US 3691421 A
Description (OCR text may contain errors)
United States Patent Decker et al.
[451 Sept. 12, 1972  DOUBLED LAYER HEATER COATING FOR ELECTRON DISCHARGE DEVICE 72 Inventors: John J. Decker, Donald R.
Kerstetter, both of Emporium, Pa.
 Assignee: GTE Sylvania Incorporated  Filed: July 15, 1971  Appl. No.: 162,992
 US. Cl. ..3l3l345, 117/217, 313/270, 313/337, 313/340  Int. Cl ..H0lj l/l4, H01j 19/06  Field of Search ..313/337, 340, 270, 345; 117/217  References Cited UNITED STATES PATENTS 3,134,691 5/1964 Kopecky ..313/340 X 3,195,004 7/1965 Hassett ..3l3/345 X 3,490,944 l/1970 Almer et a1. ..313/337 UX 3,500,454 3/1970 Emerick ..3l3/337 X Primary Examiner-David Schonberg Assistant Examiner-Toby H. Kusmer Attorney-Norman J. OMalley et al.
[5 7] ABSTRACT A double layer heater coating on a wire support comprises a first layer of aluminum oxide and an overcoating layer comprising particles of aluminum oxide, zirconium oxide or beryllium oxide overcoated with metallic tungsten which in turn is overcoated with a layer of adifferent material whereby oxidation of the tungsten is substantially prevented. The different material can be aluminum oxide, zirconium oxide, platinum, or iridium.
5 Claims, 2 Drawing Figures rmmznsmz m2 3.691.421
IN VENTORS JOHN J. DECKER & DONALD R. KERSTETTER BY WMQW ATTORNEY DOUBLED LAYER HEATER COATING FOR ELECTRON DISCHARGE DEVICE BACKGROUND OF THE INVENTION This invention relates to heaters for electron discharge device thermionic cathodes and more particularly to such heaters having a dark outer surface to improve heat radiation therefrom. Heaters having a dark outer surface have been made which include OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, an object of this invention to obviate the disadvantages of the prior art.
It is another object of this invention to enhance the performance and life of dark heaters.
The objects are accomplished in one aspect of the invention by a heater which comprises a wire structure having an insulative coating thereon which is comprised of aluminum oxide (A1 0 Overlying the aluminum oxide coating is a second layer comprised of particles selected from the group consisting of aluminum oxide, zirconium oxide and beryllium oxide, which particles are coated with metallic tungsten. Further, to substantially prevent oxidation of the tungsten, each of the particles has over the tungsten coating a coating of a different material that has a melting temperature above l,700C. Such a material can be selected from the group consisting of platinum, iridium, aluminum oxide and zirconium oxide.
Heaters of the type described above perform as well or better than the prior art type of dark heaters and have the additional advantage that the tungsten is substantially prevented from oxidizing, thus maintaining the dark color more uniformly throughout its life.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a sectional view of a portion of a heater showing the double layered coating; and
FIG. 2 is a greatly enlarged sectional view showing one of the particles of the second layer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the present invention together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in conjunction with the above-described drawings.
Referring now to the invention with greater particularity, in FIG. 1 is shown a section of a heater constructed in accordance with one aspect of the invention which comprises a wire structure or filament 10 of tungsten or molybdenum having a first layer or coating 12 thereon consisting of aluminum oxide (A1 0 The aluminum oxide is an electrically insulating material and has long been used for this purpose. As a matter of fact, many heaters utilize only this coating. However,
the aluminum oxide coating is white and its heat radiating characteristics are such that it is often necessary to operate the heater at a temperature 300 to 600C hotter than its associated cathode. To overcome this problem and lower the heater temperature while increasing the heat radiation therefrom a thermally dark layer 14 is applied over the layer 12. The layer 14 comprises particles 16 (see FIG. 2) of aluminum oxide, zirconium oxide, or beryllium oxide. The particles 16 are overcoated with metallic tungsten l8 and the tungsten layer 18 is in turn overcoated with a different material 20, the last named material being selected from the group of platinum, iridium, aluminum oxide and zirconium oxide, which substantially prevents oxidation of the tungsten. The preferred particle material 16 is aluminum oxide and the preferred material 20 is platinum.
To achieve the desired thermal radiation characteristics from the dark heater, it has been found that the tungsten should be present in an amount of about 2 to 2.5 percent or more by weight of the particles 16. Likewise, to achieve adequate oxidation protection for the tungsten, the layer 20 should be present in an amount of about 1 to 1.5 percent or more, this being particularly true when the layer 20 is platinum or iridium since any excess of material greater than necessary to perform its function would add to the cost.
To prepare the coated particles 16, metallic soaps are used.
For example, a 2 percent tungsten-l percent platinum coated particle can be made as follows:
To 36.8 gms of tungsten soap containing 22.2 percent by weight (such as tungsten resinate) add ml toluene as a dilutent. Then add 400 gm of alu minum oxide very slowly and mix until the particles are uniformly coated with the soap. Dry and coated particles at C for 4 hours to evaporate the solvent (stirring occasionally with a spatula to prevent crust formation). After drying, the soap coated mixture is sieved through a 100 mesh sieve and fired in air at 500 to 550C for 1 hour to remove the organic portion of the soap. This firing leaves a layer of tungsten oxide on the particle. After the air firing the particles are fired in dry dissociated ammonia for 15 minutes at 875C to reduce the tungsten oxide to metallic tungsten.
The tungsten coated particles (which now weigh about 408 gms) are now added to 17 gms of a platinum soap containing 26 percent Pt by weight (such as platinum resinate) which has been diluted with 100 ml toluene. Again the particles are added slowly and mixed until they are uniformly coated with the soap. Following the above-described steps of drying, sieving, firing in air and firing in dissociated ammonia are performed. While it is probable that the platinum coats out as the metal rather than the oxide, it is possible that the air firing could oxidize some of the tungsten; therefore, the final firing will reduce any oxides so formed.
The particles 16 can have a 2% percent tungsten layer and 1% percent platinum layer applied by the same process by using 45 gms of tungsten soap and 24.5 gms of platinum soap. Suitable soaps for either purpuse are Englehard No. 8629 Tungsten Resinate and Englehard No. 9450 Platinum Resinate.
The tungsten-platinum coated particles can be applied over the previously applied aluminum oxide layer by dip coating. A suitable dip coating suspension can be made as follows:
In a pebble mill mix 300 gms W-Pt coated Al O 200 ml butyl acetate 200 ml 3.5 percent solution 60 sec. nitrocellulose lacquer in amyl acetate.
Mill for hours at 60 RPM.
It will be seen from the above that there is herein provided a new and novel heater for electron discharge device thermionic cathodes. The darkened outer layer has good thermal radiation due to the tungsten and the coating over the tungsten protects the same from oxidation.
While specific examples have been given only with respect to aluminum oxide particles, it will be obvious to those skilled in the art that zirconium or beryllium oxides could be used in lieu thereof. As for plating or coating from a platinum soap, an iridium soap could be used; however, the cost would increase. For overcoating the tungsten coated particles with aluminum oxide or zirconium oxide soaps of aluminum or zirconium can be used, such as aluminum or zirconium resinate.
While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.
1. A heater for use with an indirectly heated cathode comprising a wire structure having an insulative coating thereon, said coating having a first layer in contact with said wire consisting of aluminum oxide and a second layer in contact with said first layer, said second layer comprising particles of material selected from the group consisting of aluminum oxide, zirconium oxide and beryllium oxide substantially each of said particles being substantially coated with metallic tungsten, and a coating of a different material over said tungsten whereby oxidation of said tungsten is substantially prevented, said different material having a melting temperature above about 1,700C.
2. The heater of claim 1 wherein said particles of material for said second layer are aluminum oxide.
3. The heater of claim 1 wherein said different material is selected from the group consisting of platinum, iridium, aluminum oxide and zirconium oxide.
4. The heater of claim 3 wherein said tungsten is present in the amount of about 2 to 2.5 percent by weight of said particles.
5. The heater of claim 4 wherein said platinum is present in the amount of about 1 to 1.5 percent by weight of said particles.