Effect of graphene orientation on heat transfer properties of graphene/nitrates composites by molecular dynamics simulation

Nitrates have been widely used as heat storage mediums in the concentrating solar power (CSP) systems due to their low costs and wide operating temperature ranges. However, the heat transfer performance of nitrates is unsatisfactory, and nano-fillers are generally incorporated in nitrates to further improve the power efficiency of CSP systems. In particular, the nitrates composite doped with graphene has been proven to be a potential candidate with excellent thermal conductivity. In this work, molecular dynamics (MD) simulations were performed to investigate the thermal properties and heat transfer behaviors of graphene/nitrates composites. The effects of the graphene orientation on interfacial thermal conductance and the overall thermal conductivity were deliberately examined using the non-equilibrium molecular dynamics (NEMD) method and the effective medium theory model. It is shown that the interfacial thermal conductance can be considerably enhanced from 46.36 MW·m−2·K−1 to 80.03 MW·m−2·K−1 as the angle θ between the graphene surface and the heat flux direction (i.e., z direction) decreases from 90 °to 0 °. As the angle θ decreases, the effective projection of the graphene plane in the direction of heat flow is enhanced, and more heat will be transported along the graphene plane. Such efficient heat transport along the internal direction of the graphene plane compensates the obstructing effect of heat transport across the graphene plane. The results of the vibrational density of state (DOS) clearly signify that heat flow at the interface changes from transport across the graphene plane to efficient transport along the graphene plane with the decrease of angle between graphene and heat flow. Moreover, the nitrates form a similar dense layer around the graphene for all different orientation angles, which would also promote the enhancement of the thermal conductance. Finally, the thermal conductivity of the graphene/nitrates composites with different orientations at the microscale is predicted by the effective medium theory. It is found that the thermal conductivity of the composite decreases with the orientation angle, but increases with the volume fraction and the length of the graphene. This work is thus believed to provide atomic and microscale understanding of the thermal transfer and performance of graphene/nitrates composites.