US 3175105 A
Description (OCR text may contain errors)
March 23, 1965 J. CREEDON ETAL 3,175,105
CONVERSION OF HEAT TO ELECTRICITY Filed July 28, 1961 HEAT Q i 7 g 4 RL 8 n 1' I? O O D c U U v H M13 INVENTORS,
JOHN E. CREEDON SOL SCHNEIDER 8 MORTIMER H. Z/NN.
A T TORNE Y.
United States Patent 3,175,105 CONVERSION OF HEAT T0 ELECTRIClTY John E. Creedon and Sol Schneider, Little Silver, and
Mortimer H. Zinn, West Long Branch, N..l., assignors to the United States of America as represented by the Secretary of the Army Filed July 28, 1961, Ser. No. 127,747 4 Claims. (Cl. 310-4) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.
This invention relates to the conversion of heat to elec tricity and more particularly to such conversion by means of electric discharge devices of the thermionic converter type.
Thermionic converters utilize the Edison effect to convert heat directly to electricity without the aid of moving parts or chemical reactions. Although these devices have been known for many years, the increased need for lightweight, reliable power supplies for satellites and portable field equipment has revived interest in this means for generating electricity.
A thermionic converter is essentially a diode type discharge device comprising an electron emitting cathode and an electron collecting anode. If the work function of the cathode is higher than that of the anode, power will be delivered to an external load circuit on the application of sufiicient heat to the cathode to cause thermionic emission. Such an elementary thermionic converter is inefiicient because only a small percentage of the emitted electrons reach the anode, the rest falling back to the cathode. The reason for this inefficiency is two-fold (1) no ac celerating anode voltage is available for attracting the electrons and, (2) the presence of a cloud of free electrons in the discharge space creates an electric field tending to repel electrons being emitted from the cathode. This phenomena is known as the space charge effect. The result is that emitted electrons must climb up a potential hill in order to reach the anode and only those electrons which leave the cathode at high velocity have sufficient kinetic energy to do this.
Attempts have been made in the past to overcome the space charge effect by such means as extremely small electrode spacing but this results in practical difliculties because of the close tolerances required. Further, it has been suggested in the prior art to neutralize the space charge by projecting an easily ionizable vapor into the discharge space. The vapor molecules, usually those of an alkali metal, are ionized by contact with the hot cathode. While this method reduces the space charge effect, it requires an auxiliary reservoir of alkali metal which must be thermally controlled to produce the desired amount of vapor.
The instant invention provides for eficient neutralization of space charge in a thermionic converter by novel and simple means. It is therefore an object of this invention to provide a practical and eflicient source of electrical power of the thermionic converter type.
It is a further object of this invention to provide a novel and useful two stage thermionic converter which combines two diverse discharge chambers in a manner which results in a high output power and high Carnot efiiciency.
Other objects and advantages of the invention will become apparent from the following detailed description and drawing, which is a pictorial representation of the device.
The drawing shows a two-stage thermionic converter comprising two discharge chambers 5 and 11, formed by three parallel electrodes 3, 7 and 15, and insulated side walls 4. Electrode 3 forms the cathode of the first stage iatented Mar. 23, 1965 of the converter and the top surface of intermediate elec trode 7 forms the anode thereof. The lower surface of 7 forms the cathode of the second stage of the converter and the lower electrode 15 is the anode thereof. A source of heat is applied to electrode 3, resulting in a negative temperature gradient from the top to the bottom of the device. If electrode 3 is selected to have a high work function, electrode 15 a low Work function and electrode 7 a work function intermediate the other two, power will be delivered to external load R Alternatively, the upper and lower surfaces of intermediate electrode 7 may be given different Work functions by depositing activating agents such as thorium or the oxides of barium and strontium on the electrode surfaces. The work functions are chosen to give maximum efficiency at the operating temperature.
Each of the chambers of the device is provided with self-contained means for neutralizing space charge. The principle of operation depends on the direct production of neutralizing ions by means of an ionic cathode which emits a copious supply of ions when heated. These ion emitters are similar to conventional electron emitting cathodes and can be characterized by the Richardson equation. It has been found that an eificient ion emitter consists of one of the metallic alkali alumina-silicates. At practical operating temperatures, ion emission of one to ten milliamperes per square centimeter can be obtained from these compositions. The particular alkali used determines the type of ion emitted. Although each positive ion emitted carries the same charge as a single electron. it is capable of neutralizing the field due to several hundred electrons because it remains in the discharge space much longer than does an electron because of its relatively large mass and resultant low velocity.
Referring again to the drawing, both surfaces of intermediate electrode 7 are provided with a plurality of beads, 9 and 13, of the ion emitting material specified above. The use of a plurality of small beads of ion emitting material, distributed over both surfaces of the intermediate electrode, assures an even distribution of neutralizing positive ions in both discharge chambers. While the ionic cathode may be located anywhere within the chambers, it has been found that the temperature of the center electrode is in the best range for thermionic ion emission. The total ion emitting area is chosen to provide sufiicient ions to neutralize the space charge and to replenish ion losses. Ion loss occurs due to recombination caused by electron-ion collisions and due to absorption by the electrodes and chamber walls. In accordance with one feature of the invention, ion losses in the upper chamber are reduced by selecting a type of ion and a cathode with a work function which permits contact ionization of any ions or re-combined atoms which strike the cathode. If the ionization potential of the neutralizing ions is less than the Work function of the cathode 3, contact ionization is possible provided the cathode temperature is sufiiciently high. Under these conditions, any ions or re-combined atoms which strike cathode 3 will be returned to the discharge space as ions, thus reducing ion losses. Due to the temperature gradient in the device, the cathode of the lower chamber will ordinarily not be at sufiiciently high temperature to permit contact ionization.
Other means may be provided for reducing ion loss, thereby reducing the amount of ion emitting material required on the intermediate electrode. For example, the alumino-silicates of lithium have been found to provide a very stable ion emitter. Further, lithium has the lowest atomic weight and the highest ionization potential of the five metallic alkalis. Accordingly, if one or both of the discharge chambers are filled with one of the alkali vapors with a higher atomic weight and lower ionization potential than lithium, the positive charge on the emitted lithium ions will be transferred to the vapor molecules, thereby creating heavier, slower moving ions which are more effective in neutralizing space charge, as explained above. For example, if cesium vapor with an atomic weight of 133 and ionization potential of 3.9 ev. is used as the gaseous atmosphere, the cesium ions produced will be about 19 times heavier than the lithium ions emitted by the intermediate electrode, due to the great difference in the atomic weights of these two elements.
In summary, it is seen that each discharge chamber is provided with a simple and effective means for overcoming space charge effect Without the disadvantages of the prior art devices. While each chamber of the twostage device illustrated can be used separately, the dual construction shown takes advantage of the inherent temperature gradient of the device to produce a greater output with only a slight increase in the complexity of the device. Since the upper chamber operates at high temperature, the lower chamber, in effect, uses what would otherwise be Waste heat from the upper chamber.
While a preferred embodiment of the invention has been shown and described many modifications may be made by those skilled in the art without departing from the spirit of the invention. For example, while the ion emitters have been shown as beads of material attached to the electrode surface, it is obvious that the ion emitters and the intermediate electrode may be made integral by mixing the required ion emitter and electrode materials in powder form together with a suitable binder, molding the mixture into the required shape and sintering.
Accordingly, the invention should be limited only by the scope of the appended claims.
What is claimed is:
1. A two-stage space charge neutralized thermionic converter comprising first and second discharge chambers formed by first, second and third parallel electrodes, said first discharge chamber formed by said first and second electrodes as the cathode and anode thereof and said second discharge chamber formed by said second and third electrodes as the cathode and anode thereof, means to apply heat to said first electrode, said electrodes having progressively lower work functions in the direction of heat flow, said heat being suflicient to cause substantial thermionic electron emission from both of said cath odes, said second electrode having a plurality of uniformly distributed beads of ion emitting material deposited on each surface thereof whereby said beads emit positive alkali ions for neutralizing space charge, said first electrode having a work function greater than the ionization potential of said alkali ions, and an electrical load device connected externally between said first and third electrodes.
2. The device of claim 1 in which said ion emitting material comprises a metallic alkali alumino-silicate.
3. A thermionic converter comprising two spaced, generally parallel electrodes, one electrode having a higher work function than the other, means to apply heat to the electrode of higher work function, an ionic cathode composed of an alumino-silicate of lithium disposed within said thermionic converter, said heat being sutficient to cause substantial thermionic electron emission from said electrode of higher work function and substantial thermionic ion emission from said ionic cathode, said converter filled with an alkali vapor of atomic weight greater than lithium and ionization potential less than lithium.
4. A thermionic converter comprising two spaced electrodes, one electrode having a higher work function than the other, means to apply heat to the electrode of higher work function, an ionic cathode disposed within said thermionic converter, said heat being suflicient to cause substantial thermionic electron emission from said electrode of higher work function and substantial thermionic ion emission from said ionic cathode but said heat being insuflicient to produce contact ionization at said ionic cathode, said converter filled with a vapor with an atomic Weight greater than that of the ions emitted by said ionic cathode and an ionization potential less than that of said ions, whereby the positive charge on said ions is transferred to the heavier atoms of said vapor.
References Cited by the Examiner UNITED STATES PATENTS 2,510,397 6/50 Hansell 310-4 2,975,320 3/61 Knauer 3104 3,021,472 2/62 Hernqvist 3104 3,088,851 5/63 Lemmens 313-346 3,119,059 1/64 Hall 3104 3,129,345 4/64 Hatsopoulos 310-4 MILTON O. HIRSHFIELD, Primary Examiner.
L. RADER, Examiner.