Thermal Properties Of Functionalized Graphene/palmitic Acid Thermal Storage Materials: A Molecular Dynamics Study

Energy storage technology is a key technology to solve the problem of intermittent energy supply,and is an important way to promote the development of transformational energy structure and improve the capacity and efficiency of thermal storage systems.As the core of energy storage technology,phase change materials（PCM）are the key to developing high-performance thermal storage systems.To solve the drawback of low thermal conductivity of PCM,researchers added the high thermal conductivity material graphene,but the interface thermal resistance（ITR）inhibits the degree of improvement of thermal conductivity.In recent years,the use of functionalized graphene（FG）to reduce the ITR and thus improve the thermal conductivity has gained much attention,but the effects of graphene surface functionalization on thermal properties and interfacial heat transport are still not clear enough.In this paper,we simulated and analyzed the effects of surface functionalization on the thermal properties and heat transport of phase change thermal storage materials using functionalized graphene as filler and palmitic acid（PA）as phase change matrix.The details of the study are as follows.Firstly,the simulation models of pure PA and PA/pristine graphene（PG）composite phase change materials（CPCM）were developed,the effects of temperature on the thermal property parameters of PA obtained by calculation were verified and the model reliability was verified.The effects of adding PG on the microstructure and thermal properties of the system were investigated.The addition of PG acts as heterogeneous nucleation,and the better ductility of graphene promotes the extension of molecular chains and improves the orderliness of the system.The density,specific heat capacity,and viscosity of the system were increased by the addition of graphene.A PA/FG model was developed to investigate the effect of the presence of oxygen-containing functional groups on the surface of graphene on the microstructure and thermal properties of the composite system,using PA/PG as a control.The functionalization of graphene surface leads to the bending and torsion of molecular chains and the decrease of order.However,larger interactions cause the molecules farther from the FG plane to stretch and the degree of molecular bending of the system is improved.The addition of FG increased the phase transition temperature and density and decreased the specific heat capacity compared to the addition of PG.With the increase in functionalization rate,the phase transition temperature,density,and specific heat capacity of each composite system increased,which was related to the increase in interaction energy.In addition,the addition of either PG or FG increases the viscosity of the system,which affects the convective heat transfer of the material.Finally,a heat transfer model of the PA/PG interface was developed.The causes of the ITR were clarified based on the distribution of the phonon density of states（PDOS）curve.The effects of size effect,system temperature,graphene layer number,and graphene surface functionalization on the ITR were investigated.It is found that the increase in size and the increase in system temperature have less effect on the ITR.The increase of layer number leads to the enhancement of phonon scattering and the gradual increase of the ITR,which deteriorates the heat transfer at the interface.Functionalization of graphene surface with both oxygen-containing functional groups and alkane chains can reduce the ITR,and the degree of reduction depends on the functionalization rate and the type of functionalization.The analysis shows that the reduction mechanisms are different between the two.The former relies mainly on the enhancement of the interaction energy,while the latter relies on the phonon coupling of alkane chains with PA molecules.