Pressure-Driven Structural Evolution of Nongraphitizable Bituminous Coal: Mechanistic Insights into Low-Temperature Graphitization

Abstract

This study demonstrates that applying static pressure can transform inherently nongraphitizable low-rank bituminous coal into graphitizable carbon. Under ambient pressure conditions, even after high-temperature treatment at 2600 °C, the degree of graphitization in bituminous coal remains low, indicating its nongraphitizable nature. However, after applying static pressure, a high degree of graphitization can be achieved with treatment at just 1100 °C. The study characterized the degree of graphitization in the samples using X-ray diffraction (XRD) and Raman spectroscopy, while the evolution of microstructures was observed via transmission electron microscopy (TEM). Based on analyses using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy, the structural evolution can be categorized into three stages. In the initial stage, partial detachment of side chains in bituminous coal occurs, accompanied by the progressive condensation of aromatic nuclei to form aromatic clusters and the concurrent generation of pore structures. In the intermediate stage, stress concentration at pore walls, induced by the presence of sp3-hybridized metastable carbon, leads to pore rupture. Finally, static pressure induces sp3 carbon bond cleavage, releasing carbon atoms previously confined within metastable disordered structures. Simultaneously, the elimination of edge-unstable groups and defects generates abundant clean and active graphene precursors. Under sustained static stress, these precursors align along specific orientations and rapidly grow into graphitic carbon. In summary, static pressure guides the evolution pathway of microdomains by regulating the formation and annihilation of pores within the carbon framework.