The chemistry in detonating explosives is critical to managing the nation's nuclear stockpile, and scientists at the Lawrence Livermore National Laboratory have found a way to speed up the process. The Journal of Physical Chemistry Letters features their research.
There are physical explanations for the safety characteristics of the high explosives, but they are not fully explained. There are hotspots that are formed when a shockwave interacts with micro structural defects Chemical reactions needed to initiate burning and ultimately detonation can be accelerated by ultrafast compression of pores. Engineering models for high explosives, which are used to assess safety and performance, have difficulty in describing a wide range of conditions, indicating missing physics in those models.
The team wanted to directly compute how hotspots form and grow to better understand what causes them to react.
The rate of chemical reactions increases when the temperature increases.
The simulations show that shear bands can support faster reactions. The reasons for the accelerated reactions in shear bands and hotspot were not clear.
The main advantage of simulations is their complete resolution of the atom motions.
"Simulations generate enormous quantities of data, which can make it difficult to derive general physical insights for how atom motions govern the collective material response," said a researcher.
Modern data analytic techniques were used to deal with the big data problem. The team found that the chemical reaction rates were connected to the state of the genes. Traditional thermochemistry describes the temperature as well as one of these. There is a new metric for the energy associated with the changes in molecule shape.
The shape of the molecule leads to a crystal packing that is thought to be connected to the insensitivity of the molecule.
The team's analysis shows that when a molecule is driven from its equilibrium shape, it reacts more quickly.
Mechanochemistry operates in many systems, from precision manipulation of bonds through atomic force microscopy to industrial-scale ball milling.
There is a cascade of physical processes that begin when a shock causes plastic material to change shape.
"We distinguish this kind of process, which is a downstream consequence of a long chain of events, as extemporaneous mechanochemistry."
The work shows that the rapid reactions in the hotspots and other regions of plastic deformation are caused by the chemical makeup of the molecule.
"This work provides a link between hotspot ignition chemistry and the recent 2020 discovery of shear band ignition, which provides a firm basis for formulating more general physics-based explosives models," he said. Mechanochemical effects in explosives models will allow for systematic improvements to assess performance and safety accurately and reliably.
More information: Brenden W. Hamilton et al, Extemporaneous Mechanochemistry: Shock-Wave-Induced Ultrafast Chemical Reactions Due to Intramolecular Strain Energy, The Journal of Physical Chemistry Letters (2022). DOI: 10.1021/acs.jpclett.2c01798 Journal information: Journal of Physical Chemistry Letters
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