US1838766A - Refractory insulator support for electron discharge devices - Google Patents

Description

Patented Dec. 29, 1931 UNITEDQSTATESY PATENT OFFICE CHARLES V. IREDELL, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTINGHOUSE 1 LAIP COMPANY, A. CORPORATION OF PENNSYLVANIA REFRACTORY INSULATOR SUPPORT FOR- ELEGTRON DISCHARGE DEVICES No Drawing.

This invention relates to insulating material for usein vacuum electric devices and more particularly to an insulating support for the heater element of an indirectly heated thermionically active cathode. x

The type of electron discharge device with which the invention is particularly concerned employs a cathode heated by radiation and conduction from an electrically insulated heating element energized by the passage of alternating current therethrough. The heating element of such a cathode may consist of a tungsten filament which is supported within an aperture in a cylindrical or other shaped insulator of porcelain magnesium silicate or other refractory material about which is positioned a metal cylinder coated with a thermionically active material such lll as the oxides of the alkaline earths.

The method which has largely been employed for producing the thermionically active composite cathode has been to first coat the outer metal cylinder with a mixture of alkaline earth metal carbonates suspended in an organic solvent containing an organic binder such as nitro-cellulose. Thereafterthe insulating support containing the heater element is inserted and the composite mounted in position in an electron discharge device. Followin the usual evacuation procedures the alkaline earth metal carbonates in the coating are decomposed by heat to form the desired oxide compounds. The usual practice is to energize the heater element by the passage of an alternating current therethrough which heats by radiation and conduction the insulating support. The metal cylinder upon which the alkalineearth metal carbonates are deposited is thereby heated to the operating temperature. Subsequently during operation of the device the active coating on the outer metal sleeve is brought to the desired operating temperature by a similar heating means.

The time interval involved for the heati v Application filed September 24, 1928. Serial No. 308,139.

-it is essential that the electrical resistivity of the refractory be high so as to avoid. leakage of the low frequency heating current across to the thermionically active surface.

' It is highly desirable in the art to materially shorten the time interval heretofore required to bring the cathode to operating temperature. It is inexpedient to accomplish this result by, increasing the filament temperature. The most expedient means is to decrease the cross sectional dimensions of the refractory material comprising the insulating support or to increase the heat conductivity of the refractory material or both.

. It is appreciated that the prior art has proposed the use of many refractory mate rials combining in part many desirable features. One of the best of these materials is thorium oxide (ThO which is claimed in copending application Serial No. 233,543, entitled Insulating material for vacuum electric devices, filed November 16, 1927, by John W. Marden and Frank H. Driggs and assigned to the same assignee as the present application.

With the development of the necessity of producing an insulating support having a smaller cross sectional area, higher heat conductivity and relatively greater electrical resistance at operating temperatures togetherwith greater transverse strength in the 7 formed insulating support member, it was found that owing to limitations of commercial production means it was inexpedient to utilize any of the heretofore mentioned refractory materials.

One of the reasons therefore, was that in utilizing the higher melting point refractory in furnaces. The transverse strength of sa1d refractory materials varied materially with the temperature and time interval of firing and to obtain the requisite strength therein in the reduced diameter desired required a firing temperature beyond ordinary practical commercial application. One means of overcoming this difiiculty has been disclosed in a copending application Serial No. 306,291 by Frank H. Driggs, entitled Refractor insulator for electron discharge devices, ffied September 15, 1928, and assigned to the same assignee as the present invention.

One of the objects of the present invention is to provide a refractory material having chemically inert qualities of the best of the heretofore roposed refractory materials and in addition thereto, be capable of being formed into insulators of relatively smaller cross sectional area than heretofore obtainable having a high transverse strength, high heat conductivity and high electrical resistance.

Another object of the present invention is to improve the operatin characteristics of indirectly heated catho es by effecting a substantial reduction in the time interval required to bring the composite cathode to operating temperatures.

Another object of this invention is to simplify the manufacturing procedure and improve operating characteristics of the electron discharge devices employing hot cathodes of the indirectly heated type.

Other objects and advantages will be apparent as the invention is more fully disclosed. I

In accordance with my invention I manufacture the refractory element of the composite cathode out of aluminum oxide (A1 0,) and may employ the extrusion method of manufacture substantially as described in the above mentioned copending application by J. W. Marden and F. H, Driggs, Serial No. 233,543, filed Nov. 16, 1927.

I have determined that in addition to the necessity of utilizing a relatively chemically inert refractory oxide of the type of thorium oxide, zirconium oxide, rare earth oxides and such other non-silica containing refractory materials as.boron nitride in the forming of said insulator supports which are substantially inert with respect to the tungsten heater element at elevated temperatures, the

material employed in order to commercially cathode, it is also limited by the available commercially practical firing temperatures.

This does not limit, however, the subsequent utilization of higher melting point refractories in a similar manner should higher temperature furnaces be available to effect substantial sintering of the refractor in accordance with the practice herein esta lished. -I have determined that it is impractical to employ as a refractory material for forming such insulators, refractory oxides which do not frit or sinter together at temperatures approximating 1400 to 17 00 (1, which is the highest available commercial furnace temperatures and obtained only in tungsten or molybdenum wound hydrogen furnaces. With insulators of thoria and zirconia fired at this temperature the addition of a lower melting constituent is'usually necessary to effect cohesion of the particles to impart sufficient strength to withstand subsequent handling without undue loss by fracture. In order to increase the electrical resistivity and heat conductivity the materials must be either fired for a prolonged period of time or fired at a higher temperature or as disclosed in the above identified copending application Serial No. 306,291 by F. H. Driggs bonded with a suflicient amount of a suitable bonding agent to substantially reduce the sintering temperature thereof within the temperature range commerciall obtainable Whereas t e prior processes have disclosed other refractory materials such as porcelain, silica; (magnesium silicate), thoria, zirconia,

, etc., none of these materials-may be formed so as to s ecifically accomplish the main object of thls invention, namely to provide an insulator material, substantially inert with respect to tungsten at elevated temperatures, having a melting point between 1400 C. and 2000 (1, a high electrical resistance at these temperatures, a high heat conductivity, and capable of being formed into insulators of relatively small cross sectional area having high transverse strength.

Through the use of aluminum oxide (A1 0 substantially chemically pure, I am enabled to produce the above identified prodnot. As a specific embodiment of my-process, I prefer to employ the unfused aluminum oxide such as may be purchased upon the market (C. P. grade) substantially free of silica, or other metal oxides and I preferably prefer to subject this oxide to a prefiring operation at temperatures approxlmating 1500 to 1600 C. before use. I do this because such oxides normally are not of the highest density and were it possible to obtain chemically pure prefused aluminum oxide uncontaminated by iron, silica, carbon and the like it would be still more desirable as on the subsequent firing of the extruded formed cylindrical insulators the shrinkage of the material would be reduced to a minimum. In lieu thereof I prefer to substantially preshrink the oxide by subjecting it to the maximum firing temperature to which it may subsequently be subjected.

Thereafter the fired oxide is given adry ball milling process to the extent that-100% of the material passes 200 mesh screen readily. Although it is not strictly essential to the successful use of aluminum oxide as an insulator I prefer to admix prior to ball milling approximately 2% of a lower melting point material such as talc, magnesium silicate, isolantite. and the like for the purpose ofassisting in the cementing of the particles during the firing process applied to the formed insulator support through the formation of a lower melting eutectic bond there- I then admix the sieved ball milled material with sufficient binder such as fiour paste, amylacetate, nitrocellulose, glycerine, linseed oil and the like, to form a plastic mass suitable for use in the extrusion process described in the above mentioned copending applica tion by J. W.,Marden and F. H. Driggs. Serial No. 233,543.

In the formation of the plastic mass of refractory material with suitable binder, I prefer to use a binder such as amylacetatenitrocellulose or flour paste, and preferably employ the flour paste.

The flour paste is formed of flour (wheat) 14%, water 79% and ammonia (NH OH, cone.) 7%. The flour is placed in a beaker and the water slowly'added thereto with constant stirring. The ammonia is then added with stirring and the mixture heated to a low temperature until the thin solution thickens and sets like a gel.

To form a plastic mass of alumina or the alumina 98% tale 2% mixture preferably employed in forming the insulators, I place the desired weight of the calcined, ball milled and sieved refractory material in a mortar and by means of a pestle work increments of the flour paste into the powder until a uniform plastic mass of the proper consistency is obtained. It is usual practice to use the least amount of flour paste possible but the amount used will vary appreciably depending upon the specific particle size, density or admixture of refractory material employed, and for material prepared as above described will vary from 35 to 40% by weight.

The plastic mass thus formed is placed in the mold and extruded'in the usual manner. a

In the event I utilize amylacetate nitrocellulose binder I substantially follow the same admixture procedure as above described with respect to flour paste. Care must be taken with this binder that the admixing and extrusion process be consecutively done in rapid order to prevent hardening of the plastic mass and loss of efficiency and materials in production. V p

The extruded material is cut in selected lengths, placed in molybdenum boats, air dried and then slowly heated to elevated temperatures to volatilize the bonding medium. Thereafter the insulators are fired for approximately three hours in a tungsten or molybdenum wound hydrogen furnace at temperatures approximating 1500 C. to 1600 C.

Throu h the use of this specific material I am ena led to form strong fracture resistant insulators having all the chemically inert qualities heretofore associated with the use of thoria, zirconia and the like, but having a diameter of 35 to 40 mil which is approximately 33 to 35% less than the smallest practical diameter obtained with said prior insulator materials under the same manufacturing conditions. In this manner I am enabled to substantially shorten the heating time interval required to raise the composite cathode to operating temperature from approximately 40 seconds to only 15 seconds.

a result not only of the decreased diameter but to the normal superior and higher inherent heat conductivity of the aluminum oxide material over refractory materials heretofore employed. In addition to thisfactor the increased proportionate amount of sintering obtained therein as compared to that obtained with other refractory materials at the same fusing temperatures, increases the heat conductivity of the material. This increased amount of sintering also decreases the porosity and increases the electrical resistance of the insulator to a maximum 1 This material saving in time is obtained as thereby substantially eliminating leakage of I The use of aluminum oxide also efiects a material saving in the cost of production and ease of manufacture of the insulator support and the composite cathodes prepared there- It may be noted that the first three refractory materials, porcelain, isolantite A and B are of the class which is reactive with respect to the heater element, whereas the other three materials are non-reactive with respect to the tungsten filament.

Under the available commercial firing temperatures, thoria insulators without the addition of bonding agents such as described in the above identified application by F. H. Driggs, Serial No. 306,291 may not be prepared at diameters below .060" as at the firing temperatures (1500 to 1600 C.) the transverse strength of the material is insufiicient to give commercially satisfactory results. Magnes a fired similarly is also'relatively weak at this diameter and when formed at lower diameters is strength to the alumina insulators similarly prepared. At the desired diameter (.0375) the alumina insulator is nearly three times as strong as the magnesia and nearly equal in strength of the other materials at .060 diameter.

This increase in strength permits the forming of theouter nickel sleeve around the in- .sulator bv mechanical. means thus permitting close thermal contact therewith and increase in the operating efficiency of the device. The

usual practice of forming the sleeve over a use of this ma terial is that the same after assembly into a I compositecathode and prior to the applica-,

tion of the thermionic coating in the nickel sleeve may be submitted to a hydrogen baking procedure at elevated temperatures to effect.

far inferior in transverse cleaning of the incorporated metal parts without deleterious effect on the insulator or.

the composite assembly;

The prefiring feature a plied exide also serves to s mplifyan rendermoreeom-j mercial the general method of production as by such procedure I am enabled to'utilize a relatively cheap and inexpensive as well as chemically pure product obtainable on the market, in preference to a rather" expensive impure fused product'such as alundum. By

prefiring the anhydrous aluminum oxide to a temperature approximately equal to the subsequent firing temperature I am enabled to accurately gau e and control the subseqlilient shrinkage o the formed article during 7 t e subsequent firing temperature, and thereby successfully manufacturingjsuch articles having highly uniform dimensional characteristics which is a highly desirable feature for this product, insuring the maximum manufacturing efiiciency.

As an example with material of the composition above described, namely prefired aluminum oxide, groundto pass 200 mesh, and admixed with approximately 2% binder, I will obtain on extrusion cylindrical insulators which on firing shrink approximate? 16% dimensionally when heated to 1550 for approximately 3 hours. I therefore em ploy an extrusion die having an opening of approximately 45 mil (.045 inch) with pins 7 for the internal holes a proximately 11 to 12 mil (.012 inches). T ese are cut into 28 to 28.5 mil (.028 inch) lengths prior to firing. At the conclusion of the firing operation the approximate size of the insulator will be 37.5 mil (.037 5 inches) i3% diameter. by 24 mil (.024 inch) length with internal heater holes having a diameter approximately 7 mil (.007 inches).

These proportions may be. varied within wide limits and I find that with this material I may reduce the diameter of the insulator support to approximately 5 times thediameter of the central apertures utilized for the heater element of the cathode, whereas with I the scope of my invention it is apparent that many variations in the specific embodiment may .be made therein without essentially departing from'the nature of the invention as outlined in the following claims.

What is claimed is I 1. An insulating support comprised of aluminum oxide approximately 98% and talc 2%.

2. An insulator for use in electron discharge devices comprised of a dense coherent body tember, 1928.

of aluminum oxide containing approximately 2% talc.

In testimony 5 subscribed m whereof, I' have hereunto y name this 17th day of Sep- CHARLES v. 'IREDELL.