Crystal Growth and Thermoelectric Properties of Materials with Layered and Complex Structures
Abstract
Amid escalating energy demands, the significance of thermoelectric applications, which convert waste heat into electrical energy, is receiving heightened attention. This dissertation delves into the synthesis and thermoelectric properties of layered and complex-structured materials, focusing on enhancing thermoelectric performance through two key approaches: improving the power factor and reducing lattice thermal conductivity.
ZrTiSe4, a layered van der Waals transition-metal chalcogenide, exhibits a high Seebeck coefficient of –202 μV K−1 at 300 K despite being a semimetal. This unique behavior is attributed to the triple valley degeneracy of conduction bands at the M point, leading to a large effective mass of the density of states, as demonstrated through a combination of theoretical and experimental studies. ZrTiSe4 serves as a model system to illustrate how band convergence can enhance the power factor. Additionally, its low thermal conductivity, resulting from low sound velocity, phonon-boundary scattering, and the formation of a solid solution with randomly occupied Zr and Ti atoms, positions ZrTiSe4 as a promising material for thermoelectric applications.
Fe2Ge3 with an incommensurate Nowotny chimney-ladder compound also exhibits significant thermoelectric potential. Single crystals of Fe2Ge3 grown by the chemical vapor transport method exhibit a low, nearly temperature-independent thermal conductivity of 1.9 W m−1 K−1 at 300 K. The low thermal conductivity is attributed to the scattering of heat carrying acoustic phonons by low-energy optical phonons and the involvement of degenerate overlapping optical vibrational modes known as diffusons, which contribute to thermal transport through a hopping mechanism. This material is used to demonstrate the intrinsic thermal transport mechanisms in complex crystal structures.
This dissertation also presents the synthesis and characterization of other transition-metal chalcogenides, including ZrSe3 and ZrTiTe4. Thermal transport measurement on ZrSe3 suggests that optical phonons can contribute to thermal transport at high temperatures. Measurements of Hall resistivity and the Seebeck coefficient on ZrTiTe4 reveal a bipolar carrier transport in this material, which has potential applications in transverse thermoelectric devices.