Motivation
When a high-temperature gas comes in contact with a solid surface, in addition to the convective/conductive heat flux from the gas to the surface, surface reactions may release further chemical energy to the surface and possibly degrade the surface material. Understanding gas-surface chemistry is important for the design of thermal protection systems (TPS) for hypersonic vehicles and spacecraft. High gas temperatures in the shock layer generated by a hypersonic vehicle dissociate molecules in the gas and the reactive atomic species can diffuse through the boundary layer and react with the TPS surface. As an example, the energy released by the heterogeneous recombination (surface catalysis) of atomic oxygen into molecular oxygen, can contribute significantly to the overall heat flux and heat load experienced by a hypersonic vehicle. CFD surface reaction models (surface boundary conditions) have advanced to a point where elementary gas-surface reaction mechanisms are specified along with steric factors and activation energies for each reaction. Thus, the parameters required for CFD models (and DSMC models) are directly linked to fundamental chemistry. Most often, experiments are unable to determine precise reaction mechanisms and activation energies, and so our research attempts to use computational chemistry (molecular dynamics) to determine or refine these model inputs.
Unlike gas-phase collisions (involving on a few atoms per collision), gas-surface interactions require the simulation of large atomic systems; hundreds to thousands of atoms at least. For many of our problems we use the ReaxFF interatomic potential and we collaborate closely with Prof. Adri van Duin at Penn State University. ReaxFF is a state-of-the-art potential that is fit to quantum chemistry single-point energy data (for a specific problem). The functional form of the fit enables natural bond-breaking and bond-formation (chemical reactions) with a computationally efficient empirical potential. This allows accurate molecular simulation of large gas-surface systems. Our research focuses on validating (and in most cases improving) the potential for a specific gas-surface interaction of interest, predicting realistic surface morphology (specifically surface defect chemistries), and finally predicting gas-surface interactions on such realistic surfaces. Our research started with oxygen-platinum systems where a significant amount of high-quality experimental data was available. Research now focuses on oxygen-silica systems (ceramics) important for a large number of real TPS materials used on hypersonic vehicles.
Results and Publications