| The materiale absorbs or emits heat and the temperature is almost stable,during the phase change process,which can be used to store or release energy,solving the problem of spatial or temporal mismatch of energy supply and demand,and adjust the external temperature.The organic phase change material has stable properties,large energy storage density,no toxicity and no corrosion.It is a kind of energy storage and temperature control material that is often studied at present,but it has the problems of low thermal conductivity and easy leakage.Due to its low price,high thermal conductivity and good compatibility with organic materials,expanded graphite is often used to improve the thermal conductivity of organic phase change materials.The preparation of expanded graphite mainly uses concentrated sulfuric acid and concentrated nitric acid to oxidize and intercalate the natural flake graphite,and then the high temperature treatment of the obtained expandable graphite.However,the expanded graphite obtained by this method contains an excessive amount of sulfur element,which corrodes the container in contact with it during its application.The microcapsule method is commonly used for the packaging of phase change materials,but the process is complicated.Besides the phenomenon of capsule rupture occurs in multiple thermal cycles.Epoxy resins have good adhesion and heat resistance,but there are few reports in the literature on the use of epoxy resins to encapsulate phase change materials.In this paper,we prepare the sulfur-free expanded graphite at first.And then it is compounded with n-icosane as thermally conductive filler.Finally,epoxy resin is introduced as wrapping material.This research strives to fundamentally solve the two problems of limiting the application of organic phase change materials and makes sure that the preparation is simple,the raw material cost is low,and the thermal physical properties of the material can meet the requirements of practical use.In this paper,it is finally determined through experimentation that the natural flake graphite is oxidized using ferric chloride and chromium trioxide,and the graphite flake layer is intercalated with nitromethane.Compared with the conventional chemical oxidation method,the reaction conditions are mild,the efficiency is high,and the product after high-temperature treatment has a large expansion volume.The phase,composition and morphology of the graphite products at each stage were analyzed.The results confirm that the oxidation and intercalation effect of the above reaction system.The results of the morphology analysis also showed that the expanded graphite had a rich pore structure,so that it can be used as the thermally conductive reinforcing filler adsorbs the phase change material.A phase change composite with expanded graphite content of 3% to 12% was prepared by vacuum adsorption method.Through microscopic morphology analysis,it was found that the multi-stage pores between the surface of the expanded graphite and the sheet had adsorption to the phase change main body,n-icosane.The phase transition enthalpy of the phase change composites is close to the theoretical calculation value and the phase transition temperature is only 1 °C worse than that of n-icosane,indicating that there is only a physical bond between the expanded graphite and n-icosane.When the expanded graphite content increases from 3% to 9%,the expanded graphite can gradually form a long-term heat conduction network in contact with each other in the matrix material.Compared with n-icosane,the thermal conductivity of the phase change composite material is significantly improved;Then the expanded graphite content keeps increasing to 12%,the thermal conductivity of the phase change composite reaches to 4.52 W/(m·K),which is 20.8 times that of n-icosane,but the growth rate begins to slow down.The calculated thermal conductivity calculation models of composite materials,Nielsen model and interface thermal resistance correction model,further support the conclusions above.The adsorption of n-icosane on expanded graphite is not enough to inhibit the leakage during the phase change process.The mass loss rate of phase change composites after 50 cycles is over 21%,and further wrapped with epoxy resin.The results show that when the mass ratio of epoxy resin to n-icosane is 1:1,the mass loss rate can be reduced to less than 2%,which satisfies the requirements for leakage in actual use.The phase transition enthalpy of the composite phase change material wrapped by epoxy resin decreased slightly,and the phase transition temperature was only 0.1 ℃ compared with that before epoxy resin packaging,indicating that the use of epoxy resin would not bring bad influence on the thermal properties of n-hexadecane.The results of the scanning electron microscope test show that the epoxy resin can be wrapped because its three-dimensional network structure can block small molecules of n-icosane. |
