US1874785A - Oxide coatings and method of preparing the same - Google Patents

Description

Aug. 30, 1932. H. J. MILLER ET AL OXIDE COATINGS AND METHOD OF PREPARING THE SAME Filed June 25, 1930 T/ME Patented Aug. 30, 1932 HENRY JOHANNES MILLER AND FRANK mum, on GHI'CAGQILLINOIS, assrenons TO GRIGSBY-GRUNOW COMIA NY, CHICAGO, ILLINOIS, A CORPORATION ILLINOIS OXIDE commas AND METHOD or PREPARTNGVTHE SAME Application' filed June 23,

' faces of vacuum tubes have been used for sometime. One of the outstanding weak': nesses of such coatings has been their 1nsuf ficient hardness and their consequent disen-- tegration on handling and in use. The ef-' fects of the emission of electrons have a tendency to erode the electron emitting surface and thereby reduce the useful life of atube. An object of our invention is to provide a new and improved oxide coating and a method of producing the same.

A further object is to provide an oxide coating of great hardness. I

A further object is'to produce an oxide coating of fine texture and smooth surface.

Other objects and advantages will appear as the description proceeds.

The single figure of the drawing shows a series of, experimental curves illustrating our invention. a y v In.coating an electron emitting surface or cathode, barium carbonate and strontium carbonate are mixed with a binding material which consists of nitrocellulosedissolved in methyl alcohol and amyl acetate. ing is sprayed upon the cathode by air pressure and during the spraying processpractically allof the methyl alcohol and amyl acetate evaporate so that the barium and strontium carbonates with the nitrocellulose binder reach the cathode in abomparative- 1y dry state. The cathode is'mount'ed in the tube or envelope in this condition and during the exhaustion of the tube heat is applied to the cathode to gasify the nitrocellulose and remove carbon dioxide from the barium and strontium carbonates to convert them into oxides. (BaCO =BaO plus CO and SrCO =SrO plus CO The amount of nitrocellulose used shoul be as small as possible since the vaporization of this material if. present in large quantities is apt to produce blow holes or disruptions in the coating which injuriously affect the emissivity of the coating. Still another reason for using as little of the nitrocellulose The coat- 1930. Serial No. 463,337.

binder as possible is that the spacing orseparation of the carbonate particles will be smaller andthus produce greater uniformity of the coating and providing a smooth surface of fine texture. I We have found that it is possible to great ly reduce the amount of nitrocellulose used as a binder in a coating mixture by employing a nitrocellulose having ahigh molecular Weight and high viscosity and grinding the mixture for a long period in a ball mill. t The method of producing nitrocellulose of high molecular weight and high viscosity iswell known to those skilled in the art and will therefore not be described.

In usingnitrocellulose of a high molecular weight and high viscosity we'have found that the time of grinding the inixtureof carbonates and nitrocellulose in a ball mill "increases the hardness of the coating very much, while the grinding of a mixture containing nitrocellulose of low molecular weightand low viscosity has but small effect upon the hardness of the coating. i

Referring to the drawing, the abscissas indicate the time in hours of grinding such a mixture in a ball mill and the ordinates indicate relative hardness. v r

Curves 1 and 2 represent the use of a nitrocellulose having a low molecular weight and low viscosity; curve 1 being of a mixture in which the nitrocellulose had a molecular weight and viscosity lower than in curve 2. Curves 3 and 4 show the effects of long grinding upon a mixture having nitrocellulose of a high molecular weight and high viscosity. It willb-e seen that as the time of grinding is increased for nitrocellulose mixtures of low molecular {Weight and low viscosity the hardness increased but slightly, and the expense of ball millingthe mixturefor a long period does not justify the slightly increased hardness of the resulting coating. However, as shown by curves '3 and 4, when a nitrocellulose of high molecular weight and high viscosity is used, the slope of the curve is rather steep and a very material increase in the hardness of the resulting coating is obtained. e do not wish to confine ourselves to any theory for this g market.

phenomenon, although it seems very probbe used and still have suflicient binding properties, permits a closer positioning ofthe car.- bonate particles and results in a finer texture of the coating. Also, the fact that the gasiiication of the smaller amount of nitrocellulose has less'tendency to disrupt the coatinga't the points where the gas escapes, is believed to be important.

In actual practice we have found itdesirable to ball mill the mixture of carbonates and nitrocellulose for from 12 to Q lhours The ball milling of the materials for a long er period than24l hours results in too large a percentage of metal abraded from the ball mill being incorporated in the materials which is harmful to the product 'Thisprolonged milling produces a'fvery hardcoating which is of sufiicient hardness to very satisfactorily answer the purpose of coatings in Vacuum tubes. Additional grinding beyond 24; hours does'not seem to be justified by the additional cost entailed; By this method I have produced an oxide coating for cathodes which by actual test, proved to betwenty times harder than the coatings on the cathodes of similar tubes now on the A'formula which we have found convenient in practicing our invention consists of dissolving '15 grams ofnitrocellose of high molecular weight and high viscosity in 6 5 c. c'. of amyl acetate,'and 450 c. c. of methyl alcohol. Tothis is added 750 grams of barium andstrontium carbonates. The nitrocellulose has preferably" a viscosity of the 6 order of 80-times that of water when dissolved in amyl acetate to a 3% solution. This mixture is ground for 12 to 24 hours in a ball mill and is then sprayedor otherwise applied on the cathode. The subsequent heating of the cathode upon the exhaustion ofthe tube gasifiesthe nitrocellulose and converts the carbonates to oxides, leaving an extremely hard coating of oxides of very fine textures upon the cathode. The texture and smoothness of the resulting coating greatly enhances the emissive character of the coating and is capable of effectively with r 1 standing the erosive effect of electron emission.

What We claim is new and desire to protect by Letters Patent of the United States is:

" 1. A method of producing an oxide composition which consists of dissolving nitro cellulose of high viscosity in a solvent, add

ing alkaline earth carbonates to the solution, grinding the resulting mixture at least 8 hours, evaporating the nitrocellulose, and

one percent of nitrocellulose of high viscosity in a solvent, adding alkaline earth carto oxides by the application of heat and low pressure.v

' 3. A method of producing oxide com position which consists of. dissolvingasmall percent of nitrocellulose of high molecular weight and high viscosityin a solvent, adding alkaline earth carbonates thereto, grinding the ,resultingproduct for an extended period, gasifying the nitrocellulose, and converting the carbonates to oxides bythe application of heat.

4. A method of producing an oxide composition which'consists of mixing methyl alcohol and amyl acetate, dissolving nitrocellulose of high molecularweight and high vis cosity in said mixture, adding. barium and strontium carbonates to the solution thus formed, milling the product for substantial 1y 24 hours, gasifying the nitrocellulose, and converting the carbonates to oxides by the application of-heat. I V i 5. A composition for preparing oxide coatings for cathodes comprising a solvent having .nitrocellulose of high viscosity dissolved therein, and line. particles of carbonates of barium and strontium intimately dispersed therein whereby a coating of fine texture and a high degree of hardness may be produced.

6. A composition for preparing oxide coatings for cathodes comprising approximately 15 grams of nitrocellulose of high: viscosity, 1125 cubic centimeters of a solvent mixture, and 750 grams of barium and strontium car'- bonates reduced to an intimate'mixture.

7. A composition for preparing oxide coatings for cathodes comprising approximately 15 grams of nitrocellulose of high viscosity, a solvent therefor, and approximately 7'50 cubic "centimeters of barium and strontium carbonates reduced to an intimate mixture, whereby an oxide mate-rial of fine texture and greathardness may be produced. r

converting the carbonates to oxides by the application of heat.

2. A method of producing an oxide coating which consists of dissolving less than i i

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