Preparation And Properties Of Room-temperature Salt Hydrate Form-stable Composite Phase Change Materials

The energy consumption involved in building sector has accounted for 40%of total social energy consumption,since China is on the highway of urban construction.With the purpose of improving energy utilization efficieny and energy-saving emission reduction,it has been the research focus on the methods to reduce building energy consumption without compromising the indoor comfort requirements.The application of room-temperature（15-25℃）phase change materials（PCMs）as building materials in indoor space can reduce the energy consumption of HVAC equipments,meanwhile increasing indoor thermal inertia and improving indoor comfort.Based on the background of building energy conservation,the synthesis and properties of novel room-temperature phase change materials were studied with inorganic hydrated salt targeted as the research object.（1）The ternary nitrate（LiNO₃·3H₂O-KNO₃-NaNO₃）was used as the research object to study the thermal properties of ternary nitrate under different ratios of nitrate;the effect of nucleating agent on supercooling degree of ternary nitrate PCM was discussed;Based on that,the ternary nitrate form-stable composite phase change material（CPCM）was studied by using expanded graphite（EG）as the carrier material.In order to improve the hydrophilicity of EG and its compatibility with ternary nitrate,the SiO₂ prepared by sol-gel method was adapted as modifier to obtain modified expanded graphite（MEG）.The pore structure,morphology,compositions and the adsorption capacity for ternary nitrate of EG before and after modification were analyzed.The form-stable performance and thermal properties of the obtained CPCM were analyzed.The results showed that the obtained ternary nitrate owned a suitable phase transition temperature（22.98℃）and high phase change enthalpy（218.8 J/g）under the optimized ratio of LiNO₃·3H₂O-KNO₃-NaNO₃（78.7-20.4-0.9%,mass fraction,the same as below）,and the additional 2%Ni（NO₃）₂·6H₂O as nucleating agent could reduce the supercooling degree of ternary nitrate to 3.96℃.The MEG containing 15%SiO₂ showed the best affinity to water and ternary nitrate PCM.With the addition of 20%MEG,the high-performance form-stable CPCM can be obtained with phase change temperature of 21.66℃,enthalpy of 158.7 J/g,supercooling degree of only 1.56℃and thermal conductivity of 4.565W/m·K.Moreover,the thermal performance fluctuations of the CPCM were negligible after200 thermal cycles,which indicates the prepared CPCM can be applied in the phase change regenerator.（2）Calcium chloride hexahydrate（CaCl₂?6H₂O）was chosen as PCM and two types of hydrophilic fumed silica（SiO₂）were used as carriers to prepare CaCl₂?6H₂O/SiO₂ form-stable CPCMs.Based on it,urea（CO（NH₂）₂）was adapted as the temperature regulator to obtain room-temperature CaCl₂?6H₂O-CO（NH₂）₂/SiO₂ form-stable CPCM;the form-stable performance of the CPCMs under different types and mass fractions of SiO₂ was discussed and the temperature regulating performance under different mass fractions of urea was analyzed.The thermal properties,composition and morphology of the CPCM were characterized.The results showed that the CPCM with favorable form-stability and thermal stability could be obtained with the addition of 20%HL150 or 22%A380.The phase change temperature of the CPCM can be adjusted to room-temperature with the addition of 1%urea.The phase change temperature,phase transition enthalpy and the supercooling degree of the room-temperature CPCMs were21.03℃,114.3 J/g and 1.34℃（HL150）and 20.69℃,104.8 J/g and 0.259℃（A380）,respectively.The results of thermal properties test after 200 thermal cycles indicated that the two CPCMs owned good thermal reliability.The CPCMs can be applied in building to maintain indoor temperature.