The mechanism of graphitization of pyrolytic graphite is essentially the same as that of pitch coke, described in other articles.
Graphitization of columnar and laminar deposites: The columnar and laminar deposits described above have generally a turbosratic structure in the as-deposited condition, with a large interlayer spacing as revealed by x-ray diffraction. The material graphitizes readily when heat-treated at 2500C for 90 minutes.
The 2500C heat-treatment causes the reordering of the structure. The basal planes coalesce and become more parallel and closer together. The various crystallite imperfections such as vacancies, stacking faults, dislocations, and rotational disorders, tend to heal and disappear; the crystallite size increases; the 002 line narrows considerably and becomes close to the position of the ideal graphite line as the interlayer spacing decreases to approach that of the ideal graphite crystal. This observed reduction of the interlayer spacing is attributed in part to the removal of interstitial elements, mostly carbon.
When columnar or laminar pyrolytic graphites are annealed above 2700C, usually under a pressure of several atmospheres, further ordering and stress relieving of the structure occur within each plane and between planes. The material is known as “highly oriented pyrolytic graphite” (HOPG). It is soft and structurally close to the ideal graphite crystal with an angular spread of the c axes of crystallites of less than on degree.
Graphitization of isotropic deposits: Unlike columnar and laminar pyrolytic deposits, isotropic carbon does not graphitize readily and is, in this respect, similar to vitreous carbon. Some reduction in the interlayer spacing is usually observed, but rarely does it decreases below 0.344nm. The crystallite size remains small.