Molecular Dynamics Study On Thermal Transport Properties Of Polycrystalline Graphene

Graphene,as an emerging two-dimensional material with extremely high thermal conductivity,has broad application prospects in electronic equipment thermal interface materials.However,large-scale preparation of large area,high stability,and high-quality graphene is one of the difficulties in the industry today.Currently,chemical vapor deposition is widely used internationally to synthesize graphene on copper foil in the millimeter or even centimeter scale.The chemical vapor deposition method utilizes carbon atoms to precipitate,nucleate,and grow on the substrate surface to form graphene.Grain boundaries are formed at the growth interface,usually forming polycrystalline graphene.Therefore,the study of the thermal transport properties and regulation of polycrystalline graphene is of fundamental research significance for understanding the thermal transport,thermal dissipation,and inherent phonon diffusion mechanisms of low dimensional systems,as well as guiding the design and application of related devices.In this thesis the thermal transport properties of polycrystalline graphene have been studied using molecular dynamics methods,and the main results include as follow:(1)We studied the thermal transport properties and regulatory mechanisms of single-layer polycrystalline graphene nanoribbons.The results showed that annealing temperature and rate have significant effects on the structure and thermal conductivity of polycrystalline graphene.The thermal conductivity of polycrystalline graphene nanoribbons is lower than that of single crystal graphene,and is influenced by both the length of the nanoribbons and the size of the crystal domains.However,the mechanisms of their effects are different.The increase in the overall length of the nanoribbons enhance the mean free path of phonons to promote phonon thermal transport,while the increase in the size of the crystal domains promotes phonon thermal transport by reducing phonon boundary scattering.At the tensile limit,the mechanical properties of polycrystalline graphene are much lower than those of single crystal graphene due to structural influence.The thermal conductivity of polycrystalline graphene decreases with increasing tensile/compressive strain.This is because during the strain process,compared to the pristine graphene without strain,the phonon mode is suppressed by strain,which is not conducive to phonon thermal transport,and result in the decrease of thermal conductivity.The thermal conductivity of polycrystalline graphene decreases with the increase of defects,which is due to the reduction of high-frequency phonon peaks,resulting in a decrease in phonon lifetime and a decrease in thermal conductivity.(2)The thermal conductivity and interfacial thermal resistance of double-layer polycrystalline graphene nanoribbons were studied.The results show that the in-plane phonon thermal conductivity of polycrystalline graphene is lower than that of single crystal graphene,regardless of whether it was a single or bilayer structure,and weakened with the increase of interlayer forces in the bilayer structure.This is because the interface distance between bilayer membranes decreases with the increase of interlayer interaction intensity,and the contribution of phonon bending mode is suppressed.A larger interlayer interaction intensity corresponds to a larger van der Waals interaction,which reduces the in-plane phonon thermal conductivity.The interface thermal resistance of the double-layer structure will decrease with the increase of interlayer force,the decrease of crystal domain size,and the increase of defects.The interfacial thermal resistance of polycrystalline graphene is smaller than that of single crystal graphene,and this decrease in interfacial thermal resistance can be explained by the increased friction caused by the enhancement of interface roughness and defects,resulting in an enhanced phonon coupling.Our research work provides a new solution for the heat dissipation and structural stability of graphene devices in actual production,and also provides an important reference for the application of graphene based devices in the thermoelectrics.