Approach-to-equilibrium molecular dynamics simulations have been used to study thermal transport in nanocrystalline graphene. Nanostructured graphene has been created using an iterative process for grain growth from initial seeds with random crystallographic orientations. The resulting cells have been characterized by the grain size distribution, by the number of atoms in each grain and the number of atoms in the grain boundary. Introduction of nanograins with a radius of gyration of 1 nm has led to a significant reduction in the thermal conductivity to 3% of the one in single crystalline graphene. Analysis of the vibrational density of states has revealed a general reduction of the vibrational intensities, in particular of the mode at 53 THz. This mode is additionally found to shift to lower frequencies with decreasing grain size. Vibrations of grain boundary atoms have led to the evolution of a mode at 30 THz. Thermal conductivity has been evaluated as a function of the grain size with increasing size up to 14 nm and it has been shown to follow an inverse rational function. The grain size dependent thermal conductivity can be described by a function assuming a connection in series of conducting elements and resistances for thermal transport.
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