A switchable explosive could prevent dangerous accidents

In August 2020, a fire in a warehouse in Beirut, Lebanon, caused 2750 metric tons of the explosive ammonium nitrate to detonate, killing at least 218 people and injuring around 7000. Such catastrophic accidents occur regularly despite more than a century of innovations in explosives safety—for example, a 2014 survey identified more than 500 unintentional explosions at munitions sites from 1979 to 2013. Safety developments have centered on making explosive materials progressively less sensitive to unplanned stimuli through the addition of inert materials and other strategies. But no design has created an explosive that’s completely switchable from safe to active.

Now Alexander Mueller and his team at Los Alamos National Laboratory have crafted a porous explosive that detonates only when filled with a fluid. They three-dimensionally printed lattices of an explosive containing HMX, which is used primarily for military applications. The lattices comprised 400-µm-diameter strands printed 320 µm apart with the layers arranged in one of two alternating orientations to create space between and around the HMX cylinders. Cameron Brown, a graduate student in Mueller’s lab, and colleagues then sandwiched the lattices between aluminum plates. Using a video camera and sensors, the researchers then tracked the kinetic energies of the plates and the velocity of the detonation wave as it traveled down the sample.

When the gaps in the lattice were filled with water, the sample detonated, as shown in the video, releasing energy comparable to that of other common explosives. In that case, the high-pressure detonation wave passed from HMX strand to HMX strand through the water. But without fluid, the detonation dissipated because of pressure losses at the interface between the strands and air. The dry sample generated just 2% of the energy of the water-filled system.

The researchers tested the performance of the explosive when filled with fluids of various densities, including mineral oil, salt water, and water mixed with sodium polytungstate. As the solution density increased, the detonation wave slowed by as much at 13% relative to water-filled samples, and the energy released decreased by as much as 10%. The explosive design thus offers an additional way to tune the behavior, in addition to being completely switchable from off to on. Such explosives could be stored with little risk of accidental explosions and then intentionally detonated when filled with fluid at the location and time desired. (C. B. Brown et al., Phys. Rev. Lett. 130, 116105, 2023 .)