Hypergolic Ionic Liquids Research Articles

Tension ring-functionalized bicyclic ammonium ionic liquids as hypergolic fuels with superior energy density

Hypergolic ionic liquids are a new type of liquid propellants, and the energy density is critical to propulsion performance. We propose to combine cage structures with tensile rings to design and synthesize new ionic liquid molecules. Cycloalkane-substituted 1-aza-bicyclo[2.2.2]octane and 1,4-diazabicyclo[2.2.2]octane-like ionic liquids were synthesized using dicyanamide root as an anion, and its structure and properties were determined. The obtained ionic liquids possess higher energy density up to 1.87 kJ·mL−1. The three-membered ring substituent can increase the energy density of ionic liquids by 79.8 % and reduce the ignition delay time by 52.5 % than that of straight-chain alkanes. This work provides an important basis for the design and synthesis of the new type of hypergolic ionic liquids.

N-Alkyl-1,2,4-triazole–Borane Complexes as High-Density Hypergolic Materials

AbstractHydrazine and its derivatives have served as a conventional bipropellant fuel for several decades. However, their extremely acute toxicity and carcinogenicity, high volatility, and environmental impact have attracted significant attention. In order to synthesize green bipropellant fuels with high density, high specific impulse, and good thermal stability, three novel N-alkyl-1,2,4-triazole–borane complexes were successfully synthesized by reacting alkylated 1,2,4-triazole coordinated with sodium borohydride in the presence of ammonium sulfate. During the droplet test with white fuming nitric acid, there was a relatively short ignition delay time of 98 ms. Additionally, these hypergolic fuels possessed a high density exceeding 1.10 g cm–3, and the specific impulse is ranging from 187 to 199 s, and the highest decomposition temperature reaches 153.4 °C. These results demonstrate their great potential as hypergolic fuels or hypergolic ionic liquid additives in the field of hypergolic materials.

Hypergolic ionic liquids: to be or not to be?

Hypergolic ionic liquids (HIL) - ionic liquids which ignite spontaneously upon contact with an oxidizer - emerged as green space propellants. Exploiting the previously marked hypergolic [EMIM][CBH] - WFNA (1-ethyl-3-methylimidazolium cyanoborohydride - white fuming nitric acid) system as a benchmark, through the utilization of a novel chirped-pulse droplet-merging technique in an ultrasonic levitation environment and electronic structure calculations, this work deeply questions the hypergolicity of the [EMIM][CBH]-WFNA system. Molecular oxygen is critically required for the [EMIM][CBH]-WFNA system to ignite spontaneously. State-of-the-art electronic structure calculations identified the resonantly stabilized N-boryl-N-oxo-formamide [(H3B-N(O)-CHO)-; BOFA] radical anion as the key intermediate in driving the oxidation chemistry upon reaction with molecular oxygen of the ionic liquid. These findings challenge conventional wisdom of 'well-established' test protocols as indicators of the hypergolicity of ionic liquids thus necessitating truly oxygen-free experimental conditions to define the ignition delay upon mixing of the ionic liquid and the oxidizer and hence designating an ionic liquid as truly hypergolic at the molecular level.

First Example of 1,2,5-Oxadiazole-Based Hypergolic Ionic Liquids: a New Class of Potential Energetic Fuels.

The development of liquid energetic fuels with improved properties is important topic in space propulsion technologies. In this manuscript, a series of energetic ionic liquids incorporating a 1,2,5-oxadiazole ring and nitrate, dicyanamide or dinitramide anion was synthesized and their physicochemical properties were evaluated. The synthesized compounds were fully characterized and were found to have good thermal stabilities (up to 219 °C) and experimental densities (1.21-1.47 g cm-3). Advantageously, 1,2,5-oxadiazole-based ionic liquids have high combined nitrogen-oxygen contents (up to 64.4%), while their detonation velocities are on the level of known explosive TNT, and combustion performance exceeds those of benchmark 2-hydroxyethylhydrazinium nitrate. Considering the established hypergolicity with H2O2 in the presence of a catalyst, and insensitivity to impact, synthesized ionic liquids have strong application potential as energetic fuels for space technologies.

Different Ignition Modes and Temperature Evolution of Typical Hypergolic Ionic Liquids

The hypergolic ignition behaviors of three ionic liquids [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] reacting with white fuming nitric acid (WFNA) were investigated using the drop test approach. Two high-speed cameras with/without a long-distance microscope and a time-resolved infrared camera were used to simultaneously record the hypergolic ignition process and the reaction region temperature evolution. Results showed two distinct hypergolic ignition modes. Specifically, for [BMIm][DCA] and [EMIm][DCA], some “vapor smoke” is generated after the fuel droplet contacts the WFNA pool, followed by sudden ejection of liquids induced by microexplosion. The measured temperature in the observation window is found to first increase, then decrease, and rise again. For [EMIm][CDB], no microexplosion is observed and the temperature monotonically increases before hypergolic ignition. It is inferred that microexplosion is caused by sufficient accumulation of gaseous intermediates or products underneath the liquid surface, and the monotonic or nonmonotonic increasing temperature is a characteristic of ignition modes. A conceptual model is proposed to illustrate the two ignition modes. In addition, the droplet impact velocity (U0) was varied from 1.1 to 2.1 m/s in the experiment. It is found that the vapor delay time (VDT), explosion delay time (EDT), and ignition delay time (IDT) are all reduced with U0 by at most 50%. For a given U0, the three time scales decrease in the order [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] (IDT as low as 14.4 ms).

Synthesis and properties of bio-renewable ionic salts derived from theophylline as green hypergolic fuels

Hypergolic ionic liquids (ILs) are a feasible green alternative to the toxic and volatile hydrazine-based fuel in propellant systems. However, most reported hypergolic ILs still have environmental and biological issues due to their petrochemical product sources. Theophylline is a plant-derived methylxanthine-type alkaloid with relatively high energy density. This work synthesizes a novel family of bio-renewable theophylline-derived cations for energetic ionic compounds, with dicyanamide as the hypergolic anion. Five with shorter cationic alkyl groups (less than or equal to four carbon atoms) exhibit spontaneous combustion upon contact with 100 % HNO3. Meanwhile, they possess high densities above 1.30 g·cm−3, and density-specific impulses varied in the range of 333.2–348.0 s·g·cm−3 (calculated by Gaussian 09 program), better than those of the hydrazine-based propellants. Notably, 9-butenyl-1,3,5-trimethylxanthinium dicyanamide exhibits excellent integrated properties, including the shortest ignition delay time (49 ms), most comprehensive liquid operating temperature range (−70 to 215 °C), and a high density-specific impulse (342.2 s·g·cm−3). This study is expected to pave the way for developing bio-renewable hypergolic ILs from natural sources.

Characteristics of ignition delay of hypergolic ionic liquids combined with 1-amino-4-methylpiperazine.

The ignition delay time of the hypergolic ionic liquids, 1-ethyl-3-methylimidazolium dicyanamide [EMIM][C2N3] and 1,3-dimethyl imidazolium dicyandiamide [DMIM][C2N3], can be controlled to approximately 20 ms by adding 1-amino-4-methylpiperazine while keeping the vapor pressure below 1 torr at 298 K.

ATOMIZATION AND MIXING OF BLENDED ENERGETIC IONIC LIQUID HYDROXYETHYLHYDRAZINIUM NITRATE

Hypergolic propellants are widely used in liquid rocket propulsion. Instantaneous control of combustion, lack of requirement of an ignition system, and storability for long durations are the major advantages of hypergolic propellants. The propellant community is on the lookout for replacements for the currently utilized toxic hydrazine and its derivatives. Energetic hypergolic ionic liquids are excellent candidates for such replacement. However, their high viscosity and surface tension render atomization difficult, and hence, increase the ignition delays in the combustion chamber and reduce combustion efficiency. The current study focuses on the geometrical aspects of injector design and other injection conditions that control the ignition process and combustion efficiency. These mainly depend on the extent of mixing and atomization. Shadowgraphy and patternator-based experiments were performed on doublet and triplet impingement injectors under variable injection conditions, such as impingement angle, jet diameters, and injection velocities. A blend of 50% unsymmetrical dimethylhydrazine in the energetic ionic liquid hydroxyethylhydrazinium nitrate (UHN50) shows promise as a candidate for a hydrazine replacement. A blend of 38% ethanol in glycerol (GE6238) was used as a surrogate for UHN50, whereas water was used as a surrogate for nitrogen tetroxide. The effect of variation of injection conditions was found to be contradictory for achieving atomization and mixing. An optimum criterion was obtained for both atomization and mixing.

Unraveling the initial steps of the ignition chemistry of the hypergolic ionic liquid 1-ethyl-3-methylimidazolium cyanoborohydride ([EMIM][CBH]) with nitric acid (HNO3) exploiting chirped pulse triggered droplet merging.

The composition of the products and the mechanistic routes for the reaction of the hypergolic ionic liquid (HIL) 1-ethyl-3-methylimidazolium cyanoborohydride ([EMIM][CBH]) and nitric acid (HNO3) at various concentrations from 10% to 70% were explored using a contactless single droplet merging within an ultrasonic levitation setup in an inert atmosphere of argon to reveal the initial steps that cause hypergolicity. The reactions were initiated through controlled droplet-merging manipulation triggered by a frequency chirp pulse amplitude modulation. Utilizing the high-speed optical and infrared cameras surrounding the levitation process chamber, intriguing visual images were unveiled: (i) extensive gas release and (ii) temperature rises of up to 435 K in the merged droplets. The gas development was validated qualitatively and quantitatively with Fourier Transform Infrared Spectroscopy (FTIR) indicating the major gas-phase products to be hydrogen cyanide (HCN) and nitrous oxide (N2O). The merged droplet was also probed by pulsed Raman spectroscopy which deciphered features for key functional groups of the reaction products and intermediates (-BH, -BH2, -BH3, -NCO); reaction kinetics revealed that the reaction was initiated by the interaction of the [CBH]- anion of the HIL with the oxidizer (HNO3) through proton transfer. Computations indicate the formation of a van-der-Waals complex between the [CBH]- anion and HNO3 initially, followed by proton transfer from the acid to the anion and subsequent extensive isomerization; these rearrangements were found to be essential for the formation of HCN and N2O. The exoergicity observed during the merging process provides a molar enthalpy change up to 10 kJ mol-1 to the system, which could be sufficient for a significant fraction of the reactants of about 11% to overcome the reaction barriers in the individual steps of the computationally determined minimum energy pathways.

Unsupervised Reaction Pathways Search for the Oxidation of Hypergolic Ionic Liquids: 1-Ethyl-3-methylimidazolium Cyanoborohydride (EMIM+/CBH–) as a Case Study

Hypergolic ionic liquids have come under increased study for having several desirable properties as a fuel source. One particular ionic liquid, 1-ethyl-3-methylimidazolium/cyanoborohydride (EMIM+/CBH-), and oxidant, nitric acid (HNO3), has been reported to be hypergolic experimentally, but its mechanism is not well-understood at a mechanistic level. In this computational study, the reaction is first probed with ab initio molecular dynamics simulations to confirm that anion-oxidant interactions likely are the first step in the mechanism. Second, the potential energy surface of the anion-oxidant system is studied with an in-depth search over possible isomerizations, and a network of possible intermediates are found. The critical point search is unsupervised and thus has the potential of identifying structures that deviate from chemical intuition. Molecular graphs are employed for analyzing 3000+ intermediates found, and nudged elastic band calculations are employed to identify transition states between them. Finally, the reactivity of the system is discussed through examination of minimal energy paths connecting the reactant to various common products from hypergolic ionic liquid oxidation. Eight products are reported for this system: NO, N2O, NO2, HNO, HONO, HNO2, HCN, and H2O. All reaction paths leading to these exothermic products have overall reaction barriers of 6-7 kcal/mol.

Experimental characterization of the electrospray propulsive performance for ionic liquid propellants [EMIm][DCA] and [BMIm][DCA]

A chemical-electrospray dual-mode propulsion system is significant in improving mission capability and flexibility of a spacecraft. Hypergolic ionic liquids are potential propellants of chemical and electrospray propulsion systems. In this work, the electrical propulsive performance of two hypergolic ionic liquids 1-ethyl-3-methylimidazolium dicyanamide ([EMIm][DCA]) and 1-butyl-3-methylimidazolium dicyanamide ([BMIm][DCA]) are experimentally characterized using two array-emitter electrospray thrusters. Compared with a commonly used ionic liquid propellant 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF4]), the current magnitude and current steadiness of the electrospray thruster with [EMIm][DCA] and [BMIm][DCA] are efficiently improved, especially with [EMIm][DCA] as propellant. The results show that the low viscosity is beneficial for steady and sufficient flow supply and thus favors a long-time steady operation of the thruster. For the propellant [EMIm][DCA], the maximum specific impulse is 1791 s, i.e., 27.8% higher than that of [EMIm][BF4], while the maximum thrust 32.5 μN is 128.9% greater than that of [EMIm][BF4]. For the propellant [BMIm][DCA], the thrust-to-power ratio of the thruster is approximately 82.8% higher than that of [EMIm][BF4] at a similar specific impulse of about 1120 s resulting from a much greater thruster efficiency. These results demonstrate that [EMIm][DCA] has a better performance than [EMIm][BF4] in both maximum thrust and specific impulse, and [BMIm][DCA] performs best in the aspect of thrust-to-power ratios. This work highlights the prospect of hypergolic ionic liquids and is valuable to promote their application in chemical and electrical propulsion communities.

Research on Hypergolic Ionic Liquid Propellants

Research on Hypergolic Ionic Liquid Propellants

Green Hypergolic Ionic Liquids: Future Rocket Propellants

Hydrazine and its derivatives have been used as standard propellants for spacecraft propulsion systems since the 1960s, despite being highly toxic and carcinogenic. The propellant synthesis community has constantly been looking for green alternatives for the same. Hypergolic ionic liquids (HILs) with several attractive properties, such as high energy content, high bulk density, low vapor pressure, and low toxicity, have been proposed as an alternative to hydrazine and its derivatives. In the present study, the theoretical performance of sixty-eight HILs was studied at a combustion chamber pressure of 3 MPa and a nozzle expansion ratio of 40. The specific impulse and density specific impulse of the HILs were calculated with white fuming nitric acid (WFNA), inhibited red fuming nitric acid (IRFNA), and nitrogen tetroxide (NTO) as oxidizers. The specific impulse of 2,2-dimethyltriazanium nitrate (HIL-1) was found to be 23 s higher than monomethylhydrazine (MMH), whereas its density-specific impulse was found to be 123 g-s/cm3 higher than MMH. The gains in the specific impulse and density specific impulse coupled with other desirable “green” properties for several HILs are expected to establish them as potential replacements for hydrazine and its derivatives.

Synthesis and characterization of hypergolic salts based on bis(1H-1,2,3-triazole-1-yl) dihydroborate anion

As a new generation of potential rocket fuel, hypergolic ionic liquids, especially boron-containing hypergolic ionic liquids, have emerged as attractive alternatives for unsymmetrical dimethylhydrazine (UDMH). Herein, a series of bis(1H-1,2,3-triazole-1-yl) dihydroborate (BTDB) hypergolic ionic liquids were successfully synthesized, which had a melting point below -60 °C, high density ranging from 1.134 g/cm3 to 1.173 g/cm3, and superior density-specific impulse from 374.2 to 386.5 s g/cm3. In particular, 1,3-diallyl-1H-imidazol-3-ium bis(1H-1,2,3-triazole-1-yl) dihydroborate owned an ignition-delay time of 36 ms and the enthalpy of formation about 1.78 kJ/g, exhibiting that it was a good potential for the rocket fuels.

Achieving short ignition delay and high specific impulse with cyanoborohydride-based hypergolic ionic liquids

A new family of HILs, based on substituted 1H-1,2,4-triazol-4-ium, pyrrolidinium, ammonium and pyridinium cations and a cyanoborohydride anion, is introduced, with higher heats of formation, heats of combustion and specific impulse compared to dimethylhydrazine.

Microfluidic assisted 90% loading CL-20 spherical particles: Enhancing self-sustaining combustion performance

The performance of the chemical fuel determines the altitude, range and longevity of spacecraft in air and space exploration. Promising alternatives (e.g., hypergolic ionic liquids or high-energy composites) with high-energy density, heat of formation and fast initial rate are considered as potential chemical fuels. As the high-energy density material, hexanitrohexaazaisowurtzitane (CL-20) often serves as secondary explosive with poor self-propagating combustion behaviors. Herein, 90% loading CL-20 microspheres with uniform particle sizes are precisely prepared by microfluid method, which exhibit unique hierarchical structure. The morphology, thermal behaviors, as well as combustion performance were further investigated. The results demonstrated that as-prepared spherical particles exhibit prominent thermal compatibility, and the enhanced self-sustaining combustion performance. This work provides an efficient method achieving the uniform high-energy density particles with excellent self-sustaining combustion performance.

From heart drug to propellant fuels: Designing nitroglycerin-ionic liquid composite as green high-energy hypergolic fluids

New-generation green bipropellants based on ionic liquids for aircraft have attracted the attention of researchers. However, the major defects of ionic liquid propellants are their low specific impulse and incomplete combustion, which limits their practical application. In this work, a series of novel hypergolic fluids were prepared by mixing oxygen-rich nitroglycerin (NG) with fuel-rich dicyanamide-based ionic liquids. The complete dissolution of NG in ionic liquids makes these hypergolic fluids became self-supplying oxygen composite fuels, which generate much less solid residual products than pure hypergolic ionic liquids (HILs). Moreover, the addition of NG inhibited the micro-explosion between fuel and white fuming nitric acid (WFNA), extended the contact time between fuel and oxidizer. And a proper amount of NG in the fuel could suppress the secondary combustion. These composite fuels have high density (>1 g cm−3), low viscosity (as low as 13.49 cP), wide liquid operating ranges and acceptable ignition delay time. Additionally, the specific impulses of the composite fuels were higher than pure HILs. For the first time, this work provides a simple and reliable method to control the ignition properties of HIL by adding oxygen-enriched additives.

Cationic effect on properties related to thermal stability and ignition delay for hypergolic ionic liquids

Hypergolic ionic liquids (HILs) are considered as a promising alternative of toxic hydrazine derivatives in the field of liquid bipropellants. The anionic structure plays an important role in determining the hypergolic performance of HILs. Nevertheless, the structure–activity relationship between the cationic structure and ignition properties is still unclear. In this study, the aromaticity, electrostatic potentials (ESPs), natural bond orbital (NBO) analysis, and molecular orbital gap of 25 ionic liquids based on dicyanamide anion were studied to gain more insight into the correlation between cationic structure and hypergolic properties. For the HILs based on the same type of cation with different substituents, better aromaticity leads to better thermal stability of HILs. For the HILs with different types of cations, when the ESPs distribution is relatively concentrated around a certain value, the thermal stability of HILs is relatively higher. The second-order perturbation energy (E(2)) would be responsible for ignition delay (ID) time. The larger the E(2), the more stable the system, and the longer the ID time of HILs. It is also proved for the first time that the energy gap between the HOMO of cations and LUMO of HNO3 could not be used as a method to determine whether this cation-based IL is hypergolic. All these results can be fully justified with experimental data and have important theoretical instructive significance in the design of new high-performance HILs.

Hypergolic coordination compounds as modifiers for ionic liquid propulsion

Ionic liquids (ILs) have been regarded as green hypergolic materials since 2008, and hundreds of hypergolic ionic liquids have been reported thus far. However, only few have been proved to meet practical applications, due to their complex preparation technology and long hypergolic ignition delay (ID). Herein, to improve the properties of hypergolic ignition of ILs, we demonstrate a new design strategy of hypergolic coordination compounds (HCCs). Eight HCCs were synthesized based on two commonly used hypergolic ILs components (imidazolium = IM, dicyanamide = DCA) and four transition metal ions (Mn, Co, Ni, Cu) and were characterized through infrared spectroscopy, X-ray powder diffraction, and single-crystal X-ray diffraction. The experimental results showed that these HCCs exhibited excellent hypergolic ignition properties, and the ignition and combustion properties could be modulated by changing the metal, ligand, and anion. Additionally, the effects of HCCs on the ignition and combustion of organic hypergolic IL were investigated. The Cu-based HCCs showed the best performance among these new hypergolic materials. Compared with the HCCs-free IL, the IL system with 5% of [Cu(AIM)4](DCA)2 exhibited a sharp reduction in the ID time of 1-allyl-3-methylimidazolium dicyanamide (AMIMDCA) (6 ms vs. 45 ms).

Withdrawal notice to: Guanidinium dicyanamide-based nitrogen-rich energetic salts as additives of hypergolic ionic liquids [Combustion and Flame 226 (2021) 243–247

Withdrawal notice to: Guanidinium dicyanamide-based nitrogen-rich energetic salts as additives of hypergolic ionic liquids [Combustion and Flame 226 (2021) 243–247