MULTISCALE MODELING OF REACTIVE Ni/Al NANOLAMINATES

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Date
2013-10-20
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Johns Hopkins University
Abstract
This dissertation employs multiscale modeling for the purpose of investigating reactions occurring in reactive Ni/Al nanolaminates. These are comprised of alternating layers of Ni and Al that can react exothermically upon local ignition, eventually leading to the initiation of a self-propagating reaction front with speeds that can exceed 10 m/s. A generalized thermal transport model is developed, based on the transient multi-dimensional reduced continuum formalism introduced by Salloum and Knio [31]. The generalized model accounts for an anisotropic thermal conductivity, that also depends on composition and temperature. A systematic analysis of the role and ramifications that such a generalization has on the flame front structure and dynamics is conducted, revealing that it has a dramatic impact on the ability to successfully capture experimentally observed thermal front instabilities. A multiscale analysis is then conducted in order to infer atomic intermixing rates prevailing during different reaction regimes in the nanolaminates. The analysis combines the results of Molecular Dynamics (MD) simulations with macroscale experimental observations, and leads to the construction of a new composite atomic diffusivity law. Using this composite diffusivity law, a generalized reduced model is obtained with the capability to simultaneously capture various reaction mechanisms over a wide temperature range. The generalized reduced model for single multilayers is then extended towards exploring reactions occurring in layered particle networks. A further reduction of the model is sought through identifying regimes under which spatial homogenization on the particle level would be valid. The limiting case of a single chain of particles is considered, and comparisons between the computational results of the heterogeneous and the homogeneous reduced model descriptions are carried out. These reveal a complex dependence of the reaction progress on the system properties and that simple scaling arguments, based on particle size and rates of heat transfer, are not sufficient for establishing a universal criterion of validity.
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Keywords
Multiscale modeling, Reactive nanolaminate, Self-propagating reaction, Reduced model, Reaction-Diffusion, Ignition, Thermal conductivity, Thermal contact resistance, Reactive particles, Molecular Dynamics, Heat transfer, Homogenization, Atomic diffusion, Nickel, Aluminum, Multilayer, Thermal transport, Front instability
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