Modeling of disorder and X-ray diffraction in coal-based graphitic carbons

In this work, X-ray diffractograms of about 20 coal-based graphites and carbons, with different degrees of graphitic order, have been modeled using the recent computational model of Shi et al. This model relates the crystalline parameters c (=2d(002)) and a and the crystallite sizes L(c) and L(a), to the parameters of the disorder present in carbons. These disorder parameters include the probability P for a random displacement delta between adjacent layers, the probability P-t for finding a 3R stacking fault, the strain parameters, and the preferred orientation. 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 observation of a step at 3.37 Angstrom is consistent with the recent observation of Bragg et al. in a graphite where disorder was produced by successive grinding, thermal treatments and neutron irradiation. A theoretical interpretation of this step-transition is however not yet available. Plots of d(002) against [delta(2)>], 2P, (2P+P-t) are discussed in terms of the disorder models proposed by Franklin and by Ruland. The preferred orientation evaluated from the fit varies systematically with the ratio L(c)/L(a) and the experimental X-ray anisotropy ratio. Copyright (C) 1996 Elsevier Science Ltd