A Novel Salt-mixture Containing Magnesium Chloride Hexahydrate And Its Expanded Graphite Based Composite Phase Change Materials

Latent heat storage technology with phase change materials?PCMs?as the core is an important technology to improve energy efficiency.The preparation of high performance and low cost PCMs is the key to the development and application of this technology.Magnesium chloride hexahydrate?MgCl₂·6H₂O?,an inorganic salt hydrate PCM with the melting point at117?or so,needs to be exploited and utilized urgently since it is abundant in salt lake but abandoned which causes the“magnesium pollution”.However,the shortcomings of supercooling,phase separation,liquid phase leakage after solid-liquid phase transformation and losing crystal water easily for the melting point is higher than water are seriously restricted the application of such PCM.This work focuses on preparation of high performance magnesium chloride hexahydrate phase change material.First combining MgCl₂·6H₂O with other hydrated salt to develop a new mixed salt with the melting point lower than the boiling point of water and a low degree of supercooling.Next employing hydrophilic expanded graphite?EG?to carry the mixed salt solves problems of phase separation and liquid leakage.Finally,the high performance composite PCM is obtained.Novel MgCl₂·6H₂O-NH₄Al?SO₄?₂·12H₂O mixture was firstly developed.To be more specific,A series of mixtures with different mass of MgCl₂·6H₂O and NH₄Al?SO₄?₂·12H₂O were prepared by a melt blending method.The results show that when the composition was determined to be 30%in mass fraction of MgCl₂·6H₂O,the mixture has a melting point of64.15?and a latent heat value of as large as 192.1 kJ/kg and supercooling of 0.74?.The latent heat is higher than the mixed salt contained MgCl₂·6H₂O which has been reported.The200 heating-cooling cycles verify that the mixture possesses good structure stability as well as excellent thermal reliability.Polarized microscope images and XRD show that the acicular crystals of MgCl₂·6H₂O stick into the gaps among the octahedral NH₄Al?SO₄?₂·12H₂O crystals or attach onto the surfaces of the octahedral crystals.These excellent thermal characteristics indicate this novel MgCl₂·6H₂O-NH₄Al?SO₄?₂·12H₂O mixture has great potentials in practical applications.To improve the compatibility of EG and inorganic compounds,the EG was modified with a surfactant TritonX-100.The effect of the amount of TritonX-100 on the adsorptive capacity of modified EG was explored,followed by immersing modified EG into the MgCl₂·6H₂O-NH₄Al?SO₄?₂·12H₂O mixture to prepare composite PCM.Moreover,the microstructure and thermal properties of the mixture/modified EG composite block were characterized and measured,respectively.The results indicate that the optimal mass ratio of TritonX-100 to EG is found to be around 0.10,at which the obtained modified EG exhibits the highest adsorptive capacity of 81.9%for the mixture,higher than that 52.0%of EG.The FT-IR spectrum of MEG contains the feature peaks of TritonX-100.The contact angle between the modified EG and water is around 0°,much smaller than 87.1°for EG.It is indicated that the modification with TritonX-100 makes an obvious improvement on the hydrophilicity of MEG as well as the compatibility of EG and inorganic compounds.It is shown that the composite PCM exhibits a melting temperature of 63.40°C and a melting enthalpy of 157.8 kJ/kg with a low supercooling degree.The thermal conductivity of the composite block is measured to 4.789 W/?m·K?,about ten times that of the mixture.SEM observations indicate a uniform distribution of the mixture in the modified EG block.100heating-cooling cycles suggest that the mixture/MEG composite block possesses good thermalreliability.Thesegoodthermalcharacteristicsshowthe MgCl₂·6H₂O-NH₄Al?SO₄?₂·12H₂O mixture/MEG composite block has great potentials in solar thermal utilization such as solar heater systems and heat pump water heaters.