Molecular Dynamics Study of Size Dependence of Combustion of Aluminum Nanoparticles
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Molecular Dynamics Study of Size Dependence of Combustion of Aluminum Nanoparticles Ying Li, Richard Clark, Aiichiro Nakano, Rajiv K. Kalia, and Priya Vashishta Collaboratory for Advanced Computation and Simulations Departments of Chemical Engineering and Materials Science, Physics and Astronomy, and Computer Science University of Southern California, Los Angeles, CA 90089-0242, U.S.A ABSTRACT Oxidation dynamics of three different sizes (26, 36 and 46 nm) of single aluminum nanoparticle (ANP) in oxygen environment are studied using multimillion-atom reactive molecular dynamics simulations. In the simulation, each aluminum nanoparticle is coated with an amorphous alumina shell of the same thickness (3 nm), and is ignited by heating the nanoparticle to 1100 K. The metallic aluminum and ceramic alumina are modeled by the VoterChen embedded atom model and the interatomic potential by Vashishta et al., respectively. Energy release rate and atomistic-level details of combustion of these single aluminum nanoparticles are investigated, along with the effect of nanoparticle size. The onset temperature of shell Al ejection is found to be independent of the ANP size, whereas the onset time of ejection and the time delay to the highest temperature change rate dT/dt depend on the size. INTRODUCTION Nanoparticle-based materials are used in broad scientific and industrial applications, because their nanoscale dimension can lead to special properties comparing with traditional bulk or microscale materials. The reaction of traditional thermite containing micron-size metallic particles with oxidizers is controlled by diffusion mechanisms. Metastable intermolecular composites (MICs) including nano-thermitic materials, which consist of nanometer-size fuel and oxidizer particles, are characterized by their highly exothermic reaction after ignition, such as higher energy releasing rate [1], faster flame prorogation speed [2], and decrease in ignition delay time [3]. Those unusual enhancements, sometimes accompanied by different stages of reaction in the process of oxidation [4, 5], cannot be explained by the reaction mechanism based solely on diffusion. The particular reaction mechanism of MICs therefore is an important and challenging topic in combustion theory. Aluminum nanoparticle (ANP) is a prototypical MIC. Design of ANPs by controlling spatial configurations is essential to achieve better performance in various military and industrial applications. However, the relationship between the structure and performance of ANPs is not yet fully understood. There has been much progress in the efficient low-cost production of ANPs and therefore, several experimental groups have conducted studies on ANPs to explore the possibility of systematic control of combustion reaction. Zachariah et al. observed the process of ANP oxidation by transmission electron microcopy (TEM) and performed online density measurement to reveal two regimes reactions, i.e., before the melting of aluminum and after the melting point. They also indicated the significanc
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