Nanoscale quantitative characterization of microstructure evolution of partly graphitized high rank coal: Evidence from AFM and HRTEM

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The microstructure of coal, such as matrix morphology, pore structure and carbon structure, is of great significance to the resource enrichment in coal and the study of graphitization. In this work, atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM) were employed to characterize microstructure of cores from the Upper Paleozoic coal seams in the southern Qinshui Basin. The results show that carbon structure is local parallel in alignment, with several layers stacked and the interlayer spacing is 0.338 ∼ 0.363 nm. The matrix surface fractal dimension D varies from 2.15 to 2.38, with surface roughness Ra ranging from 0.8 to 29.9 nm. The pore number ranges from 257 to 575 and the plane porosity varies from 0.75% to 4.87%. With the decrease of scan size, D and plane porosity show strong stability relative to the other parameters. The relationship between matrix surface morphology and pore structure indicates that the rough surface with low self-similarity is beneficial to the formation of pores. During coalification and graphitization, the matrix morphology of high rank coal tends to be regular and smooth while interlayer spacing of carbon structure gradually decreases. Two opposite changing trends that Ro, m with surface fractal dimension D and plane porosity suggest the existence of matrix-pore coupled evolution. The results are supplementary to the coalification and graphitization theory, and have application value in the development of resource in coal and utilization of coal derived materials.