US2041802A

US2041802A - Electron emitter

Info

Publication number: US2041802A
Application number: US678426A
Authority: US
Inventors: James R Wilson; John T Acker; Charles D Hartman
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1933-06-30
Filing date: 1933-06-30
Publication date: 1936-05-26
Anticipated expiration: 1953-05-26
Also published as: GB430169A; NL39798C; FR768485A; DE667942C

Description

May' 26, 1936.

J. R. WILSON ET'AL ELECTRON EMITTER Filed June 50, 1933 y wma. mmf,

A 7` TOR/VEY a portion of the material of the carrier or core Patented May 26,1936- lUNrrED 4STATES- aoirsoz ELEcraoN Em'rrEa James R.v Wilson, Mountain Lakes, N. J., and John 'l'. Acker, Queens Village, and Charles D. Hartman, Forest Hills, N. Y., assignors to Bell ,'Ijelephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 30, 1933, Serial No. 678,426 11 claims. (c1. 25o-27.5)

This invention relates to electron emitters and more particularly to electron' emitting cathodes of the coated type used in thermionic discharge and space discharge devices. y

The well known dull emitter or oxide coated cathode has been, for a. number of years, produced in two characteristic types, known as the uncombined type and the combined type.

'Ihe uneombined" cathode comprises a metallic carrier or core of platinum, platinum alloy, nickel or nickel alloy coated with barium and strontium compounds of the type which can beeasily decomposed by heat to the oxides. When this cathode is mounted in an enclosing vessel with the usual cooperating electrodes of an electron discharge device, the vessel evacuated, and the cathode heated, the compounds decompose to the oxides of barium andv strontium with `no chemical combination with the metallic `carrier or core. This type-of cathode has the white appearance of the oxides of barium and strontium.

The combined cathode comprises a metallic carrier or core of platinum, platinum alloy, nickel or nickel alloy coated with barium and strontium compounds of the type which can be easily decomposed by heat to the oxides. The coated carrier or core is heated in air to a high temperature, approximately 1100 C.,.which causes (1) the barium and strontium compounds to decompose to the oxides and (2) a portion of the barium and strontium oxides, the oxygen of the air and to combine chemically to form` complex comgrey or black in appearance. When this type of cathode is mounted in an 'enclosing vessel with the usual cooperating electrodes of an electron discharge device, the vessel evacuated, and the cathode heated, the complex compounds decompose to give the oxides of barium and'strontium and finely divided core material. This cathode retains its dark appearance due to the finely divided core material dispersed throughout the barium and strontium oxides. Upon further treatment under appropriate conditions inthe evacuated vessel both types of cathodes can be made thermionically active by the formation of small amounts of alkaline. earth metals, notably barium. The thermionic characteristics of these cathodes depend largely upon` these small amounts of alkaline earth metals in association with the matrix of oxides and, in the case of the combined" type, also in association with the finely divided metal from the core.

lo In the case of the "uncombined cathode thermionic activation results from the ordinary processes of outgassing the cathode and heating the anode and other cooperating electrodes by induced high frequency currents. In the case of the combined" cathode thermionic activation is 5 attendant upon a bombardment process in which the anode and other cooperating electrodes are maintained at a positive potential in order to draw space current in a gas at low pressure. The resulting bombardment of electrons and ionized l gas on the electrodes causes heating and outgassing of the parts and simultaneously enhances thesupply of thermionically active material in the cathode.

Thermlonic devices using these types of cathl odes are subject to loss of thermionic activity with use due to the loss of active material, i. e; barium metal, from the surface of the coating. 'I'he adequacy with which this surface is maintained or replenished with active material from a reserve supply in the coating matrix determines -the emission activity of the cathode and the length of its useful life.

In this connection it should be noted that the combined cathode has in the past exhibited a superior characteristic of long life and this fact has been attributed to the presence of finely divided core material in the oxide coating which acts as'a reservoir for active material. 'I'he combined" cathode, however, has not been appli. cable to many uses due to limitations of coating and activation techniques. Thefcombined" cathode technique described above involves the heterogeneous reaction between .the core surface, oxygen of the air and the solid barium and strontium oxides and the subsequent diffusion of the complex compounds, from the surface of the core to within the body of the remaining oxides. This process is also a function or the time and temperature of treatment and is difiicult to control, hence the amount and state of dispersion of finely divided core material in the coating matrix after activation varies Within wide limits. Moreover, the composition of the core materialpredetermines the kind and the amount of finely divided core material in the activated cathode, hence the process is limited in application. Furthermore the coating technique is not readily applicable to all shapes and sizes of desired cathodes. 'I'he method of activating this type of cathode is dim- 5 cult to control, time consuming and. inapplicablel in many cases, notably in vacuum tubes employing a multiplicity of control electrodes and in vacuum tubes employing the equi-potential type of cathode.

With the above ideas in mind, this invention is concerned with anelectrcn emitter of the coatedltype which is not subject to the limitations of fabrication, activation and application of the combined type and which has characteristics of life and thermionic emission superior to those of the present so-called combined and uncombined" types.

An object of this invention is to increase the operating life of oxide coated electron emitters.

Another object of this invention is to obtain saturation eillciency of emission from oxide coated emitters with low power consumption.

A further object of this invention is to obtain optimum dispersion of active material in the coating matrix of the emitter.

In accordance with this invention any refractory metal suitable for a cathode may be used as the carrier or core on which the coating is formed or deposited. A coating mixture is prepared which comprises compounds of alkaline earth metals and a compound of a metal or compounds of metals of the eighth group of the periodic system of chemical elements suspended with a binder in a suitable suspending medium. A binder is added to promote the ease of handling the cathode during the fabrication of the electron device. The binder decomposes or disappears during the subsequent heat operations. This mixture is coated or sprayed onto the carrier or core. The coated carrier or core is then heated at a low temperature to evaporate the suspending medium and cause the binder to function but not at a suiilciently high temperature to cause chemical combination between the mixture and the carrier or core material. The coated carrier or core is then mounted in an enclosing vessel with the usual cooperating electrodes of an electron discharge device. The vessel is then evacuated and the cathode heated so that the compounds are decomposed to oxides. The oxide of the metal of the eighth group, which is now finely dispersed throughout the coating, isreduced in situ byA means of a reducing gas whereby the metal is uniformly dispersed throughout the oxides of alkaline earth metals in a definite amount and in a deilnite state of dispersion. Upon electron bombardment in carbon monoxide, a portion of the alkaline earth oxides is reduced to alkaline earth metal which diffuses in and through the matrix of alkaline earth oxides and the free metal of the eighth group where it is deposited on or alloyed with the constituents of the matrix. It can be seen that by this invention, a definite and reproducible cathode of the coated type can be produced which has the optimum characteristics of thermionic activity and long life.

As a specic example of the invention, a triadic composition of barium and strontium carbonates and nickelous carbonate is mixed with a suitable organic binder, such as nitrocellulose, and a suitable organic solvent, such es amyl acetate, which gives a suspension of the proper viscosity for readily coating or spraying the type of carrier or core on which the coating is to be formed. The coated carrier is then heated at a low temperature to remove the solvent. The cathode is then mounted in a vessel` and processed as outlined above to make a thermionically active cathode.

In another specific application of the invention barium and strontium nitrates are added to the ture to fuse the nitrates. These temperatures. however. are not high enough to cause chemical combination with the carrier or core material.

In a further specic application of the invention, a mixture of barium and strontium carbonates and nickelous carbonate is suspended in a solution of Water and barium and strontium nitrates which gives a suspension of the proper viscosity for readily coating or spraying the type of carrier or core on which the coating is to be formed. The coated carrier is then heated at a low temperature to evaporate the water and then at a higher temperature to fuse the nitrates.

These temperatures, however, are not high enough to cause chemical combination with the carrier or core material. The cathode is then mounted in a vessel and processed as outlined above to make a thermionically active cathode.

This invention will be more clearly understood from the following detailed description in connection with the accompanying drawing.

Fig. 1 illustrates a coated lament made in accordance with this invention.

Fig. 2 is an enlarged view of a portion of the filament shown in Fig. 1 with a quarter sector cut away and shown in cross-section to illustrate the complex matrix of this invention.

Fig. 3 illustrates a discharge device of the three electrode type embodying an emitter made in accordance with this invention and Fig. 4 is a diagrammatic arrangement showing the various steps of the coating and transformation process of one form of the invention.

In accordance with one aspect of this invention, the objects previously set forth are accomplished by forming a matrix on the cathode core or surface i0 which includes alkaline earth oxides H, such asv barium and strontium, a metal of the eighth group I 2, such as nickel, in a free state and crystalline form, dispersed throughout the oxides, and free metallic barium or barium and strontium I3 in the matrix and on the surface thereof.

The metal I2 distributed through the oxides, primarily is introduced in the coating as a compound and serves as a depository surface for free barium in the coating or on the surface thereof and the barium either is alloyed with or is adsorbed on the nickel particles. In effect, the free nickel is covered with a skin layer or iilm of metallic barium. In view of the large number of particles in the oxide coating and their dispersed positions between the particles of oxides, it is evi'- dent that a large amount of free barium is always in reserve in the coating and this barium is supplied by diifusion to the surface to replace the lm of barium depleted during operation of the emitter.

The adequacy with which this surface is maintained or resupplied from a reserve supply in the coating matrix determines the emissive activity of the emitter and the length of life or duration of time the emitter may be considered as emitting a copious supply of electrons. The amount of surface barium and the concentration thereof will depend upon and be in equilibrium with the volume concentration of barium and nickel within .the matrix.

Another advantage of the dispersion of nickel throughout the matrix is the elimination of bright spots in the emitter. This is chieiiy due to the separation or dispersion of the free nickel in the oxides whereby fusion of the nickel is avoided.

While nickel has been speciiled as a preferable metal for association with the alkaline earth oxide. other metals of the eighth groumsingly or in combinations, may prove equally as good,

such as iron, cobalt," palladium or platinum or? earth metals, and their oxides, or other ther"-y mionicallyV active substances. y

It should be'realized that when anelectron emitter is mentioned in 'this specification it is intended that the emitter maybe employed in a variety of forms depending on the purpose for which it is to be used. For instance an incan descent or directly heated emitter may be the common form of a fllainentary conductor. An indirectly heated type may be an equipotential surface. And a cold emitter or cathode may be a metallic electrode which is rendered active by a suitable potential in combination with aconducting gas or vapor.

In order to secure the advantages of this invention and to realize the long life of the emitter,

the following description is given to illustrate more clearly how theinvention may' be practiced.

but it is to be understood that the proportions are merely suggestive and the invention is not to be bound thereby.

A fllainentary emitter til, such as a fine wire or ribbon core of nickel, platinum, or alloy thereof is preferably preglowed in a vacuum oven to remove deleterious matter and occluded gases and then subjected' to an annealing temperature treatment in an atmosphere `oi? hydrogen. The treated wire or ribbon may be coated or provided with any number of layers by dipping or spraying with a suspension of a coating mixture which may be prepared as follows:

` i Grams Barium carbonatos 255 strontium carbonate 300 .Barium nitrate 50 strontium nitrate 100 Nickelous carbonate 40 The vehicle used for such a suspension may be water of the proper proportions to give a wet mixture of the viscosity tosuit` the type of core to be coated. Instead of water the vehicle may be an organic binder, such' asv cellulose nitrate dissolved in amyl acetate. The mixture should be continually agitated during the coating of the filmentary core to secure a uniform distribution and to prevent settling of the nitrates or carbonates. Nickelous carbonate is recommended because this compound is stable at ordinary temperatures, but at the elevated temperatures used in discharge devices, it is easily reduced to the oxide and metal. Furthermore, it is readily `ob tained in pure form and requires no extraordinary precautions in handling. Under proper conditions many other nickel salts or compounds or compounds or salts ofthe other metals in the eighth group of the periodicsystem which can be easily reduced to an oxide or metal may be used.

By varying the amount of nickelous carbonate in the original mixture, filaments can be produced having different black body constants thereby making it possible to produce filaments with any desired power characteristic in shapes and sizes hitherto impossible. Inthe abovemixture the ratio of nlckelous carbonate to barium and strontium carbonatos is substantially 1 to 18.

After the nlament is coated it is heated only to a low temperature to drive od the suspension vehicle. The coated filament it is then heated in an inert atmosphere up to 675 C. tofuse the barium and strontium nitrates and form a flux or bond between the core and the coating material. At this temperature, no chemical combination with the core material occurs.' The coated filament it may be mounted in a vessel i5 with its cooperating electrodes, such as an anode i6 and grid vi l as shown in Fig. 3. The vessel is then connected to an evacuating system including the usual baking oven enclosing a header which is connected to a mercury aspirator pump backed with a mechanical oil pump.' A liquid air trap may be inserted in the system. The vessel i5 is baked for a suitable length of time to remove water vapor and other gases given off at low tem- Peratures from the vessel after which a suitable low pressure can 4be obtained in the vessel by means of the vacuum pumps. The emitter is their glowed at the proper temperature in vacuum, to decompose the nickelous carbonate to nickel oxide. The plate 'and grid of the device are heated to a temperature just below the point Where evaporation of nickel would darken the vessel. This heating may be accomplished by bombardmentor by heating the plate and vgrid byinduced high frequency current. This heat treatment is continued for five minutes. During this process the emitter is outgassed by heating to a vtemperature whereby the barium and strontium carbonates and nitrates are decomposed to the oxides. After the above transformations have occurred in the matrix, and a high degree of vacuum The initial incorporation of the nickel in the form of a compound results in'the final product of a finely divided depository for the free barium in the matrix, to serve as a reservoir for replenlshing the bariuni on the surface of the emitter which is gradually depleted during operation. 'I he free nickel in the oxide matrix tends to increase the initial quantity of free barium since the barium is readily alloyed with or adsorbed by the nickel. The free nickel also promotes the retention of the barium Within the coating matrix and tends to effect an optimum surface configuration for the highest thermionic efciency of the emitter.

While the emitter of this invention may be activated by vthe method heretofore described, other advantageous results may be obtained by activating the emitter in accordance with the process disclosed in a Patent No. 2,019,504 issued November um and strontium oxides .to produce free barium and strontium in and upon the oxide matrix. In this process, which is known as chemical activation, the free metal is dispersed throughout the the free nickel which serves as a large depository for the barium. When the chemical activation process is used in connection with the complexl matrix emitter of this invention, the activating gas is removed by further pumping of the device to a low pressure and then the device is sealed off the header of the pumping station.

The primary consideration in the production of an emitter of hi'gh thermionic efficiency in accordance with this invention is to obtain an optimum dispersion of finely divided free metal or metals throughout the coating matrix, and in fact, the total radiant emissivity of the emitter corresponds roughly to the degree of metal dispersion. In the coating matrix of this invention, the ratio of free metal, such as nickel, to barium and strontium oxides is approximately 1 to 24. This ratio has been found to give the optimum thermionic emission and life. It is of course understood that this ratio may be varied when certain other factors become more controlling than optimum emission and life.-

What is claimed is:

1. A coating composition for the production of thermionically active emitters comprising carbonates of barium and strontium, nickelous carbonate, and a binder material. i

2. A coating composition for the production of thermionically active emitters comprising carbonates of barium and strontium, nitrates of barium and strontium, a carbonate oi' nickel, and a fluid binder of nitro-cellulose in amyl acetate.

3. The method of producing a thermionically active emitter which comprises coating a core suitable for an emitter with compounds of alkaline earth metals and a compound of metals of the eighth group, fourth series, of the periodic system, subjecting said core to a heat treatment to decompose the alkaline earth and eighth group compounds to oxides, and reducing the eighth group metal oxide to metal in vacuum.

4. The method of producing thermionically active emitters which comprises coating a core suitable for an emitter with compounds of alkaline earth metals of low and high fusing points and a compound of a metal of the eighth group,

fourth series, subjecting said core to a preliminary heating to fuse said alkaline earth compounds of low fusing point, heating said core in a vacuum to convert the eighth group metal compound to oxide, heating said core in vacuum to decompose said high fusing point alkaline earth compounds to oxides, and reducing said eighth group metal oxide to metal with carbon monoxide in a vacuum.

5. The methodof producing thermionically active emitters which comprises coating a core suitable for an emitter with nitrates and carbonates of alkaline earth metals and a carbonate of the eighth group of the periodic system, subjecting said core to a preliminary heating to fuse the nitrates of alkaline earth metals, heating said core in vacuum to decompose the alkaline earth compounds and eighth group carbonate to oxides, and reducing the eighth group metal oxide to metal with carbon monoxide in a vacuum.

6. The method of producing a thermionically active emitter which comprises coating a core suitable for an emitter with barium, strontium and nickelous carbonates, decomposing the nickelous carbonate to nickel oxide, decomposing the barium and strontium carbonates to oxides, and

:2,041,802 oxidev matrix and is adsorbed upon or alloyed with y reducing the nickel oxide to nickel in the presence of carbon monoxide.

7. The method of producing' a thermionically active emitter which comprises coating a core suitable for an emitter with barium, strontium and nickelous carbonates, decomposing the nickelous carbonate to nickel oxide, decomposing the barium and strontium carbonates to oxides, reducing the nickel oxide to free nickel in an atmosphere of carbon monoxide, reducing barium oxide to metallic barium in the sa'me atmosphere, and associating the metallic barium with the nickel.

8. The method of forming layers on a filamentary core with al coating suspension of barium, strontium and nickelous carbonates which comprises agitating the coating suspension during application to maintain a relative proportion of nickelous carbonate to barium and strontium carbonates, dispersing the nickelous carbonate throughout the layers of barium and strontium carbonates, decomposing the nickelous carbonate to oxide in the absence of decomposition of the barium and strontium carbonates, decomposing the barium and strontium carbonates tooxides, converting the nickel oxide to metallic nickel to the exclusion of reduction of barium and strontium oxides, and subsequently reducing part of the barium and strontium oxides.

9. The method of coating an electron emitter with a coating suspension of barium and strontium nitrates and barium, strontium and nickelous carbonates which comprises agitating the coating suspension during application to maintain a relative proportion of nickelous carbonate to barium and strontium carbonates, heating the coated emitter `at a low temperature in air to fuse the barium and strontium nitrates, heating the emitter to convert the nickelous carbonate to nickel oxide, heating in vacuum to change the barium and strontium carbonates to barium and strontium oxides, heating the emitter in a reducing atmosphere of carbon monoxide at a low pressure to reduce the nickel oxide to free nickel, and heating the emitter in the same atmosphere to a higher temperature to reduce some of the barium oxide to free barium which is associated with the free nickel. v

10. The method of forming electron emitters which comprises coating a core with different compounds of alkaline earth metals having high and low melting points and a compound of a metal of the eighth group of the periodic system, heating the coated cathode at a low temperature to fuse the low melting point compounds of alkaline earthvmetals, heating the core in vacuum to a higherl temperature to convert the eighth group compound to oxide, heating the core in vacuum to a higher temperature to decompose the high melting point alkaline earth compounds to oxides, and reducing the eighth group oxide to metal with the aid of a reducing gas in vacuum.

1l. A cathode comprising a core metal, an uncombined coating matrix having combined characteristics on said core metal, said matrix consisting of barium and strontium oxides, finely divided particles of nickel dispersed throughout the oxides, a reservoir supply of free barium absorbed on said nickel particles, and an atomic layer of barium on the surface of said matrix.

JAMES R. WILSON. JOHN, T. ACKER. CHARLES D. HARTMAN.