Photoignited aluminum nanopowder combustion in air : oxidation characteristics and energy threshold

Metal combustion has in past years received renewed interest as result of the increased ability to manufacture and characterize nanoparticles which are known to exhibit desirable traits, mainly as a result of their high specific surface area. Unique characteristics, such as lower melting and ignition temperatures, and reduced burning times and ignition delays, make these materials interesting for an entire range of energetic applications inappropriate for conventional metal fuels. An extensive literature review on the ongoing research involving aluminum nanoparticles and nanopowder is reported. Size-dependent properties, combustion mechanism models and nano- scaled powder behavior are described in order to introduce the experimental objectives. The objective of this work was to investigate experimentally the oxidation behavior of nanosized aluminum powders in air under white light photo-ignition condition. Powder samples with three different nominal size particles of 20, 50, and 70 nm were obtained from SkySpring Nanomaterials Inc (Huston, TX) and characterized with HRSEM and TEM. The reactivity of the photo-ignited powder and the energy threshold have been measured and discussed. The presence of a natural oxide coating and, in turn, the optical properties of the powder are believed to be the cause of the found results. An investigation on the well-known two-stage combustion mechanism has been carried out and a simple model to determine the critical condition for a cone-shaped pile of nano-powder is provided.