Thermionic converter

D) L.L. Haring W.R. Oppen, Design and Development of a Pyrotechnic Heated Thermionic Converter , Final Summary Report, Contract DA-30-069-AMC-334(A), Ford Instrument Co, Long Island City (1966) E) Anon, R and D Abstracts. Vol 19, No 15, 1st—15th... 

The colloidal particles are often deposited on metallic electrodes in the form of adsorbed coatings. Rubber and graphite coatings can be formed in this way, using solvent mixtures (water-acetone) as the dispersion media. The advantage of this method is that additives can firmly be codeposited with, for example, rubber latex. Thermionic emitters for radio valves are produced in a similar manner. The colloidal suspensions of alkaline earth carbonates are deposited electrophoretically on the electrode and are later converted to oxides by using an ignition process. 

When rhenium is added to other refractory metals, such as molybdenum and tungsten, ductility and tensile strength are improved. These improvements persist even after heating above the rccrystallization temperature. An excellent example is the. complete, ductility shown by a molybdenum-rhenium fusion weld. Rhenium and rhenium alloys have gained some acceptance in semiconductor, thermocouple, and nuclear reactor applications. The alloys also axe used in gyroscopes, miniature rockets, electrical contacts, electronic-tube components, and thermionic converters. 

Verwej et al. [175] have described a procedure for the determination of PH3-containing insecticides in surface water. In this procedure the insecticide is hydrolysed to methylphosphonic acid, and the acid is concentrated by anion exchange and converted to the dimethyl ester. After clean-up on a microsilica gel column the ester is analysed by gas chromatography using a thermionic phosphorus-specific detector. Detection limit is lnmol L 1. 

Thermionic converters are high temperature devices which utilize electron emission and collection with two electrodes at different temperatures to convert heat into electric power directly with no moving parts. Most thermionic converters operate with a plasma of positive ions in the interelectrode space to neutralize space charge and permit electron current flow. Both the plasma characteristics and the surface properties of the electrodes are controlled by the use of cesium vapor in thermionic diodes. 

Thermionic energy conversion is a method of converting heat directly to electricity. A metal electrode, the emitter, is heated sufficiently to emit electrons, as shown in Figure 1. The electrons cross a narrow interelectrode gap and are collected by another metal electrode, the collector. Heat is removed from the collector so that its temperature is lower than the emitter, and the electrons striking the collector cannot be returned except by... 

Figure 1. Scheme of thermionic diode for directly converting heat into electricity. 

The unique properties of cesium play a crucial role in the operation of thermionic converters. Cesium functions both as adsorbed atomic layer to produce the required work functions on the electrodes, and as a plasma medium to form Cs ions which neutralize space charge in the interelectrode region. Cesium is desirable as the plasma medium because of its low ionization potential and large atomic mass. Since the surface adsorbed layers are continuously evaporating and being replenished by cesium atoms refluxing from the vapor, the surface properties are very stable. Thermionic converters have operated with no change in performance for more than 5 years. 

The cycle has many similarities to a Rankine cycle which uses electrons as a working fluid. Unlike the normal Rankine cycle, however, the working fluid s "heat of evaporation", approximately the emitter work function, and its "heat of condensation", approximately the collector work function, can be varied in the thermionic converter. This feature provides the converter with great flexibility in matching the operating constraints of any particular system. 

A simple analytical model of thermionic converter performance must be made before the impact of converter performance on system behavior can be studied. Fortunately, a very simple model of converter performance has been found to be sufficiently accurate for this purpose. The ideal thermionic diode serves as the basis for this model. Motive diagrams and converter current voltage characteristics for an ideal diode are shown in Figure 2. 

Figure 2. Current-voltage characteristics of an ideal thermionic converter. Top J-V curve Bottom Electron notive diagram for two different conditions.

Figure 4. Characteristics of an ignited mode thermionic converter. Ignited mode motive diagram shown at top. The actual J-V curve is displaced from the ideal curve (bottom) by an amount equal to the voltage losses in the converter.

The motive diagram for the potential energy of electrons in an ignited mode thermionic converter has a more complicated shape, as shown at the top of Figure 4. The presence of positive ions in the plasma creates a minimum in the electron motive inside the interelectrode gap. There are narrow collisionless sheaths (the order of a Debye length in thickness) at both the emitter and collector edges of the plasma. 

Figure 5. Fluid mechanical analog for ignited mode thermionic converter.

A number of one dimensional computer models have been developed to analyze thermionic converters. These numerical models solve the nonlinear differential equations for the thermionic plasma either by setting up a finite element mesh or by propagating across the plasma and iterating until the boundary conditions are matched on both sides. The second of these approaches is used in an analytical model developed at Rasor Associates. A highly refined "shooting technique" computer program, known as IMD-4 is used to calculate converter characteristics with the model ( ). 

Calculated Power and Efficiency. The simplified analytical models of thermionic characteristics have been used to project the converter efficiency and power density with the barrier index as a parameter. These projections are shown in Figures 8 and 9 as functions of the emitter temperature. The dashed lines in these two figures are for a constant current density of 10 A/cm. If the current density is adjusted to maximize the efficiency at each temperature, the calculated performance is represented by the solid lines. Typical present generation themionlc converters operate with Vg near 2.0. Ignited mode converters in laboratory experiments have demonstrated practical operation with 1.85 < Vg < 1.90. Other laboratory devices with auxiliary sources of ions and/or special electrode surfaces have achieved Vj < 1.5, but usually not under practical operating conditions. 

Hardware Experience. The efficiency of cylindrical thermionic converters can be detemined accurately because the thermal input to the emitter is easy to measure. The efficiency of actual converters is shown in Figure 10. The lines for... 

There are no inherent degradation mechanisms to limit the life of thermionic converters. The lifetime of test devices has usually been related to damage from the environment of the heat source. Converters with nuclear fuel are affected by fisson product swelling, which distorts the emitter. Flame heated converter lifetimes are controlled by the durability of the hot shell, which protects the emitter from the combustion atmosphere. 

The record holder for converter life is LC-9, a converter built for NASA by General Atomic as part of the in-core nuclear space reactor program. LC-9 operated with perfectly stable performance for over five years at an emitter temperature of 1970 K. As shown in Figure 12 ( ), LC-9 had an electrode efficiency of 17%, and generated 8 W/cm of output power (80 KW /m ). The converter was still performing stably when tests were terminated for programmatic reasons. This test illustrates well the long life capability of the thermionic converter process. 

Figure 11. Progress in increasing output power density of thermionic converters at lower emitter...

Recently, two types of surfaces with coadsorbed cesium and oxygen have shown promise for low work function operation in thermionic converters. The first type of surface is made by codeposition of a "thick layer" ( 30 A) of cesium. As shown in Figure 13 ij), if the proportions of cesium and oxygen are properly controlled, a work function as low as 1.0 ev can be obtained. The substrate material does not affect the low obtained with a thick Cs - 0 layer. This type of surface could potentially be maintained in a thermionic converter by an equilibrium mixture of cesium, oxygen, and cesium oxide. Experiments to demonstrate this in operating diodes are underway ( ). 

Thermionic conversion is a technology that needs, and can immediately use, research on high temperature properties of alkali metals. Electron transport properties of alkali vapors and characteristics of atomic clusters are particularly Important. Improved understanding in these areas could lead to performance improvements that would more than double the output power density and efficiency of cesium ignited mode thermionic converters. 

Thermionic Converter and Fuel Element Testing Summaries at Gulf General Atomic Company, " GULF-GA-C12345, California, 1972. 

Goodale, D. B., Reagan, P. Miskolczy, G., Lieb, D. and Huffman, F. N. "Characteristics of CVD Silicon Carbide Thermionic Converters", 16th Intersociety Energy Conversion Engineering Conference, Atlanta, GA, 1981. 

Hansen, L. K. and Woo, H. "Thermionic Converters with a Thick Cesium Oxide Collector", IEEE International Conference on Plasma Science, Madison, Wisconsin, 1980, 75. 

---