Nottingham, W. B.

DSpace@MIT

I. PHYSICAL ELECTRONICSProf. W. B. Nottingham W. T. Norris P. J. ShaverK. P. Luke L. E. SpragueA. QUANTITATIVE ANALYSIS OF A PHOTOEMISSIVE SOLAR-ENERGY CONVERTERAn energy converter that depends on photoemission of electrons can best be under-stood in terms of its component parts. First, the emitter itself should be of a type thathas a very high quantum yield. A suitably processed cesium antimonide surface is typi-cal of this class of photoemitters. The emitter must be located in such a manner as tomake efficient use of the solar energy received. No useful power output can be expectedfrom this converter unless the electron receiver has a very low work-function. Thisreceiving surface should also have a low photoelectric yield. The converter structuremust take on a form that minimizes the amount of light received by the electron collec-tor. The reason for these two requirements is that no matter what the actual configura-tion is, the minimum useful solar-energy frequency, or the maximum wavelength, willbe determined by the sum of the collector work-function and the voltage output. Thus ifthe output of the device is to be as high as 0. 4 volt and the work-function of the receiveras low as 1. 2 volts, the longest wavelength in the solar spectrum which could be used is760 mL. All of the various configurations of electrodes that have been proposed to meetthe requirement that the electrons be collected on a low work-function surface main-tained in the dark have additional potential barriers that limit the useful range of solarenergy to still shorter wavelengths than this.A configuration that lends itself to analysis - even though it is by no means prac-tical - is shown in Fig. I-1. Its construction and operation may be described withreference to this figure. The emitter is a semitransparent film of cesium antimonidedeposited on a glass surface with transmission far into the ultraviolet. A rectangularset of conducting bars is formed on the emitter surface; each bar is 0. 01 inch wide,,CONDUCTING GRID OF METAL FILM0.003" STAINLESS-STEEL WIRESHO ALFWAY BETWEEN GLASS PLATES0.01" SPACING BETWEEN WIRESGLASS O 0.03" SPACING BETWEEN INSIDEO \ SURFACES OF GLASS PLATES0LIGHT - -- 1 O0000O " 0O GLASSSEMITRANSPARENT FILM OF Cs5 SbFig. I-1. A possible configuration for a photoemissive solar-energy converter.(I. PHYSICAL ELECTRONICS)and the spacing between bars is 0. 1 inch. Between this photoemissive surface andanother glass wall is a set of stainless steel grid wires, 0. 003 inch in diameter and with0. 01-inch spacing from center to center. The spacing between the glass surfaces is0. 03 inch. Even though it would be difficult to accomplish, it is assumed that silver canbe deposited by evaporation on the right side of each of the stainless-steel wires. Thesesurfaces are not exposed directly to light. After oxidation this silver film is reactedwith cesium, with the idea that it can be made to have a true work-function of 1. 2 ev.The exposed side of the stainless steel is assumed to have a work-function of 4.7 ev.Photoelectrons are emitted from the semitransparent film and some of those with highenergy can surmount a potential barrier between the grid wires. These electrons arecollected on the low work-function side of the wires. After a detailed analysis to estab-lish the distribution of potential in the space between the glass and the wires, a calcula-tion was made to find the current-voltage characteristic that this cell should produceunder the most optimistic assumptions that are reasonable. The resulting current-voltagecurve is shown in Fig. 1-2. Since the power output will be the product of the current andthe voltage, the associated power curve is shown in this same figure. The maxi-mum in this curve occurs at 0. 3 volt and the power available per square centimeteris 3.7 Lw. The total solar energy received on a surface outside of the earth'satmosphere is taken as 0. 135 watt/cm 2 . The conversion efficiency computed for this30 x 10- 628 -2624 -22 -CURRENT DENSITY20 -10 o16 - 4X -614 -W 2XPOWER/ <6 0.2 0.4 0.6 0.8 1.0 1.2OUTPUT(VOLTS)Fig. I-2. Computed voltage-current curve and power output as a function of voltage.(I. PHYSICAL ELECTRONICS)configuration is therefore 0.0037 per cent.The electrode configuration shown in Fig. I-1 is not claimed to be the most efficientof all possible configurations. It was used because it contains the essential parts of thedevice and provides a means for carrying through the calculation of the final volt-currentcharacteristic shown in Fig. 1-2. If a number of optimistic assumptions had not beenmade, the efficiency of this device would have been still lower. It is therefore con-sidered realistic to predict that no photoemissive solar-energy converter is likely to bedeveloped with an efficiency greater than 0. 03 per cent.W. B. Nottingham

Physical Electronics

I. PHYSICAL ELECTRONICSProf. W. B. Nottingham J. J. Pergola P. J. ShaverK. P. Luke S. N. Salomon L. E. SpragueW. T. Norris G. L. WithbroeA. AIDS FOR THE ANALYSIS OF THERMIONIC ENERGY CONVERTERSExperimental data describing the electrical performance of the thermionic energyconverter that is operated in the presence of cesium vapor generally include:1. The emitter temperature.2. The collector temperature.3. The area of the emitter.4. The area of the collector.5. The spacing, emitter to collector.6. The cesium temperature.7. Emitter material.8. Collector material.This list represents the controllable parameters; and the principal observationsmade involve the determination of a current-voltage curve with all of the parametersheld constant, except for the applied voltage. The current then becomes the dependentvariable.The questions that an analysis would hope to answer include the following:1. What is the work-function of the emitter, and does it remain constant throughoutthe full range of applied voltage ?2. What is the collector work-function, and does it remain constant during the entirerange of applied voltage?3. Are ions generated at the emitter surface?4. Do these ions pass freely into the emitter-collector space, or are they inhibitedby the presence of a positive ion space charge at the emitter?5. If there is no positive ion space charge, is there an electron space charge thatinhibits the flow of electrons into the space?6. Do the electrons that do enter the space generate more ions by collision, andwhat is the ionization process: a single step ionization, or an atomic excitation followedby ionization? Can electrons or light ionize atoms at a surface?7. If the electrons do excite or ionize the gas, do they get their energy by passingthrough the positive ion space-charge sheath, or did they get that energy as a result ofthe application of a collector potential capable of accelerating electrons?8. Does the mode of generating ions in the space for a given applied potential dependon the sequence of events before the application of that potential?9. Does the observation of the open-circuit voltage yield information helpful to theanalysis of converter performance?(I. PHYSICAL ELECTRONICS)10. Can the reverse current characteristic of the diode aid in its analysis to giveinformation on ion production rates, or ion loss rates, as the case may be?11. Can observations taken with the collector biased positive with respect to theemitter contribute to an understanding of converter performance, since the device actsas an energy converter only when the collector is biased negative with respect to theemitter.This list covers a large number of questions which an experimenter would like tohave answered in order to understand the performance characteristics of a thermionicenergy converter. It is seldom possible to answer many of these questions, and con-siderable detailed analysis must be undertaken in order to accomplish even a tentativeanalysis of conversion performance. In every case there are certain relations thatare generally expressible in equation form which must be evaluated over and overagain.A report is now in preparation to present facts based on the latest available data,together with graphical means for obtaining answers quickly with all of the requiredaccuracy. The tables and charts include:1. Thermionic emission data as computed at various temperatures for a wide rangeof limiting barriers or work-functions.2. Cesium "pressure" data expressed in a variety of ways which include:(a) equilibrium pressure as a function of temperature of the liquid.(b) atom concentration as a function of temperature.(c) atom arrival rate as a function of temperature expressed both in terms ofthe number of atoms per unit area and the ion current that this number ofatoms would produce if all of them became ionized.(d) electron mean-free path expected in a region of some specified temperatureequal to or higher than the cesium condensation temperature.3. The relation between emitter temperature and cesium temperature for idealizedspace-charge neutralization at surfaces of different work-function as the parameter.4. Average properties of tungsten surfaces at specified temperatures in thepresence of cesium arriving at the surface and controlled by some specified condensationtemperature of the cesium. The charts presented depend on the Taylor-Langmuir dataand on Houston's data.5. Similar charts for tantalum in the presence of cesium based on Houston's data.6. Similar charts for molybdenum in the presence of cesium based on Houston'sdata.7. Examples of these aids in the analysis of converter data.8. Fundamental atomic constants and many useful combinations of these constantsare evaluated.W. B. Nottingham(I. PHYSICAL ELECTRONICS)References1. J. M. Houston, Thermionic Emission from Refractory Metals in Cesium Vapor,Report of Proceedings of the Round Table Discussion on Cathode Emission Investigationsand Experimental Techniques for Fabricating and Operating Thermionic Cells, ReportPIC-ELE-T13/3, Power Information Center, University of Pennsylvania, Philadelphia,Pennsylvania, 30 June 1961.

http://dspace.mit.edu/bitstream/1721.1/53615/1/RLE_QPR_063_I.pdf

Physical Electronics

Abstract

Similar works

Full text

Available Versions

DSpace@MIT

DSpace@MIT