Benchtop energetics progress

Abstract

Abstract.We have constructed an apparatus for investigating the reactive chemical dynamics of mgscale energetic materials samples. We seek to advance the understanding of the reaction kinetics of energetic materials, and of the chemical influences on energetic materials sensitivity. We employ direct laser irradiation, and indirect laser-driven shock, techniques to initiate thin-film explosive samples contained in a high-vacuum chamber. Expansion of the reacting flow into vacuum quenches the chemistry and preserves reaction intermediates for interrogation via time-of-flight mass spectrometry (TOFMS). By rastering the sample coupon through the fixed laser beam focus, we generate hundreds of repetitive energetic events in a few minutes. A detonation wave passing through an organic explosive, such as pentaerythritol tetranitrate (PETN, C 5 H 8 N 4 O 12 ), is remarkably efficient in converting the solid explosive into final thermodynamically-stable gaseous products (e.g. N 2 , CO, CO 2 , H 2 O…). Termination of a detonation at an explosive-to-vacuum interface produces an expanding pulse of hyperthermal molecular species, with leading-edge velocities ~ 10 km/s. In contrast, deflagration (subsonic combustion) of PETN in vacuum produces mostly reaction intermediates, such as NO and NO 2 , with much slower molecular velocities; consistent with expansion-quenched thermal decomposition of PETN. We propose to exploit these differences in product chemical identities and molecular species velocities to provide a chemically-based diagnostic for distinguishing between "detonation-like" and deflagration events. We report recent progress towards the quantitative detection of hyperthermal neutral species produced by direct laser ablation of aluminum metal and of organic energetic materials, as a step towards demonstrating the ability to discriminate slow reaction intermediates from fast thermodynamically-stable final products.

Benchtop energetics: Detection of hyperthermal species

“…Using a first generation apparatus [5,6] we were able to observe products of in-vacuum deflagration of PETN samples [7] shock initiated by laser-driven flyers [8]. However, we did not observe the hyperthermal final products expected from detonation-like events.…”

Integrated Modeling and Experimental Studies at the Meso Scale for Advanced Reactive Materials

“…Experiments by Thomas et al [32] showed that the detonation products expanding toward vacuum achieve terminal velocities in the range of 8 to 12 mm/µs. Fajardo et al [33], reported the leading-edge velocities of species through the vacuum approximately ≈ 10 mm/µs in their experiments on aluminum ablation. Lundborg [34] measured the velocity of the shock front of TNT evacuating from chambers and the velocity increased from 10 mm/µs to 20 mm/µs with decreasing pressure.…”

“…Trunov et al [13] measured adiabatic flame temperatures for the titanium-boron reaction of approximately 2400-3300 K. One expects, similar to the values measured experimentally at low temperatures [20], that the binary diffusivities of both, boron and titanium into titanium-diboride, are significantly smaller than the mixture thermal diffusivity, and that D 23 D 13 . Taking D 23 /D 13 = 10 as a base-line model value, and the estimate T f = 3000 K for the flame-sheet temperature, our formulas (33) and 34…”

Modeling of the quenching of blast products from energetic materials by expansion into vacuum

“…Experiments by Thomas et al [32] showed that the detonation products expanding toward vacuum achieve terminal velocities in the range of 8 to 12 mm/µsec. Fajardo et al [33], reported the leading-edge velocities of species through the vacuum approximately ≈ 10 mm/µsec in their experiments on aluminum ablation. Lundborg [34] measured the velocity of the shock front of TNT evacuating from chambers and the velocity increased from 10 mm/µsec to 20 mm/µsec with decreasing pressure.…”