Graduate Research Fellow, Purdue University
Hot spots are the best theory for how explosions start, I engineer these individually in crystals
Hot spot theory for the initiation of energetic materials (explosives) says that because a single shock compression wave traveling across a material doesn't account for the subsequent initiation of that explosive, there must be some sort of energy localization. There are many candidates for the points at which energy (and so higher temperatures) is localized; some include adiabatic heating, pore collapse, viscoelastic (heating from non-destructive deformation), viscoplastic (heating from plastic, destructive, deformation), crack-tip heating, and various forms of shear friction modes. Perhaps all of these modes have a part to play in the process of detonation ... and in the soup of multi-crystalline particles with voids and defects, it's hard to tell what is going on.
Which brings me to the work I will present. Low-defect (near perfect) explosive crystals have been grown and had engineered defects re-inserted. It's a shame I can't include pictures because they are quite beautiful. Defects can include holes and slots formed by our miniature drill press, or fast pulsed (femtosecond) laser. Additionally, low defect crystals can be arranged to impact each other, with the knowledge that no internal flaws are contributing to the recorded outcome.
This knowledge can be used to create models for both the more efficient use of energetic materials (mining operations, etc.), and for better explosives safety (high-performance rocket propellant sometimes has a propensity to detonate under the right conditions).
Abstract: We calculate and analyze a thermodynamic limit of a multiscale molecular dynamics based scheme that we have developed previously for simulating shock waves. We validate and characterize the performance of the former scheme for several simple cases. Using model equations of state for chemical reactions and kinetics in a gas and a condensed phase explosive, we show that detonation wave profiles computed using the computational scheme are in good agreement with the steady state wave profiles of hydrodynamic direct numerical simulations. We also characterize the stability of the technique when applied to detonation waves and describe a technique for determining the detonation shock speed.
Pub.: 07 Feb '07, Pinned: 03 Jul '17
Abstract: Five new experiments are reported that tested both detonation wave corner turning and shock desensitization properties of the triaminotrinitrobenzene (TATB) based plastic bonded explosive (PBX) LX-17. These experiments used small pentaerythritol tetranitrate (PETN) charges to initiate hemispherical ultrafine TATB (UF TATB) boosters, which then initiated LX-17 hemispherical detonations. The UF TATB boosters were placed under steel shadow plates embedded in the LX-17 cylindrical charges, which were covered by thin aluminum plates. The LX-17 detonation waves propagated outward until they reached the aluminum plates, which were instrumented with photonic Doppler velocimetry probes to measure their axial free surface velocities. X-ray radiographs and framing camera images were taken at various times. The LX-17 detonations propagated around the two corners of the steel shadow plates and into thin LX-17 layers placed between the steel and the top aluminum plates. The detonation waves were met there by weak diverging shocks that propagated through the steel plates and imparted 1-2 GPa pressures to these unreacted LX-17 layers. These weak shock waves compressed and desensitized the unreacted LX-17, resulting in failures of the LX-17 detonation waves. The hydrodynamics of double corner turning and shock desensitization in the five experiments were modeled in two dimensions using the Ignition and Growth LX-17 detonation reactive flow model. The calculated arrival times and axial free surface velocity histories of the top aluminum plates were in excellent agreement with the experimental measurements.
Pub.: 10 Feb '10, Pinned: 03 Jul '17
Abstract: Structural reactive material (SRM) is consolidated from a fine granular mixture of reactive materials towards the mixture theoretical maximum density with little porosity, thus bearing both high energy density and mechanical strength. A reactive hot spot concept was investigated for fine fragmentation of a SRM solid under explosive loading to augment air blast through rapid reaction of fine SRM fragments. In this concept, micro-sized reactive materials were distributed in a fuel-rich SRM solid, such as MoO3 particles consolidated in a particulate aluminum base in 10Al+MoO3. Intermetallic reactions of micro-sized MoO3 and nearby Al under explosive loading created heat and gas products to form microscale hot spots that initiated local fractures leading to fine fragments of the rest of Al. The SRM solid was made of a thick-walled cylindrical casing, containing a high explosive in a detonation pressure range of 25–34 GPa with a casing-to-explosive mass ratio of 1.78. Experiments in a cylindrical chamber demonstrated the presence of a large amount of fine SRM fragments, whose reaction promptly after detonation significantly enhanced the primary and near field blast wave, as compared to the results from a baseline pure Al-cased charge, thus indicating the feasibility of the concept.
Pub.: 27 Jun '17, Pinned: 03 Jul '17
Abstract: The primary goal of this research is to develop a three-term mesoscopic reaction rate model that consists of a hot-spot ignition, a low-pressure slow burning and a high-pressure fast reaction terms for shock initiation of multi-component Plastic Bonded Explosives (PBX). Thereinto, based on the DZK hot-spot model for a single-component PBX explosive, the hot-spot ignition term as well as its reaction rate is obtained through a “mixing rule” of the explosive components; new expressions for both the low-pressure slow burning term and the high-pressure fast reaction term are also obtained by establishing the relationships between the reaction rate of the multi-component PBX explosive and that of its explosive components, based on the low-pressure slow burning term and the high-pressure fast reaction term of a mesoscopic reaction rate model. Furthermore, for verification, the new reaction rate model is incorporated into the DYNA2D code to simulate numerically the shock initiation process of the PBXC03 and the PBXC10 multi-component PBX explosives, and the numerical results of the pressure histories at different Lagrange locations in explosive are found to be in good agreements with previous experimental data.
Pub.: 17 May '16, Pinned: 03 Jul '17
Abstract: Gem-trinitromethyl groups were introduced into a 1,3,4-oxadiazole ring to give the first example of a bifunctionalized single five-membered ring with six nitro groups. 2,5-Bis(trinitromethyl)-1,3,4-oxadiazole (12) has a high calculated crystal density of 2.007 g cm-3 at 150 K and a very high positive oxygen balance (39.12%) which make it a strong candidate as a high energy dense oxidizer. The dihy-droxylammonium and dihydrazinium salts of bis(trinitromethyl)-1,3,4-oxadiazole (5 and 6) exhibit excel-lent detonation properties (5, vD = 9266 m s-1, P = 38.9 GPa; 6, vD = 8900 m s-1, P = 36.3 GPa) and ac-ceptable impact sensitivities (5 20 J, 6 19 J), which are superior to those of RDX (7.4 J) and HMX (7.4 J). Such attractive features support the application potential of the gem-polynitromethyl group in the design of advanced energetic materials. Surprisingly, 2,5-bis(trinitromethyl)-1,3,4-oxadiazole (12) is more thermally stable and less sensitive than its bis(dinitromethyl) analogue, 8. The compounds were fully characterized by using IR and multinuclear NMR spectroscopy, elemental analysis, and differential scan-ning calorimetry (DSC). The structures of the energetic salts (3•H2O, 5 and 13) and neutral compounds (8 and 12) were confirmed by single-crystal X-ray diffraction.
Pub.: 20 Jun '17, Pinned: 03 Jul '17
Abstract: Understanding the fundamental processes of light-matter interaction is important for detection of explosives and other energetic materials, which are active in the infrared and terahertz (THz) region. We report a comprehensive study on electronic and vibrational lattice properties of structurally similar 1,3-dinitrobenzene (1,3- DNB) crystals through first-principles electronic structure calculations and THz spectroscopy measurements on polycrystalline samples. Starting from reported x-ray crystal structures, we use density-functional theory (DFT) with periodic boundary conditions to optimize the structures and perform linear response calculations of the vibrational properties at zero phonon momentum. The theoretically identified normal modes agree qualitatively with those obtained experimentally in a frequency range up to 2.5 THz and quantitatively at much higher frequencies. The latter frequencies are set by intra-molecular forces. Our results suggest that van der Waals dispersion forces need to be included to improve the agreement between theory and experiment in the THz region, which is dominated by intermolecular modes and sensitive to details in the DFT calculation. An improved comparison is needed to assess and distinguish between intra- and intermolecular vibrational modes characteristic of energetic materials.
Pub.: 11 Jan '16, Pinned: 03 Jul '17
Abstract: In this contribution, 5,5′-bis(trinitromethyl)-2,2′-bi(1,3,4-oxadiazole) (4) and 11 nitrogen-rich salts featuring bi(1,3,4-oxadiazole) were synthesised. Compound 4 was obtained by nitration of 2,2′-bi(1,3,4-oxadiazolyl)-5,5′-diacetic acid and the salts (6, 8–17) were prepared by facile deprotonation and metathesis reactions. All compounds were characterized by IR, multinuclear NMR spectroscopy and elemental analysis. The structures of 6, 9 and 15 were further confirmed by single crystal X-ray diffraction. The physicochemical as well as energetic properties of these compounds including density, thermal stability and sensitivity were investigated. Except for 12 and 15, most of the salts decompose at temperatures over 180 °C. The performance data from the calculated heats of formation and experimental densities indicate that many of the salts have potential applications as energetic materials. The tested sensitivities of these compounds illustrate that they are less sensitive than RDX towards impact, friction and electrostatic discharge.
Pub.: 19 Jan '17, Pinned: 03 Jul '17
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