The Study On Graphene Enhanced Thermal Conductivity Of Phase Change Materials

Phase change materials(PCM), which can be utilized in many fields such as solar energy storage, energy efficient buildings and cycling of waste heat, have attracted more and more attention recently. Organic PCM have advantages such as the ideal phase change latent heat, nontoxic and non corrosivestable, physical and chemical properties. However, low thermal conductivity and easy leaky of organic PCMs greatly reduced their energy storage efficient. In order to overcome the defect of organic phase change materials, it is necessary to improve the thermal performance of phase change materials. Graphene has many special physical properties, like high thermal conductivity. It has broad prospects to improve the thermal conductivity of phase change materials.In this research, the thermal conductivity of organic PCM has been improved by forming a composite with graphene by three different methods. Meanwhile, intensive research has been taken on the properties of resulting composite through various testing methods. The main points of this works are as followed:(1) Graphene oxide has prepared by improving Hummers method and reduced by hydrazine hydrate for the graphene nanosheets (GNs). GNs/PCM composite phase change materials has prepared with GNs and phase change material (PCM). Graphene and composites were characterized by SEM, TEM, FT-IR, XRD, DSC and thermal constant analyzer. The results were as follows:With the increase of GNs, the thermal conductivity of composites increases gradually. For the composites with filler weight of 2 wt%, the thermal conductivity is 1.8428 W/(m-K), is 10.1 times of pure PCM. After 200 times of psychro-thermal cycles, GNs/PCM composite thermal conductivity is reduced. Thermal enthalpy △Hm and △HC of GNs/PCM composite were 208.2 J/g and 195.4 J/g. GNs have little impact on the PCM of the phase change temperature and phase change enthalpy. It can be proved that there was neither chemical reaction within the composite-process, nor change in crystal structure, as could be proved by FTIR and XRD.(2) Using 160℃ hydrothermal reduction different concentrations of graphene oxide solution,This part is to preparate the graphene self-assembly aerogel (SGA), And SGA was used to prepare SGA/PCM composite phase change material by natural adsorption and vacuum impregnation. Graphene and composites were characterized by SEM, TEM, FT-IR, XRD, DSC and thermal constant analyzer. The results were as follows:When GO concentration is 2 mg/mL, thermal conductivity of GA2/PCM composite by natural adsorption and vacuum impregnation is 1.7123 W/(m-K) and 2.7324 W/(m-K) respectively, significantly higher than the thermal conductivity of pure PCM. Heat enthalpy △Hm and △Hc were 203.4 J/g and 194.5 J/g,209.1 J/g and 197.6 J/g respectively. SGA2/PCM composite is stability by vacuum impregnation than natural adsorption. SGA has no effect on the crystalline structure of the PCM, no new substance by chemical reaction in forming.(3) Using environment-friendly reductant vitamin C, graphene oxide was reducing to the three-dimensional porous graphene aerogels (VC-GA). It is showed that the preparation of VC-GA structure surface is smooth and the internal three-dimensional porous structure by SEM. VC-GA can significantly improve the phase change materials thermal conductivity of PCM. Especially,when the concentration of the initial oxidation of graphene is 3 mg/mL, VC-GA3/PCM composite thermal conductivity is 1.7324 W/(m·K), is 9.5 times than pure PCM; thermal enthalpy △Hm and △Hc were 213.5 J/g and 199.8 J/g. VC-GA to PCM have obviously improved the thermal stability of the function. VC-GA has no effect on the crystalline structure of the PCM basic, no new substance by chemical reaction in forming.