Comparative studies on structures, mechanical properties, sensitivity, stabilities and detonation performance of CL-20/TNT cocrystal and composite explosives by molecular dynamics simulation

Theoretical study on the structure, electronic properties, intermolecular interactions, and detonation performance of DAF:ADNP co-crystal

CONTEXT

Explosives have a wide range of applications in many fields due to their high energy and high density. Recently, a new synthesized co-crystal explosive DAF:ADNP presents high detonation performance and low sensitivity. This work is aimed to understand how the structure and intermolecular interactions affect the performance of the DAF:ADNP co-crystal. The results indicate that the formed π-π interactions and stronger hydrogen bonds in the co-crystal enhance its stability and its impact sensitivity is reduced. The strong intralayer H···N and H···O interactions and interlayer π-π stacking are the main driving force for the formation of the co-crystal. Compared with the pure crystals, the detonation performance of the co-crystal slightly decreases, while its sensitivity reduces.

METHODS

All calculations were used the DFT-PBE-D method with Vanderbilt-type ultrasoft pseudopotentials and plane wave (340.0 eV) in the CASTEP package. Radial distribution function were calculated by NVT-MD simulations for 100 ps with a time step of 1 fs at 298 K. Hirshfeld surfaces were generated by CrystalExplorer 3.0 and reduced density gradient analyses were performed by Multiwfn 3.0.

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.

Thermal Decomposition Kinetics and Compatibility of 3,5-difluoro-2,4,6-trinitroanisole (DFTNAN)

In this paper, the thermal decomposition behavior of 3,5-difluoro-2,4,6-trinitroanisole (DFTNAN) was studied by differential scanning calorimetry (DSC) and thermogravimetry (TG) by using different heating rates (2, 5, 10, 15 °C·min^-1). Subsequently, the kinetic and thermodynamic parameters of non-isothermal thermal decomposition of DFTNAN were calculated. The critical temperature of thermal explosion (Tb) and self-accelerating decomposition temperature (TASDT) were determined to be 249.03 °C and 226.33 °C, respectively. The compatibility of DFTNAN with a number of high explosives (cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-tetracyclo-[5.5.0.05,9.03,11]-dodecane (CL-20) and dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50)) was studied at different mass ratios using DSC. The criteria to judge the compatibility between the materials were based on a standardization agreement (STANAG 4147). The thermodynamic study results revealed that DFTNAN possessed superior thermal safety and stability. The experimental of compatibility results indicated that the mass ratios of the high explosives in the DFTNAN/RDX, DFTNAN/HMX and DFTNAN/CL-20 compositions more than 40%, 60% and 70% exhibited good compatibility, whereas DFTNAN/TKX-50 demonstrated poor compatibility.

Investigation of the solid-liquid ternary phase diagrams of 2HNIW·HMX cocrystal

The influence of temperature and solvent on the solid–liquid ternary phase diagrams of the 2HNIW·HMX cocrystal has been investigated. Ternary phase diagrams were constructed for the 2HNIW·HMX cocrystal in acetonitrile and ethyl acetate at 15 °C and 25 °C. HMX and HNIW showed inconsistent dissolution behavior and congruent dissolution behavior in acetonitrile and ethyl acetate, respectively. In the HMX–HNIW–acetonitrile system, the 2HNIW·HMX cocrystal has a narrow thermodynamically stable region at both temperatures. The cocrystal exhibits a wider thermodynamically stable region in the HMX–HNIW–ethyl acetate system. The results show that the choice of solvent has a crucial influence on the dissolution behavior of the cocrystal and the size and position of each region in the phase diagram, while the temperature has no apparent effect on the overall appearance of the phase diagram. By properly selecting the ratios, the 2HNIW·HMX cocrystal could be prepared by the isothermal slurry conversion crystallization method.

The ternary phase diagrams of 2HNIW·HMX cocrystal system in ethyl acetate at 15 °C. In the HMX–HNIW–ethyl acetate system, HMX and HNIW showed congruent dissolution behavior, and the cocrystal exhibits a wider thermodynamically stable region.

Theoretical investigations on structures, stability, energetic performance, sensitivity, and mechanical properties of CL-20/TNT/HMX cocrystal explosives by molecular dynamics simulation

In this article, the CL-20, TNT, HMX, CL-20/TNT, CL-20/HMX and different CL-20/TNT/HMX cocrystal models were established. Molecular dynamics method was selected to optimize the structures, predict the stability, sensitivity, energetic performance, and mechanical properties of cocrystal models. The binding energy, trigger bond length, trigger bond energy, cohesive energy density, detonation parameters, and mechanical properties of each crystal model were obtained. The influences of co-crystallization and molar ratios on performances of cocrystal explosives were investigated and evaluated. The results show that the CL-20/TNT/HMX cocrystal explosive with a molar ratio of 3:1:2 or 3:1:3 had larger binding energy and better stability, i.e., CL-20/TNT/HMX cocrystal explosive was more likely to be formed with these molar ratios. The cocrystal explosive had shorter maximal trigger bond length, but larger trigger bond energy and cohesive energy density than CL-20, namely, the cocrystal explosive had lower mechanical sensitivity and better safety than CL-20 and the safety of cocrystal model was effectively improved. The cocrystal model with a molar ratio of 3:1:2 had the best safety. The energetic performance of the cocrystal explosive with a molar ratio of 3:1:1, 3:1:2, or 3:1:3 was the best. These CL-20/TNT/HMX cocrystal models exhibited better and more desirable mechanical properties. In a word, the cocrystal model with molar ratio of 3:1:2 exhibited the most superior properties and was a novel and potential high-energy-density compound. This paper could provide practical helpful guidance and theoretical support to better understand co-crystallization mechanisms and design novel energetic cocrystal explosives.

Theoretical investigations on the structures and properties of CL-20/TNT cocrystal and its defective models by molecular dynamics simulation

"Perfect" and defective models of CL-20/TNT cocrystal explosive were established. Molecular dynamics methods were introduced to determine the structures and predict the comprehensive performances, including stabilities, sensitivity, energy density and mechanical properties, of the different models. The influences of crystal defects on the properties of these explosives were investigated and evaluated. The results show that, compared with the "perfect" model, the rigidity and toughness of defective models are decreased, while the ductility, tenacity and plastic properties are enhanced. The binding energies, interaction energy of the trigger bond, and the cohesive energy density of defective crystals declined, thus implying that stabilities are weakened, the explosive molecule is activated, trigger bond strength is diminished and safety is worsened. Detonation performance showed that, owing to the influence of crystal defects, the density is lessened, detonation pressure and detonation velocity are also declined, i.e., the power of defective explosive is decreased. In a word, the crystal defects may have a favorable effect on the mechanical properties, but have a disadvantageous influence on sensitivity, stability and energy density of CL-20/TNT cocrystal explosive. The results could provide theoretical guidance and practical instructions to estimate the properties of defective crystal models.