Bicyclic ammonium ionic liquids as dense hypergolic fuels

Abstract

It is critical for hypergolic fuels to be dense so as to enhance the performance of propellants and improve the loading of rockets. Considering that dense elements and bicyclic rings contribute to high density, two series of ionic liquids were prepared with 1-aza-bicyclo[2.2.2]octane-/1,4-diazabicyclo[2.2.2]octane-based cations and the dicyanamide anion. Their key properties were measured or calculated as decomposition temperature (208–322 °C), density (1.06–1.31 g cm⁻³), specific impulse (260.5 to 266.2 s) and ignition delay time (14–1053 ms). Compared with 1-aza-bicyclo[2.2.2]octane-based ionic liquids, the corresponding 1,4-diazabicyclo[2.2.2]octane-based ionic liquids exhibit a lower decomposition temperature, higher density and bigger specific impulse. As expected, 1-methyl-1-azonia-bicyclo[2.2.2]octane and 1-(prop-2-ynyl)-4-aza-1-azonia-bicyclo[2.2.2]octane dicyanamide possess higher densities (1.20 and 1.19 g cm⁻³) than the corresponding pyrrolidinium and imidazolium-based isomers (1.05 and 1.07 g cm⁻³), besides hypergolicity with white fuming nitric acid.