Study On Thermal Properties Of Graphene/n-octadecane Composite Phase Change Materials

Thermal energy storage technology can effectively improve the efficiency of energy utilization,and has played an important positive role in overcoming the energy crisis and environmental pollution.Latent heat storage mainly uses the latent heat of phase change material(PCM)to realize thermal energy storage and utilization,which is the most important thermal energy storage method at present.However,in practical applications,most of PCM have the disadvantage of low thermal conductivity,especially the traditional organic PCM,which greatly restricts the thermal response rate and system efficiency of the thermal energy storage system.With the development of nanotechnology,new nano-materials have great application potential in many fields owing to their unique volume effect,surface effect,macroscopic quantum tunneling and so on.It is expected to improve the thermal conductivity of traditional PCM by adding high thermal conductivity nanoparticles.In this paper,a novel two-dimensional carbon nanomaterial,graphene was selected as an additive for PCM.The phase transition thermal properties of graphene/n-octadecane composite PCM were investigated by experimental measurements and molecular dynamics simulations.Firstly,the graphene nanoplatelets/n-octadecane composite PCM was prepared by two-step method.The dispersion stability,micro-structure and chemical compatibility were characterized,and the effects of the addition amount of graphene nanoplatelets on the phase transition temperature,enthalpy,specific heat capacity,thermal conductivity and thermal stability of the composite PCM were compared.The study shows that the dispersion stability of the graphene nanoplatelets in the composite system is greatly improved by the addition of dispersant,and the system has not produced obvious agglomeration or sedimentation after multiple phase transformation cycles.Graphene nanoplatelets still maintain good lamellar structure and homogeneous dispersion in the n-octadecane matrix,and no chemical reaction occurs during the composite process.Compared with the significant increase in thermal conductivity,the addition of nano-graphene sheets has little effect on the phase transition temperature and latent heat of PCM,and still exhibits good heat storage performance.In addition,the composite PCM has a good thermal stability below 105?.Then,from the microscopic point of view,the molecular models of n-octadecane and three kinds of graphene/n-octadecane composites with different mass ratios were established.Molecular dynamics method was used to study the variation of thermophysical properties such as density,melting point,specific heat capacity,and thermal conductivity of each system during phase transition.The microscopic mechanism that influence the macro thermal physical parameters and the energy transfer processes in the composite systems were also discussed.It is found that the strong interaction between graphene and n-octadecane restricts the diffusion intensity of n-octadecane molecules.The variation of the self diffusion coefficient with the temperature can be used to judge the temperature of the phase transition.As the mass fraction of graphene increases,the melting point of the composite PCM decreases slightly,the specific heat capacity changes little,and the thermal conductivity increases significantly.In the composite systems,the vibration power spectra of graphene contains considerable overlap to that of n-octadecane,which is beneficial to improve the heat conduction between the graphene and n-octadecane molecules in the system.Finally,the effects of graphene and nano-Al surface on the molecular diffusion behavior and order degree of matrix PCM during the phase transition were investigated from the molecular point of view.The microscopic mechanism that affects its thermal conductivity was also analyzed.Studies have shown that the movement of graphene in the composite system is not diffusion but vibrates near the initial position.Near the surface of nano-Al,the order degree of n-octadecane molecule increases,indicating that the interaction from nano-Al surface promotes the crystallization of n-octadecane molecules.By comparison,the addition of graphene can further increase the order degree of n-octadecane molecules,and the effect of promoting the directed crystallization of n-octadecane molecules is more significant.The directed crystallization of long chain molecules is also an important factor for the enhancement of thermal conductivity.