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Explosive goes boom, but not too soon

Leavening a volatile new material with TNT yields safer substance

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4:35pm, September 6, 2011
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Look out, Road Runner: There’s a new explosive for Wile E. Coyote’s arsenal. By reining in a supersensitive explosive with good old-fashioned TNT, chemists have created a new compound that can be stored and transported safely and then quickly converted to an activated, superexplosive form.

The new “cocrystal” comprises a zigzagging chain of CL-20 and TNT that, after heating, detonates more readily than either explosive alone, researchers report online August 25 in Angewandte Chemie International Edition.

Developing explosives that have the right mix of properties is extremely difficult.

“You want an explosive to deliver a lot of destructive energy,” says Thomas Klapötke, an expert in energetic materials at the Ludwig Maximilians University of Munich in Germany who was not involved in the work. And you want it to explode fairly easily. “But then it often becomes more dangerous to handle,” Klapötke says.

Seeking a magical mix of stability, sensitivity and explosive power that would also literally provide a good bang for the buck, materials chemist Adam Matzger and his University of Michigan colleague Onas Bolton experimented with CL-20, a relatively new material developed by the U.S. Navy.

“CL-20 has wonderful power, but it’s a little sensitive — it tends to go off easily,” says Matzger. “If you’re on a ship transporting munitions you don’t want a little hit to set everything off.”

So the researchers put CL-20 in a solvent with Wile E.’s old standby, TNT.

“TNT is terrible stuff,” says Matzger. “The best thing about it is it is cheap. But you can have a pile of it and try to set it off, and it won’t explode.”

Mixed together in a solvent, the two substances form a solid crystal that is much less sensitive than CL-20, the researchers report. They tested the new material using a detonation yardstick known as a drop test, which is just what it sounds like: The researchers repeatedly dropped a weight on the new material from different heights to find the distance that causes it to explode 50 percent of the time.

Drop testing with a 2,940-gram weight revealed that, for the new crystal, this height is 99 centimeters. That’s more than twice the height of CL-20 alone, which detonates half the time from 47 centimeters. The results suggest the new material might be transported relatively safely.

When it’s time to blow something up, handlers would heat the new crystal to 136 degrees Celsius to create a supersensitive version of CL-20. The two substances separate, yielding pure TNT and CL-20 that’s peppered with structural defects that make it even more touchy than before; it has a drop height of 41 centimeters.

Half a milligram of the stuff — barely a pinch — can yield a cantaloupe-sized explosion. “It’s amazing to watch — quite a fireball,” says Matzger.

Co-crystalization looks like a very promising approach for making new explosives, says Klapötke. But first he’d like to see how the process will scale up from the laboratory to large-scale production. “At the end of the day you want 500 kilos,” he says.

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