Synthesis And Characterization Of New Dicyanamide-based Hypergolic Ionic Liquids

Hypergolic propellants as a kind of liquid propellants with chemical ignition have become important power source of rockets and space vehicles, which can simplify the design of engine and achieve the trouble-free ignition for thousands of times. With the unique properties, hypergolic propellants have become main topic in the aerospace field. Currently, hypergolic ionic liquids(ILs) with good thermal stability and combustion performance show application potential to replace traditional propellant fuels(hydrazine and its derivatives), and have attracted increasing attentions in energetic material field.In this paper, four tetramethylethylenediamine-based(TMEAD-based) and ten bis(R-imiazolium)dihydroboronium-based hypergolic ILs were designed and synthesized from TMEAD and borane-trimethylamine complex as reaction substrates, respectively. The resulting ILs were characterized by 1H and 13 C NMR, infrared spectroscopy, and elemental analysis(or high resolution mass spectrum). Their properties such as density, viscosity, thermal stability and ignition performance were studied by experimental or theoretical methods.For the TMEAD-based hypergolic ILs, their calculated frequencies are in good agreement with the corresponding experimental values. The results indicate that the alkyl substitutes of cations have a major impact on the physicochemical properties of ILs. With thermal stability up to 200 °C, the resulting ILs have heats of formation from 85.1 to 154.4 kJ mol-1. Moreover, 2-(dimethylamino)-N,N,N-trimethylethanaminium dicyanamide exhibits good combustion performance with ignition delay time(84 ms) and specific impulse(158 s).For the bis(R-imiazolium)dihydroboronium-based hypergolic ILs, their alkyl substitutes of cations have a big effect on the physicochemical properties also. Densities of ILs range from 1.04 to 1.27 g cm-3, heats of formation from 12.4 to 484.9 kJ mol-1, and decomposition activation energy from 74.72 ~ 130.82 kJ·mol-1. Among them, bis(1-allyl-1H-imidazole-3-yl)dihydroboronium dicyanamide shows best combustion performance including ignition delay time(18 ms) and specific impulse(176.7 s).In short, the obtained ILs possess relatively good hypergolic performance to be the promising alternative products of hydrazine and its derivatives. Moreover, 2-(dimethylamino)-N,N,N-trimethylethanaminium and bis(1-allyl-1H-imidazole-3-yl) dihydroboronium dicyanamide exhibit excellent combined properties, and have potential as the green fuels of liquid bipropellants.