| Phase change materials of inorganic hydrated salt have many advantages such as high latent heat,low price,safe and non-toxic,but also reflect the disadvantages of supercooling,phase separation and low thermal conductivity.Calcium chloride hexahydrate is a very important inorganic hydrated salt due to its higher latent heat(around190 J/g)and the appropriate phase transition temperature(around 29?C).The modification of calcium chloride hexahydrate mainly focuses on four directions,namely,adding nucleating agent such as strontium chloride to modify the supercooling,using thickening agents to inhibit the phase separation.,utilizing porous materials to improve the supercooling,phase separation and stability,adding high thermal conductivity particles to improve low thermal conductivity and supercooling.The composite of porous materials and the composite of high thermal conductivity material have a variety of modified functions with good effect in application.In the aspect of the porous material,diatomite was selected as a candidate with a specific surface area of 15~60 m~2/g.Because diatomite has a high absorption properties to water and the surface of a large number of silanols can form hydrogen bonds with water molecules,which can improve its adsorption of hydrated salt and the stability of the cycling property of the material.In the experiment,diatomite was firstly modified to improve its specific surface area in order to increase its adsorption capacity.The used method was an orthogonal experimental design of four factors and three levels.The calcination temperature(450?C),calcination time(3 h),sulfuric acid concentration(60%)and sulfuric acid soaking time(4 h)were selected as the best process conditions.Calcium chloride hexahydrate/diatomite/paraffin composite material was made by vacuum adsorption method.The optimal adsorption time(60 min)and temperature(40?C)of diatomite were chosen.In order to further improve the stability,the paraffin was coated on the composite material and the obtained coated composite material had the enthalpy of 108.2 J/g and the phase transition temperature of 28.9?C.The characterization of the material showed that the different parts of the composite PCM had a good chemical compatibility,and diatomite and coating paraffin inhibited the phase separation and improved the thermal stability and cycling stability of hydrated salt.In the aspect of the composite PCM with thermal conductivity material,graphene was selected due to the high thermal conductivity at about 5300 W/(m?k).Because,achieving the same enhancement of thermal conductivity,high thermal conductivity material can reduce the amount added into PCM.In order to make graphene disperse better,graphene oxide was first prepared by modified Hummers method,then graphene oxide was reduced by hydrazine hydrate and PVP was added to prepare a graphene aqueous dispersion.Graphene/calcium chloride hexahydrate(?H=180.6 J/g,T=27.8?C)was prepared by adding calcium dihydrate into the graphene aqueous dispersion.The graphene increased the thermal conductivity of hydrated salt(80%),while the enthalpy of the hydrated salt declined 2.7%.It shows that the graphene can build heat transfer network to improve the thermal conductivity of PCM.Compared with the method reported in the literature to improve the thermal conductivity of the hydrated salt,the used graphene and its application method have a better advantage.In addition,it shows that the active agent not only improves the dispersion ability of the graphene but also increases the viscosity of the system and further improves the stability of the hydrated salt.The application of the used process,namely to prepare the aqueous dispersion of additives firstly and then to use the related salts to synthesize the hydrated salts PCM is a good example to other additives that are not easily dispersed in the hydrated salt. |
PDF Full Text Request