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Gang Chen. Lee, Hohyun, 1978- Massachusetts Institute of Technology. Dept. of Mechanical Engineering. Massachusetts Institute of Technology. Department of Mechanical Engineering 2010-01-07T20:57:33Z 2010-01-07T20:57:33Z 2009 2009 http://hdl.handle.net/1721.1/50589 464614755 Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009. Page 164 blank. Includes bibliographical references. Direct energy conversion between thermal and electrical energy based on thermoelectric effects is attractive for potential applications in waste heat recovery and environmentally-friendly refrigeration. The energy conversion efficiency is related to the thermoelectric figure of merit ZT, which is proportional to the electrical conductivity, the square of the Seebeck coefficient, and the inverse of the thermal conductivity. Currently, the low ZT values of available materials restrict the large scale applications of this technology. Recently, however, significant enhancements in ZT were reported in nanostructured materials such as superlattices mainly due to their low thermal conductivities. According to the studies on heat transfer mechanisms in nanostructures, the reduced thermal conductivity of nanostructures is mainly attributed to the increased scattering of phonons at interfaces. Based on this idea, nanocomposites are also expected to have a lower thermal conductivity than their bulk counterparts of the same chemical configuration. Nanocomposites are materials with constituents of less than 100 nm in size. They can be fabricated with a low cost just by mixing nano sized particles followed by consolidation of nano sized powders. In this thesis, SiGe nanocomposites are investigated for power generation at high temperature. The material properties are characterized at different temperatures, and the optimized process conditions are explored experimentally. In addition, theoretical studies are carried out for better understanding of transport phenomena and our experimental results. (cont.) Grain boundaries in nanocomposites can scatter phonons, when their mean free paths are longer than the grain size. Mean free paths of electrons are usually shorter than the grain size of nanocomposites, so that the electrical conductivities of nanocomposites are not expected to change significantly. However, the experimental results show that nanostructures indeed affect electron transport. The grain boundary effects on electron transport are investigated to explain the experiments. Furthermore, the effects of nanosized pores are explored. Our experimental results show that pores in nanocomposites degrade the electrical conductivity more than predicted by effective medium theories. A scattering model is developed to understand the transport phenomena in porous materials. These modeling studies can also be used to guide sample preparation conditions. by Hohyun Lee. Ph.D. 164 p. eng Massachusetts Institute of Technology M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission. http://dspace.mit.edu/handle/1721.1/7582 Mechanical Engineering. Modeling and characterization of thermoelectric properties of SiGe nanocomposites Thermoelectric properties of SiGe nanocomposites Thesis application/pdf false Publication unknown unknown <Publication xmlns="https://www.openaire.eu/cerif-profile/1.1/" id="1324a648-982a-4418-a80d-db9de4e311fa"> <Type xmlns="https://www.openaire.eu/cerif-profile/vocab/COAR_Publication_Types">http://purl.org/coar/resource_type/c_1843</Type> <Language>eng</Language> <Title>Modeling and characterization of thermoelectric properties of SiGe nanocomposites</Title> <Subtitle>Thermoelectric properties of SiGe nanocomposites</Subtitle> <PublishedIn> <Publication> </Publication> </PublishedIn> <PublicationDate>2009</PublicationDate> <Authors> <Author> <DisplayName>Lee, Hohyun, 1978-</DisplayName> <Affiliation> <OrgUnit> </OrgUnit> </Affiliation> </Author> </Authors> <Editors> </Editors> <Publishers> <Publisher> <DisplayName>Massachusetts Institute of Technology</DisplayName> <OrgUnit /> </Publisher> </Publishers> <License>http://dspace.mit.edu/handle/1721.1/7582</License> <Keyword>Mechanical Engineering.</Keyword> <Abstract>Direct energy conversion between thermal and electrical energy based on thermoelectric effects is attractive for potential applications in waste heat recovery and environmentally-friendly refrigeration. The energy conversion efficiency is related to the thermoelectric figure of merit ZT, which is proportional to the electrical conductivity, the square of the Seebeck coefficient, and the inverse of the thermal conductivity. Currently, the low ZT values of available materials restrict the large scale applications of this technology. Recently, however, significant enhancements in ZT were reported in nanostructured materials such as superlattices mainly due to their low thermal conductivities. According to the studies on heat transfer mechanisms in nanostructures, the reduced thermal conductivity of nanostructures is mainly attributed to the increased scattering of phonons at interfaces. Based on this idea, nanocomposites are also expected to have a lower thermal conductivity than their bulk counterparts of the same chemical configuration. Nanocomposites are materials with constituents of less than 100 nm in size. They can be fabricated with a low cost just by mixing nano sized particles followed by consolidation of nano sized powders. In this thesis, SiGe nanocomposites are investigated for power generation at high temperature. The material properties are characterized at different temperatures, and the optimized process conditions are explored experimentally. In addition, theoretical studies are carried out for better understanding of transport phenomena and our experimental results.</Abstract> <Abstract>(cont.) Grain boundaries in nanocomposites can scatter phonons, when their mean free paths are longer than the grain size. Mean free paths of electrons are usually shorter than the grain size of nanocomposites, so that the electrical conductivities of nanocomposites are not expected to change significantly. However, the experimental results show that nanostructures indeed affect electron transport. The grain boundary effects on electron transport are investigated to explain the experiments. Furthermore, the effects of nanosized pores are explored. Our experimental results show that pores in nanocomposites degrade the electrical conductivity more than predicted by effective medium theories. A scattering model is developed to understand the transport phenomena in porous materials. These modeling studies can also be used to guide sample preparation conditions.</Abstract> <Access xmlns="http://purl.org/coar/access_right" > </Access> </Publication>