US2734857A

US2734857A - snyder

Info

Publication number: US2734857A
Authority: US
Priority date: 1951-10-11
Publication date: 1956-02-14
Anticipated expiration: 1973-02-14
Also published as: GB763049A

Description

Feb. 14. 1956 R. B. SNYDER 2,734,857

PROCESS OF COATING A CATHODE-HEATER ELEMENT Filed 061. 11, 1951 FIG. 1

Deposit Refractory Oxide Coating Air-Dry One-Half Hour Room Temperoture Dip in Non-Aqueous Siiico Solution Air- Dry One-Half Hour Room Temperature Fire in H d Bake in Air 30 Minutes Above l7l0 0, 400 C.

Fire in Hydrogen Above |7lOC.

FIG.3

INVENTOR: RUSSELL B. SNYDER hag HIS ATT N United States Patent PROCESS OF COATING A CATHODE-HEATER ELEMENT Russell B. Snyder, Chicago, Ill., assignor to The Rauland .Corporation,: a corporation of Illinois Application October 11, 1951, Serial No. 256,911 3 Claims. (Cl. 204-181) This invention relates to cathode-heater elements for electron-discharge devices and more particularly to insulated cathode-heater elements and to processes for forming the insulating coating on such elements.

In the fabricationof cathode-heater elements for indirectly-heated cathodes of electron-discharge devices, it is necessary to provide some means of insulating the electron-emissive cathode element from its indirect-heater element. In accordance with common practice in the art, the heater element is provided with an insulating coating of refractory oxide prior to insertion in the cathode memher. This insulating coating must be hard, adherent, and relatively tough to withstand all the processing operations and handling during the fabrication of the tube. Moreover, the coating must retain its insulating properties at the high operating temperatures, usually in excess of 1000 C., of the heater element. The material most commonly used for the insulating coating is aluminum oxide, sometimes referred to as alumina or Alundum, having a minimum of impurities and of very fine particle size.

Insulating coatings of this type have .been applied to heater elements in numerous manners, as by spraying, drag coating, and the process generally referred to as cataphoretic coating. Subsequent to the application of the insulating material, the heater is fired in a hydrogen or non-oxidizing atmosphere at a temperature below the fusion point of the refractory oxide insulator. While the resultant coating is relatively tough and adherent, great care has been required in the subsequent handling to avoid undesirable flaking or peeling. Moreover, insulating coatings applied in accordance with known pro cedures are rather sensitive to vibration, and tubes employing heater elements having coatings of this type are consequently susceptible to premature failure in certain applications.

It is an important object of the present invention to provide a new and improved cathode-heater element having an insulating coating which is tougher, harder, less brittle, and which better withstands vibration than those known heretofore.

It is a further object of the invention to provide a new and improved process for applying an insulating coating to a cathodeheater element.

In accordance with the invention, a new and improved process of forming an insulating coating on a cathodeheater element for an electron-discharge device comprises the steps of cataphoretically depositing on the heater element a coating comprising an electrically insulating refractory oxide, dipping the coated heater element in an alcohol solution of an organic silicate, and subsequently firing the coated heater element in a non-oxidizing atmosphere at a temperature above the fusing point of silica and below both the boiling point of silica and the boiling point of the refractory oxide.

The features of the present invention which are believed to be novel are set forth with particularity in the or in any other equivalent manner.

2 appended claims. The invention, together with further objects and advantages .thereof, may bestbe understood, however, by referenceto the following description taken in connection with the accompanying drawing, in .the several figures of which like reference numerals indicate like elements, .and .in which: I

Figure 1 is aflow diagram or process chart of a new and improved method of forming an insulating coating in accordance with the present invention;

Figure 2 is aside elevation of a new and improved cathode-heater element-constructed in accordance with the invention; and

Figure 3 is an enlarged cross-sectional view taken along the line 3-3 of Figure 2.

A cathode-heater element for an electron-discharge device may be provided with an.insulatingicoating -having the desired properties in accordance with theprocedure set forth diagrammatically in Figure 1. The metallic base member, having a high' resistivity and usually constructed either of tungsten or an alloy of tungsten :and molybdenum, is first provided with .a refractoryoxide coating in a known manner. For example, the well known cataphoretic deposition process may be employed. In accordance with this process, fine particles of aluminum oxide are suspended .ina suitable liquid medium such .as water and methanol containing small :amounts .of hydrated aluminum nitrate and hydrated magnesium nitrate. A unidirectional potential difference, which may be of the order of volts, is applied between the heater element .to be coated and another electrode which are immersed in the coating suspension. In the presence of the appliedpotential difference, the :aluminum'nitrate hydrolyses to aluminum hydroxide and nitric acid, and the aluminum hydroxide is cataphorized or caused to migrate to the heater element. In the process, particles of alumina are trapped on the surface of the heater element by the aluminum hydroxide which acts as a temporary binder.

When the coating thus deposited'has built up to the desired thickness, the heater element is removed from the coating suspension and dried in air for about one-half hour at room temperature to permit the coating to set on the base metal. At this stage in the process, the coated heater element is in precisely the same condition as those produced by conventional methods immediately prior to the hydrogen firing operation.

In accordance with the present invention, the heater element bearing the cataphorized insulating coating of alumina is dipped in a non-aqueous silica solution, such as a solution of ethyl ortho-silicate or other alkyl ester of silicic acid in an alcohol solvent. Alternatively, the non-aqueous silica solution may be applied by spraying The amount of non-aqueous silica solution thus applied is not critical; in practice, the coated heater element is preferably im mersed in the silica solution and immediately withdrawn, it being only necessary to moisten :therefractory oxide coating. The moistened coating is then permitted to dry in air at room temperature for about one half hour. Hydrolysis of the silica solution and subsequent evaporation of the solvent leaves substantially pure silica finely dispersed throughout the particles of refractory oxide.

At this stage of the procedure, either of two courses may be pursued. According to one alternative, the heater element may be directly fired in a non-oxidizing atmosphere, such as hydrogen, an inert gas, or in a vacuum, at a temperature above the fusing point of silica (about 1710 C.) and below both the boiling point of silica and the boiling point of the refractory oxide employed. By the firing operation, the silica is fused to serve as a binder for the insulating coating. The firing a 3 operation may be accomplished with the aid of a radiofrequency induction heating field if desired.

According to, the other alternative, following the application of the'non-aqueous silica solution, the coated complished much more readily prior to the fusion of the silica bond than afterwards. The heater element may be fired in a non-oxidizing atmosphere, as described above, at any subsequent stage in the assembly operation or the firing operation may be omitted entirely in whch case the fusion of the silica binder is accomplished during the initial operation of the completed tube by passage of heater current.

While the process has been described in connection with the use of pulverulent Alundum or aluminum oxide, other refractory oxides may be employed as the insulating material. The refractory oxides comprise, in addition toalumina, the oxides of zirconium, beryllium, thorium, and magnesium which are known respectively as zirconia, berrylia, thoria, and magnesia. Certain of these oxides are not readily adaptable to cataphoretic deposition, but other well known processes such as drag coating and spraying may be employed to apply the insulating material to the base metal. In the event that one of these latter processes is employed, the refractory oxide may be suspended in the non-aqueous silica solution so that both are applied simultaneously, or the ethyl ortho-silicate or other non-aqueous silica solution may be applied after formation of the refractory oxide coating.

Forming of the heater element may be accomplished .at any stage in the process prior to the firing operation,

although this may most readily be accomplished prior to the commencement of the coating process.

The significance of the particle size of the refractory oxide is Well known in the art. In general, it is essential I that the refractory oxide be pulverulent in the form of discrete particles of 50 microns or less in diameter, an average particle size of to microns being most satisfactory. Coarse particles usually result in soft coatings which are apt to yield slightly and separate when the wire is bent in forming or handling. On the other I hand, fine particles result in hard brittle coatings which often rupture if the wire is sharply bent.

A cathode-heater element constructed in accordance with the present invention is illustrated in Figures 2 and 3 and comprises a metallic base member 10 of tungsten or tungsten-molybdenum alloy, coated with a thin layer 11 of refractory oxide bonded with fused silica.

The invention is applicable to all types of cathodeheater elements, regardless of the physical configuration and the type of ultimate apparatus in which the heater is to be employed. Specifically, heater elements of various forms coated in accordance with the invention may 4 beemployed in high-vacuum receiving tubes, cathoderay tubes, gas filled tubes such as thyratrons and the like, and special purpose tubes. The coating is tough, hard and durable and may be inserted into the cathode proper with little or no risk of damage. In the completed apparatus, the risk of failure caused by damage to the coating from even the most extreme vibration is substantially obviated.

While the process has been described as employing a single refractory oxide, it is apparent that the insulating material may comprise two or more of the refractory oxides if desired. Consequently, the term refractory oxide is employed in the appended claims in both the singular and plural sense.

While particular embodiments of the present invention have been shown and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. The process of forming an insulating coating on a cathode-heater element for an electron-discharge device which comprises the steps of: cataphoretically depositing on said heater element an electrically insulating refractory oxide coating; dipping said coated heater element in an alcohol solution of an organic silicate; and subsequently firing said coated heater element in a nonoxidizing atmosphere at a temperature above the fusing point of silica and below both the boiling point of silica and the boiling point of said refractory oxide.

2. The process of forming an insulating coating on a cathode-heater element for an electron-discharge device which comprises the steps of: cataphoretically depositing on said heater element an electrically insulating refractory oxide coating; dipping said coated heater element in a solution of ethyl ortho-silicate in an alcohol solvent; and subsequently firing said coated heater element in a non-oxidizing atmosphere at a temperature above the fusing point of silica and below both the boiling point of silica and the boiling point of said refractory oxide.

3. The process of forming an insulating coating on a cathode-heater element for an electron-discharge device which comprises the steps of: cataphoretically depositing on said heater element an aluminum oxide coating; drying said coated heater element; dipping said coated heater element in a solution of ethyl ortho-silicate in an alcohol solvent; drying said heater element; and subsequently firing said coated heater element in a non-oxidizing atmosphere at a temperature above the fusing point of silica and below both the boiling point of silica and the boiling point of said aluminum oxide.

References Cited in the file of this patent UNITED STATES PATENTS 1,874,542 Ka -.11 Aug. 30, 1932 1,926,407 Ruben Sept. 12, 1933 2,158,665 ONeill May 16, 1939 2,307,0l8 Cardell Ian. 5, 1943 2,348,045 Wooten May 2, 1944 2,552,535 De Santis May 15, 1951