“Research in nanoenergetics is at a very early stage,” Texas Tech researcher Latika Menon told nanotechweb.org. “Nanoscaled energetic materials are expected to be far superior to existing bulk energetic materials due to an increased reaction interface area and drastically decreased distances between reactants, leading to much faster diffusion-limited processes.”
Techniques for making nanoscaled energetic composites currently include sol-gel processing, which results in nanoscale fuel particles “suspended” in an oxidizer matrix, and powder methods - mixing ultrafine powders of the fuel and oxidizer components. According to Menon, both these methods produce high stored energy densities but are limited by the fact that the particles have different sizes and are separated from each other by a range of distances. This can locally separate the fuel and oxidizer and inhibit self-sustaining processes.
With this in mind, Menon and colleagues used a nanotemplate approach to create a nanocomposite consisting of Fe2O3 nanowires embedded in a thin aluminium film. “If we could create a fuel-oxidizer combination in which the honeycomb was fuel and the pores were oxidizer, or vice versa, then every volume of fuel would have the same oxidizer available and the reaction volumes could be controlled over a certain range,” said Menon. “In principle, this is far superior to the powder and sol-gel methods.”
The researchers made a honeycomb-like alumina template by electrochemical anodization of an aluminium foil in an acid. They were able to tailor the diameter of the template’s pores by altering the voltage and the acid used, producing pores between 10 and 150 nm in size.
The team then electrodeposited iron inside the template pores, which they later oxidized to make Fe2O3 nanowires. After various additional steps, the researchers added a 50 nm layer of aluminium on top of the nanowires, forming a structure in which the nanowires were partially embedded in the aluminium layer.
“Aluminium oxidizes rapidly when exposed to oxygen, so we needed to discover a procedure that would place the iron oxide in direct contact with aluminium without having the aluminium in air or water,” said Menon. “We tried several approaches before learning that by making iron oxide nanowires in the templates, exposing them by chemical etching, then coating them with aluminium in an ultrahigh vacuum chamber, we could obtain iron oxide in direct contact with aluminium without aluminium oxide forming.”
Igniting samples of the nanocomposite using a butane flame, resistive heating element or a laser caused them to burn with a flame temperature of around 4000 °C, a value that did not depend on the ignition temperature. The scientists reckon that the energy released was about 0.4 J/sq. cm - around a thousand times higher than the amount released by a purely surface reaction, as for a planar film.
Now, the researchers are studying the reaction mechanism, thermodynamics and kinetics of the ignition process. “The reactions are scientifically interesting, since they are based on interdiffusion and are highly energetic,” said Menon. “We are also interested in exploring other material combinations and improved fabrication approaches that promise greater energy release upon ignition. The interdiffusion reaction also leads to incomplete reaction and possibly to new materials, which need to be investigated.”
The scientists reported their work in Applied Physics Letters.