Why some carbons may or may not graphitize? The point of view of thermodynamics

Not all carbons graphitize in equal measure. Some will develop a structure which approaches the one of perfect graphite (graphitizable carbons) upon heat treatment, while others will not (non-graphitizable carbons). The present work develops a phenomenological model for the conceptual understanding of graphitizability (capacity to graphitize). To support this model, a mathematical formalism, inspired from thermodynamics, is proposed to calculate the Ultimate Graphitizability (eta(g)) of some graphitizable and non-graphitizable carbon materials. eta(g) is the average interlayer spacing (d(002)) of a graphenic carbon following graphitization at similar to 3400 K. eta(g) can be estimated assuming a topological graphitization mechanism operating between 1700 K and 3400 K. Two independent variables define eta(g): d(002) (T alpha) and d(002) (T beta). T alpha and T beta are arbitrarily selected temperatures between 1700 K and 2550 K (the graphitization threshold). In order to better understand the parameters affecting d(002) (T-alpha) and d(002) (T-beta), new carbonization/ graphitization experimental results are presented. These suggest that d(002)(T-alpha) and d(002) (T-beta) are correlated to the oxygen/hydrogen composition ratio and the relative mesoscale crystallite orientation of some graphitizable carbons following the end of primary carbonization. (C) 2019 The Authors. Published by Elsevier Ltd.

Mots clés

transmission electron-microscopy; x-ray-diffraction; electrochemical properties; coke crystallites; phase-transitions; heat-treatment; carbonization; microtexture; sulfur; behavior