High-Temperature Oxidation of SiC-Based Composite: Rate Constant Calculation from ReaxFF MD Simulations, Part II

Abstract

Space vehicles often encounter very high temperature and harsh oxidative environments. To ensure proper thermal protection, layers composed of SiC and EPDM polymer are placed on the outer surface of the space vehicle. The O 2 and H 2 O molecules are able to oxidize the SiC network, creating SiO 2 -type structures that may form a protective layer, while also pyrolyzing and burning the EPDM polymer, causing ablation. Reactive molecular dynamics simulations nicely complement experiment, as they provide direct observation and information to calculate physical parameters such as transport diffusivities and reaction constants. In this study, rate models were developed and molecular dynamics simulated trajectories were used to extract Arrhenius parameters that describe the initial stages of transport and kinetics of SiC oxidation by O 2 and H 2 O and the combustion and pyrolysis of EPDM. The simulations showed that O 2 was able to oxidize SiC more efficiently than H 2 O, with the transport activation barrier of O 2 in the range of 40−70 kJ/mol, and for H 2 O at 125−150 kJ/mol. The oxidizer molecules created a continuous surface of SiO 2 that grew on top of the SiC layer. The O atoms insert themselves in between the Si−C bonds, causing the C atoms to migrate into a carbonaceous phase in the center of the simulation box. A model of the EPDM polymer was used for the combustion and pyrolysis simulations. An Arrhenius analysis gave activation barriers of 183 kJ/mol for combustion and 213 kJ/mol for pyrolysis. By comparison, experimental observations of EPDM and similar polymers conclude a range of ∼100−250 kJ/mol for both combustion and pyrolysis, indicating that ReaxFF can give reasonable quantitative predictions for these thermal studies, demonstrating the possibility of direct calculation of Arrhenius parameters from reactive molecular dynamics simulations.

Covalent Bonding and Atomic-Level Plasticity Increase Adhesion in Silicon–Diamond Nanocontacts

“…The simulations were carried out at 300 K using the MD code LAMMPS . Atomic interactions were calculated using the ReaxFF interatomic potential parametrized for Si, C, and H interactions. − One of the primary advantages of the ReaxFF potential is its ability to model reactive chemistry with similar accuracy as quantum mechanical calculations but with a much lower computational cost. A typical simulation starts with the lowest atom in the tip set at 0.5 nm above the highest atom on the substrate.…”

ReaxFF molecular dynamics simulations of CO collisions on an O-preadsorbed silica surface

“…The ReaxFF has been parameterized and tested for a great variety of processes: chemical reactions on solid surfaces (e.g., dissociation of water on titania [23,24], hyperthermal oxidation of Si(100) by O and O2 [25,26,27,28],..), gas-phase reactions (e.g., n-heptane pyrolysis [29], oxidation of toluene [30],..), aqueous reactions (e.g., aqueous chloride and cooper chloride/water systems [31],..), reactions in zeolites (e.g., methanol to olefin reactions [32], confined reactive water in minerals [33],..), etc. In particular, there are studies about the oxidation of silica by O and O2 [7,34,35] and oxidation of silicon carbide by O2 and H2O [36,37], which are very related with the present work (i.e., CO/O/SiO2 system).…”

An extended-Lagrangian scheme for charge equilibration in reactive molecular dynamics simulations

“…We test our extended Lagrangian QEq scheme and the parallel XRMD code using oxidation of a silicon carbide nanoparticle (n-SiC) as an example. We adopt ReaxFF parameterization by Newsome et al [36,37]. A n-SiC composed of 25 silicon (Si) and 25 carbon (C) atoms is placed in oxygen environment.…”