Study On Phononic Thermal Transport And Thermoelectric Conversion Mechanism Of Low-dimensional Nano-devices

Heat energy is a double-edged sword.On the one hand,the manipulation and utilization of heat energy is the basis of human existence.On the other hand,many practical applications of technologies in today's world are plagued by heat energy.With the increasing integration of micro-nano electronic devices,the heat generation of high energy density functional devices such as computer processors and lithium batteries has seriously affected the working efficiency,stability and lifetime of the devices.To address this challenge,people expect to find materials with higher thermal conductivity so as to realize efficient thermal management of the functional devices.In addition,increasingly serious environmental pollution,energy shortage problems urge people to find cleaner and more efficient ways of energy conversion.Therefore,thermoelectric materials have attracted much attention because they can directly convert heat and electric energy into each other.However,thermoelectric materials with high conversion efficiency usually require the material to have the lowest thermal conductivity possible.In order to take both directions into consideration,it is necessary to have a deeper and more detailed understanding of heat conduction and transformation at different scales.This makes thermal science become a research field that the scientific community pays wide attention to nowadays.The research content of this thesis focuses on the nanoscale heat transfer and related physical phenomena,and mainly studies the two major fields closely related to heat transfer,namely,the thermoelectric energy conversion and the mechanism of phonon heat transport.Firstly,the phonon heat transport mechanism at different scales is studied.By combining the first principles calculation with the linearized Boltzmann transport equation,we take the anharmonic effect into account in the phonon transport calculation,and present an effective criterion condition for distinguishing the ballistic-diffusive heat transport.We first qualitatively verified the validity of this criterion in monocrystalline silicon and graphene nanoribbon.Then,based on this criterion,we further studied the ballistic-diffusive heat transport mechanism of sixteen kinds of III-V binary compounds with face-centered cubic structure,and the effects of materials size,temperature and phonon structure on ballisticdiffusive thermal conductivity are considered.Meanwhile,the relationship between the acoustic-optical phonon band gap and the ballistic-diffusive heat transport ratio is studied.We find that materials with a large acoustic-optical phonon band gap tend to have a larger ballistic phonon transport ratio,which means that phonon transport in such materials is more likely to enter the ballistic transport region when the size,temperature is reduced.This criterion condition provides a convenient method for further understanding phonon transport behavior.Furthermore,we study the phonon heat transport mechanism and its application on the basis of nanoscale devices.Analogous to the quantum interference effect of electrons,we theoretically predict the existence of the destructive phonon interference effect in intermediately coupled molecular junctions by means of non-equilibrium Green's function method.Moreover,the theoretical calculation results show that the rotational degrees of freedom of the intermediately coupled molecular junctions can continuously adjust the strength of phonon interference effect.At different twist angles,the intermediately coupled molecular junctions have significant selective blocking effects on different phonon modes.This results in the out-of-plane phonon modes are completely suppressed,while the transmission of in-plane phonon modes can be further regulated by the twist angle.Finally,the thermal conductance of the molecular junctions can be significantly suppressed,and its thermoelectric conversion efficiency is enhanced.Different from strongly coupled molecular junctions,the configuration of intermediately coupled molecular junctions between two critical contact sites is continuously adjustable,which provides a valuable theoretical basis for designing molecular devices with more controllable quantum interference effect.Meanwhile,we also explore the novel mechanism of thermoelectric conversion.At present,thermoelectric materials based on the Seebeck effect have been widely studied.However,the conversion efficiency of thermoelectric materials still fails to meet the requirements of large-scale application caused by the coupling relations between the parameters that determine thermoelectric figure-of-merit.Therefore,we propose a thermoelectric conversion mechanism based on the combination of solid interface thermophoresis effect and piezoelectric effect.The mechanism is different from the Seebeck effect and the pyroelectric effect,and it can also convert heat energy into electric energy through the solid interface thermophoresis effect and piezoelectric effect.Through simplified theoretical model and molecular dynamics simulation,we first verify the feasibility of this idea from the perspective of theoretical and numerical calculation.Further numerical calculation by finite element method shows that the output voltage of two-dimensional piezoelectric materials driven by thermophoresis effect can be as high as 3.49 V.Finally,we are also actively looking for simpler material structures to realize thermally driven thermo-piezoelectric energy conversion.By introducing graded structures on the free-standing nanowires,we theoretically verified that the nanowires with graded structure can produce larger deflection amplitude than that of nanowires without graded structures at the same temperature condition.and further study the substrate thickness,substrate defect,nanowire diameter,the structure of the temperature on thermal gradient driven nanowire deflection amplitude.The influences of substrate thickness,substrate defects,nanowire diameter and temperature on the deflection amplitude of graded nanowire are further studied.We found that the deflection direction and amplitude of nanowires could be significantly affected by changing the substrate structure.Meanwhile,the nanowires with graded structures are very sensitive to the diameter change of nanowires.When the diameter of nanowire is small,the introduction of graded structure can suppress the deflection amplitude,while the results is the opposite when the diameter is large.This result provides a simpler method for the realization of a more controllable thermally-driven thermo-piezoelectric conversion process.