Graphitizing and non-graphitizing carbons

Graphitizing and non-graphitizing carbons (alternatively graphitizable and non-graphitizable carbon) are the two categories of carbon which are produced by pyrolysis of organic materials. They were first identified by Rosalind Franklin in a 1951 paper for Proceedings of the Royal Society . In this paper she defined graphitizing carbons as those which could be transformed into crystalline graphite by heating to a temperature of 3000°C, while non-graphitizing carbons could not be transformed into graphite at any temperature. Among the precursors which produced graphitizing carbon were polyvinyl chloride (PVC) and petroleum coke, while polyvinylidene chloride (PVDC) and sucrose produced non-graphitizing carbon. The physical properties of the two classes of carbons are quite different. Graphitizing carbons are soft and non-porous while non-graphitizing carbons are hard, low density materials. Non-graphitizing carbons are otherwise known as “chars”, “hard carbons” or, more colloquially, charcoal. Glassy carbon is also an example of non-graphitizing carbon.

The precursors for graphitizing carbons pass through a fluid stage during pyrolysis (carbonization). This fluidity facilitates the molecular mobility of the aromatic molecules, resulting in intermolecular dehydrogenative polymerization reactions to create aromatic, lamellar (disc-like) molecules. These “associate” to create a new liquid crystal phase, the so-called mesophase. A fluid phase is the dominant requirement for the production of graphitizable carbons.

Non-graphitizing carbons generally do not pass through a fluid stage during carbonization. A number of models have been put forward for their structure since the time of Franklin. Oberlin and colleagues have emphasised the role of basic structural units (BSU), made of planar aromatic structures consisting of less than 10-20 rings, with 4 layers or fewer. Cross-linking between the BSUs in non-graphitizing carbons prevents graphitization . More recently, models have been put forward which incorporate pentagons and other non-six-membered carbon rings .