Preparation And Thermal Properties Of Microencapsulated Phase Change Materials With High Heat Storage Density And Enhanced Thermal Conductivity

With the rapid development of the global economy,the shortage of fossil energy and the environmental pollution have become increasingly prominent.The exploitation of renewable energy sources such as solar energy can alleviate the burden of economic growth on energy and the environment.Phase change energy storage technology is an effective way to use solar energy,of which energy is stored or released through the melting or solidifying of phase change materials.Phase change materials have problems such as flowability and corrosion when used directly,which might be solved by microencapsulation.The ideal microencapsulated phase change materials should have the advantages of large heat storage capacity,good heat transfer performance and high stability.This thesis is dedicated to preparing microencapsulated phase change materials with high heat storage capacity and high thermal conductivity.N-dodecanol@melamine microcapsules with large energy storage density were synthesized,into which high thermal conductivity fillers such as graphene and carbon nanotubes were introduced to improve their thermal conductivity.To prepare microencapsulated phase change materials with larger latent heat and better thermal conductivity,inorganic hydrated salt Na₂HPO₄·12H₂O?DSP?with high energy storage density and thermal conductivity was used as phase change material and encapsulated by polyurea or silica,respectively.By comparing the comprehensive performance of the two mircocapsules,the microcapsule with silica shell with larger heat storage capacity and thermal conductivity was selected.Then,the hydrated salt/silica microcapsule enhanced by graphene oxide was dispersed into the base fluid to obtain the latent heat functional fluid,and its photo-thermal conversion performance was investigated.Firstly,the organic phase change material n-dodecanol was used as the core material.The microencapsulated phase change material with n-dodecanol core and melamine shell was prepared by in-situ polymerization and its thermal conductivity was enhanced by introducing graphene oxide?GO?and carbon nanotube?CNT?into the microcapsule shell.The effect of the oxidation degree of GO and the synergistic effect of GO and CNT on the morphology,phase change property and thermal conductivity of the microcapsules was investigated,and the thermal conductivity enhancement mechanism was obtained by analyzing the interfacial interaction between GO or CNT with the melamine shell and dispersion state of GO or CNT in the melamine shell.The experimental results showed that the microstructure and latent heat of the microcapsules changed little,and the average latent heat and encapsulation ratio of the microcapsule with GO-CNT was 162.9 J/g and 74.3%,respectively.However,the thermal conductivity of the microcapsules with GO or CNT increased to some extent.With addition of0.6 wt%GO-CNT filler,the thermal conductivity of dodecanol@melamine microcapsule increased to 0.3821 W/mK,increased by 195%.Although the addition of high thermal conductivity fillers such as graphene oxide and carbon nanotubes to the melamine shell improved the thermal conductivity of the microcapsule wall,the thermal conductivity of the organic core phase change microcapsule was still low.In order to compensate for this defect,inorganic hydrated salt Na₂HPO₄·12H₂O was used as the microcapsule core,and encapsulated by polyurea?PU?shell formed by toluene diisocyanate?TDI?with H₂O or diethylenetriamine?DETA?through interfacial polymerization.The effect of reaction time,reaction temperature,TDI/DETA mass ratio and core/shell ratio on the phase change property and thermal conductivity of Na₂HPO₄·12H₂O@PU microcapsule were systematically investigated.When polyurea formed by TDI and DETA was used as the microcapsule shell,the Na₂HPO₄·12H₂O@PU microcapsule had the latent heat and encapsulation ratio of 139.3 J/g and 55.4%,respectively.Its thermal conductivity was 0.3946W/mK,2 times higher than the un-modified dodecanol@melamine microcapsule.In order to further improve the thermal conductivity,the inorganic hydrated salt Na₂HPO₄·12H₂O was used as the core material and encapsulated by the inorganic silica shell.The Na₂HPO₄·12H₂O@SiO₂ microcapsule was prepared by interfacial polymerization combined with sol-gel process,and its comprehensive performance was improved by optimizing the synthetic process.The experimental results indicated that the microcapsule synthesized at 40?for 8 h with the extra water of 20%and core/shell mass ratio of 4:1 had the highest latent heat and encapsulation ratio of 177.0 J/g and 70.4%with the thermal conductivity of 0.5004 W/mK.Its latent heat and thermal conductivity were larger than those of Na₂HPO₄·12H₂O@polyurea microcapsule.Subsequently,graphene oxide?GO?and functionalized graphene oxide?FG?were simultaneously introduced into the Na₂HPO₄·12H₂O core and the silica shell to further improve the thermal conductivity of Na₂HPO₄·12H₂O@SiO₂.The effect of GO and FG on the morphology,phase change property and thermal conductivity of the microcapsules was investigated.When 0.3 wt%GO was added to the core and 0.5 wt%FG was added to the shell,the latent heat and encapsulation ratio of the graphene oxide-enhanced Na₂HPO₄·12H₂O@SiO₂ microcapsule were still above 170 J/g and 69%,but its thermal conductivity was increased to 0.6945 W/mK,38.8%higher than the un-modified Na₂HPO₄·12H₂O@SiO₂ microcapsule,4.36 times higher than the un-modified dodecanol@melamine microcapsule.Subsequently,the graphene oxide-enhanced Na₂HPO₄·12H₂O@SiO₂ microcapsules were dispersed to the base fluid to obtain a latent heat functional fluid,and its photo-thermal conversion performance was explored.