Continuous nucleation: The structure is also often dependent on the thickness of the deposit. For instance, the grain size increases as the thickness increases and the columnar-grain structure becomes more pronounced as the film becomes thicker. Such large columnar structures are usually undesirable as the deleterious effects of grain growth and columnar formation can be considerable, causing structural failure and the rapid diffusion of impurities along grain boundaries.
fig.7.6-effect of gas-phase nucleated impurities on deposited structure of pyrolytic graphite
Large grain size can be avoided by continuously adding growth sites, where new columnar growth can be generated. This effect is shown schematically in Fig.7.6. These new growth sites originate from soot particles, which are formed in the gas-phase when the pressure and supersaturation are above a certain level.
Laminar structure: The laminar structure of pyrolytic graphite consists of essentially parallel layers. It is shown in fig.7.7
Both columnar and laminar structures are optically active to polarized light and usually have similar physical properties.
fig.7.7-laminar structure of pyrolytic graphite, deposited on carbon filament
Isotropic structure: The other type of pyrolytic structure, isotropic carbon, has little graphitic characteristic and essentially no optical activity. It is composed of very fine grains without observable orientation and for this reason, it is known as isotropic carbon rather than isotropic graphite. It is often obtained in fluidized-bed deposition, possibly due to continuous surface regeneration by the mechanical rubbing action of the bed.