Synthesis and Coating of Aluminum NanoCrystals
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Synthesis and Coating of Aluminum NanoCrystals Dan A. Kaplowitz1, Jason Jouet2, and Michael R. Zachariah1 1. University of Maryland, College Park, MD 20740 U.S.A. 2. Indian Head Division-Naval Surface Warfare Center, Research and Technology Department, 101 Strauss Avenue, Indian Head, MD 20640 U.S.A.
ABSTRACT We show a low temperature gas-phase synthesis route to produce faceted aluminum crystals in the aerosol phase. Use of triisobutylaluminum whose decomposition temperature is below the melting point of elemental aluminum enabled us to grow nanocrystals from its vapor. Combustion tests show an increase in energy release compared to commercial nanoaluminum. Production of aluminum in an oxygen free environment resulted in a bare aluminum surface that was passivated in separate experiments with nickel and iron by decomposition of their carbonyl precursors. INTRODUCTION Nanoaluminum is by far the most important reactive metal due to its high heat of reaction to the oxide, and its low materials cost. Several aerosol production methods include evaporation of Al from solid pellets [1], laser ablation [1], exploding wire techniques [2-5], and gas atomization reaction synthesis [6]. These methods involve a very high temperature evaporation of elemental aluminum followed by a rapid quench. Not surprising, then, is that these methods tend to produce similar types of polycrystalline particles with primary particle sizes less than ~ 50 nm that are highly aggregated. Low temperature routes have been successfully developed by Jouet et al. [7] for decomposition of aluminum compounds in solution, but these are batch routes that don’t allow for continuous production. We explore a low temperature gas phase route, through the use of a metal organic precursor that has a decomposition temperature below the melting point of aluminum, as a means to more carefully control the nucleation and growth of nanoaluminum. We will demonstrate that thermal decomposition of triisobutylaluminum (TiBAl), under the appropriate time/temperature histories, can generate highly faceted nanocrystals of aluminum. By accomplishing this in an environment free of oxygen, we produce a bare nanoaluminum surface. To prevent oxidation on the surface of the nanoaluminum, the most common approach is to develop a coating on the material before oxidation can occur. A nonreactive passivation layer can be used for storage and can be removed when the aluminum is needed, but non-oxidizers can also be used for coating bare nanoaluminum and still have an energetic function. Nickel, for example, has successfully been coated on the surface of aluminum particles. These particles have shown decreased agglomeration [8] and decreased critical ignition temperature most likely due to the exothermic reaction between Al and Ni [9]. By producing an iron coating on bare aluminum, as well, we can compare and contrast the effects of the different metal coatings on the aluminum surface.
TIBAL EXPERIMENTAL DETAILS An aluminum synthesis scheme was developed using a continuous flow aerosol
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