Preparation And Properties Of Graphene/fatty Acid Composite Phase Change Materials

In recent years, with the aggravation of the energy crisis, more and more people have begun to study phase change materials in order to alleviate the crisis. As a common phase change material, organic fatty acid has lots of advantages, such as appropriate phase change temperature and latent heat, low price, non-toxic and corrosive, non flammable and supercooling phenomenon, etc. However, the phase change temperature of some single fatty acid is too high, which limits its application. While, mixing two or more kinds of fatty acids, forming a dual or multiple eutectic fatty acid, a composite material with lower phase change temperature can be prepared, which can be applied in more fields. Graphene, as the hottest carbon material in recent years, has very high thermal conductivity, with the advantages of large surface area and high aspect ratio. So it can be used as an additive to stabilize fatty acid phase change materials and improve its thermal conductivity, making the performance of fatty acid superiorer.Firstly, nine kinds of lauric(LA)-palmitic(PA) binary eutectic fatty acid phase change materials with different mass ratios were prepared by melt-blending method. FT-IR and SEM were applied to determine microstructure, chemical structure, respectively. TG-DTG and DSC were used to characterize the thermal performance. Then, modified Hummers method was used to synthesize graphene, and XRD, FT-IR, TG-DTG and SEM were carried out to characterize the structure, thermal performance and surface morphology of graphene. Melted LA-PA was dispered in water using sodium dodecyl sulfate(SDS) as an emulsifier and cooled to prepare a water-based latex containing negatively charges LA-PA particles. Graphene was non-covalently functionalized with cetyl trimethyl ammonium bromide(CTAB) which is a cationic surfactant, to make graphene dispersed in water stablly. A simple mixing of this latex with an aqueous dispersion of graphene, prepared with aid of acationic surfactant, results in the self-assembly of graphene on a LA-PA particle surface by an electrostatic interaction, allowing for the encapsulation of LA-PA with graphene to form a core–shell composite microcapsule.According to the requirement of phase change temperature of 9 different mass ratios of LA-PA binary eutectic fatty acids, finally 8:2 and 7:3 are determined to the optimal choice applied to heating floor in the building field. Because of the high content of the active material in the core-shell structure composite materials, so the latent heat of composites is large. The phase change temperature of the composites remains almost unchanged. Compared with before the addition of graphene, latent heat decreases, which is because the presence of graphene decrease the content of the active material in composites, but its value is more than 90%. Because of the protective shell graphene, composites have excellent stability during the process of phase transition. It is found that after 100 times thermal cycling, the phase change temperature of composites is also not changed, while the latent heat decreases a little, which is mainly attributed to the rearrangement of graphene in composites, such as stacking or gathering, hindering fatty acid crystallization. After adding thin graphene, due to its high thermal conductivity, so the thermal conductivity of composites increases significantly, and the value increases with the increase of graphene content. For example, the thermal conductivity of LA-PA binary eutectic fatty acid is 0.355, when the graphene content is 2 wt % and 5 wt %, the value of composites were increased to 0.497 and 0.562, respectively.