Preparation And Performances Of Temperatureadjusted Inorganic Salt Hydrate/Silica Composite Phase Change Materials

In recent years,the tendency of environmental protection and resource conservation has been increasing.Taking building energy conservation and carbon emission reduction as an example,the application of hydrated salt phase change materials（PCMs）in building latent heat storage system can improve heat storage efficiency and reduce the use of fossil energy.In addition to the basic problems of supercooling,corrosion and phase separation for inorganic hydrated salts as PCMs,the more important problems are the leakage in the solid-liquid phase transition process and the difficulty in matching the phase transition temperature in the work place.In this thesis,adopting fumed silica（SiO₂）as temperature-regulating and form-stable carrier,and sodium acetate trihydrate（NaOAc·3H₂O）or calcium chloride hexahydrate（CaCl₂·6H₂O）with wide raw material source and low price as main hydrated salt PCM,the preparation and properties of a series of hydrated salt/SiO₂ form-stable composite phase change materials（CPCMs）were investigated.The temperature regulation performance of SiO₂ on different hydrated salts was emphatically discussed to reveal the relationship between the pore structure of the carrier and the phase transition temperature of hydrated salt.（1）Chosen NaOAc·3H₂O as main PCM and different types of SiO₂（HL150,HL200,HL300,HL380 and A380）as porous substrates and temperature-regulating agents,A series of NaOAc·3H₂O/SiO₂ form-stable CPCMs were prepared.The influence of different types and contents of SiO₂ on the phase transition performance of the CPCMs was analyzed.Optimum ratio of SiO₂ in CPCMs was obtained.The morphology,composition and pore structure of CPCMs were characterized.The phase change temperature regulation of SiO₂on NaOAc·3H₂O was analyzed by combining pore structure parameters and DSC data.The results showed that with the increased content of SiO₂,the phase change temperature and enthalpy of CPCM decreased comparing to NaOAc×3H₂O(respectively 58.50℃,258.8 k J×kg^-1).Under the premise that the content of each type of SiO₂was 22%,the obtained CPCMs could maintain outstanding shape-stabilization（no leakage）.The phase change temperatures of HL150,HL200,HL300,HL380 and A380 based CPCMs were 53.37,52.54,51.75,50.69 and 49.24°C,respectively,and the corresponding enthalpies were 182.3,186.7,171.3,170.6 and 159.9 k J·kg^-1,respectively.Moreover,they had almost no supercooling.TG analysis and thermal cycling experiment showed that CPCMs had good thermal stability and thermal reliability.Linear fitting was performed for the difference of phase change temperature（（35）T_m）of each CPCM and the pore size in verse（1/d）of SiO₂,and the linear relationship between（35）T_m and 1/d was obtained as y=124.93x-10.06,R2=0.99,which shows a high correlation.（2）Using CaCl₂·6H₂O as the main PCM,and various types of fumed SiO₂ as the carrier and temperature regulator,a series of CaCl₂·6H₂O/SiO₂shape-stable CPCMs were prepared.The composition,morphology and pore structure of the CPCMs were characterized,and their thermal properties were analyzed comprehensively.The results indicated that,compared to CaCl₂·6H₂O(respectively 29.01℃,181.5 k J×kg^-1),the phase change temperature and enthalpy of the obtained CaCl₂·6H₂O/SiO₂ CPCMs decreased with the increase of SiO₂ content.Under the premise of fixing the content of each type of SiO₂ was 22%,the prepared CPCMs displayed excellent shaping effect.The phase change temperatures of HL150,HL200,HL300,HL380and A380 based CPCMs were 25.45,24.95,23.48,22.96 and 21.51°C,respectively,and the corresponding phase change enthalpies were 138.4,121.9,127.7,125.4 and 115.1 k J·kg^-1,respectively.The supercooling degrees of the CPCMs were limited to 1.87～3.78°C.TG analysis and thermal cycling experiment showed that CPCMs had good thermal stability and thermal reliability.For CaCl₂·6H₂O/SiO₂ CPCMs,the linear relationship between（35）T_m and 1/d was obtained as y=120.98x-11.15,R²=0.93,which showed a good correlation.The above equations between（35）T_m and 1/d provide reference for the ability of porous carrier to regulate the temperature of hydrated salt PCMs.