ReaxFF molecular dynamics study on the decomposition mechanism of AlH3 in NO2 gaseous environment.

CONTEXT: As an intermediate product of propellant decomposition, NO2 can react with AlH3 nanoparticles (AHNPs) and significantly influence the overall combustion characteristics; however, the underlying decomposition mechanism remains unclear. In this study, ReaxFF molecular dynamics (RMD) simulations were performed to investigate the decomposition and ignition processes of AHNPs in NO2 gaseous environment. The reaction mechanism of NO2 decomposition to N2 is revealed at the atomic level, and the promotional role of H, Al, and O species spilling over from AHNPs in facilitating NO2 decomposition is clarified. Furthermore, the coupled combustion behavior of dual AHNPs is analyzed. The melting of the two AHNPs delays nanoparticle cracking, particularly at low temperatures. The apparent activation energies for NO2 decomposition are 26.2 kJ/mol in S1, 18.8 kJ/mol in S2 and 23.6 kJ/mol in S3, respectively, indicating the promoting effect of the dual AHNPs in S2. Collectively, these microscopic insights clarify the gas-phase NO2 and AHNPs reaction mechanism and the influence of particle number, providing a mechanistic basis for understanding the initial decomposition behavior of composite propellants.

METHODS: The generalized gradient approximation with GGA-PBE method was used to optimize NO2 molecular geometry using Dmol3 package. The models containing AHNPs and NO2 are built by PACKMOL software. ReaxFF molecular dynamics simulations were used to study the decomposition of AHNPs in NO2 atmosphere. The ReaxFF parameters are from Mei which are used to describe AlH3. The bond order truncation cutoff is set as 0.3 to facilitate the determination of chemical bonds and products. All simulations were performed using LAMMPS software.