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PowerPedia:Atomic Battery

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The terms atomic battery, radioactive battery, nuclear battery, and radioisotope battery are used to describe a device which uses the charged particle emissions from a radioactive isotope to directly generate electricity. Devices for converting natural radioactive decay directly into electricity are nothing new. Nuclear battery technology began in 1913, when Henry Moseley first demonstrated the Beta Cell. The field received considerable research attention for applications requiring long-life power sources for space needs during the 50s and 60s. Over the years many types and methods have been developed. The scientific principles are well known, but modern nano-scale technology and new wide bandgap semiconductors have created new devices and interesting material properties not previously available.

Batteries using the energy of radioisotope decay to provide long-lived power (10-20 year) are being developed internationally. Conversion techniques can be grouped into two types: thermal and non-thermal. The thermal converters (whose output power is a function of a temperature differential) include thermoelectric and thermionic generators. The non-thermal converters (whose output power is not a function of a temperature difference) extract a fraction of the incident energy as it is being degraded into heat rather than using thermal energy to run electrons in a cycle. Atomic batteries usually have an efficiency of 0.1–5%.

Contents

Radioisotopes Used

Atomic batteries use radioisotopes that produce low energy beta particles or sometimes alpha particles of varying energies. Low energy beta particles are needed to prevent the production of high energy penetrating Bremsstrahlung radiation that would require heavy shielding. Radioisotopes such as tritium, nickel-63, promethium-147, and technetium-99 have been tested. Plutonium-238, curium-242, curium-244 and strontium-90 have been used.

Energy Production Mechanisms

Non-thermal converters

Non-thermal converters extract a fraction of the nuclear energy as it is being degraded into heat. Their outputs are not functions of temperature differences as are thermoelectric and thermionic converters. Non-thermal generators can be grouped into three classes.

Betavoltaics

Main article: BetaVoltaics

Betavoltaics is an alternative energy technology that promises vastly extended battery life and power density over current technologies. Betavoltaics are generators of electrical current, in effect a form of battery, which use energy from a radioactive source emitting beta particles (electrons). The functioning of a betavoltaic device is somewhat similar to a solar panel, which converts photons (light) into electric current. The radioactive battery (or nuclear battery) operate on the continuous radioactive decay of certain elements. These theoretical batteries last a long time.

In May 2005, a group including researchers from the University of Rochester and from the University of Toronto announced a small battery powered by the beta-particle-emitting decay of tritium and positioned the product as suitable for pacemakers or low-current electrical household devices. The device gathers energy from the beta-particles that pass through a silicon diode, in a manner analogous to photovoltaic cells. This technique is called betavoltaics and has the potential to radically increase atomic battery efficiency and energy production densities.

Direct charging generators

In the first type, the primary generators consists of a capacitor which is charged by the current of charged particles from a radioactive layer deposited on one of the electrodes. Spacing can be either vacuum or dielectric. Negatively charged beta particles or positively charged alpha particles, positrons or fission fragments may be utilized. Although this form of nuclear-electric generator dates back to 1913, few applications have been found in the past for the extremely low currents and inconveniently high voltages provided by direct charging generators.

English physicist H.G.J. Moseley constructed the first of these. Moseley’s apparatus consisted of a glass globe silvered on the inside with a radium emmiter mounted on the tip of a wire at the center. The charged particles from the radium created a flow of electricity as they moved quickly from the radium to the inside surface of the sphere. As late as 1945 the Moseley model guided other efforts to build experimental batteries generating electricity from the emissions of radioactive elements.

Optoelectric

An optolectric nuclear battery has also been proposed by researchers of the Kurchatov Institute in Moscow. A beta-emitter (such as technetium-99) would stimulate an excimer mixture, and the light would power a photocell. The battery would consist of an excimer mixture of argon/xenon in a pressure vessel with an internal mirrored surface, finely-divided Tc-99, and an intermittent ultrasonic stirrer, illuminating a photocell with a bandgap tuned for the excimer. If the pressure-vessel is carbon fiber/epoxy, the weight to power ratio is said to be comparable to an air-breathing engine with fuel tanks. The advantage of this design is that precision electrode assemblies are not needed, and most beta particles escape the finely-divided bulk material to contribute to the battery's net power.

An optolectric nuclear battery has been developed by researchers of the Kurchatov Institute in Moscow. A beta-emitter such as technetium-99 or strontium-90 is suspended in a gas or liquid containing luminescent gas molecules of the excimer type, constituting a "dust plasma." This permits a nearly lossless emission of beta electrons from the emitting dust particles for excitation of the gases whose excimer line is selected for the conversion of the radioactivity into a surrounding photovoltaic layer such that a comparably light weight low pressure, high efficiency battery can be realised. These nuclides are low cost radioactive waste of nuclear power reactors. The diameter of the dust particles is so small (few micrometers)that the electrons from the beta decay leave the dust particles nearly without loss. The surrounding weakly ionized plasma consists of gases or gas mixtures (e.g. krypton, argon, xenon) with excimer lines, such that a considerable amount of the energy of the beta electrons is converted into this light. The surrounding walls contain photovoltaic layers with wide forbidden zones as 3.g. diamond which convert the optical energy generated from the radiation into electric energy.

The battery would consist of an excimer of argon, xenon, or krypton (or a mixture of two or three of them) in a pressure vessel with an internal mirrored surface, finely-ground radioisotope, and an intermittent ultrasonic stirrer, illuminating a photocell with a bandgap tuned for the excimer. When the beta active nuclides (e.g. krypton-85 or argon-39) are excited, their own electrons in the narrow excimer band at a minimum of thermal losses that this radiation is converted in a high band gap photovoltaic layer (e.g. in p-n diamond) very efficiently into electricity. The electric power per weight compared with existing radionuclide batteries can then be increased by a factor 10 to 50 and more. If the pressure-vessel is carbon fiber/epoxy the weight to power ratio is said to be comparable to an air-breathing engine with fuel tanks. The advantage of this design is that precision electrode assemblies are not needed, and most beta particles escape the finely-divided bulk material to contribute to the battery's net power. The disadvantage consists in the high price of the radionuclides and in the high pressure of up to 10 MPa (100 bar) and more for the gas that requires an expensive and heavy container.

Thermal converters

Thermal converters outputs are functions of temperature differences, for example thermoelectric and thermionic converters. Thermal generators can be grouped into four classes.

Thermionic converter

A thermionic converter, consists of a hot electrode which thermionically emits electrons over a space charge barrier to a cooler electrode, producing a useful power output. Cesium vapor is used to optimize the electrode work functions and provide an ion supply (by surface contact ionization) to neutralize the electron space charge.

The scientific aspects of thermionic energy conversion primarily concern the fields of surface physics and plasma physics. The electrode surface properties determine the magnitude of electron emission current and electric potential at the electrode surfaces, and the plasma properties determine the transport of electron current from the emitter to the collector. All practical thermionic converters to date employ cesium vapor between the electrodes, which determines both the surface and plasma properties. Cesium is employed because it is the most easily ionized of all stable elements.

The surface property of primary interest is the work function, which is the barrier that limits electron emission current from the surface and essentially is the heat of vaporization of electrons from the surface. The work function is determined primarily by a layer of cesium atoms adsorbed on the electrode surfaces. The properties of the interelectrode plasma are determined by the mode of operation of the thermionic converter. In the ignited (or “arc?) mode the plasma is maintained via ionization internally by hot plasma electrons (~ 3300 K); in the unignited mode the plasma is maintained via injection of externally-produced positive ions into a cold plasma; in the hybrid mode the plasma is maintained by ions from a hot-plasma interelectrode region transferred into a cold-plasma interelectrode region.

Radioisotopic Thermoelectric Generator

space_battery.jpg
Radioisotopic Thermoelectric Generator
(Image from http://www.primidi.com)

A "thermoelectric converter" connects thermocouples in series. Each thermocouple is formed by the junction of two dissimilar materials, one of which is heated and the other cooled. Metal thermocouples have low thermal-to-electrical efficiency. However, the carrier density and charge can be adjusted in semiconductor materials such as bismuth telluride and silicon germanium to achieve much higher conversion efficiencies.

A simple electrical generator which obtains its power from radioactive decay, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples. RTGs can be considered as a type of battery and have been used as power sources in satellites, space probes and unmanned remote facilities. RTGs are usually the most desirable power source for unmanned or unmaintained situations needing a few hundred watts or less of power for durations too long for fuel cells, batteries and generators to provide economically, and in places where solar cells are not viable.

The radioactive material used in RTGs must have several characteristics:

  • The half-life must be long enough so that it will produce energy at a relatively continuous rate for a reasonable amount of time. However, at the same time, the half life needs to be short enough so that it decays sufficiently quickly to generate a usable amount of heat. Typical half-lives for radioisotopes used in RTGs are therefore several decades, although isotopes with shorter half-lives could be used for specialized applications.
  • For spaceflight use, the fuel must produce a large amount of energy per mass and volume (density). Density and weight are not as important for terrestrial use, unless there are size restrictions.
  • Should produce high energy radiation that has low penetration, mainly Alpha radiation. Beta radiation can give off considerable amounts of Gamma/X-ray radiation through bremsstrahlung secondary radiation production, thus requiring heavy shielding. Isotopes must not produce significant amounts of gamma, neutron radiation or penetrating radiation in general through other decay modes or decay chain products.

The first two criteria limit the number of possible fuels to less than 30 atomic isotopes within the entire isotope table of elements. Plutonium-238, curium-244 and strontium-90 are the most often cited candidate isotopes, but other isotopes such as polonium-210, promethium-147, caesium-137, cerium-144, ruthenium-106, cobalt-60, curium-242 and thulium isotopes have also been studied. Of the above, 238Pu has the lowest shielding requirements and longest half-life. Only three candidate isotopes meet the last criteria (not all are listed above) and need less than 25 mm of lead shielding to control unwanted radiation. 238Pu (the best of these three) needs less than 2.5 mm, and in many cases no shielding is needed in a 238Pu RTG, as the casing itself is adequate.

238Pu has become the most widely used fuel for RTGs, in the form of plutonium(IV) oxide (PuO2). 238Pu has a half-life of 87.7 years, reasonable energy density and exceptionally low gamma and neutron radiation levels. Some Russian terrestrial RTGs have used 90Sr; this isotope has a shorter half-life, much lower energy density and produces gamma radiation, but is cheaper. Some prototype RTGs, first built in 1958 by USA Atomic Energy Commission, have used 210Po; this isotope provides phenomenally huge energy density, but has limited use because of its very short half-life and some gamma ray production. A kilogram of pure 210Po in the form of a cube would be about 95 mm on a side and emit about 63.5 kilowatts of heat (about 140 W/g), easily capable of melting then vaporizing itself. 242Cm and 244Cm have also been studied well, but require heavy shielding from gamma and neutron radiation produced from spontaneous fission.

Americium-241 is a potential candidate isotope with a longer half-life than 238Pu: 241Am has a half-life of 432 years and could hypothetically power a device for centuries. However, the energy density of 241Am is only 1/4 that of 238Pu, and 241Am produces more penetrating radiation through decay chain products than 238Pu and needs about 18 mm worth of lead shielding. Even so, its shielding requirements in a RTG are the second lowest of all possible isotopes: only 238Pu requires less.

Most RTGs use 238Pu which decays with a half-life of 87.7 years. RTGs using this material will therefore lose 1 − 0.51 / 87.7 or 0.787% of their capacity per year. However, the bi-metallic thermocouples used to convert thermal energy into electrical energy degrade as well.

RTGs use thermoelectric couples or "thermocouples", to convert heat from the radioactive material into electricity. Thermocouples, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3-7%. However studies have been done on improving efficiency by using other technologies to generate electricity from heat. Achieving higher efficiency would mean less radioactive fuel is needed to produce the same amount of power, and therefore a lighter overall weight for the generator. This is a critically important factor in spaceflight launch cost considerations.

Energy conversion devices which rely on the principle of thermionic emission can achieve efficiencies between 10-20%, but require higher temperatures than those at which standard RTGs run. Some prototype 210Po RTG have used thermionics, and potentially other extremely radioactive isotopes could also provide power by this means, but short half-lives make these infeasible. Several space-bound nuclear reactors have used thermionics, but nuclear reactors are usually too heavy to use on most space probes.

Thermophotovoltaic cells work by the same principles as a photovoltaic cell, except that they convert infrared light emitted by a hot surface rather than visible light into electricity. Thermophotovoltaic cells have an efficiency slightly higher than thermocouples and can be overlaid on top of thermocouples, potentially doubling efficiency. Systems with radioisotope generators simulated by electric heaters have demonstrated efficiencies of 20%[2], but have not been tested with actual radioisotopes. Some theoretical thermophotovoltaic cell designs have efficiencies up to 30%, but these have yet to be built or confirmed. Thermophotovoltaic cells and silicon thermcouples degrade faster than thermocouples, especially in the presence of ionizing radiation. Further research is needed in this area.

Dynamic generators, unlike thermoelectrics, use moving parts to mechanically convert heat into electricity. Unfortunately, those moving parts can wear out and need maintenance, which may not be possible for certain applications like space probes. Dynamic power sources also cause vibration and RF noise. Even so, NASA has worked on developing a next generation RTG called a Stirling Radioisotope Generator (SRG) that uses Free-Piston Stirling engines to produce power. SRG prototypes demonstrated an average efficiency of 23%, and higher efficiency can be achieved with the use of greater temperature differentials between the hot and cold ends of the generator. The use of magnetically non-contacting moving parts, non-degrading flexural bearings, and a lubrication-free and hermetically sealed environment have, in test units, demonstrated no appreciable degradation over years of operation. Experimental results demonstrate that an SRG could continue running for decades without maintenance. Vibration can be reduced through damping and counter piston movement. The most likely future use for SRG's may be future Mars Rovers where vibration is less of a worry.

Thermophotovoltaic cells

Thermophotovoltaic cells work by the same principles as a photovoltaic cell, except that they convert infrared light (rather than visible light) emitted by a hot surface, into electricity. Thermophotovoltaic cells have an efficiency slightly higher than thermoelectric couplers and can be overlaid on thermoelectric couples, potentially doubling efficiency. The University of Houston TPV Radioisotope Power Conversion Technology development effort is aming at combining thermophotovoltaic cell concurrently with thermocouples to provide a 3 to 4-fold improvement in system efficiency over current thermoelectric radioisotope generators.

Thermophotovoltaic (TPV) energy conversion is a direct conversion process from heat differentials to electricity via photons. A basic thermophotovoltaic system consists of a thermal emitter and a photovoltaic diode cell. The temperature of the thermal emitter varies between different systems from about 900 °C to about 1300 °C, although in principle TPV devices can extract energy from any emitter with temperature elevated above that of the photovoltaic device (forming an optical heat engine). The emitter can be a piece of solid material or a specially engineered structure. A conventional solar cell is effectively a TPV device in which the Sun functions as the emitter. Thermal emission is the spontaneous emission of photons due to thermal motion of charges in the material. For normal TPV temperatures, this radiation is mostly at near infrared and infrared frequencies. The photovoltaic diodes can absorb some of these radiated photons and convert them into free charge carriers, that is electricity.

Thermophotovoltaic systems have few, if any, moving parts and are therefore very quiet and require low maintenance. These properties make thermophotovoltaic systems suitable for remote-site and portable electricity-generating applications. Their efficiency-cost properties, however, are often rather poor compared to other electricity-generating technologies. Current research in the area aims at increasing the system efficiencies while keeping the system cost low. In the design of a TPV system, it is usually desired to match the thermal emission's optical properties (wavelength, polarization, direction) with the most efficient conversion characteristics of the photovoltaic cell, since unconverted thermal emission is a major source of inefficiency. Much research and development in TPVs therefore concerns methods for controlling the emitter's properties.

TPV cells have often been proposed as auxiliary power conversion devices for regeneration of lost heat in other power generation systems, such as steam turbine systems or solar cells. Many attribute the idea of this system to the French scientist Pierre Aigrain (1956).TPV research is a very active area. Among others, the University of Houston TPV Radioisotope Power Conversion Technology development effort is aiming at combining thermophotovoltaic cell concurrently with thermocouples to provide a 3 to 4-fold improvement in system efficiency over current thermoelectric radioisotope generators

Alkali-metal thermal to electric converter

The alkali-metal thermal to electric converter (AMTEC) is a thermally regenerative electrochemical device for the direct conversion of heat to electrical energy. It is characterized by high potential efficiencies and no moving parts, which make it a candidate for space power applications. It is an electrochemical system which is based on the electrolyte used in the sodium-sulfur battery, sodium beta-alumina. The device is a sodium concentration cell which uses a ceramic, polycrystalline β-alumina solid electrolyte (BASE), as a separator between a high pressure region containing sodium vapor at 900 - 1300 K and a low pressure region containing a condenser for liquid sodium at 400 - 700 K. Efficiency of AMTEC cells has reached 16% in the laboratory and is predicted to approach 20%.

This device accepts a heat input at 900K-1300K and produces direct current with predicted device efficiencies of 15-40%. In this device sodium is driven around a closed thermodynamic cycle between two heat reservoirs at different temperatures. The unique feature of the AMTEC cycle is the isothermal expansion of sodium vapour through a solid electrolyte which causes sodium atoms to separate into sodium ions and electrons. The AMTEC thus the work of isothermal expansion of sodium vapour directly into electric power.

The converter is based on the electrolyte used in the sodium-sulfur battery, sodium beta-alumina. The device is a sodium concentration cell which uses a ceramic, polycrystalline beta-alumina solid electrolyte (BASE), as a separator between a high pressure region containing sodium vapor at 900 - 1300K and a low pressure region containing a condenser for liquid sodium at 400 - 700K. For the single cell, the open voltage of 1.37 V and the maximum power of 7.89 W and maximum power density of 0.40 W/cm2 at temperature of 1077 K have been obtained.

Efficiency of AMTEC cells has reached 16% in the laboratory. High voltage multi-tube modules are predicted (using state-of-the-art computer simulations) to be 20% to 25% efficient, and power densities up to 0.2 kWe/liter appear to be achievable in the near future. Calculations show that replacing sodium with a potassium working fluid increases the peak efficiency from 28% to 31% at 1100 K with I mm thick BASE tube. Further development will raise the power densities substantially, and raise the efficiency into the 35% to 40% range.

AMTEC requires energy input at modest temperatures, and not at a specific wavelength, it is easily adapted to any heat source, including radioisotope, concentrated solar, external combustion, or nuclear reactor. A solar thermal power conversion system based on an AMTEC has advantages over other technologies (including photovoltaic systems) in terms of the total power that can be achieved with such a system and the simplicity of the system (which includes the collector, energy storage (thermal storage with phase change material) and power conversion in a compact unit). The overall system could achieve as high as 14 We/kg with present collector technology and future AMTEC conversion efficiencies. The energy storage system outperforms batteries, and the temperatures at which the system operates allows long life and reduced radiator size (heat reject temperature of 600 K). Deep-space applications would use radioisotope heat similar to RTG's, Hybrid systems are in design.

While space power systems are of intrinsic interest, terrestrial applications will offer large scale applications for AMTEC systems. At the +25% efficiency projected for the device and projected costs of $350/kWe, AMTEC is expected to prove useful for a very wide variety of distributed generation applications including self-powered fans for high efficiency furnaces and water heaters and recreational vehicle power supplies. Cathodic protection of pipelines, remote telemetry from oil well sites are other areas where this type of electrical generation might be used. The potential to scavenge waste heat may allow for integration of this technology into general residential and commercial cogeneration schemes although costs per kilowatt-hour would have to drop substantially from current projections.

Reciprocating Electromechanical Atomic Batteries

Electromechanical atomic batteries use the build up of charge between two plates to pull one bendable plate towards the other, until the two plates touch, discharge, equalizing the electrostatic buildup, and spring back. The mechanical motion produced can be used to produce electricity through flexing of a piezoelectric material or through a linear generator. Milliwatts of power are produced in pulses depending on the charge rate, in some systems with cycles up to radio frequency regions.

The Radioisotope piezoelectric generator converts energy stored in the radioactive material directly into motion to generate electricity by the repeated deformation of a piezoelectric material. This approach creates a high-impedance source and, unlike chemical batteries, the devices will work in a very wide range of temperatures.

A piezoelectric cantilever is mounted directly above a base of the radioactive isotope nickel-63. The Milli-Curie-level nickel-63 thin film generates electrons alone. As the isotope decays, it emits beta particles. As the cantilever accumulates the emitted electrons, it builds up a negative charge at the same time that the isotope film becomes positively charged. The beta particles essentially transfer electronic charge from the thin film to the cantilever. The opposite charges cause the cantilever to bend toward the isotope film. Just as the cantilever touches the thin-film isotope, the charge jumps the gap. That permits current to flow back onto the isotope, equalizing the charge and resetting the cantilever. As long as the isotope is decaying - a process that can last for decades - the tiny cantilever will continue its up-and-down motion. As the cantilever directly generates electricity when deformed, a charge pulse is releasrd each time the cantilever cycles.

Radioactive isotopes can continue to release energy over peri­ods ranging from weeks to decades. The half-life of nickel-63, for example, is over 100 years thus a battery using this iso­tope might continue to supply useful energy for at least half that time. Researchers have demonstrated devices with about 7% efficiency with high duty cycles of 120 Hz to low duty (every three hours) cycle self-reciprocating actuators.

External articles and references

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  • Physical, Nuclear, and Chemical, Properties of Plutonium, IEER Factsheet.
  • Mortality and Morbidity Risk Coefficients for Selected Radionuclides, Argonne National Laboratory
  • A. Angelo Jr. and D. Buden," Space Nuclear Power", Krieger Publishing Company 1985 ISBN 0894640003
  • Olsen, L.C. Energy Conversion, 1973, 13, 4, 117-127 doi 10.1016/0013-7480(73)90010-7
  • N. S. Rasor, "Thermionic energy converter," in Fundamentals Handbook of Electrical and Computer Engineering, vol. II, S.S.L. Chang., Ed., New York: Wiley, 1983, p. 668.
  • G. N. Hatsopoulos and E. P. Gyftopoulos, Thermionic Energy Conversion, vol. I, (1973); vol II, (1979); MIT Press, Cambridge, MA.
  • F.G. Baksht, et al., Thermionic Converters and Low-Temperature Plasma, Russian Edition (B. Moyzhes and G. Pikus, Eds), Acad. of Sciences USSR, Moscow, 1973. English Edition (L.K.Hansen, Ed.) available as DOE-tr-1 from NTIS, Springfield, VA.
  • J. Mills and R. Dahlberg, “Thermionic Systems for DOD Missions”, Proc. 8th Symp. on Space Nucl. Power Syst., (Albuquerque, NM), pt.3, p. 1088.
  • G. M. Griaznov, et al., “Thermoemission Reactor-Converters for Nuclear Power Units in Outer Space”, Atomnaya Energiya 66, 371-383 (1989); English translation available from Plenum.
  • E. van Kemenade & W. B. Veltkamp, “Design of a Thermionic Converter for a Domestic Heating System”, Proc. 29th Intersoc. Energy Conv. Eng. Conf., Vol. 2, p1055 (1994). Also see V.I. Yarygin, Ye. A. Meleta, V.V. Klepikov, V.A. Ruzhnikov, & L.R. Wolff, “Test of a TEC-Module”, ibid, p1061.
  • N. S. Rasor and C. Warner, “Correlation of Emission Processes for Adsorbed Alkali Films on Metal Surfaces”, J. Appl. Phys. 35, 2589 (1964).
  • N. S. Rasor, “Thermionic Energy Conversion Plasmas”, IEEE Trans. Plasma Sci., 19, 1191 (1991); invited review.
  • N.S. Rasor, “Physical-Analytical Model for Cesium/Oxygen Coadsorption on Tungsten”, Proc. 27th Intersoc. Energy Conv. Eng. Conf., Vol.3, p3.529 (1992).
  • J-L. Desplat, L.K. Hansen, G.L. Hatch, J.B. McVey and N.S. Rasor, “HET IV Final Report”, Volumes 1 & 2, Rasor Associates Report #NSR-71/95/0842, (Nov. 1995); performed for Westinghouse Bettis Laboratory under Contract # 73-864733; 344 pages. Also available in total as C.B. Geller, C.S. Murray, D.R. Riley, J-L. Desplat, L.K. Hansen, G.L. Hatch, J.B. McVey and N.S. Rasor, “High-Efficiency Thermionics (HET-IV) and Converter Advancement (CAP) programs. Final Reports”, DOE DE96010173; 386 pages (1996).
  • N.S. Rasor, “The Important Effect of Electron Reflection on Thermionic Converter Performance”, Proc. 33rd Intersoc. Energy Conv. Engr. Conf., Colorado Springs, CO, Aug., 1998, paper 98-211.
  • V. Yarygin, et al., “Energy Conversion Options For NASA’s Space Nuclear Power Systems Initiative – Underestimated Capability Of Thermionics”, Proc. 2nd International Energy Conversion Engineering Conference, Providence, RI, Aug. 2004.
  • Polymers, Phosphors, and Voltaics for Radioisotope Microbatteries, by Kenneth E. Bower (Editor), et al

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