Preparation Of Two Kinds Of Phase Change Materials Composites And Study On Their Thermal Storage Properties

Phase change materials(PCMs)are able to absorb and release a large amount of latent heat during phase transition,which now has been widely used in building energy saving,indoor warmth,adjustable temperature textiles,mi litary and aerospace fields,etc.In this paper,polypropylene(PP)and boron nitride(BN)were selected as the matrix material,and two kinds of phase change composites were prepared.T he whole experiments are divided into three parts.In the first part,the PP phase change composites were prepared by dissolving the PP into p-xylene,and then using freeze-drying method to obtain the PP aerogel,which was used to adsorb phase change substances such as paraffin,stearic acid(SA)and myristic acid(MA).The SEM shows that the interior of the PP aerogels is loose and porous,and phase change substances were adsorbed into the pore network of PP aerogels.The EDS results show that the adsorption of phase change material is relatively uniform.The results of BET analysis show that the specific surface area of PP aerogels is 38.66 m2g-1,and the pore volume is 0.17 cm3g-1 when the relative pressure P/P0 is 0.97.In addition,the pore size distribution was mainly concentrated on 22.54 nm.The DSC test results show t hat the PP aerogels,whether adsorbed paraffin,or SA,MA and other phase change substances,the energy storage density of the phase change composites can achieve 75%,compared with that of the pure.After 30 times of hot and cold cycles,the adsorption ma ss fraction of PP aerogels on paraffin wax still reached 900%.After 50 times of hot and cold cycles,there is no significant decline trend of the melting enthalpy of the PP/Paraffin phase transition composites,and their DSC curves were almost overlapped.The thermal analysis results show that the thermal conductivity of PP/Paraffin phase change compound is at least twice as high as that of pure paraffin,which indicates that PP aerogels can enhance the thermal conductivity.In the second part,several pro portions of binary phase transition composites were obtained by using isopropanol to dissolve SA and different proportions of BN.The lamellar structure of boron nitride can be clearly observed by SEM and TEM.The FTIR shows that the mixed phase transition complex does not undergo any chemical reaction.The XRD results show that the boron nitride phase change composites still maintain good crystallinity.The results of DSC show that the exothermic temperature of BN phase change composites is 50 °C,and the proper addition of BN is very beneficial to improve latent heat of phase change materials.The undercooling analysis shows that the addition of BN reduces the subcooling of the composite to below 10 °C,indicating that it is helpful for reducing the degree of subcooling of the PCMs.By testing the thermal conductivity of the sample,the results show that a small addition of BN improves the thermal conductivity significantly.In the last part,the chemical reduction methods were employed to reduce the mixture solutions of the BN and graphene oxide,then the obtained BG gel was freeze-dried,and at last it was used to adsorb the phase change substances to form the BGS ternary phase change composites.Through the SEM characterization,the inside of the graphene aerogel is loose and porous,and the two-dimensional graphene sheet layer twisted and linked together.When graphene aerogels and SA form a compound,there is almost no pore of the inner,but the pores are still existing while compounding with BN.The results of FTIR show that the characteristic absorption peaks of BN and SA are obvious,and there is no new absorption peak after the complex of graphene and BN.The XRD results show that the addition of BN and graphene has no effect on the two characteristic peaks of SA.Adsorption performance test shows that the adsorption capacity of graphene aerogels was 15.11 times,while that of small amount addition of BN has a higher adsorption rate of 18.52 times.The results of DSC show that the phase transition temp erature difference between BGS and the pure phase change substances is very small.When the BN is added in a small amount,the energy storage density can reach 180 J g-1,whereas the energy density of the other complexes is about 150-160 J g-1,and the latent heat value of the ternary phase change complex is about 15% higher than that of the non-added BS composites.The thermal conductivity test results show that the thermal conductivity of the ternary phase change composites rises with the increase of the amount of BN,which indicates that the BN is beneficial to improve the thermal conductivity of the composites.