Thermoelectric Transport Properties Of Novel Layered Materials

In the present era,thermoelectric materials have become an important part of the solution to the energy crisis and environmental pollution problems.The conversion efficiency of a thermoelectric material can be expressed as a dimensionless thermoelectric figure of merit ZT=（S²σ/κ）T.However,the complex interrelationships between thermoelectric parameters make it difficult to maximize the ZT value,while low-dimensional layered structures have unique electronic and phonon properties with quantum confinement effect and the electrical and thermal transport may decoupled.Therefore,we focus on the electrical and thermal transport properties of several layered structures in this thesis,including two-dimensional layered structures and three-dimensional layered structures stacked with two-dimensional structures,based on the first principle and Boltzmann transport theory.The main studies are as follows:1.The thermoelectric properties of the 7-atom layer material ZrGe₂N₄ have been investigated.Theoretical calculations demonstrated the 7-atom layer structure is kinetic stable and thermodynamic stable.The electronic band calculation indicated that the electronic states near the Fermi plane are mainly contributed by the lighter N elements,while the phonon spectrum calculation shown that the low-frequency phonon states are mainly contributed by the heavier elements Zr and Ge.Thus the electronic transport in the 7-atom layered structure depends mainly on the lighter mass N atoms,while phonon transport is concentrated on the heavier mass Zr and Ge atoms.The structure also has a very low lattice thermal conductivity,with a value of 3.7 Wm^-1K^-1 at room temperature,which is mainly attributed to the strong phonon scattering of the Zr-based structure.The transport coefficient calculations show that Zr Ge₂N₄ has a high optimum thermoelectric figure of merit.At room temperature,the highest thermoelectric optimum values are 0.85 and 0.64 for the p-type and n-type doped structures,respectively,which increase rapidly with increasing temperature.At 1100 K,the highest ZT value is up to 3.32 and 3.93 for the p-and n-doped structures,respectively.Our research not only provides a highly efficient thermoelectric material but also propose new way to the search for high-performance thermoelectric materials with multi-atomic layered structures.2.The electronic structure and thermal transport properties of bilayer Mg X（X=S,Se,Te）have been investigated.We have exfoliated the bilayer Mg X structure from a bulk magnesium chalcogenide and calculated the stability and thermal transport properties of the structure.Unlike the unstable monolayer Mg X structure,the bilayer Mg X structure has no imaginary frequencies in the phonon band and the softening of the acoustic modes increases with the increasing mass of chalcogenide atoms.The bilayer Mg X structure has a wide bandgap,with Mg Te having a direct bandgap and a convergence of energy valleys.One can find that these structures can obtain relatively low lattice thermal conductivity at high temperatures,especially for the bilayer Mg Te.The lattice thermal conductivity is low as 5.90 Wm^-1K^-1 at 900 K.Our study revealed a stable bilayer Mg X structure,which provides a theoretical basis for the follow-up thermoelectric research.3.The anisotropic thermoelectric properties of three-dimensional layered B₃N have been investigated.The three-dimensional layered structure of B₃N was obtained by stacking using boronate and hexagonal boron nitride,and the kinetic and thermodynamic stability of the structure was verified.The mobility was calculated from the electronic structure of B₃N and the structure was found to have excellent electrical transport properties.The phonon transport properties of the laminated B₃N in different directions also indicate that the lattice thermal conductivity in the in-plane direction of the structure is 5-8 times higher than that in the out-of-plane direction.The optimum thermoelectric optimum values along c-axis vary considerably with temperature and are superior to the in-plane thermoelectric properties at high temperatures.As the temperature rises to1200 K,the highest thermoelectric figure of merit can reach 0.72.Our study provides an idea to unravel thermal and electrical transport for the layered structure.