Study On Preparation,Structure And Properties Of Form-Stable Phase Change Composites

Phase change materials（PCMs）have broad application prospects in the fields of solar energy harvesting,energy-saving buildings,waste heat recovery,intelligent fabrics,thermal management of electronic equipment because of their thermal energy storage and temperature control characteristics.However,the practical applications of PCMs are seriously restrained by the drawback of melt flow and liquid leakage during the solid-liquid phase transition process.Solid PCMs change absorb heat and turn into liquid state,which usually leads to destroy of materials dimension and environmental pollution.In addition,PCMs has poor thermal conduction performance,thus leading to low heat transfer rate and inefficiency in heat storage and heat release.Besides,most of organic PCMs are highly flammable because their molecular contains a large number of C and H elements,which makes them easy to be ignited and cause fire,and limits their applications in some field with fire-proof requirements.This paper is focused on solving the liquid flow problem of PCMs and also pay attention to improve the thermal conductance and flame retardance.Two kinds of 3-dimensional porous materials,polyvinyl formal（PVF）foam and carbon skeleton reinforced graphene aerogel（c-GA/MF）are prepared and used to prepare shape-stabilized phase change materials.In addition,two phase change microcapsules,silicone dioxide@stearic acid and magnesium phytate@1-octadecanol are prepared based on the microencapsulation strategy.The structure and properties of phase change composites are systematically investigated,and the applications of phase change products in thermal energy storage and temperature regulation are explored.As a result,this paper will build the novel technology and new theory to prepare high-performance and multifunctional shape-stabilized phase change composites.1.Polyvinyl formal（PVF）foam shape-stabilized poly（ethylene glycol）-based phase change material.PVF foams are fabricated foams were prepared using water as the pore-forming agent and formaldehyde as the crosslinking agent.The PVF foams are then used to absorb polyethylene glycol（PEG）by means of vacuum impregnation,resulting in form-stable phase change composites（PEG/PVF）.It is found that the crosslinking degree of PVF increases with the increase of formaldehyde content.With the increase of crosslinking agent and PVA concentration,the cell size of PVF decreases while the mechanical strength increases gradually.The open-cell PVF skeleton can be obtained with the CH₂O/OH ratio of 2/1 and PVA concentration of 4-5%.The PEG exhibited good interfacial compatibility with PVF framework and the phase transition enthalpy ranges from 104.1 to 158.4 J/g.It is found that the PVF framework acts as the heterogeneous nucleating agent and promotes the formation of more nuclei at the initial stage of crystallization.Meanwhile,the dense framework hinders the growth of spherulites and prolongs the overall crystallization time.In addition,due to the high mechanical strength of PVF framework,PEG/PVF exhibits excellent dimensional stability above the melting point.2.Form-stable polyethylene glycol phase change material supported by carbon skeleton reinforced graphene aerogel.The carbon skeleton reinforced graphene aerogel（c-GA/MF）with high reduction degree and large mechanical strength is prepared through performing the chemical reduction assembly of graphene oxide/ascorbic acid slurry in the melamine skeleton and high temperature carbonization process.The c-GA/MF is used to vacuum-assisted impregnate PEG to obtain a c-GA/MF/PEG phase change composites.The results show that high temperature annealing treatment can further improve the reduction degree and graphitization degree of graphene aerogel,thereby enhancing the thermal conductivity of graphene network.By pre-constructing a thermal conduction pathway,the thermal conductivity of c-GA/MF/PEG reaches 1.32 W·m^-1·K^-1 and the latent heat of phase transformation is 94.2%of PEG.The carbon skeleton can greatly enhance the mechanical strength of aerogels through the"pillar"effect and lead to large compressive strength of 66.7 k Pa,thus endowing the c-GA/MF/PEG with excellent shape-stability.It is also demonstrated that the c-GA/MF/PEG holds tremendous promise in fields of solar energy harvesting,thermal energy storage and thermal management.3.Silicone dioxide@stearic acid phase change microcapsules and large dimensional phase change composites.Phase change microcapsules（SiO₂@SA）with a stearic acid（SA）core and silicone dioxide（SiO₂）shell are synthesized by interfacial polycondensation technology.Surface modified boron nitride（m-BN）is prepared by plasma treatment and coupling agent grafting.Silicone rubber-based large dimensional phase change composites（PCCS）were prepared by adding SiO₂@SA as the phase change filler and m-BN as the thermal conductive fillers.The Si O2 shell can effectively prevent SA from leaking out,which endows PCCs with excellent form-stability and leaking prevention property.The grafted of coupling agent on m-BN surface can significantly improve the compatibility with the matrix and enhance the thermal conductance,leading to a large thermal conductivity up to 0.506 W·m^-1·K^-1.Furthermore,the phase change enthalpies of PCCs can be facilely tailored from 3.74 J/g to 18.52 J/g by changing the loading fraction of microcapsule,enabling adjustable thermal energy storage and thermal management ability.4.Bio-based flame retardant phase change microcapsules.Flame retardant phase change microcapsules（FRPCMs）with a 1-octadecanol core and a biomass-derived magnesium phytate（PAMg）flame retardant shell are prepared by the facile chelation-deposition strategy.The PAMg shell can not only encapsulate the PCM,but also enhance its flame retardance,thus solving the two problems of liquid leaking and flammability simultaneously.It is found that there is no strong interaction between the PAMg shell and 1-octadecanol,but only a simple physical combination.When the microcapsules are introduced to the epoxy resin（EP）,the main combustion parameters of EP/FRPCMs decreased significantly.The study on the flame retardant mechanism shows that the flame retardant not only works in the condensed phase through catalyzing the formation of intumescent char layers,but also acts in the gas phase through the release of some P-containing radicals to quench the combustion process.The phase change enthalpy of FRPCMs can be adjusted from 117.2 J/g to 158.5 J/g by changing the content of PAMg shell.The FRPCMs are demonstrated to possess commendable thermoregulation property and long term thermal stability.In addition,the 1-octadecanol exhibits no corrosive effect on the three metal samples,but FRPCMs show a significant corrosive effect on Cu and negligible corrosive nature to Al and SS 304.