Preparation And Properties Of Silicon Dioxide Encapsulated Molten Salt Microencapsulated Phase Change Materials

Molten salts,with advantages of high phase change temperature,high latent heat and low cost,are excellent heat transfer and storage materials.They are suitable for medium and high temperature applications,with wide application prospects in fields such as solar power generation,industrial waste heat,and waste heat recovery.However,molten salts have some disadvantages such as corrosion of equipment and leakage.Therefore,using an inert coating to encapsulate a molten salt into microcapsules can not only solve these problems,but also improve its thermal performance.In this study,microencapsulation of molten salts with different solubility properties was carried out by sol-gel method and interfacial polymerization method.Surface morphology,chemical composition and crystal structure were characterized;thermal properties were tested and improved.Experimental conditions with the best thermal storage properties were explored.By comparing the properties of the microencapsulated phase change materials synthesized by the two methods,the better synthesis method and experimental conditions with better performance of microcapsules were summarized.Ternary carbonate@silica microcapsules were synthesized by the sol-gel encapsulation method.Experimental conditions with the highest energy storage efficiency were explored by the controlled variable method.The results show that eutectic salts Li KCO₃ and Li Na CO₃were formed after heat treatment.The microcapsules with heat-treated ternary carbonates exhibited a melting latent heat of 192.2 k J/kg and an encapsulation ratio of 84.8%.The supercooling degree was reduced by 43.7%.No leakage was detected after phase change cycles,with the cycle reliability of 80.7%,indicating good thermal stability of the microcapsules.Since the sol-gel method previous mentioned is only suitable for microencapsulation of molten salts which are insoluble in alcohol,it is necessary to develop a microencapsulation method for molten salts which are soluble in alcohol.Therefore,based on the principle of interfacial polymerization,a microcapsule synthesis method suitable for molten salts soluble in alcohol was proposed,in which molten salts are only required to be soluble in water.Lithium nitrate(Li NO₃)as a model ethanol-soluble molten salt was microencapsulated in silica(Si O₂)by the novel method.Effects of doping Ti₃C₂T_x in the microcapsules on the performance were studied.Experimental parameters for the highest energy storage efficiency of microcapsules were explored by orthogonal test.The results show that microcapsules had a low supercooling degree.The thermal conductivity was increased by 78.6%by doping Ti₃C₂T_x in the microcapsules.The microcapsules exhibited a melting enthalpy of 246.9 k J/kg and a thermal reliability of 95.8%after 50 phase change cycles.Since the two microcapsule synthesis methods in this study are suitable for molten salts which are both insoluble in alcohol and soluble in water,the advantages and disadvantages of performance and process of the microencapsulated phase change materials synthesized by the two methods were compared.Na NO₃@Si O₂ microcapsules were synthesized by the sol-gel method and interfacial polymerization method respectively,and the performance of the microcapsules were improved by doping Ti₃C₂T_x.Experimental results show that the melting latent heat,thermal reliability and thermal conductivity of the microcapsules synthesized by the sol-gel method were 121.7 k J/kg,96.2%and 1.159 Wm^-1K^-1 after doping the Ti₃C₂T_x,respectively.While the microcapsules synthesized by the interfacial polymerization exhibited a melting latent heat of 134.1 k J/kg,a thermal reliability of 98.4%,and a thermal conductivity of 1.194 Wm^-1K^-1.The thermal conductivity of the microcapsules synthesized by the two methods were improved 78.6%after doping the Ti₃C₂T_x in the microcapsules.The supercooling degree of the microcapsules prepared by sol-gel method and interfacial polymerization were 8.9?and 6.8?after several phase change cycles.Therefore,from the perspective of supercooling,thermal conductivity and cycle reliability,microcapsules synthesized by interfacial polymerization exhibited better performance,showing greater advantages in application in this study.