The effects of grain size and fractal porosity on thermal conductivity of nano-grained graphite: A molecular dynamics study

The thermal conductivity of isotropic graphite is a crucial parameter in mechanical and nuclear engineering. However, obtaining its value accurately is still difficult, especially under irradiation and high ambient temperature conditions. In this study, the nano-grained graphite models with different grain sizes and arrangements are designed, and corresponding thermal conductivity is calculated with equilibrium and non-equilibrium molecular dynamics methods. The results show that the thermal conductivity of nano-grained graphite varies between 3 to 5 W/(m & sdot;K) and decreases exponentially with the increase of the number of nanograins, while the arrangement of nanograins has almost no effect on the thermal conductivity. In addition, the effect of porosity of nanopolycrystalline graphite is analyzed by constructing fractal porous structures with different fractal dimensions and stages, of which the pore characteristics are determined with the initial zero-stage structure. The thermal conductivity of porous graphite increases with increasing fractal dimension and decreasing fractal porosity and the outcomes of molecular dynamics simulations are verified with the theoretical model results.