US3551708A - Heat shielded thermionic converter - Google Patents

Description

United States Patent (72] Inventor Robert R. Hobson San Jose, Calif. [21] Appl. No. 686,112 {22] Filed Nov. 28, 1967 [45] Patented Dec. 29, 1970 [73] Assignee General Electric Company a corporation of New York [54] HEAT SHIELDED THERMIONIC CONVERTER 16 Claims, 7 Drawing Figs.

[52] U.S. Cl 310/4 [51] 1101j 45/00 [50] Field of Search 310/4; 196/39; 165/104 [56] References Cited UNITED STATES PATENTS 3,322,978 5/1967 Lary etal. 310/4 3,393,329 7/1968 Janner 310/4 38 I0 23 I I2 FORElGN PATENTS 1,358,001 3/1964 France 1,421,323 11/1965 France 6,409,864 3/1965 Netherlands.

Primary Examiner-Milton O. Hirshfield Assistant Examiner-D. F. Duggan Attorneys-Allen E. Amgott, Henry W. Kaufmann, Paul F. Prestia, Melvin M. Goldenberg, Oscar B. Waddell and Frank L. Neuhauser PATENTEUnmzsmm a SHEET 3 UF 6 NVENTOR ROEJHQT iQ. HOEWN ATTORNEY BY/Z PATENIEU UEC29 I976 SHiET l BF 6 ENVENTOQ mm m. LwW W %A KM ATTORNEY PATENTED [152291970 5517 O8 SHEET 5 OF 6 as, I0 23 Ii 12 '17 I3 39 58 swam w; RMMDM ATTORNEY PATENTEUnaczsrsm I 34551708 SHEET 6 [1F 6 MVENTGR WMQT R. WWW

Y mam ;\TTORNEY HEAT SHIELDED TI'IERMIONIC CONVERTER BACKGROUND OF THE INVENTION A thermionic converter is an electrical power generating device including an emitter electrode having an electron emissive surface of relatively high work function closely spaced adjacent the electron collective surface of relatively low work function on a collector electrode. The space between the electrodes is evacuated or filled with a suitable gas at low pressure. The electron emissive surface is operated at a high temperature, typically above l400 C. resulting in copious electron emission to the collector electrode which is operated at a much lower temperature, typically from about 700 C. to about l000 C., thus producing current flow. Such thermionic converters have inherently low output potential. A plurality of these converters are often connected in series to give a desirably higher output potential.

It is often difficult to maintain the emitter electrode at the high temperature necessary for operation. Also, there are problems of excessive heat transfer from the emitter electrode to the collector electrode during operation and of removing heat from the collector electrode to maintain the desired temperature difference between the two electrodes.

Nuclear fuel has been found to be a desirable heat source for thermionic converters. The fuel may either be a radioisotope which generates sufficient heat as it decays or it may be a fissionable material which generates sufficient heat when subjected to a high neutron flux. This type of thermionic converter is further described by Lawrence in U.S. Pat. No. 3,176,165 and by Hobson in copending U.S. Pat. application Ser. No. 517,392, both assigned to the assignee of the present invention.

In an often especially desirable configuration, the emitter electrode is in the form of a cylinder or tube enclosing the heat source with the electron emissive surface on the exterior surface of the tube and the collector electrode is in the form of a coaxial tube surrounding the emitter electrode and having the electron collecting surface on the interior wall of the tube. This is a highly compact arrangement. However, where the heat source is a radio-isotope, the cylindrical emitter electrode containing the isotope often has too much surface area for the nuclear heat to raise the emitter surface to the high temperature required for operation. A similar problem exists for many other heat sources, such as flame or hot gases introduced into a tubular emitter electrode. In each case, the emitter electrode cools too rapidly both by electron cooling and thermal radiation cooling.

Attempts have been made to conserve heat by physically separating a large heat source from the emitter electrode, and conducting the heat to the emitter electrode through straps of conductive material. The resulting structure, however, is large, complex and expensive and is inefficient due to heat losses along the path from heat source to emitter electrode.

Thermionic converters using radio-isotopes as heat sources are especially desirable since they are entirely self-contained and portable. Thus, there is a continuing need for improvements in thermionic converters to permit the use of lower power density heat sources.

It is, therefore, an object of this invention to provide a thermionic converter overcoming the above-noted problems.

It is another object of this invention to provide a thermionic converter capable of operating efficiently with low power density heat sources.

Another object of this invention is to decrease transfer of heat from the emitter electrode to the collector electrode of a thermionic converter.

The above objects, and others, are accomplished in accordance with this invention in which the surface area of the collector electrode adjacent the emitter electrode is reduced. Heat shield means are provided adjacent the emitter electrode in those areas from which the collector electrode surface has been eliminated.

A converter with small collector areas surrounded by electrically insulated heat shields operates as though it has collector and emitter areas approximately equal to the collector area. The decrease in heat loss from the emitter electrodeis sufficient to permit the use of many radio-isotopes and other heat sources which previously had insufficient heat density to operate a thermionic converter.

Any suitable radio-isotope in elemental or compound form may be used as the heat source. Pu O is preferred because it has a high melting point, a long halflife and is easily shielded. Typical radio-isotopes fuels include Pu Cm Pm", Th", Th Cm and mixtures thereof, an elemental or compound form.

Where the thermionic converter is to be subjected to high neutron flux, such as iii a nuclear reactor, any suitable fuel which generates heat under such conditions may be used. Typical materials include uranium dioxide, uranium carbide, plutonium carbide, t'horium carbide, and mixtures thereof.

The emitter electrode may comprise any suitable material such as tungsten, rhenium or molybdenum. If desired, the electron emissive surface of the emitter electrode may be coated with a highly emissive material such as barium oxide, strontium oxide, thorium oxide and mixtures thereof.

Any suitable material may be used for the collector electrode. Typical materials include stainless steel, niobium, nickel, titanium, zirconium, molybdenum and mixtures or alloys thereof.

The space between the two electrodes may be evacuated or filled with an alkali metal vapor, such as cesium.

Any suitable heat shielding material may be used to fill the recesses in the collector electrode between the discrete electron collective surface areas. It has been found that a stack made up of a plurality of highly reflective spaced metal foils, arranged parallel to the surface of the emitter electrode and electrically insulated from the collector electrode is highly effective. From about 10 to about foils may be used in the stack. Analytical studies show best results will be obtained with from about 10 to about 70 foils. An optimum balance between increasing heat losses at the shield edge with a greater number of foils and increasing heat losses through the stack with fewer foils is found at about 35 foils. Preferably, each foil has a thickness of from about 0.5 to about 2 mils, with a surface coating of an insulator such as alumina or yttria having a thickness of from about 0.5 to about 2 mils. The foils may be made from any suitable high melting temperature material. Typical materials include tungsten, tantalum, rhenium, and mixtures or alloys thereof.

The electron collective surface of the collector electrode should be reduced sufficiently to conserve emitter electrode heat sufficiently to produce efficient operation of the thermionic converter. It has been found that beneficial results are obtained where the ratio of electron emissive surface area to electron collective surface are from about 3:1 to about 50:1. Best results have been obtained where this ratio is from about 5: lto about 20:1.

The invention will be further understood upon reference to the drawings, wherein:

FIG. 1a shows a simple schematic representation of a thermionic converter according to the prior art;

FIG. lb shows a simple schematic representation of the thermionic converter of FIG. 1a modified in accordance with the present invention;

FIG. 2 shows a schematic representation, partially broken away, of a preferred embodiment of a thermionic converter;

FIG. 3 shows an alternative collector electrode configuration useful in a thermionic converter such as is shown in FIG. 2;

FIG. 4 shows an overall view ofa single cell thermionic converter power supply;

FIG. 5 shows a section through the converter of FIG. 4 showing the reduced-area collector; and

FIG. 6 shows a section through the converter of FIG. 4 showing a second embodiment of the reduced-area collector.

Referring now to FIG. 111 there is shown a conventional thermionic converter including an emitter electrode having an electron emissive surface 11 spaced adjacent the electron collective surface 12 of a collector electrode 13. Casing 14 supports emitter electrode 10 and encloses collector electrode 13. Collector electrode 13 is held adjacent to emitter electrode 10 by a support (not shown). The space between the electrodes, which is generally on the order of 0.005 inch, may be evacuated or, preferably, may contain an alkali metal vapor, such as cesium.

Emitter electrode 10 may be formed of any suitable material. Where the emitter electrode is heated externally, it may consist of any suitable metal. Typical metals include tungsten, tantalum, molybdenum, copper, silver, iron, gold, nickel or mixtures thereof. Where the heat is to be produced by nuclear fuel, the emitter electrode may consist of the fuel itself, or may consist of a metal wall of a container holding the fuel.

Collector electrode 13 is preferably made of a material having a desirably low work function relative to the work function of the electron emissive surface 11. Typical low work function materials which may be used where suitable include stainless steel, alloys of niobium and titanium, molybdenum, alloys of zirconium, or the like.

In the simple device shown in FIG. 1a, openings 15 and 16 are provided in the emitter electrode 10 and collector electrode 13, respectively, for temperature measuring devices (not shown).

FIG. lb shows a thermionic converter of the general type shown in FIG. la, modified in accordance with the present invention, Here, the area of electron collective surface 12 adjacent electron emissive surface 11 is greatly reduced. In addition, heat shielding means, generally designated 17, covers the areas adjacent to electron emissive surface 11 which are not covered by electron collective surface 12. Any suitable heat shielding material may be used. In this exemplary instance, heat shield 17 includes an insulating layer 18 of an electrical and thermal insulating material, such as alumina or yttria. This layer may be formed by coating a metal washer 19 with a mixture of the powdered insulating material, then placing the coated washer in the system. Several reflective foil washers 20 are then placed over metal washer 19. The foil washers 20 are dimpled or pinpricked to reduce thermal contact. The foil washers 20 may be held in place by means of a simple frame 21. Preferably, collector electrode 13 has a shoulder 22 extending slightly over the heat shield 17 to avoid having a black body crack between the collector electrode and heat shield 17 which would undesirably absorb heat from electron emissive surface 11.

The thermionic converters of FIG. la and 1b are both operated by heating emitter electrode 10 to temperatures in the range from about 1000 C. to about 1800 C. while maintaining collector electrode 13 at a temperature of from about 400 C. to about 900 C. When thermionic converters such as shown in FIG. 1a and 1b are thus operated, it is found that considerably more heat need be supplied to the emitter electrode in the embodiment of FIG. la than in that of FIG. 1b. Also, it is found that a greater amount of heat must be removed from the collector electrode in the embodiment of FIG. 1a than in that of FIG. lb to maintain the collector electrode 13 at the desired lower temperature. Only slight loss in electrical output is observed with the smaller collector surface area. In the embodiment of FIG. 1a the emitter surface is radiating heat to a much larger surface than is desirable for efficient operation.

A multicell embodiment which is especially preferred where the heat source is a radio-isotope is shown in FIG. 2. In this embodiment, the emitter electrode 10 is in the form of a closed end tube which contains the heat generating fuel 23. Adjacent electron emissive surface 11 on emitter electrode 10 are discrete raised portions of collector electrode 13. These raised portions are similar to those shown in FIG. lb. Each of these raised portions of collector electrode 13 has an electron collector surface 12 spaced from and conforming to electron emissive surface 11. Several individual thermionic converters are shown in FIG. 2 in axial alignment separated by conductive support members 24. These are electrically connected in series to raise the ultimate output voltage of the entire assembly. Heat shield means 17 similar to that described in FIG. 1b is included in the recessed areas between the raised portions of collector electrode 13. Heat shield17 rests on a layer of an insulating material 31 such as alumina. Heat shield means 25 is provided at the ends of tubular emittei electrode 10 to decrease heat transfer to. conductive support members 24. These conductive support members 24 hold collector electrodes 13 in longitudinal position, support and space emitter electrodes 10 and also serve to electrically connect each collector electrode in series to the next emitter electrode. Insulating layer 30 insulates conductive support member 24 from the prior collector electrode. Axial supports 26, which are conductive, serve to mechanically and electrically connect emitter electrode 10 to conductive supports 24. Typically, heat shield 25 may consist of a plurality of spaced reflective metal foils. Axial supports 26 maintain tubular emitter electrodes 10 in axial alignment. Also, pins 27, mounted on rings 28, supported on insulating layer 31 are provided to space emitter electrode 10 to maintain the desired spacing between the electrodes. As discussed above, the space between the electrodes may be evacuated or filled with an alkali metal vapor. The entire assembly is mounted in a casing 29 which is insulated from collector electrodes 13 by an insulating layer 30, which may be any suitable insulating material, such as alumina. Collector electrode 13 and heat shields 17 may be assembled by placing the heat shield inside a perforated tube, then inserting the raised portions into the perforations to form the collector electrode. I

,FIG. 3 shows an alternative embodiment of the collector electrode. This embodiment is suitable for use, for example, in the thermionic converter shown in FIG. 2. Of course,it could be adapted for use in a converter having other configurations; e.g. planar rather than cylindrical.

In FIG. 3, collector electrode 13 includes electron collective surfaces area 12 in an axial array. Between these surfaces are the heat shields 17, shown cut away. These heat shields may be the plural spaced metal foils 20 electrically insulated by insulating layer 18 as are described above. Also, the raised collector surface 12 could be formed as radial or diagonal, rather than axial, strips.

FIG. 4 shows an overall view ofa single cell thermionic converter power supply useful, for example, as a power source in outer space. Emitter electrode 10 is in the form of a cylinder containing a heat generating material, such as a radio-isotope. Collector electrode 13 surrounds emitter electrode 10. Collector electrode 13 is electrically connected to output lead 32 by electrical conductors 33 and conductive ring 34. Emitter electrode 10 is electrically connected to output lead 35 through conductor 36. A cesium reservoir 37 is provided to feed cesium vapor to the interelectrode gap. Storage tanks 38, are provided to accumulate off-gases, e.g. helium, which are generated by the decaying isotope. The entire assembly is supported by rods 39 and support plate 40. It can be seen that this is a compact, high efficient assembly which may typically have dimensions of about 4 by 14 inches.

FIG. 5 shows a cut away section of the thermionic converter of FIG. 4 showing an embodiment of an electrode assembly overcoming the prior problems.

Here, electron collective surface 12 is in the form ofa single radial raised ringlike surface. Heat shield means 17 covers the remainder of collector electrode 13 and the ends of emitter electrode 10. A disc 41 covers and supports the heat shield 17 at the ends of emitter electrode 10. The assembly of collector electrode 13, heat shield 17 and disc 41 are supported by radial seal insulators 42 at each end. The cesium vapor which fills the interelectrode gap is admitted through cesium supply tube 43. Filters 44 permit passage of the off-gases from emitter electrode 10 to storage tanks 38 but prevent transfer of fuel 23 to the storage tanks. As is apparent, this is a simple, reliable and lightweight system. I

FIG. 6 shows a second embodiment of an electrode arrangement suitable for use in a thermionic converter power supply such as that shown in FIG. 4. This embodiment is generally similar to that of FIG. 5, with an alternative collector electrode and heat shield arrangement. Here, electron collective surface areas 12 are in the form of two radial rings at the ends of cylindrical emitter electrode 10. Between these two rings and at the ends of emitter electrode are located heat shields 17. Between the two rings, behind heat shield 17, collector electrode 13 is relatively thin. Immediately behind electron collective surfaces 12 are located heavy heat dissipating rings 45. These rings 45 may be finned, if desired, to aid in dissipating heat. Additional heat shield mean 46 is located outside collector electrode I3 between the two electron collective surfaces 12. Heat shield 46 is supported by extensions 47 of collector electrode 13. This embodiment has advantages of short electrical and heat paths through emitter electrode 10 to the surface adjacent electron collective surfaces 12. The heat paths are indicated by arrows 48. The equivalent electrical and heat paths in the embodiment of FIG. 5 are relatively long. In addition, the embodiment of FIG. 6 exhibits excellent heat shielding between electron collective surface rings 12, with good heat dissipation in the area of said rings. On the other hand, the embodiment of FIG. 5 is simple and easily constructed. Each may be advantageous in a given application.

The following examples further define preferred embodiments of the thermionic converter of the present invention. Parts and percentages are by weight unless other wise indicated.

EXAMPLE I A multicell thermionic converter is constructed as shown schematically in FIG. 2. Each emitter electrode is in the form of a cylindrical container having a length of about 4.5 inches and a diameter of about 2.2 inches. These containers are formed from tungsten tubing having a thickness of about 0.05 inch. A cylindrical pellet of 90 percent dense Pu O is con tained within each container, with about 5 percent radial and 10 percent axial clearance between the pellet and the container. The isotope has a fresh power density of 3.9 W/cm. Since the half life of this isotope is 89 years, the power density at the end of one year is about 3.87 W/cm. The total area of the small discrete collector surfaces opposite each emitter is one-fifth the emitter surface area. These collector surface areas are spaced about 0.005 inch from the emitter surface. The remaining area opposite each emitter is recessed and filled with a reflective heat shield. The heat shields consist of a thin insulating layer of M 0 separating a spaced stack of twenty 2 mil molybdenum foil sheets from the collector electrode. The foil sheets are dimpled with pinpricks to reduce thermal contact. In addition, several spaced molybdenum foil sheets are placed adjacent the ends of the emitter electrodes to reduce heat loss there. A supply of cesium vapor is maintained in the interelectrode space. The emitter electrodes are operated at a temperature of about l820 K with the collector electrodes at about 975 K. It is found that this converter produces about 7.6 amps/cm of emitter surface at about 0.8

volts, from each cell, giving an energy conversion efficiency of about 12.3 percent. Where twenty cells are connected in series, the output potential is about 14 volts.

EXAMPLE [I example, the total area of the discrete collector surface areas is about one-tenth of the emitter electrode surface. with the recessed intervening areas heat shielded as inFxample I. It is found that each cell in this converter produces about 24 amps/cm of emitter area, at a potential of about 0.5 volts, with an energy conversion efficiency of about 10 percent. Thus, 10 cells connected in series produce an output potential of about 5 volts.

EXAMPLE III A single cell thermionic converter is constructed as shown in FIGS. 4 and 5. The converter has an overall length of about 14%; inches, with a diameter of about 43/ l 6 inches. A cylindrical pellet of Pu0 having a diameter of about 2.5 inches and a length of about 2.9 inches is encased in a tungsten can having a wall thickness of about 0.3 inches. Spaced adjacent this emitter electrode is a nickel collective surface as shown in FIG. 5. The electron collective surface adjacent the emitter surface has an area about one-ninth the emitter surface area. The heat shield is made up of 35 tungsten foils, each about 1 mil thick and coated with about a 1 mil layer of alumina. The interelectrode space is about 0.005 inch. This space is filled with cesium vapor. The emitter electrode is operated at a temperature of about l820 K, with the collector electrode at about 900 K. This converter is found to produce about 5.2 amps/cm of emitter surface at an output potential of about 0.8 volts, giving an energy conversion efficiency of about 12 percent.

Although specific materials, components and proportions have been shown in the above descriptions of typical embodiments, other suitable materials and arrangements, as indicated above, may be used with similar results. Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

Iclaim:

l. A thermionic converter comprising an emitter electrode having an electron emissive surface for thermal emission of electrons therefrom; a collector electrode having an electron collective surface spaced from and facing said electron emissive surface for receiving said electrons; said electron collective surface being smaller in area than said electron emissive surface and consisting of a plurality of discrete areas, said collector electrode being recessed between said areas, said recesses containing heat shield means facing said electron emissive surface whereby heat transfer from said emitter electrode to said collector electrode is reduced.

2. The thermionic converter of claim 1 including heating means to heat said emitter electrode.

3. The thermionic converter ofclaim 2 wherein said heating means comprises a radio-isotope.

4. The thermionic converter of claim 1 wherein said recesses undercut said areas so that said areas overlap said heat shield means.

5. The thermionic converter of claim 1 wherein said emitter electrode has a cylindrical exterior surface and said electron collection surface is spaced adjacent to said cylindrical exterior surface and substantially conforms thereto.

6. The thermionic converter of claim 1 wherein the ratio of electron emissive surface to electron collective surface is from about 3:1 to about 50:1.

7. The thermionic converter of claim 1 wherein said heat shield means comprises a plurality of substantially spaced reflective surfaces electrically insulated from said collector electrode.

8. The thermionic converter of claim l wherein a plurality of said emitter and collector electrode pairs are electrically connected in series to raise the output potential of the resulting array.

9. A thermionic converter comprising an emitter electrode having an electron emissive surface for thermal emission of electrons therefrom; a collector electrode having an electron collective surface spaced adjacent said electron emissive surface for receiving said electrons; said electron collective surface consisting of at least one discrete area having a total surface area from about one-fiftieth to about one-third the electron emissive surface area, the remaining area of said collector electrode being recessed, said recess containing heat shield means whereby heat transfer from said emitter electrode to said collector electrode is reduced.

10. The thermionic converter of claim 9 including heating means to heat said emitter electrode.

11. The thermionic converter. of claim 10 wherein said heating means comprises a radio-isotope.

12. The thermionic converter of claim 9 wherein said recesses undercut said areas so that said areas overlap said heat shield means.

13. The thermionic converter of claim 9 wherein said emitter electrode has a cylindrical exterior surface and said connected in series to raise the output potential of the resulting array.

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