The Study Of Phonon Transports Regulation Mechanism In Nanostructures

In this paper,We designed the nanostructures of materials to regulate their thermal conductivity,based on the wave-particle duality of phonon and the characteristics of phonon transport at the nanoscale.We analyzed the physical mechanism of the impacts of different nanostructures on the phonon transport.We proposed the structural design which is helpful to improve the performance of thermoelectric material and thermal insulation material,and to optimize the thermal management of two-dimension(2D)transistor devices.Phonon resonance effect can significantly reduce the thermal conductance in the low frequency range.In order to improve the efficiency of phonon resonance,we propose a novel phonon resonator for hindering the thermal transport of Si nanowires(NWs),based on the thread-like helical nanowalls.Results from molecular dynamics(MD)simulations reveal that the thermal conductivity(?)and phonon transmission of the thread-like Si NWs continuously decrease with the period density of the helical nanowalls.The reduction can reach as high as 36%for the NW with the maximum period density in our models,which is much larger than in the case of NWs with straight nanowalls(12%)and nanopillars(15%).With regard to suppressing the phonon propagation,the helical nanowalls as a phonon resonator exhibits superiority over the straight nanowalls or conventional nanopillars,because(1)it has larger contact area with the base structure and(2)it avoids the problem of nanopillars or nanowalls touching each other,which maintains the ability of generating localized modes.Combining phonon scattering mechanism and phonon wave interference,we investigated the inhibition of phonon transport in one-dimensional(1D)silicon/germanium(Si/Ge)NWs and two-dimensional(2D)graphene/boron nitride(Gr/h-BN)lateral heterostructures with component gradient interface,respectively.MD results show that the?of nanomaterials is tunable by changing the length of the component gradient interface.The?decreases with the increasing length of the component gradient interface.The phonon wave packet propagation analysis reveals that the composition gradient can reflect more than70%of the wave packet energy,and phonon localization is observed in the composition graded region.The existence of phonon localization in the component gradient interface was confirmed by the exponential decay of phonon transmission spectrum at a given frequency,as well as the phonon participation ratio of the Gr/h-BN lateral heterostructures.Thermal rectification was also found in the Gr/h-BN lateral heterostructures because of the mass asymmetry across the interface.The rectification ratio is the largest in the Gr/h-BN lateral heterostructures with abrupt interface.However,when the component gradient interface is introduced,the gradient components weaken the mass difference between the two sides of the interface,and the rectification ratio is thus reduced.Equilibrium molecular dynamics Simulation is used to research the thermal conductivity of three-dimensional carbon honeycombs with different sidewall widths.Anisotropic thermal conductivity along different directions were found.We noted that the local buckling introduced by the interconnections severely impedes the phonon transport.However,the interconnections bridge the gaps between the graphite layers through the sp³C-C,which results in a larger?perpendicular to the axis than the graphite cross-plane thermal conductivity.The phonon density of states shows additional modes in the optical region for the interconnection region,which is related to the structural change.We use multiscale modelings to investigate the heat dissipation in 2D transistors based on phosphorene and silicene.First,molecular dynamics(MD)simulations were used to calculate the thermal interface resistance Rint between the 2D materials(phosphorene and silicene)and dielectrics substrates(Si O₂ and Ti O₂).The calculated R_int of these systems are close to that between graphene and Si O₂ and are insensitive to the temperature.The MD values then served as inputs for finite element simulations at the device scale.It is found that the heat dissipation ability of the 2D transistors can be improved by increasing the thermal conductivities of the 2D materials as well as of the substrate.However,in contrast to the common belief,it is difficult to largely reduce the hot-spot temperature by tuning the interface thermal resistance.Finally,we show that the cooling performance of silicene/Si O₂system can be significantly improved with a few-layer graphene heat spreader.