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Molten salt is regarded as one of the most promising candidates for solar energy storage due to possessing stable properties and large energy storage densities. However, the intrinsically low thermal conductivity of molten salt has become a bottleneck for rapid heat storage and transport. The addition of nanoparticles is generally considered to be the most effective way to improve the thermal conductivity of molten salt phase change materials (PCMs) while the phase change enthalpy of the nanocomposite phase change materials usually shows two opposite trends of enhancement or weakening. What's more, the reason for the abnormal change of phase change enthalpy is not clear in the literature at present, so the mechanism of change needs to be further explored. In this paper, GNS (graphene nanosheets)@NaNO 3  (sodium nitrate) nanocomposite phase change materials were prepared by hydration ultrasonic method. The materials were characterized by SEM (scanning electron microscope) and the phase change characteristics were measured using DSC (differential scanning calorimeter). Molecular dynamics simulation was carried out to explain the mechanism for the formation of the NaNO 3  dense layer and the the noncollateral decrease of the enthalpy from the microscopic level. With the increase of GNS mass fraction, the melting point of the GNS@NaNO 3  composite phase change material decreases slightly while the phase change enthalpy decreased significantly with a non-colligative trend. A 13.81% decrease of the theoretical phase change enthalpy was observed with GNS doping ratio of 1.5wt%. The NaNO 3  clusters observed on the surface of GNS were considered to have not melted, resulting in a reduction in the phase change enthalpy. The mechanisms were further investigated by molecular dynamics simulation:The strong van der Waals attraction between GNS and NaNO 3  leads to the formation of NaNO 3  dense layer in the vicinity of GNS for 2~4 Å. With the increase of GNS mass fraction, the centroid equivalent distance between the dense layer and GNS gradually increases, which contributes to the mutual attraction between the two first increases and then weakens. When GNS mass fraction is 1.5%, the centroid equivalent distance reaches closest to the potential well position, leading to the strongest mutual attraction. In other words, the phase change enthalpy decreases most obviously at this mass fraction. Thus, conclusions could be drawn as:The type of interaction between molten salt and nano-enhancers and the position of the potential well are the fundamental reasons for the thickness of molten salt dense layer and the reduction of phase change enthalpy. The calculation of the interaction energy can be used to guide the selection of the mass fraction of the nano-enhancers, so as to avoid the loss of core material cluster and phase change enthalpy caused by the introduction of the nano-enhancers to the greatest extent. The preparation cost of the composite phase change material can also be reduced to a certain extent.

Effect and mechanism of doped graphene nanosheets on phase transition properties of sodium nitrate