"Environmentally friendly" explosives get ready for ignition
14:20 05 September 01
Damian Carrington, Glasgow
The advent of environmentally friendly explosives has moved a step closer, with German scientists solving the problem of water absorption in one promising compound - the moisture literally turns the compound into a damp squib.
"It may sound strange that military are concerned about health and safety," says Thomas Klapotke, at the University of Munich. "But 99.9 percent of missile launches and explosions take place in training, over your own territory and involving your own personnel."
Conventional explosives and missile propellants are packed with metals and halogens, which can be harmful before and after combustion. Even guns can produce smogs of lead compounds in the indoor shooting ranges now used by many police forces, because lead azide is used as the initiator.
"I would not call them environmentally friendly, but environmentally compatible," Klapotke told New Scientist. But replacing conventional explosives with compounds containing only nitrogen, carbon, hydrogen and oxygen means the only combustion products are N2, water and methane - "hot air", says Klapotke.
Klaptoke has been working with the highly explosive compound hydrozinium azide (N5H5) but the synthesis process used meant it was highly hydroscopic. The water attracted completely defuses the explosive properties.
However, Klapotke found that by crystallising hydrozinium azide directly from water-free hydrazine, the hydroscopic property was removed. The work has just been published in the journal Propulsion, Explosives and Pyrotechnics.
Hydrozinium azide is particularly promising because its combustion products are N2 and H2 alone. The lightness of these gases means that the detonation velocity is especially high and when the H2 burns to produce water just milliseconds later, a second powerful burst of energy is released.
The new generation of explosives need not only be greener, they can also be more powerful. Klapotke has found that diazyltetrozolate, which is 90 percent nitrogen by weight, is 25 percent more powerful than HMX, the most powerful explosive currently in use.
A number of groups in countries belonging to the NATO military alliance are investigating green explosives, but are taking different approaches. The German team is looking at compounds that store their energy in single nitrogen to nitrogen bonds. When detonated, the atoms form highly stable triple nitrogen to nitrogen bonds, releasing energy.
In contrast, US teams are testing materials in which the energy is stored as molecular strain in twisted chains or rings of atoms.
The key difficulty that remains to be overcome is cost. Currently green explosives are 100 times more expensive to produce than conventional ones. Klapotke believes that even at best the new generation materials will be double or triple the price, but says that the military is highly committed to developing less environmentally harmful options. "If your new explosive has metals or halogens in, its not even worth putting on the drawing board," he says.
Klapotke presented his research at the British Association's Festival of Science in Glasgow.