Thermionic conversion means

Abstract

Claims

1. IN A THERMIONIC CONVERTER, AN ELECTRON EMITTER ADAPTED TO BE HEATED TO A TEMPERATURE TO EMIT ELECTRONS AND GENERATE THERMAL RADIATION, A THERMAL-RADIATION-TRANSPARENT AND EELCTRON-OPAQUE COLLECTOR ASSEMBLY, MEANS TO SUPPORT SAID EMITTER AND COLLECTOR ASSEMBLY IN SPACED RELATION TO FORM AN INTERELECTRODE GAP THEREBETWEEN, AND MEANS ELECTRICALLY CONNECTING SAID COLLECTOR ASSEMBLY TO SAID EMITTER.
United States Patent O THERMEONIC CONVERSION MEANS Russell G. Meyerand, Jr., and Donald W. Bell, Glastonbury, and Robert H. Bullis, West Hartford, Conn.,.as signers to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed .lune 22, 1964, Ser. No. 376,728 14 Claims. (Cl. 310-4) This invention relates to thermionic converters and more particularly to apparatus and method for increasing the performance and/ or reducing the weight of thermionic systems by reducing the net thermal radiation between the emitter and the collector of a thermionic converter, that is, by reducing7 the radiated that is absorbed by the collector assembly and has to subsequently 'be rejected from the system at the collector assembly temperature. A thermionic converter is a device in which an electron emitter, operating in alkali metal or other easily ionizable environment at a temperature of about 1400 K., produces electrons by thermionic emission and these electrons so emitted pass over to a cooler, approximately 900 K., electron collector. The electrons so collected are then passed through electrical leads through an electrical load back to the emitter. In this fashion, thermal energy is converted into electrical energy which acts upon the load to produce work. The output voltage of the device is irnproved by maintaining the work function of the collector at a lower value than that of the emitter. The proximity of the emitter or cathode to the collector or anode causes the thermal radiation, primarily infrared (micron wave lengths), from the emitter to be absorbed in t-he collector of conventional thermionic converters, thereby increasing the temperature of the collector assembly and requiring excessive cooling to maintain the collector at the desired operating temperature. It is an object of this invention to teach methods and apparatus to reduce the amount of thermal energy radiated by the emitter which is absorbed by the collector assembly and thereby reduce the total amount of thermal energy which has to be rejected from the thermionic conversion system at the collector assembly temperature. Our teaching is to either use a thermal radiation reflecting collector which will reflect the emitter generated thermal radiation back to the emitter, or to use a thermal radiation transparent collector which will directly reject, due to its transparency, the emitter -generated thermal energy. Ideally, the simplest approach to reducing the amount of emitter thermal radiation absorbed by the collector assembly is to use a low emissivity collector surface to reflect the thermal radiation from the emitter back to the emitter there-by reducing the amount of thermal energy absorbed by the collector assembly and also increasing the overall eiciency of the converter, since less heat energy is required to keep the emitter at the desired operating temperature. Unfortunately, in practice it has been found that it is not possible to directly employ a low emissivity collector surface to achieve this end, since the corrosive environment produced by the alkali metal or other ionized gas atmosphere used for space charge neutralization attacks low emissivity surfaces causing significant increases in emissivity and a concomitant increase in the thermal radiation absorbed by the collector assembly which results in a decrease in converter efliciency. Therefore, it is one object of this invention to teach method and apparatus to eliminate the problems associated with achieving near ideal collector emissivities which remain consistent with time wit-hout suffering attack from the corrosive environment in the interelectrode space of the converter. It is an additional object of this invention to teach method and apparatus to reduce the amount of heat energy that has to be rejected from the thermionic system at the collector assembly temperature by employing a system which allows direct rejection from the converter of a major portion of the emitter thermal radiation. In this type of system, the overall efficiency of the converter is reduced but this loss in efhciency is far offset by the total reduction in overall system weight which occurs primarily due to a reduction in the size of the apparatus employed for heat rejection at the collector assembly temperature. It is accordingly an object of this invention to teach apparatus and method for reducing the mount of emitter generated thermal radiation which is absorbed bythe collector assembly of the thermionic converter. It is an object of this invention to teach method and apparatus for reducing the amount of emitter generated thermal radiation which is absorbed -by the collector assembly of the thermionic diode or converter comprising placing a thermal radiation reflecting surface in intimate relation to the collector structure. l lt is still a further object of this invention to teach an electron collector or anode which is transparent to the heat radiated from the emitter but opaque to the electrons being emitted from the emitter, thereby reducing the amount of heat absorbed by the assembly. It is still a further object of this invention to teach the use of a thermal radiation transparent member to protect the thermal radiation-reflecting surface from the high temperature alkali metal or other ionized gas atmosphere used for space charge neutralization .in the thermionic converter. lt is still a further object of this invention to teach a thermal radiation transparent and electron opaque anode or collector which comprises either a very fine mesh screen or a thin conducting film placed by plating or the like on the inner side of the cell wall of a thermal radiation transparent anode. It is still a further object of this invention to teach -a thermionic electron collector which is transparent to radiated heat lbut opaque to emitted electrons and which includes a thermal reflecting surface which reflects the emitter thermal radiation back to the emitter. It is a `further object of this invention to enhance the output of thermionic converters in arc-mode operation by reflecting the thermal radiation from the collector assembly back through the neutralization plasma to the emitter since this thermal radiation, primarily the infrared radiation, contains energy in the wave lengths suitable to excite cesium or other alkali atoms from the ground state. Such excited atoms can provide the ions lrequired for space charge neutralization. Further, t-he highly reflective properties of the collector assembly surface will help to strongly trap thermal radiation of these wave lengths in the interelectrode space. As more fully explained in a paper entitled An Evaluation Orf The InRadiator Approach To Nuclear-Thermionic Space Power Systems, by Donald W. Bell and Arthur I. Chalfant presented at the June l7-2=0, 1963 summer meeting of the American Institute of Aeronautics and Astronautics and published by that organization, variations in the thermal emissivity of one or both of the thermionic electrodes can have a significant effect on the converter efliciency, although the power density is only slightly affected. A Ahighly reflective collector not only permits a significant system Weight saving but it also prevents significant changes in thermionic converter performance from occurring should the thermal emissivity of the thermionic emitter vary substantially over the system operating lifetime. The effect of such variations are illustrated in the aforementioned paper, which shows that the thermionic efficiency changes only slightly with large changes in emitted thermal emissivity in the low collector emissivity range of- 0.03 to 0.05. At higher collector emissivity values, however, the reductions in eiciency with increases in emitter emissivity become more pronounced. T-he effect of such variations in emitter emissivity with time on an operating space power system would be detrimental since the converters would be operating at off-design conditions and producing less power than desired. It is, therefore, an object of this invention to reduce or eliminate the effect of thermal emissivity changes in the emitter electrode during the life of the converter. It is still a further object of this invention to reduce changes in thermionic converter performance caused by changesl in electrode emissivities during the converter lifetime by providing at least one electrode with a thermally reflecting surface and protecting the thermally refiectingsurface from corrosion. It is still a further object of this invention to improve the degree of ionization in a thermionic converter by increasing the reflectivity of the collector and thereby increasing the number of times the thermal radiation is reected Ibetween the electrodes. Other objects and advantages will be apparent from the specification and claims and from the accompanying drawings which illust-rate an embodiment of the invention. FIG. 1 is a schematic representation of a thermionic converter using a thermal radiation reflecting collector. FIG. 2 is a schematic representation of a thcrmionic converter using another embodiment of this invention and depicts one form of thermal radiation transparent and electron opaque collector. FIG. 3 is a schematic representation of another form of this invention and depicts a second form of thermal radiation transparent and electron opaque collector. FIG. 4 isa schematic representation of a thermionic converter in which the collector assembly combines both reflection of emitter thermal radiation using a thermalradiation-transparent, corrosion-resistant member and an electron-opaque structure for the electron collector which is substantially transparent to emitter thermal radiation. Referring to FIG. l, we see thermionic converter or diode which consists of cathode or emitter 12 and anode or collector assembly 14 with electrical insulators 16 therebetween. The emitter 12 and the collector assembly 14 are supported in spaced relation to define interelectrode gap 18 therebetween. Interelectrode gap 18 is evacuated and may be filled with a gas or vapor, such as cesium or other alkali metal but not limited to these gases. During operation these gases are partially ionized to prevent or minimize the creation of an electron space charge. In operation, 4heat is added to emitter 12 to lbring the emitter, which may *be made of molybdenum or other refractory material but not limited specifically to these materials, to a temperature such as 1400 K., at which electrons are emitted -by thermionic emission from surface 20, from which they pass across interelectrode gap 18 to electron collector and then flow through electrical leads 22 and load 24 and perform useful work in passing through lo'ad 24. Due to the proximity between hot emitter 12 to cold collector assembly 14, the thermal radiation, primarily infrared, from emitter 12 is absorbed by collector assembly 14 such that conventional cooling means must be used to cool collector assembly 14. This conventional cooling means may be either fins 2'6 or heat exchanger 28 which includes coolant passed through tubes 30. This description of the construction and operation of a thermionic converter is considered to be adequate for the present purpose but greater detail with respect thereto may be had by referring to U.S. Patent No. 2,980,819 and also to Direct Conversion of Heat to Electricity, edited by Joseph Kaye and John A. Welsh, John Wiley & Sons, Inc., 1960. It is the object of this invention in one embodiment to increase the efficiency of the thermionic converter by in- Cir creasing the reflection back to the emitter of the thermal radiation from the emitter, which would otherwise be absorbed by the collector assembly. This reduces the amount of emitter generated thermal radiation absorbed by the collector assembly and thereby reduces the amount of cooling required to maintain the collector assembly at a desired operating temperature and also serves to maintain the emitter at its elevated temperature, thereby reducing the amount of heat which needs to be added thereto by an external source. As best shown in FIG. l, enhanced thermal radiation reection is accomplished by providing a low ernissivity surface, such as a highly polished silver surface, but not limited thereto, 32 behind electron collector 15 of collector assembly 14 from emitter 12. Due to the action of low emissivity surface 32, the thermal radiation from emitter 12 is retiected or transmitted by surface 32 of collector assembly 14 back to emitter 12. In this fashion, a large percentage of the emitter generated thermal radiation which would otherwise be absorbed by ,the collector assembly 14 is reflected back to the emitter. To protect thermal radiation reecting surface 32 from the alkali metal vapor or other corrosive environment in interelectrode gap 18, thermal radiation transparent window 34, made of sapphire, or other suitable material, is positioned between retiecting surface 32 and electron collector 15 of collector assembly 14. The aforementioned other suitable material from which the thermal radiation transparent window 34 may be made in addition to aluminum oxide (sapphire), include single crystal beryliurnV oxide (BeO) or single crystal magnesium oxide (MgO) or any of the materials listed in the American Institute of Physics Handbook, edition by Dwight E. Gray, second edition, published by McGraw-Hill Company in 1963 and more particularly listed in FIGURE 6c-1 on page 6-47 thereof. The thermal radiation reflecting surface 32 may be made of many materials which have suitably low emissivity, for example, the materials listed in the publication Heat Transmission by William I-I. McAdams, third edition, 1954, published by McGraw-Hill Company and more particularly shown on the emissivity table at page 472 thereof, for example, silver, copper, gold, brass, aluminum or nickel. Reflecting surface 32 may either be part of collector support plate 36 of collector assembly 14 or may be positioned between reector support plate 36 and window 34, or may be placed onto window 34. Surface 32, support plate 36, window 34 and electron collector 15 are part of electron assembly 14. Preferably electron collector 15 should be made of a semiconducting oxide so as to have a low electrical resistance and be essentially transparent to thermal radiation (primarily infrared) and not subject to chemical attack from the corrosive environment in the converter interelectrode gap 18. Removal of the electron current from the electron collector 15 may be facilitated by thin wires imbedded in said electron collector or some similar fashion. In order to reduce transport losses in the interelectrode gap 18 to a minimum, the emitter and collector assembly 14 are supported in close proximity by electrical insulating supports 16. Due in part to this proximity, the micron wave length heat being emitted from emitter 12 heats by thermal radiation, the collector assembly 14 and all of this heat absorbed by the collector must be dissipated as waste heat. The dissipation of this Waste heat requires the use of a radiator, such as a conventionally nned radiator 26, or y,eat exchanger 2S to be used in connection with collector assembly 14 to maintain the temperature thereof relatively low in comparison to the temperature of emitter 12. It is a further object of this invention to teach a collector assembly which is transparent to the heat radiated by the emitter, but opaque to the electron emission therefrom. An embodiment of a transparent anode or collector is shown in FIG. 2. Referring to FIG. 2 We see emitter 12' and transparent anode or collector assembly 14' positioned on opposite sides of interelectrode gap 18. Transparent collector assembly 14 comprises very fine screen electron collector 130 which is selected to offer small cross-sectional area to emitter radiation butto be a good conductor which draws from the interelectrode plasma high electron currents, and cell wall material 132. Screen 130 may be made of any conductor which is insensitive to chemical attack, such as copper but not limited thereto, and be fabricated of a mesh suitable for high transmission of radiation and low electrical resistivity such as solid frontal area. Cell wall material 132 is selected to be transparent to radiation typical of wave lengths from emitter radiation such that after the radiation passes through screen anode 130, it will be radiated to space or any convenient heat absorption means through the transparent cell wall material 132, which may be sapphire or other suitable material. The aforementioned other suittable material from which the thermal radiation transparent cell wall material 132 may be made in addition to aluminum oxide (sapphire), include single crystal berylium oxide (BeO) or single crystal magnesium oxide (MgO) or any of the materials listed in the American Institute of Physics Handbook, edition by Dwight E. Gray, second edition, published by McGraw-Hill Company in 1963 and more particularly listed in FIGURE 6c-1 on page 6-47 thereof. Another embodiment of a transparent anode or collector is shown in FIG. 3 and is identified as 14". In this FIG. 3 embodiment, collector assembly 14 includes therrnal radiation transparent cell wall 132', which may be made of any of the materials suggested above for use as cell wall member 132 of FIG. 2 and a thin conducting film 134 used as an electron collector, which may be made of a semiconducting oxide but not limited thereto and which is preferably maintained as thin as is consistent with conducting typical diode currents of about l0 amps/ cm.2 from this surface, is placed on the cell walladjacent to emitter 12 and has electrical leads 22 projecting therefrom. Lead wires 22 are selected to offer the smallest possible cross-sectional area to emitter radiation and essentially the transparent purpose of cell 132 will not be affected by either the thin r'ilm placed on the surface thereof or the current leads 22 connected to the film. In either the FIG. 2 or FIG. 3 configurations, cell wall members 132 and 132', respectively, may have an inrared radiation surface such as surface 32 of FIG. l on the side of cell 1vall members opposite to emitters 12 and 12, respectively. The advantage of a transparent anode system is that a smaller radiating surface is needed to reject Waste heat in space power applications because with this system a major portion of the W-aste heat is in the form of. thermal radiation which is rejected at the emitter temperature rather than at the collector assembly temperature and need not be brought down to the collector assembly temperature prior to rejection. For example, consider a conventional space thermionic power system operating at 15% etiiciency with an emitter temperature of l400 K., collector assembly temperature of 900 K. This means that 15% of the heat input to the emitter is converted to useful electricity. The remaining 85% of heat input is waste energy which must be removed from the system. Rejection of a major portion of this waste heat at the emitter temperature (l400 K.) rather than the collector assembly temperature (900 K.) will significantly reduce the weight of the radiator. It should tbe noted, however, that the overall heat input to the emitter would have to be higher since the collector is not reecting heat back to the emitter. However, the weight increase required for the higher heat input would be more than offset by the 6 Weight reduction of the radiator surface in a nuclear space power application. Since it can be shown mathematically that the radiator size varies as a lower limit inversely with the temperature of the collector raised to the fourth -power (T4), the weight penalty imposed by the radiator is one of the main contributing factors to the system weight for out-of-pile thermionic space power systems as shown by the aforementioned Bell and Chalfant paper. The performance of a thermionic power system could be increased substantially by using the construction shown in FIG. 4 in which copper collecting tins, but not limited thereto, 40 cooperate to form a transparent grid collecv tor and have radiation transparent windows 42 positioned therebetween. A highly reflective coating, such as a highly polished silver surface, is placed behind a transparent window such as sapphire 42 at surface 44 to reflect the waste heat back to emitter 12'. Transparent windows 42 serve to protect the highly reflective surface 44 from the alkali metal or other corrosive material environment existing in the gap of the converter. Thermal radiation transparent window members 42 may be made from any of the materials which were previously enumerated above as tbeing ac-ceptable for use as the thermal radiation transparent window member 34 of FIGURE 1 and the thermal radiation transparent cell wall material 132 or 132 of FIGS. 2 aind 3. With respect to FIG. 4 construction and assuming a collector assembly structure with 90% transparency and a heat discharge requirement of 30 watts/cm.2 at the front of the grid collector surface, the calculated temperature drop across a l mm. copper grid is approximately 9.5 K. according to the following formulae: KAAT Q: e 1) (s.2) (o.1)AT 30 Wartt/Cmk- AT=9..3 Kl. (3) conv-iA L EC-l-EA This represents the total emissivity for a conventional operating thermionic converter. ...iii m erm-1 8) where econ., is the emissivity of a conventional converter, emmor is the total emissivity of a mirror electrode, sc is the emissivity of the cathode, eAisthe emissivity of the anode. This represents the total emissivity for a mirror-type surface in thermionic converter operation whichhas been 7 protected from the effects of the corrosive cesium environment. Then the total grid-mirror collector emissivity assuming 90% transparency is: emirror) eoonv) 0.9( 0.0268) -l- 0.1(0.935) o.oss47 The grid-mirror collector thus provides a reduction in overall emissivity by approximately a yfactor of three, which. should result in an approximate increase in eficiency of one or two points with a reduction in space power system weight of approximately 1-3 lb./kwe. as shown by the aforementioned Bell and Chalfant paper. It would be possible to use a single layer of material on the infrared-reflecting surface if the material were infrared-transparent and an electron collector with suitable work function. It is to be understood that the invention is not limited to' the specific embodiment herein illustrated and described but may be used in other ways without departure from its spirit as defined by the following claims. We claim: 1. In a thermionic converter, an electron emitter adapted to be heated to' a temperature to emit electrons and generate thermal radiation, a thermal-radiation-transparent and electron-opaque collector assembly, means to support said emitter and collector assembly in spaced relation to form an inter-electrode gap therebetween, and means electrically connecting said collector assembly to said emitter. 2. Apparatus according to claim 1 and wherein said collector assembly includes an electron collector member comprising a fine mesh screen presenting minimal -cross-sectional area to said radiation and a good electrical conducting media to said electrons. 3. Apparatus according to claim 2 and including a radiation transparent, corrosion resistant member behind said screen. 4. Apparatus according to claim 3 wherein said member is made of sapphire. 5. Apparatus according to claim 1 wherein said collector assembly is a thin conducting film placed on a radiation transparent, corrosion resistant member. 6. Apparatus according to claim 1 wherein said collector assembly is a radiation transparent corrosion resistant member with a thin conducting lm placed thereon on the surface adjacent said emitter and including a highly radiation reflecting surface adjacent said corrosion resistant member on the side opposite said emitter. 7. In a thermionic converter, a cathode, an anode, means enveloping said cathode and anode in a controlled atmosphere, means supporting said cathode and anode in spaced relation to form an interelectrode gap therebetween, said anode comprising a fine mesh screen of small crossesectional area and good electrical conductivity positioned adjacent said cathode and a radiation transparent corrosion resistant member on the opposite side of said screen `from said cathode and a highly heat reflective surface on the opposite side of said corrosion resistant member from said cathode. 8. In a thermionic converter, an electron emitter adapted to be heated to a temperature to emit electrons vand generate thermal radiation, a collector assembly, means to support said emitter and collector assembly in spaced relation to form an -interelectrode gap therebetween, means electrically -connecting said emitter and collector assembly, said collector assembly including an electron collector member communicating with said interelectrode gap and a collector support plate attached to and supporting said electron collecting member, a thermal radiation reflecting surface positioned between said collector support plate and said electron collecting member and presenting a thermal-radiation-reflecting-surface toward said emitter, and a thermal-radiation-transparent, corrosion-resistant member positionedbetween said reflecting surface and said electron collector member so that the thermal radiation generated in said emitter is `reflected from said reecting surface back to said emitter to reduce the amount of emitter generated thermal radiation absorbed by said collector assembly. 9. In a thermionic converter, an electron emitter adapted to be heated to a temperature to emit electrons and generate thermal radiation, a collector assembly, means to support said emitter and collector assembly in spaced relation to form an interelectrode gap therebetween, means electrically connecting said collector assembly and said emitter, a cooling structure, said collector assembly including a plurality of electron collecting fins joined to form a grid-type collector in spaced relation to and exposed to said emitter and with said fins attached to said cooling structure, said collector assembly further including a plurality of thermal-radiation-transparent members joined to and extending between said fins and cooperating therewith to dei-inc a thermal-radiation-transparent, electron-opaque grid anode, and said members having a plurality of thermal radiation reecting `surfaces positioned on the side thereof away from said emitter and -facing said emitter so that the thermal radiation from said emitter passes through said radiation-transparent members to said radiation reflecting surfaces from which the thermal radiation is reflected back to the emitter, thereby reducing the amount of emitter-generated thermal radiation absorbed by the collector assembly. 10. In a thermionic converter, `an electron emitter, an electron collector assembly including an electron collector, means supporting said emitter and collector assembly in electrical isolation, means enveloping said emitter and electron collector, said collector assembly including means to prevent the absorption of emitter generated thermal radiation by the collector assembly. 11. Apparatus as in claim 10 and including a thermal radiation-transparent member between said electron collector and said reflection surface. 12. Apparatus as in claim 10 wherein said electron collector assembly is essentially transparent to thermal radiation. 13. In a thermionic converter, an electron emitter, an electron collector, means to support said emitter and collector in spaced relation to establish an interelectrode gap therebetween collector support means, a highly polished silver surface behind said collector from said emitter, and a protective sapphire window between said collector and said surface. 14. In a thermionic converter, lan electron emitter adapted to be heated to a temperature to emit electrons and generate thermal radiation, a thermal radiation nonabsorptive electron collector assembly including an electron collector, means supporting said emitter and collector assembly in electrical isolation and in spaced relation to form an interelectrode gap therebetween, means enveloping said emitter and electron collector. References Cited UNITED STATES PATENTS 2,319,912 5/1943 Anderson 313-47 3,201,618 8/1965 Coleman y310--4 3,243,612 3/1966 Lyczko 310-4 3,248,577 4/1966 Hoh 310-4 FOREIGN PATENTS 919,148 2/ 1963 Great Britain. OTHER REFERENCES Gabor, D.: A New Thermionic Generator. In nature, vol. 189, Mar. 18, 1961, pp. 868-872. MILTON O. HIRSHFIELD, Primary Examiner, D. F. DUGGAN, I. W. GIBBS, Assistant Examiners. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No $5376 ,4"5'7 April 2 1968 Russell. G Meyerand Jr et al certified that. error appears in the above numbered pathereb a It 1s y on and that the said Letters Patent should read as ent requiring correcti corrected below. Column 8 lines 38 to 40 cancel "Apparatus as in claim l0 and including a thermal radiationtransparent member between said electron collector and said reflection surface" and insert In a thermionic converter, an electron emitter, an electron collector assembly including an electron collector, means supporting said emitter and collector assembly in electrical isolation, means enveloping said emitter and electron collector, and means operatively associated with said emitter and electron collector` to reduce the total emitter generated thermal radiation absorbed by the collector assembly including a thermal radiation reflection surface operatively associated with said collector assembly and a thermal radiation transparent member between said electron collector and said reflection surface Signed and sealed this 12th day of August 1969. (SEAL) Attest: EDWARD M; PLETCHERJR. WXLLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents

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