Oxidation of compression annealed pyrolytic graphites

Abstract

The reactivities of three highly crystalline, compression annealed pyrolytic graphites, designated APG142, G900, and G100 and differing in preferred orientation, crystallite size and defect content, were studied as a function of burn-off at 750°C and 20 Torr O 2 pressure. Large burn-off increments were achieved by oxidation at 900°C and about 30 Torr initial O 2 pressure. Measurements of bulk reaction anisotropy of the two principal surfaces (basal and edge) were found to correlate well with degree of preferred orientation and defect content. For two of the graphites, rate data taken on samples having wide variations in their basal surface to edge surface ratio could be normalized by expressing the rates in terms of an effective geometric edge area which takes into account the reaction anisotropy. However, the reactivity of APG142 samples, expressed in this same manner, was found to increase with increasing fraction of edge area, indicating a different mechanism for edge oxidation. Microscopic and macroscopic examination of oxidized specimens revealed a number of features including a high density (~ 10 5/cm 2) of non-basal defects in the G100 material. A number of samples of APG142 and G900 showed a pronounced terracing effect at the layer plane edges. This is consistent with a mechanism of edge oxidation involving the migration of reactive species over the basal surfaces. A principal feature observed in all these materials was the occurrence of non-uniform oxidation at the edges, resulting in the formation of slit shaped voids running parallel to the layer plane direction. The possible role of lattice vacancies, non-basal dislocations and grain boundaries in the overall oxidation process has been examined by developing idealized models involving preferential attack at these defect sites.