Oxidation Dynamics of a Chain of Aluminum Nanoparticles
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Oxidation Dynamics of a Chain of Aluminum Nanoparticles Adarsh Shekhar,1 Weiqiang Wang,1 Richard Clark,2 Rajiv K. Kalia,1,2,3 Aiichiro Nakano1,2,3 and Priya Vashishta1,2,3 1 Department of Chemical Engineering and Materials Science 2 Department of Physics and Astronomy 3 Department of Computer Science University of Southern California, Los Angeles, CA 90089, USA
ABSTRACT Multimillion-atom molecular dynamics simulations are used to investigate burning behavior of a chain of three alumina-coated aluminum nanoparticles (ANPs), where particles one and three are heated above the melting temperature of pure aluminum. The mode and mechanism behind the heat and mass transfer from the hot ANPs (particles one and three) to the middle, cold ANP (particle two) are studied. The hot nanoparticles oxidize first, after which hot Al atoms penetrate into the cold nanoparticle. It is also found that due to the penetration of hot Al atoms, the cold nanoparticle oxidizes at a faster rate than in the initially heated nanoparticles. The calculated speed of penetration is found to be 54 m/s, which is within the range of experimentally measured flame propagation rates. As the atoms penetrate into the central ANP, they maintain their relative positions. The atoms from the shell of the central ANP form the first layer, which is followed by the atoms from the shell of the outer ANP making the second layer and lastly the atoms from the core of the outer ANPs form the third layer. In addition to heating the central ANP by convection, the ejected hot Al atoms from the outer ANPs initiate exothermic oxidation reactions inside the central ANP, leading to further heating within the central ANP. During 1 ns, all three ANPs fuse together, forming a single ellipsoidal aggregate. INTRODUCTION Aluminum particles have generated much interest in recent years due to their unique properties, which are very different from bulk1. Studies have shown that on a nanoscale, aluminum particles release energy at a much faster rate than such particles on a micro or macroscale2. It has been suggested that the large surface area to volume might be the contributing factor for this phenomenon. Due to high energy-release rate, Aluminum Nano Particles (ANPs) find varied applications in explosives and other pyrotechniques3-5. Due to their high energy released to volume ratio, ANPs have been widely studied in rocket propellant formulations6. Although many experiments involving ANPs are being done7-10 to get a better understanding of the behavior of such particles, the extremely small scale, both in terms of size and time, in which the ANPs operate make it very difficult to get a clear understanding of the properties at atomistic levels. Numerical methods such as molecular dynamics simulations11-14 hold advantage over experiments when it comes to studies at atomistic levels. With the advent of computing techniques and ever increasing computing resources, researchers have used such methods to gain insight into various properties of ANPs such as size effects15, combustion mechan
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