Carbonization of various precursors. Effect of heating rate Part II: Transmission electron microscopy and physicochemical studies

During primary carbonization, various types of liquid crystal phases can demix depending on the elemental composition of the material. Their occurrence temperature was determined by TEM and elemental analyses (inflection point of (H/C)(at) versus HTT). As the factor F-LMO = (O + S-R/H) measured at anisotropy occurrence increases, (1) the size and ordering of mesophases decrease from spherical Brooks and Taylor mesophase spheres to other mesophases with a different microtexture (1000-150 nm in size), down to volumes with digitized contours (100 nm in size); (2) the temperature of mesophase formation increases and that of final local molecular orientation (LMO) domains decreases, so that the gap of temperature allowing the liquid crystal phase development narrows. At solidification, determined by Vickers microhardness, the anisotropic domain (final LMO) size decreases from micrometric mosaics down to nanometric final LMO (200 nm). It results in a good correlation between final LMO size and the factor F-LMO measured at anisotropy occurrence. Above solidification, perfect aromatic layer stacks (grains) derive from final LMO with the same diameter. The smaller the grains, the lower the graphitability. If during primary carbonization an excess of H coincides with 100% of anisotropy before solidification (Ashland 240), macroporosity develops under gas-pressure. The pore walls become oriented mosaics inside which stresses provide disclinations themselves preventing the formation of perfect grains and high graphitizability. (C) 1999 Elsevier Science Ltd. All rights reserved.