Quantifying orientation and curvature in HRTEM lattice fringe micrographs of naturally thermally altered coals: New insights from a structural evolution perspective

Abstract To visually investigate the lattice fringe arrangement and structural evolution in thermally altered coal, the observation of high-resolution transmission electron microscopy (HRTEM) lattice fringe images was employed to quantify the structural arrangement. Here, a series of six thermally altered coal samples (T6-T1: approaching the intrusion gradually with a distance from 4.4 to 0.1 m and the Ro increases from 0.75 to 3.50%) were collected, and a new approach, orientation obtained by the Linear Directional Mean and curvature calculated with Python, was carried out. The structural evolution from T6 to T1 had experienced a process that the average size of the aromatics increased (T6-T4), decreased (T4-T3), and then increased (T3-T1). In the aspect of orientation, the lattice fringe became more orderly with samples approaching the intrusion. However, the extent of orientation change process is different. This process is slight before T3, while after T3, the lattice fringe is oriented rapidly. In addition, long fringes (>1.14 nm) are preferentially aligned, followed by intermediate (0.54–1.14 nm), and short fringes (0.30–0.54 nm). In the aspect of curvature, curvature first increased and then fluctuated with samples approaching the intrusion, and peaks had a change in the distribution frequency which appeared at T4 and T2 respectively. In this process, the short and intermediate fringes contribute most to the curvature, followed by the long fringes. An “active point” is caught where the maximum segment angle of the fringe is located, indicating the non-six-membered rings or other lattice defects exist. The structural arrangement evolution of thermally altered coal experienced a tortuous process, and the eventual evolution tended to a consistent orientation and the lowest curvature. Reducing the curvature of lattice fringe in coal may be expected to provide a theoretical basis for promoting the conversion of coal to carbon materials.