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On the mechanism of low temperature oxidation for aluminum particles down to the nano-scale

: Eisenreich, N.; Fietzek, H.; Juez-Lorenzo, M.; Kolarik, V.; Koleczko, A.; Weiser, V.


Propellants, explosives, pyrotechnics 29 (2004), Nr.3, S.137-145
ISSN: 0340-7462
ISSN: 0721-3115
ISSN: 1521-4087
Fraunhofer ICT ()

The oxidation of Al-particles down to nano-scale was investigated by TG, SEM and in-situ X-ray diffraction. Al particles are usually coated by a 2 - 4 nm layer of Al2O3 which can be derived from the degree of weight increase on complete oxidation by TG-curves. The low temperature oxidation of Al particles occurs at least in two steps. The first step builds a layer of 6 to 10 nm thickness composed of crystallites of the same size independent on the initial particle size. This reaction is dominated by chemical kinetics and converts a substantial fraction of the particle if the particle sizes decrease below 1 mum, an effect carefully to be taken into account for nano-particles because of safety reasons. The second step combines diffusion and chemical reaction and proceeds therefore slowly, the slower the bigger the particles are. The kinetic parameters of these two steps can be obtained by a model taking into account both reaction steps, chemical kinetics and diffusion for spherical particles when fitting it to TG-curves. X-ray diffraction shows that particles smaller than 1 mum build gamma-and theta-Al2O3 in the first step with nano-crystalline structures which are then transformed to alpha-Al2O3.