Decomposition Mechanisms of α-Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine Nanoparticles at High Temperatures

Abstract

Density functional tight-binding molecular dynamics simulations with dispersion corrections were performed to study the adiabatic initial decomposition processes of molecular explosive α-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (α-HMX) nanoparticles (NPs) with the diameters of 1.4–2.8 nm at high temperatures from 2400 to 3000 K. The results indicate that the global thermal decompositions of the HMX NPs present great dependence on the temperature and particle size. The initial decomposition process of the HMX NPs includes two sequential stages: (i) competition between rapid expansion and unimolecular decompositions at surfaces; (ii) subsequent complex uni- and bimolecular decompositions. The main decomposition pathway at low temperatures is the isomerization reaction of the HMX molecule, which is quite different from the N–NO2 homolysis with ring opening at high temperatures. A second-order rate model was established to rationalize the global decompositions of the HMX NPs at different temperatures as well as the elementary decomposition paths. Our findings suggest that the thermal decompositions of the HMX NPs are quite different from their solid phase decompositions in both decomposition mechanisms and global kinetics. These findings provide basic understandings of initial decompositions of nanosized explosives.

Difference in the Thermal Stability of Polymorphic Organic Crystals: A Comparative Study of the Early Events of the Thermal Decay of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) Polymorphs under the Volume Constraint Condition

“…As illustrated in Table , the relative errors of densities of β-, ε-, and γ-CL-20 are 3.2, 3.8, and 4.7%, respectively, showing the reliability of the DFTB method applied to the three polymorphs of CL-20 at ambient condition. In combination with previous successful applications of the method on the decay mechanisms of EMs, such as HN 3 , TATB, HMX, TKX-50, and so forth, − ,− we believe that our simulations of this work should be reliable.…”

Polymorph-Dependent Initial Thermal Decay Mechanism of Energetic Materials: A Case of 1,3,5,7-Tetranitro-1,3,5,7-Tetrazocane

“…Three kinds of steps starting the decay of the HMX molecules are observed in our MD simulations of heating the three polymorphs, including the C–N bond breakage to open the ring­(I), the N–NO 2 fission­(II) and the secondary reaction between the intermediates and HMX reactants, or the intermediate-initiated decomposition­(III), as illustrated in Figure . These steps were also ascertained in previous work, and Reaction II was deemed dominant among all the possible steps. ,, Reactions I and II occur in all three polymorphs of HMX, and the partitioned NO 2 and the broken C–N bonds can be axial or equatorial. As exhibited in Figure , the energy barrier of the NO 2 partition depends on the extending orientation to a certain extent.…”

“…In previous studies, it was believed that the elimination of HONO is also one of the main initial thermal decomposition steps of HMX. [40][41][42]44,57,67,71,74 However, in our simulations, the HONO elimination has not been observed in a way of transferring the H atom of the CH 2 group to the O atom of -NO 2 to form a N−NO 2 H bond and simultaneously breaking it. By tracing the MD trajectories of our simulations, it is found that the HONO is formed by the reactions among the intermediate, such as H + NO 2 ⃗ HONO; meanwhile, it produces small molecular products by means of some reactions like HONO ⃗ HO + NO.…”

“…These steps were also ascertained in previous work, and Reaction II was deemed dominant among all the possible steps. 46,47,57 Reactions I and II occur in all three polymorphs of HMX, and the partitioned NO 2 and the broken C−N bonds can be axial or equatorial. As exhibited in Figure 3, the energy barrier of the NO 2 partition depends on the extending orientation to a certain extent.…”

“…DFTB has been widely used because it is an approximate density functional theory (DFT) method and possesses nearly the same accuracy as DFT, while with a much lower computational cost in contrast to DFT. In principle, DFTB is based on a second-order expansion of the Kohn–Sham total energy in DFT with respect to charge density fluctuations. , The Lennard Jones scheme was adopted in this work. , In addition, the viability of this scheme has already successfully been verified in many other calculations of EMs to reveal the reaction mechanisms against heating and shock. ,,− We validated the method applied by comparing the optimized lattice parameters with experimental observations. As listed in Table , the largest relative error of cell volume is 4.15% (α-HMX), below a usual criterion of 5%.…”