Construction And Properties Of Three-Dimensional Porous Network-based Composite Phase Change Material With Enhanced Thermal Conductivity

Phase change materials(PCMs)can realize energy storage and release by endothermic and exothermic processes during phase transition periods.Organic phase change materials have become a hot topic in the research of PCMs due to the characteristics of small volume change,high heat storage density,excellent chemical stability,non-toxic and pollution-free.However,the weakness of low thermal conductivity,thermal capture mode unicity and easy leakage during solidliquid phase transition seriously restrict the industrial application of organic PCMs.At present,the shape-stabilized composite PCMs constructed by materials compositing stratagem can not only prevent the leakage of PCMs,but also endow the PCMs with solar-/electric-/magnetic-thermal energy conversion and storage properties.Therefore,developing outstanding comprehensive performance of shape-stabilized composite PCMs is crucial to the research of phase change energy storage filed.Based on the idea of combining three-dimensional porous support support and PCMs,in this paper,three-dimensional continuous porous network structure materials with high thermal conductivity performance are designed and prepared as the supports of composite PCMs.The intermolecular and capillary forces between porous supporting materials and organic PCMs are utilized to firmly restrict the liquied flow of PCMs within supports pores.The development of threedimensional shape-stabilized composite PCMs with high thermal storage density,enhanced heat transfer efficiency and solar-/electric-/magnetic-thermal energy conversion and storage properties provide the theoretical and experimental foundation for the controllable construction and application of three-dimensional porous network-based composite PCMs in the field of energy conversion and storage.The detailed research contents are as follows:The porous melamine sponge is selected as the three-dimensional(3D)skeleton and the graphene nanosheets are selected as the thermal conductivity enhancing material.The three-dimensional continuous sponge-reduced graphene oxide porous support material is obtained by wrapping the reduced graphene oxide sheets layer by layer on the sponge skeleton by cyclic impregnation and adsorption and in situ reduction.By varying the cycle times of impregnation and adsorption of the sponge on the graphene oxide suspension,the graphene content and its distribution in the sponge-reduced graphene oxide porous support are regulated to obtain a three-dimensional porous network-based composite phase change materials with high thermal storage density(melting enthalpy 170.4 J/g),excellent thermal transfer performance(thermal conductivity of 0.46 W/m-1 K-1,84%higher than paraffin wax)and 85%efficient solar-thermal conversion efficiency.By comparing the thermal storage/transfer performance of composite phase change materials constructed with 3D porous network supports and simple disordered physical mixing,the effect on thermal conductivity enhancement and shapestabilized performance of 3D porous network supportss for phase change materials are clarified.Besides,the solar-water heating and insulation system is designed based on the thermophysical properties of composite phase change materials,which is effectively expanding the application of the resulting composite phase change materials.One-dimensional multi-walled carbon nanotubes are utilized as the building blocks of the three-dimensional continuous porous carbon nanotube aerogel network with high porosity which is constructed by adding binders and using the ice template method and high-temperature calcination.Lightweight,malleable,light-absorbing and thermally conductive polymers are used as cross-linking materials on the carbon nanotube arerogel networks and thin layers of polypyrrole are generated by in situ polymerization on the aerogel network through chemical vapor deposition to obtain carbon nanotube-polypyrrole heterogeneous porous aerogel supports materials.The thermally conductive enhanced three-dimensional porous network-based phase change composites are obtained.The thermal conductivity(0.64 W/m-1·K-1,2.56 times that of paraffin wax)and the solar/electrical-thermal energy conversion and storage efficiency(up to 90%and 91.5%,respectively)of the composite phase change materials are greatly enhanced by regulating the content of carbon nanotubes in the supports network and the formation of polypyrrole layers.The thermal conductivity enhancement mechanism of the aerogel supports material is also demonstrated by molecular dynamics simulations.Kapok fiber as the natural biomass hollow fiber is selected as the building block of the 3D aerogel network,and polypyrrole,which is lightweight,malleable,light-absorbing and has high thermal conductivity,is used as the thermally conductive enhancing material.The continuous polypyrrole coating is generated in situ on the inner and outer walls of the kapok fiber by oxidative polymerization.Simultaneously,the remaining trivalent iron oxidant in the system is correspondingly converted into Fe3O4 magnetic nanoparticles by co-precipitation technology to form polypyrrole-Fe3O4 bicontinuous functionalized hollow fibers.After that,the polypyrrole-Fe3O4@kapok composite hollow fibers are furtherly assembled into a 3D porous network supports material,and then the paraffin wax is efficiently encapsulated into the porous network supports by vacuum melting infiltration.Therefore,the thermally conductive enhanced 3D porous networkbased composite phase change materials with solar-/magnetic-thermal energy conversion and capture properties are obtained.By regulating the network density of the porous supports,the maximum melting enthalpy,excellent solar-thermal conversion and storage efficiency and the highest thermal conductivity of composite phase change materials are achieved up to 161.4 J/g,90%and 1.06 W/m1·K-1,respectively.The construction of a low-density,high-porosity,green and renewable fiber network and the formation of a polypyrrole-Fe3O4 functional coating ensure the high thermal storage density and siginificantly enhanced thermal conductivity of the composite phase change materials,which effectively balancing the thermal storage/transfer performance of the composite phase change materials.