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

Abstract

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 (ηg) of some graphitizable and non-graphitizable carbon materials. ηg is the average interlayer spacing (d002) of a graphenic carbon following graphitization at ∼3400K. ηg can be estimated assuming a topological graphitization mechanism operating between 1700K and 3400K. Two independent variables define ηg: d002(Tα) and d002(Tβ). Tα and Tβ are arbitrarily selected temperatures between 1700K and 2550K (the graphitization threshold). In order to better understand the parameters affecting d002(Tα) and d002(Tβ), new carbonization/graphitization experimental results are presented. These suggest that d002(Tα) and d002(Tβ) are correlated to the oxygen/hydrogen composition ratio and the relative mesoscale crystallite orientation of some graphitizable carbons following the end of primary carbonization.