Graphitization Mechanism of Cokes

Graphitization occurs in a series of steps which begins as the increasing temperature passes the carbonization temperature, i.e., 1200°CJ 1 Hydrogen, sulfur, and other elements, which might still be present after carbonization, are gradually removed and, as the temperature reaches 2000°C, essentially none remciinsJ ) 

Above 1800°C, the conversbn from a turbostratic structure to a graphitic structure (shown in Figs. 3.4 and 3.2 of Ch. 3) begins slowly at first then more rapidly as the temperature passes 2200°C. The gradual graphitization of the structure is readily confirmed by x-ray diffraction (Fig. 4.5). 

The crystallite size (LJ increases from 5 nm, which is atypical size for turbostratic crystallites, to approximately 100 nm or more. At the same time, the interlayer spacing (d) is reduced from 0.344 nm to a minimum of 0.335 nm, which is the spacing of the graphite crystal. 

Graphitization is accompanied by a weight loss, which is attributed to the removal of interlayer chemical species, mostly interstitieil carbon.1 Most graphitizable materials (cokes) require a temperature of 3000°C in order to reach full graphitization with a minimum value of the interlayer spacing, as shown in Fig. 4.6 and Also shown in Fig. 4.7 is the 

The process of graphitization can be accelerated by the presence of a metal catalyst or an oxidizing gas. In the latter case, obstacles to the ordering of the graphite layers, such is the more structurally disordered regions and the cross-linking bonds, are preferentially oxidized. 

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