Thermodynamic description of graphitizable carbons and the irreversible graphitization process

Abstract

The present paper develops a thermodynamic model for the calculation of the Gibbs energy of graphitizable carbons near the graphitization temperature threshold of 2550K. The graphitization threshold is the minimal heat treatment temperature for the establishment of true graphitic ordering in a graphitizable carbon. The present model represents the structure of graphitizable carbons with a simplified cluster approach. In the model, two distinct clusters share the totality of the carbon atoms in the material: Oberlin's Local Molecular Orientation clusters (LMOs) and mesoscale grain boundary clusters (mGBs). The ability of a carbon material to graphitize is proportional to the size of the LMOs (larger LMOs = more graphitizable). Strongly graphitizing carbons have a large average size for their LMOs. Thus, for these carbons, the fraction of carbon atoms in mGBs can be neglected relative to the one in LMOs. In a LMO, two phases are present in pseudo-thermodynamic equilibrium: the intercrystalline matter phase (IMP) and the coke crystallite phase (CCP). If the average crystallite diameter (La) of the CCP is sufficiently large, the model assumes that the number of carbon atoms in the IMP relative to the CCP is negligible. The paper presents an additive scheme to calculate the Gibbs energy of carbon atoms in a graphitizable carbon based on the present cluster description. The current Gibbs energy model accounts for the contribution of intercrystalline matter removal by the irreversible densification of LMOs up to the graphitization temperature threshold of 2550K.