PORTLAND, Ore. With the more than 7 billion pounds of nitrogen-based explosives sold in the United States each year for everything from military ordnance to mining, you might have thought that scientists had a good understanding of how and why they work. You would be wrong.
Scientific understanding of explosives has taken a back seat to trial-and-error methods of formulating them. The biggest obstacle to a better understanding has been the extremely short time it takes for explosives to explode. Now, scientists at Lawrence Livermore National Laboratory have solved the problem through detailed simulations on a supercomputer that slow time down long enough to conjure the quantum mechanisms that explain the violent transformation that explosives undergo.
"Perhaps the most remarkable thing here is that we are able to say anything about what happens on these time scales in a detonation—this is really new, uncharted territory," said Evan Reed, lead researcher on the project.
The Lawrence Livermore National Laboratory (LLNL; Livermore, Calif.) focused its efforts on one of the most common nitrogen-based explosives, nitromethane—the same formulation that top-fuel dragsters use at the race track. Nitromethane is oxygen poor, but when mixed with ammonium nitrate, as it was for the bombing of the Alfred P. Murrah Federal Building in Oklahoma City, the result can be extremely lethal.
Nitromethane is an optically transparent, electrically insulating material, but the researchers' detailed quantum mechanical simulation revealed that within 5 picoseconds of detonation, it transforms into an optically reflecting metallic conductor. This new state exists all along the shock wavefront, but immediately returns to being optically transparent and electrically insulating behind the wavefront.
The detailed simulations of the molecular dynamics of detonation revealed two key mechanisms that create this metallic state along the explosive's shock wavefront.
"First, the nitromethane chemically decomposes into smaller molecules that have electronic properties more conducive to conducting electrons, because their electronic states are more spread out in space," said Reed. "Secondly, the high temperature of the detonation, around 3000 Kelvin [4940 Fahrenheit], creates a large population of electronic excitation that carries currents."
The LLNL collaborated with the Massachusetts Institute of Technology (MIT) to create the world's first quantum molecular dynamic simulation of explosives during detonation. LLNL researchers included Riad Manaa, Laurence Fried and Kurt Glaesemann, working alongside John Joannopoulos from MIT. Now the researchers are proposing to design new, more powerful synthetic explosives using the simulation software.