Two new green high-energy materials (HEMs), 2,4,6-triamino-1,3,5-triazine-1,3,5-trioxide (MTO) and 2,4,6-trinitro-1,3,5-triazine-1,3,5-trioxide (MTO3N), were recently suggested by Klapötke and co-works as promising HEMs.
In our previous work, the solid states of MTO and MTO3N, which have never been synthesized before, were studied using the crystal packing models of Monte Carlo Simulated annealing with the Dreiding force-filed and the density functional theory (DFT) with dispersive forces. The most stable crystal structures of MTO and MTO3N have P21 and P21/c space-groups symmetries with high densities of 1.9 gr/cm3 and 2.0 gr/cm3, respectively. Such high densities show the efficient packing of nonpolar electronic structures of these materials. In addition, the computed heat of reactions of MTO and MTO3N and their mixture (up to 1653 kcal/kg) show exceptional performance of these materials as compare to other famous energetic materials such as RDX, HMX, and PETN.
In the present work, we studied the reaction pathways and decomposition mechanism of these new HEMs using DFT molecular dynamics simulations (DFT-MD). Our DFT-MD studies find that the first step in the decomposition of MTO is intermolecular hydrogen-transfer reaction (barrier 3.0 kcal/mol) which is followed quickly by H2O and NO release with reaction barriers of 46.5 and 35.5 kcal/mol, respectively. In contrast for MTO3N (P21/c predicted space group), we find that the first steps are a bimolecular decomposition to release NO2 (ΔH = 44.1 kcal/mol, ΔG = 54.7 kcal/mol) simultaneous with unimolecular NO2 cleavage (ΔH= 59.9 and ΔG = 58.2 kcal/mol) a unique initial reaction among energetic materials. These results suggest that MTO3N would be significantly more thermally stable (barrier >6.0 kcal/mol higher) than RDX and HMX, making it an excellent candidates for insensitive new green energetic materials. In addition, we found that the decomposition barriers of MTO (46.5 kcal/mol) and MTO3N (44.1 kcal/mol) are very close to the decomposition barriers of TKX-50 (45.1 kcal/mol) and DTTO (45.9 kcal/mol) which are known to be insensitive energetic materials. These results show that MTO and MTO3N are thermally stable materials with high densities and high performances.
In principle, first principle calculations such as DFT can provide information about the properties of energetic materials. However, study of important phenomena such as shock compression and detonation at nanometer and nanosecond length and time scales are far beyond DFT capabilities. Instead, reactive molecular dynamics simulations can be used to address such problems. ReaxFF reactive FF has been successfully used to study decomposition mechanism and shock behavior of many energetic materials. It has provided valuable information on initial chemical decomposition and subsequent energy release processes on system with millions of atoms. ReaxFF-lg is an extension of ReaxFF, in which the London dispersion (van der Waals attraction) has taken into account by adding a long-range-correction term, using the low-gradient model, proportional to 1/r6. It has resulted more accurate description of cell parameters for molecular crystals at low pressure and has been tested for several energetic materials. To test the accuracy of ReaxFF-lg for simulations of MTO and MTO3N materials, we applied it to calculate their equilibrium crystal structures and also the equation of states (EOS). Then, the cook-off simulations of the supercells of MTO and MTO3N were utilized using ReaxFF-lg to obtain an overview of their thermal decomposition as a result of increasing temperature from 50 to 5000 K.
The simulations at high temperature and in micorcanonical ensemble (NVE-MD) were utilized to study the energy release and temperature evolution along with the reaction dynamics. In addition, the chemical species were analyzed during these simulations which helps to understand the evolution of chemical processes. The reaction rates of these materials were determined via isothermal-isochoric (NVT-MD) simulations at high temperatures in the range of 1800 to 3000 K. Finally, the Chapman-Jouguet (CJ) pressures were predicted by utilizing NVT-MD simulations at elevated temperatures for different volumes of the super cells. The results are compared with other famous energetic materials such RDX, HMX, and PETN.
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