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 de mix depending on the elemental composition of the material. Their occurrenc e temperature was determined by TEM and elemental analyses (inflection poin t of (H/C)(at) versus HTT). As the factor F-LMO = (O + S-R/H) measured at a nisotropy occurrence increases, (1) the size and ordering of mesophases dec rease from spherical Brooks and Taylor mesophase spheres to other mesophase s with a different microtexture (1000-150 nm in size), down to volumes with digitized contours (100 nm in size); (2) the temperature of mesophase form ation increases and that of final local molecular orientation (LMO) domains decreases, so that the gap of temperature allowing the liquid crystal phas e development narrows. At solidification, determined by Vickers microhardne ss, the anisotropic domain (final LMO) size decreases from micrometric mosa ics 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 occurre nce. Above solidification, perfect aromatic layer stacks (grains) derive fr om final LMO with the same diameter. The smaller the grains, the lower the graphitability. If during primary carbonization an excess of H coincides wi th 100% of anisotropy before solidification (Ashland 240), macroporosity de velops under gas-pressure. The pore walls become oriented mosaics inside wh ich stresses provide disclinations themselves preventing the formation of p erfect grains and high graphitizability. (C) 1999 Elsevier Science Ltd. All rights reserved.