Study on the Initial Decomposition Mechanism of Energetic Co-Crystal 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-Hexaazaiso-Wurtzitane (CL-20)/1,3,5,7-Tetranitro-1,3,5,7-Tetrazacy-Clooctane (HMX) under a Steady Shock Wave

CL-20 exhibits high energy density, but its high sensitivity limits its use in various applications. A high-energy and low-sensitivity co-crystal high explosive prepared around CL-20 has the potential to widen the application scope of CL-20 single crystals. The initial physical and chemical responses along different lattice vectors of the energetic co-crystal CL-20/HMX impacted by 4-10 km.s(-1) of steady shock waves were simulated using the ReaxFF molecular dynamics method combined with the multiscale shock technique (MSST). The temperature, pressure, density, particle velocity, initial decomposition paths, final stable reaction products, and shock Hugoniot curves were obtained. The results show that after application of the shock wave, the energetic co-crystals successively undergo an induction period, fast compression, slow compression, and expansion processes. The fast and slow compression processes correspond to the fast and slow decomposition of the reactants, respectively. An exponential function was adopted to fit the decay curve of the reactants and the decay rates of CL-20 and HMX were compared. Overall, with increasing shock wave velocity, the response time of the reactants was gradually advanced and that of CL-20 molecular decomposition in the co-crystal occurred earlier than that of HMX after the shock wave incident along each lattice vector. The decay rate of CL-20 was highest during the fast decomposition stage, followed by that of HMX. However, the decay rate of the reactants during the slow decomposition stage was similar. The initial reaction path of the energetic co-crystal involves the dimerization of CL-20, while the initial reaction path of the shock-induced co-crystal decomposition involves fracturing of the N-NO2 bond in CL-20 to form NO2. Then, small intermediate molecules such as N2O, NO, HONO, OH, and H are formed. The final stable products are N-2, H2O, CO2, CO, and H-2. The shock sensitivities of the lattice vector in the b and c directions were the same, but lower than that of the lattice vector a direction. The minimum velocities (u(s)) of the shock wave inducing CL-20 and HMX decomposition were 6 and 7 km.s(-1), respectively. Moreover, the particle velocities behind the shock waves on the three lattice vectors showed only minor differences. The shock-induced initiation pressures of CL-20/HMX along lattice vectors a, b, and c were 16.52, 17.41, and 17.41 GPa, respectively, as determined by the shock Hugoniot relation. The detonation pressure ranged from 36.75 to 47.43 GPa.