Preparation And Thermal Conductivity Of Carbon-based Phase Change Composites

The rapid development of modern science and technology has accelerated the process of human civilization,and at the same time has brought a series of environmental and energy problems.Therefore it is imperative to find efficient clean energy to replace the traditional chemical energy.Phase change material(PCM)is a new type of high-efficiency energy storage material,which has the characteristics of high energy density,good reversibility and small temperature change.However,it also has some problems such as poor thermal conductivity and leakage during phase change,which limits its large-scale application.A large number of studies show that the thermal conductivity of phase change composites can be greatly improved by using high thermal conductivity carbon materials loaded with phase change materials.Paraffin wax(PA)is an ideal phase change material,which has the advantages of low price,environmental friendly,no supercooling and phase separation.Expanded graphite(EG)and graphene have excellent thermal stability,chemical stability and high thermal conductivity.They are usually used as thermal conductive fillers to improve the thermal conductivity of composites.In this paper,EG/PA and graphene aerogel(GA)/PA phase change composites are prepared by using PA as phase change material,and expanded graphite(EG)and graphene as thermal conductive fillers respectively.The main work of this paper includes:(1)The EG/PA phase change composites are prepared by using expanded graphite as thermal conductive filler and paraffin wax by melt blending and molding process.The effects of EG particle size,EG content,carbonization/graphitization temperature,carbon fiber(CF)and carbon nanotube(CNT)doping amount on the thermal conductivity of the materials are studied;(2)By using graphene oxide(GO)prepared by improved Hummers method as raw material,the GA is prepared by hydrothermal method.The GA was carbonized or graphitized and then compounded with paraffin to obtain the GA/PA phase change composite.The effects of GO concentration,hydrothermal time and hydrothermal times on the morphology and structure of GA are studied.The variation of GA composition and structure with carbonization or graphitization temperature is investigated.The structure-activity relationship between thermal conductivity and structure of GA/PA phase change composites was established.The main results and conclusions are as follows:(1)In the system of the EG/PA phase change composites,the thermal conductivity of the composites decreases with the decrease of the expanded graphite particle size.The thermal conductivity of the composites prepared with 150 mesh and 200 mesh expanded graphite is 27.205 and 20.922 W·(m·K)^-1,respectively;The EG content has a significant effect on the thermal conductivity of the composites.The thermal conductivity of EG/PA phase change composites in x-y direction increases linearly with the increase of EG content,and there is no obvious change in z direction;when the EG content increases to 20%,the thermal conductivity of the composites in x-y direction reaches 23.608 W·(m·K)^-1,which is 110 times higher than that of pure paraffin,and the melting enthalpy and solidification enthalpy of the composites are 187.51 J/g and189.61 J/g,respectively.(2)The thermal conductivity of EG/PA phase change composites prepared by heat-treated EG at 400?1000? is 19.762?21.369 W·(m·K)^-1,which is lower than that of the composites without heat treatment.This may be due to the formation of new lattice defects while the oxygen-containing groups on EG surface are removed by low temperature carbonization,resulting in the decrease of thermal conductivity.When the heat treatment temperature of EG rises to 1500?3000?,the thermal conductivity of EG/PA phase change composite rises to 23.532?23.905W·(m·K)^-1,which may be due to the high temperature carbonization or graphitization treatment that makes the lattice defects of EG repaired again.(3)The doping amount of high thermal conductivity particle MPCF has obvious influence on the thermal conductivity of EG/PA-MPCF phase change composites.With the increase of MPCF content,the thermal conductivity of EG/PA-MPCF phase change composites first increases and then decreases.When the content of MPCF reaches 4%,the thermal conductivity of the composite reaches 26.926 W·(m·K)^-1,and the whole MPCF is oriented along the x-y direction,which strengthens the heat conduction in the x-y direction;when the content of MPCF reaches 8%,the thermal conductivity of the composite decreases to 19.093 W·(m·K)^-1,and the random orientation of MPCF inhibits the x-y direction thermal conductivity.On the contrary,the thermal conductivity of EG/PA phase change composites doped with CNT decreases slightly,which may be due to the limited thermal conductivity enhancement effect of CNT in the case of high EG content.(4)The GA prepared by“one-step”hydrothermal method has disordered three-dimensional structure,while the s-GA prepared by“three-step”hydrothermal method has obvious layered structure,which provides a channel for directional heat transfer and a larger space for phase change materials;after carbonization at 1500?,most oxygen-containing groups on the surface of graphene sheet of GA are removed,after graphitization at 3000?,the internal lattice defects are repaired.(5)The s-GA/PA prepared by“three-step”hydrothermal method has better heat transfer performance and heat storage performance than the GA/PA prepared by“one-step”hydrothermal method.The thermal conductivity,melting enthalpy and solidification enthalpy of s-GA/PA composites prepared by 1500? heat treatment are0.727 W·(m·K)^-1,243.69 J/g and 242.67 J/g,respectively,but the proportion of graphene is only 1.3%,while the thermal conductivity of GA/PA composites prepared by the same process is 0.662 W·(m·K)^-1,and the proportion of graphene is 11%.The thermal conductivity of the composite prepared by graphitization at 3000? reaches0.825 W·(m·K)^-1.(6)The anti-leakage performance of the s-GA/PA phase change composite is better than that of GA/PA phase change composite before 3 h,but the leakage amount of s-GA/PA is larger after 3 h,which is almost the same as that of GA/PA.