Study of electrical and thermal transport in low-dimensional structures aiming for improving thermoelectric efficiency

Thermoelectric;Seebeck;Thermal conductivity;Green function;Molecular dynamics;Semiconductor;Graphene

Park, Kyeong Hyun

关键词: Thermoelectric;    Seebeck;    Thermal conductivity;    Green function;    Molecular dynamics;    Semiconductor;    Graphene;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/90893/PARK-DISSERTATION-2016.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Thermoelectric engineering has been getting much attention anticipating its potential to serve an important role in producing clean energy out of waste heat dissipated from industries, residential processes, and transportation. However, thermoelectric efficiency has been limited to a non-practical range because of the interplay among transport coefficients which measure the efficiency. With widely growing interest in thermoelectrics, understanding the electrical and thermal transport in thermoelectric devices and finding ways to improve the efficiency is critical to develop future thermoelectric applications and to contribute to solving the energy shortage that the world is facing. In this thesis, we present both electron and phonon aspects of the thermoelectric transport employing the Boltzmann transport theory, the Landauer formalism, and the element kinetic theory. We calculate the thermal conductivity of layered bismuth telluride thin films by solving the Boltzmann transport equation in the relaxation-time approximation using full phonon dispersion. The full phonon frequency and gradient are obtained from the framework of the first-principle density functional theory and effectively used to confirm the anisotropic thermal transport arising from the phonon mode softening. In addition, we introduce the phonon scattering due to van der Waals interface in bismuth telluride to fully capture the anisotropy of the material. We also study the thermal conductivity of the rippled graphene which has potential to be used as a future thermoelectric material. The ripple is formed through a deformation process performed in molecular dynamics. We introduce a technique which enables creation of the graphene sheet with evenly distributed ripples and calculate the thermal conductivity using Green-Kubo linear response theory. The study reveals significantly reduced thermal conductivity, which confirms the plausibility of the rippled graphene as a future thermoelectric material. To stimulate the thermoelectric efficiency, we suggest a strategy that limits the thermal transport with no significant influence on the electrical transport by geometry-engineering. We investigate the Seebeck coefficient, the electrical conductance, and the thermal conductivity of sinusoidally corrugated silicon nanowires as well as an enhancement of the thermoelectric efficiency. A loss in the electronic transport coefficient is calculated with the recursive Green function along with the Landauer formalism, and the thermal transport is simulated with the molecular dynamics. In contrast to a small influence on the thermopower and the electrical conductance of the geometry-modulated nanowires, a large reduction of the thermal conductivity yields an enhancement of the efficiency.