The Department of Chemistry invites you to its departmental seminar today at 4:10 p.m. in Fulmer Hall, room 201.
Dr. Timothy K. Minton from the Department of Chemistry and Biochemistry at Montana State University will present, Oxidation of Carbon at High Temperatures, Studied by Atomic Beam-Surface Scattering.
Abstract: Gas-surface interactions at high temperatures are of great importance to atmospheric re-entry of spacecraft. Hypersonic flows generate the most extreme thermal conditions experienced by any flight vehicle. For a flight vehicle to survive in this environment, it requires a thermal protection system (TPS) composed of materials that can function at extreme temperatures under harsh oxidizing conditions. Most TPS materials are based on carbon, either in pure form or in a composite. During atmospheric re-entry, these materials are exposed to partially oxidized air at temperatures that can exceed 2000 K. The fundamental reactive and non-reactive dynamics between carbon and atomic or molecular oxygen strongly impact the thermal load on these TPS materials, but they have not been studied in extreme environments such as re-entry.
Hyperthermal interactions of ground-state atomic oxygen, O(3P), with both highly oriented pyrolytic carbon (HOPG) and vitreous (or glassy) carbon surfaces were investigated with a broad range of surface temperatures from 600 K to approximately 2200 K. Beams of 5 eV O atoms were directed at surfaces, and angular and translational energy distributions were obtained for inelastically and reactively scattered products using a rotatable mass spectrometer detector. Inelastically scattered O atoms exhibited both thermal and non-thermal components. The inelastic scattering from HOPG showed mostly nonthermal (impulsive) scattering with very sharp and superspecular angular distributions. In contrast, thermal scattering was much more important on the vitreous carbon surface and the angular distributions for impulsive scattering were broader. Surprisingly, an increasing fraction of inelastically scattered O atoms was observed on both HOPG and vitreous carbon as the surface temperature was increased, which corresponded to a significant increase in thermally scattered O atoms and a concomitant shift in the angular distribution of the scattered atoms toward the surface normal. For both surfaces, CO and CO2 were produced at lower temperatures, and CO2 disappeared above 1100 K. The primary reaction product (CO) was formed through direct (nonthermal) and indirect (thermal) mechanisms, where the thermally desorbed products apparently needed to surmount a barrier before desorbing. The flux of CO reached a maximum at surface temperatures between 1500 and 1900 K, depending on heating rate, and decreased with increasing temperature. Similar non-Arrhenius behavior was observed decades ago in the oxidation of carbon with thermal O and O2, but we have explained it for the first time. The increasing thermal desorption of O atoms with temperature signifies a decrease in surface oxygen coverage, and with fewer reagent O atoms to react with carbon the reactivity of the carbon surface is limited even though the surface is being constantly bombarded with highly reactive (hyperthermal) O atoms.
