Quantifying the Structural Transitions of Chinese Coal to Coal-Derived Natural Graphite by XRD, Raman Spectroscopy, and HRTEM Image Analyses

Abstract

The macromolecular evolution of coal in proximity to intrusion zones provides a means of exploring the coal to coal-derived natural graphite (CDNG) transitions. Here, samples with different metamorphic grades from anthracite to CDNG were examined based on X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) image analyses. The anthracite aromatic layers were limited with the average layers in a stack being ∼10, with a height and width of ∼4 and ∼5 nm, respectively. The CDNG had higher crystallinity with the number of layers in a stack being ∼120, with a height and width of ∼40 and ∼70 nm, respectively. The D1/G area ratios (Raman) decreased from 6.5 to 0.3 (from anthracite to graphite). The short fringe length (0.3–1.14 nm) distributions account for >90% of anthracite, while meta-anthracite and semi-graphite had lower contributions of 88.5 and 81.5%. The fringe orientation value (the frequency of the total fringe length within the most prominent 15° bin) increased from 22.1 (anthracite) to 29.4% (meta-anthracite). For semi-graphite, the orientation was high reaching 72.5%. The high-grade CDNG had essentially 100% of fringes oriented within 15°. The fringe curvature was high in the initial anthracite (90.9% of the fringes) but this decreased to 85.5% in the anthracite of higher metamorphic grade. In meta-anthracite, the curvature decreased to 80.1% as the fringe length further increased. The fringe curvature became less frequent (51.5%) in the highly ordered CDNG. The structural evolution from anthracite to meta-anthracite followed the general coalification characteristics: fringes are increasingly longer, oriented, and linear but limited to small stacks. An extensive molecular amalgamation occurs, forming fringe curvature and dislocations in the meta-anthracite to semi-graphite transitions. With graphitization, the curvature and the dislocations decrease forming the well-ordered graphite.

Coupling Effect of Temperature, Gas, and Viscoelastic Surfactant Fracturing Fluid on the Chemical Structure of Deep Coal: An Experimental Study

“…Peak G is caused by the respiratory vibration of aromatic rings, and peak D is caused by the lattice defects of graphite, edge disorder, and low symmetry carbon structure. ,− The results show that the Raman spectrum of coal cannot be simply divided into peak G and peak D because there are other peaks that can reflect the structure of coal. Therefore, we used the Origin 2019b software to divide the Raman spectrum (800–1800 cm –1 ) of the coal sample into three Lorentzian peaks (G, D 1 , and D 2 ) and one Gaussian peak (D 3 ). ,− Taking coal sample C 8 as an example, the Raman displacement after its deconvolution is shown in Figure . The meaning of each band is shown in Table .…”

“…Coal is a short-range ordered and long-range disordered amorphous material with some internal anisotropy. ,,, As the degree of metamorphism increases, the ordering of macromolecules increases, forming a partially ″ordered″ carbon structure consisting of several layers of aromatic lamellae (mainly composed of aromatic nuclei and hydrocarbon branched chains and various functional groups). − ,− The stacked aromatic lamellae, which resemble graphite (semigraphite) structures, are referred to as ″microcrystals″. ,,,− , In the Raman spectra of coal, different peaks represent different skeletal structures of the coal, and characteristic parameters such as the placement of peaks G and D 1 , the peak difference (G–D 1 ) and the integrated intensity ratio ( I D1 / I G ) are important parameters that are utilized to assess the degree of crystallinity or defects in carbonaceous materials. ,− ,,− …”

HRTEM analysis of the aggregate structure and ultrafine microporous characteristics of Xinjiang Zhundong coal under heat treatment

“…2 is the sum of θ 1 and θ 2 . By calculating the sum of the segment angles of each fringe, the bending situation of a single lattice fringe can be reflected, thus obtaining the cumulative angle sizes of lattice stripes with different lengths 24 . Stack structure.…”

Fluctuations in Graphitization of Coal Seam-Derived Natural Graphite upon Approaching the Qitianling Granite Intrusion, Hunan, China

“…The intensity ratios and the half-height width are often used to characterize the degree of structural changes underway [7,8,10]. However, the half-height width was greatly affected by test conditions (i.e., different gratings, CCD cameras, etc.)…”

“…The heat, emanation, and dynamic effects of the intruded rock can induce graphitization in the coal seam and organic matters in the associated rock, thus forming coal seam-derived graphite. This type of graphite is also known as natural graphite derived from coal [7,8] or microcrystalline graphite [9]. As a type of graphite ores, coal seam-derived graphite has attracted significant interest from researchers due to its economic efficiency, ease of mining, and potential utilization as a cutting-edge material.…”