Thermal Conductivities And Phonon Transport Mechanisms Of Several Typical Oxide Single Crystals

Heat conduction in solids is the macroscopic reflection of the interaction of heat-carrying particles such as electrons and phonons.The thermal conductivity is not only characteristic of the lattices,but also reveals the influence of microstructures on the transport of heat-carrying particles.The oxides hold abundant structural degrees of freedom,providing a good platform to study the relationship between properties and structures of solids.Recently,with the rapid development of characterization methods such as time-domain thermoreflectance(TDTR),it has been possible to measure the thermal conductivity of millimeter-sized single crystals.In this dissertation,the thermal conductivites of several oxide single crystals with structural features(including layered structure and perovskite structure)were studied by the TDTR method,and the dissertation focuses on the relationship between the properties and structures,as well as the inherent phonon transport mechanism.By summarizing the influence of structures from atomic to micron-sized level on the properties of materials,it offers the opportunity for the in-depth understanding of heat condution rules in solids,and provides enlightenment for the development of thermoelectric materials.The main contents are as follows:1.Studying the influence of atomic interfaces on thermal conductivities of layered misfit cobalt oxides.The samples were grown by flux method.TDTR measurements were performed to study the influence of misfit interfaces and Bi O-Bi O interfaces on the c-axis thermal conductivities.It was shown that both misfit interfaces and Bi O-Bi O interfaces would lead to ultralow thermal conductivity which is close to the theoretical disorder limit,but the ultralow thermal conductivity caused by Bi O-Bi O interfaces is lower(?0.28 W m^-1 K^-1 at 300 K).Utilizing picosecond laser ultrasonics,the longitudinal acoustic velocities were studied,proving the effect of acoustic velocity on thermal conductivity.Through phonon transport mechanism analyses,it was found that without the existence of Bi O-Bi O interfaces,the main transport mechanism is phonon Umklapp scattering and phonon-boundary scattering,while the main transport mechanism is phonon boundary scattering when there are Bi O-Bi O interfaces.These results indicate that the ultralow thermal conductivity in layered cobalt oxides with misfit system should be attributed to the effect of Bi O-Bi O interfaces which enhance phonon boundary scattering;2.Studying the influence of ion intercalation on thermal conductivities of ionic superconductors A_0.5Rh O₂(A=K,Rb,Cs).The samples were grown by flux method.The effect of intercalated ions on the c-axis heat conduction properties was studied by TDTR method.It is found that the samples exhibit ultralow thermal conductivity(?0.39 W m^-1 K^-1 at 300 K),which is even lower than the theoretical disorder limit.Through analyses,the ultralow thermal conductivity was a consequence of phonon-boundary scattering and phonon resonant scattering.By first principles calculation,it was found that the local vibration of A-site ions features with resonant modes,whose frequency is inverse of ion mass.These results indicate that the origin of ultralow thermal conductivity in ionic superconductors is the phonon resonant scattering caused by the A-site ion vibration.In addition,the excitation temperature of resonant modes can be adjusted by changing the mass of A-site ions,which could be expected to be a degree of freedom to realize in-situ modulation of thermal conductivities;3.Studying the influence of spontaneous polarization on the thermal properties of freestanding perovskite BiFeO₃ single-crystalline films.The samples were obtained by oxide molecular beam epitaxy and transfer method,and a-axis axial strain was applied to the samples by the tensile holder.TDTR was used to study the evolution of c-axis heat conduction properties under strain state.The experiments showed that c-axis thermal resistances of the samples increase significantly(up to 26 times)with the applied strain.Combined with piezoresponse force microscopy and phase field simulation,the effects of increased density of ferroelectric domain walls and structural phase transition were eliminated.Assuming the effect of bound charges on the surface of samples due to spontaneous polarization,the deflection of spontaneous polarization during the stretching may change the heat transfer condition at the interface.This conjecture was also confirmed in the following supplementary experiments.These results reveal that the heat transfer properties of interfaces between ferroelectric materials and metals could be affected by the direction of spontaneous polarization,and it is possible to realize in-situ modulation of thermal conductivity by means of applying electric or strain field;4.Studying the influence of micron-sized microstructure on thermal conductivities of quasi-one-dimensional ZrTe₅/HfTe₅ single crystals.The loose microstructure samples were grown by chemical vapor transport method,and samples with dense microstructure were grown by flux method.The effect of microstructure on b-axis heat conduction properties of the system was studied by TDTR.It was found that the samples grown by CVT method exhibit loose microstructure of nanoribbons,while the samples grown by flux method exhibit dense microstructure of nanoflakes,and thermal conductivity of the latter is four times higher than the former at room temperature.First principles calculation and scanning electron microscopy showed that the difference is due to the phonon-boundary scattering of varied intensities caused by microstructure.These results show that the thermal conductivity of quasi-one-dimensional materials could be further reduced and imply the possibility of improving thermoelectric properties by adjusting the parameters during the growth process.