Quantifying the Structural Transitions of Chinese Coal to Coal-Derived Natural Graphite by XRD, Raman Spectroscopy, and HRTEM Image Analyses
Liang Yuan, Qinfu Liu, Jonathan P. Mathews, Hao Zhang, Yingke Wu
Abstract
The macromolecular evolution of coal in proximity to intrusion zones provides a means of exploring the coal to coal-derived natural graphite (CDNG) transitions. Here, samples with different metamorphic grades from anthracite to CDNG were examined based on X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) image analyses. The anthracite aromatic layers were limited with the average layers in a stack being ∼10, with a height and width of ∼4 and ∼5 nm, respectively. The CDNG had higher crystallinity with the number of layers in a stack being ∼120, with a height and width of ∼40 and ∼70 nm, respectively. The D1/G area ratios (Raman) decreased from 6.5 to 0.3 (from anthracite to graphite). The short fringe length (0.3–1.14 nm) distributions account for >90% of anthracite, while meta-anthracite and semi-graphite had lower contributions of 88.5 and 81.5%. The fringe orientation value (the frequency of the total fringe length within the most prominent 15° bin) increased from 22.1 (anthracite) to 29.4% (meta-anthracite). For semi-graphite, the orientation was high reaching 72.5%. The high-grade CDNG had essentially 100% of fringes oriented within 15°. The fringe curvature was high in the initial anthracite (90.9% of the fringes) but this decreased to 85.5% in the anthracite of higher metamorphic grade. In meta-anthracite, the curvature decreased to 80.1% as the fringe length further increased. The fringe curvature became less frequent (51.5%) in the highly ordered CDNG. The structural evolution from anthracite to meta-anthracite followed the general coalification characteristics: fringes are increasingly longer, oriented, and linear but limited to small stacks. An extensive molecular amalgamation occurs, forming fringe curvature and dislocations in the meta-anthracite to semi-graphite transitions. With graphitization, the curvature and the dislocations decrease forming the well-ordered graphite.