ENERGETIC materials—explosives, propellants, and pyrotechnics—are substances that store and release large amounts of chemical energy. They are made by either physically mixing solid oxidizers and fuels to produce a composite energetic material, such as gunpowder, or by creating a molecule that contains both oxidizing and fuel components, such as TNT. The total energy released during the chemical reaction—the energy density of the material—can be much greater for composites than for single-molecule energetic materials, but the rate at which composites release energy is much slower (that is, the power is lower). (See S&TR, October 2000, Nanoscale Chemistry Yields Better Explosives.)
Laboratory scientists have begun to remedy this trade-off between energy density and power. “With composite materials, the particles have to diffuse much further to mix, which slows the reaction,” explains Livermore materials chemist Alex Gash. “Although composites will never be like explosives, we can make them react faster by making their particles smaller.” Twenty years ago, scientists discovered that shrinking fuel and oxidizer particle size from the micrometer to the nanometer scale boosts composite reactivity by at least three orders of magnitude. Consequently, efforts to improve reactivity have focused on refining particle size and other methods for decreasing the distance particles have to travel.
Livermore mechanical engineer Kyle Sullivan studies thermites, a type of pyrotechnic composite made from a metal fuel and metal oxide that rapidly burns up when ignited. Because of the focused, intense heat thermites provide, they have traditionally been used for applications such as metal joining and cutting. Sullivan, Gash, and fellow Livermore researcher Joshua Kuntz have examined the effect of fuel size on reactivity by initiating thermite reactions in clear acrylic burn tubes and recording the resulting flame propagation with a high-speed camera. They found that below three micrometers in diameter, decreasing particle size has rapidly diminishing returns.