Thermoelectric Properties Of Nanowire Structures

Thermoelectric effects have been investigated widely for the purpose of energy harvesting as well as refrigeration and heat pumping. The efficiency of thermoelectric devices is determined by the dimensionless thermoelectric figure-of-merit ZT. Nanostructuring approaches that emerged in the 1990s have led to impressive progress in improving the figure-of-merit, benefiting mainly from both the reduction of the lattice thermal conductivity and the enhancement of the Seebeck coefficient. In this thesis, the thermoelectric properties of nanowire structures are investigated theoretically, considering the transport of both electrons and phonons. A high ZT value was predicted in the nanowire structure.We developed a combined model to calculate the thermoelectric properties of the nanowire arrays in the lateral direction. Both the quantum confinement of electrons and the boundary scattering of phonons were considered. A high ZT value was predicted, made possible by the large Seebeck coefficient and the reduced lattice thermal conductivity. The coupling effect in the nanowires supports electron transport, so that the enhancement of ZT does not diminish. The detailed analysis suggests that the nanowire volume fraction should be larger than some critical point if a high ZT value is desired in the lateral direction, as a result of the balance between the Seebeck coefficient and the electrical conductivity. Moreover, the results indicate that a high ZT value can be obtained with a larger nanowire diameter as the nanowire volume fraction is larger, which is important in experimental fabrications. Since the carrier concentration increases with increasing temperature, the larger power factor is expected to enhance the ZT value at higher temperatures. Additionally, The combined model is not limited to nanowires arrays, but can also be generalized to evaluate the thermoelectric properties of other nanostructures, such as the holey membrane and nanotube arrays. Our results may serve as guidance in developing high-ZT materials.We have also investigated the thermoelectric properties of Kondo insulator nanowires with low doping level, considering the Kondo effect in the framework of dynamical mean field theory as well as size effect of both electrons and phonons. Our findings showed that the power factors of such materials enhanced dramatically because of the Kondo effect at low temperatures. The strong repulsion of the screening clouds on the singlet resulted in the small intrinsic mean free paths of conduction electrons that preserve electron transport from the nanowire boundary scattering. Meanwhile, the phonons are severely scattered by the nanowire boundary and the lattice thermal conductivity is largely reduced. This occurrence is especially amplified at low temperatures. Consequently, the thermoelectric properties can be improved effectively in such materials to achieve ZT>1 in nanowires with diameters between 20-500 nm. In principle, the MFPs of conduction electrons and the characteristic length of the nanostructure could be balanced well by doping and fabrication technology, respectively. Hence, it is possible to design real Kondo insulators based on our proposed model for thermoelectric cooling at cryogenic temperatures. The type of calculation reported in this work can be extended to a variety of materials involving the strong correlation physics, and provide a possible path to develop high ZT materials.