Practical aspects for characterizing air oxidation of graphite

Characterization of Porosity Development in Oxidized Graphite using Automated Image Analysis Techniques

“…Although based on several simplifying approximations, the analytical method predicts the development of non-uniform oxidation profiles by oxidation with water vapor or oxygen, their time evolution to a steady state condition, and the effect of temperature. The oxidation depths calculated in this simplified model were in fair agreement with our previously reported data from direct density measurements [10].…”

“…The results and observations reported here confirm the usefulness of optical microscopy enhanced with AIA technique for observation of oxidant penetration during oxidation at various conditions, and of the distribution of oxidized zones between the surface and the bulk regions of graphite specimens. Although on different grades of nuclear graphite, the results reported here for PCEA graphite correlate well with previous observations from the ORNL group [10] on development of non-uniform density profiles in oxidized specialty grade graphite (R4-650). The results are also in very good agreement with recent theoretical estimates advanced by ORNL researchers [13,14] on penetration depth of the oxidant gas at quasi-steady state oxidation conditions.…”

“…Using direct density measurements (by layer peeling and physical measurements) the density profile of graphite specimens was measured after oxidation in air at various temperatures but at equal weight loss [10]. A larger than expected effect of oxidation conditions (temperature) was found on the density profile of oxidized samples.…”

“…Oxidation is controlled by the kinetics of the chemical process at low temperatures, but becomes diffusion-limited at high temperatures, and is strictly limited to mass transfer in the boundary layer at very high temperatures [9]. The temperature ranges specific to each of these mechanisms are not well defined [10] and are affected by sample's size and geometry [11]. When the rates of oxidation are slower than those of in-pore diffusion, oxidation is under kinetic control, and the oxidant penetrates deeper into the bulk of material, leading to (almost) uniform oxidation of specimens.…”

Note on Graphite Oxidation by Oxygen and Moisture

“…In contrast, Contescu (2008) reports much shallower oxidation depths, approximately 1 mm at 1000K for a typical structural graphite oxidized in air. Such a shallow depth is representative of a much lower D eff value than expected, approximately m = 0.1%.…”