Mechanisms of Formation of Isotropic and Anisotropic Carbons

Summary Structure in both isotropic (non-graphitizable) and anisotropic (graphitizable) carbons is based on the graphite molecule albeit in very defective forms. Isotropic carbons have lamellae arranged randomly with associations of small number to give an indication of short-range order. Anisotropic carbons have essentially approximately parallel arrangements of lamellae which subsequently assume increased order on heat treatment to give graphitic material. Isotropic carbons are generated from polymeric biomass materials, e.g. wood and synthetic resins which maintain their polymeric character on pyrolysis, without fusing, the carbon being a pseudomorph of the parent substance. Anisotropic carbons are generated from pitch and coals. These materials fuse on pyrolysis and it is from within this liquid phase that polynuclear, aromatic, discotic, nematic liquid crystals (mesophase) are generated by a process of homogeneous nucleation. Mesophase is the precursor to anisotropic structure in carbons/cokes. Pyrolysis chemistry is discussed indicating how optical texture of coal is a function of the reactivity of pyrolyzing systems. The importance of viscosity/temperature curves is described. Coal carbonization is more complex than pitch pyrolysis because of the need to depolymerise the original coal structure before liquid crystals can be generated from within the fluid coal.