(399b) Experimental and Modeling Studies of Self-Sustaining Reactions between Nanopowders

During the past several years, a significant effort has been on investigation of reaction front propagation and the rate of energy release in heterogeneous systems consisting of nanopowder reactants1,2. Substantial size reduction of each reactant powder (e.g. from micro- to nano-size) leads to increase of reaction front propagation in some systems under unconfined conditions by approximately two to three order of magnitude3. This is accomplished when nano-sized fuel and oxidizer particles are mixed. The scaling of these reactants to the nano-scale has allowed for several capabilities and applications that were not previously possible with conventional micro-sized thermite mixtures. A significant size reduction of reactant powders allows more intimate contact. As a consequence of this significant reduction of size, new issues such as dispersion and mixing of reactants, safety, and surface functionalization of fuel particles in order to minimize potential undesired reaction with oxygen and water vapor must be addressed.4 Contemporary work in this area primarily revolves around experimental effort; therefore, there is a need to develop reliable models describing propagation of reaction fronts which are accompanied by gas expansion in different geometric configurations. Prior modeling studies involving strongly exothermic heterogeneous reactions focused primarily on two general types of systems. The first considers ?gasless? conductively driven combustion fronts whereas the second describes combustion fronts propagated by exothermic reaction a solid reactant and gas which is transported to the reaction zone through the porous structure (filtered combustion).5, 6 This contribution will focus on both experimental and modeling studies of reaction front propagation in cylindrical tubes. Different cylindrical setups with one or both ends open are considered. Experimental results have revealed that the combustion front velocity in ?almost? gasless reacting system consisting of aluminum and iron oxide nanopowders is very sensitive to the place of reaction initiation within the cylindrical tube and configuration setup. In addition, another reacting system consisting of aluminum and copper oxide nano-reactants, which is characterized by partial vaporization of reaction products, was investigated using similar geometric configurations. Experimental determination of kinetic constants for both heterogeneous reacting systems was done using differential scanning calorimetry. Utilizing this data, mathematical models describing reaction front propagation in cylindrical tubes in the presence of gas expansion were developed. The effect of pressure generation due to inert gas expansion in porous matrix and/or partial product vaporization as well as reactant composition, porosity, and geometric setup on dynamic characteristics, such as temperature, pressure, conversion, reaction zone dimension, and gas velocity will be discussed.

References 1. C.E. Aumann, G.L. Skofronick, and J.A. Martin, Journal of Vacuum Science & Technology B 13(2): 1178-1183 (1995). 2. C.J. Bulian, T.T. Kerr, and J.A. Puszynski, ?Ignition Studies of Aluminum and Metal Oxides Nanopowders?, The International Pyrotechnics Society, 31st International Pyrotechnics Seminar, Fort Collins, Co, 327-338, (2004). 3. S.F. Son, H.L., B.W. Asay, J.R. Busse, B.S. Jorgensen, B. Bockmon, and M. Pantoya, ?Reaction Propagation Physics of Al/MoO3 Nanocomposite Thermites,? The International Pyrotechnics Society, 28th International Pyrotechnics Seminar, Adelaide, Australia, November 4-9, 2001. 4. J.A. Puszynski, C.J. Bulian, and J.J. Swiatkiewicz, ?The Effect of Nanopowder Attributes on Reaction Mechanism and Ignition Sensitivity of Nanothermites?, 2005 MRS Proceedings, Boston, MA, Nov 28 ? Dec 2, 2005 (in print). 5. A.G. Merzhanov, ?Theory and Practice of SHS: Worldwide State of the Art and the Newest Results,? International Journal of Self-Propagating High-Temperature Synthesis, vol. 2, no. 2, pp. 113-158, 1993. 6. A. Varma, A.S. Rogachev, A.S. Mukasyan, and S. Hwang, ?Combustion Synthesis of Advanced Materials: Principles and Applications,? Adv. in Chem. Eng., vol. 24, pp. 79-225, 1998.