Investigation of nanocrystalline structure in selected carbonaceous materials

The structural parameters of nine Indian coals were determined by X-ray diffraction (XRD) and Raman spectroscopy. The study revealed that the coals contain crystalline carbon of turbostratic structure with amorphous carbon. The stacking height (L _c) and interlayer spacing (d ₀₀₂) of the crystallite structure of the coals ranged from 1.986 to 2.373 nm and from 0.334 to 0.340 nm, respectively. The degree of graphitization was calculated to range from 42% to 99%, thereby confirming the ordering of the carbon layers with the increase in coal rank. An exponential correlation was observed among the aromaticity (f _a), the lateral size (L _a), and the rank (I ₂₀/I ₂₆), suggesting that the coal crystallites are nanocrystalline in nature. A very strong correlation was observed between the structural parameters (f _a, d ₀₀₂, L _c, the H/C ratio, and I ₂₀/I ₂₆), the volatile matter content, and the elemental carbon content, indicating the structures of coals are controlled by the degree of contact metamorphism. The Raman spectra exhibited two prominent bands: the graphitic band (G) and the first-order characteristic defect band (D). The deconvolution resulted in five peaks: G, D1, D2, D3, and D4. The intense D1 band, which appeared at ～1350 cm^?1, corresponds to a lattice vibration mode with A_1g symmetry. The D2 mode, which appeared at ～1610 cm^?1, arises from the structural disorder as a shoulder on the G band.