of an element selected i from the'group consisting of the elements anti- .mony, arsenic andE bismuthadjacent one of lsaid caps, sulisifantially-the-entire surface of said coating facing the' opposite v'cap A:and removed therefrom: by va'distance vof at leastthree quartersy of: an `inch whereby' the sealing' of'said voppositefcapfdoesnot injure saidcoating, and an alkali fmetal deposited on said coating to render Y antimony on the exposed side of said foundation.

6. A" phototube comprising a tubularV glass envelope of predetermined length, a metal cather ode support cap sealeddirectly to and closing one end of saidV envelope, an anode cap sealed directly toand closing the'other end of said envelopef'at al distance of 'at least three quarters of an inch from the junction between said cathodefcap and said envelope, said caps extending transversely of said envelope, a coatingl of anti-v mony substantiallycoextensively with the surface"-V of'said cathode cap facing said anode cap, land ai; quantity of alkali metal deposited on" said antimonycoating; v Y 7.r The `method of manufacturing a phototube having; two closure caps separated by a tubular insulatingmember comprisingLsealing one of said capsto vone end of said` member, depositing an element' selected from the groupconsistingk of the elements antimony, arsenic and bismuth on a surfacein electrical contactwithand substantially'coextensive with the cap sealed to the said one' end of `said member, then sealing the other c'ap to the other'end of said member without heating the deposited element to a temperature atl which it becomes oxidized, exhausting the envelope'formed by'said member and said closure caps, baking said envelope and sensi-tizing said deposited element with an alkalimetal.

8. The method claimed lin claim 'i' Yincluding the step of'maintainingthe deposited element at a temperature notgreater than 150 C. during the sealing of said othercap to said member.

9. The method of Ymanufacturing a phototube having an envelope of two closure caps sealed to and separated by a section of glass tubing comprising the steps-of sealing a'metal cap to one,v endof said tubing, depositing antimonyA on a surface closely adjacent and in 'metallic contact with said sealedmetal cap, sealing-1an,`

other cap on the opposite end of said tubing byheating said tubing and said other cap to facilitate sealing, simultaneouslyv cooling :said metal cap by conduction to a metallic member exterior to said envelope to maintain said de` posted antimony at a temperature less than' C. during said Ysecond-mentioned sealing'v step, evacuating and baking said envelope, and

sensitizing said deposited antimony with` analkali metal,

10. The methodof manufacturing a phototube 1 having a tubular glass envelopejclosedv at opposite ends by metal closure caps comprising thc sequential steps of welding a, cathode foundation to a metal cap, oxidizing said cap, sealing said cap to one end of said glass envelope, cleaning said foundation of any oxide thereon, depositing antimony cn said foundation while protectingV the inner Wall of said envelope against deposi-v having two closure caps separated bya tubular; insulating :member comprising sealing one of'said closure caps to one end of said tubular member,

inserting an open-ended shield within the'opeiif end of said member to within close proximity of, the closure cap sealed to said member, evaporat` ing a material which when sensitized withan alkali metal is photosensitive, condensing said material on a surface adjacent said closure cap while'the inner Walls vof said tubular niemberare protected by said shield, removing said's'hield,

sealing the other closure cap to the'open end of said insulating member andphotosensitizing` the condensed material with an alkali metal.`

' 12. The method 0f manufacturing a phototube having two metal closure caps on opposite ends' of an insulating envelope comprising fastening a cathode foundation to one closure cap, sealing said cap to one end of the envelope, inserting a shieldfmember within the open end 0f said envelope of suflicient length to extend at least' :to said cathode foundation, Aevaporati'ng and' and condensing antimony on said foundation -in the presence of a non-oxidizing atmosphere, subjecting said condensed antimony to normal at'- mospheric conditions, removing said shield mem'- ber, sealing the other closure cap to the open end of said envelope, exhausting said envelope, and sensitizing said condensed material with an alkali metal. v

ROBERT B. JANliIS.r

deposited antimony with an

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