Preparation And Performance Study Of SiO₂-based Composite Wall Phase Change Nanocapsules

Phase change materials(PCMs)can be encapsulated into containers of mirco or nano size,i.e.micro/nano encapsulation technique,which can prevent their leakage in liquid state,improving heat transfer by increasing the surface area to volume ratio,and controling the volume changes.Thus,micro/nano encapsulation can largely broaden the application area of PCMs.In order to meet the requirement of practical application,micro/nano encapsulated PCMs(MePCMs or NePCMs)should possess many ideal performances,including large phase change enthalpy,highe thermal conductivity,low supercooling,good leakage proof property and high mechanical strength.The shell materials and size of capsules are two key factors related to the properties of NePCMs.In the aspect of shell materials,researchers have successively developed organic shell,inorganic shell and organic-inorganic hybrid shell materials and so on,in order to improve thermal conductivity,leakage proof property,and mechanical strength of encapsulated PCMs.In the aspect of capsule size,compared with MePCMs,NePCMs possess much higher surface area to volume ratio,and it can't be ruptured easily.Thus,NePCMs became an important research field in recently years.Currently,facile construction of NePCMs with highly tunable composite shell in composition and microstructure,is one of the most important challenges in the thermal energy storage area,for the purpose of improving integrated performance.In this paper,three kinds of NePCMs with SiO2 based composite shell were prepared by combining interfacial hydrolysis-polycondensation of alkoxy silanes and other kind of reaction process(including electroless plating,self-assembly of graphene via electrostatic interaction,and radical polymerization of vinyl monomers)in a miniemulsion system.The obtained NePCMs have different morphologies and microstructures.The phase change property,thermal conductivity,leakage proof property and other important properties were studied.The relationship between structure and properties were analyzed.On this basis,the application performances of NePCMs inthermo-regulating materials and photo-thermal conversion latent heat functional fluid(LFTF)were explored.Mainly 4 parts were included in this thesis.(1)Metal is a kind of materials with excellent thermal conductivity and mechanical properties.A novel kind of metal coated NePCMs were prepared through electroless plating.Firstly,surface activation by dopamine was carried out on silica nanocapsules containing n-octadecane.Highly tunable PDA layer thickness was achieved by appropriately adjusting the dopamine and Tris-HCl concentration.The mass-based latent heats decline obviously after silver plating,but the decrease of volume-based latent heats was minor(about 31.89%).The NePCMs crystallize mainly based on heterogeneous nucleation and show low supercooling,by introducing n-octacosane as nucleating agent.Moreover,they can keep constant phase change properties during multiple melting/solidifying thermal cycles.The apparent thermal conductivity of the NePCMs increases significantly from 0.246 to 1.346 W/m·K,by changing the AgNO3 concentration to form continuous thermal conductive pathways.The novel silver coated NePCMs are promising for fast heat transfer and thermo-regulation applications.(2)NePCMs with graphene-SiO2 shell and flower like morphology were fabricated by combining interfacial hydrolysis and polycondensation of alkoxy silanes and self-assembly of graphene.The thermal conductivity is enhanced largely,the supercooling of the NePCMs is eliminated completely,and the latent heats is kept very well.The NePCMs possess unique flower like morphology and matrix type structure.The thermal conductivity NePCMs with graphene-SiO2 shell is enhanced by 132.9%(increased form 0.6416 W/m to 1.4941 W/m K))compared with NePCMs with SiO2 shell.The supercooling of the NePCMs is eliminated completely due to the introduction of graphene.Therefore,the novel NePCMs with high phase change enthalpy,high thermal conductivity,and low supercooling have great potential to be applied in thermal energy storage,thermal regulation,smart textiles,LFTF and so on.(3)Polymer-SiO2 hybrid shelled NePCMs were fabricated in one-pot,through sequentially executed interfacial hydrolysis-polycondensation of alkoxy silanes and radical polymerization of vinyl monomers.The NePCMs with polymer-SiO2 hybrid shell have defined core-shell structure and regular morphology.Compared with NePCMs with SiO2 shell,the NePCMs with polystyrene(PS)-SiO2 shell possess smaller size and bowl like shape,while NePCMs with poly(hydroxylethyl methacrylate)(PHEMA)-SiO2 shell possess larger size and perfect spherical shape.The polymer types have great impact on the supercooling behavior of the NePCMs.The polymer-SiO2 hybrid shell materials endow the NePCMs with improved thermal reliability,thermal conductivity,and leakage proof property.More importantly,the compressive load at yield increases remarkably from 14.7 ?N for NePCMs with SiO2 shell,to>34.6 ?N for that with PS-SiO2 shell,and 65?N for that with PHEMA-SiO2 shell.In this work,polymer and inorganic contents of NePCMs hybrid shell can be facilely tuned in a larger range,and the polymers can block the mesopores of SiO2 shell,which provide an ideal solution for improving the overall performances of NePCMs.(4)NePCMs with different shell were applied to thermo-regulating rigid polyurethane foams(RPUF)and LFTF.The obvious deterioration of mechanical performance is frequently encountered for thermo-regulating composites containing encapsulated PCMs as function additive.The thermo-regulating composite RPUF were fabricated by introducing NePCMs with SiO2 shell and PDA-SiO2 shell,respectively.The thermo-regulating RPUF containing NePCMs with PDA-SiO2 shell present higher phase change enthalpies(24.77 J/g),better compressive strength(1.18 MPa)and compressive modulus(42.83 MPa),compared with that containing NePCMs with SiO2 shell.The reason was that the abundant catechol and amine groups on the surface of SiO2-PDA hybrid shell can improve the interfacial interaction and compatibility in the composites.NePCMs with SiO2 shell and graphene-SiO2 shell were dispersed in water to make LFTF.The thermal conductivity and photo-thermal conversion efficiency were compared.The thermal conductivity(20 0C)of the LFTF containing 10 wt%NePCMs with SiO2 shell increases by 5.2%,compared with water.For LFTF containing NePCMs with graphene-SiO2 shell,the value is 12.4%.Most importantly,the photo-thermal conversion efficiency of the former was only 42%,and the latter was much improved to 81%.