Shock-induced Reactions Research Articles

Shock-induced hotspot formation and chemical reaction initiation in PETN containing a spherical void

We present results of reactive molecular dynamics simulations of hotspot formation and chemical reaction initiation in shock-induced compression of pentaerythritol tetranitrate (PETN) with the ReaxFF reactive force field. A supported shockwave is driven through a PETN crystal containing a 20 nm spherical void at a sub-threshold impact velocity of 2 km/s. Formation of a hotspot due to shock-induced void collapse is observed. During void collapse, NO2 is the dominant species ejected from the upstream void surface. Once the ejecta collide with the downstream void surface and the hotspot develops, formation of final products such as N2 and H2O is observed. The simulation provides a detailed picture of how void collapse and hotspot formation leads to initiation at sub-threshold impact velocities.

Hypo-osmotic Shock-Induced Subclinical Inflammation of Skin in a Rat Model of Disrupted Skin Barrier Function

Aging disrupts skin barrier function and induces xerosis accompanied by pruritus. In many cases, elderly patients complain of pruritus during skin hygiene care, a condition called aquagenic pruritus of the elderly (APE). To date, the pathophysiology and mechanism of action of APE have not been elucidated. We conducted the present study to test the hypothesis that hypo-osmotic shock of epidermal cells induces skin inflammation and elongation of C-fibers by nerve growth factor β (NGFβ) as a basic mechanism of APE. The dorsal skin of HWY rats, which are a model for disrupted skin barrier function, was treated with distilled water (hypotonic treatment [Hypo] group) or normal saline (isotonic treatment [Iso] group) by applying soaked gauze for 7 days. Untreated rats were used as a control (no-treatment [NT] group). Histochemical and immunohistochemical analyses revealed inflammatory responses in the epidermis and the dermal papillary layer in the Hypo group, while no alterations were observed in the Iso or NT groups. Induction of expression and secretion of NGFβ and elongation of C-fibers into the epidermis were found in the Hypo group. In contrast, secretion of NGFβ was significantly lower and elongation of C-fibers was not observed in the Iso group. These results suggest that hypo-osmotic shock-induced inflammatory reactions promote hypersensitivity to pruritus in skin with disrupted barrier function.

Shock compression response of Ti+B reactive powder mixtures

The shock compression response of Ti+2B (1:2 Ti:B stoichiometric ratio) reactive powder mixtures at ~50% theoretical material density (TMD) is investigated for shock pressures up to 5 GPa to investigate the possible shock-induced chemical reactivity of this highly exothermic mixture. The shock adiabat is produced from instrumented parallel-plate gas-gun impact experiments on encapsulated powders using poly-vinylidene fluoride (PVDF) stress gauges to measure the input and propagated stresses and wave speed in the powder. The shock compression regime is probed from crush-up to full density and onward to assess the potential onset of a shock-induced chemical reaction event in the powder mixture. A series of two-dimensional continuum meso-scale simulations on validated simulated microstructures are performed to predict the shock compression response and identify the meso-scale mechanics that are essential for reaction. The suitability of the synthetic microstructural representations is evaluated by comparing the experimental and predicted pressure traces.

Numerical simulation of shock initiation of Ni/Al multilayered composites

The initiation of chemical reaction in cold-rolled Ni/Al multilayered composites by shock compression is investigated numerically. A simplified approach is adopted that exploits the disparity between the reaction and shock loading timescales. The impact of shock compression is modeled using CTH simulations that yield pressure, strain, and temperature distributions within the composites due to the shock propagation. The resulting temperature distribution is then used as initial condition to simulate the evolution of the subsequent shock-induced mixing and chemical reaction. To this end, a reduced reaction model is used that expresses the local atomic mixing and heat release rates in terms of an evolution equation for a dimensionless time scale reflecting the age of the mixed layer. The computations are used to assess the effect of bilayer thickness on the reaction, as well as the impact of shock velocity and orientation with respect to the layering. Computed results indicate that initiation and evolution of the reaction are substantially affected by both the shock velocity and the bilayer thickness. In particular, at low impact velocity, Ni/Al multilayered composites with thick bilayers react completely in 100 ms while at high impact velocity and thin bilayers, reaction time was less than 100 μs. Quantitative trends for the dependence of the reaction time on the shock velocity are also determined, for different bilayer thickness and shock orientation.

Observations on shock induced chemistry of cyclohexane

We use double pass absorption spectroscopy to examine shock induced reactions in situ in cyclohexane and benzene at pressures up to 33.1 GPa. Reactions in cyclohexane begin by 27 GPa and complete by 33.1 GPa. Reactions in benzene are observed to begin by 12 GPa and are complete by 18 GPa. Absorption spectra indicate that the first reaction in cyclohexane occurs within or near the shock front, and that a metastable local equilibrium is reached in the post-shock state. A second process may be observed upon reshock at the lower pressures, suggesting a new equilibrium is reached post-reshock as well. Absorption bands are consistent with the formation of short radicals or fragments upon decomposition; however, spectral resolution is too low to confirm this mechanism.

Shock-Induced Chemical Reaction between Iron and Enstatite

By using a two-stage light gas gun, two experiments of shock recovery experiments with initial sample of Fe+(Mg, Fe)SiO3 (En) were conducted between 78 and 113 GPa shock pressure (the corresponding temperature is estimated as 3000~5000K). The recovered samples were analyzed by X-ray Diffraction (XRD).The XRD observation at the middle section of two recovered samples illustrates the new composition of recovered samples is (Mg, Fe)2SiO4.Comparing with the recovery experiments of MgO+SiO2 ,we can infer that iron and perovskite react to form SiO2 and (Mg, Fe)O. The experiments result indicates that reaction between liquid iron and (Mg, Fe)SiO3 perovskite may occur at the core-mantle boundary in geological history. The reaction creates a very heterogeneous zone at the base of the mantle. Si and O dissolved in liquid iron are rapidly dispersed by the flow of the liquid outer core.

Multiscale modelling on the shock-induced chemical reactions of multifunctional energetic structural materials

Multifunctional energetic structural materials (MESMs) are usually granular mixtures, which release energy due to exothermic chemical reaction initiated under shock loading conditions. The mesostructure, in terms of the size, shape, and distribution of granular mixture, plays a significant role in chemical reaction and the energy release characteristics of MESMs. However, it is difficult to model such a complex process involving thermal-mechanical-chemical responses, especially the effects of the initial mesostructures. In this paper, a multiscale modelling approach is proposed to simulate the chemical reaction of MESMs under a shock compression. The thermal-mechanical response of MESMs is first obtained from mesoscale simulations. Then, the macroscale thermochemical model for a shock-induced chemical reaction is given, in which the extent of reaction is considered. Finally, the spatial profiles of temperature and pressure from the mesoscale heterogeneous simulation are homogenized into cells as an initial state for chemical reaction and further combined with the thermochemical model in macroscale. Hence this provides insight into thermal-mechanical-chemical responses based on the initial mesostructures. Aluminum/Tungsten/Polytetrafluoroethylene granular mixture is selected to demonstrate the method and the effects of volume fraction and impact velocity on the shock-induced chemical reaction. The multiscale approach developed, which combines the mesoscale simulation and macroscale thermochemical modelling, can be used to predict the shock-induced chemical reaction of MESMs with different mesoscale characteristics over a wide range of impact velocities.

Experimental study on impact-initiated characters of multifunctional energetic structural materials

Multifunctional energetic structural materials (MESMs) are a new class of energetic materials, which release energy due to exothermic chemical reactions initiated under shock loading conditions. In order to analyze the impact-initiated process of MESMs, a quasi-sealed test chamber, which was originally developed by Ames [“Vented chamber calorimetry for impact-initiated energetic materials,” in AIAA (American Institute of Aeronautics and Astronautics, 2005), p. 279], is used to study on shock-induced chemical reaction characters at various impact velocities. The impact initiated experiments are involving two typical MESMs, Al/PTFE (polytetrafluoroethylene), W/Zr and inert 2024 Al fragment. The video frames recorded from reactive and inert material impact events have shown the process of late-time after burn phenomena. The total pressure and shock wave reflection at the wall of the test chamber are measured using high frequency gauges. The quasi-pressures inside the test chamber, which is fitting from the total pressure curves, are used to determine the impact initiated reaction efficiencies of MESMs at different impact velocities. A thermochemical model for shock-induced reactions, in which the reaction efficiency is considered, is validated against the experimental data from impact initiation. The results show that the impact velocity plays a significant role in chemical reaction and the energy release characteristics of MESMs. The theoretical calculations correlate reasonably well to the corresponding experimental results, which can be used to predict the reaction results of MESMs over a wide range of pressure satisfactorily.

Reaction synthesis of TiSi2 and Ti5Si3 by ball-milling and shock loading and their photocatalytic activities

Shock loading and high energy ball-milling are typical mechanochemical processing methods by which the chemical reaction of different substances can be initiated and the functional materials with particular physical and chemical properties were produced. In this paper, the precursors of Ti–Si powder with different stoichiometric ratios were pre-activated by ball-milling and then loaded by shock wave. The recovered samples were characterized by XRD, SEM, and DSC, and photocatalytic activity of splitting water into hydrogen was evaluated under visible light. The results indicate that shock-induced reaction of Ti–Si is easier after certainly ball-milling treatment in which the Ti5Si3 with high thermodynamic stability was produced in the precursor mixtures of Ti:Si=1:2 and the designed Ti5Si3 was also produced in that of Ti:Si=5:3. The test for photocatalytic activity shows that there has a common sequence of photocatalytic activity of the treated samples with different stoichiometric ratios including the precursor mixtures of Ti:Si=1:2 and 5:3: (ball-milling+shock loading)>shock loading>ball-milling meaning that the samples treated by both of milling and shock loading exhibit better photocatalytic activity than that of only milling or shock loading due to the greater degree of activation.

Insight into shock-induced chemical reaction from the perspective of ring strain and rotation of chemical bonds

Density functional theory BLYP/DNP and hyperhomodesmotic equations were employed to calculate ring strain energy, the bond dissociation energy of X-NO(2) (X=C, N) and the charges on the nitro groups of several four-membered and six-membered heterocycle compounds. BLYP/DNP and LST/QST + CG method were also applied to calculate bond rotational energy of X-NO(2) (X=C, N) of above mentioned compounds. It indicated that ring strain energy of four-membered heterocycle nitro compounds is apparently higher than that of six-membered heterocycle nitro compounds. Predictably, ring-opening reactions may preferentially occur for those compounds containing higher ring strain energy under shock. In addition, C-NO(2) bonds in these compounds may rotate easier than N-NO(2) bonds in response to the external shock. As for N-NO(2) bonds in these compounds, they also respond to the external shock by the rotation of N-NO(2) bonds, once to the saddle point of the rotational energy barrier, the whole molecule will become relaxed, N-NO(2) bond becomes weaker and eventually leads to the breakage. When one -C=O, -C=NH or -NH(2) group is introduced to the six-membered heterocycle, the charges on the nitro groups of the new compound decrease drastically, and ring strains increase remarkably. It can be predicted that the new compounds will be more sensitive to shock, and the viewpoint is confirmed by the experimental results of shock sensitivity (small scale gap test) of several explosives.

Thermochemical modeling of temperature controlled shock-induced chemical reactions in multifunctional energetic structural materials under shock compression

Multifunctional energetic structural materials (MESMs) are a new class of energetic materials which release energy due to exothermic chemical reactions initiated under shock loading conditions. In order to analyze shock-induced chemical reactions (SICR) for MESMs, theoretical models have been developed to calculate the Hugoniot data which include the heat released by shock temperature controlled reactions. The temperature rise of porous materials due to shock compression is first calculated using a constant volume and pressure adjustment. Then the Arrhenius reaction rate and Avrami-Erofeev kinetic models are used to calculate the extent of reaction of MESMs under shock compression. Thermochemical models for shock-induced reactions, in which the reaction efficiency is considered, are given by combining the shock temperature rise with the chemical reaction kinetics. The Hugoniot relations and temperatures are calculated by using the proposed method. The models developed have been validated against the experimental SICR data involving Fe2O3/Al, Al/Ni, and Ti/Ni mixtures. It has been shown that the theoretical calculations correlate reasonably well with the corresponding experimental and simulation results. The models presented can be used to predict the reaction results of MESMs over a wide range of pressure satisfactorily.

Orientation-dependent Constitutive Model with Nonlinear Elasticity for Shocked β-HMX Single Crystal

Abstract As orientation-dependence of shock-induced thermal responses and chemical reactions in energetic single crystals are related to anisotropic mechanical behavior, a crystal plasticity model for low-symmetric β-HMX single crystal with only limited operative slip systems has been developed. The model accounts for nonlinear elasticity, volumetric coupled with deviatoric behavior, and thermo-dynamical consistency. Simulations of planar impact on β-HMX single crystal show good agreement with existing particle velocity experimental data. An important feature of the model is its capability to capture the evolution of elastic-plastic wave profiles. The model provides new perspective to a range of complex, orientation-dependent mechanical and thermal behaviors than isotropic elastic-plastic constitutive descriptions. Pressure wave profiles show multiple fluctuations for single crystals with different orientations compared to that of isotropic medium. Simulated bulk temperature distribution and evolution with various characteristic features for different orientation single crystals are compared and discussed. Simulations for bicrystal samples show that the evolution and pattern of temperatures in different crystal pairs have great variations. Given that explosive grains are crystalline, the anisotropy of a crystal may be one of the sources of nonuniform temperature distribution.

Orientation-dependent Constitutive Model with Nonlinear Elasticity for Shocked β-HMX Single Crystal

Abstract As orientation-dependence of shock-induced thermal responses and chemical reactions in energetic single crystals are related to anisotropic mechanical behavior, a crystal plasticity model for low-symmetric β-HMX single crystal with only limited operative slip systems has been developed. The model accounts for nonlinear elasticity, volumetric coupled with deviatoric behavior, and thermo-dynamical consistency. Simulations of planar impact on β-HMX single crystal show good agreement with existing particle velocity experimental data. An important feature of the model is its capability to capture the evolution of elastic-plastic wave profiles. The model provides new perspective to a range of complex, orientation-dependent mechanical and thermal behaviors than isotropic elastic-plastic constitutive descriptions. Pressure wave profiles show multiple fluctuations for single crystals with different orientations compared to that of isotropic medium. Simulated bulk temperature distribution and evolution with various characteristic features for different orientation single crystals are compared and discussed. Simulations for bicrystal samples show that the evolution and pattern of temperatures in different crystal pairs have great variations. Given that explosive grains are crystalline, the anisotropy of a crystal may be one of the sources of nonuniform temperature distribution.

Trace element systematics and 147Sm– 143Nd and 176Lu– 176Hf ages of Larkman Nunatak 06319: Closed-system fractional crystallization of an enriched shergottite magma

Combined 147Sm– 143Nd and 176Lu– 176Hf chronology of the martian meteorite Larkman Nunatak (LAR) 06319 indicates an igneous crystallization age of 193 ± 20 Ma (2 σ weighted mean). The individual 147Sm– 143Nd and 176Lu– 176Hf internal isochron ages are 183 ± 12 Ma and 197 ± 29 Ma, respectively, and are concordant with two previously determined 147Sm– 143Nd and 87Rb– 87Sr internal isochron ages of 190 ± 26 Ma and 207 ± 14 Ma, respectively ( Shih et al., 2009). With respect to the 147Sm– 143Nd isotope systematics, maskelynite lies above the isochron defined by primary igneous phases and is therefore not in isotopic equilibrium with the other phases in the rock. Non-isochronous maskelynite is interpreted to result from shock-induced reaction between plagioclase and partial melts of pyroxene and phosphate during transformation to maskelynite, which resulted in it having unsupported 143Nd relative to its measured 147Sm/ 144Nd ratio. The rare earth element (REE) and high field strength element (HFSE) compositions of major constituent minerals can be modeled as the result of progressive crystallization of a single magma with no addition of secondary components. The concordant ages, combined with igneous textures, mineralogy, and trace element systematics indicate that the weighted average of the radiometric ages records the true crystallization age of this rock. The young igneous age for LAR 06319 and other shergottites are in conflict with models that advocate for circa 4.1 Ga crystallization ages of shergottites from Pb isotope compositions, however, they are consistent with updated crater counting statistics indicating that young volcanic activity on Mars is more widespread than previously realized ( Neukum et al., 2010).

Phase, crystal struture and sintering behavior of shock-synthesized Pb(Zr 0.95Ti 0.05)O 3 powders

With a cylindrical shock-wave-loading technique, the single perovskite-phase Pb(Zr 0.95Ti 0.05)O 3 powders (PZT 95/5) were synthesized by shock-induced chemical reactions in heterogeneous multi-material powder mixtures of Pb 3O 4, ZrO 2 and TiO 2. The phase and crystal structure of as-synthesized powders were characterized by X-ray diffraction (XRD) and fourier transform infrared (FT-IR) analysis. And the microstructure and electrical properties of PZT 95/5 ceramics prepared with as-synthesized PZT powders at different sintering temperature were analyzed. The results showed that the shock-wave-induced a large quantity of lattice defects and distortion of the crystal structure in the shock-synthesized PZT powders, which could enhance the sintering activity. Thus, the optimal density and electrical properties of PZT ceramics prepared with as-synthesized powders could be obtained at a sintering temperature of 1200–1225 °C for 3 h, significantly lower than the sintering temperature of PZT 95/5 ceramics prepared by conventional solid-state reaction.

ChemInform Abstract: Shock-Induced and Shock-Assisted Solid-State Chemical Reactions in Powder Mixtures

Shock compression of powder mixtures can lead to chemical reactions, resulting in the formation of equilibrium as well as nonequilibrium compounds, and rapid increases in temperature. The reactions occur as manifestations of enhanced solid‐state chemical reactivity of powders, caused by configurational changes and defect states introduced during shock compression. Two types of reactions are possible and can be distinguished on the basis of their respective process mechanisms and kinetics. Shock‐induced chemical reactions occur during the shock‐compression state, before unloading to ambient pressure, and in time scales of mechanical equilibrium. In contrast, shock‐assisted reactions occur after unloading to ambient pressure, in an essentially shock‐modified material, in time scales of temperature equilibration. The mechanisms of shock‐assisted reactions include solid‐state defect‐enhanced diffusional processes. Shock‐induced reactions, on the other hand, require mechanisms different from conventional solid‐state nucleation and growth processes. The complex nature of deformation of powders has precluded a detailed understanding of the reaction mechanisms of such high‐rate reaction processes. Results of controlled experiments, however, suggest that shock‐induced chemical reactions involve nondiffusional processes giving rise to mechanochemical effects and solid‐state structural rearrangements. Mechanistic concepts that distinguish between shock‐induced and shock‐assisted chemical reactions are described. The effects of configurational changes introduced during shock compression, and the influence of material properties and shock‐loading characteristics on such effects, are analyzed to identify the mechanisms of complex processes leading to chemical reaction initiation and compound formation.

Multiscale chemical reactions in reactive powder metal mixtures during shock compression

A mesoscale reaction (MSR) model is proposed as part of a multiscale numerical modeling approach to study reactive powder metal mixtures. A discrete particle simulation (DPS) incorporates details of phase morphology and arrangement which are an integral part of designing this class of materials. The DPS provides the spatial distribution for the contact sites or locations in which reactant granules come into contact. Temperature and reactant contact surfaces, estimated from DPS results, are used to initialize the MSR model. A heterogeneous granular-level reaction model is incorporated into the MSR model to describe the chemical reactions between reactant granules. Ensemble averages from the MSR model describe the bulk mixture reaction rate in the macroscale continuum model. The proposed approach is demonstrated for a powder mixture containing Ni+Al+voids. Shock-induced reactions are simulated and compared to experiments.

ChemInform Abstract: Nanoshocks in Molecular Materials

Nanoshocks are tiny but powerful laser-driven shock waves that can be used to produce large-amplitude compression in molecular materials on the picosecond time scale. When coupled with ultrafast molecular spectroscopy, the molecular response to nanoshocks can be probed in detail. Simple molecular systems (anthracene crystals) are used to characterize the nanoshock pulses. Well-characterized nanoshocks are used to study complex phenomena such as shock-induced chemical reactions, shock-induced orientation of energetic solids, and shock compression of organic polymers and proteins.

Anisotropic material model and wave propagation simulations for shocked pentaerythritol tetranitrate single crystals

An anisotropic continuum material model was developed to describe the thermomechanical response of unreacted pentaerythritol tetranitrate (PETN) single crystals to shock wave loading. Using this model, which incorporates nonlinear elasticity and crystal plasticity in a thermodynamically consistent tensor formulation, wave propagation simulations were performed to compare to experimental wave profiles [J. J. Dick and J. P. Ritchie, J. Appl. Phys. 76, 2726 (1994)] for PETN crystals under plate impact loading to 1.2 GPa. Our simulations show that for shock propagation along the [100] orientation where deformation across shear planes is sterically unhindered, a dislocation-based model provides a good match to the wave profile data. For shock propagation along the [110] direction, where deformation across shear planes is sterically hindered, a dislocation-based model cannot account for the observed strain-softening behavior. Instead, a shear cracking model was developed, providing good agreement with the data for [110] and [001] shock orientations. These results show that inelastic deformation due to hindered and unhindered shear in PETN occurs through mechanisms that are physically different. In addition, results for shock propagation normal to the (101) crystal plane suggest that the primary slip system identified from quasistatic indentation tests is not activated under shock wave loading. Overall, results from our continuum simulations are consistent with a previously proposed molecular mechanism for shock-induced chemical reaction in PETN in which the formation of polar conformers, due to hindered shear, facilitates the development of ionic reaction pathways.

Theory analysis on shock-induced chemical reaction of reactive metal

Based on the one-dimensional shock wave theory and the powder shock temperature rising model, dynamical shock response, shock temperature rising and shock-induced chemical reaction were theoretically analyzed for reactive metals under high-velocity impact by considering the effects of material compactness and impact velocity on shock pressure and shock temperature, respectively. By combining the calculated shock temperature of the power material and the chemical dynamics of shock-induced reaction, a thermo-chemical model for shock-induced reaction in reactive metal materials was presented by taking reaction efficiency into account. The calculated results by the new model presented in this paper are in agreement with the corresponding experiments by L. S. Bennett, et al. Shock-induced chemical reaction characteristics of reactive metal can be influenced evidently by material compactness, impact velocity and material kind.