In order to gain a better understanding the gas-surface reactions on heat shields during hypersonic flight through air, we have used molecular beam-surface scattering experiments to study reactions of O and N atoms on high-temperature carbon surfaces. The experiments were performed with both pulsed and continuous molecular beams of O or N atoms and, in some cases, mixed beams containing O and N atoms. The reactive scattering dynamics of O on various carbon surfaces suggest that the oxidation mechanisms on all sp2 types of carbon are similar but that surface morphology influences the relative importance of the individual mechanisms. All products exhibited the dynamical characteristics of thermal desorption. The efficiencies of the gas-surface interactions, both reactive and non-reactive, were quantified as a function of surface temperature. In addition to reacting with carbon to produce CO2 (minor product) and CO (major product), oxygen atoms may recombine on the surface to produce O2 with an efficiency that is somewhat lower than that to produce CO. Nitrogen atoms may recombine on the surface to produce N2 or react to produce CN. The recombination efficiency of N atoms is generally more than an order of magnitude higher than the reaction efficiency to produce CN. Even a small percentage of N atoms in the presence of O atoms can increase the reactivity of O atoms on a carbon surface by more than 50%. The data from these studies have been used in the formulation of oxygen-carbon ablation models.

Molecular beam studies of O and N reactions on carbon at high temperatures

ACS Spring Meeting 2021

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