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

AlH 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 AlH. This study investigated the effects of fragmentation of AlH and AlH/PVDF particles on the thermal decomposition, ignition, agglomeration, and combustion of HTPB propellants. Thermal analysis indicated that AlH and AlH/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 AlH particle and the fragmentation of the AlH/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 AlH. 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 AlH/PVDF particle, aiding in the generation of internal tensile stress, while the sublimation of AlF induced the fragmentation. Synchrotron-based X-ray imaging revealed the formation of aggregates promoted by molten Al, the splitting of AlH aggregates due to hydrogen explosion, and the enhanced fragmentation of AlH/PVDF due to the synergistic effect of hydrogen explosion and the sublimation of AlF. Compared to raw particles, the CCPs (condensed combustion products) of SP2 propellant display a 48% reduction in average size ( = 24.5 μm), whereas there is an over 89% decrease in particle size for the CCPs of SP3 propellant ( = 5.14 μm). This study contributes to understanding the fragmentation of AlH and AlH/PVDF upon ignition and combustion, providing valuable insights for the development and optimization of propellants containing AlH.