Investigation of the carbon structure of naturally graphitized coals from Central Hunan, China, by density-gradient centrifugation, X-ray diffraction, and high-resolution transmission electron microscopy

A suite of six naturally graphitized coals were separated by density-gradient centrifugation (DGC). The density profiles of anthracite and meta-anthracite samples show an average density increase from 1.50 g/mL to 1.93 g/mL with increased rank. Components of partially graphitized coals were separated into two distinct density peaks: a low-density peak at similar to 1.82 g/mL, and a high-density peak at similar to 2.0 g/mL; sample SL-3 also showed a shoulder at around 2.14 g/mL. The separated density fractions of the partially graphitized samples were examined by X-ray diffraction (XRD) and optical microscopy. This revealed that the lower density portions contain vitrinite and inertinite, whereas microcrystalline graphite along with needle and flake graphite are the primary components of the higher density portions, demonstrating that DGC can be used to separate components in metamorphosed coals. Carbon phases including local molecular oriented domains (LMODs) and turbostratic layers are observed in the partially graphitized coals under high-resolution transmission electron microscopy (HRTEM). The average density of the most graphitized coal is 2.18 g/mL, which together with the XRD pattern suggests a carbon structure close to that of graphite. In anthracite, basic structural units (BSUs) are distributed throughout the disordered carbon structure matrix; whereas graphitic lamellae are the primary constituent in the most graphitized coals, indicating the transformation of the carbon structure from amorphous-like anthracite to ordered graphite during natural graphitization. The co-occurrence of different carbon phases and structural units in the same sample demonstrates the heterogeneous nature of graphitized coals. The presence of novel carbon structures (such as BSUs, LMODs, turbostratic structure, and onion-like carbon, etc.) in the naturally graphitized coal series suggests potential use in the future development of new coal-based carbon materials.