US1762581A - Cathode for thermionic devices and method of producing same - Google Patents

Description

June 10, 1930. LA. 'MCLI. NlcoLsoN 1,762,531

CATHODE FOR THERMIONI C DEVICES AND METHOD OF PRODUCING SAME Filed Feb. 14, 1927' IN V EN TOR.

A TTORNEX Patented June 10, 1930 m oFFica ALEXANDER MCLEAN NIOOLSON, OF YORK, N. Y., ASSIGNOB TO COMMUNICATION PATENTS, INCL, OF NEW YORK, N. Y., A CORPORATION OF DELAWARE .CA'lI-IODE FOR THEBMIONIC DEVICES AND METHOD OF PRODUCING SAME Application filed February 14, 1927. Serial No. 168,242.

This invention relates to electron emitting electrodes, employed in audions, vacuum tubes, and thermionic devices, and particularly to those in which the emission of electrons results from anemissive surfaceformed by the application of a coating to a base orcore, and to methods of preparing the same.

It has for an object the provision of an electron emitting electrode having a comparatively thin and regular coating or emissive surface of substantially uniform depth and in which the electronically inactive and physically unstable portions have been reduced to a minimum.

Another object is the provision of an electron emitting electrode surface whose thickness and therefore capacity for emission or activity may be regulated during the process of production and increased as desired while remaining closely adherent in itself and to the core and which coating is substantially entirely active at comparatively low operating temperatures.

Another object is the provision of a method of decreasing and reducing to a minimum the quantity of substantially electronically inactive and mechanically or physically unstable components in the emissive coating of a completed filament or electrode.

Another object is the provision of a method of decreasing and reducing to a minimum the quantity of substantially electronically inactive and mechanically or physically unstable components in the emissivc coating of a completed filament or electrode.

Another object is the provision of a method of producing electron emissive coatings on a conductive base or core in which the surface produced is regular, substantially entirely active and conductive, and may be made of any desired thickness with out becoming non-adherent.

Generally stated, the accomplishment of the above objects is based'upon my discovery that the surface formed by coating a base or core is made up of two components, one of which is essentially the source of the electron-emitting activity of the filament While the other is substantially non-essential as regards the emissive characteristics of the filaments. This essential component,

I have found, comprises substantially a thin I film formed adjacent the conductor and adhering particularly well to the latter. I believe this film to be essentially a chemical. combination, as alloyed or compounded with the composition of the core, but regardless of this phase of the matter it may be said to be closely attached to or associated with the core.- Working upon this basis, I have achieved theabove objects by treating the coated conductor or core after the application of a coating composition so as to remove the physically unstable and chemically uncombined constituents from the surface.

More particularly this treatment according to my invention comprises, the subjecting of the coated and baked conductor to a dissolving process, such as a leeching in water or in an alkaline solution, whereby the readily soluble and physically unstable components are separated from the emissive surface, leaving the active portion thereon. In this manner any desired depth of completed emissive surface which is closely adherent may be procured by repeatingthe successive steps of coating, baking and leeching a sufficient number of times. The surface remaining on the conductor after this leeching treatment may be termed substantially the under-coat of the originally applied coating and is remarkably superior to the original coating formed merely by application and baking.

To these and other ends, the invention consists in further features all of which will be more fully described and thoroughly understood with the aid of the description to follow, the novel features being particularly set forth in the appended claims.

In the drawing, Figure 1 shows in greatly magnified form the relative thicknesses of the undercoat or adhering portion 3 and the overcoat or unstableportion 4 of a thermionic'coating applied to a fiat ribbon filamentary core 1, the latter having been found suitable for use as an electron emitting cathode.

employing a heated filament, as exemplified by the three electrode vacuum tubes containing a filament, and input and output electrodes, and whose operating characteristics are directly dependent upon the composition or substance used as the filament or cathode. Of the factors influencing the choice of a cathode, the electron aflinity (ratio of electron evaporation constant to the electron charge) is probably the most important and for general uses of thermionic devices, this factor is maintained as small as possible since the power dissipated in heating the cathode to obtain a definite thermionic current decreases as the electron afiinity is decreased. Thus, a low value of the electrode aflinity constant implies economy of operation and increased lifepf both filament and tube, because of the lower temperature at which the filament or cathode can be operated to produce the desired saturation current values. To this end, the cathode is preferably treated to obtain a low value of the electron aflinity and one of such treatments is the Wehnelt process which comprises coating a conductive and highly re-' fractory core, as platinum, with the oxide of one of the alkaline earth metals, these latter compositions having been found to possess high emissivity or activity characteristics, or in other words low electron aflinity values.

Another filament characteristic which is determinative in the choice of a filament which will operate at low temperatures producing suitable activity values is the heat characteristic of the emissiveconductivit surface whlch should beas high as possible, since the surface. derives its heat substantially entirely by radiation and diffusion from the conductive heated core and the heat conductivity of the surface will thus determine the temperature to which the'core must be heated to produce a given temperature in the coating.

In accordance with my invention, I have found that the emissive or activity characteristics of coated filaments are dueessentially to a thin almost imponderable undercoat film 3 formed immediately adjacent the conducting core while the remainder 4 of the coated surface, which for the purpose of description may be termed the overcoat, does not exert any material beneficial influence upon the o eration of the filament On the other han this overcoat portion possesses anundesirable high resistance thereby causing a loss of power from unnecessary heat-, ing and further possesses an irregular surface. while the undercoat film is comparadiscovery, my improved process includes a treatment of the coated and baked conductor whereby the overcoat portion is removed and the undercoat film unafiected, leaving essen-. tially only the thin undercoat film, and the steps of the coating, baking and overcoat removal treatment are repeated until a finishedcoating surface of the desired thickness has been built up. By this process, I

am .able to produce a thermionically active 7 filament having an active surface of uniform depth thus appreciably reducing the likelihood of occurrence of bright spots in the-heated filament, and possessing a regular even surface which closely approximates the desired regularity of an uncoated conductor surface. Furthermore, the desirable strong adherence of the coating in itself and to the core is not reduced by increasing the thickness of the surface and successively applied undercoat films adhere strongly to each other, thus making possible the production of any desired thickness of surface or degree of activity. The life of the filament is also materially increased, since the entire coating is highly electrically conductive, thereby implying low temperature of. operation.

In the preferred treatment, the conductor after being coated and baked is passed through a dissolving or leeching treatment in a solvent which will dissolve the overcoat portion without materially affecting the undercoat film. Solvents which have been. found satisfactory, include water and various alkaline solutions, as sodium hydroxide, potassium hydroxide and ammonium hydroxide. The duration of leeching treatment will of cours'e depend upon the amount of coating applied andupon the strength and temperature of the solvent, but as a general rule I have found that the greater numlength and 0.615 grams weight which had been given a coated and baked surface of 0.035 grams Weight and found first that after five minutes leeching there remained an undercoat film weighting 0.005 grams and second that a continuous leeching of the same coated and baked filament for as long as fifty hours resulted in no further removal of the coating.

Various materials may be used as the actual coating composition, but I have found the use of a mixture of a highly active material with a binder material, as disclosed in 'my copending applications, Serial No. 108,676, filed May 12th, 1926; No. 108,961, filed May 13th, 1926; No. 154,035, filed December 10th, 1926; and in my joint patent with E. C. Hull, No. 1,209,324, dated December 19th, 1916. Briefly these disclosures comprise the use of an emissive surface including both an alkaline-earth material, particularly a compound of barium, and a material which may or may not possess any activity characteristics but which serves as a binder between the active alkaline earth material and the material of the core. The binder materials disclosed and claimed, as above listed, comprise compounds of strontium, the rare earth group (titanium, zirconium, cerium, thorium, lanthanum, praseodymium, samarium, yttrium, europium, gadolinium, terbium, dysprosium, holmium, erbium,thulium, ytterbium, lutecium,) and the group contained in the fourth series of the periodictable between groups three and eight inclusive (scandium, titanium, uranium, chromium, manganese, iron, cobalt, nickel), the oxides of these metals being preferable. When such materials are employed as the .coating compositions, I have found that the undercoat film is. particularly adherent and believe that this film or layer is composed of the compounds of the coating metals used with the core material employed.

Noble metal cores, .and particularly those composed essentially of platinum as platinum and platinum-iridium alloys, are especially satisfactory for use in accordance with my invention.

Myimproved process is especially advantageous in that the activity or emissivity properties ofthe coated filament may be inleeching a sufficientnumber of times, since the removal of the inactive overcoat portions makes it possible to bring fresh material into contact with the core and the conductive undercoat film to thereby form a fresh layer of the latter. Without the re-' moval of the overcoat, it would be substantially impossible to increase the thickness of the essential undercoat film, as there could then be no interaction between the core composition and the uncombined portions of th applied coating composition.

What I claim is 1. The method of producing an electron emitting electrode which comprises coating a conducting core with an electron emissive material, baking the coated core, andremoving the uncombined coating material.

2. The method of forming an electron emissive electrode which comprises repeating the successive operations of coating an electrically conductive core with an electron emissive. material, baking the coated core and removing the chemically uncombined coating material to-produce a coating surface of the desired thickness.

3.. The method of producing a thermionically active filament which includes the step of removing the uncombined portions of the coating.

4. The method of'producing a coated electron emissive filament which includes the step of leeching the coated conductor to remove the uncombined constituents.

5. The method of producing a coated electron emissive filament which includes the steps of coating a conductive core with an electron emissive material, baking the material thereon, and leeching the baked conductor to remove the overcoat portion.

6. The method of making an electron emitting cathode, Which comprises coatinga conductive core with a composition containing an alkaline earth-metal compound, baking said coated conductor, to combine a portion of the coating with the core composition, and subjecting the cathode element to a dissolving treatment to remove the uncombined portion of the "coating.

This specification signed this 9th day of February, 1926.

ALEXANDER McLEAN NICOLSON.

creased to any desired value by repeating v

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