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dc.contributor.advisorShi, Li, Ph. D.
dc.contributor.advisorZhou, Jianshi
dc.creatorChen, Xi, active 21st centuryen
dc.date.accessioned2015-01-15T20:29:41Zen
dc.date.issued2014-12en
dc.date.submittedDecember 2014en
dc.identifier.urihttp://hdl.handle.net/2152/28047en
dc.descriptiontexten
dc.description.abstractThermoelectric (TE) materials, which can convert temperature gradients directly into electricity and vice versa, have received renewed interest for waste heat recovery and refrigeration applications. Higher manganese silicides (HMS) are promising p-type TE materials due to the abundance of the constituent elements, environmental friendliness, and good chemical stability. The objective of this dissertation is to establish a better understanding of the structure-TE properties relationship of HMS with a complex Nowotny chimney ladder structure. The focus of this work is on the investigations of the phonon dispersion of HMS crystals and the effects of chemical doping and nanostructuring on the TE properties of polycrystalline HMS. HMS crystals have been synthesized by the Bridgeman method for inelastic neutron scattering measurements of the phonon dispersion. In conjunction with density functional theory calculations, the results clearly show the presence of numerous low-lying optical phonon branches, especially an unusually low-energy optical phonon polarization associated with the twisting motions of the Si helical ladders in the Mn chimneys. The obtained phonon dispersion can be used to explain the low and anisotropic thermal conductivity of HMS crystals. (Al,Ge) double doping was found to be effective in modifying the electrical properties of HMS polycrystals. The peak thermoelectric power factor occurs at an optimized hole concentration of 1.8~2.2×10²¹ cm⁻³ at room temperature. On the other hand, Re substitution can suppress the lattice thermal conductivity to approach the calculated minimum value corresponding to the amorphous limit. Meanwhile, the thermoelectric power factor does not markedly change at low Re content of x ≤ 0.04 although it drops considerably with increasing Re content. Hence, the peak ZT has been improved to ~0.6 in both systems. The effects of nanostructuring on the TE properties have been studied in the cold-pressed samples and ball-milled samples. The thermal conductivity was reduced remarkably by decreasing the grain size. It is found that the grain size effects are more significant at low temperature. However, it is difficult to reduce the grain size to less than 50 nm without the formation of impurity phases by ball milling. These facts limit the ZT enhancement of the nanostructured HMS at high temperatures in this study.en
dc.format.mimetypeapplication/pdfen
dc.language.isoenen
dc.subjectThermoelectricen
dc.subjectHigher manganese silicidesen
dc.subjectPhonon dispersionen
dc.subject(Al,Ge) dopingen
dc.subjectRe dopingen
dc.subjectNanostructuresen
dc.titleSynthesis and thermoelectric properties of higher manganese silicides for waste heat recoveryen
dc.typeThesisen
dc.date.updated2015-01-15T20:29:42Zen
dc.description.departmentMaterials Science and Engineeringen
thesis.degree.departmentMaterials Science and Engineeringen
thesis.degree.disciplineMaterials Science and Enigneeringen
thesis.degree.grantorThe University of Texas at Austinen
thesis.degree.levelDoctoralen
thesis.degree.nameDoctor of Philosophyen


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