Study On Phonon Ballistic-Diffusive Heat Conduction In Both Micro-and Macro-Levels

With the increasing power density,a reliable thermal design is of crucial importance for the further development of electronic devices.Moreover,the research of nanostructured thermal functional materilas has been a hot topic in academic circles.A more in-depth understanding of nanocale thermal transport is still needed to handle the two issues above.In dielectric and semiconductor materials,phonons are the dominant heat carriers.When the characteristic length of a structure is comparable to the phonon mean free path,the classical Fourier heat conduction law becomes invalid,and the ballistic-diffusive heat conduction emerges.In the present dissertation,a systematical study in both micro-and macro-levels is conducted on the phonon ballistic-diffusive heat conduction using the phonon Monte Carlo(MC)simulation,Boltzmann transport equation(BTE),and experimental methods.The underlying microscopic mechanisms for the macroscopic non-Fourier phenomena in ballistic-diffusive regime are uncovered,and the predictive models that characterize the non-Fourier phenomena are also given;additionally,the least action principle and optimization of heat conduction are investigated.A phonon MC simulation program is developed for the phonon ballitic-diffusive heat conduction.A two-step phonon MC method is proposed to efficiently calculate the effective thermal conductivity of large-area periodic nanostructured materials.Numerical experiments show that the two-step MC method can greatly reduce the computational cost without significant deviations.Two types of slipe boundary conditions in the ballistic-diffusive heat conduction are systematically studied.Ballistic transport will cause boundary temperature jump(BTJ).The predictive model for BTJ is derived and verified.BTJ will increase with the enhancement of ballistic transport,and the phonon property mismatch at the boundary can further enhance it;moreover,the ballistic-interface coupling effect can cause the thermal resistance superposition principle invalid.The interactions between phonons and boundary(interface)lead to the boundary heat flux slip.The heat flux slip will become more significant with the enhancement of phonon diffusive reflections at boundaries,and the phonon property mismatch at boundaries can also canuse the heat flux slip;in addition,it is found that the interfacial effect can be used to achieve the two-way tuning of the in-plane effective thermal conductivities of nanofilms.The effective thermal conductivities of some typical nanostructures are systematically studied both theoretically and experimentally.An effective thermal conductivity model for the multiply-constraint nanostructures is derived for BTE,and verified by the phonon MC simulations.It is proven that the effective thermal conductivity of a cross-plane nanofilm with internal heating is lower than that of the indetnical nanofilm under temperature difference.The two-step phonon MC method is used to study the anisotropic heat conduction in nanoporous structures,and it is found that the dependence of effective thermal conductivity on boundary roughness can be influenced by the geometries.An experimental system for thermal property measurements is built and used to study the effective thermal conductivities of silicon carbide(SiC)nanowires and graphene papers,and the relevant models above are used to analyze the exmperimental date.The least action principle of heat conduction is studied.It is found that the function that characterizes the irreversibility of heat conduction process is not unique.Except for entropy production,entransy dissipation rate can also serve as the irreversibility function in this case.By contrast,the action of heat conduction is unique,and only the entransy dissipation based least action principle can derive Fourier's law.As for the steady-state heat conduction optimization,the entransy dissipation extremum can give the result under both the artificial and physical constraints,while the entropy production extremum cannot;moreover,the influence of ballistic transport on the optimal result is studied by using the BTJ model.As for transient heat conduction optimization,the simulated annealing(SA)method is used.It is found that both the entropy production and entransy dissipation extremums become in-applicable in this case.A convolution integral based entransy dissipation is introduced,and its extremum is found to be corresponding to the optimal thermal conductivity distribution in this case.