A methodology for analysis of 002 lattice fringe images and its application to combustion-derived carbons

A technique is presented for semi-quantitative digital analysis of 002 lattice fringe images of carbon materials. The automated technique yields statistics on fringe length and tortuosity, as well as new quantitative parameters describing the mode, degree, and length scale of orientational order among graphene layers. The technique is applied to combustion-derived carbons with special emphasis on the nanostructural characterization of a variety of solid fuel chars as a function of residence time. During the combustion of pulverized coal and biomass (120 ms at 1700-1900 K), the main features of char nanostructure are established early, near the end of the rapid, in-flame pyrolysis. Only for the high-volatile bituminous coals is the nanostructural order in the young chars significantly altered by the subsequent char combustion process. A noteworthy general observation is the presence of two distinct length scales for orientational order in most chars. At short length scales (<30 Angstrom) a high degree of orientational order is observed reflecting the presence of small molecular orientation domains. At larger length scale (>30 Angstrom), a lesser but significant degree of orientational order is still present among the distinct domains. Quantitative analysis of this low-grade long-range order using Maier-Saupe theory indicates that it cannot arise by a liquid crystal mechanism. Long-range orientational order in these samples can thus be classified as 'mesophasic', exhibiting a high degree of order arising through liquid crystal formation in the fluid phase of pyrolysis (or retention of anisotropy in very high rank coals), and 'statistical', perhaps arising from elongational strain during carbonization. (C) 1999 Elsevier Science Ltd. All rights reserved.