Scanning Electron Microscopy


Carbonaceous materials differ according to their chemical composition (types of heteroatoms), their structure and their microtexture. So, it is interesting to find methods for characterizing them. We choose to correlate data from Raman microspectroscopy, optical microscopy and transmission electron microscopy. First, we use both graphitizable or non-graphitizable reference carbon series of simple chemical composition in order to follow the structural transformation of the carbonaceous materials according to the evolution of these data throughout heat-treatment of these samples. Then, the coals of different ranks are studied.

The Raman results are correlated with those from electron microscopy, particularly by plotting, for the graphitizable series, the diameter of the aromatic layers La (determined from lattice fringes and 11 dark-field) versus the specific surface of the Raman band characteristic of the defects centered at about 1350 cm-1 (ratio between surfaces of this band and the whole spectrum). For the coals, the half-maximum width of the band at about 1600 cm-1 has been plotted versus the reflectance.

We conclude that the evolution of carbonaceous materials, throughout heat-treatments or natural processes, is possible only because different types of defects are progressively removed. These defects are heteroatoms, tetrahedral carbons, isolated and crosswise basic structural units or BSU (one, two or three stacked in parallel polyaromatic structures, less than 10 A in size) and defects in aromatic layers. The elimination of these defects permits the rearrangement of the BSUs and the establishment of an organization which can possibly reach the triperiodic order according to the series.

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