Investigation On Phase Change Properties And Thermal Conduction Performances Of Carbon Nanocomposite Form-stable Phase Change Materials

With the increasing problems of energy crisis and environmental pollution,more and more attention has been paid to the development,utilization and storage of new energy.Phase change energy storage is an effective way of heat storage,which has the characteristics of high energy storage density and stable working temperature.However,the thermal conductivity of phase change materials is poor,and there are leakage problems in the process of solid-liquid phase change,which restrict the promotion and use in the field of energy storage.In view of the above problems,the carbon nanomaterials / paraffin form-stable phase change materials were studied by means of experiments and molecular dynamics simulation,and their phase change properties and micro thermal conduction performance were discussed.The main research contents and conclusions are summarized as follows:1.The molecular dynamics method was used to investigate heat transfer characteristics of paraffin in solid,liquid and crystal states.The influence of temperature on the interfaces heat transfer of crystal paraffin was discussed.The results show that the order of paraffin molecules is the key factor affecting the thermal conductivity.The high order arrangement of crystal paraffin leads to the interfacial thermal conductance between the blocks.Due to the effect of van der Waals force,there is thermal fluctuations between the interfaces with the change of temperature.which leads to the coupling and decoupling characteristics of heat flow at the individual molecular scale,and the thermal coupling of the interfaces is conducive to the interfacial heat transfer.2.The phase change properties and thermal conduction performances of graphene / paraffin form-stable phase change materials were studied by experiments and molecular dynamics simulation.The experimental method was adopted for the preparation and characterization of graphene nanosheets/paraffin phase change materials.The thermal properties of the composite phase change materials prepared by solution mixing,vacuum adsorption and vacuum mixing were compared and analyzed,and the photothermal conversion properties of the composite phase change materials with different graphene nanosheets content were discussed.The results show that the adsorption capacity of paraffin prepared by solution mixing method is the largest,reaching 84.67%,the latent heat value of phase change is the highest,the thermal cycle stability is the best,and the thermal conductivity is 11.64 times higher than that of pure paraffin;The addition of graphene nanosheets significantly improves the photothermal conversion performance of paraffin,the conversion efficiency of photothermal is 2.4 times higher.The molecular dynamics method was used to investigate the adsorption process of graphene to paraffin,the adsorption behavior,phase change and thermal conduction of paraffin by porous graphene structures with different density were discussed.The results show that the mass fraction of paraffin with different composite structures decreases with the increase of the density of porous graphene structures.Due to the interaction between graphene and paraffin,there are dense phases around the graphene sheets,which has negative influence on the latent heat,stretching and bending behavior of paraffin,but it has little influence on diffusion behavior.The higher the density,the lower the thermal storage efficiency of the composite phase change materials,and the lower the deviation from the theoretical value.But the existent of dense phase leads to the increase of the order degree of paraffin molecules in this region,and graphene sheets form a heat conduction network,thus the thermal conductivity increases.Under the effect of these two aspects,the optimum density of the porous graphene structure is 0.5 g/cm3,the latent heat of the composite material is 84.17 J/g,and the thermal conductivity is 0.51 W/(m·K).3.The heat transfer characteristics of the nano capsules formed by paraffin filling carbon nanotubes were studied by molecular dynamics,and the effects of different diameters on heat transfer were discussed.The results show that the change of interfacial thermal conductance and axial thermal conductivity are related to the mass fraction of paraffin,but not to the tube diameter.The higher the paraffin mass fraction,the smaller the interfacial thermal conductance and axial thermal conductivity.The filling degree can be increased by adding pressure,the increase of filling degree leads to the increase of latent heat,and the decrease of interfacial thermal conductance and axial thermal conductivity.