First Principles Study Of Anisotropic Thermal Conductivity And Thermoelectric Properties Of Hexagonal Layered Materials

The transport and conversion of heat in crystalline materials are important research directions in materials science,physics,and related fields.With the increasing power density of micro/nano-electronic devices,effective thermal management techniques have become an urgent priority.Rapid and directional heat transfer is essential for preventing thermal failure in these devices.On the other hand,the directed movement of charge carriers in a temperature gradient can convert waste heat into electrical energy,enhancing direct conversion between thermal and electrical energy and improving energy utilization efficiency.Therefore,the development of efficient anisotropic thermoelectric conversion technologies holds significant scientific value in the fields of electronics,information,energy,and beyond.Layered materials,characterized by their anisotropic crystal structure and bonding modes,typically exhibit low interlayer thermal conductivity,providing an opportunity to decouple the interdependent thermoelectric coefficients.Layered materials also exhibit significant differences in thermal transport properties in both the in-plane and out-of-plane directions,making them an ideal platform for exploring anisotropic thermal management.In this thesis,we present a systematic study of different hexagonal layered materials using first-principles computations and the Boltzmann transport equation,focusing on electronic structure,phonon spectrum,lattice thermal conductivity,and thermoelectric transport coefficients.We focus on electronic structure,phonon spectrum,lattice thermal conductivity,and thermoelectric transport coefficients to provide a theoretical explanation for the physical origins of anisotropic thermal transport in layered materials.Furthermore,we identify two new potential thermoelectric pnictides with improved thermoelectric performance.Our research findings provide a theoretical guideline for the experimental synthesis of novel thermoelectric materials.The main research contents are as follows:1.Research on anisotropic thermal transport of layered materials In Se,MoS₂,and h-BN.Recent research on the anisotropic thermal transport properties of layered materials often concentrated on the transport mechanism of individual materials.There is a lack of systematic research on the sources of anisotropic thermal transport in layered materials,including the effects of different structural units,interlayer coupling,and harmonic/anharmonic parameters on the anisotropy of thermal transport.Despite the significant anisotropic thermal conductivity of layered materials,there is still a lack of theoretical guidance on how to design and optimize the anisotropy ratio of thermal conductivity in these materials.In addition,the potential of the directional heat transfer mechanism in layered materials has not been fully understood.To address these critical scientific issues,we analyzed the thermal transport properties of three different hexagonal layered materials（In Se,MoS₂,and h-BN）using the density functional theory（DFT）.We found significant differences in the lattice thermal conductivity of these layered structures in the in-plane and out-of-plane directions,exhibiting a wide range of anisotropy ratios（9.4-107.7）.From phonon transport analysis,we found that the anisotropic thermal transport of layered materials mainly comes from two factors:i)the lattice thermal conductivity of layered materials is primarily controlled by harmonic parameters（phonon frequency,phonon group velocity,and atomic mass）;and ii)the phonon group velocity is the controlling parameter due to which these structures have different anisotropic thermal transport ratios.The findings of our study provide insights into the physical mechanism of anisotropic thermal transport in layered materials and offer theoretical guidance for the experimental realization of anisotropic phonon thermal transport in layered materials.2.Research on the anisotropic thermoelectric transport properties of layered NaBeAs and NaBeSb.Thermoelectric materials offer an environmentally friendly and feasible way to generate electricity by converting thermal energy into electrical energy.These materials provide a platform for clean energy technology.Using first-principles calculations,we have discovered two novel hexagonal layered materials,NaBeAs and NaBeSb,which exhibit excellent thermoelectric properties.Our calculations show that both these materials exhibit excellent thermodynamic and dynamic stability.Significantly,the compounds present an extremely low lattice thermal conductivity in the z-axis direction,more likely due to the combined effect of low phonon group velocity,weak chemical bonding in the structure,and strong three-phonon scattering.In addition,both materials exhibit high band degeneracy at the conduction band edges,indicating good electronic transport properties.Owing to their low thermal conductivity and excellent electrical transport properties,NaBeAs and NaBeSb exhibit remarkable n-type thermoelectric performance.At the optimal n-type doping level,NaBeAs and NaBeSb can achieve high ZT values along the z-direction,reaching 1.82 and 4.26 at 600 K,respectively.Our results indicate that NaBeAs and NaBeSb have the potential to become medium-temperature,high-performance thermoelectric materials.