Additive-promoted hypergolic ignition of ionic liquid with hydrogen peroxide

Combustion and Flame 214 (2020) 426–436

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Additive-promoted hypergolic ignition of ionic liquid with hydrogen peroxide Vikas K. Bhosale a,b, Junyeong Jeong a, Jonghoon Choi b, David G. Churchill b,∗, Yunho Lee b, Sejin Kwon a,∗ a

Department of Aerospace Engineering, Korea Advanced Institute of Science and Technology, KAIST, 291 Daehak-ro, Daejeon 34141, Yuseong-gu, Republic of Korea Department of Chemistry, Korea Advanced Institute of Science and Technology, KAIST, 291 Daehak-ro, Daejeon 34141, Yuseong-gu, Republic of Korea

b

a r t i c l e

i n f o

Article history: Received 24 September 2019 Revised 30 October 2019 Accepted 13 January 2020

Keywords: Ionic liquid Hypergolic fuel Green fuel Bipropellant

a b s t r a c t The exploration of environmentally friendly hypergolic combinations of ionic liquid fuels and hydrogen peroxide as an oxidizer offers opportunities to replace commonly used conventional toxic, corrosive and carcinogenic hydrazine-based liquid hypergolic combinations. Various research opportunities await regarding the detailed investigations of green hypergolic ionic liquids (HILs) with rocket grade hydrogen peroxide (RGHP, >85% H2 O2 ). In this work, the combustion of HILs 1-ethyl-3-methyl imidazolium cyanoborohydride ([EMIM][BH3 CN]) and 1-allyl-3-ethyl imidazolium cyanoborohydride ([AEIM][BH3 CN]), and HIL-additive mixtures with 95% H2 O2 has been investigated. A new additive, 1,3-dimethyl imidazolium copper iodide ([diMIM]n [Cu2 I3 ]n ) was synthesized successfully; a structural investigation was performed through the use of single-crystal x-ray diffraction analysis through which a crystal density of 3.22 g/cm3 was revealed. The physicochemical properties (density, viscosity, and decomposition temperature) as well as performance parameters (ignition delay and specific impulse) of 2 to 15 wt% of [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] were determined. A 15 wt% of [diMIM]n [Cu2 I3 ]n exhibited ignition delay time (IDT) values of 13 and 29 ms under fuel-rich and oxidizer-rich conditions, respectively; these IDT values are 100 times lower than those for [EMIM][BH3 CN]. Interestingly, [diMIM]n [Cu2 I3 ]n [EMIM][BH3 CN] combinations revealed high density (>0.98 g/cm3 ), good thermal (>200 °C) and chemical stability, and 5.6–6.0% higher density specific impulse than those found for unsymmetrical dimethyl hydrazine. The additive-promoted hypergolic combustion of HIL with RGHP opens a new avenue to the replacement of conventional toxic hypergolic combinations. © 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Auto-ignition of fuels with storable oxidizers is a characteristic property of fuels designated as ‘hypergolic fuels’ [1]. Hypergolic fuels have advantages over non-hypergolic fuels because the rocket engine can ‘start’ and ‘restart’ easily without any ignition source; therefore, the total mass of the rocket engine can be reduced. Such fuels have been used in satellite launch vehicles, missiles, orbital maneuvers, etc. [2]. Since 1947, hydrazine and its derivatives (monomethylhydrazine, MMH and unsymmetrical dimethylhydrazine, UDMH) have been used or tested as hypergolic liquid fuels with various conventionally storable oxidizers such as white fuming nitric acid (99% HNO3 , WFNA), red fuming nitric

∗

Corresponding authors. E-mail addresses: [email protected] (D.G. Churchill), [email protected] (S. Kwon).

acid (99% HNO3 , RFNA), nitrogen tetroxide (N2 O4 ), mixed oxides of nitrogen (MON-10), etc. [2–4]. However, these hypergolic combinations are extremely toxic, corrosive and carcinogenic; this leads to more hazards to personnel during required handling, because the compounds possess high vapor pressure and may generate highly toxic vapors [5–7]. Therefore, rocket scientists are looking to promising and less hazardous storable liquid hypergolic propellant combinations to replace non-optimal conventional hypergolic propellant combinations. Researchers have been investigating several non-toxic hypergolic fuel combinations, but a limited number of fuels have been discovered that achieve desirable rocket performance [8]. The development of new non-toxic hypergolic fuel combinations without, at the same time, compromising overall rocket engine performance is still challenging in current research. Therefore, intense research is underway to develop a green hypergolic propellant combination. Recently, hypergolic ionic liquids (HILs) have emerged as a green hypergolic fuel for propellant applications [9–13]. HIL is the salt

https://doi.org/10.1016/j.combustflame.2020.01.013 0010-2180/© 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

V.K. Bhosale, J. Jeong and J. Choi et al. / Combustion and Flame 214 (2020) 426–436

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Table 1 Physicochemical properties of hypergolic fuels. Hypergolic fuels

Tm °C

Td °C

ρ (g/cm3 )

Ƞ (mPas)

Vapor pressure (kPa)

IDT (ms)

[EMIM][BH3 CN] [15] [AEIM][BH3 CN] [33] UDMH [34]

−71 <−50 −57

247 265 62

0.98 0.95 0.79

19.0 17.0 0.492

0.00138a 0.00075a 13.7b

4 3 –

Tm : melting temperature, Td : decomposition temperature, ρ : density at 25 °C, Ƞ: viscosity at 25 °C, Hf : heat of formation, IDT: ignition delay time was measured with WFNA. a Vapor pressure of ionic liquids was calculated using TGA method at 120 °C (Supporting information SI-F). b Vapor pressure at 20 °C Ref. [35].

composed of an organic cation and organic or inorganic anion; it has a melting temperature below 100 °C [14]. These liquids possess low vapor pressure, are easy to synthesize in sufficient yields, are liquid over a wide temperature range, possess comparable density and viscosity, are non-flammable and, significantly, exhibit ultrafast ignition when they come in contact with storable oxidizers [9,15–23]. As of a few years ago, our research group has been working on the development of green hypergolic fuel combinations [10,24–27]. Recently, B–H bond-rich HILs and zwitterionic green hypergolic fuel combinations with green oxidizer rocket grade hydrogen peroxide (RGHP, > 85% H2 O2 ) has been an area of intense investigation [10]. Hydrogen peroxide, a household product and environmentally friendly oxidizer that has been used successfully in bipropellant and monopropellant applications because it has a preferable low vapor pressure and produces non-toxic gases upon combustion [8,28,29]. Schneider et al. [30] investigated the hypergolic combination of aluminum-borohydride-based HILs and 98% hydrogen peroxide; however, these fuels have limited applications due to their high viscosity. Gozin et al. [31,32] investigated highly soluble promoters, such as closo-icosahedral periodoborane and iodocuprate (polymeric copper iodide anion with heterocyclic cation) in HIL. These promotors are promising because they aid to lower the ignition delay time (IDT) of HIL from 10 0 0 to 32 ms upon combustion with 95% H2 O2 . However, there is still a pressing need to lower the IDT of HILs with 95% H2 O2 . Hence, searching for the right combination of inherently reactive or additive-promoted hypergolic fuels with H2 O2 having low IDT is a current challenge in rocket fuel research. In this paper, various additive-promoters were used to promote the hypergolic ignition of HIL with 95% H2 O2 . HILs 1-ethyl3-methyl imidazolium cyanoborohydride ([EMIM][BH3 CN]) and 1allyl-3-ethyl imidazolium cyanoborohydride ([AEIM][BH3 CN]) were chosen for the investigation because they remain in the liquid phase over a wide range of temperature (−71–265 °C), have low vapor pressures (≤ 0.0014 kPa), possess sufficiently high density (> 0.95 g/cm−3 ) and exhibit both, low viscosity (~17 mPas) and good thermal stability (> 200 °C) (Table 1) [33]. Interestingly, they exhibit ultrafast ignition with nitric acid (oxidizer) because the B– H bond-rich moiety contains chemical energy, which helps to initiate the ignition with the chosen oxidizer. The B–H bond-rich moiety also assists to initiate the ignition of fuels with RGHP [10]. The additive-promoters, such as cobalt acetate, manganese acetate, copper chloride, and sodium borohydride were used (Fig. 1) to promote the hypergolic ignition of HIL [EMIM][BH3 CN] with 95% H2 O2 . In addition, a highly soluble iodine-rich additive-promoter 1,3-dimethyl imidazolium copper iodide ([diMIM]n [Cu2 I3 ]n ) was also developed to lower the IDT of the given HIL. Finally, the physicochemical properties and combustion behavior of HIL-additive were investigated. 2. Experimental approach Caution! While performing hypergolic drop testing research, we the authors, did not experience an explosion but did experience a

Fig. 1. Structures of HILs and additives.

couple of unexpected events. Because, rocket grade hydrogen peroxide can explode violently under excess concentration or quantity of hydrogen peroxide under oxidizer-rich conditions. Copper ion concentration leads to a continuous decomposition of hydrogen peroxide. Hence, steps to consider and review safety precautions need to be taken and proper guidance should always be followed when research is conducted regarding hypergolic drop testing, even on small scale. The ignition drop test should be conducted using a proper experimental setup including a ventilated fume hood to avoid an accident. In addition, the mishandling of rocket grade hydrogen peroxide can cause injury to persons such as damage to skin including burns which could be severe. 2.1. Materials and methods Chemicals including 1-methyl imidazole (Sigma-Aldrich), 1allyl imidazole (97%, Across organics), Bromoethane (98%, SigmaAldrich), sodium borohydride (98%, Alfa Aeasar), THF (99%, Merck), sodium cyanoborohydride (95%, Sigma-Aldrich), ethyl acetate (≥99.5%, Merck) and acetonitrile (Samchun Pure Chemical Co., Ltd.) were used in the synthesis of HILs. The additives such as cobalt(II) acetate (99%, Sigma-Aldrich), manganese(II) acetate (98%, Sigma

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Scheme 1. Synthesis of 1,3-dimethyl imidazolium copper iodide ([diMIM]n [Cu2 I3 ]n ).

Aldrich), copper(II) chloride (anhydrous, Sigma-Aldrich), sodium borohydride, and [diMIM]n [Cu2 I3 ]n were used to prepare the catalytic reactive hypergolic fuels. The density and viscosity of 2– 15 wt% [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] were measured by using I.S.O. 649 standard hydrometer and SV-10 VIBRO viscometer, respectively. A Parr-1261 bomb calorimeter was used to measure the heat of combustion of the fuels. All chemicals were used as received without any further purification. 1 H and 13 C NMR spectra of all fuels were recorded in CDCl3 or DMSO-d6 using an Avance 400 MHz Bruker NMR spectrometer. Chemical shifts found in the NMR spectra are reported relative to the singlet peak assigned for Me4 Si. The FT-IR spectra of compounds were recorded by using Nicolet iS50 ATR FT-IR spectrometer (Thermo Fisher Scientific Instrument) with light accuracy of < ± 0.07%. The DSC-TGA was measured by using LABSYS Evo simultaneous TGA-DSC analyzer as a reference with a heating rate of 10 °C min−1 under N2 atmosphere (gas flow rate 50 mL/min). A 95% H2 O2 was purchased from Shanghai HABO Chemical Technology Co., Ltd. China and refractometer (PR-50HO, ATAGO/Japan, measuring accuracy ± 0.5%) was used to measure the concentration of hydrogen peroxide prior to the hypergolic test experiment and which was found to be 94.7% (± 0.2). All the hypergolic test experiments were conducted with the same concentration of H2 O2 . The heats of formation for the HILs and [diMIM]n [Cu2 I3 ]n were determined by using a bomb calorimeter (Supporting Information, SI-E); the theoretical specific impulse was evaluated by using NASA-CEA software [36,37]. HILs, [EMIM][BH3 CN] and [AEIM][BH3 CN] were synthesized by the reported methods [15,33]. The additives viz., cobalt acetate, manganese acetate, copper chloride, and sodium borohydride were used to lower the ignition delay of [EMIM][BH3 CN]. Further, a new additive 1,3-dimethyl imidazolium copper iodide [diMIM]n [Cu2 I3 ]n was prepared by a two-step synthetic process (Scheme 1). It was used in the combustion trials to facilitate a faster ignition of [EMIM][BH3 CN]. Step 1: Synthesis of 1,3-dimethyl imidazolium iodide [diMIM]I In a 250 mL round bottom flask, 1-methyl imidazole (15.5, 188 mmol) and iodomethane (32.1 g, 226 mmol) were dissolved in a sample of dry THF (200 mL) and stirred at room temperature (25-30 °C) for 2 h. A precipitate was found to form. Then, the precipitate was washed three times with THF to obtain a 1,3-dimethyl imidazolium iodide as a white solid, yield: 40.4 g, 95 %. Characterization: 1 H NMR (DMSO-d6 ): 9.04 (1H, -C-H), 7.66 (2H, C–H), 3.81 (6H, –CH3 ) ppm.13 C NMR (DMSO-d6 ):140.08, 126.51, 38.90 ppm (spectra are given in the SI-B section). Step 2: Synthesis of 1,3-dimethyl imidazolium copper iodide [diMIM]n [Cu2 I3 ]n A powder of CuI (5.94 g, 31.2 mmol) was dispersed in a 100 mL round bottom flask containing [DiMIM]I (4.66 g, 20.7 mmol)

methanol (50 mL) solution; the reaction mixture was refluxed for 2 h. After, the reaction mixture was cooled to room temperature, the precipitate was separated and washed with cold methanol (3 × 3 mL). A white powder of a pure compound 1,3-dimethyl imidazolium copper iodide was obtained, yield: 8.34 g, 70%. Characterization: 1 H NMR (DMSO-d6 ): 8.98 (1H, -C-H), 7.63 (2H, C–H), 3.80 (6H, –CH3 ) ppm.13 C NMR (DMSO-d6 ): 140.07, 126.55, 38.84 ppm (spectra are given in SI-B). 2.2. Hypergolic drop testing The experimental setup of the hypergolic drop test is illustrated in Fig. 2. The hypergolic ignition process of the HIL and HILadditives fuels with 95% H2 O2 were recorded. The additives were used to promote the hypergolic ignition of HILs with 95% H2 O2 . In order to consider the safety precautions, the experiment was conducted inside an ignition delay drop test setup; it was placed in the fume hood containing air atmosphere at 298.15 K. The test setup was open on the top to allow for the removal of the combustion gases after the ignition test performed. In addition, the polyacrylate sheet door system was used for the ventilation of air and place the sample into an appropriate position. The mixing of the fuel and oxidizer impacts ignition performance; hence, the distance between the fuel/oxidizer drop and oxidizer/fuel pool was fixed at 110 mm by using a vertical and horizontal stand (Fig. 2). Under fuel rich conditions, a watch glass (diameter 40 mm) containing the fuel-pool was placed horizontally on the experimental stand; the oxidizer line was mounted on a vertical stand. The oxidizer-rich condition, fuel line was mounted on a vertical stand; the watch glass or beaker (hight 45 mm × 32 mm diameter) containing the oxidizer-pool was placed horizontally on the experimental stand. The syringe pump was monitored to ensure the dispensing of the 40 μL droplet of fuel/oxidizer into the 200 μL oxidizer-pool/fuel-pool and it was kept constant for all experiments (except Sections 3.2 and 3.3, a 50 0 0 μL oxidizer pool was used in a 25 mL beaker). A k-type thermocouple with GL220 elctronic device was used to measure the temperature of oxidizer pool. The release height of the fuel/oxidizer droplet was maintained at a constant distance during the experiment to ensure the same fall velocity; the impact, therefore, of the drop was consistant so that it would not change the ignition performance from one test to another. Each test was performed three times under the same experimental conditions. The hypergolic ignition process was recorded by using a high-speed charge-coupled device (CCD) camera (X-StreamTM XS) which acquires 10 0 0 frames per second (fps). The LED lamp was used as the light source to attain clear images. The size (3.0 mm) and velocity (0.6 m/s) of the droplet were measured by careful estimates based on the images captured by the high-speed camera. The velocity of droplet was measured just before the impact to the oxidizer/fuel pool.

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Fig. 2. Schematic of ignition delay measurement setup.

3. Results and discussion HILs [EMIM][BH3 CN] and [AEIM][BH3 CN] were synthesized by the following literature methods [15,33], the spectroscopic data were in good agreement with the reported information (SI-A). In addition, a new additive [diMIM]n [Cu2 I3 ]n was developed and its structural information and application of hypergolic fuel research was discussed in detail. 3.1. Synthesis of [diMIM]n [Cu2 I3 ]n and X-ray crystallography The water-insoluble polymeric ionic salt of [diMIM]n [Cu2 I3 ]n was synthesized by refluxing the [diMIM]I and CuI in methanol. Further, the structure was confirmed by using 1 H and 13 C NMR spectroscopy, and use of single-crystal X-ray diffraction. The thermal properties were measured by using DSC-TGA; [diMIM]n [Cu2 I3 ]n was found to be stable up to 335 °C (Td 5% ) (SID). White crystals of the ionic salt were grown by slow addition of methanol into the solution of [diMIM]n [Cu2 I3 ]n in dimethylsulfoxide (DMSO). The [diMIM]n [Cu2 I3 ]n crystals (CCDC 1,948,887) were found to crystallize in the orthorhombic space group Pbcn and carries a crystal density of 3.22 g/cm3 measured at 120 K (SI-C). Figure 3 demonstrates the arrangement of atoms within the single crystal structure. The symmetric unit contains the interconnected four copper and six iodide ions and two 1,3 dimethyl imidazolium moieties. Each copper atom is tetrahedrally coordinated with four iodide atoms in a distorted tetrahedral geometric fashion, whereas one iodide is coordinated to three Cu centers. The Cu–I bond lengths are found to be between 2.5477(5) and ˚ The bond angles of Cu–I–Cu are between 53.303(17) 2.7978(6) A. and 112.20(2)°, whereas I–Cu–I bond angles formed are between 97.384(17) and 116.486(19)°. Adjacent copper atoms are connected through an edge-sharing mode to form a polymeric [Cu2 I3 ]n − cluster which is charge stabilized by 1,3-dimethyl imidazolium cation. The molecular formula of the crystal structure is C5 H9 N2 Cu2 I3 , relevant to formulation of [diMIM]n [Cu2 I3 ]n . 3.2. Combustion of HIL with hydrogen peroxide Auto-ignition of fuel with an oxidizer is the primary characteristic of hypergolic fuels. The ignition performance of fuels, therefore, is measured in terms of an ‘ignition delay time (IDT)’. It is the time

interval between the physical contact of fuel with the surface of oxidizer and first visible flame. Based on IDT, the performance of various hypergolic fuels can be compared conveniently and have indeed been compared previously [9,38–41]. In the 1950s, the acceptable time limit of hypergolic fuel IDT ≤50 ms was designed in order to avoid the “hard” start of the rocket engine. IDT values that are longer result in accumulation of fuel in the combustion chamber compartment and generate unwanted pressure spikes during combustion [2]. In this study, green HILs, [EMIM][BH3 CN] and [AEIM][BH3 CN] were taken under consideration to measure the IDT with oxidizer-rich conditions containing 50 0 0 μL of 95% H2 O2 in a 25 mL beaker. These HILs are ultrafast in their ignition (<5 ms) with oxidizers, such as WFNA and RFNA; however, these oxidizers are both highly toxic and corrosive [23]. Hence, the combustion tests of these HILs were conducted with 95% H2 O2 at room temperature by using a hypergolic drop test setup. As described in Section 2.2, IDTs of HILs, [EMIM][BH3 CN] and [AEIM][BH3 CN] were measured under oxidizer-rich conditions having 50 0 0 μL of 95% H2 O2 and revealed smooth ignition with longer values of >10 0 0 ms; these longer IDT values may be due to the weakly acidic nature of 95% H2 O2 . Further, the IDT of HILs was lowered by a change in temperature in the oxidizer pool. The temperature of the 95% H2 O2 pool was increased from 25 to 85 °C through the use of a subsequent internal combustion reaction of HILs with oxidizer. The k-type of thermocouple was used to record the temperature of the oxidizer-pool in a GL220 electronic device. The desirable temperature of oxidizer-pool was achieved by the initial combustion of IL with 95% H2 O2 . Furthermore, fuel was dropped into the oxidizer at experimental temperature and the IDT was then measured. The temperature probe was not kept for longer times in the oxidizer pool because it may start to react with the oxidizer. Samples of fresh oxidizer were taken for each experiment. Figure 4 reveals that the IDTs of HILs were lowered by increasing the temperature of the 95% H2 O2 pool. The HILs [EMIM][BH3 CN] and [AEIM][BH3 CN] exhibited IDTs of 116 and 173 ms, respectively, at 85 °C; this is ten times lower than the IDTs found for HILs at 25 °C. As per thermodynamic principles, the rate of reaction is doubled when the temperature of the reaction is raised by 10 °C [42]. Hence, HILs exhibit lower IDT at high temperature. Shortening the IDT of HILs by increasing the temperature of 95% H2 O2 is not a feasible method in rocket propulsion because 95% H2 O2 may be decomposed at higher temperatures. Therefore, further

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Fig. 3. (a) Molecular structure and (b) crystal packing diagram of [diMIM]n [Cu2 I3 ]n .

studies were conducted to find a suitable additive-promoter to help lower the IDT and increase the combustion efficiency of HILs. HIL, [EMIM][BH3 CN] was considered for further combustion study because it has better IDT than [AEIM][BH3 CN]. 3.3. Combustion of HIL-additive with hydrogen peroxide A limited number of known reactive fuels are hypergolic with RGHP; therefore, the additive or promoter has been used to promote the hypergolic ignition of fuel with RGHP [24,27,32,43–45]. HILs of borohydride rich functionals are reactive with 90 and 98% H2 O2 with IDT <30 ms in nitrogen atmosphere [30]; however, they are hygroscopic and required to prepare a dry atmosphere. Hence, it is essential when developing a green hypergolic fuel for it to be hydrolytically stable, of low viscosity and deliver short IDT with RGHP. In similar a way, our research group has also been involved in developing non-toxic hypergolic fuel combinations by the

addition of sodium borohydride additives in the fuel mixture with lowering IDT <10 ms [24]. Again, drawbacks for sodium borohydride, because it is moisture sensitive. Further, the effort has been continued to search for additive-promoted HIL fuel combustion. In this study, additives, such as cobalt acetate, manganese acetate, copper chloride, sodium borohydride, and [diMIM]n [Cu2 I3 ]n were used to lower the IDT of HIL, [EMIM][BH3 CN]. The 5 wt % of each additive was stirred in a 5 mL glass bottle containing [EMIM][BH3 CN] for 24 h. The green color of copper chloride was observed in the solution containing [EMIM][BH3 CN]; however, the evolution of gas bubbles was also observed during the dissolution. The additives sodium borohydride, cobalt acetate, manganese acetate were partially immiscible. Interestingly, 5 wt% [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] was completely miscible. Further, IDT of HIL-additive was measured by dropping it into the 50 0 0 μL of 95% H2 O2 in a 25 mL glass beaker. Table 2 presents the ignition behavior of HIL-additives with 95%

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Table 2 Combustion behavior of HIL-additives with 95% H2 O2 . HILs

Catalyst

wt% additives in HIL

IDTa (ms)

Observation Solubility of additive in HIL

[EMIM][BH3 CN] [AEIM][BH3 CN] [EMIM][BH3 CN] [EMIM][BH3 CN]

– – CuCl2 Mn(CH3 COO)2

– – 5 5

1400 (±100) 1500 (±100) 139 (±9) 395 (±12)

– – Soluble and unstable in HIL Limited solubility

[EMIM][BH3 CN]

Co(CH3 COO)2

5

887 (±45)

Limited solubility

[EMIM][BH3 CN] [EMIM][BH3 CN]

NaBH4 [diMIM]n [Cu2 I3 ]n

5 5

73 (±8) 87 (±9)

Limited solubility Highly soluble, no precipitation

a

Smooth combustion Smooth combustion Violent combustion! Smooth combustion but continuous decomposition of hydrogen peroxide was observed Smooth combustion but continuous decomposition of hydrogen peroxide was observed Vigorous combustion Moderate noise during combustion

Oxidizer-rich 50 0 0 μL 95% H2 O2 .

1600

[EMIM][BH3CN]

1400

[AEIM][BH3CN]

1200

IDT (ms)

Combustion HIL-additive

1000 800 600 400 200 0

20

30

40

50

60

70

80

90

o

Temperature of 95% H2O2 ( C) Fig. 4. Ignition delay time of HILs at different temperatures of oxidizer (95% H2 O2 ).

H2 O2 . A violent ignition of 5 wt% CuCl2 was observed at 139 ms. However, the HIL-additives of cobalt acetate and manganese acetate exhibited smooth combustion at 887 and 395 ms, respectively. After subsequent addition of cobalt acetate and manganese acetate of HIL mixture, a continuous decomposition of 95% H2 O2 was observed. The ˂5 wt% NaBH4 exhibited IDT about 73 ms with vigorous combustion; the 5 wt% [diMIM]n [Cu2 I3 ]n revealed an IDT of about 87 ms. In conclusion, the additive-promoter NaBH4 exhibits a shorter ignition delay than that for [diMIM]n [Cu2 I3 ]n ; however the solubility of NaBH4 was about <5 wt% and destabilization occurs upon longer storage [46]. Therefore, the highly soluble additive-promoter [diMIM]n [Cu2 I3 ]n is considered for detailed combustion investigations, because, the iodine-rich promoter can help in lowering the IDT of HILs [31,32]. 3.4. Physicochemical properties and IDT of [diMIM]n [Cu2 I3 ]n -[EMIM][BH3 CN] With respect to liquid hypergolic fuels, viscosity, density and thermal stability are all equally yet independently vital parameters for consideration in rocket engine applications. Low viscosity fuels are recommended for rocket fuel applications to help facilitate rapid mass transfer and ease in mixing with the oxidizer. Hence an increase in propellant combustion efficiency results [15]. Higher density fuels have high loading capacity contained within a minimal volume; this allows more fuel to be packed into

the fuel tank. It helps therefore to increase the (mission) range and density specific impulse performance. Therefore, the density, viscosity, and decomposition temperature of various proportions such as 2, 5, 7, 9, 12 and 15 wt% of [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] (Fig. 5) were measured (Table 3). The crystal density of the additive is 3.22 g/cm3 . Further, the density of the HIL-additive mixture was 0.98–1.02 g/cm3 from 2 to 15 wt%, respectively. This was found to be higher than in the hydrazine-based fuels revealing the higher loading capacity of fuel in the storage tanks. Nevertheless, viscosity increases from 19.2 to 44.0 mPas. The thermal properties of [diMIM]n [Cu2 I3 ], [EMIM][BH3 CN] and 2–15 wt% of [diMIM]n [Cu2 I3 ]n -[EMIM][BH3 CN] was measured by using DSC-TGA analysis under a nitrogen flow of 50 mL/min (SI-D). A characteristic endothermic peak of [diMIM]n [Cu2 I3 ] was observed at 189 °C and 372 °C corresponding to the melting and decomposition temperature, respectively. However, [EMIM][BH3 CN] exhibited exothermic peak at 265 °C and 415 °C resemblance to decomposition temperature. The similar exothermic peaks of 2 to 15 wt% [diMIM]n [Cu2 I3 ]n -[EMIM][BH3 CN] mixtures were observed and these mixture were thermally stable up to Td , 244 °C. However, the TGA study reveals the initial decomposition temperature of [EMIM][BH3 CN] was lowered by increasing the concentration of [diMIM]n [Cu2 I3 ]n (Fig. 6 and Table 3). The measurement for IDT for [diMIM]n [Cu2 I3 ]n -[EMIM][BH3 CN] mixtures was performed in oxidizer and fuel-rich conditions with different [diMIM]n [Cu2 I3 ]n concentrations. During the course of experiments, the volume of oxidizer/fuel pool (300 μL) and falling drop of oxidizer/fuel (40 μL) was kept constant. In addition, the container (watch glass) of the oxidizer/fuel pool was also identical in all experiments. Because, the IDTs vary with different physical parameter [47,48]. The 2–15 wt% [diMIM]n [Cu2 I3 ]n helped to lower the IDT of [EMIM][BH3 CN] from 126 to 29 ms and 47– 13 ms under oxidizer-rich and fuel-rich conditions, respectively (Fig. 7). Furthermore, the IDT of higher concentration (>15 wt%) of [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] was measured in fuel-rich and oxidizer-rich conditions (Fig. 7); however, IDT was not changed under fuel-rich condition. Moreover, the higher concentration of additive ≥15 wt%, [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] exhibited the violent decomposition with oxidizer under oxidizer-rich as well as fuel-rich conditions; therefore, the additive composition <15 wt% of [diMIM]n [Cu2 I3 ]n can be safe to study hypergolic application. Figures 8 and 9 presents the sequence of hypergolic images of 15 wt% [diMIM]n [Cu2 I3 ]n in oxidizer-rich rather than fuel-rich conditions. Interestingly, the 15 wt% [diMIM]n [Cu2 I3 ]n exhibited a lower IDT of [EMIM][BH3 CN] under fuel-rich conditions rather than when the conditions were oxidizer-rich. This may be due to the pre-ignition energy because the ignition of fuel requires a sufficient amount of heat energy to be

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Table 3 Physicochemical properties and ignition delays of HIL-additive hypergolic fuel. Hypergolic fuels

Td ( °C)

η (mPa s)

ϱ (g/cm3 )

IDT, ms Oxidizer richness

[EMIM][BH3 CN] 2 wt% [diMIM]n [Cu2 I3 ]n + 98 wt% [EMIM][BH3 CN] 5 wt% [diMIM]n [Cu2 I3 ]n + 95 wt% [EMIM][BH3 CN] 7 wt% [diMIM]n [Cu2 I3 ]n + 93 wt% [EMIM][BH3 CN] 10 wt% [diMIM]n [Cu2 I3 ]n + 90 wt% [EMIM][BH3 CN] 12 wt% [diMIM]n [Cu2 I3 ]n + 88 wt% [EMIM][BH3 CN] 15 wt% [diMIM]n [Cu2 I3 ]n + 85 wt% [EMIM][BH3 CN]

264 244 242 242 240 237 234

0.9752 0.9890 0.9965 1.0050 1.0174 1.0190 1.0250

Fuel richness

>1000 126 (±10.0) 57 (±5.0) 50 (±5.0) 41 (±4.0) 38 (±2.0) 29 (±1.5)

16.6 19.2 23.8 28.6 31.6 39.3 44.0

>1000 47 (±2.0) 30 (±1.0) 21 (±0.6) 17 (±0.6) 14 (±0.2) 13 (±0.2)

Td : decomposition temperature, ϱ: density at 25 °C, η: viscosity at 25 °C, IDT: ignition delay time was measured with 95% H2 O2 .

Fig. 5. Blends of additive, 2–15 wt% of [diMIM]n [Cu2 I3 ]n with an HIL, [EMIM][BH3 CN].

110 100

Td(5%)

HIL

90

Additive

Weight (%)

80 70

Additive HIL A B C D E F

60 50 40 30

B E

C

D F

A

20 10 50

100

150

200

250

300

350

400

450

500

Temperature (°C) Fig. 6. TGA analysis of A: 2 wt% [diMIM]n [Cu2 I3 ]n + HIL, B: 5 wt% [diMIM]n [Cu2 I3 ]n + HIL, C: 7 wt% [diMIM]n [Cu2 I3 ]n + HIL, D: 10 wt% [diMIM]n [Cu2 I3 ]n + HIL, E: 12 wt% [diMIM]n [Cu2 I3 ]n + HIL, F: 15 wt% [diMIM]n [Cu2 I3 ]n + HIL, HIL: [EMIM][BH3 CN], Additive: [diMIM]n [Cu2 I3 ]n .

generated during the pre-ignition reaction. During the oxidizer rich combustion process, the amount of heat energy generated during the pre-ignition reaction may be immediately absorbed by unreacted hydrogen peroxide (due to high latent heat of H2 O2 , 1260 kJ/kg); therefore, the IDT values are longer. However, under fuel-rich combustion, the total heat energy generated during the pre-ignition reaction may be promptly available for the ignition of fuel and therefore was found to exhibit a shorter IDT.

Gozin and coworkers [31] proposed the mechanism of hydrogen peroxide decomposition in the presence of polymeric copper iodide; the strongly exergonic (G = −14.0 and −6.5 kcal mol−1 in two steps) release was enough to prompt the combustion of fuel. Overall, the additive, [diMIM]n [Cu2 I3 ]n (IDT 13 ms) was found to promote faster ignition than either closo-icosahedral periodoborane salts (IDT 17 ms) [32] or [diMIM]n [Cu2 I3 ]n (IDT 24 ms) [31].

V.K. Bhosale, J. Jeong and J. Choi et al. / Combustion and Flame 214 (2020) 426–436

433

Fig. 7. Ignition delay of HIL-additive under oxidizer-rich and fuel-rich conditions.

3.5. Specific impulse and density specific impulse Specific impulse (Isp ) and density specific impulse (ϱIsp ) are important and relevant parameters to assess help the performance of rocket engines. The specific impulse is the amount of thrust produced by the rocket engine per unit weight of flow rate of propellant (fuel plus oxidizer) [2]. It measures the potential efficiency of particular fuel and oxidizer combinations. The combustion of propellants in a rocket engine converts chemical energy into useful work. We should note that the specific impulse does not only depend on the propellant combustion; rocket engine parameters are also significant. The high heat of formation of propellant and low molecular weight of combustion products are favorable to enhance the efficiency of the rocket engine [2,49]. √ Equation, Isp ~ (Tc /M) states that the specific impulse is directly proportional to the combustion temperature (Tc ) and inversely proportional to the molecular weight of exhaust combustion products (M) [2]. Lowering the molecular weight of the combustion gases leads to an increase in the average kinetic energy of gas molecules, and therefore increases the specific impulse of the fuel.

Fig. 9. High-speed camera image sequence of hypergolic ignition with 15 wt% additive-HIL under fuel-rich condition.

In addition, the physical parameters such as oxidizer-to-fuel ratio (O/F), chamber pressure (Pc ), ambient pressure (Pe ) and expansion ratio (Ae /At , ratio of nozzle exit area to the nozzle throat area) are also significant parameters to help monitor the propulsive performance of the rocket engine. The higher specific impulse may ultimately increase the final velocity of the space vehicle because the specific impulse usually has a direct effect on the vehicle’s overall performance [2]. Therefore, it is necessary to design a rocket engine and propellant compositions with as high a specific impulse as possible. In our previous work, a set of theoretical specific impulses of HILs with different oxidizers was evaluated and compared with those for hydrazine-based fuels [34]. The study revealed that HILs deliver high specific impulse with H2 O2

Fig. 8. High-speed camera image sequence of hypergolic ignition with 15 wt% additive-HIL under oxidizer-rich condition.

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244 242 240

Isp (s)

238 236 234 232 230 228 226

2.0

2.5

3.0

3.5

4.0

4.5

5.0

O/F 2 wt% additve + HIL,

5 wt% additve + HIL,

,

10 wt% additve + HIL HIL,

7 wt% additve + HIL,

12 wt% additve + HIL,

15 wt% additve + HIL,

UDMH.

Fig. 10. Specific impulse of hypergolic fuels (additive: [diMIM]n [Cu2 I3 ]n , HIL: [EMIM][BH3 CN]). Table 4 Theoretical performance of samples containing HIL-additive evaluated by using NASA-CEA. Hypergolic

[EMIM] [BH3 CN]

fuels Optimum O/F Isp (s) ϱ (g/cm3 ) ϱIsp (s g/cm3 )

3.5 241.2 1.289 311.0

wt% [diMIM]n [Cu2 I3 ]n in HIL, [EMIM]BH3 CN]

UDMH

2

5

7

10

12

15

3.5 240.7 1.295 311.6

3.5 239.9 1.297 311.3

3.5 239.4 1.301 311.4

3.5 238.6 1.305 311.4

3.5 238.0 1.306 310.8

3.5 237.1 1.308 310.1

3.5 243.2 1.204 292.9

Isp : specific impulse, ϱ: density of propellant, ϱIsp : density-specific impulse of propellant.

oxidizer under optimized conditions; their performance was better than hydrazine-based hypergolic fuels. In this work, the theoretical specific impulse and densityspecific impulse of different fuel compositions such as 2, 5, 7, 10, 12, 15 wt% additive, [diMIM]n [Cu2 I3 ]n in HIL, [EMIM][BH3 CN] was evaluated with 95% H2 O2 by using chemical equilibrium application (CEA) codes developed and reported by researchers at NASA [36,37]. The information viz., heat of formation and elemental composition of fuel and oxidizer, ambient temperature (298.15 K), fuel and oxidizer ratio (2.0 to 5.0), Pc = 25 atm, Pe = 1 atm. and Ae /At = 4 and frozen flow conditions during expansion were considered throughout the calculations. The heat of formation of [diMIM]n [Cu2 I3 ]n and [EMIM][BH3 CN] was calculated by using values of the heat of combustion (SI-E). The heats of formation of [diMIM]n [Cu2 I3 ]n , [EMIM][BH3 CN] and H2 O2 were found to be −179.0, 236.0 and −188.0 kJ/mol, respectively. Figure 10 depicts the change in the specific impulse of fuels with a change in the composition of [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] at different O/F ratios. The specific impulse increases when O/F increases from 2.0 to 3.5 and the specific impulse decreases when O/F increases from 3.5 to 5.0. The maximum specific impulse of fuels was observed at O/F, 3.5 (optimum O/F, Table 4). The specific impulse of [EMIM][BH3 CN] was 241.2 s; it was gradually lowered to about 1.7% for the sample containing 15 wt %

[diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN]. This diminution may be due to the addition of high molecular weight elements such as copper and iodine. Further, the density-specific impulse of fuels was estimated by considering the density of propellant (fuel and oxidizer) [34]. Eq. (1) calculates the density of propellant by considering the density of IL-additive and 95% H2 O2 (1.42 g/cm3 ), and an optimum O/F ratio.

 propel l ant

  oxidizer ×  f uel 1 + OF   =  f uel × OF + oxidizer

(1)

where, ϱpropellant , ϱfuel and ϱoxidizer are the density of the propellant, fuel and oxidizer, respectively. O/F is the ratio of oxidizer to fuel. Consequently, ϱIsp values were also estimated by considering the ϱpropellant ,

Isp =  propel l ant × Isp

(2)

The 2–15 wt% of [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] have 5.6– 6.0% higher density specific impulse scores than those for UDMH because they have higher propellant density (>1.30 g/cm3 ). 4. Chemical stability of fuel The additive [diMIM]n [Cu2 I3 ]n reported in this current investigation appears to be a very promising to help lower the

V.K. Bhosale, J. Jeong and J. Choi et al. / Combustion and Flame 214 (2020) 426–436

-B-H

-C-H

1570

2328

[EMIM][BH3CN]

2326

2166

[diMIM]n[Cu2I3]n

3500

3000

=C-H

2166

0 day, 32 wt.% [diMIM] [Cu I ] : n 23n [EMIM][BH CN] 3

-C-H

1570

2328

[EMIM][BH CN] 3

-C=C-

2166

30 days, 32 wt.% [diMIM] [Cu I ] : n 23n

Transmittance (%)

-C≡N

435

1570

1565

2500

2000

1500

1000

500

-1 Wavenumber (cm ) Fig. 11. FT-IR spectrum of [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] and 32 wt% [diMIM]n [Cu2 I3 ]n -[EMIM][BH3 CN].

IDT of [EMIM][BH3 CN]. However, it is essential to investigate the storability of the additive in the fuel at atmospheric conditions for extended time. Therefore, the chemical stability of [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] was investigated by using FT-IR spectroscopy. A solution of higher concentration of additive 32 wt% [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] was prepared. This solution was stored in a 5 mL airtight sample tube for thirty days at 20 °C. Further, spectra of pure samples of [diMIM]n [Cu2 I3 ]n and [EMIM][BH3 CN] were recorded by using FT-IR. Consequently, the spectrum of freshly prepared and stored solution after 30 days of additive-HIL were recorded (Fig. 11). It was found that there is no chemical change in spectral information, except the transmittance of –C≡N bond. The stretching frequency assigned to the respective alkyl –C–H, –B-H, –C≡N, aromatic –C=C– bonds were observed at 320 0–290 0, 2328–2326, 2166 and 1570 cm−1 , respectively. However, the –B–H bond of the pure sample of [EMIM][BH3 CN] and with [diMIM]n [Cu2 I3 ]n -[EMIM][BH3 CN] stretches at 2326 cm−1 and 2628 cm−1 , respectively. We propose that this may be due to intermolecular hydrogen bonding between the cyanoborohydride and copper iodide anions; these interactions, therefore, sense to weaken the B-H bond signal and oscillation found at higher frequency. Moreover, the bending frequency of the aliphatic and aromatic C–H bond vibrations were observed in the range of 120 0–50 0 cm−1 . The vibrational frequency of the Cu–I bond was not observed in the ATR-IR spectrum because it oscillates in the far IR region. In conclusion, spectral information of the additive-HIL solution of [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] led us to conclude that the blend was chemically stable for 30 days.

ited IDT values of about 116 and 173 ms, respectively at 85 °C. A new additive-promoter, [diMIM]n [Cu2 I3 ]n was successfully developed. The single-crystal X-ray diffraction study of [diMIM]n [Cu2 I3 ]n obtained and revealed important structural information with a crystal density of 3.22 g/cm3 . The sample containing 2–15 wt% mixtures of [diMIM]n [Cu2 I3 ]n -[EMIM][BH3 CN] revealed high density (>0.98 g/cm3 ), comparable viscosity (<44 mPas) and good thermal stability (>200 °C) as compared with those values for trials with UDMH. An increase in the concentration of additive from 2 to 15 wt% [diMIM]n [Cu2 I3 ]n in [EMIM][BH3 CN] helps to lower the IDT time in both oxidizer-rich and fuel-rich conditions. A 15 wt% [diMIM]n [Cu2 I3 ]n result in a lower the IDT of [EMIM][BH3 CN] of 13 and 29 ms in fuel-rich and oxidizer-rich conditions, respectively. The density specific impulse of [diMIM]n [Cu2 I3 ]n -[EMIM][BH3 CN] exhibited values 5.6–6.0% higher than those for UDMH. The promising properties and performance of additive-promoted hypergolic HIL could help to promote the safer discovery of hypergolic combinations with RGHP so to replace conventional toxic propellant combinations.

5. Conclusions

The research was supported by basic science research program through the National Research Foundation of Republic of Korea (NRF) funded by the Ministry of Education (NRF2018R1D1A1A02049748) and Defense Acquisition Program Administration and Agency for Defense Development, Republic of Korea under the contract UD180046. The author, Dr. Vikas K. Bhosale is

The different additives which promoted hypergolic ignition of HILs with oxidizer, 95% H2 O2 were investigated. The effect of the temperature of oxidizer-pool on the hypergolic ignition of HILs was studied. The [EMIM][BH3 CN] and [AEIM][BH3 CN] samples exhib-

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments

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