Stability, reactivity and decomposition kinetics of surface passivated α-AlH₃ crystals

As a novel high energy fuel, aluminum hydride (AlH₃) has great potential in the field of solid propellants because of its high hydrogen capacity, which can significantly improve the specific impulse of solid propellants. In order to improve the stability of α-AlH₃, hydrochloric acid has been used to stabilize AlH₃ and the stabilization mechanism has been investigated. Various characterization techniques including scanning electron microscopy, X-ray electron spectrometer, X-ray diffraction, thermal analysis, and vacuum stability test have been employed to investigate the morphology, crystal structure, thermal stability, and decomposition kinetics of raw and passivated α-AlH₃. The results showed that the honeycomb-like structures could be formed on the surface of α-AlH₃ after passivation. First of all, the initial decomposition temperatures of the passivated samples were slightly increased. In particular, for the optimized sample with 105 min passivation time (AlH₃-105min), the initial decomposition temperature (173.4 °C) is increased by 5.6 °C. Moreover, the total decomposition time (1652 min) is improved by about 50% than that of the raw sample (1098 min). Besides, the decomposition activation energies (E_a) of passivated samples are much higher than that of the raw sample (84.8 kJ mol⁻¹), in which the optimized sample (AlH₃-105min) reaches 107.1 kJ. mol⁻¹. The decomposition kinetics model may change from 3-D nucleation and nucleus growth model to 2-D nucleation and nucleus growth model. It demonstrates the passivated samples have a lower decomposition rate and higher thermal stability. The stabilization mechanism is as follows: removing the impurities on the surface and accelerating the hydrolysis reaction of AlH₃ to generate complete and dense oxide layers.