Extending Aluminum Hydration Surface Treatments from Nanoparticles to Microparticles

The combustion mechanism of aluminum (Al) particles depends on the alumina (Al2O3) passivation shell surrounding the metal core. Hydrating Al2O3 to synthesize an Al(OH)3 shell alters the particle’s reaction mechanism and energy release behavior. While the Al2O3 shell of aluminum nanoparticles (nAl) has been successfully hydrated to produce an Al(OH)3–Al shell–core structure, extending shell processing to micrometer-sized aluminum (μAl) particles did not follow the same hydration procedure. The difference in particle surface energy is responsible for the parameters controlling hydration kinetics. The μAl particles with lower surface energy successfully achieved surface hydration by controlling only temperature and time in suspension. The nAl particles, with 91.4% higher surface energy than μAl, required control of aqueous pH to achieve similar surface hydration as μAl. One objective of this study is to identify surface reaction mechanisms that are a function of particle size (and surface energy). A second objective is to assess the feasibility of using surface-hydrated particles for energy-generating applications. This objective is accomplished through accelerated aging tests using thermal equilibrium analysis to investigate particle reactivity and safety tests including electrostatic discharge, impact, and friction sensitivity. Results show that all surface-hydrated particles passed safety testing, but the surface-hydrated nAl experienced significant aging while the μAl particles negligibly aged. An explanation for the distinct aging behavior as a function of particle size is provided and based on hydration kinetics and equilibrium results.