Microstructure Characteristics and Thermal Expansion Behavior of Coal Measures Graphite
LI Huan-tong1, ZHANG Qian1, ZOU Xiao-yan2*, ZHANG Wei-guo1, LIN Ke-jin1
1. College of Geology and Environment, Xi'an University of Science and Technology, Xi'an 710054, China
2. College of Urban, Rural Planning and Architectural Engineering, Shangluo University, Shangluo 726000, China
Abstract：Graphite is a layered carbon material composed of sp2 hybridized carbon atoms. Coal-measure graphite, a cryptocrystalline form of graphite derived from coal through high-temperature metamorphism, exhibits significant anisotropy. In this study, four samples from the anthracite-coal measure graphite series were selected and analyzed for their microstructural characteristics and thermal expansion behavior using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and ultraviolet-visible-near-infrared diffuse reflectance testing (UV-Vis-NIR). The quantitative relationship between these properties was also investigated using in situ high-temperature XRD. The results show that as the graphitization degree increases, the microcrystalline size and stacking height along the c-axis direction of coal-measure graphite significantly increase, and the anisotropic characteristics become more pronounced. The ID1/ID2 ratio, which corresponds to different defect types, varies, with the basal plane exhibiting a higher ID1/ID2 ratio than the edge plane. HRTEM reveals that the aromatic layers of highly metamorphosed anthracite exhibit local orientation domains, with fewer stacking layers and unstable extension lengths, and that amorphous carbon is present on the microcrystalline surface or edges. Oxygen-containing functional groups are mainly concentrated in the defective regions of graphite microcrystals. As the graphitization degree increases, the oxygen content decreases, and the proportion of carbonyl groups（C═O） decreases. The presence of polycyclic aromatic hydrocarbons in coal-measure graphite -shifts the π—π* transition absorption peak to approximately 208 nm, with a simulated color distribution ranging from Cool Gray 9 CP to 2 CP. Higher graphitization degrees are associated with stronger reflectivity. Regarding thermal expansion behavior, the thermal expansion coefficient of graphite shows significant anisotropy, with the coefficient parallel to the basal plane being lower than that perpendicular to it. As the temperature rises, the expansion of graphite microcrystals along the c-axis is regulated by the corresponding increase in-the interlayer spacing. In contrast, in the a-axis direction, the formation of lattice defects decreases the average microcrystalline size (W-Hsize). The higher the graphitization degree, the more complete the microstructure, the more pronounced the anisotropy, and the more significant the anisotropic thermal expansion behavior.
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