Nanothermite foams: From nanopowder to object

Abstract

Nanothermites are combustible mixtures made up of a metallic oxide and a reducing metal. The high combustion velocities of nanothermite loose powders (0.1–2.5km/s) arise from the propagation of heat through their porosity by the advection of hot gases and particles. The pressing of nanothermite powders alters their performances by decreasing their internal porosity, which makes their integration in pyrotechnic systems difficult to achieve. This article reports on a chemical method to prepare a nanothermite (Al/WO3) in the form of solid and porous foams. For this purpose, Al and WO3 nanoparticles are enclosed in a reactive alumino-phosphate matrix (Al2O3/AlPO4) which is formed by the reaction between phosphoric acid and aluminum. The cementing of the nanopowders avoids the dispersion of nanoparticles and increases at the same time the interfacial contact between the fuel (Al) and the oxidizers (WO3; AlPO4). Nanothermite foams are very insensitive to impact and friction and far less sensitive to electrostatic discharge than conventional nanothermites. However, they can be easily ignited by an open flame. Their combustion is extremely fast and propels incandescent particles. Nanothermite foams are actually the missing link between the nanothermite powders and their integration in various functional pyrotechnic systems as igniters or detonators.