Abstract:

The thermal decomposition mechanisms of cyclotrimethylene trinitramine (RDX) and its derivatives are investigated using ReaxFF reactive molecular dynamics simulation at high temperatures (2000, 2500, and 3000 K). It is shown that the first pyrolysis step of RDX and its derivatives is the N―NO₂ homolytic cleavage to produce NO₂; however, the subsequent reaction mechanism is completely different due to the different sixmembered rings and side chain groups. In these four model systems, NO₂ and NO molecules are common intermediates in the thermal decomposition process, which eventually transform to N₂ in the following reactions. The most stable products are N₂, H₂O, and CO₂ in the thermal process, among which N₂ has the maximum molecular number (more than 20). The numbers of cracked H₂O and CO₂ molecules are very different in the four model systems due to the different C/N and H/O ratios. At different temperatures, the maximum numbers of carbon atoms in the carbon clusters are all small in the unit cell simulations of the four systems. In the further super cell simulation for RDX and RDX-D2 the numbers of carbon atoms reach about 30 and 16, respectively, which are higher than those from the unit cell simulation. In both the unit and super cell simulations for RDX-D1 and RDX-D3, the carbon cluster cannot be formed and there exist only small carbon molecular fragments. Therefore, the initial molecular structure and elemental ratio have a large effect on the production of carbon clusters.

Key words: RDX and its derivatives, Thermal decomposition, ReaxFF, Carbon clusters, Molecular dynamics