Large Scale Computational Chemistry Modeling of the Oxidation of Highly Oriented Pyrolytic Graphite

Abstract

Large scale molecular dynamics (MD) simulations are performed to study the oxidation of highly oriented pyrolytic graphite (HOPG) by hyperthermal atomic oxygen beam (5 eV). Simulations are performed using the ReaxFF classical reactive force field. We present here additional evidence that this method accurately reproduces ab initio derived energies relevant to HOPG oxidation. HOPG is modeled as multilayer graphene and etch-pit formation and evolution is directly simulated through a large number of sequential atomic oxygen collisions. The simulations predict that an oxygen coverage is first established that acts as a precursor to carbon-removal reactions, which ultimately etch wide but shallow pits, as observed in experiments. In quantitative agreement with experiment, the simulations predict the most abundant product species to be O2 (via recombination reactions), followed by CO2, with CO as the least abundant product species. Although recombination occurs all over the graphene sheet, the carbon-removal reactions occur only about the edges of the etch pit. Through isolated defect analysis on small graphene models as well as trajectory analysis performed directly on the predicted etch pit, the activation energies for the dominant reaction mechanisms leading to O2, CO2, and CO product species are determined to be 0.3, 0.52, and 0.67 eV, respectively. Overall, the qualitative and quantitative agreement between MD simulation and experiment is very promising. Thus, the MD simulation approach and C/H/O ReaxFF parametrization may be useful for simulating high-temperature gas interactions with graphitic materials where the microstructure is more complex than HOPG.

Flow-Tube Oxidation Experiments on the Carbon Preform of a Phenolic-Impregnated Carbon Ablator

“…A closer analysis of the micrograph reveals that the pit size is ≈1-2 μm, roughly of the order of magnitude of 10 6 carbon atoms. The formation of pits was observed by several authors during previous experiments [32,33] and simulations [34] on graphite-based materials and discussed in detail in [35]. Oxidation reactions mainly occur at the location of a crystalline defect of the pseudographitic structure.…”

Inelastic and Reactive Scattering Dynamics of Hyperthermal O and O2 on Hot Vitreous Carbon Surfaces

“…27,34,35 In an earlier study in our laboratory on hyperthermal O-atom interactions with a 503 K HOPG surface, we attributed much of the O 2 signal to an Eley−Rideal reaction, 27 34 and we now agree that the earlier attribution of a significant fraction of the O 2 signal to an Eley−Rideal reaction was incorrect because a detailed consideration of the data for the observed nonthermal O and O 2 reveals that the scattering dynamics for these two products were similar and are best interpreted as impulsive scattering from a smooth surface. 50 Thus, we no longer believe that our earlier data support the computational results that the Eley−Rideal reaction to produce O 2 is the dominant reaction pathway under continuous bombardment of an HOPG surface by hyperthermal O atoms.…”

Burning Graphene Layer-by-Layer

“…All molecular dynamics simulations were carried out using the ReaxFF 45 force field as implemented on LAMMPS (large scale atomic molecular massive parallel simulator) 46 . The specific ReaxFF parametrization used during this work have been specifically tailored to describe combustion phenomena thus it is perfectly suited for the study of this problem 45 46 47 48 . Typical molecular dynamics considered a timestep of 0.1 fs and a total simulation time of ~150 ps.…”