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Acta Astronautica xxx (2016) xxx-xxx

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Synthesis and testing of hypergolic ionic liquids for chemical propulsion S.V. Stovbun, A.N. Shchegolikhin, S.V. Usachev, S.V. Khomik, S.P. Medvedev ⁎⁠ Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygin Str., Moscow 119991, Russia

ABSTRACT

Keywords: Ionic liquid Hypergolicity Ignition Propulsion

Synthesis of new highly energetic ionic liquids (ILs) is described, and their hypergolic ignition properties are tested. The synthesized ILs combine the advantages of conventional rocket propellants with the energy characteristics of acetylene derivatives. To this end, N-alkylated imidazoles (alkyl = ethyl, butyl) have been synthesized and alkylated with propargyl bromide. The desired ionic liquids have been produced by metathesis using Ag dicyanamide. Modified hypergolic drop tests with white fuming nitric acid have been performed for N-ethyl (IL-1) and N-butyl propargylimidazolium (IL-2) ionic liquids. In the modified drop tests, high-speed shadowgraph imaging is used to visualize the process, and the temperature rise due to ignition is monitored with a two-color photodetector. It is shown that the ignition delay is shorter for IL-1 as compared to IL-2. The ignition of IL-1 occurs in two stages, whereas the combustion of IL-2 proceeds smoothly without secondary flashes.

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ARTICLE INFO

1. Introduction

Energetic ionic liquids (EILs) can be viewed as a subclass of the broader class of high-energy materials (HEMs). By the most general definition, HEMs are chemical compounds containing large amounts of stored chemical energy that can be released when the materials are subjected to certain stimuli (e.g., heat, impact, friction, or electrostatic discharge). Typical HEMs include explosives, pyrotechnic compositions, jet fuels, and rocket propellants. In view of increasing concerns about environmental and safety issues, great effort has been invested in the development of eco-friendly and insensitive HEMs [1,2]. Recent research and development in new effective HEMs has focused on nitrogen-rich heterocyclic compounds (e.g., imidazole, pyrazole, triazole, tetrazole, and 1,2,4,5-tetrazine) since these materials have higher heats of formation, densities, and thermal stability as compared to their carbocyclic analogues [3–7]. In particular, one of the most recently developed subclasses of HEMs is that of energetic ionic salts [8–11]. A high-energy organic salt of this kind generally consists of a nitrogen-rich cation (e.g., guanidinium, imidazolium, triazolium, or tetrazolium) and a bulky anion containing at least one energetic functional group, such as −NO2⁠ , −N3⁠ , or –CN. Such HEMs are quite similar to ordinary ionic liquids because (1) both are ionic materials; (2) the cations most frequently used in both are nitrogen-containing heterocycles; and (3) their cation and anion structures can be designed, synthesized, and modified according to the intended use of the material. It is obvious that the flexibility inherent in the molecular design of ionic



Corresponding author. Email address: [email protected], [email protected] (S.P. Medvedev)

http://dx.doi.org/10.1016/j.actaastro.2016.11.047 Received 18 September 2016; Accepted 30 November 2016 Available online xxx 0094-5765/ © 2016 Published by Elsevier Ltd.

liquids offers broad opportunities for synthesizing numerous EILs with required properties β The very concept of energetic ionic liquids implies that this subclass of HEMs essentially subsumes low-melting ionic materials whose properties are similar to those of explosives, pyrotechnic fuels, or rocket propellants. A wide variety of EILs with different structural formulas synthesized to date can be successfully used as "green" explosives or propellants [13–16]. As a matter of fact, laboratory synthesis of IL-based HEMs is currently booming around the world. It should be noted that most of the myriad energetic salts based on nitrogen-rich heterocycles that have been synthesized over the past decade and have proved to be highly effective explosives cannot be categorized as EILs because they are not low-melting, with melting points above 100 °C. Such materials are not addressed in the present study, which focuses only on energetic salts that fall under the definition of EILs given above. However, data on other ionic materials can be found in recent reviews [15,17,18]. In 2001, energetic hydrazinium salts were patented, and their use as rocket propellants was explored [19]. A year later, Hammerl et al. reported synthesizing three protonated hydrazinium azide derivatives with melting points below 100°C, which can be categorized as EILs, but without referring to the concept of EIL [20]. In 2003, Drake et al. described a number of heterocycle-based salts where a protonated heterocycle was paired with an NO3⁠ −, ClO4⁠ −, or N(NO2⁠ )2⁠ − anion [21]. The authors wrote: "Many of these salts have melting points well below 100 °C, yet high decomposition onsets, defining them as new, highly energetic members of the well known class of materials identified as

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thesis of acetylene derivatives in bulk is much more feasible than that of strained-ring compounds. It has been shown in our earlier studies that acetylenic bromides are highly versatile synthons, permitting molecular design of various new heterocyclic compounds and fine tuning of functional characteristics thereof [25,26]. Accordingly, propargyl bromide is tested in this work as the simplest acetylene synthon for preparing a number of new EILs. When hypergolic pairs are selected, hypergolicity is assessed by measuring ignition delay times in various tests. Ignition delay is defined as the elapsed time between the first fuel–oxidizer contact and the appearance of a luminous flame. In the most popular method, known as the hypergolic drop test, a fuel droplet falls into an oxidizer pool contained in a crucible or (less frequently) vice versa. The droplet volume is much smaller than the pool volume. As correctly noted in [27], a test of this kind does not definitively measure the intrinsic ignition delay and only characterizes the ignition behavior of a fuel–oxidizer pair under particular test conditions. Nevertheless, this technique is well suited to the purpose of preliminary testing of fuel–oxidizer pairs for hypergolicity. Although test instrumentation varies between studies, high-speed video recording of fuel–oxidizer interaction is a common practice. The main objective of this study are to demonstrate the feasibility of preparative synthesis of new ionic liquids suitable as components in hypergolic liquid rocket propellants and to measure their respective ignition delays. Ignition delay measurements are performed using a modified drop test.

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ionic liquids." That was the paper in which the concept of EIL was originally introduced. Their results inspired the synthesis of more and more new EILs with diverse molecular structures and varying energy characteristics [12]. Results obtained by other research groups soon confirmed their potential use as high explosives and rocket propellants [13–15,22]. In 2008, dicyanamide-based ionic liquids were proposed for the first time as candidates to replace hydrazine and its derivatives as state-of-the-art fuels for bipropellant applications [23]. The discovery of these new IL-based hypergols stimulated the development of novel hypergolic bipropellant systems. Currently, new IL-based propellants of this kind are being developed at an unprecedented scale. Most researchers agree that the term ionic liquid is almost synonymous with the concept of green or safe. However, the advent of EILs has radically changed the attitude towards ionic liquids. Depending on the intended application, highly energetic ionic components of a salt can be selected individually to formulate the composition of a target EIL, making it a highly explosive or flammable material. As a consequence, new factors have come into play that have necessitated a revision of the conventional view of ionic liquids and effectively facilitated the emergence of EIL research and development as a new branch of materials chemistry. As distinct from conventional ionic liquids (green and safe solvents), the very nature of EILs suggests that use should be made of their high reactivity (e.g., in explosive formulations) or hypergolicity (spontaneous ignition upon contact with oxidizers). Fortunately, the design and synthetic approaches used to produce conventional ionic liquids are fully applicable to low-melting EILs. Moreover, the general synthetic strategy allows fine-tuning of the energy characteristics and hypergolicity of the target energetic material while preserving the inherent properties of ionic liquids. Furthermore, EILs actually constitute a new class of IL-based materials having high heats of combustion. The unique properties of EILs (ionic nature, wide liquid-state temperature range, structural diversity) can be used to obviate or minimize the disadvantages of conventional HEMs, such as polymorphism and impact sensitivity. The use of EILs as a new generation of HEMs is advantageous in various respects. In particular, (1) the unique properties of ionic liquids (low melting point, wide liquid-state temperature range, low viscosity, and high thermal stability) are retained and are more pronounced in EILs as compared to conventional HEMs; (2) the volatility of EILs is extremely low (often impossible to measure), which prevents both material losses due to evaporation and therefore toxic effects of EIL vapors on humans; (3) the liquid nature of EILs effectively eliminates the problems associated with polymorphism, which are commonly encountered when solid HEMs are used; (4) the lower sensitivity of most EILs to external stimuli (impact, friction, heat) significantly facilitates storage, handling, shipping, and processing of EILs and makes them much safer. In summary, EILs successfully combine characteristics of energetic salts with the unique advantages of ionic liquids. The physicochemical properties of EILs determine their effectiveness and practical applicability as a new generation of HEMs. By analogy with conventional ionic liquids, the characteristics of EILs can easily be tuned by selecting appropriate cation–anion pairs. The use of different substituted heterocycles and combinations of energetic functional groups can substantially modify the properties and performance of EILs, including melting point, energy density, thermal stability, detonation properties, and sensitivity to external stimuli. More importantly, their "dual functionality" can be exploited to design the molecular structure of cations and anions independently, which makes it possible to predict the hypergolicity and safety of the target ionic liquid with good accuracy. It has been shown in [24] that acetylenes have the highest heat of formation per gram of a hydrocarbon compound and a higher heat of formation per gram than many strained-ring compounds, e.g., cubane or quadricyclane. Also of importance is the fact that preparative syn

2. Synthesis and testing procedures To prepare for the study, two N-alkyl-substituted imidazoles (alkyl=ethyl, butyl) were synthesized and alkylated with propargyl bromide. The desired ionic liquids were produced by metathesis using silver dicyanamide. Fig. 1 schematizes the reaction pathway for the synthesis of propargyl-containing ionic liquids used in this study. The values of physicochemical parameters for the synthesized ILs were measured. The densities of both ILs are approximately 1100 kg/ m3⁠ . The viscosity of IL-1 is 0.5 kg/(m·s), and that of IL-2 is 2 kg/(m·s). The measured wetting angles are shown in the table (Table 1). Hypergolic ignition tests of these N-ethyl and N-butyl propargylimidazolium ionic liquids (IL-1 and IL-2, respectively) with white fuming nitric acid (WFNA) are performed using a modified drop test procedure inside a fume hood. Fig. 2 shows the test setup. The fuel and oxidizer are brought into contact and mixed in a 10×10-mm cross section, 50 mm high cuvette containing 5–10 ml of WFNA (oxidizer). A shad

Fig. 1. Schematic of synthesis of ionic liquids.

Table 1: The comparison between water and ionic liquids wetting angles. Glass

Water 4.32 Polytetrafluoroethylene 109±3.2

2

IL−1 27.43±3.7

IL−2 27.85±4.0

70.8±0.38

90.2±0.48

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tion. Note also that the oxidizer starts to boil (frame 3). In particular, the liquid volume in the frame 5, where the fuel reacts with the oxidizer, is approximately 40 times the droplet volume. Ignition (frame 4) and combustion take place in the gas phase above the oxidizer surface and the fuel–oxidizer mixing region. The images here show that the flame front gradually moves away from the region where the fuel and oxidizer mix and boil. However, combustion does not end at this stage. At approximately 10 ms after the ignition, another "flash" is observed and the flame spreads over the region occupied by fuel and oxidizer. This process is illustrated by the two rightmost images in Fig. 4. The occurrence of two "flashes" is confirmed by the IR emission detected with the photodiode. Fig. 5 shows the emission signals recorded at two wavelengths in this experiment and the temperature curve calculated from the emission data, where two "flashes" are clearly seen. Experiments on IL-2 have shown that the corresponding ignition delay is 387 ms, which is longer by almost an order of magnitude. As in the test discussed above, ignition is preceded by mixing, boiling, and outflow from the mixing region. Furthermore, IL-2 burns much longer than IL-1, and its combustion proceeds smoothly without secondary flashes. The temperature curve obtained during ignition and combustion of IL-2 is shown in Fig. 6.

Fig. 2. Modified drop test setup for measuring ignition delay: 1 – cuvette; 2 – high-speed camera; 3 – shadowgraph optical system; 4 – two-color photodetector.

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owgraph technique is used to record images of the droplet's fall, mixing, and ignition with a Mikrotron-1362 high-speed digital camera at a frame rate of up to 30,000 fps. A parallel light beam crossing the cuvette is formed by using a light source and a doublet lens. This technique not only distinguishes between liquid and gaseous states but also visualizes gas density gradients, making it possible to observe otherwise invisible details of flow structure. The temperature of combustion products is measured with a silicon-germanium "sandwich" two-color photodiode in the visible and near IR spectral range. The measurement principles and photodetector design are described in [28].

4. Conclusions

The possibility of synthesis of imidazolium-based ionic liquids suitable for use in hypergolic liquid propellants has been demonstrated. High-speed shadowgraphy combined with temperature measurement has been used to determine ignition delay times. It is found that changing an alkyl group in the imidazole ring can lead to a significant change in fuel properties. Control of ignition delay and combustion characteristics by modifying the chemical structure of the fuel is important for designing reliable rocket engines.

3. Results and discussion

Fig. 3 shows a selection of shadow images recorded at a frame rate of 5000 fps. In the drop tests, the droplet velocity just prior to impact varied between 0.8 and 1 m/s. It is clear that, after the droplet comes in contact with the oxidizer (frame 2), fast fuel–oxidizer mixing occurs both before and after igni

Fig. 3. Ignition of an IL-1 droplet upon contact with WFNA, with an ignition delay of 31 ms (frame 4). Frame rate =5000 fps; exposure time =2 µs.

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Fig. 4. Flame development in a mixture of IL-1 with WFNA.

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Fig. 5. Time variation of photodetector output and temperature during ignition and combustion of IL-1 mixed with WFNA.

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