The combustion of lithium-aluminum alloy fuels in oxygen and water vapor

The combustion of a lithium-aluminum alloy was investigated using optical diagnostics and a computational flame model. Chemical kinetic mechanisms were developed to model combustion of the Li-Al alloy in oxygen and in water vapor. The alloy was heated inductively in a crucible and burned with air, oxygen/argon mixtures, and water vapor in a counterflow diffusion flame. Bulk alloy temperature was held relatively constant during combustion, allowing alloy temperatures in the 1000–1400 K range to be studied. The flame temperature profile was measured using a resonance line reversal method, and the relative lithium atom population in the flame was measured using an absorption method. The products of combustion were collected and analyzed using x-ray diffraction (XRD) and field-emission scanning electron microscopy/energy dispersive x-ray spectroscopy (FESEM/EDS). The primary condensed-phase products of combustion in both oxygen and water vapor are Li₂O, γ-LiAlO₂, and β-Li₅AlO₄; in addition, some solid LiOH is produced during combustion in water vapor. Although aluminum monoxide (AlO) gas was detected at ignition in air and water vapor in a limited number of tests, evidence indicates that aluminum does not burn in the vapor phase during steady-state combustion of the alloy. Experiments and counterflow diffusion models both show that the lithium burns in the vapor phase and aluminum reacts on the surface or in the bulk phase to form γ-LiAlO₂, and β-Li ₅AlO₄. Models indicate that Li₂O is the primary gas-phase product in dry oxidizers and that H₂ and LiOH are the primary gas-phase products in water vapor. This work provides the first experimental evidence that the aluminum in a lithium-aluminum alloy reacts in the liquid phase, the first chemical kinetic mechanisms to describe lithium vapor-phase flames, and the first chemical kinetic mechanisms for combustion of a metal alloy.