MODELING OF DISORDER AND X-RAY-DIFFRACTION IN COAL-BASED GRAPHITIC CARBONS

In this work, X-ray diffractograms of about 20 coal-based graphites an d carbons, with different degrees of graphitic order, have been modele d using the recent computational model of Shi et al. This model relate s the crystalline parameters c (=2d(002)) and a and the crystallite si zes L(c) and L(a), to the parameters of the disorder present in carbon s. These disorder parameters include the probability P for a random di splacement delta between adjacent layers, the probability P-t for find ing a 3R stacking fault, the strain parameters, and the preferred orie ntation. Plots among d(002) L(c), and L(a) lead us to conclude that a step transition exists in graphites near d(002)=3.37 Angstrom where a two-fold increase in L(c) is observed for a fixed L(a). This observati on of a step at 3.37 Angstrom is consistent with the recent observatio n of Bragg et al. in a graphite where disorder was produced by success ive grinding, thermal treatments and neutron irradiation. A theoretica l interpretation of this step-transition is however not yet available. Plots of d(002) against [delta(2)>], 2P, (2P+P-t) are discussed in te rms of the disorder models proposed by Franklin and by Ruland. The pre ferred orientation evaluated from the fit varies systematically with t he ratio L(c)/L(a) and the experimental X-ray anisotropy ratio. Copyri ght (C) 1996 Elsevier Science Ltd