Browsing by Subject "Nanomechanics"

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    The Influence of 3D Interfaces on Mechanical Behavior of Nanolaminated Bimetallic Composites
    (2024-01) Cheng, Justin
    Cu/Nb nanolaminates containing 3D interfaces (3D Cu/Nb) are used in this study to demonstrate the effects of controlled interface structure on mechanical behavior and unit deformation activity in nanostructured alloys. 3D interfaces are internal boundaries that exist on length scales relevant to unit deformation mechanisms and contain nanoscale chemical and crystallographic heterogeneities in all spatial dimensions. 3D interfaces are a new method to manipulate alloy microstructure whose effects on plastic deformation have not been previously explored in depth. Elucidation of the link between 3D interface structure and mechanical behavior will provide key insights into nanoscale metallic deformation allowing for materials that exhibit near-theoretical strengths while also being highly deformable. The exploration of these themes requires understanding of a wide range of topics in physical metallurgy, which is reflected in the structure of this thesis. Chapter 1 begins with a high-level overview of the motivation and methodology of this work. Chapters 2 introduces fundamental concepts of metallic deformation at the macroscale and the atomic scale. Chapter 3 explores the participation and influence of interfaces in atomic scale deformation and ties the nanoscale to the mesoscale by discussing previous findings about atomically sharp 2D interfaces on nanocrystalline alloy mechanical behavior. Chapter 4 introduces the experimental methods required to characterize 3D interfaces structurally and mechanically. Chapter 5 presents structural characterization results, while Chapter 6 presents mechanical characterization results. Chapter 6 contains findings from mechanical testing, while also providing discussion connecting 3D interface structure detailed in Chapter 5 to observed 3D Cu/Nb mechanical behavior. The information from these techniques are crucial to forming structure-behavior relationships detailing the effect of 3D interfaces on unit deformation, but they cannot probe the atomic scale alone, so synthesis of computational results with experimental results is also discussed in Chapter 6. Chapter 7 concludes with a summary of key findings of this and proposes future work addressing new scientific issues raised by this work.
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    Nanoscale mechanics of helical and angular structures: exploring and expanding the capabilities of objective molecular dynamics
    (2014-06) Nikiforov, Ilia Andreyevich
    Objective molecular dynamics (OMD) is a recently developed generalization of the traditionally employed periodic boundary conditions (PBC) used in atomistic simulations. OMD allows for helical and/or rotational symmetries to be exploited in addition to translational symmetry. These symmetries are especially prevalent in nanostructures, and OMD enables or facilitates many simulations that were previously dicult or impossible to carry out. This includes simulations of pristine structures that inherently possess helical and/or angular symmetries (such as nanotubes), structures that contain defects (such as screw disclocations) or stuctures that are subjected to deformations (such as bending or torsion). OMD is already a powerful method, having been coupled with the quantum-mechanical density functional-based tight-binding (DFTB) method, as well as with classical potentials. In this work, these capabilities are used to investigate electromechanical properties of silicon nanowires, treating the mechanical simulation results in the context of continuum mechanics. The bending of graphene is studied, and the underlying molecular orbital mechanisms are investigated. The implications of the results on other simulation methods used to study bending of graphene are discussed. OMD is used in an experimental-theoretical collaboration studying the kinking of graphene and boron nitride nanoribbons. The simulations elucidate and quantify the underlying mechanism behind the kinking seen in experiments.Although theoretically, as a generalization, OMD can match or exceed the capabilities of PBC in all cases, OMD is a new method. Thus, practical implementation must be tackled to expand the capabilities of OMD to new simulation methods and simulation types. In this work, OMD is expanded to allow coupling with self-consistent charge (SCC) DFTB, by developing and implementing the required summation formulas for electrostatic and dispersion interactions. SCC-DFTB is an improved form of the standard DFTB method which includes explicit consideration of charge transfer between atoms. This allows for improved description of heteronuclear materials. To demonstrate this capability, proof-of-concept calculations are carried out on a boron nitride nanotube, a screw-dislocated zinc oxide nanowire, and a single-helix DNA molecule.Finally, preliminary development of heat current calculations under OMD is presented. Heat current calculations are used for calculating thermal conductivity of materials from equilibrium molecular dynamics. So far, heat current calculations have been implemented for the pairwise Lennard-Jones potential. The next development (not yet implemented) is the extension of the heat current calculation under OMD to the Tersoff interatomic potential. The challenges and considerations involved are discussed.