Preparation And Energy Storage Performance Of Composite Phase Change Materials Containing Magnesium Chloride Hexahydrate-magnesium Nitrate Hexahydrate Eutectic Salt

Magnesium chloride hexahydrate（MgCl₂·6H₂O）possesses the advantages of relatively large latent heat and low cost,making it a very promising phase change material（PCM）.Exploring MgCl₂·6H₂O as a PCM is especially important in China due to its large amount.However,the shortcomings of supercooling and phase separation inherent in hydrate salts greatly limit the pratical application of MgCl₂·6H₂O.Furthermore,it is easy to lose the hydrates during the phase change due to the high phase change temperature around 110℃,which requires complicated seal systems.The thesis focuses on the developments of high-performance MgCl₂·6H₂O based PCMs that is mixed with Mg（NO3）2·6H₂O to obtain a eutectic salt with lower melting point,followed by combining with SiO2 and expanded graphite（EG）to prepare two composite PCMs.The composition and properties of the eutectic salt of magnesium chloride hexahydrate and magnesium nitrate hexahydrate were characterized by DSC,temperature curves,XRD and EDS.The shortcomings of the eutectic salt as a phase change energy storage material are improved by the combination with hydrophilic fumed silica and expanded graphite.The composition of the composites was determined by liquid leakage experiments.The surface morphology of the composites was observed by SEM.The composition and structure of the composites were determined by EDS,XRD and IR.The thermal stability of the composites was tested by TG.Hot Disk detected the thermal conductivity.The thermal reliability of the material was tested by heating/cooling cycling experiment.The experimental results show that the composition of the eutectic salt consisting of magnesium chloride hexahydrate and magnesium nitrate hexahydrate is within an interval of 35-50 wt% magnesium chloride hexahydrate,rather than an exact proportion.The phase transition temperature of the eutectic salt is 57-61 ℃,and the latent heat is 112-128 kJ/kg.Specifically,the phase change temperature of the eutectic salt（with 41.3 wt% magnesium chloride hexahydrate,ES）is 58.9 ℃,and its melting enthalpy is as high as 127.45 kJ/kg.But the ES suffers from great loss of phase change enthalpy during heating-coolign cycles.After 100 and 200 cycles,the remaining phase change enthalpy of ES was only 62.8 % and 53.1 %.By combining with 15 wt% SiO2,the obtained composite PCM exhibits a phase transition temperature of 54.3 ℃,melting enthalpy of 88.13 kJ/kg,and supercooling degree of only 2.9 ℃.The enthalpy of phase change after 100 heating-cooling cycles maintains 90.8 % for the composite PCM,much more than the ES.The phase transition temperature and enthalpy of the composite PCM with 10 wt% EG are 57.2 ℃ and 89.84 kJ/kg,respectively,and its supercooling degree is only 1.8 ℃.After 200 heating-cooling cycles,the eutectic salt/EG composite PCM shows unchanged in phase change temperature,and its phase change enthalpy maintains 92.6 %.The thermal conductivity of the eutectic salt/EG composite PCM is 3.243 W/（m·K）at packing density of 1200 kg/m³,7.84 times that of the eutectic salt.It is shown that the energy storage densities of the SiO2-and EG-based composite PCMs are 158.6 and 161.7 MJ/m³,respectively,and both the composite PCMs exhibit little supercooling,no phase separation,and good thermal reliability.