Research has found that mechanically driven chemistry speeds up reactions in explosives


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Scientists at Lawrence Livermore National Laboratory (LLNL) and the Center for Biomaterials and the Department of Materials Engineering at Purdue University used simulations on the LLNL quartz supercomputer to reveal a general mechanism that speeds chemistry into detonating the explosives necessary to manage the nation’s nuclear stockpile. Their research was featured in the July 15th issue of Journal of Physical Chemistry Letters.

Non-sensitive high explosives based on TATB (1,3,5-triamino-2,4,6-trinitrobenzene) offer improved safety properties compared to conventional explosives, but physical explanations for these safety properties are unclear. The initiation of detonation is understood to arise from hotspots that form when shock waves interact with microstructural defects such as pores. The ultra-rapid pore pressure causes an extreme localized temperature rise, accelerating the chemical reactions needed to initiate and eventually combustion. blast. Engineering models of insensitive high explosives – used to assess safety and performance – are based on the hotspot concept but have difficulty describing a wide range of conditions, indicating a lack of physics in those models.

Using a supercomputer simulation of atomically-resolved interactive molecular dynamics at a large scale, the team aimed to directly calculate how hotspots form and grow to better understand why they interact.

Chemical reactions generally accelerate when temperature is increased, but there are other potential mechanisms that can influence reaction rates.

“Recent molecular dynamics simulations have shown that regions of intense deformation in plastics, such as shear bands, can support faster interactions,” explained LLNL author Matthew Cronblood. “Similar accelerating rates were also observed in the first hotspot interactive molecular dynamics simulations, but the reasons for the accelerating interactions in the shear and hotspot bands were not clear.”

The main advantage and predictive power of molecular dynamics simulation comes from the complete accuracy of all motions of an atom during a dynamic event.

“These simulations generate huge amounts of data, which can make it difficult to derive general physical insights into how the motions of atoms govern the collective response of materials,” said Ali 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 are correlated with chemical reaction rates. One of these is temperature, which is a good concept from traditional thermochemistry. Another important descriptor is a newly proposed measure of the energy associated with molecule shape distortions, that is, the stress energy within the molecule.

“In ambient conditions, TATB molecules adopt a planar shape, and this shape results in a highly flexible crystalline encapsulation that is thought to be associated with the unusual sensitivity of TATB,” said Brenden Hamilton of Purdue University.

The team’s cluster analysis revealed that molecules in a hotspot that snap off their equilibrium planar shape react more quickly; Mechanical deformations of the particles in regions of high plastic flow lead to a mechanochemical acceleration of the rates.

Mechanically driven chemistry (mechanical chemistry) is known to operate in many systems, from micromanipulation of bonds through atomic force microscopy “tweezers” to industrial scale ball milling.

The mechanical chemistry that operates in shock explosives is not directly triggered, but results from a complex series of physical processes that begin when shock causes deformations in plastics.

“We characterize this type of process—in which mechanochemistry is a direct consequence of a long series of events—as mechanochemistry outside of time,” said Hamilton, “and this contrasts with the widely studied mechanochemistry in which the initial catalysis directly stimulates a mechanochemical reaction. .”

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

“This work provides a quantitative link between hotspot ignition chemistry and LLNL’s recent 2020 discovery of shear band ignition, which provides a firm basis for formulating more general physics-based explosive models,” Kroonblawd said. “Incorporating chemical-mechanical effects into explosive models will improve their material basis and allow for systematic improvements to accurately and reliably assess performance and safety.”

Simulations explain the detonation characteristics of TATB

more information:
Brenden W. Hamilton et al, Improvised mechanochemistry: ultrafast shock wave induced chemical reactions due to intramolecular stress energy, Journal of Physical Chemistry Letters (2022). DOI: 10.1021 / acs.jpclett.2c01798

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