US3401297A

US3401297A - Thermionic cathodes for electron discharge devices with improved refractory metal heater wires

Info

Publication number: US3401297A
Application number: US481818A
Authority: US
Inventors: Feinleib Morris
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1965-08-23
Filing date: 1965-08-23
Publication date: 1968-09-10
Anticipated expiration: 1985-09-10
Also published as: DE1589024A1; GB1142728A; DE1589024B2

Description

P 1968 M. FEINLEIB THERMIONIC CATHODES FOR ELECTRON DISCHARGE DEVICES WITH IMPROVED REFRACTORY METAL HEATER WIRES Filed Aug. 25, 1965 TEMPERATURE c BRIGHTNESS INVENTOR.

MORRIS FEINLEIB ORNEY United States Patent O THERMIONIC CATHODES FOR ELECTRON DIS- CHARGE DEVIQES WITH IMPROVED REFRAC- TORY METAL HEATER WIRES Morris Feinleib, Los Altos, Calif, assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Aug. 23, 1965, Ser. No. 481,818 8 Claims. (Cl. 313-340) ABSTRACT OF THE DISCLOSURE The present invention provides tube and cathode designers with improved refractory metal heater wires and improved thermionic cathodes by the utilization of a plurality of minute refractory metal particles on the heater wire itself either in a bare or non-insulated case or in a coated or refractory insulated type of heater wire. The improved heater wire operates at a reduction in heater wire operating temperature for a given input power and hence increased longevity and emissivity for the heaters. In addition, the roughened surface of the heater wire enhances the adherence of the refractory insulation coating on the heater wire.

This invention relates in general to electron discharge devices, and more particularly to improved thermionic cathodes incorporating refractory metal heater wires having a plurality of refractory metal particles disposed thereon.

Of particular interest to designers of electron discharge devices such as pentodes, triodes, klystrons, traveling wave tubes, etc. is the thermionic emission portion thereof and in particular the cathode assembly which may be of any of the conventional types such as dispenser cathodes, oxide cathode, etc., generic to each of which is the incorporation therein of a refractory metal heater wire element utilized to initiate thermionic emission from the electron emission surface thereof.

The present invention provides tube and cathode designers with improved refractory metal heater wires and improved thermionic cathodes by the utilization of a plurality of minute refractory metal particles on exterior surface on the heater wire itself either in a bare or noninsulated case or in a coated or refractory insulated type of heater wire. The advantages to be derived from the utilization of the teachings of the present invention with regard to providing a plurality of minute refractory metal particles on the heater wire surface in a bare or noninsulated heater type are reduction in heater wire operating temperature for a given input power and hence increased longevity and emissivity for such bare heaters. The advantages to be derived from roughening a bare or as received refractory metal heater wire and applying a refractory insulation coating thereon either by conventional cataphoretic deposition techniques or by chemical vapor deposition techniques is a somewhat similar reduction in heater operating temperature for a given input power and a considerable enhancement in the refractory insulation coating itself with regard both to its adherence n the heater wire itself and with regard to its physical condition.

The present invention particularly teaches the application of minute refractory metal particles on a bare or as received heater wire as a preferred means of roughening the heater wire to achieve the aforementioned beneficial advantages in both noninsulated and insulated cases.

It is therefore an object of the present invention to provide an improved thermionic cathode for electron discharge devices.

A feature of the present invention is the provision of a thermionic cathode incorporating a refractory metal heater wire having a plurality of refractory metal particles deposited on the bare wire surface.

Another feature of the present invention is the provision of a novel thermionic cathode heater assembly and method of making the same wherein the refractory metal heater wipe is coated with a plurality of refractory metal partic es.

Another feature of the present invention is the provision of a novel thermionic cathode heater assembly and method of making the same wherein a refractory metal heater wire is coated with a plurality of refractory metal particles which in turn are provided with a refractory insulation coating.

These and other features and advantages of the present invention will become more apparent upon perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a fragmentary cross-sectional view partly in elevation depicting an electron discharge device of the klystron type incorporating the improved thermionic cathode of the present invention.

FIG. 2 is a cross-sectional view partly in elevation of the thermionic cathode embodied in the device of FIG. 1 taken along the lines 2-2 in the direction of the arrows.

FIG. 3 is a cross-sectional view of an enlarged refractory metal heater wire having a plurality of minute refractory metal particles deposited thereon according to the teaching of the present invention.

FIG. 4 is an enlarged cross-sectional view partly in elevation of the improved refractory insulation coated refractory metal heater shown in FIG. 2 taken along the lines 44 in the direction of the arrows.

FIG. 5 is an illustrative graphical portrayal depicting input power vs. temperature for a bare as received refractory metal heater relative to a bare refractory metal heater having a plurality of refractory metal particles deposited thereon.

Referring now to FIG. 1 there is depicted an electron discharge device 6 of the klystron type incorporating a thermionic cathode which incorporates the teachings of the present invention. The klystron 6, depicted in FIG. 1, is of a conventional reflex type incorporating a standard refiector assembly 7 disposed at the downstream end portion thereof and mounted on a main body block 8 which incorporates a thermionic cathode assembly 9 at the upstream end portion thereof. A conventional tuner mechanism 10 is utilized to vary the resonant frequency of re-entrant cavity 11 in a manner well known in the art. Electromagnetic wave energy is extracted via output waveguide 12 through an alumina or the like dielectric vacuum sealed window assembly 13. For the particular details of such a high frequency electron discharge device see US. Patent No. 3,097,323 by F. L. Salisbury issued July 9, 1963 and assigned to the same assignee as the present invention.

Turning now to FIG. 2 there is shown in cross-sectional view partly in elevation a thermionic cathode assembly 9 depicted in the exemplary electron discharge device 6 of FIG. 1. The thermionic cathode assembly 9 depicted in FIG. 2 is of the type which is generically known as the oxide cathode and is utilized as a representative example of conventional thermionic cathodes utilizing refractory metal heater wires such as the impregnated dispenser cathode. Thermionic cathodes such as the oxide cathode 9 incorporate an electron emission surface 16 disposed in heat transfer relationship with respect to a refractory metal heater wire such as the coated wire 17 dipsosed within any suitable tubular refractory metal mounting member 18. The insulated or coated refractory metal heater 17 depicted in FIG. 2 is preferably of tungsten Wire 19 and can also be made of rhenium and rhenium-tungsten alloys. The tungsten heater wire 19 is provided with a coating of a plurality of minute refractory metal para ticles 20 as best seen in FIGS. 3 and 4, and in the in sulated or coated type of heater an exterior refractory insulation coating of alumina (A1 is provided thereon.

Separate mounting means may be utilized to support the refractory metal heater within the support sleeve 13.- See for example US. Patent 2,990,495 by R. S. Spencer issued June 27, 1961, and assigned to the same assignee as the present invention wherein an uncoated or bare refractory metal heater is supported within a tubular mounting sleeve and spaced therefrom. The coated heater wire 17 when provided with an intermediate layer composed of a plurality of refractory metal particles such as tungsten, molybdenum or rhenium as shown best in FIGS. 3 and 4 has been found to result in several mechanical and thermal advantages in comparison to prior art thermionic heaters incorporating an alumina or the like refractory insulation coating on a bare or as received heater.

Turning to FIG. 5, an illustrative graphical portrayal of the thermal emissivity improvements and enhanced or reduced operating temperature improvements achieved by the deposition of a plurality of refractory metal particles on a refractory metal heater is depicted. Curve A represents a simple coiled tungsten heater without the refractory metal particles deposited thereon having a wire diameter of approximately .006 inch and Curve B represents a similar tungsten wire provided with a plurality of tungsten particles having approximately 1 micron diameters and total thickness of approximately .0005 inch to .001 inch deposited thereon. Acceptable results can be obtained with thicknesses which are less than .0005 inch, such as .0003 inch although we have found the aforementioned range to produce good results. In order to achieve a meaningful comparison between the two, both heaters were positioned in a Bell jar and optical pyrometer readings were taken for various input powers. The significant reduction in optical pyrometer or brightness temperature for the roughened heater represented by Curve B in comparison with the unroughened or bare heater represented by Curve A are self-evident. Hence for any given input power in watts a significant reduction in heater wire temperature is achieved and hence heater wire life is increased correspondingly and the thermal emissivity characteristics thereof are enhanced correspondingly.

It has been determined that considerable mechanical advantages are also derived with the provision of the plurality of refractory metal particles on the bare heater wire in conjunction with a coated heater such as 17 as depicted in FIG. 4. Problems have always been encountered in coated heaters having refractory insulation coatings 21 of alumina or the like in conjunction with cracking thereof and poor adherence to a bare heater wire. It has been determined that the provision of the refractory metal particles 20 results in considerable reduction in observed Cracking of either cataphoretically deposited or chemical vapor deposited (CVD) alumina coatings. Furthermore, much better adherence of such alumina refractory insulation coatings has been observed in practice when a plurality of refractory metal particles have been deposited on the bare wire to form an intermediate layer. This has been experimentally verified by utilizing tweezers etc. in trying to strip off the alumina insulation coating. Furthermore, the thermal emissivity characteristics of the roughened heater wires with the refractory insulation coatings thereon have still resulted in observed brightness temperatures considerably reduced in comparison to the coated smooth wires. Although the observed reduction in brightness temperature for a given input power has not been as great as in the simple roughened bare wire heater case wherein reductions in brightness temperatures as much as 200 C. or better have been observed, experimental results have shown that reductions in brightness temperature for a given input power of around 120 C. for the cataphoretically deposited alumina and reductions of around 100 C. for the CVD deposited alumina result.

Examples of methods for applying the plurality of minute refractory metal particles of tungsten, rhenium, mo-

lybdenum, or an alloy thereof, to a tungsten or rhenium or an alloy thereof refractory metal heater are the followmg:

A simple dipping process wherein the bare refractory metal heater is dipped into a solution of tungsten powder, xylene and methacrylate may be used. The bare wire is dipped a sufficient number of times so as to completely cover the outer surface thereof in the same manner as taught in co-pending US. application Ser. No. 340,478, by Paul W. Camenzind filed J an. 27, 1964, and assigned to the same assignee as the present invention. The proportions of tungsten powder and xylene in the solution are not critical. The percentage by weight of methacrylate solution should be preferably between 0.1%5% preferably substantially 0.3%. The percentage of tungsten powder can be between 35 and 75 percent by weight, the balance of the solution being made up of xylene. The percentage of tungsten powder is preferably in the order of 60% by weight. The tungsten particles are preferably around one micron average diameter and preferably are less than 200 mesh for optimum results. The coated bare heater wire is fired in a controlled inert or reducing atmosphere such as a hydrogen furnace at around 1500 C. for approximately 1 minute after which the heater has been found ready for utilization.

Another method taught by the present invention for applying a plurality of refractory metal particles on a refractory metal heater is the following cataphoretic deposition technique. The suspension for cataphoretic coating includes around 12 grams tungsten powder made by the Wah-Chang Company (denoted as Wah-Chang grade C-3); approximately 0.4 gram of yttrium nitrate approximately 0.6 gram of H 0; approximately cc. of methanol and an additional 0.1 gram of aluminum nitrate Al(NO .9I-I O with an applied DC potential of around 50 volts in a deposition chamber. The above parameters have been found quite satisfactory for achieving good particle coatings in approximately 4 to 5 seconds. The tungsten heater is the cathode; the anode is made of aluminum. Agitation is used to keep the particles in suspension. A coating thickness ranging from .0005 inch to .001 inch is achieved. After the particles are applied to the bare heater wire, the coating is sin-tered such as by firing in around 1650" C. for approximately 30 minutes in a wet disassociated ammonia atmosphere. Good results have also been achieved by redipping the sintered roughened refractory metal heaters into a solution of approximately 4 grams of H WO in 25 cc. of concentrated NH OH, dried, and refired at around 1650 C. with the objective of achieving further consolidation of the coating such as to improve the resistance to flaking as well as the adherence of the particles both to each other and to the bare refractory metal heater wire.

After the heater has been roughened (preferably by the aforementioned latter method, the refractory insulation coating is then applied for coated heaters. This refractory insulation coating has a thickness for cataphoretically deposited alumina of preferably between .002 and .003 inch and for CVD alumina of preferably between .001 and .002 inch. A suitable catap-horetic deposition method for alumina can be found in the following work, The Oxide Coated Cathode by G. Herrmann, and S. Wagener, vol. 1, Chapman and Hall, Ltd., 1951, particularly pages 43 to 53. A suitable method for chemically vapor depositing alumina on the roughened tungsten heater wire can be found in the following article, The Deposition of Oxide on Silicon by the Reaction of a Metal Halide With a Hydrogen-Carbon Dioxide Mixture by S. K. Tung and R. E. Caffrey, Transactions of the Metallurgical Society of AIME, pages 572577, vol. 233, March 1965.

In the case of the coated heaters wherein, generally speaking, the coated heater wire itself is snugly fitted within a refractory metal support tube such as a nickel tube 18 shown in FIG. 2, thermal conduction as well as thermal radiation heating is employed for transferring heat from the heater wire to the nickel cathode button 15. Since radiation emissivity characteristics of the bare heater wire are extremely important the transfer of ratdiant energy from the coated heater to the back or the absorption side of the button is extremely important and it is also very important that the heater not be shorted out such as might occur if flaking and cracking of the alumina coating occurred in use. In both the cataphoretic and CVD deposited cases wherein an intermediate roughened layer was utilized as shown in FIG. 4 between the refractory insulation and the bare refractory metal heater wire improved adherence of the alumina coating to the Wire was achieved as Well as less cracking of the alumina insulation coating in use. Naturally, the improved thermal emissivity characteristics as exemplified by the reduced operating temperature encountered in use at various input powers as exemplified in FIG. 5 speaks fo-r itself with regard to increased longevity and improved thermal radiation characteristics of coated heaters produced according to the techniques of the present invention.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed:

1. A thermionic cathode assembly for electron discharge devices including a cathode mounting sleeve, a thermionic cathode mounted on said cathode mounting sleeve, a refractory metal heater wire disposed within said cathode mounting sleeve in a heat transfer relationship with respect to said thermionic cathode, and means to roughen the surface of said heater Wire and to improve the radiation of heat therefrom, said means comprising a plurality of minute refractory metal particles bonded to said surface, adjacent ones of said particles being in mutually contacting relationship whereby said heater wire operating temperature is lowered in use and whereby said heater wire emissivity is enhanced in use.

2. The thermionic cathode assembly defined in claim 1 including a refractory insulation coating disposed about said heater wire having said plurality of minute refractory metal particles bonded thereto whereby a refractory insulation coated refractory metal heater is achieved.

3. The thermionic cathode assembly defined in claim 1 wherein said plurality of refractory metal particles bonded to said bare refractory metal heater wire form a coating of at least .003 inch thick and wherein said plurality of refractory metal particles have a particle size which is less than 200 mesh.

4. A thermionic cathode assembly for utilization in electron discharge devices including a thermionic cathode mounting sleeve, a thermionic cathode mounted on said mounted sleeve, a refractory metal heater wire of tungsten disposed within said cathode mounting sleeve in heat transfer relationship with respect to said thermionic cathode, said tungsten heater Wire having a layer of minute tungsten particles disposed on the surface thereof and forming a particle coating thereon whereby said heater Wire surface is roughened, adjacent ones of said particles being in mutual-1y contacting relationship and a layer of refractory insulation material over said layer of minute tungsten particles.

5. A thermionic cathode assembly for utilization in electron devices including a cathode mounting member, a thermionic cathode mounted on said cathode mounting member, a refractory insulation coated refractory metal heater wire disposed within said cathode mounting member in heat transfer relationship with respect to said thermionic cathode, said refractory insulation coated thermionic heater wire including a refractory metal core made of a metal selected from the group consisting of tungsten and rhenium and alloys thereof, and an intermediate layer of particles of material selected from the group consisting of tungsten, rhenium, molybdenum and alloys thereof, adjacent ones of said particles being in mutually contacting relationship and an exterior layer of refractory insulation material disposed on said intermediate particle coating, said refractory insulation coating being made of alumina.

6. The thermionic cathode assembly defined in claim 5 wherein said intermediate layer of refractory metal particles has a thickness falling Within the following range; .0005 inch to .001 inch, and wherein said alumina refractory insulation coating has a thickness falling within the following range: .001 inch to .003 inch.

7. A thermionic cathode assembly for utilization in electron discharge devices including a nickel cathode button, said nickel cathode button having a thermionic emissive material disposed thereon, a nickel tubular support sleeve bonded to said nickel button, a coiled tungsten heater wire disposed within said nickel support sleeve and intimately contacting the internal surface of said nickel support sleeve, said coiled tungsten heater wire being provided with a plurality of minute tungsten particles disposed in a layer on the bare heater Wire surface, adjacent ones of said particles being in mutually contacting relationship and an alumina refractory insulation coating disposed in intimate contact and surrounding said particle coated heater Wire.

8. A thermionic cathode assembly for utilization in high frequency electron discharge devices including a hollow tubular cathode mounting sleeve, a cathode electron emission surface disposed on said sleeve, a refractory metal heater wire disposed within said sleeve in heat transfer relationship With respect to said cathode electron emission surface, said refractory metal heater Wire having a roughened surface composed of a layer of minute refractory metal particles sintered on the heater wire, adjacent ones of said particles being in mutually contacting relationship said plurality of minute refractory metal particles having average diameters of less than 200 mesh, and a refractory insulation coating disposed on said particles whereby an insulation coated heater Wire is formed.

References Cited UNITED STATES PATENTS 3,029,360 4/1962 Etter 313-340 3,161,540 12/1964 Kingsley et al. 117-217 3,195,004 7/1965 Hassett 313-311 X 3,227,911 1/1966 Heil 313-340 X 3,268,305 8/1966 Hagadorn et al. 313-355 X 3,328,201 6/1967 Scheible 117-217 X r JOHN W. HUCKERT, Primary Examiner.

R. F. POLISSACK, Assistant Examiner.