handbook of carbon, graphite, diamond and fullerenes

Carbonization and Graphitization 73
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 as described in Ch. 3, Sec. 2.0 and shown in Fig. 3.4.
The carbonized material is often called "amorphous" or "baked carbon".
It is without long-range crystalline order and the deviation of the
interatomic distances of the carbon atoms (from the perfect graphite crystal)
is greater than 5% in both the basal plane (ab directions) and between
planes (c direction), as determined by x-ray diffraction.
Amorphous carbon is hard, abrasion resistant, brittle, and has low
thermal- and electrical-conductivities. In afew cases, these characteristics
are desirable and amorphous carbon is found in applications such as
contacts, pantographs, current collectors and brushes for operation onflush
mica commutators, as well as in special types of carbon-carbon J 1]
In most instances however, amorphous carbon is only the intermediate
stage in the manufacture of synthetic graphite products.
2.2 Precursor Materials and Their Carbon Yield
The carbon yield is defined as the ratio of the weight of the
carbon residue after carbonization to the weight of the material prior to
carbonization.
Typical carbon yields of common and potential precursor materials are
shown in Table 4.1 .PJPJ These yields are not fixed but depend to a great
degree on the heating rate, the composition of the atmosphere, the
pressure, and other factors (see below). The nature of the carbon yield,
given in the last column, i.e., coke or char, is reviewed in the following
section on graphitization.
Effect of Pressure on Carbon Yield and Structure: The nature and
the length of the carbonization cycle are important factors in controlling the
carbon yield. For instance, the effect of gas pressure can be considerable.
Figure 4.1 shows this effect on the yield of three grades of coal-tar pitch, with
various softening points. 141 In this particular case, high pressure more than
doubles the yield. Pressure can also modify the structure of the resulting
carbon and change its graphitization characteristics.I 5 1