Aromatic cluster and graphite-like structure distinguished by HRTEM in thermally altered coal and their genesis

Abstract

Graphite-like structures have a significant effect on the conversion of coal to carbon materials. To observe and analyze the different kinds of structures in thermally altered coal, HRTEM and XRD were employed in this work. Results showed that two different kinds of structures, aromatic clusters and graphite-like structures, could be distinguished in all thermally altered coal samples, even in a thermally altered coal sample having a Ro of 2.45%. The HRTEM images showed that aromatic clusters were made up of aromatic layers with different dimensions and directions. Some of the aromatic clusters showed a ringed arrangement, which is a benefit for both vertical stacking and lateral extension. The La/Lc value of aromatic clusters is smaller than 1, and the d of aromatic clusters is about 0.40 nm. Graphite-like structures are in the form of extended, multi-layered, uni-directional structural units. Compared with the aromatic clusters, graphite-like structures have obviously smaller interlayer spacing, more layers, larger extended size, and larger stacking height. The La/Lc of graphite-like structures is much larger than 1. All chemical structural parameters of the whole coal calculated from XRD data are between those of aromatic cluster and graphite-like structure, indicating that structural characteristics calculated by XRD are a mixture of both aromatic cluster and graphite-like structure. The genesis of different kinds of structures is related to heat, because the in situ-heating HRTEM showed that the aromatic layers transformed in different ways after heating. Surface layers transformed towards the graphite-like structure, while internal aromatic layers formed the aromatic clusters.