Shock-induced Reactions Research Articles

Premixing degree-dependent reaction mechanisms of premixed Ni/Al nanolaminates under shock loading

Understanding the intermetallic reaction is critical for reactive metal systems, among which Ni/Al nanolaminates have attracted extensive interest. A long-standing open question is how nanostructure such as premixed interlayer affects the reaction process. Here, we employ molecular dynamics simulations to investigate the effects of premixing degree on the shock-induced reaction mechanisms and reactivity for premixed Ni/Al nanolaminates. The multiple exothermic processes are identified, namely, the Ni–Al mixing driven by diffusion, the B2-NiAl crystallization in premixed interlayer, and the grain coarsening driven by grain boundary migration. Intriguingly, it is found that the specific exothermic processes depend strongly on the premixing degree. As the premixing degree increases, the B2-NiAl crystallization and the grain coarsening appear sequentially. The maximum crystallinity and grain size increase linearly and exponentially with premixing degree, respectively. Furthermore, inspired by the differences in exothermic processes, the intrinsic mechanism for the weakening effects of premixed interlayer on reactivity is elucidated. The B2-NiAl crystallization in premixed interlayer decreases the reaction heat and further the final adiabatic temperature, while the appearance of grain coarsening produces additional heat and alleviates the weakening effect. These findings can provide valuable insights into the nanostructure–reactivity relationship for reactive intermetallic materials.

The propagation behavior of reaction wave for Ni/Al clad particle composites under shock loading.

Prior studies indicate that the reaction wave can propagate from the impact surface, but the possibility and the influencing factors of the reaction wave formation are still unclear. This work investigates the propagation behavior of the shock-induced reaction wave for Ni/Al clad particle composites with varying stoichiometry (from 0.5 to 0.75 of the Ni mole fraction) through molecular dynamics simulations. It is found that the solid-state reaction processes with or without wave propagation strongly depend on the conjunction of stoichiometry and shock intensity. Within the cases of wave propagation, the calculated propagation velocity (in the range of 135-170m/s) increases linearly or exponentially with the Ni mole fraction. Furthermore, the thermodynamic criteria for the reaction wave formation, including Al melting at the collision surface and higher temperature gradient, are established by analysis of the shock-induced high-entropy layer. In addition, microstructural characterization reveals the intrinsic mechanisms of the propagation of the reaction wave and the formation of additional reaction wave, namely, the dissolution of Ni into Al and the coalescence of reaction zones. Apart from the propagation behavior, the initial stoichiometry influences the crystallization-dissolution of B2-NiAl during reaction processes, notably through an exponential growth relationship between maximum crystallinity and the Ni mole fraction. These findings may provide a physical basis for improving traditional reaction rate models to break through phenomenological understanding.

Shock-induced energy release reaction characteristics of Nb17Zr33Ti17W33 high entropy alloy

Nb17Zr33Ti17W33 high entropy alloy is a novel class of energetic structural material. In addition to exhibiting insensitivity and mechanical strength commensurate with inert metal materials, it demonstrates the capacity to release non-self-sustained chemical energy under high-speed dynamic load. In this study, the energy release characteristics of the Nb17Zr33Ti17W33 high-entropy alloy material under impact were investigated using the dynamic energy release test method, which involves ballistic gun-induced bullet-target interaction in a quasi-closed reaction vessel. The energy release process and governing laws of the impact event have been meticulously analyzed, resulting in the development of a comprehensive impact compression chemical energy release model. The direct ballistic tests and the recovered products revealed that Nb17Zr33Ti17W33 experienced chemical reactions during the impact loading, primarily the oxidation of the Zr element, which resulted in the release of significant chemical energy. This study elucidates the deformation response mechanism of Nb17Zr33Ti17W33 under high-speed impact loading, providing a crucial reference for further insights and applications of high-entropy alloy materials.

Shock-induced chemical reaction characteristics of PTFE-Al-Bi2O3 reactive materials

A ternary system of PTFE/Al/Bi2O3 is constructed by incorporating PTFE-based reactive material and thermite for enhancing the energy release of the PTFE-based reactive material. The effects of Bi2O3 in the PTFE/Al/Bi2O3 on both mechanical properties and the energy release were investigated through various tests such as thermogravimetry-differential scanning calorimetry, adiabatic oxygen bomb test and split Hopkinson pressure bar test. The microstructure observed through scanning electron microscope and X-ray diffraction results are used to analyze the ignition and reaction mechanism of PTFE/Al/Bi2O3. The results indicate that the PTFE/Al/Bi2O3 are capable of triggering the exothermic reaction of molten PTFE/Bi2O3 and Al/Bi2O3 over the PTFE/Al reactive materials, thereby promoting reactions. The excessive aluminum in the ternary system is beneficial for increasing energy release. The ignition of shock-induced chemical reactions in PTFE/Al/Bi2O3 is closely related to the material fracture. The dominant mechanism for hot-spot generation under Split Hopkinson Pressure Bar test is the frictional temperature rise at the microcrack after failure.

SPH simulation of shock-induced chemical reactions in reactive powder mixtures

Reactive materials (RMs) are a mixture of two or more nonexplosive solid components in which, under extreme conditions, self-sustaining chemical reactions occur with the release of a high amount of energy. Knowing the kinetics of chemical reactions in RMs plays the key role in their efficient applications. In this work, we propose a macroscopic mathematical model of shock-induced chemical reactions based on smoothed particle hydrodynamics (SPH) for particle-scale simulation. The model includes the elastic-plastic deformation of materials, heating due to deformation and chemical reactions, diffusion of components, and heat transfer. The test calculations performed for the zero- and first-order reactions in the Al/S mixture showed a need for a more reliable determination of criteria and constants of chemical reactions. For this purpose, we propose a schematic experimental arrangement for the experimental and theoretical determination of the critical reaction pressure and the rate constant of the reaction. The type and corresponding constants of chemical reactions can be determined, for example, by the maximum likelihood method. We theoretically determined the zones of chemical reaction with different modes and found the influence of the projectile velocity and the intensity of shock waves on the development and completion of chemical reactions.

Atomic insights into shock-induced alloying reaction of premixed Ni/Al nanolaminates.

In material processing and handling processes, premixed interlayer often replace the ideal Ni/Al interface, which would become a new origin of alloying reaction. This work investigates shock-induced reaction mechanism and kinetics of premixed Ni/Al nanolaminates with molecular dynamics simulations and theoretical analysis. The reaction is found to be driven by the crystallization evolution in premixed interlayer and the diffusion of premixed atoms. Among them, multi-stage reaction patterns are strongly manifested by the crystallization evolution characteristics. Specifically, "crystallization-dissolution-secondary growth" and "crystallization-dissolution" of B2 phase respectively correspond to the solid-state and solid-liquid reaction cases, where crystallizations are fitted well by Johnson-Mehl-Avrami kinetics model. Interestingly, the different growth mechanisms of B2 grain are revealed, namely nuclei coalescence and atomic diffusion. Moreover, the analysis of microscopic diffusion theory indicates a certain non-random diffusion nature for solid-state reaction initiation, but near-purely random diffusion for solid-liquid reaction initiation. The diffused Al atoms possess a limited diffusion coefficient and enhanced diffusion correlation, resulting in extremely slow mixing rate in Ni layer. In addition, the influence law of Ni concentration in premixed interlayer on reactivity parameters can be quantitatively described by a quadratic function.

The reactive flow evolution of the polymer-bonded explosive PBX 9502: Experiments and model validation in extreme pressure regimes

The shock-to-detonation transition properties of the triaminotrinitrobenzene based PBX 9502 high explosive (HE) are experimentally and computationally explored in extremely high input pressure conditions. These include both slightly sub-Chapman–Jouguet and overdriven input pressure conditions, namely, ∼25 and ∼31 GPa, respectively. Our experiments capture the transient buildup of a shock-induced reaction via measurement of HE and polymethyl methacrylate window interface particle velocity profiles for a variety of sample thicknesses for this insensitive HE. These observations necessitate extremely thin explosive samples, and the high rates of reaction provide a considerable challenge to optical diagnostics. Samples at these thicknesses also provide an opportunity for evaluation of potential micro-structure effects on the resulting shock-to-detonation-transition measurements. To address this, the thin samples are also characterized via x-ray micro-computed tomography. Finally, a pair of previously established continuum-level detonation performance modeling approaches for PBX 9502 were used to analyze the experiments. The employed model variants crucially differ in their definition of each model’s empirical reaction rate functional form, utilization of shock state quantities, and local flow variable dependencies. As a result, the present experiments provide a novel platform to evaluate the quantitative and qualitative consequences stemming from these modeling choices in a challenging initiation scenario, largely beyond the chosen calibration range of either model. This new experimental information will provide a platform for both improved physics and model parameterizations for this well-studied explosive.

Effects of shock-induced chemical reaction on equation of state for Ni/Al energetic structural material

The equation of state (EOS) for energetic structural materials (ESMs) has been drawn a great attention due to the absent comprehensive understanding on the effect of the shock-induced chemical reaction. In this paper, the shock compression behavior of Ni/Al ESM is investigated by developing the EOS, which mainly considers the effects of the chemical reaction and the reaction products. The chemical reaction is based on the Avram-Erofeev kinetic law and the Arrhenius equation. The study concerns the shock pressure, the relative volume, the temperature, and the chemical reaction during the shock compression. The effects of the initial porosities, the stoichiometric ratios and inert additives were mainly discussed. The results showed that high porosity would induce high temperature rise. Different stoichiometric ratios would produce different temperature rise. When the stoichiometric ratio Ni: Al ​= ​1:1, the temperature rise is highest. In addition, the inert additive material would obviously reduce the temperature rise. Finally, the developed model improved the temperature calculation, compared with the existing model.

The reaction mechanism and interfacial crystallization of Al nanoparticle-embedded Ni under shock loading

The shock-induced reaction mechanism and characteristics of Ni/Al system, considering an Al nanoparticle-embedded Ni single crystal, are investigated through molecular dynamics simulation. For the shock melting of Al nanoparticle, interfacial crystallization and dissolution are the main characteristics. The reaction degree of Al particle first increases linearly and then logarithmically with time driven by rapid mechanical mixing and following dissolution. The reaction rate increases with the decrease of particle diameter, however, the reaction is seriously hindered by interfacial crystallization when the diameter is lower than 9 nm in our simulations. Meanwhile, we found a negative exponential growth in the fraction of crystallized Al atoms, and the crystallinity of B2–NiAl (up to 20%) is positively correlated with the specific surface area of Al particle. This can be attributed to the formation mechanism of B2–NiAl by structural evolution of finite mixing layer near the collapsed interface. For shock melting of both Al particle and Ni matrix, the liquid-liquid phase inter-diffusion is the main reaction mechanism that can be enhanced by the formation of internal jet. In addition, the enhanced diffusion is manifested in the logarithmic growth law of mean square displacement, which results in an almost constant reaction rate similar to the mechanical mixing process.

Atomistic study on reaction kinetics and reactivity of Ni/Al clad particles composites under shock loading.

In prior research on shock-induced reaction, the interfacial crystallization of intermetallics, which plays an important role in solid-state reaction kinetics, has not been explored in detail. This work comprehensively investigates the reaction kinetics and reactivity of Ni/Al clad particle composites under shock loading with molecular dynamics simulations. It is found that the reaction acceleration in a small particle system or the reaction propagation in a large particle system breaks down the heterogeneous nucleation and continuous growth of B2 phase at the Ni/Al interface. This makes the generation and dissolution of B2-NiAl show a staged pattern consistent with chemical evolution. Importantly, the crystallization processes are appropriately described by the well-established Johnson-Mehl-Avrami kinetics model. With the increase in Al particle size, the maximum crystallinity and growth rate of B2 phase decrease and the value of the fitted Avrami exponent decreases from 0.55 to 0.39, showing a good agreement with the solid-state reaction experiment. In addition, the calculations of reactivity reveal that the reaction initiation and propagation will be retarded, but the adiabatic reaction temperature can be elevated when Al particle size increases. An exponential decay relationship is found between the propagation velocity of the chemical front and the particle size. As expected, the shock simulations at non-ambient conditions indicate that elevating the initial temperature significantly enhances the reactivity of large particle systems and results in a power-law decrease in the ignition delay time and a linear-law increase in the propagation velocity.

Shock-induced energy localization and reaction growth considering chemical-inclusions effects for crystalline explosives

Chemical inclusions significantly alter shock responses of crystalline explosives in macroscale gap experiments but their microscale dynamics origin remains unclear. Herein shock-induced energy localization, overall physical responses, and reactions in α-1,3,5-trinitro-1,3,5-triazinane (α-RDX) crystal entrained various chemical inclusions were investigated by the multi-scale shock technique implemented in the reactive molecular dynamics method. Results indicated that energy localization and shock reaction were affected by the intrinsic factors within chemical inclusions, i.e., phase states, chemical compositions, and concentrations. The atomic origin of chemical-inclusions effects on energy localization is dependent on the dynamics mechanism of interfacial molecules with free space volume, which includes homogeneous intermolecular compression, interfacial impact and shear, and void collapse and jet. As introducing various chemical inclusions, the initiation of those dynamics mechanisms triggers diverse decay rates of bulk RDX molecules and hereby impacts on growth speeds of final reactions. Adding chemical inclusions can reduce the effectiveness of the void during the shock impacting. Under the shockwave velocity of 9 km/s, the parent RDX decay rate in RDX entrained amorphous carbon decreases the most and is about one fourth of that in RDX with a vacuum void, and solid HMX and TATB inclusions are more reactive than amorphous carbon but less reactive than dry air or acetone inclusions. The less-dense shocking system denotes the greater increases in local temperature and stress, the faster energy liberation, and the earlier final reaction into equilibrium, revealing more pronounced responses to the present intense shockwave. The quantitative models associated with the relative system density (RDsys) were proposed for indicating energy-localization mechanisms and evaluating initiation safety in the shocked crystalline explosive. RDsys is defined by the density ratio of defective RDX to perfect crystal after dynamics relaxation and reveals the global density characteristic in shocked systems filled with chemical inclusions. When RDsys is below 0.9, local hydrodynamic jet initiated by void collapse dominates upon energy localization instead of interfacial impact. This study sheds light on novel insights for understanding the shock chemistry and physical-based atomic origin in crystalline explosives considering chemical-inclusions effects.

Multiscale modeling of the shock-induced chemical reaction in Al/Ni composites

Multiscale modeling of the shock-induced chemical reaction in Al/Ni composites

Compressive Mechanical Properties and Shock-Induced Reaction Behavior of Zr/PTFE and Ti/PTFE Reactive Materials.

Existing research on PTFE-based reactive materials (RMs) mostly focuses on Al/PTFE RMs. To explore further possibilities of formulation, the reactive metal components in the RMs can be replaced. In this paper, Zr/PTFE and Ti/PTFE RMs were prepared by cold isostatic pressing and vacuum sintering. The static and dynamic compressive mechanical properties of Zr/PTFE and Ti/PTFE RMs were investigated at different strain rates. The results show that the introduction of zirconium powder and titanium powder can increase the strength of the material under dynamic loading. Meanwhile, a modified J-C model considering strain and strain rate coupling was proposed. The parameters of the modified J-C model of Zr/PTFE and Ti/PTFE RMs were determined, which can describe and predict plastic flow stress. To characterize the impact-induced reaction behavior of Zr/PTFE and Ti/PTFE RMs, a quasi-sealed test chamber was used to measure the over-pressure induced by the exothermic reaction. The energy release characteristics of both materials were more intense under the higher impact.

The Anisotropic Chemical Reaction Mechanism of 1,3,3-trinitroazetidine (TNAZ) under Different Shock Wave Directions by ReaxFF Reactive Molecular Dynamics Simulations.

1,3,3-Trinitroazetidine (TNAZ) has good thermal stability and low shock sensitivity, among other properties, and it has broad prospects in insensitive ammunition applications. In this study, a molecular dynamics calculation based on the ReaxFF-lg force field and multiscale shock technique (MSST) was used to simulate the shock-induced chemical reaction of TNAZ with different shock wave directions. The results showed that the shock sensitivity of TNAZ was in the order of [100] > [010] > [001]. There were significant differences in molecular arrangements in different shock directions, which affected the reaction rate and reaction path in different directions. The molecular arrangement in the [010] and [001] directions formed a “buffer” effect. The formation and cleavage of bonds, formation of small molecules and growth of clusters were analyzed to show the effect of the “buffer”. The polymerization reactions in the [010] and [001] directions appeared later than that in the [100] direction, and the cluster growth in the [010] and [001] directions was slower than that in the [100] direction. In different shock loading directions, the formation and cleavage mechanisms of the N-O bonds of the TNAZ molecules were different, which resulted in differences in the initial reaction path and reaction rate in the three directions

Formation of Novel Bimetal Oxide In2V2O7 through a Shock Compression Method.

Bimetal oxides with a chemical formula of A2B2O7 have received much attention from plenty of research groups owing to their outstanding properties, such as electronic, optical, and magnetic properties. Among the abundant element combinations of cations A and B, some theoretically predicted compounds have not successfully been synthesized in experiments, such as In2Zr2O7, In2V2O7, etc. In this study, a novel tetragonal pyrochlore-like In2V2O7 nanopowder has been reported for the first time. In2O3 and VO2 powders mixed through ball milling were reacted to form In2V2O7 by shockwave loading. The recovered sample is investigated to be nanocrystalline In2V2O7 powder through various techniques, such as X-ray diffraction, scanning electron microscopy, X-ray energy spectrum analysis, and transmission electron microscopy. The formed In2V2O7 is indexed as a tetragonal cell with a = b = 0.7417 nm and c = 2.1035 nm. Moreover, the formation mechanism of In2V2O7 through a shock synthesis process is carefully analyzed based on basic laws of shockwave. The experimental results also confirm that a high shock temperature and high shock pressure are the two key factors to synthesize the In2V2O7 nanopowder. Our investigation demonstrates the high potential application of a shock-induced reaction on the synthesis of novel materials, including the preparation of new bimetal oxides.

Hot spot ignition and growth from tandem micro-scale simulations and experiments on plastic-bonded explosives

Head-to-head comparisons of multiple experimental observations and numerical simulations on a deconstructed plastic-bonded explosive consisting of an octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine crystal embedded in a polymeric binder with a 4 ns duration 20 GPa input shock are presented. Hot spots observed in high-resolution direct numerical simulations are compared with micro-scale shock-induced reactions visualized using nanosecond microscope imaging and optical pyrometry. Despite the challenges and limitations of both the experimental and simulation techniques, an agreement is obtained on many of the observed features of hot spot evolution, e.g., (1) the magnitude and time variation of temperatures in the hot spots, (2) the time to fully consume the crystals (∼100 ns) of size (100–300 μm) employed in this study, and (3) the locations of hot spot initiation and growth. Three different mechanisms of hot spot formation are indicated by simulations: (1) high-temperature hot spots formed by pore collapse, (2) lower temperature hot spots initiated at the polymer–crystal interface near corners and asperities, and (3) high-temperature reaction waves leading to fast consumption of the energetic crystal. This first attempt at a head-to-head comparison between experiments and simulations not only provides new insight but also highlights efforts needed to bring models and experiments into closer alignment, in particular, highlighting the importance of distinctly three-dimensional and multiple mechanisms of the hot spot ignition and growth.

Chemical reaction of Ni/Al interface associated with perturbation growth under shock compression

The exothermic reaction of Ni/Al laminates always starts from the interface, and the role of interfacial instability in the shock-induced chemical reaction has not been clarified. This work reports the Richtmyer–Meshkov (RM) instability growth, atomic diffusion, and chemical reaction of Ni/Al interface under shock compression based on atomistic simulations. For shocking from Al to Ni, the interface experiences finite collapse and exhibits weak localized reaction. The diffusion of solid Ni to molten Al will be inhibited due to the formation of NiAl phase, and continuous inter-diffusion occurs with the melting of Ni. For shocking from Ni to Al, a small amount of NiAl structure is formed due to the atomic residue during defect collapse. RM instability growth is observed at higher shock intensity, which significantly promotes the atomic mixing and results in a power-law increase in the number of diffusing atoms. Meanwhile, the chemical reaction propagates rapidly from the vortex to the head of the spike accompanied by the decomposition of many clusters, with the nonlinear development of RM instability. The number and the size of Ni clusters no more satisfy the simple power-law relationship for which we propose an improved power-law distribution. Interestingly, the growth of nanoscale perturbation approximately satisfies the logarithmic law with time, but the linear growth stage is inhibited due to significant inter-diffusion, especially for the small wavelength. Thus, the mixing width and the reaction degree are positively correlated with the initial wavelength in our simulation scale, which is contrary to the RM growth law of the free surface.

Energy Release Characteristics and Reaction Mechanism of PTFE/Al/Bi2O3 Reactive Materials under Drop-Hammer Test

To obtain the influence of the Bi2O3 particle content of a PTFE/Al/Bi2O3 reactive material (later referred to as PAB) on its shock-induced chemical reaction (SICR) characteristics, five kinds of PAB with different Bi2O3 contents were prepared; the reaction process in a drop-hammer test, recorded using a high-speed camera, was analyzed. The ignition and reaction mechanisms of PAB under mechanical impact were analyzed based on the thermochemical reaction characteristics and the microstructure. The results show that with an increase in Bi2O3 content, the shock-induced chemical reaction duration and the sensitivity of PAB increase, and then decrease. When the Bi2O3 content is 9%, the impact sensitivity is the highest and the reaction duration is the longest. The heating at the crack tip is responsible for PAB ignition under long-pulse low-velocity impact. During ignition, PAB undergoes several physicochemical changes such as the melting of PTFE, a PTFE/Bi2O3 reaction, an Al/Bi2O3 reaction, pyrolysis of the melted PTFE, and a C2F4/Al reaction; moreover, the presence of Bi2O3 decreases the excitation threshold of the reactive material, which facilitates the propagation of the reaction and improves the degree of the reaction and overall energy release of the reactive material.

Effect of dynamic high-pressure loading on decomposition reaction and negative thermal expansion of silver oxide

The effect of dynamic high-pressure loading on the decomposition reaction and negative thermal expansion of Ag2O was investigated by X-ray diffraction (XRD) analysis and differential scanning calorimetry (DSC). The XRD pattern of the sample shocked at 6.4 GPa indicated that the sample was composed of cubic phase Ag2O and metallic Ag. These XRD patterns indicated that the shock-induced decomposition reaction of Ag2O occurred when the sample was shock-loaded at 6.4 GPa and above. The DSC curves of the shocked Ag2O revealed that an additional exothermic reaction occurred at around 478 K in addition to an endothermic reaction at around 700 K, which corresponds to the decomposition of Ag2O. The exothermic reaction at around 478 K was probably caused by the release of shock-induced residual energy. Synchrotron XRD performed from 300 to 130 K clarified the suppression of negative thermal expansion in the shocked Ag2O.

Study on energy release characteristics and penetration effects to concrete targets of Hf-based amorphous alloys

In this paper, the energy release characteristics and penetration effects of Hf-based amorphous alloys subjected to high velocity impact were investigated. The quasi-sealed test chamber system was used to record the impact energy release process of Hf-based amorphous alloy fragments shotted by 14.5 mm ballistic gun at elevated impact velocities. The reaction behaviors were observed by the high-speed camera and measured by the history curves of the impact reaction pressure using a piezoresistive pressure sensor. The penetration experiments of jacketed long rod projectiles (LRPs) of Hf-based amorphous alloys launching into concrete targets were carried out by 30 mm ballistic gun with velocities about 1400 m/s, where the experiments with inert steel jacketed LRPs were taken as a contrast. The video frames from the impact events were recorded by the high-speed camera. The results demonstrated that obviously shock-induced chemical reactions took place in Hf-based amorphous alloy fragments during the impact process, where the impact velocity played a significant role. Compared with the traditional steel jacketed LRPs, both the penetration depth and the crater of Hf-based amorphous alloy jacketed LRPs to concrete targets were significantly increased, which indicated that Hf-based amorphous alloy could be used to increase the damage effects of warheads.