Boron carbide could light way to less-toxic green pyrotechnics.
Call it a flash of inspiration. A US Army team of pyrotechnics experts has discovered that a compound long dismissed as inert could replace the toxic metal currently used in green-coloured fireworks.
Chemist Jesse Sabatini and his colleagues at Picatinny Arsenal in New Jersey, found that boron carbide matches the green-light-emitting performance of the barium-based compounds used in today's fireworks.
Replacing the barium with boron would cut the amount of toxic material released by fireworks. This is particularly important in places where displays take place every day, such as at theme parks, where the compounds can accumulate. However, it is army personnel who look set to benefit the most from the discovery. Pyrotechnics are used heavily by the military, in signal flares on the battlefield as well as on the training ground to simulate the explosions and gunfire of combat.
"In the military world, where you use large amounts of pyrotechnics every day, it [barium exposure] becomes a real issue," says Thomas Klapötke, who researches clean pyrotechnics at the University of Munich in Germany and was not involved in the new study.
Burn boron, burn
Sabatini was tasked by the US Army with coming up with a cost-effective, barium-free alternative to the green-light-emitting hand-held M125A1 signal flare. These flares are based on a combination of barium nitrate and polyvinyl chloride (PVC) which burn to form barium chloride and emit green light. Aside from the toxicity of the barium, burning PVC has its own environmental issues, because it releases toxic polychlorinated biphenyls.
For a barium- and chlorine-free replacement, the team looked to boron. As the powdery form of the element combusts, it forms boron oxide and releases green light. Unfortunately it burns so fast that the green light is lost too quickly to be of use.
The team found they were able to extend the burn time by adding crystalline boron into the mixture. But crystalline boron is expensive.
"It seemed that we were on the edge of a breakthrough, so we decided to look up 'exotic' boron compounds," says Sabatini. "We saw some really classic chemistry papers from back in the 1950s and '60s, which said that even though boron carbide is quite unreactive at room temperature, at high temperatures it's actually reactive."
Adding increasing amounts of boron carbide to amorphous boron successfully extended the burn time, the team reports in Angewandte Chemie1. Eventually they decided to try it pure and were "very surprised when it ignited", Sabatini says. Pure boron carbide proved to have the best burn time of all.
Getting the green light
Klapötke is also surprised by the performance of boron carbide. "It is thermally very stable — chemists would say it is dead, it doesn't do anything. And so I guess nobody ever tried to use it this way," he says.
Boron carbide looks like a viable replacement for barium nitrate, Klapötke adds, because it ticks all the pyrotechnic boxes with long burn times, reasonable spectral purity, and high luminous intensity. It is also used in the real world as an industrial abrasive so there are fewer hurdles in terms of toxicity and environmental data to adopting it. Perhaps most importantly, boron carbide has a cheap price — cheaper than barium nitrate — and ready availability.
For the commercial fireworks industry as well as the military, cost is crucial to adopting new pyrotechnic formulations, says Martin Van Tiel, who runs Van Tiel Pyrotechnics, a fireworks company based in Waikato in New Zealand.
"For anything new, cost and performance are the key — if it costs twice as much no-one is going to be interested," Van Tiel says. "Throughout the history of fireworks there's been continual development — people will be quite happy to try it. The fireworks industry can be quite quick."
Sabatini, J. J., Poret, J. C . & Broad, R. N. Angew. Chem. Int. Edn. advance online publication doi:10.1002/anie.201007827 (6 April 2011).
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Crow, J. Cleaner, greener fireworks. Nature (2011). https://doi.org/10.1038/news.2011.222