Ignition and Combustion Behaviors of Nanocomposite Al/MoO 3
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Ignition and Combustion Behaviors of Nanocomposite Al/MoO3 John J. Granier and Michelle L. Pantoya Department of Mechanical Engineering Texas Tech University Lubbock, Texas Tech University ABSTRACT Flame propagation in Al/MoO3 thermites is measured as a function of bulk density and initial sample temperature. The composites are composed of nano-scale reactant particles mixed and pressure molded to between 49 and 73% of the theoretical maximum density. A relationship between cylindrical die pressure and final pellet density is derived. Experiments are also performed by initially pre-heating samples to a uniform temperature ranging from 20 to 200 ºC. Ignition sensitivity was determined by measuring the ignition delay time and temperature using a 50-W CO2 laser and thermocouples, respectively. Combustion wave speeds were measured using high-speed imaging diagnostics. Results allow comparison of combustion behaviors associated with nano- vs. micron-scale particle composites. The nano-scale particle composites are extremely sensitive to ignition, especially when initially preheated. Combustion wave speeds for the compressed nano-composites were found to double when compared to the micron-scale composites. INTRODUCTION Research has shown that reducing the reactant particles from micron to nano dimensions drastically alters the ignition sensitivity and burn velocities of Al/MoO3 and Al/Fe2O3 thermite composites [1-2]. Bockmon et al [1] showed combustion wave speeds as large as 1000 m/s for nano-Al/MoO3 at low bulk densities (i.e., < 10 % TMD) and in confined tubes. As compressed pellets, the velocity is dramatically reduced to 5-10 m/s [2]. As the powder media becomes compacted, void space is reduced and convective mechanisms are impeded. Flame spreading through channels becomes less dominant, resulting in slower wave speeds. One goal of this study is to examine the effects of bulk density of compressed Al/MoO3 samples on reaction propagation. By increasing the bulk density, the amount of convective pathways available for flame spreading should be decreased. Based on previous studies [1, 2], if convective pathways are significantly reduced, the combustion wave speed should decrease. Granier and Pantoya [2] showed that ignition time was reduced by two orders of magnitude for nano versus micron-particle Al/MoO3 compressed pellets (see Fig. 1A). They suggest that the nano-particle composites are more sensitive to ignition based on the lower melting temperatures associated nano-scale particles [2]. This work also compared the combustion wave speed of nano-Al based composites to micron-Al based composites (Fig. 1B). Although Fig. 2B indicates higher velocities in the 10 and 20 µm particle Al / MoO3 samples, this behavior results from longer laser exposure time and an effective pre-heating of the reactants prior to flame propagation measurements [2]. Based on theses results [1, 2], the nano-Al/MoO3 pellet velocities are expected to be higher than the micron-Al composites. This may be true if the flame can be initiated u
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