Coalescence of Al₂O₃/Al, MgO/Mg, and MgO/Al two nanoparticles during combustion

Metal and metal oxide aggregates are easily sintered by migration-diffusion in combustion, and the aggregates hold a complex morphological and structural variability that increases with temperature. The underlying thermomechanical response remains a challenging mystery for active nanoparticles. Here, a computational strategy based on ReaxFF molecular dynamics were performed to shed much light on the coalescence behavior of Al2O3/Al, MgO/Mg, and MgO/Al nanoparticles - a doublet of two spheres. The complex sintering phenomenon was further quantitatively described in terms of coordination number, charge distribution, density distribution, atomic displacement, radius ratio, shrinkage, and sintering time, etc. Results showed that the Al2O3/Al and MgO/Mg two nanoparticles undergo collision and coalescence, and the two were connected through the neck. MgO/Al aggregates are unstable at higher temperatures (T > 1200 K) and segregate at junctions, followed by re-coalescence in the simulation box. A stable neck structure can be formed when enough O atoms are dissolved in MgO nanoparticles or at a lower temperature. The diffusion of Mg, Al, and O atoms in aggregates is driven by concentration gradients and self-generated electromotive forces. The Al2O3 passivation layer forms and hinders the diffusion of O atoms, and the oxygen-rich regions are distributed in the neck (Al2O3/Al) or the contact surface on the side of the Al nanoparticles (MgO1.5/Al) during sintering. In MgO/Al aggregates, Mg atoms tend to wrap Al nanoparticles by surface diffusion.