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

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.

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.