Heat Transport Low Dimensional Systems Of Molecular Dynamics Simulation

Thermal conductivity due to phonons in nanostructures such as nanotube, nanowire, thin film and graphene is different from that of bulk materials. In the bulk materials, phonons transport diffusively so that thermal conductivity is a constant, independence of material size and geometry and obeys the Fourier’s law. It’s the intrinsic property of the material and depends only on composition of material and environment temperature. However, both theoretical and experimental studies in the past two decades have demonstrated that thermal conductivity in nanostructure relies on material size and geometry. Theoretical calculations have illustrated that the length dependence of thermal conductivity behaves as power-law for one-dimensional system and increase logarithmically for the two-dimensional system.Using molecular dynamic methods, we investigate thermal transport in low-dimensional system. Lammps as successful molecular dynamic simulation software can fulfill the computations efficiently.First, Silicon nanowires(SiNWs) is chosen as a model. With Tersoff potential, we use equilibrium molecular dynamics methods to elucidate the relationship between thermal conductivity and the crossover area of SiNWs. Our calculation demonstrates that thermal conductivity is not a constant. Due to the high rate between surface area and volume in SiNWs the scattering by the rough wire surface inhibits thermal conductivity of SiNWs such that thermal conductivity of SiNWs is much lower than that of bulk materials, depending on the material size. With the increase of the crossover area, thermal conductivity of SiNWs rises considerable.Next, we investigate thermal transport in graphene nanoribbons(GNRs). We use the empirical Airebo potential for the interaction between carbon atoms. Non-equilibrium molecular dynamics method is employed to study the relationship between thermal conductivity and system size in one or two dimensions. We consider the4-AGNRs as a one-dimensional system. Our results demonstrate that the relationship between thermal conductivity and system size is power-law. With the increase of the width of GNRs, the GNRs can be considered as a quasi-two-dimensional system, such as20-AGNRs and40-AGNRs. We find that the relationship between thermal conductivity and system size behaves as logarithmic. Finally, we employ the airebo potential and the non-equilibrium molecular dynamics methods to study the relation between thermal conductivity and the layer number of GNRs. We find that thermal conductivity in single-layer GNRs is obviously larger than that of multi-layer GNRs, independent of the chirality(zigzag or armchair). However, when the layer number increases beyond two, thermal conductivity changes little.