US2457515A - Insulating coating compositions and method of making - Google Patents

Description

Patented Dec. 28, 1948 INSULATING COATING COMPOSITIONS AND METHOD OF MAKING John T. Acker, Queens Village, N. .Y.-,,.assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.., a corporation of New York Application November 13, 1941, Serial No. 418,894

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This invention relates to coatings for filaments and particularly to insulating coatings for heater elements of indirectly heated cathodes incorporated in electron discharge devices.

In such devices, for example of the ultra high frequency type, having diminutive electrode assemblies to match a definite short wave-length range or of other types of small structure but high heater voltage rating and in which the emitter is of the indirectly heated type, the energizing element or heater within the emitter is naturally of minute dimensions. This heater element is insulated from the cathode or emitter by a coating of suitable insulating material which is suspended in a liquid in order to .afiix a desirable amount of insulation to the fine wire by spraying, dipping or painting the substance thereon.

Various difiiculties have been encountered heretofore, even with insulating material of the finest particle size obtainable and with concentrations thereof in various liquids suitable for application by the usual methods. When the mixture contains a preponderance of solids the weight thereof with high viscosity produces undesirable results due probably to surface tension effects on the small diameter wire. If the concentration of solids is decreased the amount of coating for a given number of coats is inefficient. Even with a rich mixture, that is, of a creamy paste character, which has been found to serve the purpose required, the undesirably large number of successive coatings necessary to attain the required thickness of insulation is ineiiicient, time consuming and uneconomical.

An object of this invention is to efliciently apply refractory insulating material on very fine wire to produce a uniform, coherent and highly practical coating thereon.

Another object of the invention relates to increasing both the fluidity of the mixture and the concentration of solids in the mixture to a high degree.

A further object of the invention is to decrease the number of coats required for a given thickness of insulation.

Another object is to increase the hydrogen ion concentration of the coating mixture whereby the fluidity of the mixture is materially increased to facilitate the coating of very fine Wire filament by machine methods.

The coating mixture or suspension contemplated by this invention involves a refractory insulating material, for instance, aluminum oxide of fine particle size, which is mixed with water or any other suitable liquid carrier to form a suspension which may be applied by dipping or painting the heater Wire. The consistency of the suspension should be a creamy paste for effective results in the coating procedure. The amount of oxide solids capable of being incorporated in the suspension may be increased more than 5.0. per cent and the fluidity also increased by increasing the hydrogen ion concentration of the mixture with the addition of an ionizing medium, such as nitric or hydrochloric acid, without changing the consistency of the suspension.

The hydrogen ion concentration of the mixture imparts high fluidity to the suspended particles .Of oxide so that they adhere to very fine Wire of the order of .001 inch diameter, and the char? acter of the coating mixture overcomes inherent surface tension when the wire is traveling through the liquid carrier in a machine coating process, whereby the number of coatings to secure the desired thickness of insulation is material1 reduced. 1 As a specific example of the coating compositiOn of this invention for producing a uniform layer of insulation onvery fine tungsten wire of the order of .001 inch diameter, 1000 grams of aluminum oxide of very small particle size, such as 900 mesh, is mixed with 460 cubic centimeters of distilled water to form a creamy paste. -By adding, for example, 40 cubic centimeters of nitric or hydrochloric acid to the mixture, the fluidity is considerably increased so that 659 grams of aluminum oxide may be added to the mixture to increase the solid content approximately 65 per cent. That is: with approximately less than 10 per cent addition of fluid the solids may be increased more than 50 per cent.

This is achieved by the action of the nitric acid which increases the hydrogen ion concentration or pH value of the carrier liquid so that the viscosity is materially decreased. The acidulated solution should have a hydrogen ion concentration in the range between .15 and 1.5 normal, the concentration in the suspension given above being between .77 and .99 normal. The composition adheres to the fine wire so that three or four coatings or layers are sufiicient to attain the required thickness of insulation on the wire, whereas heretofore it was necessary to apply ten or more coatings for the same degree of insulation. The reduced number of coatings materially avoids lumpy appearing insulation which is the result of dirt, lint or other foreign matter becoming embedded in the insulation during the coating process. Furthermore, the suspension is not sensitive to temperature changes which might affect the solid substance being precipitated out of solution nor is the suspension limited by hydrolysis since any degree of hydrogen ion concentration may be obtained by varying the acid quantity in solution.

The acid content is usually mixed with the carrier liquid and the solid content added to the consistency of a creamy paste. The specific quantities are not critical so that various proportions may be prepared depending on the character of coating desired, the particle size and kind of insulating material, and the diameter of the wire to be coated.

The coated wire is fired at a temperature of 1750 C. in hydrogen or other inert atmosphere after each layer is applied, to facilitate the sintera suspension of creamy consistency, a quantity of ing of the insulation on the wire. This procedure eliminates preliminary baking at a lower temperature prior to the sintering temperature and results in providing a refractory insulation covering for the wire in which the aluminum oxide particles are tenaciously bonded to the wire without any reaction products which might contaminate the coating, the water and acid constituents being partially removed by evaporation and completely removed during the firing period as diffusible vapors.

While the strong hydrochloric and nitric acids are preferred due to their high concentration of hydrogen ions, any other suitable acid maybe employed with satisfactory results for increasing the viscosity of the suspension, whereby'a definite coating density and thickness may be attained with a lesser number of successive layers.

Although the invention has been disclosed with respect to a definite coating suspension for application to specific diameter wire for heater type cathodes, it is, of course, understood that the invention is not limited to this aspect since various changes may be made in the character and material of the coating solids and solutions or carriers and that other types of wire may be coated to achieve the results of this invention. Therefore, the scope of the invention is only to be limited by the appended claims.

What is claimed is: i

1. The method of preparing a coating mixture for wire elements which comprises mixing finely divided aluminum oxide and water in proportions toform a suspension of creamy consistency, adding hydrochloric acid to the suspension to increase the fluid content by of the order of 10 per cent, and adding finely divided aluminum oxide to the suspension to increase the solid content by of the order of 50 per cent.

substantially 10 per cent by volume of said water of a strong inorganic acid and an additional quantity of finely divided aluminum oxide of the order of 50 per cent of said first quantity of aluminum oxide, applying the resultant mixture to the wireelement in successive coatings, and firing each coating in an inert atmosphere to bond the oxide to the wire element and to remove the water and acid.

. 1 JOHN T. ACKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 424,353 Preussner Mar. 25, 1890 449,619 Euricht Mar. 31, 1891 834,633 Neubarger et a1. Oct. 30, 1906 1,001,413 Keppeler et a1 Aug. 22, 1911 1,376,523 Formby May 3, 1921 1,409,939 Formby Mar. 21, 1922 1,591,563 Ruff July 6, 1926 1,606,481 Rockow Nov. 9, 1926 1,719,988 Myers July 9, 1929 1,877,531 Reichmann Sept. 13, 1932 1,908,792 Ruben May 16, 1933 1,934,091 Reichmann Nov. '7, 1933 2,041,802 Wilson et al May 26, 1936 2,075,876 Von Wedel Apr. 6, 1937 2,097,157. Jameson Oct. 26, 1937 2,115,828 Prescott May 3, 1938 2,142,331 Prescott -1--- Jan. 3, 1939 2,215,295 Morrill Sept. 17, 1940 2,292,065 Elsey Aug. 4, 1942 FOREIGN PATENTS I Number Country Date 364,529 Great Britain 1931 373,520 Great Britain 1932 OTHER REFERENCES Searle Refractory Materials (1924), pages 96 and 97..