Mesoscale Computational Analysis on the Reactivity of Heterogeneous Energetic Materials with Electromechanical Properties

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Shin, Ju Hwan
Zhou, Min
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Extensive research was performed in the past to investigate how energetic materials (EM) ignite via thermomechanical dissipation produced by viscoplastic deformation, friction, and fracture. Yet, little is known regarding the other types of excitation leading to ignition. Electromechanically induced dissipation is one notable example of such mechanisms that have become increasingly vital to understand in the recent development of a novel, multifunctional stimulus capable of sensitizing and triggering ignition. To quantify and better understand how certain electromechanical properties of EMs can alter their ignition behavior, a two-step, multiphysics framework spanning multiple timescales is developed. The numerical simulations first track the development of electric field (E-field) in the material under external mechanical load over the microsecond timescale. The model uses a coupled mechanical-electrostatic framework for computing the stress, strain gradient, and E-field distributions of P(VDF-TrFE)/nAl (i.e., a composite film consisting of nanoaluminum particles embedded within PVDF-TrFE binder) possessing flexoelectric and piezoelectric properties. The attainment of sufficient E-field intensity within the material is then used as part of the input for the subsequent analysis, wherein dielectric breakdown and exothermic reaction processes are simultaneously resolved over the nanosecond timescale based on an electrodynamic-chemical-thermal framework. Here, the breakdown process is explicitly modeled as the irreversible transition of the material from its initially dielectric phase into conductive phase wherever the local E-field exceeds the dielectric breakdown strength. The transient current flow leads to resistive dissipation and temperature rise in these highly localized regions of breakdown, resulting in the formation of thermal hotspots that eventually serve as critical sites for the activation and the progression of exothermic reactions. The chemical reaction is modeled as a single-stage, forward kinetic process involving the catalyzed decomposition and direct pyrolysis of the PVDF-TrFE binder, followed by the exothermic fluorination of the Al particles. The reaction rates are characterized using a temperature-dependent Arrhenius equation. The species transport (i.e., Al, PVDF-TrFE, and products) is modeled as diffusion and advection driven by the pressure gradient. The analyses focus on the effects of load intensity and microstructural attributes, such as Al particle size, particle volume fraction, void size, and porosity level, on the mechanical-electrical-chemical-thermal processes with the determination of conditions for ignition being of particular interest. Further, the individual contributions of the electromechanical properties to the overall ignition behavior are delineated using poled (flexoelectric and piezoelectric) and unpoled (flexoelectric only) specimens. The ignition times (i.e., minimum time required for ignition to occur) of poled specimens are found to be ~10% shorter than those of unpoled specimens, consistent with the accompanying experiment. Smaller particle and void sizes also promote a more rapid ignition. Similarly, higher particle volume fraction is shown to lead to a quicker ignition event, owing to the intensified flexoelectric behavior at the macroscopic level and higher chemical energy stored in the aluminum. The aforementioned framework enables a systematic establishment of the material behavioral trends and the microstructure-reaction relations.
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