Regulation On Thermal Transport Mechanism Of Carbon-based Two-Dimensional Nanomaterials

Since the discovery of carbon-based two-dimensional materials,which are considered to be a type of revolutionary material that may affect the future development of science and technology due to their unique optical,electrical,mechanical and chemical properties,they have witnessed increasing interest in both theoretical research and application fields.The thermal properties of these materials have also attracted more and more attention in various fields of scientific research.High thermal conductivity and interfacial thermal conductance can be benifical for solving the heat dissipation problem of nano-devices,while low thermal conductivity can enhance the thermoelectric figure for thermoelectric applications.In this paper,graphene(GE),C₃N(CN),BC₃(BC),g-C₃N₄(GCN),graphene/g-C₃N₄(GE/GCN)van der Waals heterostructure,graphene/hexagonal boron nitride(GE/BN)in-plane heterostructure,graphene/C₃N(GE/CN)in-plane heterostructure are focused.The instrinsic thermal conductivity,thermal resistanceinterfacial thermal conductance are investigated in depth using molecular dynamics(MD)simulation methods.This paper has achieved the following important results:First,using MD methods we revealed the heat transfer mechanism of 2D materials represented by graphene.In order to ensure the reliability of simulated models as well as calculation programs,the intrinsic thermal conductivity of GE are explored.When the length of GE is small,especially less than its phonon mean free path,the thermal conductivity increases sharply as the length increases.As the length of GE increases further,the thermal conductivity increases nonlinearly.When the length is infinite,the thermal conductivities of GE along the armchair and zigzag directions are 2553.88 and 2587.74 Wmm^-1K^-1,respectively.The thermal conductivity of GE decreases with increasing temperature,layer number,coupling strength,strain,vacancy defects and doping concentration.Biaxial strain has a greater effect on the thermal conductivity of GE than uniaxial strain.Compared to nitrogen and 13carbon doping,boron doping has the most significant negative effect on the thermal conductivity of GE.Second,using MD methods we revealed the BC heat transfer mechanism with same structural characteristics to CN for the first time,and the different heat transfer mechanisms of the two materials under different influencing factors were analyzed.BC exhibits a lower thermal conductivity than CN due to stronger flexural acoustic phonon-defect scattering rates and weaker interatomic bonding stiffnesses.The thermal conductivities of these materials exhibit decreasing trends in response to increases in temperature and defect ratio,and the temperature effect in BC is more substantial than that in CN,while the defect effect in BC is less substantial than that in CN.These two deformation modes cause different effects on the thermal transport behaviours of BC and CN:the effect of uniaxial compressive strain is slightly negative,while the effect of uniaxial tensile strain is initially positive and then negative.Remarkably,the impact of uniaxial and biaxial tensile strains on thermal transport was stronger in BC than in CN.Moreover,the biaxial strains result in a more severe reduction in thermal conductivity than the uniaxial strains.Third,using MD methods we revealed the in-plane and cross-plane thermal transfer mechanisms of two GCN structures(i.e.,TGCN and HGCN).The in-plane thermal conductivities of the TGCN and HGCN monolayers along the armchair direction are 55.39 and 17.81 Wmm^-1K^-1,respectively.The cross-plane thermal resistance decreases with increasing layer number and reaches asymptotic values of 3.6×10-10 and 9.3×10-10 m2K/W at 40 layers for TGCN and HGCN,respectively.Similar to CN and BC monolayers,the in-plane thermal conductivity can be effectively manipulated by changing the temperature and applying strain,while it is insensitive to the number of layers.Moreover,the cross-plane thermal resistance decreases monotonically with temperature and coupling strength,and can be modulated by external strain.Surprisingly,the cross-plane tensile strain reduces the thermal resistance of HGCN.Forth,using MD methods we revealed the interfacial thermal conductance modulation mechanism of GE/GCN heterostructures.From simulation results,the interfacial thermal conductance at 300 K is 1.28×108 WK^-1m^-2,which is higher than the interfacial thermal conductance of 0.83×108 WK^-1m^-2 in the reverse direction,indicating that thermal rectification exists at the interface.The interfacial thermal conductance is dependent on the temperature,coupling strength and mechanical strain.In particular,the biaxial strain exerts a more significant effect on thermal transport across the interface than uniaxial strain.Moreover,the interfacial thermal conductance is independent of the vacancy defect concentration.Fifth,using MD methods we revealed the interfacial thermal conductance modulation mechanism of the GE/BN in-plane heterojunctions.The GE/BN heterostructure has a remarkably high interfacial thermal conductance,and thermal rectification occurs at the interface.The results also show that the interfacial thermal conductance is effectively modulated by strain and defect engineering.The atomic defect location can affect the phonon transmission at the interface.Interestingly,compared with the nitrogen doping effect,the boron doping defect can more effectively facilitate vibrational coupling at the interface in the GE sheet.Finally,using MD methods we revealed the enhancement mechanism of the interfacial thermal conductance G and effective thermal conductivity keff of GE/CN in-plane heterojunctions.The value of G for the GE/CN heterojunction at room temperature is 31.49 GWm-2K-1 when heat transfers from CN to GE,which is larger than the value of 28.62 GWm-2K-1 in the reverse direction,indicating that thermal rectification exits at the interface.The keff values of the GE/CN nanoribbon along the direction from GE to CN and the reverse direction are 1346.51 Wmm^-1K^-1 and 1183.60 Wmm^-1K^-1,respectively.In addition,the G and keff of heterojunctions are effectively manipulated by changing the temperature,doping with nitrogen,applying strain and employing a substrate.The thermal energy transport across GE/CN interfaces is enhanced by increasing the size,temperature,nitrogen doping concentration,and compressive strain perpendicular to the heat flux direction or by depositing the materials on an amorphous silicon dioxide substrate.Furthermore,increasing the temperature and compressive strain are efficient methods to increase keff.