Preparation And Characterization Of High Thermal Conductivity Microencapsulated Phase Change Materials

Microencapsulated phase change material（MEPCM）is an energy storage material that encapsulates the phase change material（PCM）in the form of a core material in the shell material,which can prevent leakage during the phase change process.Meanwhile,MEPCM could increase the heat transfer area,thermal stability,thermal conductivity,minimize the external influence of the environment and reduce the volume change of PCM during the phase change period.It has broad application prospects in solar energy utilization,waste heat utilization,energy saving consumption reduction and CO₂emission reduction.However,the current thermal conductivity of MEPCM is low,which seriously limits its heat storage/heat release rate and restricts its application.Therefore,the use of high thermal conductivity materials（such as graphene oxide（GO））as thermal conductivity enhancement materials has received extensive attention in the field of MEPCM preparation research in recent years.In order to prepare high thermal conductivity phase change material microcapsules,the polymerization kinetics of polymethyl methacrylate（PMMA）,the preparation and characterization of MEPCM and the preparation and characterization of GO thermally conductive MEPCM were completed in this thesis.Firstly,the polymerization kinetics of PMMA was studied by emulsion polymerization with azodiisobutyronitrile（AIBN）as initiator.The polymerization of PMMA belongs to free radical polymerization.The key to polymerization is related to the decomposition rate of initiator,which is affected by the concentration of initiator and reaction temperature.Therefore,the single variable method was used to calculate the decomposition rate of the initiator at different initiator concentrations at 65-80°C,and the relevant experiments were designed to verify.The results show that when the initial decomposition rate of the initiator is 0.002-0.004mmol/L s,PMMA particles with uniform dispersion can be obtained.The thermal stability of PMMA particles is the best when the initiator content is 1 wt.%of the monomer at 70°C,and the initial degradation temperature is about 301°C.Secondly,microcapsules were prepared based on the above optimal polymerization conditions.The effects of shell-core ratio and crosslinking agent on the morphology,phase transition characteristics and thermal stability of microcapsules were investigated.It was found that the shell-core ratio significantly affected the morphology of microcapsules.With the increase of core material content,the sphericity of microcapsules was better.When the shell-core ratio was 5:5,the depression of microcapsules was the most serious,and the sphericity of microcapsules was the best at 3:7.However,the microcapsules collected after drying adhere to each other,especially when the shell-core ratio is 3:7,the shell strength is insufficient and the adhesion is serious,and the SEM morphology is broken.After adding crosslinking agent PETRA,the morphology and shell strength of microcapsules were improved.In addition,the phase transition and thermal stability of the microcapsules were characterized.It was found that the microcapsules obtained the highest latent heat value of 223.6J/g when the shell-core ratio was 3:7,but the loss of MMA polymerization process exceeded the theoretical latent heat value.The initial degradation temperature of the microcapsules prepared at a shell-core ratio of 5:5 was216°C,which was 27°C higher than that of pure octadecane.Finally,using polyvinyl alcohol and GO as surfactants,0-0.5wt.%GO was added to the MEPCM shell based on the mass of MEPCM to explore the effect of GO addition on the performance enhancement of microcapsules.FTIR results showed that GO was successfully added to MEPCM.SEM morphology analysis showed that the use of GO and polyvinyl alcohol as surfactants contributed to the polymerization of PMMA and obtained microcapsules with good dispersion.The results of DSC showed that the content of GO/MEPCM core material was between 78%and 92%,which exceeded the theoretical core material ratio of 70%.Increasing the GO content could reduce the loss of MMA monomer and make the core material content of MEPCM closer to the initial core material ratio.At the same time,the initial degradation temperature of microcapsules increased by 8-21°C with the addition of GO.When the GO content accounted for 0.2wt.%of MEPCM,the initial degradation temperature of GO/MEPCM increased by 21°C compared with that without GO.By comparing the thermal conductivity of MEPCM and GO/MEPCM,it was found that the addition of GO improved the thermal conductivity of microcapsules.When the GO content was0.5wt.%,the thermal conductivity of microcapsules increased to 1.04W/m K.Meanwhile,the mechanical properties of GO/MEPCM samples increased the Young’s modulus from 0.1GPa to 0.3GPa,and the hardness increased from 0.03GPa to 0.13GPa with 0.5w.t.%GO.