Thermal Decomposition of Solid Cyclotrimethylene Trinitramine

Abstract

In this paper, we show how a traditional solid-state chemistry approach, applied to the essentially molecular problem of the energetic barrier for decomposition, gives qualitatively new results and, in fact, brings very new perspectives in the detonation initiation theory development at large. Quantum-chemical simulations of the thermal decomposition of solid cyclotrimethylene trinitramine (RDX) by means of the Hartree−Fock method combined with the cluster and periodic models are performed. We found that the dissociation of the RDX molecule in the bulk crystal is characterized by the different energetic barriers for cleavage of N−NO2 bonds, unlike the gas-phase molecule, where all three of the energies are equal. It is also shown that a rupture of the N−NO2 chemical bond requires less energy for an isolated molecule than for a molecule placed in the bulk of the solid. The situation changes if the molecule is close to the free surface of the crystal. In this case, less energy is required to break the bond than for a bulk molecule. Mechanisms of solid RDX decomposition, the relevant experimental data, and possible applications of the results obtained are discussed in great detail. We also discuss how the conclusion obtained can serve for the better understanding of the well-known mechanism of pore collapse, of different sensitivities to detonation initiation of porous, solid, perfect, and defective explosives, and other processes that take place in hot spots. The mechanisms of the thermal decomposition of solid energetic materials are discussed with the illustrating example of RDX.

Thermodynamic Simulation of the RDX–Aluminum Interface Using ReaxFF Molecular Dynamics

“…The focus until now, has been on the molecule structure, reaction pathways and intermediates of the gas-and solid-phases of RDX during the thermal decomposition, [17][18][19][20][21] and the shock-induced initial chemical events of RDX crystal. [22][23][24] The oxidation growth kinetics of Al particles, 25,26 the morphology and thickness of oxide films on Al surface have also been studied using computational models.…”

Modeling Defect-Induced Phenomena

“…It has been also found that the energy for RDX N-NO 2 dissociation is strongly dependent on the environment of the molecule. Thus, an isolated RDX molecule with the "crystal geometry" exhibits some difference in energies for the cleavage of the equatorial and the axial bonds against all equal energies for the gas-phase molecule [80]. This is due to bond length changes and symmetry reduction.…”

“…In the course of the thermal decomposition of solid RDX [80], among all possible reaction pathways, we considered only the N-NO 2 bond rupture channel as the most supported by experiments [81][82][83]; in fact, the nature of the initial step is still a subject of debate. Corresponding activation energies vary from 24.7 to 52.1 kcal mol −1 and reported pre-exponential factors vary from 10 17 to 10 20 s −1 for overall decomposition [84].…”

“…Surfaces were modeled by cutting the crystal in specific directions in accordance with larger crystal faces observed during the crystal growth process. Then, various clusters were constructed to study how the decomposition energy of the molecule placed on a surface differs (if at all) from that placed in a bulk crystal [80].…”

“…As was mentioned before (see Section 8.1), the dissociation energy of the isolated molecule may or may not be related to that of a molecule in the bulk crystal. Indeed, it has been shown for RDX (see Section 8.2.1) that the activation barrier is different for the solid-state and the isolated (gas phase) molecule and that the decomposition barrier near a defect is lower than in a perfect crystal [80]. Defects formed by change of orientation of molecules in the solid phase may play a significant role in the activation of the decomposition process [109].…”