Reaction mechanism of aluminum nanoparticles in explosives under high temperature and high pressure by shock loading.

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.

Chemical reactions of a CL-20 crystal under heat and shock determined by ReaxFF reactive molecular dynamics simulations.

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.

A quantum-based molecular dynamics study of the ICM-102/HNO3 host-guest reaction at high temperatures.

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.

Thermal stability mechanism via energy absorption by chemical bonds bending and stretching in free space and the interlayer reaction of layered molecular structure explosives.

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.

Decomposition and Energy-Enhancement Mechanism of the Energetic Binder Glycidyl Azide Polymer at Explosive Detonation Temperatures.

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.

Effect of density on the thermal decomposition mechanism of ε-CL-20: a ReaxFF reactive molecular dynamics simulation study.

The explosive detonation reaction occurs when explosives are compressed by different shock strengths, and the degree of compression affects the chemical reaction of the detonation process. The thermal decomposition mechanism of explosives under different compression densities has thus attracted significant research interest, and a better understanding of this mechanism would be helpful for determining the mechanism of the detonation reaction of explosives. In this study, a ε-CL-20 supercell was constructed, and the thermal decomposition was calculated at different compression densities and temperatures using molecular dynamics simulations based on the ReaxFF-lg reactive force field. We analyzed the effect of density on the main elementary reaction, which consists of the initial reaction and the formation of final products. In addition, we studied the effect of density on the generation of clusters and the reaction kinetics of the thermal decomposition. The results indicate that the initial reaction pathway of the CL-20 molecule is the cleavage of the N-NO2 bond at different densities and that the frequency of N-NO2 bond breakage decreases at high density. As the density increases, clusters easily form and are resistant to decomposition at the later stage of thermal decomposition, which eventually leads to a decrease in the number of final products. Increasing the initial density of CL-20 significantly increases the reaction rate of the initial decomposition but hardly changes the activation energy of the decomposition.

Thermal Decomposition Mechanism of CL-20 at Different Temperatures by ReaxFF Reactive Molecular Dynamics Simulations.

Hexanitrohexaazaisowurtzitane (CL-20) has a high detonation velocity and pressure, but its sensitivity is also high, which somewhat limits its applications. Therefore, it is important to understand the mechanism and characteristics of thermal decomposition of CL-20. In this study, a ε-CL-20 supercell was constructed and ReaxFF-lg reactive molecular dynamics simulations were performed to investigate thermal decomposition of ε-CL-20 at various temperatures (2000, 2500, 2750, 3000, 3250, and 3500 K). The mechanism of thermal decomposition of CL-20 was analyzed from the aspects of potential energy evolution, the primary reactions, and the intermediate and final product species. The effect of temperature on thermal decomposition of CL-20 is also discussed. The initial reaction path of thermal decomposition of CL-20 is N-NO2 cleavage to form NO2, followed by C-N cleavage, leading to the destruction of the cage structure. A small number of clusters appear in the early reactions and disappear at the end of the reactions. The initial reaction path of CL-20 decomposition is the same at different temperatures. However, as the temperature increases, the decomposition rate of CL-20 increases and the cage structure is destroyed earlier. The temperature greatly affects the rate constants of H2O and N2, but it has little effect on the rate constants of CO2 and H2.

Mechanism of the improvement of the energy of host-guest explosives by incorporation of small guest molecules: HNO3 and H2O2 promoted C-N bond cleavage of the ring of ICM-102.

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/HNO3, ICM-102/H2O2, 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 H2O2 and HNO3 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 (HNO3 and H2O2) 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.

Polymerization Effects on the Decomposition of a Pyrazolo-Triazine at high Temperatures and Pressures.

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 ß-HMX and lower than that of TNT.