Experimental Study on Microstructural Changes of Nanoaluminum Particles during the Slow Heating Process in Vacuum and Aerobic Environments

The potential incorporation of aluminum nanoparticles (ANPs) in explosive and propellant mixtures has led to increasing interest in this area of research. The microstructural alterations of ANPs encompass the phase transition of the alumina shell and variation in the thickness of the alumina shell. These alterations are contingent upon thermal expansion, redox reactions between aluminum and oxygen, and modifications to the crystal structure. In order to provide a more detailed explanation of the effects of these factors on the microstructural changes of ANPs, ANPs of equal particle size (110 nm) were slowly heated from 25 to 700 °C under vacuum and aerobic conditions, respectively. The results indicate that, under vacuum conditions, the transformation of crystal form is a slow process of change; within the temperature of 250–260 °C, crystal transformation begins to occur and the proportion of transitional Al2O3 (T-Al2O3) in the entire alumina shell gradually increases as the temperature rises, and when the temperature is 670 °C, the area of T-Al2O3 crystal form accounts for 75.32 ± 2.80% of the aluminum oxide shell plane. In parallel, the thickness of the alumina shell (Tas) is compared under vacuum and aerobic conditions. It was observed that under vacuum conditions, Tas fluctuates greatly with increasing temperature, following a pattern of first increasing and then decreasing; as the temperature increases from the ambient temperature to 450 °C, Tas increases from 4.07 nm to the maximum value of 13.5 nm. In aerobic conditions, oxygen diffuses through the aluminum oxide shell to the aluminum core, resulting in the rapid oxidation of the aluminum at the shell core interface. This forms a denser oxide layer, and Tas experienced a relatively smooth transition from 4.07 (25 °C) to 6.04 nm (700 °C). The results provide data for revealing the phase transformation and shell thickness variation of ANPs during the slow heating process and for better understanding the microstructural changes of ANPs, which is of great significance for avoiding dust explosion accidents and improving the energy utilization efficiency of ANPs in explosive propellants.