US1871352A - Electron discharge device and method of manufacture - Google Patents

Description

Patented Aug. 9, 1932 UNITED STATES EMIL GIDEON WIDELL, OF BLOOMFIELD NEW JERSEY, ASSIGNOR TO WESTINGHOUSE I LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

ELECTRON mscnnnen-nnvrcn AND METHOD on ivrnnurnorunn' No Drawing.

This invention relates to the art of manufactu ring electron discharge devices and more particularly relates to the art of manufacturing electron discharge devices employing hotcathodes of the oxide coated type.

In the manufacture of such electron discharge devices it is customary to form the cathode coating of relatively stable but thermally dissociable compounds of the alkaline earth metals in the desired proportion, and to effect the thermal dissociation thereof during the exhaust procedure in the manufacture of the device.

The most successful of these thermally dis sociable compounds heretofore employed has been the carbonate compounds" such as is described in copending application b Duncan MacRae, Serial No. 587,270, filedv eptember 11, 1922, entitled Electron discharge devices and method of making, which application is assigned to the same assigneeas the present invention.

The customary procedure in the manufacture of electron discharge devices employing this type of coated cathode is toeffect entire evacuation of the device and degasifieation of the co-operating electrodes and the glass envelope of the device prior to effecting the thermal dissociation of the carbonates in the coating. The carbonate compounds are then heated to the dissociationtemperature in the relatively high vacuo by incandescing the heating element of the cathode and the gases evolved from" the coating are removed as liberated. This procedure effects ready and complete breakdown of these compounds. At the conclusion of the decomposition of the carbonates and evacuation of all the evolved gases the usual metallic clean-up agent or getter such as magnesium or misch-metal is vaporized. within the device and the device sealed from the pumps. v 7

Under these general manufacturing conditions the use of easily oxidizable metals as the heater element of the cathode, such as tungsten, molybdenum, has not been successful due to the generally severe oxidizing conditions existant during the manufac'- turing procedure. For this reason the noble nmetals such as platinum or the non-oxidiz- Application filed February 15, 1929. .Serial No. 340,342.

ing metals such as nickel and nickelbase alloys have heretofore been utilized as the v heater element in oxide coated cathodes.

It has been foundthat the electronic emis sivity of the oxide coated cathode, prepared by the abovegeneral methodsmaybe materially benefited by'effecting an ionization of the carbon dioxide gases liberated by the decomposing carbonate compounds in a manner such as, is disclosed and claimed in U. 8. Patent 1,648,958 (R. E. Myers) issued November 15, 1927, which patent is assigned to the same assignee as thepresentapplication. Thebeneficial results obtained through the ionization of the carbon dioxide was as: cribed to the formation of a proportion of ionized carbon monoxide (CO) gas Within the device which reacted in some unknown manner to benefit the thermionic efficiency of filament.

Prior to the above identified Myers patent there was issued on June 1, 1926, to J. E. Harris, U. S. Patent 1,586,558 which disclosed the use of ionized carbon monoxide (CO) gas in the activating of hot cathodes of the oxide coated'type. The method of forming the cathode coating is not described in this patent but from associated patents by the same inventor it is believed to be dis similar from the carbonate methods utilized in the Myers patent.

It is to be noted that in both of these identified prior art processes the use of or the application of the ionized carbon oxides to effect the activation of the oxide coated cathode was made at a time subsequent to the forming by thermal dissociation or otherwise, of the alkaline earth metal oxides comnate or to avoid.

It is one of the objects of this invention to improve and simplify the manufacture of electron discharge devices employing hot cathodes of the oxide coatedtype and to im- 7 prove the thermionic efficiency of the said hot cathode.

Another object of this invention is to provide a means of liberating within the electron discharge "device a desired proportion of a suitable reducing gas such as hydrogen, carbon monoxide, etc.

Still another object ofthis invention is to provide a reducing atmosphere within electron" discharge devices employing hot cathodes of the oxide coated type, during the period in the exhaust procedure that the oxidecoating of said cathode is being formed.

- Another object of this invention is to provide a cathode coating comprised of alkaline earth metal compounds and comprised at least in part of a thermally dissociable com: ponent which liberates on thermal dissociation a proportion at least of a gas reducing with respect to metallic oxides.

Other objects and advantages will become apparent as the invention is more fully disa closed.

In accordance with the objects of my invention I have determined that the beneficial results heretofore obtained upon the thermionic efficiency of hot cathodes of the oxide coated type by the processes of Myers and Harris hereinbefore identified, was due primarily to the reducing action of the ionized carbon monoxide gas generated or present during the ionization step in the respective processes, upon the cathode coating or upon the surface oxides of the metal parts associated with saidcathode either in the cathode electrode or in the co-oper ating electrodes within the electron discharge device incorporating said cathode.

There are many theories which have been advanced to explain the effect produced upon the hot cathode by the ionized carbon monoxide and carbon dioxide gases and the true explanation thereof is not at this time apparent, but whatever the theory that may beinvolved or the true explanationof the reactions occurring, I have found that I may substantially avoid or eliminate the ionizing feature of the prior art seasoningor activating processes above identified by the provision within the device of a means of liberating a suitable or desired proportion or amounts of areducing gas such as hydrogen, carbon monoxide, at the time of or simultaneously with the thermal dissociation of the alkaline earth metal compounds comprising the cathode coating.

.- I have further determined that by providelement or said refractory metal surface and more particularly I have determined that I may readily and easily employ such heater elements as tungsten and molybdenum for such cathodes and may apply said oxide coating directly to the surface thereof without the deleterious efiects upon the thermionic efficiency of the hot cathode that heretofore has been observed.

There are many ways of efiecting the liberation of the desired reducing gas orof jintroducing the said gas into the device during the decomposition of the cathode coating, such as by incorporating a thermally dissociable compoundsuch as the carbonyl compounds, metal hydrides, rare earth metal oxalates, etc., in thermal relationship to one of the co-operating electrodes of the device, or by introducing a stream of such reducing gas from an exterior source in a mannerwell known and practiced inthe incandescent lamp industry in the manufacture of gas filled incandescent lamps, and glowing said cathode thereinl Both of these broad methods have commercial disadvantages which it. is one of the objects of this invention to overcome. i

I have found that for the purpose of my invention, the thermionic efliciency of an oxide coated cathode, is materially increased and augmented if the evolution of the desired reducing gas takes place simultaneously with and in intimate association with the decomposing alkaline earth metal compounds of the coating.

I therefore prefer to introduce the'reducing gas by the thermal, dissociation of a chemical compound which is incorporated within or thermally associated with the alkaline earth metal compounds comprising the cathode coating. This also proves a ready and'easy manner of introducing the desired reducing gas, at the'time of and during the period the gas is desired.

Those compounds which I have found useful for the purpose of my invention are the oxalate compounds of the more strongly basic elements which form refractory oxides, which are non-reactive with respect to the alkaline earth metal oxides ultimately formed in the cathode coating and to the oxides of the metallic heater element thereof and non-reducible by the metallic components of said 'hot cathode. I may also employ if desired metallic compounds such as ca-rbonyls, l

h'ydrides, etc., the metal component of which,

such as nickel, is reactive with respect to alkaline earth metal oxides in vacuo during the operating life of the cathode to liberate the alkaline earth metal of the said oxide compound for electron emission purposes.

I prefer however for the purpose of this invention to utilize the above mentioned oxallate compounds and as the easiest and most simple-specific embodiment of my invention will describe the process I employ in the manufacture of electron discharge devices. utilizing a. hot cathode of the oxide coated type, in which the cathode coating is comprised at least in part of the oxalate such. alkaline earth metal oxalates may be readily prepared or obtained upon the market in a substantially pure condition, are of similar physical and chemical characteris tics to the carbonate compounds of the alkaline earth metals and may be substituted in part or in whole for the alkaline, earth metal carbonates heretofore employed without requiring alteration-or substantial variation in the usual coating procedures employed in the art.

The amount of alkaline earth metal oxalates added to the usual carbonate compounds may be Varied quite widely dependent upon the particular metal base or core member on which they are to be coated and upon the specific type of cathode being formed, such as directly or indirectly heated, and will also be influenced somewhat by the amount of coating applied and the particular type of V in tube within which the cathode is to be utilized, aswell as by other lesser factors.

One ofthe specific and marked advantages that may be obtained by the practice of my invention is that it provides a means of producing an oxide coated cathode which is comprised of a tungsten filament heater element having the oxide coating adjacent the surface thereof, and an oxide coated cathode of the indirectly heated type which employs a tungsten filament as the heater element therein, with or without the inter-position of a refractory insulator member between the said heater element and the electron emitting oxide coated metal surface.

As a specific embodiment of the practice of my invention I will describe the specific com-- heated oxide coated cathode.

bination of materials I employ in the cathode coating and the manufacturing process I employ in the production of an electron discharge device employing a hot cathode of the oxide coated type utilizing a'tungsten filament as the heater element thereof and having said coating applied directly to the surface thereof. I

Heretofore in the art there havebeen' but two methods devised for the use of tungsten filament as the heater elementof a directly One of these methods is the so-called barium azicle process which provides that the tungsten filament be first oxidized and then'this oxidized surface is reduced by vaporized barium metal formed Within the evacuated electron discharge device by the thermal dissociation and decomposition of barium azide which has been in corporated therein inthermal relationship to I the anode of the device. The volatile products of the decomposition of the barium azide are removed b-yevacuation or by suitable getters. The second method utilizes the tungsten filament as a heater element but provides thereon an interposing substantially stable metallic coating such as nickel, chromium or the non-oxidizing noble metals.

The nickel or noble metal coated filaments have been employed where the direct application of the coating to the tungsten filament is desired but in such composite cathodes the temperatures to which such a cathode may be incandesced is limited :to the vaporization point of the interposed nickel or noble metal coating. The chromium coated or other nonoxidizing tungsten filament has been employed in the art as a heater element in the indirectly heated type catho'deas it has been found that such coatings are essential to protect the incandesced filament from the oxidizconditions present during the decomposition of the alkaline earth metal compounds comprising the cathode coating.

When the cathode coating is applied di rectly to the surface of the tungsten I prefer to utilize a mixture of alkaline earth metal carbonates and oxalates which contain relatively a larger proportion of oxalates than carbonates, or I may apply to the surface of the tungsten a coating comprised entirely of the oxalates if desire The specific combination employed will depend upon existing manufacturing conditions, the efiiciency of the exhaust, the size of filament, depth of coating and the particular device within which the cathode is to be employed. It is relatively immaterial from an electronic or thermionic eliiciency standpoint as to the specific combination employed except that the ratio of the barium to the strontium and calcium oxides be maintained in sub- 7 sta ntially the same proportions as heretofore found effective for best results, namely barium and strontium oxides in approximately about .0007 inches.

equal proportions with calciumoxide, if present, in amounts up to 10 to 20 per cent by weight of the whole.

It is commonly believed that the barium component of the filament, either as free barium metal or as the barium oxide, is the true activating or electron emitting constituent of the coating under the present operating temperature conditions of this type of oxide ode and said devicehaving substantially the same thermionic and electrical characteristics as is developed in the type tube known to the trade as a 201A radio receiving tube.

In such a device the filamentary heater element must be of the proper size so that 105 millimeters length at 6.0 volts will give the proper thermionic operating temperature for the cathode coating. This requires that the filament diameter should be approximately Such a small diameter is exceedingly difficult to coat by the drag methods disclosed in the above identified copending application by Duncan MacRae Serial No. 587,270, due to surface tension effects. Moreover with the relatively high percentage of oxalate in the coating composition it is undesirable to expose the same to the heating step of the drag method of coating.

The coating composition is therefore preferably suspended in the organic solvent such as amylacetate,diethyl carbonate, diethyl oxalate etc., with or without the addition of proportions of lower boiling alcohols. To this solvent is added a small proportion (approximately 3.0 per cent) of an organic binder material such as intro-cellulose. This type of fluid suspending medium is non-reactive with respect to either the tungsten filament core member of the cathode or to the components of the cathode coating, and moreover, may be applied readily to the metal surface without the necessity of applying heat during the coating operation.

The specific admixture I prefer to use with this small diameter filament is comprised entirely of approximately equal amounts barium and strontium oxalates. This. admixture is used because with this size filament there is a limit to the amount of coating that maybe applied to the surface so as to obtain the requisite heating effect under the current velope.

rating conditions specifiedfor this type tube. As it is highly desirable to prevent any surface oxidation of the tungsten filamentary core member, it is advisable to utilize as much of the oxalate compound as possible. To this admixture, however, I may add proportions of alkaline earth carbonates if desired.

The suspension of the alkaline earth metal compounds are then applied to the metal surface in the usual manner such as by dipping, spraying or painting thereon. Prior to coating, the surface of the tungsten filament shouldv be preferably cleansed and straightened by subjecting the same to an oxidizing and then a hydrogen fiashing? or annealing operation in the customary manner. This operation facilitates subsequent handling and mounting operations in the assembly of the electron discharge device. I have however, formed cathodes from the black, unannealed drawn filament and have noted no deleterious effects. due to the presence of the graphite drawing lubricant thereon, upon the thermionic efficiency of the hot cathode subsequently produced. 7

Following the coating of the cathode the same may be mounted in spaced relation to the co-operating electrodes, sealed into a glass envelope and the entire assembly subjected to the usual exhaust procedure. At the conclusion of the exhaust, the glass envelope and the co-operating electrodes are suitably degasified and a suitable.getter such as magnesium vaporized within theen- Theftungsten heater element is then incandesced by the passage of an electric current therethrough and the gases evolved are evacuated as rapidly as possible.

The reaction involved in the thermal dis sociation of the oxalates is substantially as follows:

Ba(Sr.Ga) C O +Heat= Ba(Sr.Ca) 0+ CO +CO.

Approximately equal amounts of carbon dioxide and carbon monoxide are formed and the total volume of gases is approximately. double in volume to thatwhich would be obtained by the use of carbonate compounds alone. This increase may be readily taken care of by proper adjustment of the rateof exhaust or of the time interval to allow for the increased volume of gases.

Following the complete evacuation of the device and elimination of the evolved gases from the heated cathode the device may be sealed off and based in the usual manner. It will be found that the oxide coated tungsten cathode is substantially thermionically active without effecting the so-called ionization steps of the priorart processes and the thermionic efiiciency thereof will be of an extremely high order and the life and efficiency equally as good as the best oxide coated cathode heretofore prepared using nickel, platinum or other non-oxidizing metal bases. It is believed that the reason for the successful application of this invention to the forming of an oxide coated tungsten filamentary cathode is substantially due to the presence of the reducing gas carbon monoxide at the time of and simultaneouslywith the formation of the strongly basic alkaline earth metal oxides in the coating. It is believed that the presence of this reducing gas substantially prevents the formation by oxidation of the strongly acidic tungsten oxides upon the surface of the heater element and consequent interaction of the barium, strontium and calcium oxides with t -e tungsten oxides to form thermionically inactive compounds such as the tungstates, 'IZOOlYbClELlZGS, etc., which may not subsequently be thermally decomposed by heating to higher tempera: tures. t

In this manner there is formed or main tained a substantially oxide free metal surface upon the heater element of the cathode for the strongly basic alkaline earth metal oxides to deposit upon. It is apparent that whereas in the specific embodiment I have described the use of a tungsten heater element, other metal heater elements would be similarly acted upon.

In the past there has been considerable confusion regarding the effect of, the advantage of or the use of an oxidized surface upon the heater element of an oxide coated cathode.-

Whereas in the past it did not appear that such an oxidized surface deleteriously effected the thermionic activity of an oxide coated cathode having a nickel heater element, it did deleteriously efiect the thermionic efiiciency of the same, requiring a greater wattage input to the heater element in order to obtain therefrom the same milli-amperes per square milli-meter surface area as may be obtained from the same oxide coated metal surface which is substantially oxide free or brightly metallic in appearance.

The advantages of the practice of this invention upon a nickel heater element is not at once apparent and may only be detected with respect to the efiect produced upon the thermionic efiiciency of the cathode. lVith the more easily or readily oxidized metal particularly those forming strongly acidic oxides the efiect is more pronounced, and is amply illustrated by the specific embodiment herein described.

As an additional application of the inventive feature of the present invention mention may be made of the procedure that may be followed in the manufacture of an oxide coated cathode of the indirectly heated type such as is described and shown in copending application entitled Electron discharge device with indirectly heated cathode, Serial In said 'copen'ding application the cathode No. 292,116 filed by John Marden and structure 'is substantially comprised 'of a nickel cylindrical sleeve coated exteriorly with the alkaline earth metal carbonate compoundsand heated by radiation by a tungsten filament centrally disposedwithin said nickel cylinder. Following the usualexhaust procedure the carbonate compounds in the cathode coating are decomposed by incandescing the tungsten filament and heating the nickel sleeve by radiation. During the decomposi tion the heated filament is subjected to erosion by oxidation which in some cases materially reduces the'diameter thereof and alters thereby the current carrying characteristics.

By the practice of my invention and the provision within said cathode coating of a proportion of alkaline earth metal oxalates, the tungsten heater element then may be incandesced in a reducing atmosphere anderosion of the filament reduced to a minimum, and the filament characteristics maintained highly constant.

Another application of the present invention may be noted in the manufacture of elec tron discharge devices of the indirectly heated type employing refractory insulators interposed between the heater element and the nickel sleeve. The Marden and Lederer copending application above identified was di- V rected to a device in-Which this insulator is not employed. However when such an insulator is employed it is customary to coat the'heater element with a non-oxidizing metal such as chromium in a manner such :asdescribed in copending application by John Humphreysliamage Serial No. 276,340, filed May 9, 1928, entitled Heater element for electron emission device. Said copending application is also-assigned to the same assignee as the present application.

Through the application of the present invention and the provision within the device of a gas reducing with respect to the tungsten, the deleterious erosion of the filament during the decomposition of the cathode coating to (eliminate Which erosion the chromium coating has heretofore been used) is substantially prevented and the filament characteristics of this type heater may be maintained highly constant.

It is apparent that there may be many variations made in the specific embodiment of the present invention without essentially departing from the nature of the same as set, forth in the following claims.

What is claimed is: I i i 1. The method of forming a thermionically active alkaline earth metal oxide coating on a tungsten metal surface which comprises coating saidsurface initially With thermally dissociab'le compounds of said alkaline earth metalsvcontaining a proportion of a compound thermally dissociable at least in part to yield a reducing gas during formation of the oxide coating by thermal decomposition of the initial coating and thereafter effecting thermal dissociation of the coating.

2. The method of manufacturing an-electron discharge device. having an electron emitting cathode of the oxide coated type a which comprises coating said cathode atleast in part with alkaline earthmetal oxalates,

- incorporating the coated cathode in the device, removing the atmospheric gases from the device, incandescing the cathode to effect thercoating including an alkaline earth metal mal decomposition of the cathode coating, removing the evolved gases, vaporizing there-. in a suitable metallic sealing 01f the device.

3. A coating mixture for use in forming hot cathodes of the oxide coated type comprised of alkaline earth metal carbonates and a pro portion of an oxalate compound of one of I the strongly basic elements.

4. The method of preventing surface oxidation of a metallic heater element of a thermionic cathode-of the oxide coated type during the formation of the coating by decomposition of alkaline earth compounds on said heater element composed of a metal in a class comprising tungsten and molybdenum which comprises applying to said heater elementa oxalate capable ofliberating a gas, reducing With respect to surface oxides of tungsten and molybdenum and thermally dissociating said coating whereby the reducing gas liberatedprevents the formation of said surface oxides. V

In testimony whereof, I have hereunto subscribed my name this 5th day of" February,

EMIL cannon WIDELL.

getter and thereafter