( ommun ca, ons Oxidation of Pyrolytic Carbon 1000 ~ to 1S00~ and Oxygen Pressures of 2 to 38 Torr

In a recent paper (1) we presented a study of the oxidation characterist ics of spectroscopic grade graphite having a density of 1.6. Below 800 ~ a chemical control led react ion mechanism was found. Above 800 ~ the react ion was l imited by either a chemical control led process or gas diffusion con-trol led process. The t rans i t ion between the two mechanisms depends on the pressure, sample size, and the nature of the reaction system. Pyrolyt ic carbon (2), hav ing a density of about 2.1, is a more dense form of carbon than the spec-troscopic grade graphites and is h ighly anisotropic. Pyrolyt ic carbon is near ly impermeable to gases, and its oxidation rate is said to be less than that of spectroscopic grade graphites by a factor of 1/3 to 1/10 (2). Theoret ical ly the oxidation rates should depend on the porosity and permeabi l i ty if the in-ternal surfaces of the graphite part ic ipate in the reaction. This difference in oxidation rates should not exist in the gas diffusion control led region of oxidation. This paper shows that pyrolyt ic and a spectro-scopic grade of graphite oxidize with near ly iden-tical rates in the gas diffusion region of the oxida-t ion process. The pyrolyt ic graphite has the fol lowing analyse