Effects of graphite microstructure evolution on the anisotropic thermal conductivity of expanded graphite/paraffin phase change materials and their thermal energy storage performance

Abstract

The thermal conductivity of paraffin phase change materials (PCMs) is greatly enhanced by filling expanded graphite, whereas the microstructure of graphite in the PCM is intensively affected by the fabrication method. The effect of graphite microstructure evolution on the thermophysical properties of the expanded graphite (EG)/paraffin composite PCM (CPCM) is not considered and remains unclear. In this paper, the microstructure evolutions of graphite during the CPCM fabrication process are evaluated through SEM, XRD, Raman, and TEM. Anisotropy degree is proposed from the SEM morphology to quantitatively illustrate the distribution evolution of graphite sheets in the paraffin matrix. The results show that the microstructure integrity of graphite is deteriorated when infiltrating the liquid paraffin into the EG porous bulk. Notably, the interlayer spacing of graphite is expanded as inferred from the TEM pattern. With increasing density, the microstructure integrity of graphite in the CPCM can be gradually improved. Anisotropic thermal conductivity is identified in the CPCM, and the thermal conductivity in the parallel direction reaches 20.8 W/(m•K) at 60 °C, which is almost 70 times of the paraffin wax. The high thermal conductivity in the CPCM can be mainly attributed to the synergetic effects induced by the intrinsic high thermal conductivity of graphite and the high anisotropy degree of graphite sheets in the paraffin matrix. In addition, the temperature-time curve of the CPCM shows the solid region, mushy region, and liquid region in the parallel direction. While in the normal direction, it only shows the solid region during the same test period. The lower temperature rise reflected in the temperature-time curve confirms that the thermal energy storage performance of CPCM is dominated by the thermal conductivity.