Research finds mechanically driven chemistry speeds up reactions in explosives

Scientists at the Lawrence Livermore National Laboratory (LLNL) Center for Energetic Materials and Purdue University’s Department of Materials Engineering have used simulations run on LLNL’s Quartz supercomputer to discover a general mechanism that speeds up the chemistry in the detonation of explosives critical to managing the nation’s nuclear stockpiles. Their research appears in the July 15 issue of the Journal of Physical Chemistry Letters.

TATB (1,3,5-triamino-2,4,6-trinitrobenzene)-based insensitive high explosives offer improved security properties compared to more conventional explosives, but physical explanations for these security characteristics are not clear. clear. Explosive initiation is understood to arise from hot spots that form when a shock wave interacts with microstructural defects such as pores. The ultra-rapid compression of the pores leads to an intense and localized increase in temperature, which accelerates the chemical reactions necessary to initiate burning and, ultimately, detonation. Engineering models for insensitive high explosives, used to assess safety and performance, are based on the critical point concept, but have difficulty describing a wide range of conditions, indicating that physics is missing from those models.

Using large-scale atomically resolved reactive molecular dynamics supercomputer simulations, the team set out to directly calculate how hot spots form and grow to better understand what makes them react.

Chemical reactions generally speed up with increasing temperature, but there are other potential mechanisms that could influence reaction rates.

“Recent molecular dynamics simulations have shown that regions of intense plastic deformation, such as shear bands, can support faster reactions,” explained LLNL author Matthew Kroonblawd. “Similar accelerated rates were also observed in early reactive molecular dynamics simulations of hotspots, but the reasons for the accelerated reactions in shear bands and hotspots were unclear.”

The main advantage and predictive power of molecular dynamics simulations comes from their complete resolution of all atomic motions during a dynamic event.

“These simulations generate huge amounts of data, which can make it difficult to gain general physical insights into how atomic motions govern the collective material response,” said Ale Strachan of Purdue University.

To better deal with this big data problem, the team turned to modern data analysis techniques. Through cluster analysis, the team found that two molecular state descriptors were connected with chemical reaction rates. One of these is the temperature, which is well understood from traditional thermochemistry. The other important descriptor is a recently proposed metric for the energy associated with deformations of the molecule’s shape, that is, the intramolecular strain energy.

“Under ambient conditions, TATB molecules take on a planar shape,” said Purdue University’s Brenden Hamilton, “and this shape leads to a highly resistant crystal packing that is thought to be related to TATB’s unusual insensitivity.”

The team’s cluster analysis revealed that molecules in a hotspot that are kicked out of their equilibrium planar shape react faster; the mechanical deformations of the molecules in regions of intense flow of plastic material lead to a mechanochemical acceleration of the rates.

Mechanically driven chemistry (mechanochemistry) is known to operate in many systems, ranging from the precise manipulation of bonds via atomic force microscopy “tweezers” to industrial-scale ball milling.

The mechanochemistry operating in shocked explosives is not triggered directly, but results from a complicated cascade of physical processes that are initiated when a shock induces deformations in the plastic material.

“We distinguish this type of process, in which the mechanochemistry is a later consequence of a long chain of events, as extemporaneous mechanochemistry,” Hamilton said, and “this contrasts with the more widely studied premeditated mechanochemistry in which the initial stimulus directly induces a mechanochemistry. reaction.”

The work provides clear evidence that the mechanochemistry of deformed molecules is responsible for accelerating reactions at critical points and in other regions of plastic deformation, such as shear bands.

“This work provides a quantitative link between hot spot ignition chemistry and the recent LLNL 2020 discovery of cut-band ignition, providing a firm foundation for formulating explosive models based on more general physics,” Kronblawd said. “The inclusion of mechanochemical effects in explosive models will improve their physical basis and enable systematic improvements to assess performance and safety accurately and reliably.”