Graphitization of Chars

Graphitization of Aromatics. As mentioned above, not ail 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 axisof growth asshown in Fig. 4.3. Thischaracteristic prevents the formation of extensive graphitic planes and of a liquid mesophase.l l 

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 ofthe 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.1 1 The catalytic process involves the dissolution of carbon particles at the catalyst sites and the precipitation of greiphite. 

During carbonization of PAN, an extensive-random network of graphitic ribbons is formed which heis a stable configuration. I n addition, it is speculated that some tetrahedral (sp (diamond) bonds are formed (which would account for the hardness of these materials). 1 1 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 preventfurther ordering ofthe staicture, regardless of the graphitization temperature. 

Rgure 4.8. Basd plane spacing of PAN-based and pitch-based fibers as afunction of graphitization temperature. l 

Some polymeric films such as polyimide (Kapton), polyacrylonitrile (PAN) and polyfurfural alcohol, when carbonized between sheets of the mineral montmoiilionite, form a two-dimensional graphitic structure with highly oriented layer planes which graphitizes readily 

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