Sriram Goverapet Srinivasan
Published: 2014
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The primary focus of this work was to study the chemistry and dynamics of hyperthermal collisions of oxygen atoms with carbon based materials of the kind witnessed in the Low Earth Orbit (LEO) environment and their pyrolysis through atomistic simulations using the ReaxFF reactive force field and to develop ReaxFF potentials for such applications. In particular, ReaxFF was used to study the oxidative erosion of graphene, graphite and diamond subjected to collisions with energetic oxygen atoms at elevated surface temperatures. Prior to these simulations, the ReaxFF C/H/O potential was validated against quantum chemical (QC) data for the energetics associated with the loss of a CO2 molecule from a model graphitic system and for various other chemical reactions occurring during the collision of a hyperthermal oxygen atom with a pristine and defective graphene sheet and a diamond slab. ReaxFF based simulations suggested that the breakup of a graphene sheet and graphite structure upon hyperthermal oxygen atom impact could be divided into distinct regimes. Graphene erosion proceeded through the formation of epoxides on the surface followed by the creation and growth of vacancy defects while the breakup of graphite occurred through the formation of epoxides on the top layer, creation and growth of vacancy defects on the top layer followed by epoxide formation on the bottom layer, creation of defects and their growth on the bottom layer. As such the breakup of graphite was observed to be a layer by layer event with the rate of growth of defects much larger along the basal plane directions compared to the axial direction. With increase in temperature, the rate of mass loss from graphite was observed to increase. While the impact of the oxygen atoms occurred at hyperthermal energies, the chemical reactions leading to mass loss from graphite were thermal in nature. Furthermore, molecular dynamics simulations of carbon loss from graphite at various surface temperatures upon hyperthermal oxygen atom collisions were used to obtain an Arrhenius type rate law for the carbon atom loss rate under such conditions. Further, the direction dependent etching properties of graphite exposed to hypothermal atomic oxygen collisions were also investigated. These simulations revealed that graphite basal planes are poorly resistant to energetic oxygen atom etching while the armchair and zigzag edge surfaces are an order of magnitude more resistant to energetic oxygen atom etching. To compare the response of diamond surfaces with graphite, energetic oxygen atom etching of low index diamond surfaces namely, diamond (100), diamond (111) and diamond (110) were carried out at various surface temperatures using the ReaxFF C/H/O potential. ReaxFF simulations on small oxygen terminated diamond slabs indicated that a variety of functional groups such as ethers, peroxides, oxy radicals and dioxetanes can form on the surface, in agreement with earlier experiments and first principles based calculations. Successive oxygen collisions on larger reconstructed diamond surfaces showed that all the low index surfaces can be etched by hyperthermal atomic oxygen with diamond (100) showing the lowest etching rate and diamond (110) presenting the largest etching rate. The erosion yield of these surfaces is in good agreement with experimental results. The simulations performed here have been used to obtain an Arrhenius type rate law for the mass loss from these surfaces under such conditions. Although diamond surfaces can be etched by energetic oxygen atoms, they were found to be more than two orders of magnitude more resistant to oxidative erosion as compared to graphite basal planes. These simulations suggest that diamond thin films are promising materials for the surface of space crafts exposed to LEO conditions and in general, the ability of ReaxFF to be used as an effective tool to screen or characterize materials for applications in extreme environments.In order to study the interaction of hyperthermal atomic oxygen with silica surfaces, a widely used material for the thermal protection system of high speed aircrafts, the ReaxFFSiO potential was extended to describe oxygen -- silica gas surface interactions by harvesting model clusters representative of a reconstructed (001) silica surface and surface defects on silica, obtaining density functional theory (DFT) based potential energy curves for the approach of an atomic and molecular oxygen to these clusters followed by re-parametrization of the ReaxFFSiO potential against this data. The new potential, ReaxFF-SiO/GSI, can be employed for accurate molecular dynamics simulations of oxygen -- silica gas surface interactions.The thermal fragmentation of a large fullerene molecule was studied through molecular dynamics simulations in order to understand the mechanisms underlying the pyrolysis of carbon based materials. While the performance of the ReaxFF C/H/O potential for the chemistry of graphite and diamond oxidation was very good, its description of the mechanical deformation of carbon condensed phases was not satisfactory. Thus ReaxFF C/H/O was re-parameterized against DFT data for the equation of state of graphite, diamond, the formation energies of defects in graphene and amorphous carbon phases from fullerenes. The newly developed ReaxFF potential (ReaxFFC-2013) was used in the molecular dynamics simulation of the thermal fragmentation of a C180 molecule. The simulations indicated that the thermal fragmentation of these giant fullerenes can be classified into two distinct regimes -- an exponential regime followed by a linear regime. In the initial exponential regime, the molecule shrinks in size but retains the cage like structure while in the final linear regime, the cage opens up into an amorphous phase, resulting in an acceleration of the decay process. Arrhenius parameters for the decay of the molecule in both the regimes were obtained by carrying out simulations at various temperatures. While the decay of the molecule occurred primarily via the loss of C2 units, with increase in temperature, the probability of loss of larger fragments was found to increase. The newly developed potential along with the methods used in this study can readily be extended towards the full computational chemical modeling of the high temperature erosion of graphitic rocket nozzles and ablation of carbon based spacecraft materials during atmospheric reentry. Finally, to explore the possibility of developing carbon based materials resistant to oxidative erosion through the impact of hyperthermal oxygen atoms, oxygen interaction with boron doped graphene was considered. Model clusters representative of boron doped graphene were used to obtain DFT based potential energy curves for the approach of an atomic oxygen to these clusters. This dataset can now be used to parameterize ReaxFF to describe oxygen -- boron doped graphene gas surface interactions.The research work reported in this dissertation lays out a clear strategy to develop a ReaxFF reactive potential and to apply it to study the oxidative degradation and pyrolysis of materials subjected to extreme conditions. Further it provides a straightforward way to extract Arrhenius type parameters from molecular dynamics simulations for the erosion of materials under such conditions. These parameters can be used directly in mesoscale simulation schemes such as Direct Simulation Monte Carlo (DSMC), thereby providing the vital link between atomic scale and macro scale in bottoms up materials design approach.
