Reactive Molecular Dynamics Simulations of Carbon-Containing Clusters Formation during Pyrolysis of TNT
H Liu and X Dong and YH He, ACTA PHYSICO-CHIMICA SINICA, 30, 232-240 (2014).
ReaxFF molecular dynamics simulations of trinitrotoluene (TNT) pyrolysis show that use of the ReaxFF/lg potential function, which adds the London dispersion term, gives superior results in equilibrium density calculation of energetic materials. According to our calculations using limited time steps, the main products are NO2, NO, H2O, N-2, CO2, CO, OH, and HONO, and H2O, N-2, and CO2 are the final products. We also used ReaxFF potential functions to simulate the same process to conduct a comparative analysis. The main and final products are consistent with those obtained using ReaxFF/lg, but the kinetics are different. Both ortho-NO2 homolytic cleavage and C-NO2 -> C-ONO rearrangement homolysis are thermodynamically favorable pathways in the early thermal decomposition of TNT. However, C-NO2 -> C-ONO rearrangement homolysis is less favorable kinetically than C-NO2 homolysis, since C-NO2 is the weakest bond in TNT. Soon after their formation, NO2 and NO participate in secondary reactions and eventually form N-2. Pyrolysis to form OH and other small molecules promotes the formation of H2O. Aromatic ring fission does not take place until most of the attached groups have interacted or are removed, and increasing the temperature accelerates main-ring fission and further decomposition to form CO2; this is the major reason for CO2 distribution fluctuations under high-temperature conditions. When the TNT molecules in the unit cell are almost completely decomposed, the potential energy of the system is significantly attenuated. The maximum amount of carbon-containing clusters formed in the thermal decomposition is more dependent on density than on temperature. Moreover, the simulation results show that coagulation of carbonaceous intermediates occurs before the TNT decomposes completely. These studies show that the simulation of TNT pyrolysis using the ReaxFF/lg reactive force field can provide detailed kinetic and chemical information, which are helpful in understanding the detonation of energetic materials and assessing their security.
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