handbook of carbon, graphite, diamond and fullerenes

84 Carbon, Graphite, Diamond, and Fuilerenes
The kinetics of graphitization of cokes has been shown to be a growth
process and not a nucleation-based process with usually a single-valued
activation energy, reported at 962 ± 60 kJ/mol (230 ±15 kcal/mol) in the
2300 - 2900°C temperature range for typical industrial cokes.t 17 l
3.5 Graphitization of Chars
Graphitization of Aromatics. As mentioned above, not all aromatic
hydrocarbons form coke. Some, such as phenanthrene and biphenyl, do not
graphitize and are considered char formers. These compounds are branched
aromatics (as opposed to the linear structure of the coke-former aromatics)
with a preferred axis of growth as shown in Fig. 4.3. This characteristic prevents
the formation of extensive graphitic planes and of a liquid mesophaseJ 2 '
Graphitization of Aliphatics. Chars are produced by the carbonization
of aliphatic hydrocarbons (compounds with an open-ended chain) and
of most polymers. They do not graphitize readily. Their turbostratic
structure and the random arrangement of their crystallites (shown in Fig. 3.4
of Ch. 3) remain essentially unchanged, regardless of the temperature and
duration of the heat-treatment. However, in some cases the graphitization
of chars can be enhanced by a radiation treatment or by the presence of a
metallic or mineral catalyst.! 18 ' The catalytic process involves the dissolution
of carbon particles at the catalyst sites and the precipitation of graphite.
Graphitization of Polymers. As mentioned above, most polymers,
such as those listed in Table 4.1, are char-formers and do not graphitize to
any extent (with the notable exception of polyvinyl chloride), although some
reduction in the interlayer spacing (d) is usually observed. This d-spacing,
however, does not shrink below 0.344 nm for most polymers. This is shown
in Fig. 4.8, where the d-spacing of carbonized polyacrylonitrile (PAN) is
plotted as a function of graphitization temperature. The crystallite size (LJ
remains small, going from - 1.5 to - 2.8 nm.t 19 ' Pitch-based fiber, on the
other hand, are cokes and graphitize more readily (see Ch. 8).
During carbonization of PAN, an extensive-random network of graphitic
ribbons is formed which has a stable configuration. I n addition, it is speculated
that sometetrahedral (sp 3 ) (diamond) bonds are formed (which would account
for the hardness of these materials) J 2 ' The presence of diamond structure in
a similar material, diamond-like carbon, is well established (see Ch. 14). The
diamond structure is reviewed in Ch. 2, Sec. 3.0. These two factors, ribbon
network and tetrahedral bonds, would prevent further ordering of the structure,
regardless of the graphitization temperature.