Developing novel bulk materials according to the structure-property relationship and nanoengineering the existing materials to reduce the lattice conductivity K
L are the two major strategies to achieve high figure-of-merit thermoelectric materials. Following these strategies, this work conducts comprehensive theoretical investigations on the thermoelectric transport in bulk tin chalcogenides, skutterudites and Si-based nanostructures at the atomic level, which is crucial for further improving the performance of these promising thermoelectric materials. Using the first-principles calculations combined with Boltzmann transport equation, both thermal and electrical transport properties of SnSe and SnS have been investigated and good agreements with the experiments are observed. Due to the distinct layered lattice structure, SnSe and SnS exhibit similarly anisotropic thermal and electrical transport behaviors. Mode-wise phonon analysis shows that the anisotropic and low K
L of both materials is due to the group velocities and high anharmonicity. For both materials, short mean-free-path (MFP) optical phonons dominate the thermal transport. Meanwhile, the existence of secondary conduction band valleys of low effective masses near the band edges will significantly enhance the cross-plane power factor, leading to much superior thermoelectric performance in n-doping materials. Using the same approach, K
L of CoSb
3 is predicted and analyzed. The MFP corresponding to median K
L accumulation is much longer than that predicted from the kinetic theory, indicating the importance of frequency dependence of MFP and providing an opportunity to reduce K
L by nanoengineering. The importance of optical phonons is highlighted. Important optical modes usually involve two or more pnicogen atoms moving synchronously due to strong covalent bonds. The effects of elemental substitution and nanoengineering on K
L are further investigated, demonstrating an effective strategy to depress the phonon transport by multiple scattering mechanisms. Further, the thermal transport in nanoporous Si and Si-based nanocomposites have been investigated using molecular dynamics simulations and lattice dynamics to evaluate the potential of using them as environmentally friendly alternatives to conventional thermoelectric materials. Significant anisotropy and junction effect in thermal transport are found in nanoporous Si with inhomogeneous pore pitches. The junction effect is attributed to the phonon dispersion mismatch and can be quantitatively modeled by the elastic waves, implying the importance of phonon wave behavior at nanoscale. A structure-based two-part model is also successfully developed to predict K
L in nanoporous structures. Meanwhile, K
L of Si-based nanocomposites can be reduced to the alloy limit by embedding nanoinclusions of similar lattice constants but different lattice structures with respect to the matrix, mainly due to the acoustic phonon density of states mismatch. The theoretical investigations in this thesis expand the fundamental understanding of thermoelectric transport in bulk and nanostructured semiconductors, which can provide guidance for further enhancement of thermoelectric materials and beyond.
| Date of Award | 2015 |
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| Original language | English |
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| Awarding Institution | - The Hong Kong University of Science and Technology
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