Graphitizing and non-graphitizing carbons

Graphitizing and non-graphitizing carbons (alternatively graphitizable and non-graphitizable carbon) 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.[1]. 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 (PVC) and petroleum coke. Polyvinylidene chloride (PVDC) 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 (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 production of graphitizable carbons.[2]

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 (BSU), 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[3]. More recently, some have put forward models that incorporate pentagons and other non-six-membered carbon rings[4]

See also

References
  1. R.E. Franklin (1951). "Crystallite growth in graphitizing and non-graphitizing carbons". Proceedings of the Royal Society A. 209: 196–218. doi:10.1098/rspa.1951.0197.
  2. H. Marsh and M.A. Diez (1994) " Mesophase of Graphitizable Carbons" In: Shibaev V.P., Lam L. (eds) Liquid Crystalline and Mesomorphic Polymers. Springer, New York, NY doi: 10.1007/978-1-4613-8333-8_7
  3. A. Oberlin (1984). "Carbonization and graphitization". Carbon. 22: 521–541. doi:10.1016/0008-6223(84)90086-1.
  4. P.J.F. Harris (2013). "Fullerene-like models for microporous carbon". Journal of Materials Science. 48: 565–577. doi:10.1007/s10853-012-6788-1.
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