Atomistic-scale investigations of hyperthermal oxygen-graphene interactions via reactive molecular dynamics simulation: The gas effect

Abstract

Hyperthermal atomic oxygen (AO) bombardment to thermal protection system surface has been identified to impact the aerodynamic heating significantly, due to complex chemical reactions at the gas-solid interface, e.g., surface catalysis recombination, oxidation, and ablation. Previous investigations have focused on the surface effects of the AO collision process, while the influence of impacting gas characteristics remains unclear under various non-equilibrium aerodynamic conditions. This work conducts a reactive molecular dynamics (RMD) study of AO collisions over graphene surface, by considering the incoming gas at different translational energies (0.1 ≤ Ek ≤ 10 eV), incident angles (θ = 15°, 30°, 45°, 60°, 75°, and 90°), and O/O2 ratios (χO2 = 0.00, 0.25, 0.50, 0.75, and 1.00). The RMD results indicate that for AO normal incidence, the predominant reactive products of O2, CO, and CO2 molecules are produced due to the synergistic catalytic recombination and surface ablation reaction effects. A maximum recombination performance is identified around 5-eV AO incidence. For off-normal AO incidence, the recombination coefficient increases with the increase in incidence angle from 15° to 60° due to the larger perpendicular components of translational energy and then decreases smoothly. With the increase in O2 mole fraction, the surface reflection probabilities increase, which result in the decrease in both catalytic recombination and ablation activities. Via revealing the atomistic-scale mechanism of gas effects on the surface under hypersonic non-equilibrium conditions, this work sheds light for the future design and optimization of thermal protection materials.