| The application of paraffin based phase change materials,which are mainly composed of straight alkanes and their mixtures,has been limited in the fields of heat storage and energy management due to its low thermal transport performance.With the development of new nano technology,many nano additives such as carbon nano materials and metal nanoparticles are added into phase change materials to form composite phase change materials to improve the thermal transport performance of phase change materials and improve the thermal conductivity of traditional phase change materials.In order to study the micro behavior of alkanes in composite phase change materials and their interaction with carbon nanomaterials and metal nanomaterials,MD simulation method and phonon transport theory were used to study the graphene /paraffin composite phase change materials and their micro behaviors on Cu nano surface.The main contents and conclusions are as follows:The amorphous octadecane system and crystal octadecane system were constructed by using octadecane as the matrix material.By using the comparative method,the distribution of end distance and torsional angle of the two systems,the diffusion coefficient and thermal conductivity of the system were analyzed.The microstructure and thermal physical properties of the two systems during the phase transformation were studied.The results show that the phase transition temperature of octadecane in crystal system is about 300K-310 K,while that of n-octadecane in amorphous system is about310K-320 K.It is an effective method to improve the order of phase change materials.The composite phase transition material of graphene / n-octadecane was established by adding graphene to paraffin(n-octadecane).The mechanism of graphene to the thermal and physical properties of the composite system was further explained by calculating and analyzing the thermal conductivity of the composite system and the vibration power spectrum of graphene and n-octadecane.Based on the analysis of the influence of graphene on the micro behavior of noctadecane molecular chain,the surface structure of n-octadecane Nano Cu(100)and that of n-octadecane / graphene Nano Cu(100)was constructed.The effects of graphene and Cu nano surface on the molecular micro behavior of alkanes during the phase transition were studied by molecular dynamics simulation.The results show that the micro mechanism of adding metal nanoparticles to improve the thermal properties of phase change materials lies in that not only the metal nano materials themselves have high thermal conductivity,but also the interaction between metal nano surface and alkane molecules promotes the directional crystallization of alkanes on the metal surface.Graphene,as a kind of high heat conduction carbon nanomaterial with excellent performance,is also a good nucleator,which can further promote the directional crystallization of alkane molecules in the composite system,and then improve the thermal conductivity of the composite phase change materials in the whole system.The amorphous octadecane system and crystalline octadecane system were constructed with n-octadecane as the matrix material.The end distance distribution,twist angle distribution,diffusion coefficient and thermal conductivity of n-octadecane in the two systems were analyzed by comparison method.The microstructure and thermophysical properties of the two systems during phase transition were studied.It is proved that improving the order degree of phase change materials is an effective way to enhance the thermophysical properties of materials. |
