Geometric effects control isothermal oxidation of graphite flakes

Abstract

The oxidation, in a neat oxygen atmosphere, of high-purity and highly crystalline natural graphite and synthetic Kish graphite was investigated. The physico-geometric model function of the kinetic rate equation was experimentally determined by isothermal thermogravimetric analysis at 650 °C. Analytic solutions for basic flake shapes indicate that this function strictly decreases with conversion. However, for both samples the experimental data trend was a rapid initial increase followed by the expected decrease to zero. High resolution field emission scanning electron microscopy, of partially oxidized flakes, provided plausible explanations for this discrepancy. Rapid development of macroscopic surface roughness during the initial stages of oxidation was evident and could be attributed to the presence of catalytic impurities. Large fissures along the planes of the natural graphite and the initiation, growth and coalescence of internal cavities in the Kish graphite were observed. Flake models incorporating the latter two features are difficult to analyse analytically. However, a facile probabilistic approach showed that reasonably good agreement with experimental data was possible.