Graphitizing and non-graphitizing carbons


Graphitizing and non-graphitizing carbons are the two categories of carbon produced by pyrolysis of organic materials. Rosalind Franklin first identified them in a 1951 paper in Proceedings of the Royal Society.. In this paper, she defined graphitizing carbons as those that can transform into crystalline graphite by being heated to 3000°C, while non-graphitizing carbons don't transform into graphite at any temperature. Precursors that produce graphitizing carbon include polyvinyl chloride and petroleum coke. Polyvinylidene chloride and sucrose produce non-graphitizing carbon. 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. This fluidity facilitates the molecular mobility of the aromatic molecules, resulting in intermolecular dehydrogenative polymerization reactions to create aromatic, lamellar molecules. These “associate” to create a new liquid crystal phase, the so-called mesophase. A fluid phase is the dominant requirement for production of graphitizable carbons.
Non-graphitizing carbons generally do not pass through a fluid stage during carbonization. Since the time of Rosalind Franklin, researchers have put forward a number of models for their structure. Oberlin and colleagues emphasised the role of basic structural units, made of planar aromatic structures consisting of less than 10-20 rings, with four layers or fewer. Cross-linking between the BSUs in non-graphitizing carbons prevents graphitization. More recently, some have put forward models that incorporate pentagons and other non-six-membered carbon rings