โ Scribed by Jorge F.Rey Boero
No coin nor oath required. For personal study only.
Chang and Rusnak [1] have recently reported a study on the air oxidation behaviour of carbon-carbon composites. Reaction rates were thermogravimetrically measured on 5 mg samples of several materials powdered to sizes below 5 pm, at temperatures between 580 and 700ยฐC. Activation energies ranging between 35 and 44 kcal/mol were obtained, which supports zone I reaction conditions [2]. By changing sample weight, the authors showed that the reaction is, at 65O"C, free of interparticle diffusion limitations.
Oxidation rates were also measured on bulk samples with dimension 3.18 x 2.54 x 0.64 cm. In this case, the experiments were performed in a quartz tube furnace and the samples were periodically removed in order to measure the weight loss.
Although good Arrhenius plots are shown in the paper, the apparent activation energies determined from them, range between 10.5 and 29.5 kcal/mol. A value of 10.5 kcal/mol was obtained between 480 and 600ยฐC for sample A, which is the most reactive one. The authors claim that this low activation energy value is due to stagnant film diffusion limitations on the reaction.
It is quite unusual to find such a low value at the temperatures used by the authors. Moreover, it is hard to understand how external diffusion conditions could exist for a bulk sample, when the powdered sample was shown to react in zone 1 conditions and free of interparticle diffusion limitations at a higher temperature.
This note aims to point out that the experimental approach used is not rigorous enough and ignores two serious considerations which may invalidate completely the conclusions reached.
We have studied the oxidation of bulk carbon specimens with air and found [3] that, even for reaction rates lower than those used by Chang and Rusnak, special caution must be taken to keep constant the temperature of the sample. This is not surprising in view of the strongly exothermic nature of the C-O2 reaction. In consequence, the procedure of weighing before and after the oxidation in a tube furnace, does not seem to be adequate to determine kinetic parameters of the reaction.
However, as heat evolution increases with the reaction rate, heating of the sample would tend to increase the measured value of the activation energy. Therefore, despite the objections about their procedure on general grounds, the anomalous low value reported should be explained in terms of some other artifact.
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