The oxidation of graphite I. The observation of oxidation on synthetic graphite before and after irradiation

Abstract

Ultra-thin sections of a synthetic graphite have been examined in the electron microscope before and after oxidation in air at a temperature of 500 °C. In unirradiated material, micrographs of individual areas showed that oxidative attack occurred at grain boundaries and at pores between individual crystallites. Parallel experiments on material which had been irradiated to a dose of 8.7 x 10 19 n .v .t. again showed etching at pores and at grain boundaries, but, in addition, in the majority of specimens, a shallow surface pitting of the entire (0001) faces of the individual crystallites took place. The density of these shallow pits varied from 5 x 10 10 /cm 2 to 5 x 10 u /cm 2 . Variations in reaction rate were observed and were traced to impurity which, especially when metallic, greatly increased the oxidation rate. In unirradiated graphite, this impurity appeared to be absorbed only at pores and intercrystallite boundaries, oxidation effects being observed only at these sites. In irradiated material impurity was also absorbed at sites on the (0001) crystal faces where oxidation produced etch pits of widely varying depth and density. Channelling of the (0001) layer nets occurred in both irradiated and unirradiated specimens. Annealing of the irradiated graphite at 650 °C for 1 h produced no significant change in the pattern of attack by oxygen. The observed etch pit density measured on the (0001) layer nets was 8 x 10 10 cm 2 . A mechanism for pit formation relating pit density and vacancy content is proposed. The general appearance of irradiated samples differed from the unirradiated in that the well-defined moire patterns characteristic of thin sections of synthetic graphite were absent in irradiated samples, but reappeared when the samples were annealed at 650 °C. This change is related to the known behaviour of interstitial atoms in graphite on annealing the materials,