HRTEM observation of morphological and structural evolution of aromatic fringes during the transition from coal to graphite

Abstract The maximum reflectance of vitrinite or graphite microcrystals in Yongan Coalfield, Fujian Province, China, ranges from 5.33% to 10.21%, comprising meta-anthracite, semi-graphite, and graphite. The morphological characteristics of aromatic fringes during different evolution stages were quantitatively analyzed using high-resolution transmission electron microscopy and image processing techniques. The results show that a nonlinear evolution of aromatic fringe morphology in the process of coal graphitization is manifested in four aspects, involving the rapid connection and growth, enhancement of ordering alignment, increase of stacking and development of straightening. In meta-anthracite samples, 3 × 3 aromatic rings representing short aromatic fringes are dominated, a majority of aromatic fringes were curved, disjoint or separate, and show a turbostratic layers, with poorly aligned. The spacing between adjacent aromatic fringes exceeded the range of graphitized carbon, and structural defects, such as nanopores were generally visible. In semi-graphite samples, in addition to 3 × 3 aromatic rings, the proportion of >8 × 8 aromatic rings represented longer aromatic fringes increasing significantly, along with ordering alignment and straightening. Stacking with two more layers increased and the spacing between adjacent aromatic fringes decreased to the range of graphitized carbon, and the structural defects decreased considerably. In graphite sample, the proportion of >8 × 8 aromatic rings increase significantly, the length, stacking, and ordering alignment of aromatic fringes increased abruptly, the spacing between adjacent aromatic fringes remained within the range of natural graphite, and a rapid graphite-like structure with flat aromatic fringes formed. A stage evolution of aromatic fringe morphology, whereas remaining almost constant of mainly organic elements, which imply the evolution of aromatic fringes in meta-anthracite originated from ring condensation dominated by late coalification, graphitization starts at semi-graphite stage and enhanced during graphite stage. The making up, originating from linkup and pileup, considered as a special mechanism for the graphitization of aromatic fringes.