| Under the background of increasingly severe challenges brought by energy problems to today's human society,Phase change material(PCM)is a practical way to solve the imbalance between energy supply and demand,so the relevant research on PCMs has become the focus of research.However,there are still many limitations in the practical application of PCMs.Common organic PCMs have a prominent disadvantage:low thermal conductivity,which leads to a low thermal response rate of the energy storage system and a delay in the processes of heat absorption and heat release.With the development of PCMs,composite PCMs with high thermal conductivity fillers can improve the above shortcomings,but it also brings a new problem: the stability of composite systems is usually poor,and the thermal conductivity will be weakened by the agglomeration and deposition of fillers,and the service life will be shortened.In this paper,two methods are tried to solve the dispersion problem of composite phase change materials:(1)The composite PCM filler graphene was chemically modified to enhance its ability of stable dispersion in n-octadecane;(2)N-octadecane was adsorbed by graphene aerogel(GA)to form a stationary composite phase change material.In this paper,the thermophysical properties of graphene/n-octadecane composite PCMs were studied by combining experiment and simulation.Stearyl alcohol modified graphene(MG)was prepared by chemical modification,and its dispersion and microstructure in n-octadecane were studied by particle size analysis and SEM.The MG was mixed with n-octadecane to form a composite PCM.In order to study the influence of graphene material content on the thermal storage performance of composite phase change materials,MG samples with mass fraction of0%,1%,2%,3% and 4% were prepared in this paper,and their thermal properties were characterized and studied by fourier transform infrared spectroscopy(FTIR),differential scanning calorimetry(DSC)and thermal conductivity analysis.The experiment shows that:(1)The MG / n-octadecane composite PCMs prepared in this paper have good dispersion;(2)The addition of modified graphene did not significantly affect the thermal energy reserve of PCM;(3)When the MG mass fraction reached 4%,the thermal conductivity of the composite phase change material increased by 131.9%compared with n-octadecane.Graphene aerogel(GA)/ n-octadecane composite PCMs were prepared by reduction self-assembly method and vacuum impregnation method.The conclusion of experiment is as follows:(1)The micro-pore structure inside the graphene aerogel provides the adsorption force to compound with n-octadecane,which can improve the thermal stability of n-octadecane and the adsorption force of GA increased with the increase of content;(2)GA itself has a high thermal conductivity,which can be used as the thermal conductivity framework of the composite PCMs to significantly improve the thermal conductivity.When the content of GA is 13.99wt%,the thermal conductivity increases by 306.2%.However,the addition of a large number of GA will reduce the latent heat of the composite PCMs.Then,molecular dynamics simulation study was carried out on two types of graphene composite PCMs,and the influence mode of internal energy migration and microscopic mechanism on macroscopic thermal physical properties of composite PCMs were explored from the microscopic perspective.Analysis of simulation results shows: The thermal conductivity of the composite PCMs was higher than that of n-octadecane and increased with the increase of the content of graphene materials.From the perspective of molecular dynamics,the reasons are as follows: The addition of graphene materials will lead to more concentrated distribution of end to end distance and torsion of n-octadecane molecules,as well as reduced values of radial distribution function and self-diffusion coefficient,indicating that the arrangement of n-octadecane molecules is affected,resulting in an increase of thermal conductivity of n-octadecane. |
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