Theoretical And Experimental Studies On The Thermal Conduction Properties Of Layered Materials

For a long time,heat conduction in solid materials has been playing an important role in the fields of both fundamental science and applications.By studying the macroscopic heat conduction properties,we can get a deep understanding on the transport behaviors of microscopic particles and elements,such as electrons,phonons,excitons,polarons and magnons,as well as their interactions.In recent years,with the development of nanotechnology and precision measurement technology,people have extended the scale of thermal transport researches to several nanometers,and discovered many different phenomena from the thermal transport properties of the traditional bulk materials.Among them,two-dimensional layered materials with very low thermal conductivity,strong anisotropy and abnormal size-dependent thermal conductivity is gradually becoming a hot topic in the research field of nano-scale heat conduction.Based on these novel properties,layered materials are expected to be widely applied in thermoelectric materials,thermal storage and other thermal logic devices.In addition,layered materials and their two-dimensional structures have shown promising applications in the electrical transport,opto-electronic response as next generation semiconductors.However,there are relatively fewer studies on the intrinsic thermal properties of such layered materials,and it also lacks a mature theoretical model to explain the phonon transport mechanism.In this thesis,we have studied the thermal conductivity of molybdenum telluride(MoTe2)and tungsten telluride(WTe2)material systems,the BiCaCoO and BiCuSeO layered oxide materials by means of the time-domain thermoreflectance(TDTR)measurement.A series of theoretical models of thermal transport and lattice dynamics are used to describe the effects of crystal structures,elastic constants,chemical components and microscopic defects on the thermal conductivity and phonon scattering mechanisms in these layered materials.Meanwhile,we have also built up a theoretical model to calculate the thermal conductivity of thin-plated nanostructures,and introduce the idea of phonon modulation by phononic crystals to realize the manipulation of thermal conductivity.The contents of this thesis are summarized as follows:(1)Based on the principles of pump-probe technique,we set up a TDTR measurement system.Using this technique,we have carried out a series of experimental calibrations on the thermal conductivities of several standard samples to confirm the stability and reliability of our TDTR system.In addition,we have developed the picosecond ultrasonics and time-resolved Brillouin scattering measurements to get the elastic constants and thermal conductivity simultaneously,in order to facilitate the theoretical analysis based on these relevant properties.(2)For the preparation and characterization of materials,we have successfully prepared the MoTe2 single crystals and the alloy materials,the BiCaCoO polycrystals and the BiCuSeO single crystals by the chemical vapor transport(CVT)and flux methods.By means of a series of material characterizations,we have obtained the compositions,structural and electrical transport properties of these samples.(3)Using the TDTR method,we have measured the temperature dependence of thermal conductivities in the MoTe2 material system,and compared the effects of the composition,phase structure and defects on the thermal conductivity.In addition,we observe the hysteresis properties of thermal conductivity,electrical conductivity and Raman vibration modes for the first time in metastable materials,and verify that these novel effects are mainly caused by the structural phase transitions at specific temperatures.Combined with the Boltzmann transport equation,we analyzed the phonon scattering mechanisms in these materials and found that the intensive phonon interface scattering in the mixed phases plays a key role in the reduction of thermal conductivity.(4)By the TDTR system,we have measured the thermal conductivities of BiCaCoO and BiCuSeO layered oxide materials,and discovered the lowest thermal conductivity in similar oxide materials,which almost reaches the low limit of thermal conductivity in a disordered system.Based on the theoretical analysis,we find that the mean free paths of the phonons are lower than the lattice constants.We have claimed that the misfit structures and weak binding forces in the layered materials can lead to the strong phonon scatterings.(5)In order to study the in-plane thermal transport properties of layered materials,we have developed a theoretical model for calculating the thermal conductivity of nanostructured thin-plates.In this theoretical system,we develop a traditional lattice dynamics theory by a long-wavelength approximation.It is found that the dispersion of long-wavelength phonons can be obtained by the corresponding elastic wave equation,which facilitates the calculation of thermal conductivity of materials in nano-scale.In addition,we have designed a silicon based phononic crystal nano-structure and calculated the thermal conductivities of this artificial structure at different temperatures using the Boltzmann transport theory.The relationship between the periodic structure and the phonon dispersion is also analyzed.These studies above do not only build a foundation for wider applications of layered materials in the fields of thermoelectric materials,thermal management and thermal logic devices,but also establish a more comprehensive understanding for the study of heat transport in similar materials.This study will benefit the practical applications of layered materials in quantum transport,energy conversion,logic calculation and information storage,which are of great research values for the development of the new-generation functional materials.