Fabrication And Characterization Of Alkylated Graphene Oxide-modified Phase-change Microcapsules

Energy crisis is one of the most important factors which restricted human society to develop in the 21th century.Unreasonable use for a long time has led to the shortage of energy resourses and serious damage of nature environments.Therefore,it has becoming a hot topic at present to respond to greenhouse effect,treat atmosphere pollution,search for clean energy and alternative energy,and pursue the equilibrium between human society and nature environment.Phase-change materials（PCMs）can absorb/release latent heat during the phase-change process while the temperature remains in a narrow range,which is helpful to realize the purpose of temperature regulation,energy saving and energy storage.To solve the problems existing in the use of organic PCMs,such as low thermal conductivity,serious super cooling,leakage and corrosion,the microencapsulation technique was adopted in this thesis,and the phase-change microcapsules with outstanding heat-storage capacity and thermal conductive properties were fabricated by incorporating graphene.First of all,nano-graphene platelets（NPGs）were oxidized and grafted with with alkyl chain to form alkylated graphene in order to improve its compatibility with organic core material.Then,the microencapsulated PCMs（MEPCMs）using melamine-formaldehyde（MF）resin and silicon dioxide as the shell materials were fabricated via in situ polymerization and interfacial polymerization,respectively,by incorporating alkylated graphene into the core material.The morphology and structure of graphene oxide and alkylated graphene were analyzed using atomic force microscopy（AFM）,Fourier transform infrared spectroscopy（FTIR）,X-ray diffraction（XRD）and Raman spectroscopy,respectively.In addition,the dispersion of alkylated graphene in the melted octadecane was observed.The morphology and size of microcapsules were examined using optical microscope（OM）,field emission-scanning electron microscope（FE-SEM）,and laser particle size analyzer（Zetasizer）.The structure and compositions of microcapsules were characterzed using XRD,Raman spectroscopy,and FTIR.The phase-change properties,thermal cycling performances and thermal stability were analyzed using differential scanning calorimeter（DSC）and thermogravimatric analyzer（TGA）.Main conclusions were listed as follows.（1）Graphene oxide（GO）,octadecyl isothiocyanate-grafted GO（iGO）and octadecylamine-grafted GO（GO-ODA）with the platelet thickness of¹ nm have been successfully fabricated.After alkylation modification,the GO became smaller,and in the damaged hexagonal lattice of carbon atoms of graphite domain was partially recovered.In comparison with iGO,GO-ODA showed a better compatibility with n-octadecane.（2）Through the encapsulation of the mixture of GO-ODA and n-octadecane using MF resin,phase-change microcapsules with good thermal conductivity was successfully prepared.The introduction of graphene leads to the decrease in both microcapsule size and thermal stability.Compared with the pure MEPCMs,the thermal conductivity of the MEPCM containing 0.5%GO-ODA increased by 36.84%,and maintained the enthalpy higher than207.2 J g^-1.The results of infrared thermal imaging analysis indicates that the phase-change microcapsules can efficiently regulate the ambient temperature.Moreover,in the aspect of response to the change of ambient temperature,the graphene-modified MEPCMs is more sensible than the pure MEPCMs.（3）The SiO₂-shelled MEPCMs,fabricated at different emulsifying speeds,have close phase-change enthalpies,encapsulation efficiency and thermal stability.The encapsulation efficiency and phase-change enthalpies of microcapsules are larger than 70%and 170J g^-1,respectively.The same amount of graphene has no obvious effect on the morphology and size of microcapsules fabricated at different emulsifying speeds,besides the slight decrease in enthalpy and encapsulation efficiency.After filling various amounts of graphene,the size,phase-change enthalpy and encapsulation efficiency of microcapsules were decreased but the thermal transfer ability was increased.Compared to that of MEPCMs without GO-ODA,the thermal conductivity of MEPCMs doped with 0.5%GO-ODA increased by 36.36%.