Mesophase Mechanisms in the Formation of Graphitic Microstructures,

Since most graphitic materials form by means of a liquid-crystal mesophase transformation, their microstructures differ fundamentally from those of conventional metals or ceramics. The microconstituents may be viewed as mesophase fossils, with lamelliform morphologies free of grain boundaries, but with the disclinations and distorted layers characteristic of liquid crystals. Hot-stage studies of the carbonaceous mesophase in the fluid state have demonstrated such typical liquid-crystalline behavior as spherulecoalescence, disclination reactions, and orientation fluctuations. However, the mesophase usually congeals to coke while under mechanical deformation, e.g., by bubble percolation in the delayed coker, so that the acicular and lamellar constituents of needle coke are nonequilibrium microstructures locked into place as the mesophase hardens. Mesophase carbon fibers, formed by the drawing and quenching of mesophase pitch, represent extreme examples of deformed mesophase morphologies. Disclination models can be constructed for the internal structures of mesophase carbon filters.