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A: Kinetics, Dynamics, Photochemistry, and Excited States
Ultrafast Shock-Induced Reactions in Pentaerythritol Tetranitrate Thin Films Samuel D Park, Michael R. Armstrong, Ian T Kohl, Joseph M. Zaug, Robert Knepper, Alexander S. Tappan, Sorin Bastea, and Jeffrey J Kay J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05387 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018
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The Journal of Physical Chemistry
Ultrafast Shock-Induced Reactions in Pentaerythritol Tetranitrate Thin Films Samuel D. Park,∗,†,‡ Michael R. Armstrong,¶ Ian T. Kohl,† Joseph M. Zaug,¶ Robert Knepper,† Alexander S. Tappan,† Sorin Bastea,¶ and Jeffrey J. Kay∗,§ †Sandia National Laboratories, Albuquerque, New Mexico 87185 ‡Present address: U.S. Naval Research Laboratory, Washington, D.C. 20375 ¶Lawrence Livermore National Laboratory, Livermore, California 94550 §Sandia National Laboratories, Livermore, California 94550 E-mail:
[email protected];
[email protected] 1
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Abstract The chemical and physical processes involved in the shock-to-detonation transition of energetic solids are not fully understood due to difficulties in probing the fast dynamics involved in initiation. Here, we employ shock interferometry experiments with sub-20 picosecond time resolution to study highly textured (110) pentaerythritol tetranitrate (PETN) thin films during the early stages of shock compression using ultrafast laser-driven shock wave methods. We observe evidence of rapid exothermic chemical reactions in the PETN thin films for interface particle velocities above ∼1.05 km/s as indicated by shock velocities and pressures well above the unreacted Hugoniot. The timescale of our experiment suggests that exothermic reactions begin less than 50 ps behind the shock front for these high-density PETN thin films. Thermochemical calculations for partially reacted Hugoniots also support this interpretation. The experimentally observed timescale of reactivity could be used to narrow possible initiation mechanisms.
Introduction Shock waves can uniquely generate conditions of extreme pressure-temperature in materials, which can initiate rapid physical and/or chemical changes. These rapid processes are of great importance in energetic materials, particularly in the initiation of detonation in solid high explosives. 1–3 In solid and crystalline high explosives, physical changes due to shear stresses that form under uniaxial compression complicate possible initiation mechanisms. For example, defects are known to significantly alter the reactivity and impact sensitivity of energetic solids. 4 The defects act as hot spots, where energy in the form of heat and stress can localize and induce rapid molecular decomposition. The formation of shear bands have also been postulated through simulations to affect initiation sensitivity, in which viscous flow within the shear bands creates a high temperature environment. 5 The microscopic events occurring behind a shock are nontrivial especially for solids. The transition from mechanical 2
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to chemical energy, whether it is through thermal 6–8 or collisional 9–11 mechanisms, remains an active area of study. Timescale is one important aspect as it helps elucidate possible initiation mechanisms in energetic solids, but it remains unknown primarily due to resolution limitations in traditional measurement techniques. Over the past few decades, improvements in experimental techniques have allowed diagnosis of the state of materials on shorter and shorter timescales to gain insight into fast dynamic processes. 12 Recently, ultrafast experiments with picosecond time resolution have been employed to elucidate the timescale of exothermic chemical reactions in energetic liquids and polymers. 13–16 Such experiments can provide insight towards phenomena such as phase changes 17 and chemical reactions. 18 The capability of resolving the shock-to-detonation transition would be valuable for understanding the complex shock initiation mechanisms in energetic solids. Furthermore, ultrafast experiments provide a more direct link to timescales investigated in current theoretical methods, such as molecular dynamics simulations, 5,11,19–21 and it is imperative to explore even shorter timescale processes for a fundamental understanding of shock initiation in energetic materials. This will benefit engineering applications of energetic materials and may also lead to safety improvements in their handling and use. Pentaerythritol tetranitrate (PETN) is an ideal high explosive that is widely used in defense and demolition applications. 22 Because PETN has a relatively high oxygen balance, it undergoes minimal carbon condensation that can inhibit rapid chemical energy release when detonated. 23,24 Unfortunately, the properties that make PETN desirable as an explosive also impede efforts to probe its reaction zone using traditional experiments. 23 There are many reports of the dynamics of initiation in PETN in the literature. For example, evidence of decomposition behind the plastic wave were first reported by Halleck and Wackerle. 25 Dick et al. 26 reported intermediate velocity transitions, which were attributed to partial decomposition and were further investigated using time-resolved emission spectroscopic measurements. 27 Dick et al. 28 also demonstrated that the shock sensitivity of PETN single crystals strongly depended on the crystal orientation of the shock propagation direction, with the 3
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h110i propagation direction being the most sensitive. A steric-hindrance model was formulated to explain the anisotropic sensitivity of PETN, where the shear stresses from the uniaxial strain of a shock wave can not dissipate without dislocation-mediated plasticity. Although these previous experiments were not able to directly capture the onset of chemistry behind the shock (reaction zone) in PETN, the accumulated results point to a plausible initiation mechanism. However, without experiments to directly probe the reaction zone of PETN, the timescale of initiation remains under debate and, consequently, a matter of speculation for theoretical models regarding its mechanism. Here, we present Ultrafast Shock Interrogation (USI) measurements of laser-driven shock compression of thin-film PETN samples. USI is a shock interferometric technique that dynamically quantifies the shocked state of the sample (interface particle and shock velocities) with sub-20 picosecond time resolution from the time-dependent interference using spectral interferometry with linearly chirped pulses. 29 Shock waves are generated by laser ablation of an aluminum film in contact with the sample. Shock compression of the sample leads to a change in the refractive index of the material, with a discontinuity at the shock front; the pulses reflect off of both the shock front and the reflective ablator acting as the piston, generating a time-dependent interference. This time-dependent interference is encrypted in the phase difference between two collinear pulses separated in time. Thus, the time delay between the pulses controls the fidelity and time resolution of the experiment. The linear chirp on the pulses maps the frequencies to a time axis, allowing the time-dependent phase difference to be measured with a spectrometer in a single shot. 30 The PETN thin films used in our experiment are near (≥ 97%) unstrained crystal density and exhibit a highly preferred (110) orientation, allowing straightforward comparison with previous measurements on single crystals of PETN shocked along the h110i direction. At low shock compression (up < 1.05 km/s), we observe shocked states in the sample that lie along the unreacted Hugoniot and compare favorably with previously reported measurements on shock-compressed PETN single crystals. At high shock compression (up > 1.05 km/s), we observe a large volume ex4
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pansion that we attribute to rapid exothermic reaction of the material. Our results indicate that exothermic chemical reactions in PETN thin films begin less than 50 ps after shock arrival and set new significantly faster upper limits on the timescale of initial reactions in PETN.
Experiment PETN thin film samples were prepared using a custom high-vacuum deposition system that is described elsewhere. 31 Each sample consisted of a 25 mm diameter #2 glass coverslip (∼200 µm thick) with ∼1.2 µm aluminum and 3–4 µm PETN layers. The aluminum and PETN films were deposited in the same system without breaking vacuum, using electron beam and effusion cell deposition sources, respectively. PETN deposited in this configuration is dense (≥ 97% unstrained crystal density, estimated from SEM images of fracture cross-sections), smooth (surface roughness
