Chemical delays are used to time explosions in military and commercial (especially mining) applications. They operate by controlling the propagation of an exothermic reaction, allowing timing of the initiation of the resulting explosion.
In our lab, we are developing reactive multilayer particles, which provide a promising alternative to currently used chemicals (azides) which can be quite hazardous.
Figure 1. Reactive multilayer particles produced by vapor deposition.
Reactive multilayer foils contain hundreds of alternating layers of elements or alloys that mix exothermically. Because the individual layers often measure just nanometers in thickness, the elements can mix rapidly and transform to stable compounds, producing a sudden burst of heat. The chemical mixing of the layers can be initiated with a small pulse of energy such as an electrical spark and the reaction can propagate along a foil at rates varying from 1 m/s to 20 m/s. Even the low end of this range, however, is too fast for most delay applications.
Recently we have demonstrated an ability to produce reactive multilayer particles by vapor deposition. The reactivity of the individual particles is controlled by the thickness and chemistry of their nanoscale layering, and the velocity of the reaction in powdered compacts can be controlled by varying the size, shape, and compaction of the particles. The size and shape of the particles are controlled by utilizing different substrates and their final geometries are very uniform as shown in Figure 1.
Figure 2. Propagation of an exothermic reaction front in a compact of reactive multilayer particles.
The velocity of self-propagating reactions in loose compacts of these particles (see Figure 2) is significantly slower than identical reactions in continuous foils (~3 orders of magnitude lower for Ni/Al foils with a 50 nm bilayer spacing). The lower velocities suggest the multilayer particles may serve as effective “green” materials for chemical delays. The particles can also serve as model materials for studying self-propagating reactions in loose compacts, as shown in Figure 2. With these layered particles, all of the chemical mixing occurs within a given particle, not between particles. Thus, the reaction velocity is dominated by heat transfer between particles, not mass transfer. This enables us to study the modes of heat transfer between particles in a controlled and simplified manner.