Hypergolic ionic mixtures with task-specific ions: A new strategy to improve performances of ionic liquids as propellant fuels

Abstract Task-specific dicyanamide anion ([DCA]−), cyanoborohydride anion ([CBH]−) and 1-ethyl or 1-allyl-3-methylimidazolium cations ([EMIM]+ or [AMIM]+) were mixed in different ratios to form hypergolic ionic mixtures. Their viscosities, densities, decomposition temperatures, specific impulses were determined and calculated. Ignition delay time was obtained by a drop test recorded by a high-speed camera. Theoretical calculations and molecular dynamics simulations were applied for exploring their energy and microstructure. The results indicated that ionic nature and ionic ratio in mixtures were two decisive factors for properties of ionic mixtures. Specially, mixture [EMIM][CBH]0.7[DCA]0.3 possessed the shortest ignition delay time of 3 ms with white fuming nitric acid (WFNA) as oxidizer, which was shorter than that of pure ionic liquid [EMIM][DCA] (32 ms) or [EMIM][CBH] (8 ms). However, with a reverse ratio of anions, mixture [EMIM][CBH]0.3[DCA]0.7 possessed a very long ignition delay time of 373 ms and showed a different phenomenon in ignition process. Thermogravimetric analysis indicated that mixture [EMIM][CBH]0.7[DCA]0.3 decomposed faster than mixture [EMIM][CBH]0.3[DCA]0.7. The radial distribution functions (RDFs) suggested that the H-bond interactions in mixtures obviously were influenced by ionic ratios. As a new design strategy, hypergolic ionic mixtures can not only combine properties of different ions, but also show more prominent performances in a specific ionic ratio than each parent ionic liquid.