US1981652A - Method of coating electrodes - Google Patents

Description

Nqv. 20, 1934,

G. E. LONVG HEAJIN'G I HYDROGEN PR EGLOWING ELECTRODES SPRAYING I H COLLOIDAL GRAPHITE DRYING HEATING IN VACUUM INVENTOR 6. ELONG 9mm 644ml;

A T TORNE V I 45 of the grid to prevent emission therefrom.

Patented Nov. 20, 1 934 I UNITED STATES PATENT OFFICE METHOD OF COATING ELECTRODES George E. Long, Orange, N. .L, assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a. corporation of New York Application April 28, 1931, Serial No. 533,577

3 Claims. (01. 250-275) This invention relates to a method of coating constant to dissipate the heat generated in the electrodes for use in connection with electron device. emitting electrodes of electron discharge devices. In accordance with this invention the elecin high power electron discharge devices, partrodes, namely, the grid and the anode, are coat- 5 ticularly of the oscillator type, the use of high ed by a method which forms a coalescent hard anode and grid voltages results in pronounced film of carbon on the grid or the anode. The

heating of the refractory metallic electrodes. carbon coating having a high black body con- Usually thecathode associated with the anode stant serves to dissipate the heat generated in and grid is provided with a highly thermionic the electrodes thereby rendering these electrodes material either combined with the 'core or uncooler and overcoming to a great extent second- 65 combined in the form of a coating. Under such ary emission from the grid electrode. When temperature condition, the grid or control elecparticles ofthermionic material are deposited on. trode becomes an emitter of electrons. When the the grid from the cathode, these particles are grid emits electrons, a phenomenon .known as prevented from introducing primary emission efblocking effect occurs, thereby paralyzing the f'ects, due presumably to the formation of car it continued operation of the device. The grid hides which are not thermionically active and emission generally falls into two distinct classes are sufiiciently stable at the operating temperaand may be defined as primary emission, due to tures encountered in the device so as not to particles. of thermionic material being thrown aifect the operation of the device. Furthermore, off the cathode and deposited on the hot grid the carbon film, while in effect a high resistance .iiwhere they form a source of electrons, and seccoating, does not detract from the proper funcondary emission, due to the high temperature of tioning of the wire grid as a control electrode the grid and the velocity of the impinging elecin the flow of electrons from the cathode to trons causing the emission of secondary electrons the anode.

from the grid. Naturally, this adverse condition The drawing shows diagrammatically the proc- 3) of the grid should be overcome if the grid is to ess steps required to obtain the results of this perform its normal function of controlling the invention.

flow of electrons from the cathode to the anode. The method of coating the electrode with 2,

However, when attempts are made to overcome -film of carbon consists in pre-glowing by heatemission from the grid by coating this electrode ing the grid or anode in a vacuum or reducing at with a black substance to maintain the grid atmosphere to completely free the electrode of cooler and also to combine with thermionic maoccluded gases and deleterious substances. The terial so as to form inactive compounds, the electrode is then coated with a colloidal solution characteristics of the device are changed due to of g p k-IWI1 Commercially as Aquadag the difference in the diameter of the lateral wires' a d fi y t Carbon is fi t e ect ode to of the grid and the spacing between adjacent surface in a vacuum or reducing atmosphere of wires. Furthermore, the thick black coatings hydrogene fi m Of carbon formed on the usually in the form of oxides of the metal of the Surface by s fix thod is relagrid practically insulate the grid from the other tlvely hard and Smooth and firm1y electrodes and it does not assume its normal funcherem to surfzffie; Furthermore, the diamtion until the insulating properties of the coatgg i gg gfig g :5; gsggg ig igigfi i mg are broken down by heatmg Slmnafly some istics of the device in which his associated with metals, such as molybdenum, do not react fath d d d I bly when coated with an oxide of the metal a 6 an Vora In accordance with this invention the anode 100 or grid is first deoxidized by heating the metallic 'lhe principal obiect of this invention is to body in a reducing gas Such as hydrogen, or in f m pnmary a emlsslon from the a vacuum to remove occluded gases and oxy en grid inelectron d1SC -r forming compounds which may deleteriously af- Anotlfler ObJeFt of the mve1 1t1n to jiorm feet the carbonizing process. This step produces 10 Protectwe coatlflg on the r whlch W111 P a clean and bright surface on the electrode which aifect the electrical characteristics of the device. is then ready f the carbonizing treatment; The

A 1 6 obl c 0 e Invention 15 0 form carbonizing of the wire grid or the anode conprotective coating on an electrode, such as an sists in spraying or otherwise coating the suranode, the coating having a high black body face of the grid or anode with a'concentratedl I slightly ammoniacal solution of colloidal graphite commerciallyknown as Aquadag. The sprayed electrodes are allowed to dry and are then subjected to a fixing" treatment in order to insure complete coalescence between the carboncoating and the surface of the electrode. This fixing process may be carried out by either of two methods of heat treatment.

In the first method the sprayed electrode is placed in a reducing oven and heated to a temperature from 500 to 800 C. while hydrogen or other reducing gas is injected into the oven. The

' flxing" process may also be carried out in a methods.

and fixing process high vacuum at a temperature from 600 to 900 C. In either fixing process the colloidal graphite coating. on the electrode is formed into a closely adherent film on the electrode which film is, very diiiicult to remove even by abrasion A major advantage of this method is that while the coating is of sumcient thickness to accomplish the desired result of considerably lowering primary and secondary emission, it is thin enough that it does not in any manner change the electrical characteristics of the devices. The

controlling of the dimensions of the grid struc-.

ture is a primary requisite in electron discharge technique since a change in diameter of the grid wire of 0.002 inch will materially affect the dynamic and electrostatic characteristics of the device. For this reason a large number of carbon producing compounds are objectionable since they result in a relatively thick coating of caricon on thegrid and therefore cause variations in the spacing between the wires of the grid electrode.

trodes which are usually associated with low temperature coated filaments.

The pre-glow treatment of the electrodes in accordancewith this invention, prior to the carbonizing and flxing" treatments prevents alteration or the carbon film on the electrodes exposure to air prior to inserting the electrodes within an evacuated vessel. Furthermore, the pre-glowed carbonized electrode insures freedom from oxygen forming compounds which may combine with thermionically active material under electronic action of the cathode and deleteriously affect the cathode coating material.

It is of course understood that other metals such as iron and alloy metals generally employed as electrodes in electron discharge devices may during the ,fixing treatment and subsequent be carbonized by the process as herein described.

Therefore, the invention is only to be limited within the scope of the'appended claims.

What is claimed is:

L The method of carboniaing an electrode for an electron discharge device which comprises preglowing the electrode to remove occluded gases, coating the electrode with a colloidal graphite solution, and heating the electrode at temperatures from 500 to 900 C.

2. The me hod oicarbonizing an electrode which comprises heatingthe electrode in a reducing atmosphere ;.to remove occluded gases,

coating the electrode with colloidal graphite,

fixing the graphite coating in a reducing atmosphere, and heating the electrode at a temperature from 500 to 800 C. while in the reducing atmosphere.

3. The method of carbonizing an electrode.

which comprises heating the electrode in a vacu-' um to remove occluded gases, spraying the electrode with a concentrated ammoniacal solution of colloidal graphite drying in air, and finally heating the electrode in an evacuated vessel within a range of temperatures from 600 to 900 C. to fix the carbon to the electrode surface.

.GEORGE E. LONGr- V use

Images