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Understanding Deformation Mechanisms in Architectured Nanomaterials by Predictive Modeling and Simulations
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Architectured materials with interconnected nanoscale framework of open cells have shown great potential applications as novel energy convertors and biological implants. Among all the nanolattice architectures, nanoporous gold (np-Au) and three-dimensional graphene honeycomb (3D-GH) structure have arose most interest from researchers and engineers due to their low density, high ultimate strain and high stress. However, lacking comprehensive understanding of their morphologies and mechanical properties hinders their broad applications, so studies on the mechanical properties, especially the relationship between their nanoscale structures and their mechanical behaviors, are crucial to their engineering applications. We focus on the effects of lattice defects, brought by different fabrication procedures, on np-Au mechanical properties and orientation-dependence of 3D-GH deformation behaviors. For np-Au, we investigate the effect of lattice defects on plastic Poisson’s ratio of np-Au using molecular dynamics simulations. We create atomistic np-Au models using two methods: the annealing method and the spinodal decomposition method. Molecular dynamics (MD) simulations of the annealing process obtain high densities of defects in the modeled sample while spinodal decomposition acquire low densities of lattice defects. Embedded Atom method Potential is employed to describe the pairwise interactions among molecules. MD simulations of uniaxial compressions show that the highly defective np-Au has elastic compliance and early yielding, but exhibits more significant hardening than the low defective one at plastic region. Lattice defects can amalgamate, impeding the formation slipping planes. The introduction of lattice defects into np-Au also decrease effective Poisson’s ratio and enhance tension-compression asymmetry. Uniaxial compression of np-Au samples that have the same porous morphology but distinct defect densities shows that a high density of defects results in lower values of plastic Poisson’s ratio. We elucidate this observation by linking the nanoscale tension-compression asymmetry in plasticity to the synergistic interplay between defects and surface from an energy perspective.To study mechanical properties of 3D-GH, we develop an algorithm to construct zigzag-bonded sp2sp3 3D-GH structures, which is theoretically the most stable 3D-GH structures. We investigate size-dependent deformation of 3D-GH structures using MD simulations on uniaxial compressions along in-plane armchair directions, in-plane zigzag directions and out-of-plane directions. The potential we employ in our simulations to describe carbon atom interactions is the Adaptive Intermolecular Reactive Empirical bond Order, which is a combination of reactive empirical bond-order potential of covalent bonding, and non-bonded potentials and torsion. All have shown that 3D-GH structure is a highly elastic material and the effective Young’s modulus of 3D-GH decreases with the increase of cell size. Mechanical properties and deformation behaviors of the zigzag-bonded sp2sp3 3D-GH structure are highly anisotropic: effective Young’s moduli and yield along the out-of-plane direction are significantly higher than in-plane values. Theoretic predictions at continuum level can well predict the deformation patterns of MD simulations at nanoscale. However, the theoretic predictions of stresses at continuum level are much lower than MD simulation results because of the stronger chemical bonds in 3D-GH made between molecules at atomic level.This dissertation demonstrates the effects of defects and arrangements of the building blocks at nanoscale on the associated energy evolution and the macroscopic mechanical behaviors of architectured nanomaterials. The results indicate that controlling nanoscale morphologies and orientations of building blocks in architectured nanomaterials might be useful for tuning their mechanical behaviors.