Shock initiation of explosives: High temperature hot spots explained

Abstract

We investigated the shock initiation of energetic materials with a tabletop apparatus that uses km s−1 laser-driven flyer plates to initiate tiny explosive charges and obtains complete temperature histories with a high dynamic range. By comparing various microstructured formulations, including a pentaerythritol tetranitrate (PETN) based plastic explosive (PBX) denoted XTX-8003, we determined that micron-scale pores were needed to create high hot spot temperatures. In charges where micropores (i.e., micron-sized pores) were present, a hot spot temperature of 6000 K was observed; when the micropores were pre-compressed to nm scale, however, the hot spot temperature dropped to ∼4000 K. By comparing XTX-8003 with an analog that replaced PETN by nonvolatile silica, we showed that the high temperatures require gas in the pores, that the high temperatures were created by adiabatic gas compression, and that the temperatures observed can be controlled by the choice of ambient gases. The hot spots persist in shock-compressed PBXs even in vacuum because the initially empty pores became filled with gas created in-situ by shock-induced chemical decomposition.

Hot Spot Chemistry in Several Polymer‐Bound Explosives under Nanosecond Shock Conditions

“…The omitted velocities are chosen such that no significant changes are seen between shown data sets. In the lower velocity regime, the temperature dynamics appear consistent with previously published results on PETN‐based PBXs where single shots were shown rather than the dynamics shown here which were taken from the averaged spectral radiance of many shots . The high impact velocity regime shows a greatly increased cooling rate at early times due to driving a shock near the von‐Neumann spike pressure and generating an overdriven detonation condition which has recently been discussed .…”

“…Recently, simulations of PETN and RDX which include chemical reaction have observed hot spot temperatures on the order of ∼4000 K for collapsing pores which are 10–100 nm in diameter . These temperatures are consistent with those our group observed recently in PBX systems based on PETN . As reported previously, the hot spot temperatures were observed concurrent with impact without a measurable temperature rise time, indicating the local temperatures under shock compression reached 4000 K in <4 ns, which corresponds to a heating rate of >10 12 K/s.…”

“…Representative 2D tomogram slices of RDX‐ and TNT‐based PBXs are shown in Figure b–c. Computer tomography with 1 μm pixel resolution did not show void spaces in X‐PETN, or X‐RDX suggesting that samples have, at most, sub‐micron void spaces similar to prior work . X‐TNT and X‐TATB tomograms showed larger crystals and occasional void space consistent with their lower observed density.…”

Hot Spot Chemistry in Several Polymer‐Bound Explosives under Nanosecond Shock Conditions

“…The omitted velocities are chosen such that no significant changes are seen between shown data sets. In the lower velocity regime, the temperature dynamics appear consistent with previously published results on PETN‐based PBXs where single shots were shown rather than the dynamics shown here which were taken from the averaged spectral radiance of many shots . The high impact velocity regime shows a greatly increased cooling rate at early times due to driving a shock near the von‐Neumann spike pressure and generating an overdriven detonation condition which has recently been discussed .…”

“…Recently, simulations of PETN and RDX which include chemical reaction have observed hot spot temperatures on the order of ∼4000 K for collapsing pores which are 10–100 nm in diameter . These temperatures are consistent with those our group observed recently in PBX systems based on PETN . As reported previously, the hot spot temperatures were observed concurrent with impact without a measurable temperature rise time, indicating the local temperatures under shock compression reached 4000 K in <4 ns, which corresponds to a heating rate of >10 12 K/s.…”

“…Representative 2D tomogram slices of RDX‐ and TNT‐based PBXs are shown in Figure b–c. Computer tomography with 1 μm pixel resolution did not show void spaces in X‐PETN, or X‐RDX suggesting that samples have, at most, sub‐micron void spaces similar to prior work . X‐TNT and X‐TATB tomograms showed larger crystals and occasional void space consistent with their lower observed density.…”

Thermo-chemo-mechanically coupled cavity dynamics and the emergence of multi-phase bursts

“…For the RDX decomposition considered here t ≈ 38 µs and T ≈ 7150 K, and thus the temperature can increase to high values rapidly. We note that rapid temperature spikes to such values have previously been reported in shock loaded polymer bonded explosives [70]. In the case of the fastest reaction rate (smallest A −1 r ) that resembles an explosive loading, the temperature oscillations are close to adiabatic.…”

The Potential Energy Hotspot: Effects of Impact Velocity, Defect Geometry, and Crystallographic Orientation

“…We use identical geometries here to enable a direct comparison between TATB (considered an insensitive explosive) and HMX (a high-performance material). The extreme temperatures (>7000 K) found in HMX following the collapse of diamond-shaped pores elongated along the shock propagation direction corresponds well to the theoretical maximum temperature and recent experimental reactive hotspot measurements from Bassett et al − These high temperatures are possibly related to the jetting and gasification of material into the void, which is later recompressed by the shockwave. Holian et al showed for simple one-dimensional (1D) shocks in a model system that jetting occurs when the energy embedded by the shock is greater than the crystal cohesive energy: 1/2 mU p 2 > E coh .…”

Hot Spot Chemistry in Several Polymer‐Bound Explosives under Nanosecond Shock Conditions

“…The omitted velocities are chosen such that no significant changes are seen between shown data sets. In the lower velocity regime, the temperature dynamics appear consistent with previously published results on PETN‐based PBXs where single shots were shown rather than the dynamics shown here which were taken from the averaged spectral radiance of many shots . The high impact velocity regime shows a greatly increased cooling rate at early times due to driving a shock near the von‐Neumann spike pressure and generating an overdriven detonation condition which has recently been discussed .…”

“…Recently, simulations of PETN and RDX which include chemical reaction have observed hot spot temperatures on the order of ∼4000 K for collapsing pores which are 10–100 nm in diameter . These temperatures are consistent with those our group observed recently in PBX systems based on PETN . As reported previously, the hot spot temperatures were observed concurrent with impact without a measurable temperature rise time, indicating the local temperatures under shock compression reached 4000 K in <4 ns, which corresponds to a heating rate of >10 12 K/s.…”

“…Representative 2D tomogram slices of RDX‐ and TNT‐based PBXs are shown in Figure b–c. Computer tomography with 1 μm pixel resolution did not show void spaces in X‐PETN, or X‐RDX suggesting that samples have, at most, sub‐micron void spaces similar to prior work . X‐TNT and X‐TATB tomograms showed larger crystals and occasional void space consistent with their lower observed density.…”

Thermo-chemo-mechanically coupled cavity dynamics and the emergence of multi-phase bursts

“…For the RDX decomposition considered here t ≈ 38 µs and T ≈ 7150 K, and thus the temperature can increase to high values rapidly. We note that rapid temperature spikes to such values have previously been reported in shock loaded polymer bonded explosives [70]. In the case of the fastest reaction rate (smallest A −1 r ) that resembles an explosive loading, the temperature oscillations are close to adiabatic.…”

The Potential Energy Hotspot: Effects of Impact Velocity, Defect Geometry, and Crystallographic Orientation

“…We use identical geometries here to enable a direct comparison between TATB (considered an insensitive explosive) and HMX (a high-performance material). The extreme temperatures (>7000 K) found in HMX following the collapse of diamond-shaped pores elongated along the shock propagation direction corresponds well to the theoretical maximum temperature and recent experimental reactive hotspot measurements from Bassett et al − These high temperatures are possibly related to the jetting and gasification of material into the void, which is later recompressed by the shockwave. Holian et al showed for simple one-dimensional (1D) shocks in a model system that jetting occurs when the energy embedded by the shock is greater than the crystal cohesive energy: 1/2 mU p 2 > E coh .…”

Hot Spot Chemistry in Several Polymer‐Bound Explosives under Nanosecond Shock Conditions

“…The omitted velocities are chosen such that no significant changes are seen between shown data sets. In the lower velocity regime, the temperature dynamics appear consistent with previously published results on PETN‐based PBXs where single shots were shown rather than the dynamics shown here which were taken from the averaged spectral radiance of many shots . The high impact velocity regime shows a greatly increased cooling rate at early times due to driving a shock near the von‐Neumann spike pressure and generating an overdriven detonation condition which has recently been discussed .…”

“…Recently, simulations of PETN and RDX which include chemical reaction have observed hot spot temperatures on the order of ∼4000 K for collapsing pores which are 10–100 nm in diameter . These temperatures are consistent with those our group observed recently in PBX systems based on PETN . As reported previously, the hot spot temperatures were observed concurrent with impact without a measurable temperature rise time, indicating the local temperatures under shock compression reached 4000 K in <4 ns, which corresponds to a heating rate of >10 12 K/s.…”

“…Representative 2D tomogram slices of RDX‐ and TNT‐based PBXs are shown in Figure b–c. Computer tomography with 1 μm pixel resolution did not show void spaces in X‐PETN, or X‐RDX suggesting that samples have, at most, sub‐micron void spaces similar to prior work . X‐TNT and X‐TATB tomograms showed larger crystals and occasional void space consistent with their lower observed density.…”

Thermo-chemo-mechanically coupled cavity dynamics and the emergence of multi-phase bursts

“…For the RDX decomposition considered here t ≈ 38 µs and T ≈ 7150 K, and thus the temperature can increase to high values rapidly. We note that rapid temperature spikes to such values have previously been reported in shock loaded polymer bonded explosives [70]. In the case of the fastest reaction rate (smallest A −1 r ) that resembles an explosive loading, the temperature oscillations are close to adiabatic.…”

The Potential Energy Hotspot: Effects of Impact Velocity, Defect Geometry, and Crystallographic Orientation

“…We use identical geometries here to enable a direct comparison between TATB (considered an insensitive explosive) and HMX (a high-performance material). The extreme temperatures (>7000 K) found in HMX following the collapse of diamond-shaped pores elongated along the shock propagation direction corresponds well to the theoretical maximum temperature and recent experimental reactive hotspot measurements from Bassett et al − These high temperatures are possibly related to the jetting and gasification of material into the void, which is later recompressed by the shockwave. Holian et al showed for simple one-dimensional (1D) shocks in a model system that jetting occurs when the energy embedded by the shock is greater than the crystal cohesive energy: 1/2 mU p 2 > E coh .…”