Scientists at the Lawrence Livermore National Laboratory (LLNL) Energetic Materials Center and Purdue University Materials Engineering Department have used simulations run on the LLNL supercomputer Quartz to discover a common mechanism that speeds up chemistry when detonating explosives. essential to the management of the country’s nuclear stockpile. Their research is featured in the July 15 issue of the Journal of Physical Chemistry Letters.
TATB (1,3,5-triamino-2,4,6-trinitrobenzene) refractory explosives offer improved safety properties over more conventional explosives, but physical explanations for these safety features are not clear. Explosive initiation is believed to arise from hotspots formed when a shock wave interacts with microstructural defects such as pores. Ultra-rapid compression of pores leads to an intense localized spike in temperature, which accelerates the chemical reactions needed to initiate combustion and ultimately explosion. Technical models for refractory explosives — used to assess safety and performance — are based on the hotspot concept, but struggle to describe a wide range of conditions, pointing to missing physics in those models.
Using large-scale atomically resolved supercomputer simulations of reactive molecular dynamics, the team wanted to directly calculate how hot spots form and grow to better understand what makes them react.
Chemical reactions generally speed up when the temperature is increased, but there are other possible mechanisms that can influence the reaction rate.
“Recent molecular dynamics simulations have shown that regions of intense plastic deformation, such as shear bands, can support faster reactions,” explains LLNL author Matthew Kroonblawd. “Similar accelerated velocities were also observed in the first 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 about how atomic motions determine the collective material response,” said Ale Strachan of Purdue University.
To better address this big data problem, the team turned to modern data analytics techniques. Through cluster analysis, the team found that two molecular state descriptors were related to chemical reaction rates. One of them is the temperature, which is well understood from traditional thermochemistry. The other important descriptor is a newly proposed metric for the energy associated with deformations of the molecular shape, ie the intramolecular stress energy.
“Under environmental conditions, TATB molecules take on a planar shape,” said Brenden Hamilton of Purdue University, “and this shape leads to a highly resilient crystal packaging that is thought to be related to the unusual refractoriness of TATB.”
The team’s cluster analysis revealed that molecules in a hotspot that are driven out of their planar equilibrium shape react faster; mechanical deformations of molecules in areas of intense plastic material flow lead to a mechanochemical acceleration of velocities.
Mechanically driven chemistry (mechanochemistry) is known to work in many systems ranging from precision manipulation of bonds through atomic force microscopy “tweezers” to industrial scale ball milling.
The mechanochemistry that works in shock explosives is not activated directly, but is the result of a complicated cascade of physical processes that begins when a shock causes plastic deformations of material.
“We distinguish this kind of process — where mechanochemistry is a downstream consequence of a long sequence of events — as improvised mechanochemistry,” Hamilton said, and “this contrasts with the more widely studied premeditated mechanochemistry in which the initial stimulus directly induces a mechanochemical response.”
The work provides clear evidence that mechanochemistry of deformed molecules is responsible for accelerating reactions in hot spots and in other areas of plastic deformation, such as shear bands.
“This work provides a quantitative link between hotspot ignition chemistry and the recent LLNL discovery of shearband ignition in 2020, providing a solid foundation for formulating more general physics-based explosive models,” Kroonblawd said. “Incorporating mechanochemical effects into explosives models will improve their physical foundations and enable systematic improvements to accurately and reliably assess performance and safety.”
Brenden W. Hamilton et al, Extemporaneous Mechanochemistry: Shock-Wave-Induced Ultrafast Chemical Reactions Due to Intramolecular Stress Energy, The Journal of Physical Chemistry Letters (2022). DOI: 10.1021/acs.jpclett.2c01798
Lawrence Livermore National Laboratory
Quote: Study finds mechanically driven chemistry speeds up reactions in explosives (2022, August 1), retrieved August 2, 2022 from https://phys.org/news/2022-08-mechanical-driven-chemistry-reactions-explosives.html
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