Chemical reaction initiation and hot-spot formation in shocked energetic molecular materials

Abstract

A theoretical model is developed to describe the nature of molecular energy transfer and chemical reactivity in shocked energetic materials. The intent is to develop a fundamental understanding of the sensitivity of secondary explosives, which are solids consisting of large organic molecules. Because secondary explosives are stable molecules with large barriers to chemical reaction, before reactions can occur, a sizable amount of energy must be transferred from the shock produced phonons to the molecules' internal vibrations by multiphonon uppumping. The dominant mechanism for uppumping is anharmonic coupling of excited phonon modes with low frequency molecular vibrations, termed doorway modes. Quantitative calculations are presented which show the extent and rate of multiphonon uppumping caused by shock excitation. A simple expression is derived to describe how the rate of uppumping increases with shock pressure. It is shown that uppumping is complete, and Arrhenius kinetics become valid, within -30 ps or 100-1 70 nm behind the shock front. The time dependence of chemical reactivity behind the front is calculated using reaction rate laws for the decomposition of nitramine explosives. The sensitivity of explosives is examined under the conditions of greatest interest: the regime of relatively weak shock waves (p = 1-10 GPa) characteristic of accidents. A mechanism for hot spot formation, based on defect induced local increases in anharmonic coupling, is discussed, and the conditions for ignition and spreading of reactions from hot spots are evaluated.

Theoretical evaluation of sensitivity and thermal stability for high explosives based on quantum chemistry methods: A brief review

“…To clarify this, a theoretical estimation of the number of doorway modes for explosives has been carried out by Tokmakoff et al [63] based on the theory of Dlott and Fayer. It has been suggested that the energy transfer rate is proportional to the number of normal mode vibrations.…”

Single-crystal, polarized, Raman scattering study of the molecular and lattice vibrations for the energetic material cyclotrimethylene trinitramine

“…The specific pathway by which the mechanical energy of the shockwave funnels into a molecular excitation, whether vibrational or electronic, is a current topic of debate. [1][2][3][4][5][6][7][8][9][10] The goal of this research is to provide an unambiguous determination of the Raman-active vibrations for cyclotrimethylene trinitramine ͑RDX͒, a common secondary explosive. Secondary explosives are advantageous to study because they are relatively insensitive to mechanical initiation compared to primary explosives.…”

“…For secondary explosives, it has been proposed that low-energy molecular vibrations, termed "doorway modes," are critical to detonation initiation. [5][6][7] These necessarily anharmonic vibrations provide the bridge between the shock-populated phonons of the material and the "uppumping" process through which the upper vibrational states of the molecule are increasingly populated to the point of decomposition. However, before a theory involving a complicated manifold of anharmonic vibrational interactions can be substantiated, the static vibrational properties of secondary explosives must first be experimentally well defined.…”

A model for mechanochemical transformations: Applications to molecular hardness, instabilities, and shock initiation of reaction

“…1,2 Shock compression of an energetic material is well characterized by translational overheating ͑phonon quasitemperature͒. In a model proposed by Dlott and Fayer, 3 it is assumed that there is continuous energy flow from the compressively heated phonon bath into vibrational doorway modes mediated by a multiphonon up-pumping mechanism that heats the molecules to temperatures at which chemical bonds break. This mechanism requires times of the order of hundreds of picoseconds for a ladder of vibrational states to be climbed and to reach vibrational thermal equilibrium.…”