Discovery of Carbon Nanotube-Like Structures in Tectonically Deformed Coals and Discussion of Their Formation Mechanisms

【Objective】 Carbon nanotubes have attracted extensive attention due to their unique structures and properties, yet their artificial synthesis usually requires harsh conditions such as high temperatures (700~2 300 ℃), stable carbon sources and catalysts, leading to their rare occurrence in nature. As a carbon-rich medium, coal can form a variety of ordered carbon structures during its metamorphic-deformational processes, which serves as an ideal carrier for exploring the formation of natural carbon nanotubes. 【Method】 In this study, typical tectonically deformed coals from Pingdingshan and Huaibei were selected as the mainresearch objects. After demineralization and chloroform extraction treatments, high-resolution transmission electron microscopy (HRTEM) was employed for observation, and in-situ carbon nanotube-like structures were unexpectedly discovered. Furthermore, X-ray diffraction (XRD) and Raman spectroscopy were combined to systematically characterize the structures of these formations. 【Results】 For the first time, in-situ carbon nanotube-like structures were discovered in medium-rank tectonically deformed coal. Most of these structures are hollow with parallel tube walls, presenting a linear or curled morphology, and are mainly multi-walled with 2~18 layers, with a diameter range of 0.4~6.0 nm and an interlayer spacing of single carbon layers of approximately 0.4 nm. The distribution of G bands (1 595, 1 599 cm^-1) and D bands (1 354, 1 364 cm^-1) in the Raman spectra is highly consistent with that of graphite; XRD analysis showed that the d₀₀₂ interplanar spacing is 0.343~0.348 nm, which further confirms its graphite-like crystal structure. The study also found that organic solvent extraction can effectively enrich such structures and enhance their observability. 【Conclusions】(1)Analysis suggests that intense tectonic stress, especially shear stress, through the synergistic effects of mechanochemical effects, element migration and frictional heat, significantly reduces the energy barrier for the ordering and curling of aromatic layers in coal. Thus, it drives the formation of carbon nanotube-like structures from organic carbon under temperatures far lower than those required for traditional synthetic methods. (2) This discovery not only enriches the system of natural carbon materials, but more importantly, proposes a “Tectonic Stress-Chemistry” formation pathway that is independent of extreme thermo-catalytic conditions. It provides a key theoretical basis and a new approach for the development of green bionic preparation technology of carbon nanotubes based on stress regulation. (3) The key to future research lies in quantitatively uncovering the mechanochemistry action process and the dynamic mechanism underlying the formation of coal-based carbon nanotube-like structures.