US1842161A - Electron emission material - Google Patents

Description

Patented Jan. 19, 1932 UNITED STATES PATENT OFFICE MARK N. FREDENBUR-GH, OF EAST ORANGE, NEW JERSEY, ASSIGNOR T0 WESTING- HOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA ELECTRON EMISSION MATERIAL No Drawing.

lates to hot cathodes of the oxide co/ated typewhich are utilized in said devices and com- 5 prises essentially an improvement in the process of forming said cathodes.

In the preparation of oxide coated cathodes for electron discharge devices one ofthe methods used in applying the alkaline earth metal carbonates is that of spraying or painting the metal base of the cathode with a suspension of the carbonates in some volatile organic solvent or solvents in which is dissolved some binding materlal such as nitrocellulose. When the organic solvent has evap orated the carbonates are held ontothe cathode by the binding action of the nitrocellulose. Subsequently this organic binder is removed from the coating by heating the same .in a vacuum during the exhaust procedure in the manufacture of an electron discharge device, whereupon a bonding action is thereafter established between the coating material and the metal base by effecting decomposition of the carbonates therein and conversion to oxides which react with the surface oxides of the metal base to bond thereon. v

In the application of this general method of manufacturing oxide coated cathodes it has been customary to employ in forming the suspension of the carbonate material, organic solvents such as amylacetate, butylacetate or mixtures of these solvents with'common alcohols, such as ethyl or methyl alcohol, to which have'been added the desired proportion of nitrocellulose.

Whereas heretofore such methods, have yielded excellent results and cathodes coated in this manner have had a long and useful electroniclife, such results have been variable from lotto lot and day to day, depending agreat deal upon production conditions.

When such, suspension material is immedlately employed when made excellent results are obtained therefrom. However, when such suspensions have been allowed to stand unused for various lengths of time from only a matter of a day to a week or more, the electronic efliciencyof the cathodes prepared from the aged material decreases as the age there- Applicationfiled July 20, 1928. Serial K01 294326.

of increases. t is believed that such an effect is produced by some interaction between the solvent and the suspended carbonates or the solvent and the residual absorbed or ad sorbed impurities of the carbonate, or that it may be produced by decomposition of the solvent material to form deleteriousnon-dissociable or non-decomposable compounds, which are deleterious to the electronic efliciency of the cathode prepared therefrom.

It is one of the objects of this invention to eliminate the factor of non-uniformity in product in the preparation of hot cathodes of the oxide coated type.

It is another object of this invention to eliminate the lowered emission from oxide coated cathodes that may be due to the aging of the organic suspension employed in the manufacture of said cathodes.

Another object of the invention is to provide asuspension of electron emitting matrial in an organic suspending medium having substantially-all the advantages of prior suspensions and which is non-deteriorating with age to form compounds detrimental to the electronic emission of the alkaline earth metal component thereof.

Another object of this invention is to pro- 'customary to employ as carbonate material the commercial C. P. grade of alkaline earth metalcarbonates purchasable on the market. These materials were mechancally admixed in any desired proportion andsuspended in the suspending medium (organic or inorganic) and applied in any desirable manner to the surface to be coated. In general the factor of adsorbed or absordedimpurities were not allowed for and as a consequence such 1mpurities were variable in amounts and in identity, depending upon the method of production and prior history of the particular material employed.

I have determined that there is usually present in such commercial grades of alkaline earth metal carbonates appreciable amounts of adsorbed or absorbed impurities which are detrimental to the electronic operating efficiency of the hot cathode of an electron discharge device. Such impurities moreover are not substantially removed from such commercial grades of carbonate material by any washing or purifying process except by solution and reprecipitation of the carbonate under such controlled conditions of dilution of solution, temperature and mechanical agitation that are set forth in my copending applications Serial Nos. 292,037, filed July 18, 1928 and 293,801 filed July 18, 1928 entitled Electron emission material, which applications are assigned to the same assignee as the present invention. I

Although these impurities which may be generally classified as being alkali metal compounds such as the chlorides, nitrates or bydroxides, etc., depending upon the general method of production lnitially employed, may deleteriously affect the operating elliciency of the hot cathode, they are not wholly responsible for. the observed loss of emission due to the aging of the organic suspension material comprising the coating, as I have determined that such a deteriorating effect is obtained when an especially purified carbonate material such as may be prepared by my copending applications Serial Nos. 292,037 and 293,801 above mentioned, is employed in the formation of such an organic sus ension.

lthough it is believed that the cause of the loss in electron efliciency from such organic suspensions of alkaline earth metal carbonates is due to some sort of chemical interaction or deterioration of the organic solvent to yield compounds which are deleterious to the subsequent electronic life of the hot cathode, it is obvious that it is extremely diflicult to state just what the interaction or chemical combination might be, and I am not prepared at this time to predict or to calculatewhat reaction does take place.

I have determined, however, that by eliminating the occluded and adsorbed impurities heretofore inadvertently included in the alkaline earth metal carbonates by prior methods, by means of the special purification methods above identified and thereafter substituting for the usual organic solvents heretofore employed an organic solvent which is nonreactive with respect to the alkaline earth metal carbonates; or which if reactive therewith (over a period of time) would yield as reaction products compounds totally dissociable in vacuo by heat at the normal seaswhich if dissociable or decomposable under the influence of atmospheric conditions of moisture, temperatures, etc., would yield as decomposition products compounds readily dissociable and vaporizable in vacuo, the va riable results heretofore obtained through the use of this general method of applying electron emitting surfaces is substantially eliminated and hot cathodes made from my improved material are uniform in operating efliciency from lot to lot and day to day irrespective of the age of the coating suspension or variationsin atmospheric conditions of moisture, temperature, etc. The solvent employed should be also readily vaporizable in air and have a reasonable solubility towards a suitable binder material such as nitrocellulose. I have determined that the organic solvents diethyl carbonate ((C H CO3) or diethyl oxalate (C O (C H are examples of organic solvents fulfilling the provisions as above noted and may be employed in forming organic suspensions of alkaline earth metal carbonates for use in coating cathode surfaces and that suspensions employing these solvents either singly or admixed in any proportion are substantially stable for prolonged periods of time and show no evidence'of deterioration or interaction of the solvent with the alkaline earth metal carbonates to yield compounds deleterious to the electron efficiency of the hot cathode prepared therefrom.

By-utilizing the especially purified product as prepared by my copending applications in combination with the diethyl carbonate or diethyl oxalate, a coating composition is obtained which is superior in every respect to that heretofore obtained by prior processes, and cathodes prepared therefrom give a higher initial electron emission per unit area of surface, have a longer life, and a uniformity in production elficiency not heretofore observed.

As a specific embodiment of the nature of my invention I will describe the method I employ in forming a suspension of carbonates from material such as may be produced by my process described in copending application Serial No. 292,037, above identified. Such material is relatively dense and coarsely crystalline and requires a minimum of fluid dilutant to form a suitable suspension whereas the carbonate material produced as by my process described in copending application Serial No. 293,801, above identified, is rela tively light and fiulfy and requires a maximum of fluid dilutant to form a suitable suspension for coating purposes.

I first form a solution of nitrocellulose for use as a binder material as follows:

Nitrocellulose (parlodion) 8.0 gms. Diethyl carbonate (C. P.) 160.0 m. l.

oning temperatures of the hot cathodes; or Diethyl oxalate (C.P.) 40.0 m. 1.

The nitrocellulose (parlodion) is placed in a glass bottle, the solvents are added and the mixture agitated until the nitrocellulose is thoroughly dissolved. With the use of parlodion which is a very purehigh grade commercial nitrocellulose product, the solution will perfectly clear and will not need any filtering. The bottle should be kept tightly stoppered. This solution will be referred to as binder solution #1.

I next prepare the carbonate coating suspension as follows:

Carbonate mixture 100 gms. Binder solutionffirl 50 m.l. Diethyl carbonate 50 m.l.

The coarsely crystalline carbonate mixture previously prepared as by my copending application Serial No. 293,037, above identified, comprised of approximatel' 50% barium carbonate, 30% strontium car onate and 20% calcium'carbonate, is placed in a porcelain ball mill of 1 liter capacity, to which are added 700 grams of flint pebbles (size to 7 diam.). The nitrocellulose binder and diethyl carbonate are added and the jar tightly sealed. The mill is then rotated for from 2 to 15 hours at a speed approximating 100 R. P. M. The milled mixture is then poured into any suitable container and is ready for use.

The time interval of the milling operation is dependent upon what use is being made of the resultant product. For use in spray methods of coating, a degree of subdivision must be obtained which will permit the coating composition to flow freely through the spray orifice. For painting and dipping operations sucha degree of subdivision is not required. Starting with the type of coarsely crystalline rather brittle material that is obtained by the process disclosed in my copending application I am enabled to regulate the degree of'subdivi'sion. of the particles and maintain the same uniformly from lot to lot and day to day, or vary it at will depending upon the ultimate usethereof.

While I have specifically described and disclosed a method of forming a suspension of alkaline earth metal carbonates in an organic solvent containing, a suitable binder therein, I do not wish to .be limited thereby to the use of the specific components described therein. .1 may utilize asa solvent material employed and the application de sired thereof. I may also employ in addition to the above mentioned dilutants, a proportion of lower boiling anhydrous alcohols, particularly when the suspension is employed in the so-called spray method of coating cathodes. Y

These variations and all others that do not essentially depart from the nature of my invent-ion are anticipated as ma fall within the scope of the'invention as de ned by the followingclaims.

What is claimed is: i

1. As an article of manufacture, a coating composition for use in forming hot cathodes of .the oxide coated type comprised of alkaline earth metal carbonates substantially free from adsorbed or absorbed chemical impurities, av proportion of nitrocellulose binder and diethyl carbonate as a solvent and dilutant materiaL- 2. As an article of manufacture, a coating composition for-use in forming hot cathodes of the oxide coated class comprised of intimately admixed barium, strontium and calcium carbonates, a small proportion of nitrocellulose binder, and an organic solvent therefor comprised of one of the organic esters of diethyl carbonate and diethyl oxalate, in suflicient amount to form a suspension for said carbonates.

3. As an article of manufacture, a suspension. medium for alkaline earth metal carbonates for use in coatinghot cathodes of the oxide coated type comprised of approximately equal amounts of diethyl carbonate and a solution of 20% diethyl oxalate and 80% diethylcarbonate in which is dissolved a small proportion of nitrocellulose binder.v

4. The process of forming a suspension of alkaline earth metal carbonates in an organic solvent for use in the manufacture of hot cathodes of the oxide coated type, which comprises forming a mixture of alkaline I earth metal carbonates in any desirable pro-' portion substantially free of adsorbed and in diethyl carbonate containing a proportion of diethyl oxalate.

In testimony whereof, I have hereunto subscribed my name this 19th day of July, 1928. MARK N. FREDENBURGH.

forming of the carbonate suspension either diethyl carbonate or diethyl oxalate alone or admixed in an proportion. I may also vary the quantity 0 the proportionate admixtures of carbonate .to organlc suspension medium or utilize greater or lesser amounts of the binder material at will, dependent upon the