Combustion Mechanism And Kinetics Of Aluminum Nanoparticles

Aluminum nanoparticles（nano-Al） have high volume energy density, adiabatic combustion temperature and burning rate. Hence it is widely applied in the special fields like propellant, explosive, thermite. Nowadays, the fundamental research on nano-Al combustion is still insuffient and the combustion mechanism is ambiguous. To explore the underlying mechanism and combustion kinetics, a high-temperature expereimental setup based on the multi-elecment non-premixed flat flame burner is developed to support the burning of nano-Al. Meanwhile, to realize the approximately single-particle combustion, the well-dispersed aeroborne nanoparticles are generated through a series of processes including ultrasonically dispersing, atomization, diffusion drying and size cutting.The imaging and spectral analysis, combined with in-situ sampling are applied to diagnose the particle’s combustion. The spectral analysis indicates that the flame radiation is dominated by thermal radiation when the ambient temperature（Tg） is below 1900 K, implying a heterogeneous combustion mode. The Planck’s law is used to fit the thermal radiation signal and derive the particle temperature（Tp）. When Tg is 1300 K, Tp reaches 1758 K. Using a simplified model to explain this large difference between Tp and Tg, the thermal accommodation coefficient should be as small as 0.05. The morphologies of burning particles are analyzed with the transmission electron microscopy. The special hollow structure is detected, confirming the outward diffusion of Al during reaction. The amorphous oxide shell also transforms to crystallized structures.The oxidation mechansim and kinetics are dervied from the thermogravimatic（TG） analysis. The activation energies of diffusion in amorphous and γ alumina are 106 k J/mol and 150 k J/mol, respectively. The crystallization kinetics is derived from the differential scanning calorimetry results. It shows that at 1330 K, the crystallization time is around 1 ms, on the same order of burning time. Hence it is reasonable to find the crystallized Al₂O₃ in the burning particles. TG analysis indicates the governing process of oxidation is varying from the ionic diffusion to the polymorphic phase transition and finally changing back to ionic diffusion. The phase transition of alumina promotes the reaction through generating micro-scale defects in the oxide shell to enhance the ionic diffusion. During combustion, the crystallization of alumina and melting of aluminum both help reduce the diffusion barrier in the oxide shell. Hence the governing process becomes the chemical adsorption. The activation energy is around 35 k J/mol.Molecular dynamics simulation is performed to investigate the atomic-scale mechanism of ionic diffusion and crystallization. The activation energy of Al cations is around 100 k J/mol, close to the result from the TG experiment. The temperature-time-transformation diagram of crystallization is obtained, indicating the fastest crystallization temperature is around 1250 K.Simplified modeling is porformed to find the impact of nanoparticle’s agglomeration on ignition. It indicates that small agglomerates are hard to ignite fully due to the fast heat transfer with ambience. However, with the agglomerate become larger, the particle temperature increases, finally realizing the full ignition. The ignition temperature also becomes lower than the melting point of Al（e.g. 790K） due to agglomeration. But at high temperature, the particles in the agglomerate will sinter together to burn like large micron particles, leading to a longer burn time.Finally the mechanism of laser-particle（Al/Al₂O₃） ablation is explored to judge the feasibility of heterogenesous laser diagnositics during the nano-Al combustion. It finds that the laser induced incandescence is not suitable for the Al/Al₂O₃ system.