US2401738A - Phototube and method of manufacture - Google Patents

Description

June 11, 1946. J. B. DIFFENDERFER' 2,401,738

PHOTOTUBE AND METHOD OF MANUFACTURE Filed Jan. 1,1943

S HsO QB 3noentor Jot-m B.'DIFFGHDRFR l attorney Patented June 11, 1946 PHOTOTUBE AND METHOD OF MANUFACTURE John B. Difienderier, Verona,

N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 1, 1943, Serial No. 471,049 r 4 Claims. (01. 250 165 My invention relates to photoelectric tubes and their method of manufacture and particularly to tubes incorporating metals of the second subgroup of the fifth group of the'periodic system photosensiti zed with metals of the alkali metal roup.

Several types of phototubes are known including those of thesilver-silver oxide-caesium type but such tubes have relatively high sensitivity in the infra-red region of the spectrum but 'low sensitivities in the visible range andfespecially in the blue portion of the VisibIespectrum. In addition this type of phototube is difficult to manufacture and manufacturingcost is relative high.

Copending application, Serial No. 379,010; filed by Robert B. Janes and Alan M. Glover February 15, 1941, describes a method of manufacturing phototubes incorporating metals such as antimony, arsenic and bismuth wherein the metal is deposited by vaporization and condensation upon a metal foundation, the deposited metal and foundation subjectedto the atmosphere containing a minimum ofwater vapor followed by sealing in the utilization envelope and sensitizing with an alkali metal. In manufacturing tubes under this prior method it was found that subjecting the cathodes to high humidity atmospheric conditions, especially when at an elevated temperature, resulted in very poor sensitivity. Consequently, the excellent sensitivity. obtained by my invention described below is very unexpected in View of the fact that the processinherently involves subjecting the coated cathodes to water and Water vapor.

It is an object of my invention to provide. a phototube havinghigher photosensitivity to blue light than the tubes made heretofore. It is also an object to provide a tube having exceptionally high photosensitivity to blue lightand an improved method wherein the processes of manufacturing such a tube are simplified and controllable to a high degree of accuracy, thereby assurin good reproducibility in the manufacture of a large quantity of tubes, and it is a still further object to. provide a manufacturing process for producingsuch tubes consisting of a minimum number of readily controlled steps. f .wInEaccordanc with my invention I provide a specially prepared cathode foundation on which I deposit antimony or another'equivalent metal suchas arsenic or bismuth preferably by an electrolytic process. cathode foundation to a high gloss 'so that its surface has alinirror-like finish. Such polishing provides a surface which under a magnification of More particularly, I polish the about fourteen diameters is entirely free of surface scratches, cracks, crevices and sharp fissures. Furthermore, diifuseol reflection from such a surface is negligible and the specular reflection approaches l 00per cent. v

Th se an ther b ects. e tu es and. e s am tag f m nve ti n w ll b a arent when taken in connection with the following descripn thereo and t o a oompa i ine raw n w rein e in lefl ur i a lon ud na i w r 1 of a tuoomado ina o danoe w h mri o ion ,R fo in o the drawin I p ovid n o velope l enclosing all electrically conducting foundation or cathode 2 on which a photosensitive surface is formed and an anodeiexposed to the photosensitive surface so thatit may receive electrons which are liberated from the. oathode uns olith nfl eoo of light The cathode nd mode hi h are ad o ni k l o o he base metal, are preferably supported by a stem or press 4 preferably of lead glass and in which the current carrying leads or support rods 5 to the cathode are sealed; The tube shown in the aw i ed wi a bu a n B hrou h which the tube may be evacuated and a source f of caesium or other alkali metal is provided withtion, leaving a brightly polished surface x a metal tab 8 so in the bulb I orin an adjacent interconnecting e ope; altho I refe to vid a qua ty of caesium bearing compoundsuch as an activating pellet TWh l'Ch isin good thermal contact with 7 that caesium may be liberated from the pellet by suitable heating treatment.

In accordance with the principal teaching of my invention a metal foundation such as nickel is providejdwith a, high glossy finishpreferably y de e oo lviio roces in a n-so io ive ele t e hi h emoves a rt n of the foundation, leaving a smooth regular non-fissured r a en ono me vo le t t wh c will remove the outer layers of such a nickel foundais a one to one solutionby Volume offconcent'rated sul-,

phuri'c acid n water; with the metal foundation connected tofthe positive terminal of a directcurrent source, the surface of the f oundation is renioved'in tlii 's' solution by electrolytic-action in few minutes; providing a mirrorlike surface as distinguished from amat Jor etched surfaceproduced by ordinary acidetching.

In the manufacture or conventional phototubes it has lon been considered, partieularlyin the case or silver silver oizide alkali metal phototubes" it is an observed fact, that 'roughen ing or acidetching ofthe'cathode foundation produces higher photosensitivi ty than'with an untreated 3 "or smooth surface. In accordance with my in: vention, however, I have found the direct opposit to be true Where the foundations are coated with a metal selected from the second sub-group of the fifth group of the periodic system.

While I do not wish to be limited tolany par ticular theory explaining the improved sensitivity using cathodes made in accordance with my invention, it appears that removal of the metal from the surface of the foundation in the polishing bath probably leaves a surface that is free of oxides and all contaminations and leaves no recesses on the foundation surface which may catch and retain contaminating elements. Furthermore, there may remain after the bath polishing a crystalline structure highly conducive to electron interchange or flow between the foundation and the subsequently applied coating of antimony, arsenic or bismuth. In addition, the coating does not peel from the foundation and apparently has a higher bond thereto than when applied to a non-polished surface.

' As an example of one preferred method of preparing the metal foundation, I provide the oathodefoundation as shown in the drawing preferably formed to whatever ultimate shape is desired and assembled with the side rods or supports 5. I then immerse the cathodes in a deplating solution, connect the foundation as an anode to the positive terminal of a direct current source 'anddeplate at a currency density of from one to five amperes per square inch of foundation surfaceexposed to the solution for from. two to five-minutes or until the surface of the foundation acquires a highly specular mirror-like finish; Preferably, a number of foundations are simultaneously deplated, the current density being maintained with respect to the total area .exposed to the deplating solution. Following the deplating operation, I immediately wash the foundations with distilled water, allowing them to remain wetor; depending upon the following treatment, I thoroughly rinse the cathodes in methanol followed by drying. I

Another deplating solution or electrolyte with which particularly good results may be obtained comprises a'low water content solution of fifty to ninety percent phosphoric acid by volume, the remainder being sulphuric acid, the water content being approximately by weight of the volume of phosphoric acid. ,.-The deplating in this solution may be'as previously indicated at a current density of about five amperes per square inch of foundation surface for a period of about two minutes. However, since in electrochemical reactions the amount of deposition or removal of metal at an electrode is a function both of time and current density, these two factors may be 'an aqueous solution or in a 4 mechanical methods such as rouge or very fine abrasives for producing the high finish.

I prefer, and have obtained the best results in practicing my invention, to provide the antimony or other metal coating on the specularly polished foundation by an electrolytic process wherein the foundation, or-preferably a plurality of founda tions, are electroplated simultaneously either in solution wherein the solventis of the organic type such as acetone, preferably utilizing as a solute the chloride of the metal to be deposited. Such a process is disclosed by Glover and Janes in their copending application, Serial No. 471,050, filed concurrently herewith.

More particularly, the preferred coating process comprises electroplating the antimony upon the foundation in an acidified solution containing varied atwill; The foundations are then washed in water preferably containing a Wetting agent such as sulphonated alcohol known commercially as Duponol.? followed by immersion in water or by rinsing in methanol and drying in the event that the alternate process of coating the foundation withantimony or other metal as described below is followed. 7

The cathodefoundation following the electrolytic polishing has a bright specular finish and when optically magnified at 14 diameters there is a total absence of sharply defined scratches and crevices whereas a mechanically rolled or acidetched nickel foundation has 7 a mat surface. There may of course be other solutions thatwill produce the same degree of high'polish as well as antimonytrichloride. This solution may be either aqueous or of the organic type such as the following:

Aqueous solution:

SbCla 100 g./liter j I-ICl (conc.) In excess to dissolve the water insoluble white oxychlorides Duponol .25 g./1iter Organic solution:

SbClz 100 g./ liter HCl (conc.) 17 cc./liter f Duponol .25 g./liter The aqueous solution is maintained strongly acid to dissolve any oxychlorides whichmay be formed during the electrolysis, and preferably the solution contains a small amount of the wetting agent such as Duponol.

When utilizing'the. aqueous. plating solution-the foundations while still wet with water following the electrolytic polishing operation are :immersedin the electrolyte, connecteclto the. negativev terminal of'a direct current source and plated at a low current density such as between 0.01 and? 0.05

ampere per square centimeter of exposed foundation surface. For a cathode such as utilized in the RCA 929 phototube and measuring by 7 the plating energy may vary from-.1 to 4 coulombs per foundation. which,.using an electrochemical equiv'alentof antimony of 0.415 millis gram per coulomb, corresponds to from 0.07 to 0.22 milligram of antimony per square centimeter. of cathode surface.

The maximum time. the foundations are im-.

- mersedt in the plating solution is not critical so that the minimum rate of plating maybevery 10w although the maximum rate of depositionis limited by the non-uniformity of plating at high current densities. The maximum current density is preferably not .greater than 0.05 ampere persquare centimeter of exposed foundation sur. face. I More rapid plating produces a spotty surface, andupon final sensitization the. sensitivity may be. lower than optimum.

Following the plating operation foundations are removed fromthe platinglsolue tion immediatelyrand. are immediately :rinsed either. vwith distilledflwater. in the case 10f:v an aqueous solution oriwithacetone.or;methanol in thescase of anorganicsolution, followed'byimmediate rinsing, .with methanol. If.;.the. rinsingoperations are performed immediately after the plating; operation-, thecathode surfaces arenof.

uniformi color and without stain marks, wherea as. anyintermediate drying-of the foundations produces a discoloration and consequent poor aeomss sensitivity. The foundations at this point a pear grayish' with a slight blueish cast and still have a very smooth surface which is specularly reflective. Following the rinsing, the foundations are immediately dried preferably in air at a temperature not exceeding 105 C. Natural drying at room temperature is preferred inasmuch as actual tests indicate that somewhat poorer results are obtained. by oven drying at 105 C. for periods in excess of five minutes. If this temperature is exceeded, the grayish color with a slight blueish castis changed and the foundations appear somewhat pinkish, this condition produced at this point not favoring high sensitivity on final processing. f V t During the initial manufacture of p'hototubes wherein the foundation was coated with antirno'ny priorto scalin in its utilization envelope it was not expected that high sensitivity could be obtained due to the presence of water and water vapor to whichthe cathodes were subjected during the plating operation whether the. solution was aqueous or' of an organiccharacten, Thus, even the water present in an organic solution was expected in view of prior results disclosed in the saidganes and Glover copendirig application to produce cathodes of very low sensitivity. Very unexpectedly, however, the process of my present invention produces high sensitivity cathodes, although care, must be taken to minimize the time the wet foundations are exposed to the atmosphere and, in addition, the foundations following drying must not be subjected to an atmosphere having high water vapor content especially at elevated temperatures such as encountered during sealing of the cathodes within the utilization envelopes. Consequently, I prefer to maintain a humidity of less than 80% at 25 C. which corresponds to a desired humidity below 8 grains of water vapor per cubic foot of air, particularly during the sealing of the cathode foundation in the utilization envelope. The satisfactory results obtained are believed to be due to the fact that antimony does not oxidize when subjected to normal atmospheric conditions as it has been impossible to find any trace of antimony oxide on the antimony film following such treatment.

The antimony, arsenic or bismuth may be deposited by alternative methods, for example such as that disclosed by Janes and Glover in their copending application, Serial No. 379,010, filed February 15, 1941. Reference to that application is made for an explanation of the method and it need not be explained herein.

Following the deposition of antimony film either by the preferred or alternate processes described above the cathodes are sealed into individual envelopes such as the bulb I with the concave side facing the anode 3 and the envelopes exhausted. No oxygen whatsoever is introduced within the tube during the exhaust or activating cycles and in fact, the introduction of oxygen substantially ruins the sensitivity of the completed tube. The tube is then baked to remove occluded gases and referably a small portion of the antimonoy film is vaporized to provide a clean antimony surface. The baking step both with respect to temperature and time may be controlled to produce a very slight vaporization of a portion of antimony, the baking being continued until a particular color is obtained, this color being a very light pink. This color change from a steel gray on the specularly polished foundation to a just perceptible pinkish color may be due to some chemical reaction or to a new and 6 clean surface, being formed on theantimony film. Wh 'bbih' me a d temp atii erecr ticelj e tures in the production of the very slight pink; ish color which isv desired, a somewhat lower temperature fora slightly longer time produces an equivalent color change although with :an'oven temperature of 275. C.'to 325,C.,p e e a b y 9 ,1

tered at 300 C.,the color changeappearsafter the tubes are baked for one half hour. The data givenabove'are for tubes raised from room' temperature by lowering the oven ,overj the" tu es after it has reached the indicated temperatureal lowing it to remain forthe time indicated. "The color change appears firstat the'edgesof the cathode foundation apparently due to fth cori' duction of heat to the cathede by the support side rods and serves as an excellentfiiidicaltiQn of the degree of baking "Contiri; heating either at a higher temperature 'or f or' a prolonged time is detrimental as indicated above andfmay be evidenced by the slightly pinkish color deepening into a red for which the final results are very poor. On the other hand, a bake-out temperature of lessthan'2'l5 C. rejsultsin' insufiicierit removal of the residualo ccluded gases" the envelope and electrode j 's ucture f f A Following thci baking the" caesium bearing compound such as the pellet lis flashed to lib erate' caesium within theenvelope 1. During the liberation of caesiumf iiitliinlthe e'nvel opef the temperature arms-env lope and electrode st'rzuc ture is maintaihedjat such a temperature asfto cause an alloying ofthe alkali metal'with the antimony film. The temperature to produce such alloying may vary over wide limits, extending from room temperature up to 200 C. and since the cathode is within the envelope, this cathode is maintained at substantially the same temperature. Immediately upon the liberation of caesium, the photosensitivity of the cathode increases, such photosensitivity being substantially non-existent prior to liberation of caesium, and such increase is observed even when the bulb l is maintained at room temperature. Following the liberation of caesium within the tube, the

tube is baked in an oven at a temperature of C. to C. until the photosensitivity has reached a, maximum. Some tubes reach a maximum photosensitivity following a baking of a few minutes, whereas other tubes require baking for several hours. Continued baking after maximum sensitivity has been obtained does no harm to the tubes so that the bulb I may be sealed off from the exhaust system so that a large quantity of tubes may be baked together in a large oven for a time sufficient for all the tubes to attain maximum photosensitivity.

My invention is of particular merit in the manufacture of phototubes of the secondary, emission type such as that known commercially as the RCA 931 photo-electric multiplier. as well as the cathode of such a tube may be electrolytically coated with the antimony, arsenic or bismuth in the same manner as disclosed above except that the thickness of the coating on the dynodes is preferably from one and one-half to three times the thickness of the coating as described above of the cathode. Such difference in thickness provides a greater ratio of allcali metal to the antimony, arsenic or bismuthcoating for the cathode than for the dynodes as described in J anes copending application, Serial No. 360,255, filed-October 8, 1940.

While I have described my invention in connection with the use of antimony, it is to be un- The dynodesderstood that I do not Wish to be limited to this particular metal, since I have found arsenic and bismuth to be satisfactory as an equivalent of antimony. Therefore, while I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only certain specific applications for which my invention may be-employed, it will be apparent that, my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure 'used and the purpose for which itis employed without departing from the scope thereof as set forth in the appended claims.

, I claix'ni I 1. The method of manufacturing a light sensitive electrode comprising immersing a metal foundation in a non-corrosive electrolytic bath, deplating a portion of the metal foundation under a cu'rrent density which will provide a highly specular surface on said foundation, coating the specular surface of said foundation with a metal selected from the group consisting of antimony, arsenic and bismuth, and sensitizing said coating with an' alkali metal.

, 2. The method of manufacturing a light sensitive electrode. comprising electrolytically polishing a metal'foundation; electrolytically depositing antimony on said foundation'to a weight between 0.07 and 0.22 milligramper square centimeter of coated foundation surface and sensitizsaid coating withan alkali metal.

3. The method of manufacturing an electron emissive cathode comprising electrolytically p01 ishing a nickel cathode foundation, electrolytically depositing antimony on said foundation, while immersed in a plating solution containing a salt of antimony, at an electrolytic current density not exceeding 0.05 ampere per square centimeter of foundation surface being plated, removing said foundation from said solution and immediately rinsing and drying said foundation, and sensitizing the depositedantimony with an alkali metal.

'4. The method of manufacturing a light sensitive cathode comprising polishing in an electrolytic solution a cathode foundation to a high mirror-like finish as contrasted with a mat rolled finish, removing said foundation from said solution and while still wet immersing said foundation in an electrolytic solution containing an antimony salt as a; solute, electrolytically plating antimonyon said foundation to a thickness cor-5 responding to a weight of antimony between 0.07 and 0.22 milligram per square centimeter, removI- ing said foundation from said electrolytic'solwtion and'immediately rinsing said foundation and drying said foundation at a temperaturern'ot exceeding 105 C., sealing said foundation in a utiliz ation envelope, evacuating said envelope and sensitizing said antimony with an alkali metal. 7

JOHN B. DIFFENDERFER."

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