Kinetic Catalysis Model for Silica Surface: Chemical Energy Accomodation

This report results from a contract tasking University of Rome as follows: Among the approaches to predicting surface recombination and oxidation there are three classes , from the simplest to the more fundamental: 1. empirical recombination rates, as obtained from experiments on single TPS materials, i.e., gamma = gamma(T), where gamma is the recombination probability, usually measured as a function of temperature, but that depends also on pressure. 2. Kinetic models, where recombinations (but also oxidation) are described through a series of kinetic steps (elementary reactions) among species present in the shock layer and the particular TPS material. 3. Ab-initio (semi-classical and Q.-M.) calculations, whereby a single species atom or molecule trajectory is simulated and the gas interface is reproduced as an atomic layer. Each class of models has many variants: for instance, kinetic models may use experimental data for the rates corresponding to each step, or deduce them from the surface potentials of each atom, as approximated in various ways; ab-initio calculations may rely on potentials surfaces obtained from Q.M. and using the Born-Oppenheim approximation, or others; and so on: currently, the family tree of recombination has grown to a respectable size. Hence the need for examining in some detail each class of models, to gauge their range of applicability, complexity of use (for instance, in aerothermal CFD codes), and potential for future development. Some of these issues have been in fact explored by this writer for an ESA TRP project, yielding valuable insights as to the future of efficient modeling. The original document contains color images.