Effects of Vacancy Defects on the Structural Stability and Thermal Decomposition of CL-20/MTNP Cocrystal: A Reactive Molecular Dynamic Study

Abstract

ReaxFF-lg reactive molecular dynamics was used to study the effects of vacancy defects on the structural stability and thermal decomposition behavior of CL-20/MTNP (2,4,6,8,10,12-hexanitrohexaazaisowurtzitane/1-methyl-3,4,5-trinitropyrazole) cocrystal. The results show that the binding energy drops monotonically with the increasing vacancy concentration, showing that vacancies can weaken intermolecular interactions and reduce lattice stability. The N-NO2 bond cleavage in CL-20 is the favored initiation step and the defects do not change the main mechanism. The vacancies can induce an increase in the atomic mobility that induces early bond breaking and facilitates early fragmentation. The increasing temperature can accelerate the rate of the formation and breakdown of short-lived nitrogen containing intermediates, while the vacancy defects directly impact their maximum abundance and lifetimes and decrease the concentration of certain stable final products like N2. The vacancy has a dual role in the reaction kinetics: k1 increases with the increasing vacancy concentration and temperature, but k2 increases with the increasing temperature and decreases with the increasing vacancy concentration, which implies that the vacancies promote early decomposition but inhibit the progression of the intermediate phase reaction and release of energy. Our research may be useful in assisting in the defect-controlled design and safety consideration of energetic cocrystals.