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

Mechanical behaviors of CL-20 under an impact loading: A molecular dynamics study

Study on the dynamic process of CL-20 crystal under impact is critical for the safe utilization of energetic materials under extreme conditions. Herein, the mechanical and structural evolution of CL-20 under the impact of a diamond ball is investigated by using molecular dynamics simulation. The considerations are given to the effect of different impact velocity, impact direction and impact angle. It is found that a high impact velocity results in a large indentation depth and force, as well as more significant energy transition and the formation of a large number of molecular fragments. Moreover, CL-20 exhibits weak anisotropy along different impact directions due to the crystalline distribution anisotropy. Furthermore, the mechanical response of CL-20 is angle-dependent, which is caused by the discrepancy in local molecular re-arrangement. These results may enhance the understanding of the mechanical behavior of CL-20 and promote its wide applications.

Investigation of dislocation and twinning behavior in HMX under high-velocity impact employing molecular dynamics simulations

CONTEXT

Under the ReaxFF/lg force field, the multiscale shock technique (MSST) was employed to investigate the decomposition behavior of perfect, dislocated, and twinned octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) at a velocity of 11 km/s. This study aimed to analyze the changes in system temperature, bond formation and breaking, variations in the number of small molecules, and the number of clusters. The results indicated that the sensitivity of dislocated HMX was the lowest, while the sensitivity of twinned and perfect HMX was comparable. Comparing the formation and breaking of bonds in HMX during the shock process, it was found that the change in the number of bonds in dislocated HMX was similar to that in perfect HMX, whereas twinning accelerated the breaking of bonds. By analyzing the changes in small molecular fragments (CO, CO2, H, H2, H2O, N2, N2H, NH2, NO, NO2, and O) during the shock process of HMX, it was found that dislocation had a relatively minor effect on the small molecular fragments, while twinning promoted the generation of CO, H, NO, and O and accelerated the decomposition of NO. A comparison of the number, weight, and atomic ratio of clusters under perfect, dislocated, and twinned conditions revealed that under the influence of shock, the number of clusters initially increased sharply and then decreased slowly. Meanwhile, compared to the perfect and dislocated explosives, the number of clusters under the twinned structure was significantly fewer, indicating that the twinned structure could reduce cluster formation. The proportion of oxygen to carbon in the twinned HMX was lower than that in the perfect and dislocated explosives, possibly due to the higher content of small molecular fragment O in twinned HMX.

METHODS

Different structures of HMX crystals were constructed, including twinned defect structure (with a supercell containing 6458 atoms), dislocation defect structure (with a supercell containing 2352 atoms), and perfect structure (also with a supercell containing 2352 atoms). The modeling of defect crystal structures was carried out using the Atomsk software. For the twinned defect structure, we first constructed a mirror symmetric structure of the original configuration and then merged these two structures together. For the dislocation defect structure, we shifted a segment of the originally ordered perfect crystal structure by a certain distance using Atomsk. Before conducting the simulations, we performed geometric optimization of the models using the conjugate gradient (CG) algorithm and carried out 10 ps of NVT and NPT simulations to equilibrate the energy, temperature, and other parameters within the system. Finally, a 50-ps MSST impact simulation was performed using Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS).

Machine learning quantitatively characterizes the deformation and destruction of explosive molecules

Although explosives have been widely used in mines, road development, old building demolishing, and munition explosions; currently, how chemical bonds between atoms break and recombine, how the molecular structure is deformed and destroyed, how the reaction product molecules are formed, and the details for this rapid change process in explosive reactions are not yet fully understood, which limits the full use of explosive energy and safer use of explosives. This paper presents a quantitative model of molecular structure deformation using machine learning algorithms as well as a qualitative model of its relationship with molecular structure destruction, based on a molecular dynamics simulation and detailed analysis of the shock-loaded ε-CL-20, providing new perspectives for explosive community research. Specifically, the quantitative model of molecular structure deformation establishes the quantitative relationship between the molecular volume change and molecular position change, and between molecular distance change and molecular volume change using the machine learning algorithms such as Delaunay triangulation, clustering, and gradient descent. We find that the molecular spacing in explosives is strongly compressed after being shocked, and the peripheral structure can shrink inward, which is beneficial to keep the cage structure stable. When the peripheral structure is compressed to a certain extent, the cage structure volume begins to expand and is then destroyed. In addition, hydrogen atom transfer occurs within the explosive molecule. This study amplifies the structural changes and the chemical reaction process for explosive molecules after being strongly compressed by a shock wave, which can enrich the knowledge of the real detonation reaction process. The analysis method based on quantitative characterization using machine learning proposed in this study can also be used to analyze the microscopic reaction mechanism in other materials.

Mechanism of the Impact-Sensitivity Reduction of Energetic CL-20/TNT Cocrystals: A Nonequilibrium Molecular Dynamics Study

High-energy low-sensitivity explosives are research objectives in the field of energetic materials, and the formation of cocrystals is an important method to improve the safety of explosives. However, the sensitivity reduction mechanism of cocrystal explosives is still unclear. In this study, CL-20/TNT, CL-20 and TNT crystals were taken as research objects. On the basis of the ReaxFF-lg reactive force field, the propagation process of the wave front in the crystals at different impact velocities was simulated. The molecular dynamics data were used to analyze the molecular structure changes and initial chemical reactions, and to explore the sensitivity reduction mechanism of the CL-20/TNT cocrystal. The results showed that the chemical reaction of the CL-20/TNT cocrystal, compared with the CL-20 single crystal, is different under different impact velocities. At an impact velocity of 2 km/s, polymerization and separation of the component molecules weakened the decomposition of CL-20. At an impact velocity of 3 km/s, the decay rates of CL-20 and TNT in the cocrystal decreased, and the intermediate products were enhanced, such as nitrogen oxides. At an impact velocity of 4 km/s, the cocrystal had little effect on the decay rates of the molecules and formation of CO₂, but it enhanced formation of N₂ and H₂O. This may explain the reason for the impact-sensitivity reduction of the CL-20/TNT cocrystal.

Decomposition mechanism of 1,3,5-trinitro-2,4,6-trinitroaminobenzene under thermal and shock stimuli using ReaxFF molecular dynamics simulations

To obtain atomic-level insights into the decomposition behavior of 1,3,5-trinitro-2,4,6-trinitroaminobenzene (TNTNB) under different stimulations, this study applied reactive molecular dynamics simulations to illustrate the effects of thermal and shock stimuli on the TNTNB crystal. The results show that the initial decomposition of the TNTNB crystal under both thermal and shock stimuli starts with the breakage of the N-NO₂ bond. However, the C₆ ring in TNTNB undergoes structural rearrangement to form a C₃-C₅ bicyclic structure at a constant high temperature. Then, the C₃ and C₅ rings break in turn. The main final products of TNTNB under shock are N₂, CO₂, and H₂O, while NO,  N_2, H₂O and CO are formed instead at 1 atm under a constant high temperature. Pressure is the main reason for this difference. High pressure promotes the complete oxidation of the reactants.

Reactive molecular dynamics study on the thermal decomposition reaction of a triple-base solid propellant

The study of the combustion property of newly designed propellant by means of computational simulation is an efficient pathway for assessment and could avoid exposure to hazardous chemicals. An RDX-modified triple-base solid propellant formula was proposed in this study. Reactive molecular dynamics simulations employing ReaxFF-lg force field were performed to explore the thermal decomposition property of the propellant for a variety of temperatures. The reaction kinetics of the system and major ingredients were analyzed, and the apparent decomposition activation energies were calculated. The population of decomposition intermediates and products is thoroughly investigated. H₂O is consumed at high temperatures indicating a water-gas reaction that could reduce carbon clusters during the combustion of solid propellant. The water-gas reaction, as well as the population of H₂ at high temperature, points out the way of adjusting the formula of the propellant, which is adding fuel and oxidizer to improve combustion temperature and oxygen balance.

Thermal kinetics, thermodynamics, decomposition mechanism, and thermal safety performance of typical ammonium perchlorate-based molecular perovskite energetic materials

In this work, we report the thermal kinetics, thermodynamics, and decomposition mechanism of AP-based molecular perovskite energetic materials and estimate their thermal safety performance. Typical AP-based molecular perovskite energetic materials, (H2dabco)[NH4(ClO4)3] (DAP-4), (H2pz)[NH4(ClO4)3](PAP-4), (H2mpz)[NH4(ClO4)3](PAP-M4), and (H2hpz)[NH4(ClO4)3] (PAP-H4), were synthesized and characterized. These were studied using differential scanning calorimetry (DSC). The results show that all of the obtained AP-based molecular perovskite energetic materials have higher thermal decomposition temperatures, and the peak temperatures are more than 360 °C. All follow random nucleation and growth models. Other thermodynamic parameters, such as the reaction enthalpy (ΔH), entropy change (ΔS), and Gibbs free energy (ΔG), show that they are generally thermodynamically stable. Moreover, their adiabatic induced temperatures were obtained; TD24 of DAP-4, PAP-4, PAP-M4, and PAP-H4 were 246.6, 201.2, 194.5, and 217.5 °C, respectively. This study offers an important and in-depth understanding of the thermal decomposition characteristics of AP-based molecular perovskite energetic materials and their potential applications.

Alleviating the stability–performance contradiction of cage-like high-energy-density materials by a backbone-collapse and branch-heterolysis competition mechanism

Key role of cage-like conformations in alleviating the stability–performance contradiction of HEDMs.