US3426221A - Thermionic converter - Google Patents

Description

-; 1969 w. E. HARBAUGH THERMIONIC CONVERTER Filed March 29, 1966 TEMP. PRES. CURVE FOR SINGLE RES AT TEMP; TE

PRES; TEMF? CURVE RES. AE AT E'M. TEMP.

le'po 2000 T TEI E2 TEMPERATURE,K

m T E3 wmnmmmmm INVENTOR.

WILLIS E. HARBAUGH BY F/g.2

United States Patent 3 Claims ABSTRACT OF THE DISCLOSURE Thermionic generator having a radioisotope heated emitter, a controlled temperature collector adjacent thereto, and equal area adsorption means that are specifically located near the collector and emitter at different temperatures for the interchange of alkali metal vapor therebetween as the emitter temperature decreases due to the radionuclide decay of the radioisotope heat source.

This invention relates to thermionic energy converters and more particularly to thermionic energy converters containing a source of gas or metal vapor. The invention described herein was made in the course of, or under a contract with the US. Atomic Energy Commission.

In the field of electrical power generation thermionic converters are an important and growing electrical energy source. These converters comprise an electron tube having a cathode and an anode for converting heat energy to the cathodeto electrical energy in the form of a voltage produced by the tube itself between its emitter cathode and collector anode terminals. The materials of the cathode and anode are usually chosen to provide a cathode having an electron work function substantially higher than that of the anode thereby to produce an internal electric field for accelerating electrons from the cathode to the anode. In order to facilitate travel of electrons from the cathode to the anode, the space charge of the electrons between the cathode and anode may be neutralized by the use of positive ions. One source of such ions may be an alkali metal vapor having a low ionization potential, such as cesium, introduced into the interelectrode space.

Various proposals have been made or used to provide the required vapor, including the systems in which the alkali metal vapor has been introduced from an envelope containing a liquid pool of alkali metal but this has required an appendage communicating with the interelectrode space of the device. Also, heating of the alkali metal to a propitious temperature, and/or precise external control has been required to optimize the cesium vapor pressure for various emitter temperatures. It has addition-ally been advantageous to provide automatic selftracking internal means for optimizing the cesium vapor pressure for various emitter temperatures.

Accordingly, it is an object of this invention to provide an improved pool-less thermionic converter and method of operating the same characterized by means providing automatic tracking of the cesium pressure within the converter.

It is another object to provide a pool-less thermionic generator having more than one adsorption area for the interchange of interelectrode alkali metal vapor therebetween wherein the rate of change of the pressure of the vapor with respect to the emitter temperature is a function of the adsorption areas.

It is another object to provide in a pool-less thermionic generator two metal vapor adsorption areas that provide the vapor pressure at an optimum value for a wide range of emitter temperatures without the need for external con- 3,426,221 Patented Feb. 4, 1969 'ice trol so as to provide a simple, self-tracking, effective and practical thermionic converter system.

It is another object of this invention to provide a poolless thermionic converter having two large high tempera ture alkali metal adsorption areas that act together to establish thermionic converter pressure according to the ratio of their areas and the coverage of alkali metal on their surfaces.

This invention provides a method and apparatus for the practical and efiicient production of electrical power from heat derived from the radionuclide decay of radioisotopes for a long operating lifetime, remote power load for space applications. The method and construction involved in this invention utilize standard and well known techniques and apparatus and is highly flexible for a broad range of applications, alkali metals, electrodes, emitter temperatures, vapor pressures, and heat sources. More particularly, this invention involves the use of two pool-less, large, high temperature, alkali metal adsorption means that are specifically located near a collector and emitter at ditferent temperatures and have large equal area-s that interchange alkali metal vapor therebetween as the emitter temperature changes. The adsorption areas are arranged in a thermionic converter in one embodiment, to be saturated with alkali metal vapor in the ratio of about one half gram in the adsorption areas to about 10 micrograms of alkali metal vapor outside the adsorption areas. With the proper selection of adsorption areas and maintenance of less than a monolayer of alkali metal vapor on surfaces outside the adsorption areas, the desired pool-less simple, effective and efiicient self-tracking electrical power generation is achieved over a wide emitter temperature range.

The above and further novel features of this invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings. It is expressly understood, however, that the drawings are not intended as a definition of the invention but are for the purpose of illustration only.

In the drawings where like parts are referenced alike:

FIG. 1 is partial cross-section of the thermionic converter of this invention;

FIG. 2 is a graphic representation of pressure in torr vs. temperature in K. of the system for self-control of the cesium pressure in the apparatus of FIG. 1;

FIG. 3 is a schematic illustration of the processing apparatus for the converter of FIG. 1.

During the development of this invention a thermionic generator was tested having no metal vapor reservoir, but this device was unsuccessful since vapor material was lost due to the extreme reactivity of the vapor with the internal generator components or other causes and the electrical output was critically dependent on a constant vapor pressure. Thus this generator was not suitable for use with a decaying radioactive source. Other tests showed problems existing with internal reservoirs. It was discovered in accordance with this invention, however, that particular communicating adjacent internal adsorbers, at the collector and emitter temperatures, self-track thus to be particularly suited for a decaying heat source.

This invention is particularly adapted for the production of electrical energy from the heat resulting from the radionuclide decay of a radioisotope. The converter and system of this invention, however, may also use other heat sources. The converter of this invention is suitable for use in space for energizing radios or other scientific payloads but it also may be used in any location where a dependable, long life or remote electrical power source is desired.

In understanding this invention reference is made to FIG. 1, which shows a thermionic energy diode converter 11 having an anode emitter 13, cathode collector 15 and interelectrode alkali metal vapor to provide adequate space charge neutralization. This is a cylindrical thermionic converter having nickel as the collector material, molybdenum as the emitter material and synthetic sapphire as the material for insulator 17 between the two electrodes 13 and 15. An envelope 19, comprising portion 21 made from NiFeCo alloy, such as Kovar brand alloy, and nickel portion 23 prevents any loss of the alkali metal vapor, since any loss of this vapor material to the atmosphere will cause the power output of the converter to drop.

Flexible metal diaphragm provides differential expansion and contraction between the converter parts to maintain a constant interelectrode width in gap 27. Ceramic insulators 17 and 31 separate the working central portion 33 of the converter 11 from the top portion 35 and bottom portion 37 thereof. Emitter contact ring 39, having bolt holes 41 therein provide support for the whole converter structure and cooling jacket 43 thereof, which has means M for circulating a suitable cooling fluid therein to maintain a constant collector temperature. One means M is the heat pipe disclosed in U.S. Patent 3,229,759, but variable pumping means, thermostatically controlled shutter means or variable heating or cooling means may also be used. Pinch off tube 45 provides means for introducing the interelectrode vapor. The interelectrode vapor material enters the converter 11 through tube 45 and the tube is then closed.

A suitable heat source, such as a radioisotope sources, placed inside the cylindrical chamber 51 of emitter 13 provides heat energy to drive electrons from the emitter 13 to the collector 15 so as to produce an electrical power producing electrical differential between these two electrodes, while remaining to provide adequate space charge neutralization.

This invention provides two internal adsorption means 61 and 63 for the vapor to provide automatic tracking of the vapor pressure within the converter 11. To this end these adsorbers are large in area, advantageously equal in area, adsorber 61 is adjacent to the emitter 13 at the temperature of the emitter 1'3, adsorber 63 is adjacent to the collector 15 at the temperature of the collector 15 and there is an interchange of vapor material between the adsorbers 61 and 63 as the emitter temperature de creases due to the radionuclide decay of source S, whereby the adsorbers provide automatic optimum self-tracking of the vapor pressure as the emitter temperature decreases. The system of this invention changes the Cs pressure automatically for a constant emitter temperature and for new emitter temperatures by automatically tracking to the new required Cesium pressures. This system works best with a constant collector temperature but normally with a radioactive heat source the emitter temperature changes only slightly with even less change in the collector. However, this invention will automatically track much better than the systems known heretofiore even if both the collector and emitter temperature change.

In understanding 'how these adsorbers 61 and 63 provide the desired self-tracking, reference is made to FIG. 2. Curve PP illustrates that thermionic converters require a particular pressure for every emitter temperature for optimum performance. An adsorber at emitter temperature does not provide the proper interelectrode pressure for optimum converter operation since it would follow the pressure temperature relationship B-B. By utilizing a first adsorption means 61 adjacent the emitter 13 and a second large area adsorption means 63 located at the collector 15 which is essentially at constant temperature the two adsorbers 61 and 63 work together to establish converter pressure according to the ratio of their areas and the coverage of the vapor on their surfaces.

The relationship that determines the pressure is A 9 +A =K=c0nstant where A is the area of the emitter adsorber 61, A in the area of the collection adsorber 63, G is the vapor coverage of the emitter ad- 4 sorber 61 and 6 is the vapor coverage of the adsorber 63.

The fact that the total amount of vapor material in the system must be constant means that any change in emitter temperature will require an interchange of cesium between the adsorbers 61 and 63 since the emitter adsorber 61 cannot now follow a constant coverage curve. The modifying effect of the collector adsorber 63 can be made greater or smaller, depending on the size of its area compared to that of the emitter temperature adsorber 61. The proper ratio of adsorber areas to make the resultant converter pressure equal to the desired pressure curve P-P can be solved graphically from the constant coverage curves of FIGURE 2. Actual tests have shown that the desired ratio is very close to unity for optimum converter operation. Moreover, during operation these adsorbers 61 and 63 provide automatic tracking of the vapor pressure at this optimum value for a wide range of emitter temperatures, such as are provided by the decay of the radioactive source in emitter 13, without the need for external control. In one embodiment, the cathode anode spacing is 0.025 cm.

Cesium may be lost by chemical reactions with residual gases or converter materials and it can be lost by physical means such as capillary attraction and condensation. Thus the recited materials and sintered tungsten particles forming adsorbers 61 and 63 are employed. Advantageously the adsorbers 61 and 63 comprise metal sponges having pore walls formed by sintering the tungsten with other materials such as aluminum oxide particles, both particles being 10- cm. in diameter and having in each adsorber a volume of 1 cm. 3 and an adsorbing area of 10,000 cm. so that more times vapor material atoms than elsewhere in the detector can adsorb in the adsorbers. In one embodiment having an inside volume of 2 cm. outside the adsorbers, there is one-half gram of Cs in each adsorber and 10 micrograms of Cs outside the adsorbers. For normal converter life, the pore walls should have a large area for adsorbing at least ten times as many cesium atoms as are present elsewhere in the converter. In the rest of the converter, the inside metal surfaces of the device are covered with less than a monolayer of cesium so that no liquid or pool exists in the device and this coverage of the internal surfaces of the device is an equilibrium state between the arrival and evaporation rate of the alkali metal vapor at these surfaces.

The greater affinity of the vapor atoms for surfaces other than the vapor material in its liquid form makes it feasible to store the vapor material in an adsorbed state within the device at a relatively wide temperature range, extending from a temperature slightly higher than the normal condensation temperature of the vapor material to any substantially higher temperature at which the converter is in operation. Thus, the temperature at which the vapor material pressure is optimum for best converter efiiciency may be used even though substantially higher or lower than one or more elements of the converter.

Advantageously, the binding energy of the vapor material atoms to the sponge pore walls is 2.00 electron volts at 553 K. whereas the binding energy of the vapor material to a liquid pool thereof is only 0.75 electron volt at that temperature.

In establishing the proper cesium pressure initially, specific processing techniques were developed. To this end the converter 11 is advantageously processed in an ultrahigh vacuum system of at least 10" torr to reduce residual gases and to achieve a high degree of vapor material purity. With background gas at a minimum the desired precise amount of material for the vapor enters converter 11 thr ugh tube 45 and a suitable machine pinches tube 45 to seal the vapor material inside converter 11. To this end, the converter envelope is maintained at a high temperature and a hot pinch-off is made to maintain the vacuum at least as low as 1 torr.

The converter of this invention advantageously comprises a small envelope diameter and sapphire emittercollector insulation. Electron beam welds and tungsten inert gas welds are used only. Where brazes were used, these were iron-palladium and nickel-palladium brazes without gold or other conventional brazing alloys.

Advantageously, the assembly system has two pumping systems 101 and 102 and suitable means 103 for providing heater bombardment power and envelope heating power. Also, the converter is connected to distillation apparatus 105 for the vapor material through the nickel pinch-off tube by a bakeable flange 107 therefor. The vapor material als connects to a liquid-nitrogen cold trap 109 and then to an exhaust manifold 111. A second liquid-nitrogen cold trap 113 provides a safety device to trap radioactive vapor material before it gets to the pump 102. The pumping systems are of the getter-ion type with a pumping speed of 15 liters per second. The second independent pumping system exhausts the heater bombardment chamber.-117.

Processing follows the steps, comprising bakeout of the entire exhaust systems at 500 F., degassing of the converter elements at a low pressure of l-' torr, admission andcontinuous distillation of cesium into the converter, maintenance of the converter interelectrode material in its vapor state i.e. in its Ball-of-Fire" m de raising of the emitter temperature to between 1300 C. and 1350 C. and pinch-off of the converter from the exhaust system while operating and providing a minimum envelope temperature of 150 C. higher than the optimum vapor material temperature. Advantageously pinch off is with the emitter at 1200" C. and the envelope at 450 C.

In the operation of the converter of this invention, the radioactive heat source heats the emitter to 1350 C. while the cooling jacket maintains the envelope at a temperature below that and above 280 C., e.g. 450 C. Advantageously cesium is the interelectrode material having a critical condensation temperature of 270 C. One advantageous initial temperature at the collector is 600 C. and 1350 C. at the emitter. The pressure at this temperature is 1 torr. However, depending on the emitter temperature which determines the propitious operating pressure, the Cs pressure can be anywhere between .1 torr and about 5 torr. The latter pressures involve emitter temperatures from 1260 C. to 1800 C. This produces electron flow from the emitter to the collector and a voltage across the converter output leads.

It has been found in accordance with this invention that heavy undesirable, liquid pools of alkali material are prevented. In this regard less than a mono-layer of the vapor material and suitable high temperature adsorbers are used. Also, the lack of welds and materials that contain residual oxides have prevented the retention of vapor material therein. Moreover, the molybdenum and nickel electrodes and supports and the main shell have been free from attack by the cesium while the metalized seals and sapphire insulators have prevented undesirable attacks from the vapor material. Additionally, the careful degassing and sealing of the converter at the required vapor pressure have enabled the converter to prevent undesirable losses of the vapor material and initial pressure thereof for long periods of time so as to provide a substantially constant'power output over the wide temperature range produced by a decaying radioisotope heat source.

The converter of this invention has the advantage of being useful with a wide range of heat sources such as radioisotopes, nuclear fission, solar radiation and fossil fuel where heat output has often varied, as well as systems having sump controls and the like for providing a constant heat input. The system of this invention is particularly advantageous for both large and small, and low and high power output systems and to this end it is operable to produce a substantially level power output over a wide range of heat inputs. Also, it contains vapor material whose pressure varies optimally with temperature while this temperature maintains all the inter-electrode material vaporized. Additionally, the converter of this invention has a high degree of compatibility with a Wide range of highly reactive interelectrode vapor materials and provides little or no loss of vapor mate-rial so as to maintain a substantially optimum interelectrode vapor pressure over long periods of time.

What is claimed is:

1. The method of operating a thermionic gas tube having a collector, an emitter and an interelectrode vapor, comprising: baking said tube at 500 F., degassing said converter elements at 10 torr, admitting continuously distilled cesium into said converter while maintaining said cesium in its vapor state in said converter, closing said converter to maintain a constant cesium vapor pressure in said converter, and maintaining a differential temperature between said emitter and collector above the condensation temperature of said cesium while adsorbing a large portion of said cesium adjacent said emitter and collector substantially at the respective temperature thereof to produce a. substantially constant electrical power output from said converter between said emitter and collector over a wide emitter temperature range.

2. A thermionic energy converter for producing electrical power from a heat source, comprising a cylindrical inner molybdenum emitter, a closely spaced cylindrical outer nickel collector, a pure interelectrode cesium alkali metal vapor, a sapphire insulator between said emitter and collector, respective porous first and second metal sponges adjacent said emitter and said collector formed from sintered tungsten particles for adsorbing a major portion f said alkali metal vapor material, a nickel envelope enclosing said elements having means for maintaining a constant collector temperature, and a radioactive heat source inside said emitter for vaporizing all said alkali metal vapor and heating said emitter and collector to differential temperatures above the normal condensation temperature of said alkali metal material to produce electron flow between said emitter and collector for producing a substantially constant voltage therebetween over a wide emitter temperature range.

3. The invention of claim 2 in which the relationship A 0 +A 6 =K=a constant where A is the area of the emitter adsorber, A is the area of the collector adsorber, 6 is the vap r coverage of the emitter and 0 is the vapor coverage of the collector, A and A being equal and larger in area than said collector and emitter respectively.

References Cited UNITED STATES PATENTS 2,510,397 6/1950 Hansell 3104 2,980,819 4/1961 Feaster 313212 3,300,661 1/1967 Talaat 310-4 MILTON O. HIRSHFIELD, Primary Examiner.

D. F. DUGGAN, Assistant Examiner.

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