Ignition Behavior of α-AlH₃

Abstract

An experimental investigation on the ignition behavior of α-Aluminum Hydride (α-AlH₃) has been conducted. The ignition characteristics were determined through the use of two separate modified T-jump experiments. In each experiment, a small amount of material was placed onto a platinum wire, which was heated rapidly through resistive heating. As a result both ignition and hydrogen release temperatures were studied for heating rates ranging from 10⁴ to 10⁵ K/s. The hydrogen release temperature was determined in vacuum, and ignition was studied at ambient pressure in air, CO₂, and mixtures of argon with oxygen. Both the hydrogen release and ignition temperature increased as the heating rate increased. Hydrogen release temperatures ranged from approximately 650 to 1200 K, whereas ignition was observed to range from below the melting temperature of aluminum (933 K) to approximately 1500 K. Activation energies for hydrogen release were ∼27 kJ/mol, and are well below those reported by others at much lower heating rates. This result is consistent with the affects of higher heating rates transitioning the rate-limiting step from one of chemical kinetics to intraparticle hydrogen diffusion. For conditions in which the particles would ignite it was found that the environment did not play a significant role in the ignition temperature, beyond a critical oxygen mole fraction of X_O2 > 0.05. Ensemble average burning times were found to decrease by a factor of about 3 when the oxygen mole fraction was increased from 0.1 to 0.5.

Keywords:

• Alane
• Aluminum hydride
• Decomposition
• Ignition

ACKNOWLEDGEMENTS

This research effort was sponsored through Center for Energetic Concepts Development (CECD), a collaborative effort between the Naval Surface Warfare Center–Indian Head Division (NSWC-IHD), and the University of Maryland, College Park (UMD). Partial support was also obtained from the Army Research Office (ARO) and the Defense Threat Reduction Agency (DTRA).