A deep understanding of explosive sensitivities and their factors is important for safe and reliable applications. However, quantitative prediction of the sensitivities is difficult. Here, reactive molecular dynamics simulation models for high-speed piston impacts on explosive supercells were established. Simulations were also performed to investigate shock-induced reactions of various high-energy explosives. The fraction of reacted explosive molecules in an initial supercell changed linearly with the propagation distance of the shock-wave front. The corresponding slope could be used as a reaction rate for a specific shock-loading velocity. Reaction rates that varied with the shock-loading pressure exhibited two-stage linearities with different slopes. The two inflection points corresponded to the initial and accelerated reactions, which respectively correlated to the thresholds of shock-induced ignition and detonation. Therefore, the ignition and detonation critical pressures could be determined. The sensitivity could then be a quantitative prediction of the critical pressure. The accuracies of the quantitative shock sensitivity predictions were verified by comparing the impact and shock sensitivities of common explosives and the characteristics of anisotropic shock-induced reactions. Molecular dynamics simulations quantitatively predict and rank shock sensitivities by using only crystal structures of the explosives. Overall, this method will enable the design and safe use of explosives.

Aluminum nanoparticles (ANPs) can greatly improve the power of explosives. However, the rapid reaction mechanism of ANPs under simultaneous high temperature and high pressure by shock loading is not fully understood. In this study, a detonation wave was generated by impact of an explosive supercell on the reflect wall, and the reflected wave was eliminated by changing the end-boundary velocity. In this way, the problem of long time simulation under extreme pressure was solved and reaction molecular dynamics simulations of ANPs in explosives under shock or detonation were then performed with the ReaxFF force field. The results showed that the ANP crystal structure first transformed under shock loading, and diffusion oxidation of the ANPs then occurred. The reaction rate of the ANPs under high-temperature and high-pressure conformed to an exponential function of the pressure and oxide-shell thickness. Finally, the ANPs were stretched and disintegrated with expansion of the detonation products, which further accelerated ANP oxidation. A thicker oxide shell and a wax covering on the ANPs limited diffusion of the O and N atoms into the ANPs, which slowed down the oxidation reaction of the ANPs. A wax covering also prevented direct contact of the ANPs with the explosive, weakening the effect of the ANPs on the reaction of the explosive. This work is of great importance to deeply understand the reaction mechanism and energy-release law of aluminized explosives.

纳米铝颗粒作为新式储氢及储能材料,在环保方面有重要意义,其可以在真空中由激光烧蚀废铝制造.为了分析激光烧蚀制造纳米铝颗粒反应过程,更好地指导纳米铝颗粒的制备,将双温模型与分子动力学结合,利用Lammps分子动力学软件对烧蚀及溅射过程进行了模拟,获得了激光烧蚀过程中溅射团簇的种类、数目及体积的变化情况.在整个模拟过程中,团簇的主要变化依次为溅射原子结合生成大团簇、大团簇裂解为小团簇及团簇分解生成铝原子.

High-power pulsed lasers provide an ingenious method for launching metal foils to generate high-speed flyers for high-pressure loading in material science or aerospace engineering. At high-temperature and high-pressure laser-induced conditions, the dynamic response of the metals and the mechanism of flyer formation remain unclear. In this study, the overall process of the laser-driven aluminum flyer, including laser ablation, rupture of metal foil, and the generation of the flyer was investigated by molecular dynamics combined with the two-temperature model. It was found that under high laser fluence (over 1.3 J/cm 2 with 200-fs laser pulse duration), the laser induced a shock wave with a peak pressure higher than 25 GPa, which led to shear bands expanding from the edge of the laser ablation zone in the foil. Compared with the cases of low laser fluence less than 0.5 J/cm 2 , the shear band induced by high laser fluence promotes the rupture of the foil and results in a high-speed flyer (> 1 km/s) with better flatness and integrity. In addition, the shock wavefront was found to be accompanied by aluminum crystal phase transformation from face-centered cubic (FCC) to body-centered cubic structure. The crystal structure reverts with the decrease of pressure, therefore the internal structure of the generated flyer is pure of FCC. The results of this study provide a better understanding of the laser-induced shock effect on the foil rupture and flyer quality and forward the development of the laser-driven flyer.

A method is proposed to assess the thermal safety of solid propellant charges by measuring the tem-perature and pressure of propellant specimens during cook-off tests and determining the reaction kinet-ics. In this work, two propellants-ammonium perchlorate/hydroxy-terminated polybutadiene/aluminum (AP/HTPB/Al), and ammonium perchlorate/hydroxy-terminated polyether/aluminum (AP/HTPE/Al)-are taken as examples to demonstrate this method in detail. Specifically, by implementing two kinds of cook-off tests-viz., multipoint temperature monitoring, and combustion pressure measurement-the ther-mal reaction temperature before ignition and the combustion pressure after ignition of the two propel-lants were respectively determined. Then, the kinetic parameters of the thermal decomposition reaction model and the parameters of the combustion reaction model were numerically simulated and calibrated to achieve mathematical descriptions of the entire process of cook off of two solid propellant charges. On this basis, the technology of grid-node separation calculation was employed to simulate and predict the rupture of a solid rocket motor case, thereby quantitatively describing the severity of the cook-off reaction. The results demonstrate that, at a heating rate of 1 K min -1, the ignition positions of the two propellant charges in the solid rocket motor are located in the annular area where the side wall and the front of the casing are joined. Compared with those of the AP/HTPE/Al charge, the ignition time of the AP/HTPB/Al charge is longer (12,714 s for AP/HTPB/Al vs. 9701 s for AP/HTPE/Al), the temperature of the casing before ignition is higher (501.5 K for AP/HTPB/Al vs. 466.2 K for AP/HTPE/Al), the reaction after ignition is more intense, and the deformation of the casing is more serious. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Laser-driven flyer technology is a new dynamic high-pressure loading approach for accelerating metal as a high-speed flyer. The flyer velocity can be effectively increased using a multi-pulse laser. However, the effect of interactions between the multi-pulse laser and the metal foil on flyer formation is not clear. Based on atomic-scale dynamics combined with the two-temperature model, this paper models for the first time the entire process of using a multi-pulse laser to form a high-speed flyer. It was found that the velocity, thickness, and integrity of the flyer are different for multi-pulse than for single pulse. For a fixed number of pulses, the velocity and integrity of the flyer can be increased by appropriately increasing the delay time. However, if the delay time is too long, the shock wave generated by the second pulse will cause the flyer to suffer from secondary shock loading, and the integrity of the flyer is destroyed. If the delay time between each laser beam is fixed, the energy of each beam and the resulting pressure of the shock wave can be reduced by increasing the number of pulses. In this case, the flyer does not undergo strong impact loading and the integrity of the flyer is improved. The shock wave caused by laser pulse can result in the crystal transformation from FCC to BCC or HCP, which enhances the formation of flyer. The results of this study are important for understanding the dynamic response of a metal subjected to a multi-pulse laser and for developing laser-driven flyer technology.

Host–guest materials exhibit great potential applications as an insensitive high-energy–density explosive and low characteristic signal solid propellant. To investigate the mechanism of the improvement of the energy of host–guest explosives by guest molecules, ReaxFF-lg reactive molecular dynamics simulations were performed to calculate the thermal decomposition reactions of the host–guest explosives systems ICM-102/HNO 3 , ICM-102/H 2 O 2 , and pure ICM-102 under different constant high temperatures and different heating rates. Incorporation of guest molecules significantly increased the energy level of the host–guest system. However, the initial reaction path of the ICM-102 molecule was not changed by the guest molecules. The guest molecules did not initially participate in the host molecule reaction. After a period of time, the H 2 O 2 and HNO 3 guest molecules promoted cleavage of the C–N bond of the ICM-102 ring. Stronger oxidation and higher oxygen content resulted in the guest molecules more obviously accelerating destruction of the ICM-102 ring structure. The guest molecules accelerated the initial endothermic reaction of ICM-102, but they played a more important role in the intermediate exothermic reaction stage: incorporation of guest molecules (HNO 3 and H 2 O 2 ) greatly improved the heat release and exothermic reaction rate. Although the energies of the host–guest systems were clearly improved by incorporation of guest molecules, the guest molecules had little effect on the thermal stabilities of the systems.

In this work, based on the ReaxFF-lg reactive force field, the effects of temperature and wax binder on RDX thermal conductivity were studied by using molecular dynamics simulation (MD). The non-equilibrium molecular dynamics (NEMD) method was used to calculate the heat transfer process of RDX crystals with different sizes and at different temperatures and for RDX/wax mixtures with different ratios. The contribution of acoustic and optical phonons to the thermal conductivity was calculated using a thermal conductivity decomposition procedure. The heat transfer process of the RDX supercell and the RDX/wax mixtures were analyzed via calculation of the vibrational density of states. The thermal conductivity of the RDX increased with increasing temperature while the temperature was below 298 K, and decreased slightly with increasing temperature while the temperature was above 323 K. The interfacial thermal resistance between RDX and wax was the main factor that led to a decrease in the thermal conductivity of the RDX/wax mixed explosives. Acoustic phonons played a major role for heat transfer in RDX and mixed RDX explosives. An increased temperature resulted in more frequent expansion or torsion of the bonds in the optical modes, resulting in a slight reduction in the thermal conductivity.

The contradiction between energy and safety of explosives is better balanced by the host-guest inclusion strategy. Understanding the reaction mechanism of the host-guest explosive is necessary. To deeply analyze the role of the small guest molecules in the host-guest system, a quantum-based molecular dynamics method was used to calculate the initial decomposition reaction of the new host-guest explosive ICM-102/HNO3 against the pure ICM-102 at several high temperatures. The incorporation of HNO3 had no significant influence on the initial decomposition step of ICM-102. Conversely, the earliest intramolecular hydrogen transfer reaction is delayed partly because the H and O atoms of HNO3 connect with the O and H atoms of ICM-102, respectively. As the reaction proceeds, guest molecules get heavily involved in the reaction and increase the reaction rate. The generation rate and quantity of the small oxidizing molecules in the final product were increased significantly in the ICM-102/HNO3 system. These mechanisms revealed that HNO3 molecules inhibit the early stages of the initial decomposition of ICM-102 to some extent, and play an important role in accelerating the decomposition subsequently.

The layered crystal structure of the explosive ICM-102 (2,4,6-triamino-5-nitropyrimidine-1,3-dioxide) exhibits an extremely low sensitivity, and furthermore, the layering is observed to induce typical anisotropy. To elucidate the anisotropic initial reaction mechanism, ReaxFF-Ig reactive molecular dynamic simulations were performed to investigate the shock-induced reaction of ICM-102 by the piston impact on the supercell directly along different directions at various velocities. A novel method is proposed, which eliminates the boundary reflection at the supercell edge and studies prolonged simulations of the explosive reaction within a small supercell. When subjected to shock loadings along the x and y axes, which are parallel to the multilayers, the layered structure is at first observed to bend prior to undergoing a dimerization reaction via intermolecular O-O or O-H bond formation between the ICM-102 molecules in the same layer. When subjected to shock loading along the z axis, which is perpendicular to the multilayers, the interlayer space is first compressed leading to a dimerization reaction via N-O or C-N bond formation between the ICM-102 molecules from different layers. The energy for dimerization of the molecules in the same layer is lower, and hence, dimerization is observed to be easier. The reaction of ICM-102 is the most intense when the shock loading along the x axis is of the same strength as the shock loading along the other directions. A critical pressure is observed when the reaction rate of ICM-102 changes from slow to fast regardless of the shock loading direction. The critical pressure correlates well with shock sensitivity. The most sensitive orientation of ICM-102 is x axis > y axis > z axis.

With ultrashort duration and ultrahigh energy, femtosecond laser (fs-laser) pulses are very promising for the precision machining of energetic materials. Compared with the mechanical machining methods of energetic materials, fs-laser machining technology has the advantages of high safety, high precision, and absence of pollution. A deep understanding of the mechanisms between fs-lasers and energetic materials is the basis for the development of fs-laser machining technology. In this paper, the method of reactive molecular dynamics (ReaxFF-MD) was adopted to calculate the fs-laser ablation process of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX, a high explosive compound), and the ablation mechanisms of HMX under different fs-laser energies were studied. The results show that the fs-laser ablation mechanisms of HMX are related to the laser power density. When the laser power density is high enough (3.4 x 10(14) W/cm(2), 1.0 mJ/pulse), HMX undergoes ionization or decomposition reactions at the picosecond level (similar to 7.65 ps) and produces a high temperature and pressure plasma. Many N, H, and O single atoms and their ionic products occur along with some small molecular fragments of NO2, H2O, CO2, N-2, H-2, NH, NH2, CO, OH, CNO2 and very few toxic products of NO and HNO2. In this case, the removal process of HMX occurs via a phase explosion mechanism. As the laser energy decreases, the ionization degree of ablation products decreases, in which the number of monatomic and ionic products decreases, while the number of toxic small molecules (such as NO, HNO2, and HNO) increases. When the laser power density is relatively low (0.34 x 10(14) W/cm(2), 0.1 mJ/pulse), the removal process of HMX occurs via a photomechanical mechanism, and the compound escapes as intact initial HMX molecules. When the laser power density is close to the ablation threshold of the explosive, the HMX molecules only undergo a melting state to some extent without escaping from the surface of the crystal. Therefore, the fs-laser can be used in the precise machining of explosives and preparation of high-purity energetic nanomaterials by a reasonable selection of fs-laser energy.

Replacing existing inert binders with energetic ones in composite explosives is a novel way to improve the explosive performance, on the proviso that energetic binders are capable of releasing chemical energy rapidly in the detonation environment. Known to be a promising candidate, the reaction mechanism of glycidyl azide polymer (GAP) at typical detonation temperatures higher than 3000 K has been theoretically studied in this work at the atomistic level. By analyzing and tracking the cleavage of characteristic chemical bonds, it was found that at the detonation temperature, GAP was able to release a large amount of energy and small molecule products at a speed comparable to commonly used explosives in the early reaction stage, which was mainly attributed to the decomposition of azide groups into N2 and the main chain breakage into small fragments. Moreover, N2 generation was found to be accelerated by H atom transfer at an earlier reaction step. The dissociation energy of the main chain was lowered with structure deformation so as to facilitate the fragmentation of the GAP chain. Based on this analytical study of reaction kinetics, GAP was found to have higher reactivity at the detonation temperature than at lower temperatures. The small molecules' yield rate is of the same order of magnitude as an explosive detonation reaction, indicating that GAP has the potential to improve the performance of composite explosives. Our study reveals the chemical decomposition mechanism of a typical energetic binder, which would aid in the future design and synthesis of energetic binders so as to achieve both sensitivity-reducing and energy-enhancing performance goals simultaneously.

Layered molecular structure explosives have the characteristic of great thermal stability. Understanding the mechanism of thermal stability and the reactions of layered molecular structure explosives can provide new ideas for the design of thermally stable explosives. In a molecular dynamics simulation of thermal decomposition of the layered molecular structure explosive 2,4,6-triamino-5-nitropyrimidine-1,3-dioxide, we find that the layered molecular structure provides free space for chemical bond deflection and expansion so that the external energy absorbed by chemical bonds on nonbenzene rings can be converted into angle bending energy and bond-stretching energy, which makes chemical bonds difficult to break and increases the thermal stability of the explosives. In the layered molecular structure explosive reactions, hydrogen-oxygen-bonded interlayer dimerizations and hydrogen interlayer transfer reactions are dominant.

Abstract 4‐amino‐3‐aminopyrazole‐8‐trinitropyrazolo‐[5, 1‐c] [1, 2, 4]triazine (PTX, C5H2N8O6) has good detonation performance, thermal stability and low mechanical sensitivity, which endow it with good development prospects in insensitive ammunition applications. To study the effects of polymerization on the decomposition of PTX, the reaction processes of PTX at different conditions were simulated by quantum chemistry and molecular dynamics methods. In this paper, the effects of polymerization on the decomposition of PTX were studied in terms of species information, reaction path of PTX, bond formation and bond cleavage, evolution of small molecules and clusters, and kinetic parameters at different stages. The results show that under the high‐temperature and high‐pressure conditions, the initial reaction path of unimolecular PTX in the thermal decomposition is mainly the cleavage of C−NO2 bonds. At the same time, there are many polymerization reactions in thermal decomposition process, which may greatly affect the reaction rate and path. The higher the degree of polymerization, the larger equilibrium value of potential energy, the less energy release of thermal decomposition. Compared with the activation energy of other explosives, the activation energy of PTX is higher than that of β‐HMX and lower than that of TNT.

Studying the chemical reactions of hexanitrohexaazaisowurtzitane (CL-20) under heat and shock is helpful to understand its sensitivity and shock initiation mechanism. In this work, several molecular dynamics simulations were performed under three different conditions: high temperature, high temperature and pressure, and shock. The formation and breakage of chemical bonds, changes of bond lengths, and initial reactions were analysed. It was found that the main small-molecule product of CL-20 during initial decomposition under the three different conditions was always NO2, but the generation pathways were different. At high temperatures, NO2 was generated by the direct cleavage of N-NO2 bonds. In contrast, high pressure and shock promoted the transfer of O atoms to N atoms connected to NO2, leading to the breakage of N-NO2 bonds. Almost all NO2 originated from the transfer of O atoms under the shock conditions.

Novel host-guest/multicomponent energetic materials can be obtained by embedding hydrogen- or nitrogen-containing oxidizing small molecules between the molecules of high-energy explosives, which can improve their explosive energy. To better understand the mechanism of oxidizing small molecules in the reaction and improve the energy, ReaxFF-lg reactive molecular dynamics simulations were performed to investigate the thermal decomposition reaction at different temperatures of the CL-20/H2O2 solvate formed by embedding H2O2 in the cavity of CL-20. We propose an analytical method to investigate the mechanism of H2O2 in the CL-20 reaction by tracing the interactions between the H and O atoms of H2O2 and the C, H, N, and O atoms of CL-20. During thermal decomposition of CL-20/H2O2 CL-20 and H2O2 first separately decompose, and then, the decomposition products react. The H atoms, O atoms, and hydroxyl (HO) groups generated by H2O2 decomposition connect with the O atoms of nitro groups, leading to N-O bond cleavage. The O atoms generated by H2O2 decomposition connect with C atoms, leading to C-N bond cleavage, which catalyzes 'destruction of the CL-20 cage structure and increases the CL-20 decomposition rate. Eventually, the H and O atoms of H2O2 mainly bond to the O and C atoms of CL-20, respectively, which causes generation of greater amounts of H2O and CO2 and increases the heat released. These mechanisms increase the detonation velocity and pressure of explosives. The proposed analytical method can be used to investigate the reaction mechanisms of other host-guest/multicomponent energetic materials.

Researchers have been striving to determine the connection between the microscopic chemical reactions and macroscopic detonation laws of explosives. In this study, we performed reactive molecular dynamics simulations of the shock-induced explosion of the 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane explosive. The results show that detonation is mainly determined by the rapid irreversible cleavage of the C–N and C–H bonds. Such C–N and C–H bond cleavages determine the early formation of N2 and H2O. The detonation reaction occurs when the cleavage rates exceed 3.11 and 4.15%/ps for the C–N and C–H bonds, respectively. A higher shock velocity results in higher cleavage rates of these bonds, but it also leads to more atoms being trapped in clusters. However, the decomposition rate of these clusters is mainly affected by the decrease in the density, not by the shock velocity, indicating that the late detonation reaction is mainly based on the characteristics of the explosive.

To understand the initial chemical reaction mechanism of the heterogeneous explosive hexanitrohexaazaisowurtzitane (CL-20), it is necessary to study the shock initiation mechanism of this nanovoid- containing crystal. In this paper, supercells of CL-20 with different void sizes were constructed. The chemical reactions induced by different impact velocities were calculated using molecular dynamics based on the ReaxFF-lg reactive force field. The effects of impact velocities and void sizes on the chemical reactions of the CL-20 crystal were discussed. The initial reaction of CL-20 molecules around the voids was analyzed, and the evolution of the formation and breakage of chemical bonds as well as the elementary reactions were also obtained. It is found that under an impact, the CL-20 molecules around the voids first undergo polymerization of the N-O bonds and then breakage of the C-N, N-N, and C-H bonds occurs. Increased void size and impact velocity lead to higher temperature "hot spots" and more intense chemical reactions, but have little effect on the breaking sequence of chemical bonds in the CL-20 molecules.

The technology of laser-driven flyer can be applied in research fields such as explosives initiation. Flyer velocity is an important indicator to measure the impact initiation ability of laser-driven flyer. A one-dimensional numerical simulation model of laser-driven flyer is established. The phase transition of the aluminum film, the laser energy absorbed by plasma, the dynamic mechanical response inside the flyer and the interaction between the flyer and air are considered. The plasma-driven flyer process are simulated by using the weighted essentially non-oscillatory (WENO) finite difference scheme, level set equation and ghost fluid method. It is found that at the end of laser irradiation, the aluminum film exhibits three regions: gas phase, gas-solid mixture phase and solid phase. High pressure plasma forms the gas phase region, and its width represents the ablation depth of aluminum film. The solid phase region forms a flyer under the action of the plasma. During the acceleration process, the flyer undergoes intense compression and tension. The flyer velocity can lie improved by using double-pulse laser.

4,10-Dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0.0(5,9).0(3,11)]dodecane (TEX) is a new type of cage structured explosive with good explosive performance, good thermal stability and low production cost. TEX has high application value in casting and press-packed explosives. A TEX supercell model was constructed using the reaction molecular dynamics (MD) method. Based on the ReaxFF/lg reactive force field, an MD simulation of the thermal decomposition process of TEX explosives at different temperatures (2000 K, 2500 K, 3000 K, and 3500 K) was performed. The calculations were carried out to analyze the initial reaction pathways, small molecule products, destruction of the TEX cage structure and formation of clusters. The structural characteristics and elemental composition of the clusters were studied, and the reaction kinetic parameters of TEX at different reaction stages were calculated. During the thermal decomposition process, the N-NO2 bond in the TEX molecular structure breaks first, then the adjacent C-O bond is stretched, and finally the cage structure is gradually destroyed. The main decomposition products are small molecules (NO2, NO, H2O, CO2, N-2, H-2, HNO2 and HNO) and clusters (C12H12N6O12 and C18H13N7O14). The effect of temperature on the clusters is twofold. On the one hand, when the temperature is low, the cage structure of the TEX molecule is difficult to destroy and is not conducive to cluster growth. On the other hand, when the temperature is high, clusters are generated in the TEX supercell system but are subsequently rapidly decomposed. Throughout the whole process, H, N and O will gradually escape from the cluster, but O is more confined to the cluster, which will affect the subsequent autoxidation process of TEX to some extent. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.