Preparation And Properties Of Heat Conduction Enhancement RGO Aerogel-Based Composite Phase Change Materials

In recent years,phase change energy storage technology has attracted wide attention because of its ability to resolve the contradiction between supply and demand of thermal energy,alleviate the energy crisis,better conservation and efficient use of energy.Phase change materials（PCMs）are one of the keys to realize phase change energy storage technology,which have the characteristics of high energy density,adjustable temperature and stable energy output,and have great application prospects in energy storage systems.Although some researches have been done on PCMs,the problems of unstable thermal cycling,low thermal conductivity and low heat storage and release efficiency have not been solved well.In particular,the problems of low thermal conductivity and unstable thermal cycling have seriously restricted the widespread application of PCMs.In order to enhance the heat conduction and improve the thermal cycling stability of PCMs,several high-performance graphene-based anisotropic aerogels for heat conduction enhanced network were prepared in this paper by regulating the self-assembly process of graphene oxide（GO）nanosheets.When applied to phase change energy storage,they can not only provide excellent heat conduction pathway,but also better adsorb PCMs,and the heat conduction pathway can maintain good stability in the thermal cycling process,without collapse and agglomeration.The research results of this paper can provide theoretical basis and new ideas for the development of new composite PCMs with high thermal conductivity.A series of reduced graphene oxide（rGO）aerogels were prepared by hydrothermal reduction by adjusting the polarity of the solution in the GO dispersion liquid system.The characterization results show that the aerogels prepared under different polar solutions have very different chemical properties,mechanical properties and internal microstructures.In a certain range,the volume of the wet gel increases linearly with the decreasing of the polarity of the solution.At the same time,the decrease of the polarity also leads to an increase in the thermal weight loss of the aerogel and a gradually increasing O/C value.The sp² C content of graphene gradually decreases,and the I_D/I_G value increases in the Raman spectrum detection.In the weakly polar solution system,the reduction process of GO is relatively slowed down,and during the reduction process,as the reduction degree of GO increases,the degree of free distortion of rGO increases.During self-assembly,the number of contact points and contact modes between the sheets increases,and finally a honeycomb aerogel with uniform pore structure is formed,which exhibits good elasticity and can be repeatedly compressed.In the strongly polar solution,rGO is flat and lamellar,and finally self-assembled into a gully aerogel with a certain anisotropy,showing brittleness and basically no elasticity.By simulating the"ocean current",the GO dispersion solution was treated under directional heating to generate a temperature gradient,thereby forming convection inside the solution.As a result,the GO nanosheets were aligned during the reduction process and self-assembled into an anisotropic rGO aerogel.The effects of microwave vacuum treatment on the structure and performances of aerogel were studied.The results showed that the O and N atoms and H₂O content of rGO decreased significantly after microwave treatment.However,with the extension of microwave treatment time,the improvement of rGO sheets’apparent performance and the reduction of I_D/I_G value were not significant,and the average in-plane crystal size L_a decreased instead.After a short time（1min）of vacuum microwave treatment,the axial thermal conductivity of rGO/paraffin composite phase change material reached 1.07W/（m K）with the rGO content of 0.32 vol.%,and the thermal conductivity enhancement efficiency reached 995%,which could reduce the supercooling degree by 1.9℃.A one-step heat flow method was used to directly incorporate pristine/native graphene nanosheets（GNs）into the self-assembly process of anisotropic aerogels.Then after freeze-drying and vacuum microwave treatment,the anisotropic rGO/GN aerogel（AGG）was obtained.The effects of different content of GNs and microwave treatment on the structural properties of AGGs and the thermal properties of rGO/GN/paraffin composite phase change materials（GGPCMs）were studied.The results showed that microwave treatment could significantly reduce the oxygen and nitrogen content and increase the sp² domain,but at the same time,the graphene layer of the pore wall expanded and partially exfoliated from the aerogel pore wall.Moreover,with the extension of treatment time,resulting in a large number of pore defects in the rGO sheet and even broken into fragments.The enthalpy of fusion,the enthalpy of solidification and the initial melting temperature of GGPCMs all decreased with the increase of AGG content,but the melting range（the temperature difference between the beginning of melting and the end of melting）,the solidifying range（temperature difference from the beginning of solidification to the end of solidification）,but the subcooling degree increased with the decrease of the relative content of GNs in AGGs.The maximum axial thermal conductivity of GGPCMs was 1.45 W/（m K）and the thermal conductivity enhancement efficiency was 884%（AGG content was 0.53 vol.%）.Thermal imaging showed that GGPCMs had obvious advantages in heat transfer.The anisotropic rGO/boron nitride（BN）aerogels（AGB）were prepared by one-step heat flow method,and the rGO/BN/paraffin composite phase change materials（GBPCMs）were obtained after vacuum adsorption of paraffin.The compatibility of AGB with paraffin,effects on heat conduction enhancement,GBPCMs thermal cycling stability and heat storage/release efficiency were studied.The results showed that the the aerogel rigidity increased with the increase of BN content,and the pore wall could better resist the volume shrinkage during paraffin solidification,making the GBPCMs shrinkage and deformation smaller in the process of thermal cycling and showing better thermal stability.When the mass ratio between GO to BN was 1:20,the axial thermal conductivity of GBPCM was 1.68 W/（m K）,which was 504%higher than that of pure paraffin.After 50 thermal cycles,the paraffin leakage was 3.1%and the enthalpy loss was 2.7%.Current test indicated that the heat conduction pathway in GBPCMs still maintains good integrity under the action of repeated melting and solidifying of paraffin.When simulating the actual temperature variation,AGBs could advance the paraffin phase change time by about 1210s and improve the paraffin phase change efficiency by about 47%.