The carbonization process, also known as pyrolysis, can be defined as the step in which the organic precursor is transformed into a material that is essentially all carbon. The mechanism of carbonization is reviewed below in general terms. Additional information on the carbonization of specific materials is given in subsequent chapters.
Carbonization cycle: carbonization is basically a heating cycle. The precursor is heated slowly in a reducing or inert environment, over a range of temperature that varies with the nature of the particular precursor and may extend to 1300C. The organic material is decomposed into a carbon residue and volatile compounds diffuse out to atmosphere. The process is complex and several reactions may take place at the same time such as dehydrogenation, condensation and isomerization.
The carbon content of the residue is a function of nature of the precursor and the pyrolysis temperature. It usually exceeds 90 weight % at 900C and 99 weight % at 1300C.
The diffusion of the volatile compounds to the atmosphere is a critical step and must occur slowly to avoid disruption and rupture of the carbon network. As a result, carbonization is usually a slow process. Its duration may vary considerably, depending on the composition of the end-product, the type of precursor, the thickness of the material, and other factors. Some carbonization cycles, such as those used in the production of large electrodes or some carbon-carbon parts, last several weeks. Others are considerable shorter, such as the carbonization cycle to produce carbon fibers, since these fibers have a small cross-section and the diffusion path is short. The specifics of each cycle will be reviewed in more detail in the following chapters.
Characteristics of carbonized materials: After carbonization, the residual material is essentially all carbon. However, its structure has little graphitic order and consists of an aggregate of small crystallites, each formed of a few graphite layer planes with some degree of parallelism and usually with many imperfections. These crystallites are generally randomly oriented.
The carbonized material is called “amorphous” or “baked carbon”. It is without long-range crystalline order and the deviation of the interatomic distances of the carbon atoms is greater than 5% in both the basal plane and between planes, as determined by x-ray diffraction.
Amorphous carbon is hard, abrasion resistant, brittle, and has low therml- and electrical-conductivities. In a few cases, these characteristics are desirable and amorphous carbon is found in applications such as contacts, pantographs, current collectors and brushes for operation on flush mica commutators, as well as in special types of carbon-carbon composite.
In most instances however, amorphous carbon is only the intermediate stage in the manufacture of synthetic graphite products.