Reactive Molecular Dynamics Study of Oxidation of Aggregated Aluminum Nanoparticles
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Reactive Molecular Dynamics Study of Oxidation of Aggregated Aluminum Nanoparticles Ying Li1, 2, Rajiv K. Kalia1, Aiichiro Nakano1, and Priya Vashishta1 1
Collaboratory for Advanced Computing and Simulations, Department of Chemical Engineering & Materials Science, Physics & Astronomy and Computer Science, University of Southern California, Los Angeles, CA 90089-0242, U.S.A. 2 Argonne Leadership Computing Facility, Argonne National Laboratory, Argonne, IL 60439, U.S.A. ABSTRACT Oxidation behavior of aggregated aluminum nanoparticles (Al-NPs), specifically the combustion propagation, is studied, when only part of the aggregated Al-NPs is heated to 1100 K and the rest of the system is kept at 300 K. Here, multi-million atoms molecular dynamics (MD) simulation reveals the sintering/coalescence phenomena for the different diameters (D = 26, 36 and 46 nm) aggregated systems. Various consuming rates of core aluminum are investigated for different layers and different diameters aggregated systems. The formation of Al2O3 fragments outside the shell (the largest covalently bonded aluminum-oxide cluster) structure is confirmed from AlO and AlO2 intermediates. The smaller size of Al-NPs results in faster trend of transition from Al-rich to O-rich for most outside small clusters. However, more core aluminum reacts with shell oxygen leads to faster decreasing of the ratio of O/Al in the shell fragment for larger Al-NPs system. INTRODUCTION Aluminum has high combustion enthalpy and low cost. It has been employed as energetic additives in propellants and explosives for many years, especially when its size comes to nanoscale level [1, 2]. Aluminum nanoparticles (Al-NPs) have exhibited lots of interesting among nanoenergetic materials, because of its high reactive power, low vapor pressure, low melting temperature and a thin oxide layer around it to prevent from simultaneous combustion [3, 4]. However, there are puzzles remaining about the combustion behavior of Al-NPs. Levitas, V. I. proposed a melt-dispersion mechanism [5] to explain the short burn time of Al nanoparticles after fast heating (107 oC/s) at T = 1100 oC. Experiments by Rai, A. et al. [6] showed the presence of hollow particles flowing the oxidation. So they developed a model, which considerate the internal pressure gradient on the oxidation process in order to explain the observation. All those explanations were trying to understand the combustion mechanism based on a single aluminum nanoparticle model. Nevertheless the oxidation was not accomplished in an independent state, as we can see from transmission electron microscopy images that Al-NPs are closely placed [6, 7], which makes the description more difficult. Even though, Egan, G et al. [8] have observed the threshold temperature in a manner of consistent heating for aggregated AlNPs. But there are still many other aspects affect the combustion behaviors of aggregated AlNPs. Besides experimental studies, couple of simulations also were conducted to study the properties of Al-NPs. Alavi, S. et al. [9] studied the
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