Structural order evaluation and structural evolution of coal derived natural graphite during graphitization

Abstract The structure of onset phase of coal derived natural graphite (CDNG for short), semi-graphite, and CDNG subjected to different metamorphic grade were investigated using XRD, Raman spectroscopy, and HRTEM to trace their structural evolution during graphitization. The transitional phases with different structural order exist in graphite field. A classification of these transitional phases is of particular importance to achieve the rational utilization of CDNG. The d(002)-spacing is used to distinguish the pre-phases of highly ordered CDNG (ordered CDNG) and highly ordered CDNG in graphite field. The asymmetry index (AI) of (002) reflection of XRD patterns and Raman parameters D1/G area ratio and FWHM of G band correlate well with d(002)-spacing of CDNG in the zone of semi-graphite to graphite, thus they are also proposed to evaluate the structural order of CDNG. For structural evolution of CDNG during graphitization, at the beginning, the BSUs composed of stacking aromatic layers arrange face to face and tend to link together laterally. Then adjacent BSUs link laterally to form long-range wrinkled layers. With graphitization, the wrinkled layers evolve to flat lamellae and turbostratic structure gradually disappears. Finally, the dislocations in stacking and in-plane defect of flat aromatic layers decrease to attain triperiodic graphite structure.