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

• Published in: International journal of heat and mass transfer Vol. 155; p. 119853
• Main Authors: Wang, X.L., Li, B., Qu, Z.G., Zhang, J.F., Jin, Z.G.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.07.2020; Elsevier BV
• Subjects: Anisotropy; Anisotropy degree; Energy storage; Evolution; Graphite; Heat conductivity; Heat transfer; Integrity; Interlayers; Liquid paraffins; Microstructure; Microstructure evolution; Morphology; Paraffin wax; Phase change materials; Sheets; Thermal conductivity; Thermal energy; Thermal energy storage; Thermophysical properties; Phase change materials; Thermal conductivity; Thermal energy storage; Anisotropy degree; Microstructure evolution
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2020.119853

•Anisotropy degree is proposed from structure evolution of expanded graphite.•Microstructure integrity of graphite is deteriorated by infiltrating paraffin.•Thermal conductivity anisotropy is found in expanded graphite/paraffin material.•Enhanced thermal conductivity results from high anisotropic structure of graphite.•Thermal energy storage performance can be improved by expanded graphite. 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.