A study on nano-graphene oxide surface modification for the design of paraffin/graphene oxide phase change material

Incorporation of surface modified nanoparticles into paraffin can produce a phase change material (PCM) with superior thermal conductivity and better heat storage capacity. In the present research, for the first time, graphene oxide (GO) nanoparticles were modified with ethoxytrimethylsilane (ETMS) to improve their dispersion and stability in paraffin-based PCMs. Composite PCMs with different amounts of unmodified and modified GO (MGO) nanoparticles (0.25 wt%, 0.5 wt%, and 1 wt%) were prepared and evaluated with various analyses including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), atomic force microscopy (AFM), thermal conductivity measurement, thermogravimetric analysis (TGA), viscosity test, and melt fraction measurement. FTIR results confirmed the successful decoration of ETMS on the surface of GO nanoparticles. AFM observation corroborated that the MGO nanoparticles have an average thickness of 1-2 nm, which is almost similar to that of a single layer GO. DSC results demonstrated that compared to pure paraffin, the melting enthalpy decreases by 6-10 % with the inclusion of unmodified nanoparticles, while it increases by 0-8 % with the inclusion of modified nanoparticles. TGA results verified that the thermal stability in nanocomposites is improved compared to pure paraffin, and this improvement is more significant for the samples with modified nanoparticles. That initial degradation temperature increased about 13-17 degrees C and 22-26 degrees C for nanocomposites with GO and MGO nanoparticles, respectively. Rheological study confirmed that the viscosity and modulus of nanocomposites with MGO were much higher than those of pure paraffin and nanocomposites with GO nanoparticles and the loading of 0.25 % MGO leads to the greatest increase in viscosity and modulus. Repeated viscosity measurement disclosed that the viscosity stability of nanocomposites with MGO is higher than the nanocomposites with GO. Thermal conductivity measurement revealed that thermal conductivity in nanocomposites is enhanced compared to pure paraffin by 13-39 %, and this enhancement is more evident for the samples with unmodified nanoparticles. Melting fraction measurement proved that the melting rate in nanocomposites is higher than pure paraffin and the fastest melting is obtained for the sample with 0.25 % nanoparticles. The time required for complete melting was reduced by 12-25 % in nanocomposite with 0.25 % GO compared to the pure paraffin.