Insights into the Fragmentation of Aluminum Hydride and Its Effect on Combustion and Agglomeration of HTPB Propellant

AlH₃ has gained considerable attention as a fuel additive due to its ability to offer high specific impulse and superior combustion performance. However, few studies have focused on the fragmentation and agglomeration behavior of AlH₃. This study investigated the effects of fragmentation of AlH₃ and AlH₃/PVDF particles on the thermal decomposition, ignition, agglomeration, and combustion of HTPB propellants. Thermal analysis indicated that AlH₃ and AlH₃/PVDF can accelerate the decomposition of ammonium perchlorate by abundant active sites for the adsorption of the decomposition intermediates. Single-particle combustion uncovered the mechanism behind the directional spray of molten Al from the AlH₃ particle and the fragmentation of the AlH₃/PVDF particle. The melting of porous Al induces particle shrinkage due to solid-liquid interfacial tension and the structural restoration of the oxide shell, which consequently results in the sealing of cracks in the oxide shell of AlH₃. Additionally, the accumulation of internal tensile stress leads to the reopening of these cracks and the directional ejection of the molten Al. The flexible oxide shell contributes to a smaller minimum normalized diameter of the AlH₃/PVDF particle, aiding in the generation of internal tensile stress, while the sublimation of AlF₃ induced the fragmentation. Synchrotron-based X-ray imaging revealed the formation of aggregates promoted by molten Al, the splitting of AlH₃ aggregates due to hydrogen explosion, and the enhanced fragmentation of AlH₃/PVDF due to the synergistic effect of hydrogen explosion and the sublimation of AlF₃. Compared to raw particles, the CCPs (condensed combustion products) of SP2 propellant display a 48% reduction in average size (<i>D</i>₅₀ = 24.5 μm), whereas there is an over 89% decrease in particle size for the CCPs of SP3 propellant (<i>D</i>₅₀ = 5.14 μm). This study contributes to understanding the fragmentation of AlH₃ and AlH₃/PVDF upon ignition and combustion, providing valuable insights for the development and optimization of propellants containing AlH₃.