The Computational And Theoretical Study On Adjustable Thermal Resistor And Phonon Coupling Based On Graphene

Energy issues are related to the major issues of national economy and environmental protection.In present,efficiency of energy utilization is still very low and amounts of energy is wasted in the form of waste heat.Therefore,the disposal and utilization of waste heat is of great significance to resolve the energy crisis.On the other hand,the highly integrated and minimized electronic devices make heat dissipation restrict the development of semiconductor industry.In recent years,owing to its super-high thermal conductivity and flexible mechanical properties,graphene has been widely used in the field of thermal management,thermal modulation and thermal conversion.As the dimensional size enters at nanoscale,heat transfer theory in bulk materials is not valid.Thus,the research on thermal transport properties of nanosized graphene can promote the application of graphene in the field of thermal management and conversion.First of all,in recent years,phononic(thermal)devices such as thermal diodes,thermal transistor,thermal logical gates,and thermal memories have been studied intensively.However,tunable thermal resistor have been demonstrated yet.Here,we propose an instantaneously adjustable thermal resistor based on folded graphene.Through theoretical analysis and molecular dynamics simulations,we study the phonon-folding scattering effect and the dependence of thermal resistivity on the length between two folds and the overall length.Furthermore,we discuss the possibility of realizing instantaneously adjustable thermal resistor in experiment.Our studies bring new insights into designing thermal resistors and understanding the thermal modulation of 2D materials by adjusting basic structure parameters.Secondly,research on thermal conductivity of different phonon groups in graphene is beneficial to the understanding of of thermal transport in graphene.Therefore,it attracts much attention of scientific community.In this chapter,thermal conductivities of different phonon groups in graphene are calculated using non-equilibrium equilibrium molecular dynamics(NEMD)method.our results revealed that the in-plane phonon group in graphene dominates.Interestingly,by comparing the total thermal conductivity with the sum of outof-plane phonon group and in-plane phonon group,it is found that the coupling between out-of-plane phonon group and in-plane phonon group is relatively weak.This is consistent with the recent experimental results.Then,the design of graphene-based composite with high thermal conductivity requires a comprehensive understanding of phonon coupling in nanosized graphene.We firstly extended the two-temperature model to coupled phonons based on phonon Boltzmann Transport Equation(BTE).The study identified two new physical quantities,the phonon-phonon coupling factor and the phonon-phonon coupling length,to characterize the strength of phonon couplings quantitatively.Besides,we successfully studied the coupling between in-plane phonon group and out-of-plane phonon group in graphene based on molecular dynamics simulation and the extended two-temperature models.Interestingly,it was found that the coupling between in-plane phonon group and out-of-plane phonon group is relatively weak,which is consistent with the recent experimental results.Moreover,our proposed coupling length has an obvious size dependence in graphene.Our studies can not only observe the nonequilibrium between different groups of phonon but explain theoretically the thermal resistance inside nanosized grpahene.Finally,organic thermoelectric(TE)materials open up a brand new perspective on thermoelectricity due to the inferior thermal conductivity.The overlap of ?-? orbit,commonly existing in organic small-molecular semiconductor,produces high electronic mobility comparable to inorganic electronics.Here we proposed a strategy to utilize the overlap of ?-? orbits to increase the TE efficiency of layered materials.Through firstprinciples electronic structure calculations and classical molecular dynamics simulations,we show that layered polymeric carbon nitride has the maximum ZT up to 0.67 at 300 K along the cross-plane direction.High ZT comes from low-dimensional band structures and predominantly cross-plane power factor,both of which are resulted from the overlap of?-? orbits.In this chapter,we mainly focus on the thermal conductivity of polymeric carbon nitride and its temperature dependence.