Effect of α-AlH3 content on ignition and combustion characteristics of multicomponent mixtures

Abstract

Aluminum hydride (alane, AlH3) is a promising energetic material that can significantly increase the specific impulse of propellants while lowering the combustion temperature. In this work, a CO2 laser was adopted to investigate the effect of α-AlH3 content on the ignition and combustion characteristics of AlH3–Al, AlH3–ammonium perchlorate (AP), AlH3–Al–AP, and AlH3–Al–AP–hydroxyl-terminated polybutadiene (HTPB) mixtures (namely, samples DAl, DAP, T, and Q, respectively). The DAl samples formed a mushroom flame at the beginning of combustion due to the Kelvin-Helmholtz effect under the heat flow generated by the combustion of H2 released from AlH3. The rapid hydrogen release caused the flame to break away from the sample to form a “dark zone”, whose height gradually shortened. The high AlH3 content promoted the decomposition of HTPB to C4 species, resulting in a “dark zone” for Q samples during ignition. The DAP and T samples combusted intensely with lower ignition delay time (ti) and combustion time (tc) and higher maximum combustion temperature (Tmax) than the other two samples due to the AP. The combustion temperatures of the mixed samples were effectively decreased, while the combustion rates were also accelerated by partially replacing Al with AlH3. As the rate of AlH3:Al was increased from 1:3 to 3:1, the Tmax of the DAl, T, and Q samples decreased by 113, 276, and 128 K, respectively, whereas tc decreased by 16.18 %, 32.11 %, and 8.18 %, respectively. However, the ti of the Q samples increased with increasing AlH3, probably because a large amount of hydrogen release exacerbated the effect of the porous layer, thus hindering the escape of the gas phase products. In addition, only weak Al(g) characteristic emission spectra were observed during combustion, which is due to the HTPB adhesion largely inhibiting the combustion of metallic fuels.