Nanothermites: Synthesis and Combustion

Nanotechnology is a hot topic at the moment, and is opening up a host of advances in many fields, such as controlling the structure of energetic materials. Nanoscale materials are known to differ from micrometer-scale materials in their physical, chemical and electrical properties. Melting point, heat of fusion, boiling temperature, among other thermophysical and thermodynamic properties, change radically when particle diameters are below 10 nm. Nanoparticles of interest have characteristic dimensions generally between 1 and 100 nm. A spherical particle with a diameter of a few nanometers contains only a few thousand atoms. Consequently, the ratio of the number of atoms on the surface of the particle to the atoms in the rest of the particle increases considerably as the diameter of the particle decreases. Since surface atoms have a lower coordination number, their thermophysical properties are very different from those of non-surface atoms. When the ratio becomes significant, the material as a whole has the properties of surface atoms. For example, a macroscopic lump of gold is an inert material. However, if this lump of gold is ground into particles 1 to 5 nm in diameter, it acquires excellent catalytic properties. An important feature of nanoscale materials is therefore the increase in the specific surface area of the material, enabling enhanced reactivity. This increased reactivity has led to interest in certain metal nanoparticles for various energy applications. It is clear that many areas of combustion will soon be influenced by nanotechnology, due to future fuels, explosives and reactive materials with nanometric components. The advantages are not without their drawbacks. For example, some industries are at risk of a type of explosion known as dust explosion. In short, metal particles are attractive candidate fuels for various propulsion and energy conversion applications, mainly because of their high energy density.

In the past, numerous projects have been carried out on "slurry fuels" involving the combustion of micrometer-sized particles, as grinding technologies at the time were unable to reach the nanometric scale. However, micron-sized particles have a number of disadvantages, such as high ignition temperatures and a tendency to agglomerate, resulting in generally low combustion efficiency. In French, these "slurry fuels" are quite often referred to as "boues", which clearly indicates that they are not easy to process. Nanoparticles, although they can agglomerate, have much lower ignition temperatures and are therefore very attractive due to their unique and favorable properties. As a result, there is growing interest in the use of metallic nanoparticles in propulsion and energy conversion systems, in the broadest sense. Without even mentioning propulsion, nanoenergetic materials...