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Microfabricated Thermionic Electron Emitters Microfabricated Thermionic Electron Emitters K. Blair Huffman Stanford University To fulfill the undergraduate Honor’s Program requirements for the Department of Electrical Engineering June 2014 I certify that I have read this Honor’s thesis and that, in my opinion, it is fully adequate in scope and quality to fulfill the requirements for the undergraduate Honor’s Program for the Department of Electrical Engineering _______________________________ Roger T. Howe, Principal Adviser I certify that I have read this Honor’s thesis and that, in my opinion, it is fully adequate in scope and quality to fulfill the requirements for the undergraduate Honor’s Program for the Department of Electrical Engineering _______________________________ R. Fabian Pease, Second Reader Acknowledgements I would like to first thank Professor Roger Howe for his constant advice and assistance on this project and on my academic aspirations. None of this would have been possible without his support throughout my undergraduate career, and I cannot thank him enough. Prof. Howe’s energy group has been a tremendous resource for me. I would especially like to thank Dr. Justin Snapp for helping me with the fabrication. Next, I would like to thank Professor Fabian Pease and Dr. J Provine for introducing me to nanofabrication during my freshman year. They both helped me find my passion and have helped me pursue it during my undergraduate career. I would also like to thank the Department of Electrical Engineering for providing me with support on my project over the summer in 2013. I would like to thank the STEM fellowship, the American Indian Science and Engineering Society, Google, and Intel for providing funding. The School of Engineering’s Engineering Diversity Programs Office has given me tremendous support throughout my career. Dr. Noé Lozano and his staff introduced me to engineering and have been huge supporters of my academic pursuits. They have provided me with a community of scholars that have been crucial for my success at Stanford. They always had confidence in me even when I did not have confidence in my own abilities. Lastly, I would like to thank my parents, Bill and Susan Huffman for their continuous support. They are my biggest fans and support me in all that I do, even if they do not know what thermionic energy converters do. They have provided me with a fantastic education, love, and support, to help me reach my future goals. Abstract Thermionic energy converters (TECs) convert very high temperature to electricity without any moving parts. Specifically, the heat gives rise to electron emission from one electrode (‘emitter’) to a nearby electrode (‘collector’) and the resulting current can drive a load. To produce a useful current, both the temperature and the work functions of the emitter and collector must be sustainably controlled. Previous work established that the gap between the two electrodes should be in the range of 1 - 10 µm to minimize space charge at larger gaps and excessive heat transfer at smaller gaps. The microfabricated thin-film emitter is thermally isolated from the substrate by a poly-SiC suspension. This thesis describes the redesign of the suspended emitter and its fabrication process. The suspended plate is optimized for vertical emission by eliminating the recessed emitter surface in the previous design. Furthermore, the process includes the etching of apertures in the silicon substrate, so that underside of the suspended plate can be heated by laser illumination. The thermal resistance of the emitter suspension beams is increased by 50% in the new designs. The five-mask fabrication process is in progress in the Stanford Nanofabrication Facility. Table of Contents 1. Introduction .................................................................................................................... 1.1 Motivation.............................................................................................................................. 1 1.2 History of Thermionic Energy Converters ............................................................................ 2 1.3 Physics of Thermionic Energy Converters ............................................................................ 5 1.4 Thesis Overview .................................................................................................................... 8 2. Thermionic Emitter Redesign ....................................................................................... 2.1 Motivation.............................................................................................................................. 9 2.2 Emitter design process......................................................................................................... 11 2.3 Micro Thermionic Energy Converter Test Assembly Design ............................................ 17 2.4 Chapter Summary ................................................................................................................ 20 3. Fabrication ...................................................................................................................... 3.1 Introduction.......................................................................................................................... 22 3.2 Layout design ...................................................................................................................... 23 3.3 Fabrication Methodology .................................................................................................... 27 3.4 Chapter Summary ................................................................................................................ 30 4. Next Steps ........................................................................................................................ 4.1 Introduction.......................................................................................................................... 32 4.2 Device Testing .................................................................................................................... 32 4.3 Chapter Summary ................................................................................................................ 36 5. Conclusion ....................................................................................................................... 5.1 Summary.............................................................................................................................. 37 5.2 Future Work ........................................................................................................................ 38 Chapter 1 Introduction 1.1 Motivation Many energy conversion methods, from internal combustion or Stirling engines to steam turbines, are based on taking heat and turning it into electricity. While our current methods use mechanical moving parts as an intermediary step in conversion, thermionic energy converters (TECs) perform the same task without any moving mechanical parts. TECs have the potential to have a competitive efficiency compared to the Stirling engine Figure 1: Illustration of the functionality of a thermionic energy converter. Its functionality is based on the concept of electrons evaporating from the emitter and being collected by the collector, creating a current. [1] 1 Figure 2: The maximum energy conversion efficiency versus the emitter temperature for collector work functions of 0.5, 1.0, and 1.5 eV. The emitter work function, ϕ E, is assumed to be TE[K]/750eV. The inset shows the exergy efficiency (also known as the second-law efficiency), i.e., the energy conversion efficiency divided by the Carnot efficiency, 1- TC/TE, for the corresponding emitter and collector temperatures [12]. [2]. They can be used as a topping stage in tandem energy converters because the TEC’s rejected heat can be hot enough to power conventional heat engines [3]. Previous work on TECs has primarily focused on space applications [4]. With the use of wafer-scale processes, fabrication cost will drop and terrestrial applications become feasible. Terrestrial applications include solar concentrators, nuclear power plants, and photon enhanced thermionic energy conversion in solar applications [2]. 1.2 History of Thermionic Energy Converters When a material is heated, electrons can escape from the surface of the material by overcoming the material’s work function. Thomas Edison was one of the first to discover this phenomenon, known as thermionic emission, in 1880 when he was working on filaments for his light bulbs. His filament burned for a couple of hours and coated the 2 interior of the bulb with carbon. More importantly, Edison noticed that the bulb carried a charge after the interior of the bulb had been coated [5]. He concluded that charge carriers had been emitted from the heated filament. In 1915, W. Schlichter first conceptualized thermionic energy conversion [6]; and in 1941, two Soviet researchers, M. Y. Gurtovoy and G. I. Kovalenko, were the first to demonstrate a working thermionic energy converter [7]. The former Soviet Union decided to investigate thermionics, and in the 1960s, launched a full-scale program to develop and test in-core thermionic reactors [4]. The program cumulated with an operational thermionic converter on the TOPAZ-II space reactor, Figure 3 [8]. These converters had an operational efficiency of 6% with emitter temperatures of 1700 K, Figure 3: TOPAZ-II thermionic space reactor at the Kirkland Air Force Base [1] collector temperatures of
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