Synthesis of Bayerite-Passivated Aluminum Particles and Their Combustion in Solid Propellants

The combustion mechanism of aluminum (Al) particles depends on the alumina (Al2O3) passivation layer surrounding the metal core. Hydrating Al2O3 into bayerite (Al(OH)3) can alter the interfacial reactions and energy conversion behavior of the metal fuel particles. This study explores chemical weathering of the surface of micron-sized aluminum (μAl) particles using temperature and time in solution as parameters for shell hydration in aqueous suspensions. Using X-ray diffraction, microscopy, spectroscopy, and thermal analysis, the formation of an Al(OH)3 shell is confirmed on μAl particles with greater than 96 wt % Al core retained. A detailed mechanism for surface hydration is discussed. The hydrated metal particles were then evaluated as a potential solid composite propellant fuel. Burn rate experiments were performed in a windowed pressure vessel, and condensed phase combustion products were recovered. Results show that the burn rate of hydrated μAl (1.068 cm/s) is 17.75% higher compared to standard μAl (0.907 cm/s) at 10.34 MPa with hydrated μAl showing a clear particle suspension entrained near the burning surface. The hydrated shell enables oxidizing gas generation in proximity to the exposed Al core contributing to more complete combustion and accelerated burn rates at higher pressures. The solution-based hydration technique alters interface properties and affects the reaction mechanism and combustion demonstrated for propulsion applications.