US3551727A - Thermionic converter having a low work function collector electrode - Google Patents

Description

United States Patent lnventors Appl. No. Filed Patented Assignee THERMIONIC CONVERTER HAVING A LOW WORK FUNCTION COLLECTOR ELECTRODE References Cited UNITED STATES PATENTS Cobine Micks Oxley et al.. Coleman Webster Cohen Meyerand et al.. Katz et al Kerstetter et al.

Primary Examiner-John W. Huckert Assistant ExaminerAndrew J. James Attorneys-Joseph l. Hirsch, Ronald Zibelli and Paul Enlau 3l3/311X 3l3/3llX 313/341X 310/4 313/3l1X 310/4 5 Claims, 3 Drawing Figs.

[1.8. CI 313/346, 313/3111310/4 Int. Cl. HOlj 1/14, 2 HOlj 19/06 Field ofSearch 313/311, 346;310/4, 10,11

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THERMIONIC CONVERTER HAVING A LOW WORK FUNCTION COLLECTOR ELECTRODE BACKGROUND OF THE INVENTION 1. Field of the Invention The field of attto which the present invention pertains is broadly low work function electrodes and methods of forming them. A particular field of art in which such electrodes are used is thennionic converters.

emitted therefrom into the vapor to support an electrical discharge. Suchelectrons are then received by an anode or colector electrode and then returned to the cathode via an external electrical circuit. In effect, such device converts a portion of the heat energy supplied to the emitter electrode to electrical energy because of the difference between the high work function" of the emitter electrode and the low work function" of thetollector electrode. As used herein, the term work function" of the electrode may be defined simply as the energy required to completely remove an electron from the surface of the electrode. In the case of thermionic converters, the work function of the electrode is substantially determined by the character of the surface of the electrode and the vapor utilized. At present, the vapor normally-used in thermionic converters is an alkali metal vapor such as cesium vapor and the electrode material is a refractory metal, usually tungsten or rhenium. Thus, a combination of cesium vapor with a rhenium collector pennits high temperature operation and achieves a relatively low work function of about 1.5 electron volts for the collector, both of which are necessary for efficient operation of the thermionic converter. For example, with such collector work function of about 1,5electron volts, a typical voltage output for a thermionic converter is 0.7 volts for 28 ampere per cm. load. But, if the collector work function" can be reduced by just 0.1 electron volts, then the output voltage can be correspondingly increased by 0.1 volts and such voltage'increase repi'esents a 14 percent increase in the efficiency of the thermionic converter. Consequently, substantial research effort has been expended to produce electrode materials which will provide a low work function; however, to date, low work function" materials have not been developed which can withstand the severe operating conditions such as are present in thermionic converters. For example, it has been found that thin films of insulated metal oxides such as aluminum oxide achieve a work function of 1.1 electron volts with cesium vapor, but such films could not be operated at high temperature, i.e., at temperatures above about 150C.

SUMMARY OF THE INVENTION Consequently, an object of the present invention is a high temperature, low work function composite surface for electrodes. Anotherobject of the present invention is a thermionic converter which includes a collector electrode having a high temperature, low work function" composite surface. Still another object of the present invention is a method of forming a'high temperature, low work function" composite surface.

In general, the present invention involves a composite surface comprisinga thin layer of refractory metal containing by weight less thanl percent insulating metal oxide. The present invention also involves a collector electrode of a thermionic converter whicli'has a composite surface comprising a thin layer of refractory metal containing by weight less then 1 percent insulating ,metal oxide. Finally, the present invention involves the method of forming such surface by forming a vapor deposition filament of refractory metal and positioning the surface substrate adjacent and exposed to the filament. Then a plate of insulating metal oxide is positioned adjacent and exposed to both filament and substrate. The filament is heated to evaporate the refractory metal and deposit it on the substrate and to evaporate and deposit a portion of the insulating metal oxide with the refractory metal on the substrate.

BRIEF DESCRIPTION OF THE DRAWING In order to facilitate understanding of the present invention reference will now be made to the appended drawing of a preferred specific embodiment of the apparatus used in the method of the present invention. Such drawing should not be construed as limiting the invention, which is properly set forth in the appended claims. In the drawings:

FIG. 1 is a cross-sectional view of the specific embodiment of the apparatus used in the method of the present invention shown in FIG. 2 taken along the line 1-1 of FIG. 2.

FIG. 2 is a cross-sectional view of the apparatus shown in FIG. 1 taken along line 2-2 of FIG. I.

FIG. 3 is a cross-sectional view of a thermionic converter constructed according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention involves in its broadest aspects a high temperature, low work function composite surface comprising a thin layer of refractory metal containing by weight less than 1 percent insulating metal oxide. Such layer should be less than 0.01 inches thick and preferably have a thickness in the range of about 0.0005 inches to 0.002 inches. Refractory metals which may be used in the composite surface include rhenium, tungsten, molybdenum, tantalum, with rhenium being preferred. The insulating metal oxide which may be used in the composite surface include aluminum oxide, beryllium oxide, and magnesium oxide, with aluminum oxide being preferred. The concentration by weight of the insulating metal oxide in the refractory metal layer should be less than 1 percent and is, preferably, in the range of about 0.10 percent to 0.20 percent. The composite surface should be formed as a layer on a suitable refractory metal substrate such as molybdenum.

A specific application of the above described composite surface is in a thermionic converter having an emitter electrode, a collector electrode, and a vapor contained between such electrodes and adapted to support an electrical discharge therebetween. In such thermionic converter, a collector electrode having the above described composite surface and comprising a thin layer of refractory metal and containing by weight less than 1 percent insulating metal oxide, provides a substantially improved performance, i.e., the voltage output and operating efficiency of the thermionic converter are substantially increased.

The method of forming the high temperature, low work function composite surface of the present invention is illustrated by the apparatus shown in FIGS. 1 and 2. As shown therein, the apparatus 10 includes a flat coil 11 of rhenium with its leads 12 extending perpendicular to the plane of the coil 11 and to the rear of the coil 11. The coil 11 is supported by its leads 12 being mounted on and extending through a plate 13 of aluminum oxide positioned adjacent to and parallel to the coil 11. Mounted on the plate 13 by struts 14 is an inner cylindrical tantalum shield 15 which extends parallel to the leads 12 and surrounds the leads l2 and coil 11. The coil 11, leads 12, plate 13, and inner shield 15 are in turn mounted within an outer cylindrical tantalum shield 16 by attaching the plate 13 to the outer shield 16 by tabs 17. The substrate 20 to be coated by the apparatus 10 is positioned adjacent and parallel to the coil 11 on the opposite side of the plate 13, i.e., at the front of the coil 11.

Thus, the method of forming the composite surface of the present invention involves forming a vapor deposition filament of a refractory metal such as coil 11. Then, the surface substrate 20 is positioned adjacent and exposed to such filament. Also, a plate 13 of insulating metal oxide is positioned adjacent and exposed both to the filament l1 and the substrate 20. Finally, the filament 11 is heated to evaporate and deposit both the refractory metal and the insulating metal oxide on the substrate so that a composite surface is deposited on the substrate.

A specific example of the method and composite surface of the present invention involves positioning a molybdenum substrate approximately oneeighth inch from one side of a rhenium coil and an aluminum oxide plate approximately one-half inch from the opposite side of said coil. The coil is then heated in a vacuum to 2,700C. for about one-half hour. The resulting rhenium film deposited on a molybdenum substrate had a thickness of approximately 0.001 inches and contained approximately 0.13 percent aluminum oxide by weight. Such composite surface'was then tested by including the surface into an electron emission test vehicle. The results of such testing showed that the composite surface exhibited a workfunction of 1.2 electron volts. For comparison, a rhenium metal surface was formed under the same conditions except the aluminum oxide plate was absent so that there was no resulting aluminum oxide in the deposited surface. The resulting measured work function was 1.5 electron volts.

In FIG. 3 there is shown a thermionic converter 21. The thermionic converter 21 includes a collector electrode 22 having the above described composite surface, an emitter electrode 23 spaced apart from the collector electrode and a vapor 24 contained in the space between the collector electrode 22 and the emitter electrode 23 and adapted to support an electrical discharge therebetween.

There are a number of features of the present invention which clearly show the significant advance the present invention represents over the prior art. However, only a few of the more outstanding features will be pointed out to illustrate the unexpected and unusual results obtained by the present invention. One feature of the present invention is the high temperature, low work function" composite surface which may be used on the collector electrode of a thermionic converter and which is stable despite the severe operating conditions to which it is subjected. Because of such feature, a substantial inintended that the invention be I imite'd thereto. All substitutio'ns, alterations, and modifications of the present invention which come within the scope of the claims or to which the present invention is readily susceptiblewithoutdeparting from the spirit and scope of this disclosure are considered part of the present invention.

We claim: I

1. In a thermionic converter comprising an emitter electrode, a collector electrode and a vapor contained between said electrodes and adapted to support the electrical discharge therebetween, the improvements comprising a collector elec; trode having a composite surface, comprising a thin layer of refractory metal containing by weight less than 1 percent of an insulating metal oxide.

2. A thermionic converter as stated in claim 1 wherein said layer is less than about 0.01 inches thick.

3. A thermionic converter as stated in claim 1 wherein said refractory metal is rhenium.

4. A thermionic converter as stated in claim 1 wherein said insulating metal oxide is aluminum oxide.

5. A thermionic converter as stated in claim 1 and wherein the refractory metal is rhenium, the insulating metal oxide is aluminum oxide and the concentration by weight of the aluminum oxide is in the range of about 0.10 percent to 0.20 percent.

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