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Title:Scale-bridging molecular dynamics simulations of plasma-surface interactions
Author(s):Harpale, Abhilash
Director of Research:Chew, Huck Beng
Doctoral Committee Chair(s):Chew, Huck Beng
Doctoral Committee Member(s):Levin, Deborah; Geubelle, Philippe H.; Johnson, Harley
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
molecular dynamics
atomic scale simulations
thermal protection system
Abstract:Plasma processing of materials plays a vital part in electronic, aerospace, automobile, metal manufacturing and biomedical industries. Plasma-surface interactions can be divided into two categories: controlled and erosive. Controlled plasma interactions are used to pattern the surfaces of materials to achieve desirable electronic and mechanical properties in an ionizing chamber. Examples include focused ion beam (FIB) milling of silicon thin films and plasma patterning of graphene. Erosive interactions typically involve damage to surfaces in close proximity to high temperature, ionized gases in an uncontrolled environment. Examples include plasma facing components in reactor vessels and thermal protection system in re-entry spacecraft. Due to the complex chemistry at the plasma surface boundaries, it is difficulty to either control or predict the effect of plasma on the patterning or erosion of the material. In this dissertation, atomic scale simulations coupled with micromechanics models are used to study the patterning of graphene and the ablation of thermal protection systems resulting from controlled and erosive plasma-surface interactions, respectively. Scalable and precise nanopatterning of graphene is an essential step for graphene-based device fabrication. Hydrogen-plasma reactions have been shown to narrow graphene only from the edges, or to selectively produce circular or hexagonal holes in the basal plane of graphene, but the underlying plasma-graphene chemistry is unknown. In part I of this dissertation, we characterize the surface patterning of graphene via low energy hydrogen plasma treatment across a range of ion energies, using scale-bridging molecular dynamic simulations. Our results uncover distinct etching mechanisms, operative within narrow ion energy windows, which explain the various plasma-graphene reactions observed experimentally. For monolayer graphene, specific ion energy ranges are demonstrated for stable isotropic (∼2 eV) versus anisotropic hole growth (∼20-30 eV) within the basal plane of graphene, as well as for pure edge etching (∼1 eV). For multilayered graphene, our results demonstrate the initial development of columnar holes, which transition to stepped-edge holes at higher fluence due to cumulative effects of basal-plane etching. The contributions of thermal radicals and dehydrogenation effects on the hole growth process are also discussed. In part II of this dissertation, multi-scale simulations are used to study the erosive role of high temperature plasma generated by the shock heating of ambient gases on the surfaces of high velocity atmospheric re-entry space craft. We specifically examine the ablation of AVCOAT, which is a composite thermal protection system (TPS) material attached to the leading edge of the Orion multi-purpose crew vehicle. Phenol formaldehyde resin used as the matrix material in AVCOAT is modeled and its pyrolysis kinetics are determined using ReaxFF-based molecular dynamics (MD) simulations. Our MD simulations show that bulk pyrolysis starts at temperatures of ∼500 K, and exhibits a temperature dependence that follows the Arrhenius law. The pyrolysis process initiates with the removal of -OH functional groups and -H atoms from aromatic C rings within the bulk phenolic resin to release H2O, followed by breaking of these C rings to release C-based fragments. Through the calculation of rate constants associated with C-C bond breaking during the pyrolysis process, we determine the effective surface recession rates of phenolic resin as a function of temperature. The surface recession rates from MD are used to inform a thermal material response model, capable of predicting the char thickness, temperature and gas blowing rates of AVCOAT TPS at macroscopic length-scales. Our model predictions of the char thickness and temperature distributions, under a variety of heat loads, are in good agreement with prior experiments.
Issue Date:2018-03-16
Rights Information:Copyright 2018 Abhilash Harpale
Date Available in IDEALS:2018-09-04
Date Deposited:2018-05

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