Synthesis, Characterization And Thermal Analysis Of Hydrated Salt Micro-encapsulated Phase-change Materials

Energy and environment is one of the important themes in the world, while the heat storage technology is the key in improving energy efficiency and protecting environment. Latent heat storage technology can solve the mismatch of the energy supply and demand on time and space, and become the effective means to improve energy efficiency. Based on the advantages, PCMs (phase-change materials) have a wide application in solar energy, balance of the power, recycling of waste heat, industrial and civil construction heating, air conditioning fields. Meanwhile, in order to overcome its own defects of leakage, supercooling and phase separation. Microcapsules have broaden the application fields of the PCMs which use film-forming materials to coat the solid or liquid core material (PCMs), and made to be particles with core-shell structures. In this thesis, we selected two different crosslinking monomers to generate different polymer as the microcapsule wall material, and used the same core material of sodium phosphate dodecahydrate (Na2HPO4·12H2O, DSP), and generate the hydrated salt microencapsulated phase change materials (micro-PCMs) by two kinds of preparation methods (solvent evaporation method and in-situ polymerization). The influence of different process parameters on the microcapsule performance was discussed and the optimum technological parameter was determined. Meanwhile, the composition analysis, apparent morphology and thermal properties of different crosslinking monomers were comparative studied. Based on the above results, the optimal methods and processes were summarized. Ultimately, the influence of coating form to the supercooling degree of core material hydrated salt was also explored.Firstly, methyl methacrylate (MMA) crosslinked with ethyl acrylate (EA) generated poly methyl methacrylate (PMMA) as coating polymer by the solvent evaporation method. And the influences of the different process parameter, material ratio, monomer types on the thermal properties of microcapsules were studied. The results showed that, the reaction temperature has little effect on the particle size distributions, but surface morphology. Temperature about85℃, reaction time about240min and ultrasonic power of500W are the most admirable parameters for the microencapsulation process.Secondly, urea was crosslinked with formaldehyde generated urea-formaldehyde resin (UF resin) as coating polymer by the in-situ polymerization method. The results showed that, the micro-PCMs were core-wall structure with good morphology and small particle size of500nm, which could release latent heat of121.2J/g under41.5℃. And the molar ratio of urea and formaldehyde have strongly effected on the morphology of the resulting mierocapsules. Meanwhile, thermal conductivity of the micro-PCMs is between the shell material and core material. The weight loss was less than10%when temperature range heated from30to84℃.Meanwhile, through the comparison of the thermal performance of the micro-PCMs by two different crosslinking monomers using different methods, some conclusion could be got. A novel modified PMMA microcapsules containing DSP were successfully prepared by MMA crosslinked with EA. The microcapsules equipped with good performances of heat transfer and thermal storage properties, but some shrinkage and holes appeared on the apparent morphology, which was likely caused by the organic solvent volatilization or residual during preparation, and may lead to the leakage of the microcapsule after multiple heating-cooling testing. This is an urgent problem needed to be resolved. Besides, UF resin micro-PCMs composed of urea crosslinked with formaldehyde as polymer shell, which equipped with good apparent morphology and stable thermal performance. It is worth noting that, the Tm about51℃and41.5℃has been increased significantly relative to pristine DSP (about17℃and7.5℃), while their latent heats of melting are decreased. It is mainly due to the interface interactions between the core material and shell material, and maybe the core material complex changes to be Na2HPO4·7H2O confined into the microcapsules during the preparation process, which makes it suitable for different applications, especially in the thermal field. In the end, according to the microcapsulation of the hydrated salt, which indicates there is an obvious supercooling phenomenon in pure Na2HPO4·12H2O and the super-cooling degree of up to15℃. At the same time, there is no significant subcooling phenomenon appeared in the micro-PCMs. The possible reason to explain this phenomenon is that relative large specific surface area of microcapsule can accelerate nucleation of hydrated salt. In view of the both as-prepared micro-PCMs possessed good thermal properties, they became potential micro-PCMs for thermal energy storage.