Shock-induced Reactions Research Articles

Probing the Impact Energy Release Behavior of Al/Ni-Based Reactive Metals with Experimental and Numerical Methods

Reactive metals (RMs) are a new class of material that can withstand mechanical loads and chemically react to release large amounts of heat under strong impact loading. They are gradually becoming widely used in defense and military fields, including for high-efficiency warheads and reactive armor. For the numerical simulation method considering the combined mechanical-thermo-chemical process for the impact energy release behavior of the RMs, the Al/Ni-based RMs were investigated in this work by combining experiments, theoretical calculations and a numerical simulation. Three kinds of Al/Ni-based RMs (Al-Ni, Al-Ni-CuO and Al-Ni-MoO3), were prepared using the hot-pressing forming process. Firstly, the compressive behavior and the parameters of the Johnson-Cook constitutive model were obtained using a mechanical testing machine and split Hopkinson pressure bars (SHPB). Secondly, the parameters of the equation of state (EOS) under the medium and low pressure conditions of the Al/Ni-based RMs, which were was seen as porous mixtures with high theoretical material density percentages (TMD%), were calculated based on the cold-energy superposition theory and the Wu-Jing method. Third, the impact energy release behaviors of the three RMs were studied with direct ballistic tests. The shock temperatures at different impact velocities were calculated based on the existing shock-induced chemical reaction thermo-chemical model while considering the chemical reaction efficiency, the relationship between the shock temperature and the extent of the chemical reaction was established, and the parameters of the relevant chemical kinetic equations were fitted. Finally, the user’s subroutines defining the material model were implemented to update the stresses in the solids elements in LS-DYNA. The model was based on the Johnson-Cook constitutive model with consideration of the mechanical-thermo-chemical coupling effect, which was verified by the experimental results. The results show that the constitutive model developed in this work can describe the impact energy release behavior of the Al/Ni-based RMs.

Sensitivity of Al‐PTFE upon Low‐Speed Impact

AbstractAl‐PTFE (Aluminum‐Polytetrafluoroethylene) is a promising composite as a kind of impact initiated energetic materials. The impact sensitivity of Al‐PTFE was characterized by drop weight tests. By varying the sintering temperature, Al mass ratio and Al particle size, the sensitivity of Al‐PTFE changes significantly from 25.78 J to 87.34 J. It was found that in general Al‐PTFE specimens with higher strength tend to be more sensitive to low‐speed impact. The SHPB test was carried out on the baseline Al‐PTFE specimen, and the stress ignition threshold was found to be 116 MPa, which is not high enough to trigger a shock‐induced reaction (SIR). Moreover, multiple evidences, including the high‐speed camera records, prove that crack‐induced ignition mechanism, rather than SIR, dominates the ignition of Al‐PTFE upon low speed impact.

The shock-induced chemical reaction behaviour of Al/Ni composites by cold rolling and powder compaction

Al/Ni composites are typical structural energetic materials, which have dual functions of structural and energetic characteristics. In order to investigate the influence of manufacturing methods on shock-induced chemical reaction (SICR) behaviour of Al/Ni composites, Al/Ni multi-layered composites with 3–5 cold-rolling passes and Al/Ni powder composites were obtained. Microstructural observation using scanning electron microscopy (SEM) and two-step impact initiation experiments were performed on the four Al/Ni composites. Furthermore, mesoscale simulations, through importing SEM images into the finite element analysis to reflect the real microstructures of the composites, were performed to analyse the particle deformation and temperature rise under shock compression conditions. The experimental results showed the distinct differences on the SICR characteristics among the four Al/Ni composites (i.e. by 3, 4 and 5 cold-rolling passes and powder compaction). The manufacturing methods provided the control of the particle sizes, particle distribution and the content of the interfacial intermetallics at scale of different microstructures, which ultimately affected the temperature distribution, as well as the contact between Al and Ni in Al/Ni composites under shock loading. As a result, the Al/Ni powder composites showed the highest energy release capacity among the four composites, while the energy release capability of Al/Ni multi-layered composites decreased with the growth of rolling passes.

ReaxFF molecular dynamics simulations of shock induced reaction initiation in TNT

Thermodynamic pathways and reaction initiation mechanisms of shocked TNT (2, 4, 6-trinitrotoluene, formula C6H2(NO2)3CH3) with shock velocities in the range of 6 -10 km⋅s-1 using the first-principles-based ReaxFF reactive force field molecular dynamics and the multiscale shock technique (MSST) are reported in this paper. The decomposition reactions occur at a shock velocity of 7 km⋅s-1 or higher. The shock initiation pressure, 25.1 GPa, is obtained from Rankine−Hugoniot relation. According to the link between macroscopic shock initiation and microscopic chemical reaction events, the formation of TNT-dimer and decomposition to C7H5O5N3 are the dominant initial route for shock induced reaction initiation. At shock speeds equal to or higher than 8km⋅s-1, TNT-dimer is formed and subsequently decomposed to C7H5O5N3, NO2 and NO. The quantity of NO2 molecules reaches maximum when TNT molecules decompose completely. Furthermore, when NO2 molecules are consumed fully, the volume of reaction system begins to expand. TNT molecules are dimerized at each shock condition, and the quantity of dimers is the largest at a shock initiation velocity of 7 km⋅s-1. Finally, the formation and evolution of carbon-containing clusters in shocked TNT are analyzed.

Shock-induced reaction synthesis of cubic boron nitride

Here, we report ultra-fast (0.1–5 μs) shock-induced reactions in the 3B-TiN system, leading to the direct synthesis of cubic boron nitride, which is extremely rare in nature and is the second hardest material known. Composite powders were produced through high-energy ball milling to provide intimate mixing and subsequently shocked using an explosive charge. High-resolution transmission electron microscopy and X-ray diffraction confirm the formation of nanocrystalline grains of c-BN produced during the metathetical reaction between boron and titanium nitride. Our results illustrate the possibility of rapid reactions enabled by high-energy ball milling possibly occurring in the solid state on incredibly short timescales. This process may provide a route for the discovery and fabrication of advanced compounds.

Investigation on shock-induced reaction characteristics of a Zr-based metallic glass

The shock-induced reaction behavior of ZrCuNiAl bulk metallic glass (BMG) was investigated by means of a modified quasi-sealed test chamber. A series of quasi-sealed chamber impact experiments were conducted through launching ZrCuNiAl BMG fragments into the test chamber. It was shown that the semi-sphere shape of the modified quasi-sealed chamber suits the expanding manner of reactions. The results demonstrated that the BMG fragments underwent chemical reaction when impacted onto steel target. The energy released by reaction led to significant pressure rise inside the quasi-sealed chamber. It was shown that the amount of energy increased with the increasing of impact velocity. A relationship between the overpressure inside the chamber and the released chemical energy was derived to analyze the effect quantitatively.

Three-dimensional reacting shock–bubble interaction

We investigate a reacting shock–bubble interaction through three-dimensional numerical simulations with detailed chemistry. The convex shape of the bubble focuses the shock and generates regions of high pressure and temperature, which are sufficient to ignite the diluted stoichiometric H2−O2 gas mixture inside the bubble. We study the interaction between hydrodynamic instabilities and shock-induced reaction waves at a shock Mach number of Ma=2.83. The chosen shock strength ignites the gas mixture before the shock-focusing point, followed by a detonation wave, which propagates through the entire bubble gas. The reaction wave has a significant influence on the spatial and temporal evolution of the bubble. The misalignment of density and pressure gradients at the bubble interface, caused by the initial shock wave and the subsequent detonation wave, induces Richtmyer–Meshkov and Kelvin–Helmholtz instabilities. The growth of the instabilities is highly affected by the reaction wave, which significantly reduces mixing compared to an inert shock–bubble interaction. A comparison with two-dimensional simulations reveals the influence of three-dimensional effects on the bubble evolution, especially during the late stages. The numerical results reproduce experimental data in terms of ignition delay time, reaction wave speed and spatial expansion rate of the bubble gas. We observe only a slight divergence of the spatial expansion in the long-term evolution.

Investigation of the shock-induced chemical reaction (SICR) in Ni + Al nanoparticle mixtures

Molecular dynamics (MD) simulations are used to investigate the shock-compression response of Ni + Al spherical nanoparticles arranged in a NaCl-like structure. The deformation and reaction characteristics are studied from the particle level to the atomic scale at various piston velocities. Shock-induced chemical reactions (SICRs) occur during non-equilibrium processes, accompanied by a sharp rise in temperature and rapid mixing of atoms. The preferentially deformed Al particles form a high-speed mass flow relative to the Ni at the shock front, which impinges on the Ni particles, and mixing of Ni and Al atoms occurs immediately at the interface. The particle velocity dispersion (PVD) that appears at the shock front has important implications for the initiation of shock-induced chemical reactions. We show that dislocations are mainly generated at the beginning of particle deformation or at the shock front, and do not directly affect the occurrence of SICRs. The intimate contact of the molten Al and the amorphous Ni is found to be critical to the subsequent reactions for the extensive mixing of Ni and Al. We conclude that the mechanisms of SICRs involve mechanochemical processes near the shock front and subsequent interdiffusion processes.

Reaction Behavior of Polytetrafluoroethylene/Al Granular Composites Subjected to Planar Shock Wave

The shock-compression responses of PTFE (polytetrafluoroethylene)/Al granular composites subjected to planar shock waves of various pressures are investigated. A 57-mm diameter single-stage gas-gun and 50-mm diameter plane-wave lenses are employed to perform planar shock wave experiments. High frequency manganin piezoresistance stress gauges are used to monitor the stress (regarded as pressure in consideration of the high pressure state) at four Lagrangian positions of the PTFE/Al granular composites specimens. Planar shock wave experiments show characteristics of densification at measured input pressure of 0.5 GPa to 1.27 GPa using single-stage gas-gun and shock-induced reaction (SIR) indicated by growth of shock pressure and specific volume expansion at measured input pressure of 7.29 GPa to 12.25 GPa using plane-wave lenses. The pressure and relative volume states behind the shock wave front are calculated from the experimental recorded pressure profiles using Lagrangian analysis method, which are used to determine the reaction ratios under different shock pressures by comparing with partial reacted Hugoniot calculations. It was shown that the reaction ratios obtained in this research have good agreement with the thermochemical modeling calculations. The corresponding results indicate that the shock-induced reactions of PTFE/Al granular composites occur in the shock wave rising period and the reaction ratios are intimately related to the shock wave pressure.

Investigation on shock-induced reaction characteristics of an Al/Ni composite processed via accumulative roll-bonding

An Al/Ni composite was produced via accumulative roll-bonding (ARB) process up to four cycles. The shock-induced reaction behavior of the material was investigated by means of a quasi-sealed test chamber. The results demonstrated that the Al/Ni composite fabricated by ARB underwent chemical reaction when impacted onto steel target. The energy released by reaction led to significant pressure rise inside the quasi-sealed chamber. It was shown that the amount of energy increased with the increasing of impact velocity. A relationship between the pressure inside the chamber and the impact velocity was derived to analyze the effect quantitatively. The results of this study represented the potential of Al/Ni composite processed by ARB as reactive fragments.

Experimental Study on Reaction Characteristics of PTFE/Ti/W Energetic Materials under Explosive Loading.

Metal/fluoropolymer composites represent a new category of energetic structural materials that release energy through exothermic chemical reactions initiated under shock loading conditions. This paper describes an experiment designed to study the reaction characteristics of energetic materials with low porosity under explosive loading. Three PTFE (polytetrafluoroethylene)/Ti/W mixtures with different W contents are processed through pressing and sintering. An inert PTFE/W mixture without reactive Ti particles is also prepared to serve as a reference. Shock-induced chemical reactions are recorded by high-speed video through a narrow observation window. Related shock parameters are calculated based on experimental data, and differences in energy release are discussed. The results show that the reaction propagation of PTFE/Ti/W energetic materials with low porosity under explosive loading is not self-sustained. As propagation distance increases, the energy release gradually decreases. In addition, reaction failure distance in PTFE/Ti/W composites is inversely proportional to the W content. Porosity increased the failure distance due to higher shock temperature.

Shock-Induced Reaction Characteristics of an Al/Ni Composite Processed via Accumulative Roll-Bonding

Al/Ni composite is widely recognized as a member of multifunctional energetic structural materials (MESMs), which could release energy due to exothermic chemical reactions initiated under shock loading conditions. In this study, an Al/Ni composite was produced via accumulative roll-bonding (ARB) process up to four cycles. The shock-induced reaction behavior of the material was investigated by means of a quasi-sealed test chamber. The results demonstrated that the Al/Ni composite fabricated by ARB underwent chemical reaction when impacted onto steel target. The energy released by reaction caused significant pressure rise inside the quasi-sealed chamber. It was shown that the amount of energy increased with the increasing of impact velocity. The results of this study represented the potential of Al/Ni composite processed by ARB as reactive fragments.

The Energy Release Characteristics of Shock-Induced Chemical Reaction of Al/Ni Composites

In this paper, the energy release mechanism of shock-induced chemical reaction (SICR) of Al/Ni composites was investigated. The Al/Ni composites with two additives, namely Teflon (PTFE) and copper (Cu), were considered in both theoretical calculations and experiments to investigate their influence on SICR characteristics of Al/Ni composites system. Assuming the SICR process is controlled by shock temperature rising in the materials, the reaction efficiency of Al/Ni composites was calculated by Arrhenius reaction rate and Avrami–Erofeev reaction models. Hugoniot curves and the temperature rise under shock compression were calculated to analyze the mechanism of the influence on SICR characteristics by additives. Impact-initiated experiments of Al/Ni composites were carried out to study SICR characters at various impact velocities. The parameters of SICR model were determined by using corresponding experimental results. The calculation results showed that for the SICR process of Al/Ni, the critical shock tem...

A computational analysis of the utility of chemical reactions within protective structures in mitigating shockwave-impact effects

Purpose The purpose of this paper is to introduce and analyze a new blast-wave impact-mitigation concept using advanced computational methods and tools. The concept involves the use of a protective structure consisting of bimolecular reactants displaying a number of critical characteristics, including: a high level of thermodynamic stability under ambient conditions (to ensure a long shelf-life of the protective structure); the capability to undergo fast/large-yield chemical reactions under blast-impact induced shock-loading conditions; large negative activation and reaction volumes to provide effective attenuation of the pressure-dominated shockwave stress field through the volumetric-energy storing effects; and a large activation energy for efficient energy dissipation. The case of a particular bimolecular chemical reaction involving polyvinyl pyridine and cyclohexyl chloride as reactants and polyvinyl pyridinium ionic salt as the reaction product is analyzed. Design/methodology/approach Direct simulations of single planar shockwave propagations through the reactive mixture are carried out, and the structure of the shock front examined, as a function of the occurrence of the chemical reaction. To properly capture the shockwave-induced initiation of the chemical reactions during an impact event, all the calculations carried out in the present work involved the use of all-atom molecular-level equilibrium and non-equilibrium reactive molecular-dynamics simulations. In other words, atomic bonding is not pre-assigned, but is rather determined dynamically and adaptively using the concepts of the bond order and atomic valence. Findings The results obtained clearly reveal that when the chemical reactions are allowed to take place at the shock front and in the shockwave, the resulting shock front undergoes a considerable level of dispersion. Consequently, the (conserved) linear momentum is transferred (during the interaction of the protective-structure borne shockwaves with the protected structure) to the protected structure over a longer time period, while the peak loading experienced by the protected structure is substantially reduced. Originality/value To the authors’ knowledge, the present work is the first attempt to simulate shock-induced chemical reactions at the molecular level, for purposes of blast-mitigation.

Morphological changes of olivine grains reacted with amino acid solutions by impact process

Early oceans on Earth might have contained certain amounts of biomolecules such as amino acids, and they were subjected to meteorite impacts, especially during the late heavy bombardment. We performed shock recovery experiments by using a propellant gun in order to simulate shock reactions among olivine as a representative meteorite component, water and biomolecules in oceans in the process of marine meteorite impacts. In the present study, recovered solid samples were analyzed by using X-ray powder diffraction method, scanning electron microscopy, electron probe microanalysis, and transmission electron microscopy with energy-dispersive X-ray spectrometry. The analytical results on shocked products in the recovered sample showed (1) morphological changes of olivine to fiber- and bamboo shoot-like crystals, and to pulverized grains; and features of lumpy surfaces affected by hot water, (2) the formation of carbon-rich substances derived from amino acids, and (3) the incorporation of metals from container into samples. According to the present results, fine-grained olivine in meteorites might have morphologically changed and shock-induced chemical reactions might have been enhanced so that amino acids related to the origin of life may have transformed to carbon-rich substances by impacts.

Quantum Nuclear Effects in Stishovite Crystallization in Shock-Compressed Fused Silica

Quantum nuclear effects include zero-point energy for each vibrational mode and potentially significant deviations of the heat capacity from classical values. While these effects play important roles in shock-induced chemical reactions and phase transitions, they are absent in classical atomistic shock simulations. To address this shortcoming, we have studied the quantum nuclear effects for stishovite crystallization in shock-compressed fused silica by employing the quantum bath multiscale shock technique, which couples the shock simulation with a colored-noise Langevin thermostat. We find that this semiclassical approach gives shock temperatures as much as 7% higher than classical simulations near the onset of stishovite crystallization in silica. We have also studied the impact of this approach on the kinetics of stishovite crystallization and the position of high-pressure silica melt line. We further describe a systematic way of setting up the parameters for the quantum thermal bath and quantum bath mu...

Shock compression response of highly reactive Ni + Al multilayered thin foils

The shock-compression response of Ni + Al multilayered thin foils is investigated using laser-accelerated thin-foil plate-impact experiments over the pressure range of 2 to 11 GPa. The foils contain alternating Ni and Al layers (parallel but not flat) of nominally 50 nm bilayer spacing. The goal is to determine the equation of state and shock-induced reactivity of these highly reactive fully dense thin-foil materials. The laser-accelerated thin-foil impact set-up involved combined use of photon-doppler-velocimetry to monitor the acceleration and impact velocity of an aluminum flyer, and VISAR interferometry was used to monitor the back free-surface velocity of the impacted Ni + Al multilayered target. The shock-compression response of the Ni + Al target foils was determined using experimentally measured parameters and impedance matching approach, with error bars identified considering systematic and experimental errors. Meso-scale CTH shock simulations were performed using real imported microstructures of the cross-sections of the multilayered Ni + Al foils to compute the Hugoniot response (assuming no reaction) for correlation with their experimentally determined equation of state. It was observed that at particle velocities below ∼150 m/s, the experimentally determined equation of state trend matches the CTH-predicted inert response and is consistent with the observed unreacted state of the recovered Ni + Al target foils from this velocity regime. At higher particle velocities, the experimentally determined equation of state deviates from the CTH-predicted inert response. A complete and self-sustained reaction is also seen in targets recovered from experiments performed at these higher particle velocities. The deviation in the measured equation of state, to higher shock speeds and expanded volumes, combined with the observation of complete reaction in the recovered multilayered foils, confirmed via microstructure characterization, is indicative of the occurrence of shock-induced chemical reaction occurring in the time-scale of the high-pressure state. TEM characterization of recovered shock-compressed (unreacted) Ni + Al multilayered foils exhibits distinct features of constituent mixing revealing jetted layers and inter-mixed regions. These features were primarily observed in the proximity of the undulations present in the alternating layers of the Ni + Al starting foils, suggesting the important role of such instabilities in promoting shock-induced intermetallic-forming reactions in the fully dense highly exothermic multilayered thin foils.

Numerical modelling of shock-induced chemical reactions (SICR) in reactive powder mixtures using smoothed particle hydrodynamics (SPH)

Shock compaction of reactive powder mixtures to synthesize new materials is one of the oldest material processing techniques and has been studied extensively by several researchers over the past few decades. The quantitative connection between the shock energy imparted and the extent of reaction that can be completed in the small time window associated with the passage of the shock wave is complicated and depends on a large variety of parameters. In particular, our understanding of the complex interplay between the thermo-elasto-viscoplastic behaviour of the granular constituents and their temperature dependent, diffusion-limited reaction mechanism may be enriched through careful numerical simulations. A robust numerical model should be able to handle extremely large deformations coupled with diffusion mediated fast reaction kinetics. In this work, a meshfree discrete particle numerical method based on smoothed particle hydrodynamics (SPH) to simulate shock-induced chemical reactions (SICR) in reactive powder mixtures is proposed. We present a numerical strategy to carry out reactions between reactant powder particles and partition the obtained products between the particles in a manner that accounts for the requirement that the total mass of the entire system remains constant as the reactions occur. Instead of solving the reaction–diffusion problem, we propose a ‘pseudo-diffusion’ model in which a distance dependent reaction rate constant is defined to carry out chemical reaction kinetics. This approach mimics the actual reaction–diffusion process at short times. Our numerical model is demonstrated for the well-studied reaction system Nb + 2Si NbSi2. The predicted mass fractions of the product obtained from the simulations are in agreement with experimental observations. Finally, the effects of impact speed, particle arrangement and mixing ratio on the predicted product mass fractions are discussed.

Influence of additives on microstructures, mechanical properties and shock-induced reaction characteristics of Al/Ni composites

Granular composites containing aluminum (Al) and nickel (Ni) are typical structural energetic materials, which possess ideal combination of both mechanical properties and energy release capability. The influence of two additives, namely Teflon (PTFE) and copper (Cu), on mechanical properties and shock-induced chemical reaction (SICR) characteristics of Al/Ni material system has been investigated. Three composites, namely Al/Ni, Al/Ni/PTFE and Al/Ni/Cu with same volumetric ratio of Al powder to Ni powder, were processed by means of static pressing. Scanning electron microscopy was used to study the microstructure of the mentioned three composites. Quasi static compression tests were also conducted to determine the mechanical properties and fracture behavior of the mentioned three composites. It was shown that the additives affected both compressive strength and fracture mode of the three composites. Impact initiation experiments on the mentioned three composites were performed to determine their shock-induced chemical reaction characteristics by considering pressure histories measured in the test chamber. The experimental results showed that the additives had significant effects on critical initiation velocity, reaction rate, reaction efficiency and post-reaction behavior.

Explosive solid-state synthesis in the Al-S system: Influence of dispersity and duration of shock loading

Explosive solid-state synthesis in the Al-S system was numerically simulated in terms of a multi-component model. Explored was the influence of initial dispersity and shock duration on the dynamics of shock-induced reactions. Depending on conditions, the reaction was found to proceed in a three-stage, two-stage or one-stage mode.