Heat-Initiated Oxidation of an Aluminum Nanoparticle
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Heat-Initiated Oxidation of an Aluminum Nanoparticle Richard Clark, Weiqiang Wang, Ken-ichi Nomura, Rajiv K. Kalia, Aiichiro Nakano and Priya Vashishta1 1
Collaboratory for Advanced Computing and Simulations, Department of Chemical Engineering & Materials Science, Department of Physics & Astronomy, and Department of Computer Science, University of Southern California, Los Angeles, CA 90089, USA ABSTRACT Multimillion-atom reactive molecular dynamics simulations were used to investigate the mechanisms which control heat-initiated oxidation in aluminum nanoparticles. The simulation results reveal three stages: (1) confined burning, (2) onset of deformation, and (3) onset of small cluster ejections. The first stage of the reaction is localized primarily at the core-shell boundary, where oxidation reactions result in strong local heating and the increased migration of oxygen from the shell into the core. When the local temperature rises above the melting point of alumina (T=2330K), the melting of the shell allows deformation of the overall particle and an increase in heat production. Finally, once the particle temperature exceeds 2800-3000 K, small aluminum-rich clusters are ejected from the outside of the shell. The underlying mechanisms were explored using global and radial statistical analysis, as well as developed visualization techniques and localized fragment analysis. The three-stage reaction mechanism found here provides insight into the controlling factors of aluminum nanoparticle oxidation, a topic of considerable importance in the energetic materials community. INTRODUCTION The aluminum nanoparticle (ANP) remains an important, if elusive, subject of scientific inquiry. The broad range of important applications (from use in propellants to explosives) requires that a fundamental ANP model be developed which determines the optimal conditions under which ANP-based materials may be synthesized and used. However, despite considerable experimental and theoretical study, a consensus regarding the controlling mechanisms remains lacking. The experimental research community has provided considerable targeted analysis into these and related systems [for example, 1-8]. From this wealth of information, several models have been constructed which have been used to explain much of the experimental phenomena seen. However, due to the relevant spatial dimensions (atomistic to microns), the speed of the reactions (~ps), and the extreme temperatures involved (>3000 K), a comprehensive model remains elusive. To provide greater insight into the inner workings of these systems, many have applied a range of computer simulation techniques, including density functional theory (DFT) and molecular dynamics (MD) [9-11]. In particular, MD has been shown to provide a considerable advantage in simulating the very fast reactions important in these systems. This method, as well as the interaction potentials it relies on, have been well validated for the aluminum bulk metal (EAM potential) [12], the alumina oxide (Vashishta 3 body potential) [1
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