Study On Preparation And Properties Of Adipic Acid-based Composite Thermal Storage Material

Phase change thermal storage technique has great application prospects in many fields such as solar thermal application, industrial waste heat and waste heat recovery, air conditioning energy conservation and building heating owing to its superior advantages of large heat storage capacity, small changes in temperature, compact heat storage device and easy to control and manage and so on. Adipic acid is the most valuable applications dibasic acid in the aliphatic dicarboxylic acid, and the phase transition temperature is151～155℃, and the latent heat is260J/g. For the shortcomings of low thermal conductivity of adipic acid, excellent thermal conductivity of expanded graphite is used as reinforcement to improve the thermal conductivity properties of it, and study the thermal cycling stability of adipic acid composite PCMs. For a single phase transition temperature of adipic acid, adipic acid and sebacic acid complex were designed as binary system, to obtain a array of diverse phase change temperature, meeting the different requirements of the different heat storage temperature.Adipic acid as the PCM, expanded graphite as the heat conduction reinforcement, adipic acid/expanded graphite composite phase change thermal storage material was prepared by mechanical mixing and melted adsorption method. The structure and properties of composite were investigated by FT-IR, DSC, Thermal Constants Analyzer, TG, FE-SEM and Thermal cycling test. The obtained results show that, adipic acid can be absorbed into the expanded graphite effectively, and exhibited good compatibility with expanded graphite. The maximum difference between calculated enthalpy and test enthalpy is4.52%/5.54%for calculated enthalpy in melting/freezing process respectively. Among expanded graphite4wt.%-8wt.%, the thermal conductivity increase more than2times, and the latent heat are more than220J/g. At200℃, the mass change of adipic acid is-1.04%, but adipic acid/8wt.%expanded graphite is-1.38wt.%.After1000cycles no chemical changes of pure adipic acid,melting/solidification phase transition enthalpy were attenuated by11.19%/11.68%. After1000cycles of adipic acid/8wt.%Expanded graphite composite PCM, melting/solidification phase transition enthalpy were attenuated by21.67%/19.31%. Thermal storage/exothermic experiments show that, pure adipic acid was dominated by the convective heat transfer during the heat storage/exothermic phase change process, but adipic acid/8wt.%expanded graphite composite PCM was dominated by heat conduction owing to expanded graphite effect, and its central position exothermic time was reduced79.33%. FE-SEM results showed that, pure adipic acid always remain better columnar crystalline structure after1000cycles. For adipic acid/8wt.%expanded graphite composite PCM, adipic acid almost completely adsorbed on the surface of expanded graphite and filling in the gap space of the expanded graphite structure after1000cycles, which makes the composite PCM surface relatively higher density.Different proportions of adipic acid/sebacic acid step cooling curve results showed that both can form binary eutectic system. When sebacic acid content is70wt.%, it reached the lowest point. DSC results showed that, when sebacic acid content is70wt.%, the binary system appears eutectic point. It occurred a single endothermic peak melting/solidification exothermic peak during heating/cooling process. The relative difference of step cooling curve results was6.12%/3.80%to DSC results of the melting point/freezing point, which can show that step cooling curve test results are reliable. XRD and IR results showed that the adipic acid/sebacic acid binary system without a chemical reaction, only physical together. Expanded graphite can greatly improve the thermal conductivity of adipic acid/sebacic acid3/7ratio(eutectic point), and when the expanded graphite content is8wt.%, it increased8.17times. FE-SEM results showed that, with increasing content of sebacic acid, binary system the particles of lamellae increased gradually, columnar structure of the particles reduced accordingly. Meanwhile, near the eutectic ratio the distribution of composite particles tended to be more uniformly, which is conducive to the formation of eutectic.