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 similar to 1350 cm(-1), corresponds to a lattice vibration mode with A(1g) symmetry. The D2 mode, which appeared at similar to 1610 cm(-1), arises from the structural disorder as a shoulder on the G band.