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dc.contributor.advisorGang Chen.en_US
dc.contributor.authorNi, George (George Wei)en_US
dc.contributor.otherMassachusetts Institute of Technology. Department of Mechanical Engineering.en_US
dc.date.accessioned2014-12-08T18:54:12Z
dc.date.available2014-12-08T18:54:12Z
dc.date.copyright2014en_US
dc.date.issued2014en_US
dc.identifier.urihttp://hdl.handle.net/1721.1/92171
dc.descriptionThesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.en_US
dc.descriptionCataloged from PDF version of thesis.en_US
dc.descriptionIncludes bibliographical references (pages 78-81).en_US
dc.description.abstractThermoelectric materials are a promising class of direct energy conversion materials, usually consisting of highly doped semiconductors. The key to maximizing their thermal to electrical energy conversion lies in optimizing three inter-related material properties, thermal conductivity, electrical conductivity, and Seebeck coefficient. All three properties are affected by the carrier concentration of the thermoelectric material. In practice, tedious trial-and-error testing is needed to determine the optimal carrier concentration for the maximum figure-of-merit, ZT. Theory and computer simulations of thermoelectric properties can aid the determination of new thermoelectric materials, but several challenges remain. The bandgap is a key piece of bandstructure information, but is difficult to determine for heavily doped thermoelectric materials. Under heavy doping conditions, the effective mass and bandgap both change due to the formation of Urbach band tails and other defect states within the bandgap. Furthermore, bandgaps of heavily doped materials are difficult to observe optically, due to significant amounts of carriers in defects states within the bandgap. Conventional optical measurement techniques relying on transmittance change require extremely thin samples, on the order of microns for thermoelectrics. Photoacoustic spectroscopy is used in this work to optically probe the thermoelectric bandgap, without the need to produce thin samples. Photoacoustic spectroscopy allows simultaneous measurements of the thermal conductivity and optical absorption coefficient. In this work, a relative measurement is devised to reduce the need to carefully control experimental parameters such as light input and microphone gain. Semiconductor theory is discussed to account for the band-filling effects, and a method is proposed to extrapolate the true electronic bandgap from the Burstein-Moss shift of the absorption edge due to heavily doping.en_US
dc.description.statementofresponsibilityby George Ni.en_US
dc.format.extent81 pagesen_US
dc.language.isoengen_US
dc.publisherMassachusetts Institute of Technologyen_US
dc.rightsM.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.en_US
dc.rights.urihttp://dspace.mit.edu/handle/1721.1/7582en_US
dc.subjectMechanical Engineering.en_US
dc.titlePhotoacoustic measurement of bandgaps of thermoelectric materialsen_US
dc.typeThesisen_US
dc.description.degreeS.M.en_US
dc.contributor.departmentMassachusetts Institute of Technology. Department of Mechanical Engineering
dc.identifier.oclc897137965en_US


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