Theoretical Study Of Phonon Thermal Transport In Nanostructures

With the development of Micro-Electro-Mechanical System(MEMS)technology,the feature size of materials has been reduced to nanoscale.The physical properties of MEMS components are obviously different from those of bulk materials due to the size effects and surface effect.Since the reliability and efficiency of the devices are strongly dependent on the heat conduction in nanomaterial,exploring phonon thermal transport in nanostructures is of great significance for the design of MEMS.When the feature size of materials reduces to nanoscale,the thermal conductivity is no longer the essential attribute of material but depends on dimensions,shape and boundaries.Therefore,the classical theory ofthermal transport is no longer applied to nanostructure in the low dimensions.Quantum-mechanical theory is used to investigate the phonon thermal transport.In this paper,the background and current research status of two-dimensional materials are first introduced.Then we present the theories and calculation methods of phonon transport,including a lattice dynamics simulation for phonon dispersion of crystal materials and firstprinciples calculations.These methods will be used to study the phonon properties and thermal conductivity of silicon nanothin film and single atomic layer materials in the following chapters,respectively.The large scale lattice dynamics calculation is used to calculate the phonon dispersion of the smooth silicon film.The validity of this technique was confirmed by comparing the phonon dispersion between the super unit cell mode and the single unit cell mode.The results show that the thermal conductivity of the nano-thickness silicon thin film is two orders of magnitudes smaller than that of the bulk silicon.The technique was performed to investigate the phonon dispersion relation of two-dimensional porous silicon thin film.The effects of the porosity and the thickness of the silicon phononic crystals on the phonon dispersion were further investigated.The results show that the phonon group velocity for all modes decreases sharply with increasing porosity,and thatincrease of the thickness mainly increases the group velocity of ZA modes.The thermal properties and phonon scattering mechanism of two-dimensional single atom layer materials are discussed with first-principles calculation method.In the first step,the thermal conductivity and phonon scattering mechanism of graphene and silicone are investigated,including phonon dispersion relation,relaxation time,three-phonon scattering phase space and the mode contribution to the total thermal conductivity.It is found that the thermal and phonon properties of silicene is very different from that of graphene.Then,we study the thermal transport properties of four different monolayer nitrides consisting of ш-A group elements,boron,aluminum,gallium and indium.For h-BN,h-AlN,h-GaN and h-InN,the results implies that the phononic band gaps increase and thermal conductivity decrease with increasing atomic number.These trends are determined to relate to atomic mass and radius.At the end of the paper,it is a summary and prospect of the whole thesis.