Helping Soldiers Disappear in a Burst of Smoke

When an imminent threat means troops need to move, sometimes the most powerful cover is a smokescreen. Not a figurative smokescreen, but an actual burst of smoke that hides soldiers—and even tanks—from enemy eyes. Commonly created by smoke grenades, these bursts are valuable only as long as the enemy can’t see through them.

A new type of smoke grenade that can obscure sensors in the visible and infrared ranges burns during a test.
Image Credit: US Army.

Historically, smoke grenades obscured mainly visible light, the wavelengths that human eyes can detect unaided. Today, that isn’t enough. Prying eyes looking through a rifle scope or goggles equipped with thermal imaging systems can essentially “see through” these smoke screens by detecting infrared light given off by people and objects. Then, they turn differences in infrared light into visible images.

Infrared smoke grenades do exist, but there isn’t a one size fits all bomb—one that obscures both kinds of light. That’s what the military wants. If one exploding canister could hide soldiers in both infrared and visible light, that translates into fewer devices, less weight, and greater efficiency. Ideally, the material should also be nontoxic, so that soldiers aren’t breathing in harmful gases.

Traditional smoke grenades contain a mixture of hexachloroethane, zinc oxide, and aluminum. Under combustion, the mixture releases what’s called HC smoke—a smoke that obscures visible light, but which turns out to be toxic to humans. In the quest for a safer option, researchers are now turning to TPA smoke instead, produced by a mixture containing the key ingredient terephthalic acid.

In order to better protect the health and safety of soldiers, researchers at the U.S. Army Edgewood Chemical Biological Center (ECBC) are working on a nontoxic smoke grenade that can simultaneously obscure visible light and infrared light. In work presented this week at meeting of the American Chemical Society (ACS) in New Orleans, ECBC scientist Zach Zander discussed a new approach. It all began when a colleague asked, “What is this? Can I blow it up?” on seeing a sample of UiO-66, a material another group was studying.

UiO-66 is a type of metal organic framework or MOF. An MOF is a physical structure created by a network of metallic nodes linked to organic molecules. MOFs have large surface areas and are very porous. The pores are like empty holes that can be filled with other things, making them attractive for applications such as storing carbon dioxide.

The metallic nodes in UiO-66 are Zirconium and the organic compounds are terephthalic acid. Zander and his colleagues predicted that the metallic nodes would absorb infrared light. Assuming that UiO-66 burns in such a way that the framework breaks apart and the terephthalic acid is freed, the researchers realized that its smoke could potentially obscure both kinds of light at the same time.
After some analysis, the team was ready to put this idea to the test. They packed UiO-66 and pyrotechnics inside of a grenade and BOOM. The explosion produced a cloud of white smoke inside of the test chamber. The chamber was lined with sensors that recorded the amount of visible light, infrared light, and ultraviolet light that made it through the smoke.

The smoke produced in this test didn’t yield great results in the visible spectrum, but it seems to be on the track overall. It blocked infrared light and some visible light. Surprisingly, the smoke blocked some ultraviolet light too. The amount of visible light that made it through the smoke suggests the explosion didn’t completely free the terephthalic acid. To remedy this, the team plans to adjust the pyrotechnics so that the material burns hotter and try again.

This research could save lives. It’s really important. But seriously, how can I get paid to ask: What is this? Can I blow it up?

Kendra Redmond

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