Preparation And Properties Of Form-stable Composite Phase Change Materials With SiO₂ Skeleton

In this thesis, fatty acid/SiO2,fatty alcohol/SiO2 and PEG/SiO2 form-stable phase change materials(PCMs) were synthesized. Tetraethyl orthosilicate (TEOS) acted as silica precursor to form Si-O-Si network structure which served as supporting materials. Ultrasound can fasten the mixing process of two-phase system, therefore ultrasound is an alternative way to promote development of sol-gel without using the surfactants and co-solvents. And different catalysts can make the sol-gel process under control. So with ultrasound assisted and different catalysts we made a straightforward and convenient way to synthesize form-stable composite PCMs with SiO2 skeleton.Fatty acid/SiO2 and fatty alcohol/SiO2 form-stable (PCM) were synthesized by sol-gel process under ultrasound assisted. The influence of ratio of TEOS to H2O, ultrasound power, different catalysts and the maximum PCM component loading weight were discussed in detail. The results showed that the preparative process was 300W ultrasound power, TEOS:H2O=1:20(mol/mol), adjusted pH of system for 2-3 with hydrochloric acid, translucent sol formed 5min later, added fatty acid or fatty alcohol to the sol, adjust pH of system for 8 with ammonia, the gel formed rapidly, then dried the gel to get the product. These form-stable PCMs were studied by FT-IR, DSC, XRD, TG. The results showed the phase change enthalpy of stearic acid/SiO2 (61%), palmitic acid/SiO2(62%), myristic acid/SiO2(62%), octadecanol/SiO2(63%), tetradecanol/SiO2(62%) were 89.3J/g,133.2J/g,97.3J/g,121.7J/g, 109.4J/g. And these form-stable PCMs had good thermal stabilities even 100℃higher than their phase transition temperature. The PCM component and SiO2 skeleton combined physically, because of the bound of SiO2 skeleton, the crystal properties of PCM component slightly declined.PEG/SiO2 form-stable (PCM) were synthesized by sol-gel process under ultrasound assisted. Simulation of PEG's molecular weight with different prepation conditions showed better preparative process was 300W ultrasound power, TEOS:H2O=1:10 (mol/mol), adjusted pH of system for 2-3 with hydrochloric acid, translucent sol formed 5min later, added PEG to the sol, adjust pH of system for 8 with Na2CO3 solution, the gel formed rapidly, then dried the gel to get the product. These form-stable PCMs were studied by FT-IR, DSC, XRD,POM, TG, SEM, TEM. The results showed that the phase change enthalpy of PEG1000/SiO2(79.1%), PEG2000/SiO2(75.4%), PEG4000/SiO2(76.3%), PEG6000/SiO2(76%) were 111.1 J/g, 109.4J/g,108.1J/g,102.8J/g. These PEG/SiO2 form-stable PCMs had good thermal stabilities below 300℃, and the phase-change behaviors of PEG/SiO2 PCMs didn't change after repeated 50 thermal cycling. The PEG and SiO2 skeleton combined physically in nano level, and PEG/SiO2 PCMs were kind of long columnar particles.The temperature adjustment time of 3g PEG1000/SiO2, PEG2000/SiO2, PEG4000/SiO2, PEG6000/SiO2 composite PCMs were 16.5min,10.5min, 10min,11.5min; temperature adjustment range were 26～30℃,33～38℃,44℃,41-46℃. The densities of PEG1000/SiO2, PEG2000/SiO2, PEG4000/SiO2, PEG6000/SiO2 PCMs under 30Mpa press molding were 1.29 g/cm3,1.28 g/cm3,1.27 g/cm3,1.24 g/cm3. The volume thermal densities of PEG1000/SiO2, PEG2000/SiO2, PEG4000/SiO2, PEG6000/SiO2 PCMs were 143.3 J/cm3, 140.0 J/cm3,137.3 J/cm3,127.5 J/cm3.