Disodium Hydrogen Phosphate Heptahydrate @ Silicon Dioxide Microencapsulated Phase Change Materials

With the features of high phase change enthalpy,big volume energy storage density,high coefficient of thermal conductivity and low price,inorganic salt hydrate as phase change material?PCM?for thermal energy storage during phase process is one of the effective means of improving energy efficiency,reducing energy consumption as well as improving environmental pollution.However,the direct use of salt hydrate PCM will result in some problems such as supercoiling,phase separation and corrosion.Microcapsulation of PCM is an effective solution for solving the shortages above.Due to the disadvantage of organic shell with low thermal conductivity and the characteristics of inorganic silica shell with excellent heat transfer performance,based on building energy conservation,this thesis carried out the experimental study of inorganic silica coating salt hydrate PCM,which has very important theoretical guiding significance and practical application value.Firstly,using ethanol as the anti-solvent,and disodium hydrogen phosphate dodecahydrate?DSP?as main regent,the disodium hydrogen phosphate heptahydrate?Na₂HPO₄·7H₂O?nanocrystals were prepared by anti-solvent technique.The effects of the crystallization temperature,crystallization concentration?the volume ratio of disodium hydrogen phosphate saturated solution and ethanol?as well as agitation rate on the nanocrystal grain size were discussed.Secondly,using disodium hydrogen phosphate heptahydrate nanocrystals as the core,and the silica synthesized by the hydrolysis and condensation of ethyl orthosilicate?TEOS?as the shell,Na₂HPO₄×7H₂O@SiO₂ MEPCMs were prepared by sol-gel and chemical precipitation method,respectively.The effects of agitation speed,reaction time,the amount of ammonia and the mass ratio of core and shell on the phase change enthalpy of MEPCMs were investigated systematically.The morphology,composition,crystalline phase and the thermo-physical performances of MEPCMs were characterized by Scanning electron microscope and Energy dispersive spectrometer?SEM-EDS?,Fourier transform infrared?FT-IR?,X-ray diffractometer?XRD?,Differential scanning calorimetry?DSC?,Thermogravimetric analysis?TG?and Hot Disk constants analyzer,respectively.The results showed that,the crystallization temperature had a significant effect on the crystal grain size,when the temperature was above 25?,the crystal grain size markedly increased.The crystallization concentration not only affected the crystal grain size,more importantly influenced the number of the crystal water in disodium hydrogen phosphate hydrate,which resulting in the difference of the phase transformation temperature and enthalpy.Similarly,the agitation rate influenced the dispersion and the crystal grain size in the antisolvent.Under the optimized conditions of the crystallization temperature of 25 ?,crystallization concentration of 0.20 and the agitation speed of 1600 rpm,the crystal grain size was about 272.4 nm.The experimental results indicated the phase change enthalpy of the Na₂HPO₄×7H₂O@SiO₂ MEPCM synthesized by sol-gel method or chemical precipitation technique had a tendency to increase and then decrease with the increase of the agitation rate,the reaction time,the amount of ammonia and the mass ratio of core and shell,respectively.Due to adding silica sol directly in the precipitation method,the microencapsulation time shortened,however,the encapsulation efficiency was low.Appropriate preparation conditions of the MEPCM synthesized by sol-gel method were a molar ratio of 1TEOS : 0.59 DSP : 50.50H₂ O : 0.91NH₃?H₂O under the reaction time of 4h at 25?.The synthesized MEPCM with flat cylinder structure and the particle size of about 8 ?m had a phase change latent heat of 159.8 kJ/kg and encapsulation rate of 82.40%.Moreover,the preparation of MEPCM could reduce the supercoiling of salt hydrate to some degree.Due to the presence of inorganic silica shell,the phase change microcapsules displayed an excellent thermal conductivity?0.3838 W/m×K?,and good thermal stability.Appropriate synthesis conditions of the MEPCM synthesized by chemical precipitation method were a molar ratio of 1TEOS : 0.79 DSP : 67.33H₂ O : 1.21NH₃?H₂O under the reaction time of 3h at 25?.The synthesized MEPCM with irregular shape and particle size of about 6 ?m had a phase change enthalpy of 89.94 kJ/kg,encapsulation rate of 46.38% as well as the thermal conductivity of 0.3678 W/m·K.