Download or read book Microfabricated Thermionic Energy Converters written by Jae Hyung Lee and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Thermionic energy converters (TECs) are heat engines that convert heat directly to electricity at very high temperatures. This energy conversion process is based on thermionic emission--the evaporation of electrons from conductors at high temperatures. In its simplest form, the converter consists of two electrodes in the parallel plate capacitor geometry, and it uses the thermionically emitted current to drive an electrical load. This dissertation presents research on five key areas of microfabricated thermionic energy converters ([mu]-TECs). First, the numerical calculation of the emitter-collector gap that maximizes the power conversion efficiency of thermionic energy converters (TECs) is discussed. Thermionic energy converters require emitter and collector work-functions that are relatively low, to reach useful efficiencies at typical operating temperatures of 1000 - 1500 oC. The optimum arises because efficiency drops both at very large gaps, due to space-charge limitations on the TEC current, and at very small gaps, due to the increased heat loss via near-field radiative heat transfer. The numerical calculation results show that, for typical TECs made with cesiated tungsten electrodes, the optimal gaps range from 900 nm to 3 [micrometers]. I then discuss several prototypes of mechanically and thermally robust [mu]-TECs, including the stress-relieved emitter design, emitter-collector structural design, as well as a recent approach for the stand-alone (encapsulated) [mu]-TECs. Thermionic emission from the SiC emitter was demonstrated for the first time. The stress-relieved design emitters were analyzed, and the work-function of the SiC emitter was estimated at temperatures of up to 2900K. Also described are both the planar and the U-shaped suspension for microfabricated TECs ([mu]-TECs). Our initial planar [mu]-TECs achieved emitter temperatures of over 2000 K with incident optical intensity of approximately 1 W/mm2 (equivalent to 1000 Suns), remained structurally stable under thermal cycling, and maintained a temperature difference between the emitter and the collector of over 1000 K. Conformal sidewall deposition of poly-SiC on a sacrificial mold is used to fabricate stiff suspension legs with U-shaped cross sections, which increases the out-of-plane rigidity and prevents contact with the substrate during the heating of the suspended emitter. By extending the conventional technique of cesium coatings to SiC, we reduce the work-function from 4 eV to 1.65 eV at room temperature. Subsequently, we tested [mu]-TECs with both barium and barium oxide coatings. The coatings reduced the work-function of the SiC emitter to as low as ~2.14 eV and increased the thermionic current by 5-6 orders of magnitude, which is a key step toward realizing a efficient thermionic energy converter. Encapsulation of [mu]-TEC was achieved by an anodic bond between pyrex and the silicon substrate with via feedthroughs. Last, I introduce the photon-enhanced thermionic emission (PETE) concept, and show why a single crystal photo-emitter is needed. I cover my recent fabrication development of smart-cut layer transfer using Spin-on-Glass (SoG). In addition, a novel layer transfer technology that can transfer any device materials onto the glass substrate, which I call "Anything on Glass, " is briefly described. I, then, describe how the first demonstration of the photon-enhanced thermionic emission (PETE) from the microfabricated emitter was achieved. The p-type SiC emitter was used to demonstrate PETE in an uncesiated and microfabricated sample, bringing this energy conversion approach closer to practical applications.

Download or read book Application of Microfabrication Technology to Thermionic Energy Conversion Progress Report No 1 1 May 31 October 1979 written by and published by . This book was released on 1980 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: In a theoretical and experimental program to evaluate those areas where three-dimensional microfabrication techniques could be important for improving methods of thermionic energy conversion, effort in the first reporting period has been directed toward a theoretical study of microstructures of electrodes for thermionic energy converters. The properties of a cesiated tungsten thermionic energy converter were analyzed with electrode temperatures compatible with a flame-generated heat source (T/sub c/ = 1650°K and T/sub a/ = 700°K), in order to estimate the efficiency, power production, and appropriate electrode spacing for microfabricated devices. The analysis yielded a maximum efficiency of 16 percent and corresponding electrical power of 11 W/cm2, requiring an emission current of 18 A/cm2. The study revealed that to attain these parameters, electrode spacing must be approximately 1 .mu.m, and that such a close-spaced diode with cesiated tungsten electrodes would operate approximately as a vacuum diode. That is, the principal function of the cesium would be to control the work function of the electrode surfaces. Operating at the point of peak efficiency, little space-charge limitation of the emission and little plasma resistance would be produced, because the atom/atom and electron/atom mean free paths would be larger than the interelectrode space.

Download or read book Application of Microfabrication Technology to Thermionic Energy Conversion Final Report 1 April 1979 31 March 1981 written by and published by . This book was released on 1981 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The first-year effort emphasized study of the kind of microstructures that could improve the performance of thermionic converters. Two ideas considered to have a fair chance of success emerged from this study: (1) use of a very closely spaced diode to eliminate the space-charge limitation of electron flow from emitter to collector, cesium vapor being used to control the work function of the emitter; and (2) use of field emission electrons, injected into a relatively large diode gap from microcathodes built into the collector, to produce ions to neutralize the space charge. The gas in the diode gap would be a mixture of cesium (to control the emitter work function) and xenon to optimize the ionization. A number of schemes were attempted to build closely spaced diodes with spacing in the 1 to 5 .mu.m range, which overcame the problems of lateral differential expansion, surface irregularities on the electrodes, and heat loss down the pillars holding the gap spacing. Theoretical studies on using field emitter electrons to produce the space charge neutralizing ions showed that this approach was feasible. However, the program was terminated before any experimental work could be initiated in this area.

Download or read book Application of Microfabrication Technology to Thermionic Energy Conversion Progress Report No 6 November 1 1980 January 31 1981 written by and published by . This book was released on 1981 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Effort was directed toward the fabrication of a micron-spaced thermionic converter diode. This technique demonstrated that interelectrode spacings down to 1.5 .mu.m could be obtained. Several methods of duplicating the emitter and collector surfaces were also investigated. Two new techniques are proposed; both stem from an earlier idea of using evaporation, photolithography, and etching techniques. These two fabrication methods yielded a one-piece diode structure with a thick-film copper collector, eliminating the need to physically duplicate the electrode surfaces and realign the electrodes. Effort has also been directed toward a more detailed theoretical analysis of micron-spaced thermionic converter performance. Taking into account heat losses through the interelectrode support structure, it is likely that the maximum energy conversion efficiency may be greatest at a spacing somewhat larger than 1 micron (.mu.m), but less than 10 .mu.m.

Download or read book Application of Microfabrication Technology to Thermionic Energy Conversion Progress Report No 5 August 1 1980 October 31 1980 written by and published by . This book was released on 1980 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Considerable effort was directed toward the fabrication of a micron-spaced thermionic converter. In the process, a new technique has evolved which appears more promising and has been investigated theoretically. In both techniques, electrodes of similar differential thermal expansions are utilized to prevent shear stresses from disrupting the spacing pillars. The newer technique has additional advantages that simplify the fabrication of the diode structure, thus making it more practical. Progress is described.

Download or read book Application of Microfabrication Technology to Thermionic Energy Conversion Progress Report 4 1 May 1980 to 31 July 1980 written by and published by . This book was released on 1980 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Two applications of microfabrication technology to thermionic converters have been investigated theoretically. The first is a novel method of maintaining micron or submicron spacings over large areas (>1 cm2), using metals of different expansion coefficients to eliminate the shear stresses on the insulating pillars separating the electrodes. The second uses low-voltage field-emission sources to create ions in a large (approx. 1 mm) interelectrode gap for space charge neutralization. The theoretical results for both these approaches are highly encouraging.

Download or read book Thermionic Energy Converter written by Dr. Walter L. Knecht and published by . This book was released on 1960 with total page 62 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Application of Microfabrication Technology to Thermionic Energy Conversion Progress Report 2 written by and published by . This book was released on 1980 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Two diode designs are being studied that require basically different fabrication techniques. The fabrication of electrodes was begun to test the concepts and the feasibility of these designs. This work was done with polished molybdenum electrode surfaces and vapor-deposited alumina insulator structures. Initial results were obtained in the close spacing of molybdenum electrodes with alumina insulators. Spacings were demonstrated with 1/2-inch diameter discs in the range of 3 to 5 .mu.m, using 1-.mu.m thick insulators. The deposition of thin alumina films on a molybdenum substrate was demonstrated, upon which arrays of thin film electrodes about 1/32-inch in diameter were deposited. A second approach uses a thin film as one of the electrodes. An analysis of the thermal and electrical transport properties of 1-.mu.m thick tungsten film shows that edge-connected films about 1 mm square in area could withstand the stresses generated by differential thermal expansion when the film is attached to the opposite electrode by stand-off insulators.

Download or read book Low Work Function Material Development for the Microminiature Thermionic Converter written by Donald Bryan King and published by . This book was released on 2004 with total page 116 pages. Available in PDF, EPUB and Kindle. Book excerpt: Thermionic energy conversion in a miniature format shows potential as a viable, high efficiency, micro to macro-scale power source. A microminiature thermionic converter (MTC) with inter-electrode spacings on the order of microns has been prototyped and evaluated at Sandia. The remaining enabling technology is the development of low work function materials and processes that can be integrated into these converters to increase power production at modest temperatures (800 - 1300 K). The electrode materials are not well understood and the electrode thermionic properties are highly sensitive to manufacturing processes. Advanced theoretical, modeling, and fabrication capabilities are required to achieve optimum performance for MTC diodes. This report describes the modeling and fabrication efforts performed to develop micro dispenser cathodes for use in the MTC.

Download or read book The Development of an Improved Thermionic Energy Converter written by and published by . This book was released on 1965 with total page 192 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Energy Research Abstracts written by and published by . This book was released on 1982 with total page 1696 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Feasibility of Thermionic Energy Converters written by Edwin Booth Forrest and published by . This book was released on 1972 with total page 98 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Renewable Technologies written by Mohammad Islam and published by LAP Lambert Academic Publishing. This book was released on 2010-06 with total page 204 pages. Available in PDF, EPUB and Kindle. Book excerpt: Thermionic energy converter (TEC) is a device to generate electricity from heat in a silent, vibration-free operation. Non-idealities such as negative space-charge effect and electron reflection from collector surface adversely effect output current density and device efficiency. Using a range of thin film deposition techniques such as rf plasma magnetron sputtering, electrochemical deposition and electroless process, different surface structures were developed and tested for suppression of electron reflection using Cs/O vapor plasma TECs. Principles of both vacuum and solution based synthesis of thin films, and promising device configurations for thermionic energy converters are presented. This book overviews evolution of thermionics technology in a comprehensive manner, identifies key issues and challenges to be addressed, and suggests feasible solutions based on a sensible choice of materials and processes. The contents of this book are useful for teaching at both undergraduate and postgraduate levels as well as for researchers working on thin film synthesis and characterization for applications in energy, microelectronics, and surface engineering industries.

Download or read book Thermionic Energy Conversion written by George N. Hatsopoulos and published by . This book was released on 1979 with total page 696 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book THERMIONIC ENERGY CONVERTERS written by I. I. DAMASKINA and published by . This book was released on 1962 with total page 1 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Application of Semiconductors to Thermionic Energy Converters written by Daniel C. Riley and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Thermionic energy conversion (TEC) is a direct heat-to-electricity conversion technology with the potential to leapfrog state-of-the-art solid-state conversion in efficiency and power density. In a thermionic energy converter, electrons evaporate from a hot electrode, the cathode, into a vacuum gap and are collected by a cooler electrode, the anode, to generate electric current. In the 1960s-1970s numerous groups reported thermionic converters with power densities above 10 W/cm^2 and conversion efficiencies of ~15%. However most of this work was tied to the US space-nuclear program which ended in 1973, and thermionics research has never fully recovered. As a result two central challenges yet remain in thermionics: (1) High operating temperatures necessary to produce electric current result in difficult materials challenges, and (2) low operating voltages due to losses associated with space charge and high anode work functions. However, new opportunities to tackle these challenges are available as a result of the breathtaking rise of semiconductor fabrication technology. In this work I present a new physical mechanism called photon enhanced thermionic emission (PETE). This concept is an improvement on thermionic emission by using light to boost the average energy of carriers in a hot p-type semiconductor cathode. Additionally, unlike in a photovoltaic cell, the waste heat from recombination losses and sub-bandgap light absorption is utilized to heat the cathode. Thus a PETE cathode can produce efficient electron emission at lower temperatures than a thermionic cathode. I will describe theoretical calculations showing that a PETE device may exceed 40% solar power conversion efficiency, and the conversion efficiency may exceed 50% if a PETE device is used in tandem with a solar thermal backing cycle. I will also describe an experimental demonstration of the PETE effect in an ultra-high vacuum photoemission measurement. In the cathode of an energy converter based on photon-enhanced thermionic emission (PETE) photoexcited carriers may need to encounter the emissive surface numerous times before having sufficient thermal energy to escape into vacuum and therefore should be confined close to the surface. However, in a traditional planar geometry, a thin cathode results in incomplete light absorption. Nanostructuring has the potential to increase light capture and boost emission by decoupling the lengths associated with photon absorption and electron emission. Nanostructures may complicate the properties of the emissive surface; therefore, the effect of nanostructuring on emission efficiency needs to be studied. In this work I describe results from a suite of simulation tools we have developed to capture the full photoemission process: photon absorption, carrier transport within the active material, and electron ballistics following emission. I show that the theoretical efficiency of a negative electron affinity emitter may be increased with nanostructures if light absorption and electron escape ballistics are considered. I then describe measurements of the photoemission efficiency of fabricated nanostructures that were designed based on the results of the simulation suite. I will also present a fundamentally new method to increase the operating voltage of a TEC by lowering the anode work function using the surface photovoltage effect. When a semiconductor surface is illuminated, photo-excited carriers form an internal dipole, or surface photovoltage (SPV), in the band-bending region and begin to flatten the bands near the surface. This SPV is analogous to the photovoltage in a photovoltaic cell and can reduce the effective work function of the material. I will describe an experimental demonstration using the SPV effect to produce a low work function surface. I will also describe a proof-of-concept demonstration of the SPV effect applied to improve the I-V characteristics of thermionic device. This generic physical process extends across materials systems and forms a realistic path to ultra-low work functions in devices to enable efficient thermionic energy conversion.

Download or read book Thermionic Energy Conversion Processes and devices written by George N. Hatsopoulos and published by MIT Press (MA). This book was released on 1973 with total page 288 pages. Available in PDF, EPUB and Kindle. Book excerpt: Good,No Highlights,No Markup,all pages are intact, Slight Shelfwear,may have the corners slightly dented, may have slight color changes/slightly damaged spine.