Aluminum nanoflakes: relations between microstructure and reactivity

Abstract

Aluminum powders are used as a component in propellant formulations, explosives and pyrotechnics [1]. Their reactive and propulsive properties are linked to the exothermic reaction of aluminum with an oxidant. Powders with a high specific surface area (higher than 10 m 2 /g) i.e. aluminum nanopowders (Al‐NPs) have a higher reactivity than micronic powders. Moreover, we have shown that shape and nanostructuration of the grains influence the reactivity [2]. The reactivity of the powders is characterized by thermogravimetric (TG) analyses. In this work, the powders which are obtained by high energy ball milling, show a flake like morphology (fig. 1). The influence of the milling parameters and additives on the morphology, structure and reactivity of the powder was investigated. Among the milling parameters, we have changed the size of the balls, the atmosphere of the process and the post‐treatment of the powder. The nanoflakes are highly dispersed in lateral size and thickness; the biggest ones have micronic lateral sizes (1‐4 mm) and thickness less than 200 nm, and the smallest ones 100 nm in lateral size and 30 nm in thickness (fig. 2). The nanoflakes contain several crystallites with an orientation relationship; a [110] texture was observed (fig. 3). They are surrounded by an amorphous layer of aluminum oxide, whose thickness depends on the size of the crystallite. Small crystallites exhibit thicker amorphous layer at the edges than on basal planes (fig. 4), this could induce different oxidation rates of the powder. To remove the passivation agent used in the milling process, the powder is rinsed several times. This is a rather tedious process, so a simple annealing of the powder was tested as an alternative to remove paraffin. Heating the powder at 420°C led to an early crystallization of the Al 2 O 3 amorphous layer, which is an unwanted effect. In‐situ heating in a transmission electron microscope, under inert atmosphere, are in progress, in order to follow the crystallisation of the amorphous alumina layer and the different phase transitions, which are predicated to play an important role in the oxidation mechanism [3].