Predicting deformation mechanisms during high speed impact of Ag nanoparticles




Chitrakar, Tushar Vijay

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A number of aerosol deposition methods are currently used to produce thick films by impacting particles onto a substrate at high velocities. Though these processes operate at a similar range of velocities, there are significant differences in the sizes of the particles used in the aerosol. Conventional aerosol deposition methods deposit 0.1–40 μm sized-particles, whereas the laser ablation of microparticle aerosol process uses very fine 2–40 nm nanoparticles (NPs) to produce thick films. For particles smaller than 0.1 μm, deformation mechanisms that occur upon impact have not been studied previously in a systematic manner. In this dissertation, molecular dynamic simulations are used to study the time-evolution of deformation mechanisms that occur at very small timescales and high strain rates during high speed impact of Ag NPs. The defect evolution and the underlying mechanisms for deformation are systematically studied and documented by varying the NP size, the NP impact velocity, and the NP crystallographic orientation relative to the substrate. A wide range of microstructures ranging from polycrystalline to epitaxial morphologies are observed for these simulations. Because epitaxial deposition by particle impact has not been experimentally obtained, considerable attention is given to understanding the factors that are predicted to lead to epitaxy. Disordering is an important mechanism because it can play a role in epitaxial growth at high deposition velocities. A critical parameter is proposed to predict disordering that occurs upon impact. An alternative method to obtain epitaxial deposition at lower deposition velocities is also explored. The goal of this dissertation is to develop a thorough understanding of the available processing parameters for controlling the microstructure for a single NP deposition event. The impact studies in this dissertation provide fundamental guidelines needed to ultimately understand the formation of thick films where thousands of particles are impacted to produce a film.


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