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0896-H04-02.1

Influence of Activating Compounds on Oxidation Mechanisms of Aluminum Powders Curtis E. Johnson, Timothy J. Foley, and Kelvin T. Higa Chemistry and Materials Division, Research Department, NAVAIR China Lake, CA 93555-6106, U.S.A. ABSTRACT This study involves an investigation of the mechanism underlying activating effects of inorganic additives on the oxidation of aluminum. The oxidation of nano aluminum powders in air was characterized by variable temperature x-ray powder diffraction and thermogravimetric analysis. For a 33-nm aluminum powder the aluminum oxide produced by air oxidation was poorly crystallized until the sample was heated to above 1050°C, where the alpha-alumina phase crystallized. For a mixture of the aluminum with cryolite, crystallization of oxide phases is enhanced, with Na-Al-O phases evident at 550°C and above. Fluorine is lost from the sample between 550 and 850°C, presumably due to reaction with moisture to produce HF. In a similar experiment with aluminum and silver molybdate, the only crystalline product was alphaalumina, which was observed at 550°C and higher. The general result is that alpha-alumina, the ultimate product in most cases, crystallized at a lower temperature when activating agents were present. INTRODUCTION Metal fuels are used in energetic material formulations to boost energy content and performance. The full potential of metal fuels has not been realized due to slow kinetics that result in incomplete combustion in the desired time frame. The oxidation and combustion of aluminum particles can be promoted with the addition of certain inorganic compounds, including metal fluorides and metal oxides. Sodium hexafluoroaluminate (cryolite) has shown activating effects on aluminum combustion and oxidation in several laboratory tests, including thermite and intermetallic reactions, bench-top powder burning experiments, and thermogravimetric analysis experiments in air [1]. However, in some combustion tests no activating effect is observed, and it is not yet clear whether cryolite activation will be effective in an energetic material application. An understanding of the mechanism of the activation process should allow one to assess the potential utility of the activating agents. EXPERIMENT The aluminum powders were obtained from Technanogy (33 nm Al) and Valimet (H-2 Al). Activating agents were precipitated from aqueous solution, then dry-mixed with the aluminum powders. X-ray powder diffraction patterns were obtained using a Scintag PAD V diffractometer (Cu Kα). For variable temperature x-ray analysis, data were collected during 60 min isotherms at 25°C, 350°C (20°C/min ramp), 550°C (3°C/min ramp), and 650, 750, 850, 950, 1050, and 1150°C (20°C/min ramp). Samples were also characterized using a TA Instruments 2950 Thermogravimetric Analyzer and an Electroscan Environmental Model E-3 electron microscope. RESULTS AND DISCUSSION The mechanism of the air oxidation of aluminum was investigated for several activating agents by conducting variable temperature x-ray diffract