(487a) Energetic Characteristics and Condensed Phase Controlled Reaction Mechanism of Hydrogenated Magnesium Nanoparticles

Magnesium Hydride nanoparticles offer the potential for high-performance reactive material due to their high combustion enthalpy and are promising fuel additives for propellants, pyrotechnics, and explosives. However, their fundamental energy release mechanism and kinetics have not been explored yet in detail. Low temperature plasma processing was implemented to hydrogenate Mg nanoparticles with hydrogen in-flight. It was found that MgH₂ has a different reaction mechanism compared to Mg, for which the ignition initiation is dependent on the outward vaporization of Magnesium released from the core, whereas the ignition of MgH₂ is initiated at a temperature lower than its vaporization temperature, indicating a condensed phase dominated reaction with a gas phase oxidizer like Potassium Perchlorate. Constant volume combustion experiment revealed that peak pressure and pressurization rate were increased, and burn time significantly decreased for MgH₂ compared to Mg. Temperature-Jump (T-Jump) Ignition showed that the ignition temperature was lowered by ~210°C for MgH₂ compared to Mg with KClO₄ oxidizer, and it was observed from the T-Jump Time of Flight mass spectrometer that ignition temperature was very close to the hydrogen release temperature of MgH₂. The oxygen signal from mass spectrometer decreased for MgH₂ compared to Mg with KClO₄ oxidizer, indicating a condensed phase reaction. The release of hydrogen might have caused defects, dislocations, or cracks at the Mg/MgO interface, facilitating this condensed phase reaction. In-situ XPS performed on MgH₂ revealed that the deconvoluted ratio of MgO from O1s to Mg1s decreased with the hydrogen release, indicating vacancy formation on the oxide layer, which can enhance oxygen ion transport from oxidizer and initiate ignition, followed by the outward diffusion of Mg at a lower temperature reacting with the oxidizer corresponding to the final step of combustion. TGA/DSC and XRD were also performed to observe the two-step reaction mechanism. Moreover, different compositions of MgH₂ on the composite and different sizes were studied to tune the energy release and ignition temperature of the thermite system. Thus this work presents the detailed combustion mechanism study and tuning energy release of MgH₂.