Thermoelectric Conversion Mechanism And Performance Control In Quasi-one-dimensional Nanostructures

Energy shortage and the environmental pollution problems have become increasingly serious, the development of high performance thermoelectric material and high thermoelectric conversion efficiency technique has attracted great attention around the world. Quasi-one-dimensional nanostructure materials can reduce the lattice thermal conductivity significantly due to the strong phonon boundary scattering, and also can improve the power factor by the quantum confinement effect, so as to improve the thermoelectric performance observably. In the present thesis, we investigate the thermal transport properties and thermoelectric properties in the quasi-one-dimensional nanostructure materials by using the nonequilibrium molecular dynamics method and the first-principles calculations combined with the Boltzmann transport equation or the nonequilibrium Green’s function method. Some meaningful and interesting results have been obtained.First of all, we investigate the phonon thermal transport properties in In As nanowires with different size and growth directions by using nonequilibrium molecular dynamics methods. The results show a remarkable anisotropy for the thermal conductivity in In As nanowire. It is found that the thermal conductivity along [110] growth direction is about three times larger than that along [100] or [111] direction. With the increase of temperature, the thermal conductivity along [110] direction decreases significantly. However, the thermal conductivity along other two directions is not sensitive to temperature. Moreover, we find a crossover from ballistic to ballistic-diffusive thermal transport for a certain length of In As nanowire. A brief physical analysis of these results is given. It is suggested that the anisotropy of thermal conductivity is common for nanowires with zinc blende structures.Secondly, by using the nonequilibrium Green’s function method, we study the thermoelectric properties of In As nanotubes. The results show that In As nanotube with a certain internal diameter has much higher figure of merit（ZT） value than nanowire due to the enhancement of quantum confinement effect leading to the increase of the power factor S2 G. The ZT value of In As nanotube can reach 1.74, which is about three times greater than that of nanowires. Moreover, it is found that the ZT values of In As nanotubes decrease rapidly with the increase of internal diameter, which results from the rapid increase of phonons thermal conductance due to the “red shift” of low-frequency optical phonon modes.And then, the thermoelectric properties of multiple core-shell nanowires are investigated by using nonequilibrium Green’s function method and molecular dynamics simulations. The results show that the thermoelectric performance of multiple core-shell NWs can be improved observably with the increase of shell number compared with the single component NWs due to the significant reduction of phonon thermal conductance. The ZT value of multiple core-shell NWs can reach three times greater than that of the single component Ga Sb NWs at room temperature. Moreover, the ZT values of both the core-shell NWs and single component NWs are increased with the increasing temperature, but the ZT value of core-shell NWs increases more slowly than that of single component NWs. These results show that the single component NWs is suitable as thermoelectric material at much high temperature, but the multiple core-shell NWs is more suitable as thermoelectric material at room temperature.Next, the thermoelectric properties of the four typical graphyne nanoribbons are systematically investigated by using the nonequilibrium Green’s function method. The results show that the β-, γ-, and（6, 6, 12）-graphyne nanoribbons exhibit excellent thermoelectric performance, their ZT values are more than 10 times larger than that of graphene nanoribbons due to the large power factor and low phononic thermal conductance. Moreover, the influence of defect on the thermoelectric properties in β-graphyne nanoribbons is also studied. We find that the defective β-graphyne nanoribbons have much higher ZT value than pristine β-graphyne nanoribbons. The ZT value of defective β-graphyne nanoribbons can reach 1.64, which is about six times greater than that of pristine β-GYNR, which results from the rapid decrease of phononic thermal conductance due to the strong phonons scattering by defect and the intensive localization of phonons, meanwhile the power factor is not deteriorated.Finally, by using first-principles calculations combined with the phonon Boltzmann transport equation, we systematically investigate the phonon transport of monolayer WSe₂. Compared with other 2D materials, the monolayer WSe₂ is found to have an ultralow thermal conductivity due to the ultralow Debye frequency and heavy atom mass. The room temperature thermal conductivity for a typical sample size of 1 μm is 3.98 W/m K, which is one order of magnitude lower than that of Mo S2. And the room temperature thermal conductivity can be further decreased by about 95% in 10 nm sized samples. Moreover, we also find the ZA phonons have the dominant contribution to the thermal conductivity, and the relative contribution is almost 80% at room temperature, which is remarkably higher than that for monolayer Mo S2. This is because the ZA phonons have longer lifetime than that of LA and TA phonons in monolayer WSe₂.