High-Yielding and Continuous Fabrication of Nanosized CL-20-Based

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High-Yielding and Continuous Fabrication of Nano-Sized CL-20 Based Energetic Co-Crystals via Electrospraying Deposition Chuan Huang, Jinjiang Xu, Xin Tian, Jiahui Liu, Liping Pan, Zhijian Yang, and Fude Nie Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01568 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Crystal Growth & Design

High-Yielding and Continuous Fabrication of Nano-Sized CL-20 Based Energetic Co-Crystals via Electrospraying Deposition Chuan Huang,† Jinjiang Xu,† Xin Tian,† Jiahui Liu,† Liping Pan,† Zhijian Yang*,† and Fude Nie*,† †

Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, 621900, China

ABSTRACT: Energetic co-crystals, especially CL-20 based co-crystals, have attracted a wide range of attention due to their low sensitivity and impressive detonation performance. In this study, a series of nano-sized CL-20 based energetic co-crystals were successfully fabricated by electrospray deposition. For CL-20/TNT nano-cocrystals, the influence of different solvents on the morphology and crystal structure of as-prepared co-crystals were investigated. The results showed that all the electrosprayed CL-20/TNT samples were partial formation of co-crystals and particles obtained from ketone had smaller size than those obtained from ethyl solvents. In contrast, electrosprayed CL-20/DNB nano-cocrystals had completely formed the co-crystal structure proved by DSC and PXRD. Moreover, Terahertz (THz) result confirmed the formation of intermolecular hydrogen bonds. Additionally, we have fabricated the CL-20/TNB co-crystals for the first time by using electrospray method. The PXRD and DSC results confirmed the formation of this novel energetic co-crystal. Expectedly, all the electrosprayed nano-sized CL-20 based co-crystals exhibited visible reduced impact sensitivity compared with raw CL-20. The electrospray, thus, *Corresponding author: Dr. Zhijian Yang and Prof. Fude Nie, Email: [email protected] (Zhijian Yang), [email protected] (Fude Nie).

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can offer a flexible and versatile approach for continuous and high-yielding synthesis of nano-sized energetic co-crystals with preferable safety performance, and also provide an efficient screening to quickly distinguish whether two energetic materials can form co-crystal. Keywords: Electrospray deposition; energetic co-crystals; nano-sized; CL-20; continuous

1

INTRODUCTION

Energetic materials (EMs), specially involved in monomolecular nitramine materials such

as

2,4,6,8,10,12-hexanitrohexaazaisowurtzitane

hexahydro-1,3,5-trinitro-1,3,5-triazine

(CL-20),

(RDX)

and

1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) etc., which can quickly release a large amount of stored chemical energy when actuated, are widely used in military field as explosives, propellants and pyrotechnics.1-3 As one of the most powerful explosives, CL-20 is a promising substitute for the commercially used RDX and HMX (its energy is 14-20 % higher than HMX). However, its large scale application in explosive field has been limited by the part of the intrinsic high sensitivity.4 Well known, one of the effective approaches to solve this problem is developing energetic co-crystals, which can offer an opportunity to produce new materials with novel properties compared with the original components.5 Generally, potential energetic co-crystals are composed of a high-energy explosive and a less sensitive one (or inert materials) in a specified ratio, which are formed by hydrogen bonds and π-π stacking.6, 7 The physicochemical properties of these co-crystals, not only sensitivity


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but also the other performances such as melting point, detonation velocity and thermal stability, are different from both the pure components and physical mixtures. Moreover, reduction of crystal size is believed to be another feasible method to decrease the sensitivity of EMs since smaller crystals contain less amount of defects and inclusions.8,9 When reduced to nanoscale, these high explosives exhibit significantly lower sensitivity to exterior stimuli in comparison to larger particles. Considering that, combining co-crystallization and nanotechnology to fabricate nanoscale energetic co-crystals would be very attractive to tailor the performance of EMs. Recently, some engineering methods have been involved into fabricating nanoscale energetic co-crystals, mainly including ultrasonic spray-assisted electrostatic adsorption (USEA) method10, spray flash evaporation (SFE) process11 and bead milling method12. Among these methods, the USEA and SFE are both involved in using the high temperature and high speed gas flow, which could bring low product yielding and cause unfavorable polymorphous problems.13,14 As a typical solid state reaction approach, the bead milling, which synthesizes co-crystal depending on the input of mechanical energy and produces on a large scale, is a flexible and versatile method.15 However, this approach presents some limitations that are the formation of irregular crystal shape and impurity. Here, one versatile and efficient technique, the electrospray deposition, has been introduced to synthesize the energetic co-crystals. Similar to spray drying process, the electrospray deposition has also involved the atomization of liquid solution into small droplets, and subsequently solvent in these



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droplets evaporates quickly in the flying process finally forming dry particles. However, droplets generation in electrospraying depends on the electrical force overcoming the surface tension of solution instead of physical force used in spray drying. In addition, for the electrospraying, since droplets are charged, most of them will fly to the collector under the electrical field, resulting in high-yielding product. Currently, the electrospray deposition has been extensively applied to fabricate fine particles, fibers and films.16-18 In this work, we firstly report using the electrospray deposition to synthesize the CL-20

based

nano-cocrystals.

Three

classes

of

nitrobenzene

explosives,

2,4,6-trinitrotoluene (TNT), 1,3-dinitrobenzene (DNB) and 1,3,5-trinitrobenzene (TNB), have been employed as the donors for the preparation of the CL-20 based co-crystals. Among these donors, TNT and DNB have been previously reported to form co-crystals with CL-20 in a molar ratio of 1:1, thus, CL-20/TNT and CL-20/DNB with the reported equimolar ratio have been used to fabricate the co-crystals.19-21 We have assessed the effect of different solvents on the formation of co-crystal detailedly by taking CL-20/TNT as an example. The morphology and properties of as-prepared samples were evaluated by Scanning Electron Microscopic (SEM), Powder X-ray diffraction (PXRD), Fourier Transform infrared spectroscopy (FT-IR) and Differential Scanning Calorimetry (DSC). Considering the similar structure of these three donors, a 1:1 molar ratio of CL-20 and TNB has been utilized to successfully prepare the novel co-crystal CL-20/TNB in nanoscale for the first time by such electrospray deposition.


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2

EXPERIMENT SECTION

2.1 Materials. CL-20 (ɛ-phase) was provided by Liaoning Qing Yang Chemical Industry Co., Ltd of China. TNT and DNB were offered by East Chemical Industry Company. TNB was synthesized and purified in our institute. Acetone, ethyl acetate, 2-butanone and n-butyl acetate were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received. 2.2 Preparation of nanosized co-crystals by electrospray deposition. CL-20 and TNT with molar ratio of 1:1 were dissolved in acetone, ethyl acetate, 2-butanone and n-butyl acetate, respectively. An equimolar ratio of CL-20 and DNB or TNB were dissolved in acetone. The concentration in solution for each test was typically 100 mg mL-1. All solutions were stirred at room temperature until obtaining a transparent solution. After that, the as-prepared solutions were sprayed by an electrospray setup (SS-2535H, Beijing Ucalery Technology Development Co., Ltd). Specifically, 3 mL solutions were loaded in a 5 mL syringe which was equipped with a 27 Gauge stainless steel nozzle. The distance between the nozzle tip and substrate (aluminum foil) was maintained 20 cm to insure the formation of dry particles. The voltage used between the nozzle tip and substrate was 15-18 kV (the nozzle and the substrate was set to 5-8 kV and -10 kV, respectively). The solution flowed out the nozzle at a rate of 0.05 mm min-1. The electrospray process was carried out at 30 ± 2 oC. 2.3 Characterization. The cone-jet operation process was recorded by a high speed camera (FASTCAM Mini UX100, Photron, Japan) with 50 frames per second. The



sample morphology was performed by the field emission scanning electron microscopy (FE-SEM, Sigma HD, ZEISS) operating at 5 kV. Powder X-ray diffraction pattern of samples was recorded from 5o to 50o with step size of 0.02 o and a scanning rate of 1.0 o/min at ambient conditions by an X-ray diffractometry system (D8 Discover, Bruker) using Cu-Kα radiation with a voltage of 40 kV and a current of 40 mA. The chemical characteristics of the samples were evaluated by Fourier transform infrared spectroscopy measurements (Tensor27, Bruker). The 2-3 mg samples mixed with a certain amount of KBr were pressed to a thin disc before tested. Spectra was recorded in the range of 4000-400 cm-1 with 4 cm-1 resolution and 128 transients. The intermolecular hydrogen bond of products was detected by a home-made terahertz (THz) spectrum, which contains a femtosecond fiber laser producing 100 fs laser pulse with a central wavelength of 1550 nm and a photoconductive antenna detector. Before testing, a certain amount of samples comprising the products and polyethylene with a 1:1 mass ratio were grinded and then pressed to form a pellet of 5 mm in diameter and approximate thickness of 1 mm. The THz spectra was recorded from 0.1 to 3.0 THz. Thermal properties of samples were performed on a differential scanning calorimetry (DSC1, Mettler Toledo). Samples (8-10 mg) were heated in a sealed aluminum pan from 30 to 180 oC at a constant heating rate of 10 oC min-1. All tests were performed under a nitrogen atmosphere with a flow rate of 40 mL min-1. The structures were relaxed at the M06-2x/6-311G(d) level of theory,22 using the Gaussian 09 program.23 The unit cells of β-CL-20, CL-20/TNT co-crystal and


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CL-20/DNB co-crystal were built according to the crystal parameters from Cambridge Crystallographic Data Centre (β-CL-20 obtained from ref24 , CL-20/TNT co-crystal obtained from ref19, CL-20/DNB co-crystal obtained from ref21). The calculated PXRD patterns were performed in Materials Studio 6.0 using the reported CIF data. The Rietveld method was employed to analyze the XRD quantitative phase of electrosprayed products.25 This method can reduce the interference caused by extinction and preferred orientation of crystals. With this method, the absolute error of the polymorphs contents determination can be reduced to 5%.26 The impact sensitivity was performed according to BAM (Bundesanstalt für Materialprüfung) method.27 Typically, a small amount of samples were confined between two co-axial cylinders guided by a ring. The impact energy was calculated by the fall height and weight of impacting hammer. The smallest impact energy was determined when the explosion of samples occurred in at least once in six consecutive trials.

3

RESULTS AND DISCUSSION

3.1 Preparation and solvent effects of CL-20/TNT nano-cocrystals. The schematic diagram of electrospray process is illustrated in Figure 1a. During the electrospray process, the co-crystal solution was emitted from the nozzle to form Taylor cone when encountered the appropriate electrical force. Generally, to obtain a stable and controllable spraying process, the electrospray process is operated in cone-jet mode. And importantly, the monodisperse particles can be obtained under this mode. Through adjusting the process parameters, a cone-jet mode was captured and



exhibited in Figure 1b. Clearly, a conical shape appeared on the tip of nozzle, and then the solution jet broke into droplets. These charged droplets induced by electrical field had been subsequently deposited on the substrate. Figure 1c shows the photograph of deposited substrate. It is obviously seen an approximate ellipse within the aluminum foil indicating most of particles were collected. After measuring the products collected from the substrate, the yield was up to 88.3 %, which is significantly higher than the product yield from the spray dryer.13

Figure 1 (a) Schematic diagram of electrospray deposition; (b) Photograph of cone-jet mode captured by high speed camera; (c) Photograph of as-electroprayed substrate. To assess the flexibility of electrospray method and evaluate the influence of solvents on the formation of CL-20/TNT co-crystal particles in the electrospray process, several solvents including acetone, ethyl acetate, 2-butanone and n-butyl acetate were employed and studied. These solvents possess different properties such as surface tension, conductivity and boiling point, exhibiting important effects on the


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formation of stable cone-jet and morphology of prepared particles. For each solvent, we can adjust the applied voltage (from 15 kV to 18 kV) to form the stable cone-jet when utilized the constant distance between the tip of needle and substrate. Figure 2 shows the SEM images of raw materials and CL-20/TNT co-crystal particles fabricated by electrospray from these solvents. Clearly, both of raw ɛ-CL-20 (Figure 2a) and TNT (Figure 2b) showed the high crystallinity and have a size distribution up to hundreds of micrometers. As expected, all prepared products were fine particles, although the used solvents have different boiling points (ranging from 56 oC to 126 oC) which had a significant effect on the evaporation rate of electrosprayed droplets. Among these products, particles fabricated from acetone (Figure 2c) and 2-butanone (Figure 2d) had similar morphology and size distribution ranges around 200 nm. While, those CL-20/TNT co-crystal particles prepared from ethyl acetate (Figure 2e) and n-butyl acetate (Figure 2f) presented a quite different morphology and greater size distribution (1µm to 2µm) in comparison to those from ketone solvents. It is believed that crystal nucleation and growth are mainly influenced by the solvent-solute interaction28, solubility29, rate of supersaturation30 and the volatility of solvent31. Since the time of flight of droplets in electrospray process was very short, the rate of supersaturation would not play a key role on the effect of crystal morphology. In general, high solvent volatility is beneficial for rapid and massive nucleation, resulting fine particles, while low solvent volatility corresponds to large crystal size.32 In addition, high solubility also prefers the small particles. In our case, the used ketone solvents have the high volatility and high solubility for CL-20 and TNT in



comparison to ester solvents, corresponding well to the observed SEM results. However, for results obtained from ethyl acetate and n-butyl acetate, we believe that the solvent-solute has predominant effect on the crystal size since ethyl acetate has the higher volatility and solubility but obtained the larger crystal size comparing to n-butyl acetate. Generally, the solvent-solute interaction involves van der Waals and hydrogen bonding.28 The strength of solvent-solute van der Waals interaction can be mainly evaluated by the dipolar polarizability and the strength of hydrogen bonding can be evaluated by the hydrogen bond donor ability. According to the reported results28, the value of hydrogen bond donor of ethyl acetate and n-butyl acetate are both zero, and the dipolar polarizability of ethyl acetate and n-butyl acetate are 55 and 46 respectively, indicating the van der Waals interaction can be the key solvent-solute interaction in our experiments.


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Figure 2 SEM images of raw materials (a) ɛ-CL-20, (b) TNT, and electrosprayed CL-20/TNT co-crystal particles from: (c) acetone, (d) 2-butanone, (e) ethyl acetate, (f) n-butyl acetate. The PXRD patterns of the electrosprayed CL-20/TNT products from different solvents and raw materials are illustrated in Figure 3 together with the calculated PXRD patterns for CL-20/TNT co-crystal (obtained from reported CIF data19). The similar trend is observed in all PXRD patterns of electrosprayed CL-20/TNT. In these patterns, the presence of TNT can be distinguished particularly from the diffraction peaks at 2θ of 8.9 o, 17.7 o and 25.1 o, corresponding to typical TNT. Additionally, the



typical diffraction peaks of co-crystal at 9.8 o, 12.5 o, 14.3 o and 21.6 o are visible in all products, indicating the formation of co-crystal phase. These suggest electrosprayed CL-20/TNT from these four solvents can just partially from the co-crystal, which is in accordance with the results as reported11. According to the ref 11, rapid crystallization as SFE process can cause a structure distortion involving the interaction between CL-20 and TNT molecules and those between the adjacent CL-20 molecules, resulting in the partial formation of co-crystal. Herein, it is interesting that the ɛ-CL-20 is not detected, instead, β-CL-20 (typical diffraction peaks at 13.7 o, 24.1 o

and 28.2 o) appears in all products. To probe the formation of β-CL-20, we carried

out the electrospray process of single ɛ-CL-20 without TNT using acetone and ethyl acetate as the solvent. The result (Figure S1 in Supporting Information) shows that the products were β-CL-20 and almost no ɛ-CL-20 was observed. In fact, CL-20 initially prefers β-form when crystallizing in most highly supersaturated solution since the β-form has the lowest lattice energy in all polymorphs,33 subsequently, the β-form transforms gradually to ɛ-form. In our experiments, the electrospray process was transient and the crystallization was depended on the rapid evaporation of solvent, directly obtaining the β-CL-20 and had no more time to transform to ɛ-CL-20. Quantitative analysis of the crystal phase of CL-20 in products performed by PXRD Rietveld method (Table S1) shows that the co-crystal content in products is not very high, approximately ranging from 10 wt% to 29 wt%.


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Figure 3 PXRD patterns of raw-CL-20 (a), β-CL-20 (b), raw TNT (c), calculated CL-20/TNT co-crystal (d), electrosprayed CL-20/TNT co-crystal from acetone (e), from 2-butanone (f), from ethyl acetate (g) and from n-butyl acetate (h). The calculated CL-20/TNT co-crystal PXRD pattern was obtained from the CIF.19 CL-20/TNT co-crystal structure exhibits a large number of CH hydrogen bonds involving nitro group oxygen atoms with CH protons (one of these is shown in Scheme 1). These hydrogen bond interactions alternately connect CL-20 and TNT molecules to form the zigzag chains, which propagate through the crystal to complete the cocrystal.19,20 The structure of raw materials and electrosprayed CL-20/TNT products was investigated by FT-IR spectra (Figure S2). Clearly, the C−H stretching



vibration, NO2 symmetric stretching vibration and NO2 deformation stretching vibration of CL-20 are shifted from 3046 cm-1, 1330-1630 cm-1 and 943 cm-1 to 3040 cm-1, 1326-1590 cm-1 and 941 cm-1, respectively, while the C−H stretching vibration and NO2 symmetric stretching of TNT are shifted from 3100 cm-1, 1530 cm-1 to 3110 cm-1, 1523 cm-1, respectively. Similarly, the partial formation of CL-20/TNT co-crystal is also confirmed by the DSC results (Figure S3). All DSC curves present two endothermic peaks, a weaker one at 80.6 oC corresponding to pure TNT melting, and a stronger one at 139.1 oC corresponding to the melting of CL-20/TNT co-crystal. This thermal property is in consistence with the reported result which fabricates the CL-20/TNT co-crystal using the SFE method, verifying the partial formation of co-crystals.11

Scheme 1 Structures of CL-20, TNT, DNB and TNB, and hydrogen bonding present in CL-20/TNT and CL-20/DNB co-crystals, which were obtained from the reported CIF data.19,21


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3.2 Preparation and characterizations of CL-20/DNB nano-cocrystals. Besides the CL-20/TNT co-crystal, we have also fabricated the CL-20/DNB co-crystal products by the electrospray deposition. Figure 4b exhibits the SEM image of as-prepared CL-20/DNB co-crystals. It is clear that the size of these products ranges from 100 to 500 nm, while the size of raw DNB (Figure 4a) and ɛ-CL-20 (Figure 2a) is around 100 µm. To discriminate the crystal phase of electrosprayed products, the collected samples were analyzed by PXRD. As shown in Figure 5, the PXRD pattern of electrosprayed CL-20/DNB products presents significant difference with both the two raw materials and β-CL-20, and extremely coincides with the calculated pattern of CL-20/DNB co-crystal (peaks at 5.3o, 10.5o, 11.7o, 13.9o, 17o, 25.5o, 26.6o, 28.6o), indicating the formation of pure co-crystal phase.21

Figure 4 SEM images of DNB (a), and electrosprayed CL-20/DNB products from acetone (b) According to the previously reported CL-20/DNB co-crystal data,21 Scheme 1 exhibits one illustrative intermolecular hydrogen bond between CL-20 and DNB, which involves into the nitro group from DNB with adjacent hydrogen atom from CL-20. To probe the intermolecular reactions in the electrosprayed CL-20/DNB, THz



spectroscopy was employed to analyze the spectra of CL-20, DNB and as-prepared CL-20/DNB products, respectively.34 The resulted spectra presents that the as-prepared CL-20/DNB products exhibit some distinctive peaks at 1.22 and 1.77 THz (Figure 5b), where both CL-20 and DNB have negligible peaks, verifying the formation of intermolecular hydrogen bonds between CL-20 and DNB.

Figure 5 PXRD patterns of raw CL-20, β-CL-20, raw DNB, calculated CL-20/DNB co-crystal, electrosprayed CL-20/DNB co-crystal from acetone(a), Terahertz spectra of electrosprayed CL-20/DNB co-crystal and raw materials (b) and DSC curves of electrosprayed CL-20/DNB co-crystal and raw materials (c). The calculated


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CL-20/DNB co-crystal PXRD patterns from the CIF.21 Additionally, the DSC curve of as-prepared CL-20/DNB product is also distinct from the raw materials (Figure 5c). More specifically, only one endothermic peak of electrosprayed CL-20/DNB is observed at 129.5 oC, significantly higher than the melting temperature of DNB (93 oC) and lower than the phase transition temperature of ɛ-CL-20 (171.7 oC).35 This further proves the formation of CL-20/DNB co-crystal in pure phase by such electrospraying method. Additionally, we employed the FT-IR to investigate the structure of electrosprayed CL-20/DNB cocrystal and raw materials (Figure S4). In the spectra, the C−H stretching vibration, NO2 symmetric stretching vibration and NO2 deformation stretching vibration of CL-20 are shifted from 3046 cm-1, 1330-1630 cm-1 and 943 cm-1 to 3034 cm-1, 1324-1624 cm-1 and 940 cm-1, respectively, while the C−H stretching vibration and NO2 symmetric stretching of DNB are shifted to 3103 cm-1 from to 3110 cm-1 and to 1610 cm-1 from 1624 cm-1, further implying the successful fabrication of CL-20/DNB co-crystals. 3.3 A novel nano-sized co-crystal of CL-20/TNB. As a typical nitrobenzene explosive, TNB possesses considerable power (higher than TNT and DNB) and is known as an insensitive energetic material.36 This compound has similar molecular structure compared to TNT and DNB, which can both form co-crystals with CL-20. Accordingly, we deduce that TNB could form co-crystal with CL-20 by similar synthesis route. Combining the molar ratio and intermolecular interactions of CL-20/TNT and CL-20/DNB co-crystals, an equal molar ratio of CL-20 and TNB was selected and the possible intermolecular hydrogen bonds of CL-20 and TNB have



been postulated (Scheme 2). The results show CH hydrogen bonds of CL-20 and TNB could be established between the nitro group of TNB and adjacent CH groups of CL-20 and/or their nitro groups and the adjacent hydrogen atoms.

Scheme 2 Two possible intermolecular hydrogen bonds between CL-20 and TNB. Similarly, we utilized electrospray method to prepare the CL-20/TNB products. SEM analysis shows the size of these products ranges from 200 to 600 nm (Figure 6b), while the raw TNB shows a flake shape with tens of micrometers (Figure 6a). The formation of co-crystal structure of electrosprayed CL-20/TNB products could be confirmed by the inspection of PXRD patterns. From Figure 6c, the PXRD patterns of electrosprayed products show a significant difference to raw materials, including the presence of new intensive peaks at 8.5o, 10.7o, 14.5o and 22o, with the absence of most intensive peaks at 23o for TNB and 12.7o for ɛ-CL-20, validating the formation of new crystalline phase. To our best knowledge, the CL-20/TNB co-crystal has never been reported. It is regretted that the single crystal structure of this novel energetic co-crystal cannot be obtained since current characterizations are unavailable to analyze such fine product in nanoscale. Nevertheless, PXRD combined with DSC and FT-IR results can confirm the formation of CL-20/TNB co-crystal. Notably, thermal analysis of electrosprayed CL-20/TNB shows an endothermic peak at 138.5 oC


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(Figure 6d), losing a melting endotherm of TNB (at 124.5 oC) and a phase transition endotherm of ɛ-CL-20 (at 171.7 oC).35 This complements well the PXRD data, suggesting the formation of co-crystal between CL-20 and TNB. Additionally, the FT-IR spectra (Figure S5) of electrosprayed CL-20/TNB has also been investigated. In comparison to raw materials, the C−H stretching, NO2 symmetric stretching and NO2 deformation stretching of CL-20 in electrosprayed CL-20/TNB products are shifted to 3040 cm-1, 1346–1604 cm-1 and 953 cm-1, respectively. And for TNB, the C−H stretching and NO2 symmetric stretching are shifted from 3106 to 3116 cm-1 and from 1620 to 1604 cm-1, respectively. These could prove the formation of intermolecular interaction between CL-20 and TNB.

Figure 6 SEM images of TNB (a) and electrosprayed CL-20/TNB products from acetone (b), PXRD pattern (c) and DSC curves of electrosprayed CL-20/TNB co-crystal and raw materials (d).



3.4 Impact sensitivity of electrosprayed nano-sized co-crystals. To explore the potential application of electrosprayed nano-sized explosives, safety performance should be one of most concerned properties. Generally, smaller crystal size, smoother morphology, higher purity, less crystalline defects and reduction of concomitant polymorphic phases of crystalline energetic materials present the decrease of sensitivity.8,37,38 In addition, cocrystallization is also known as an efficient strategy to reduce the sensitivity of high explosives.19-21 Herein, for safety testing, the impact sensitivity was evaluated according to the BAM method. By this method, the raw ɛ-CL-20 exhibits impact energy of 4 J, and the three donors have impact energy of 15 J (TNT), 39 J (DNB) and 14 J (TNB), respectively (Figure 7). Expectedly, all these three electrosprayed co-crystals exhibit more impact energy (7.5 J, 9 J and 8 J, respectively, corresponding to CL-20/TNT, CL-20/DNB and CL-20/TNB co-crystals), namely reduced impact sensitivity in comparison to CL-20 and physical mixtures of nano-sized components and raw materials (listed in Table S2). In all electrosprayed products, CL-20/TNT co-crystals present lowest impact energy that could be attributed in the presence of polymorphic phases, since the presence of β-CL-20 can definitely increase the sensitivity. Nevertheless, the impact energy of CL-20/TNT products is significantly higher than the physical mixture of nanoscale CL-20 and TNT, despite of the partial formation of CL-20/TNT co-crystals. Furthermore, the other possibility for the reduced sensitivity could be the homogeneous microstructure of electrosprayed CL-20/TNT. According to the hot spot theory, hot spots are formed at internal defects and intergranular voids.39 When encountered the mechanical stimuli,


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homogeneous microstructure possessing the lower intergranular voids can lead to a more uniform distribution of incident energy, thus lowering the sensitivity. Comparing to the physical mixing of nanoscale CL-20 and TNT, the electrosprayed CL-20/TNT were formed by quick crystallization in each single droplet and various crystal phases can mix uniformly, resulting in the more homogeneous microstructure. It is also worth noting that the impact energy of physical mixtures of nanoscale raw materials (≥ 5J) is higher than the physical mixture of raw materials with microscale (< 5J), which is consistent with the smaller size energetic crystal with higher impact energy.

Figure 7 Impact sensitivity of electrosprayed co-crystals and raw materials by BAM method.

4

CONCLUSIONS Summarily, we have demonstrated that electrospray can be used as a versatile and

flexible strategy for the high-yielding and continuous deposition of the CL-20 based nano-sized energetic co-crystals. For the preparation of CL-20/TNT co-crystal,



solvent played an important role on the morphology and size of co-crystal particles and had negligible influence on the crystal structure. All the electrosprayed CL-20/TNT samples showed partial formation of co-crystal. For CL-20/DNB, 100– 500 nm co-crystal particles were prepared. The intermolecular hydrogen bonds between CL-20 and DNB were detected by THz spectra, and the PXRD and DSC tests showed that the as-prepared CL-20/DNB had completely formed the co-crystal in pure phase. Furthermore, we have successfully prepared a novel CL-20/TNB co-crystal for the first time with the size ranging from 200 to 600 nm using the electrospray method. The PXRD and DSC results confirmed the formation of co-crystal at the molar ratio of 1:1. Additionally, the electrosprayed nano-sized CL-20 based energetic co-crystals exhibited improved insensitivity performance compared with raw CL-20, raising promising performance for their application.

Supporting information description PXRD patterns of electrosprayed CL-20 using acetone and ethyl acetate as the solvent. Variety of crystal content in products obtained by Rietveld method. The DSC curves of electrosprayed CL-20/TNT. The FT-IR of electrosprayed CL-20/TNT, CL-20/DNB and CL-20/TNB. The impact sensitivity of various physical mixtures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Z.J. Yang); [email protected] (F.D. Nie)


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Notes: The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Doctoral Fund of Ministry of Education of China (2016M592711) and National Natural Science Foundation of China (11502243). The authors acknowledge the support of the Analytical and Testing Center of Southwest University of Science and Technology. The authors also greatly appreciate the contribution from Dr. Zhaohui Zhai in the THz test.

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For Table of Contents use only

High-Yielding and Continuous Fabrication of Nano-Sized CL-20 Based Energetic Co-Crystals via Electrospraying Deposition Chuan Huang, Jinjiang Xu, Xin Tian, Jiahui Liu, Liping Pan, Zhijian Yang, Fude Nie Table of Contents graphic and synopsis:

Energetic co-crystals, especially CL-20 based co-crystals, have attracted a wide range of attention due to their low sensitivity and impressive detonation performance. In this study, a series of nano-sized CL-20 based energetic co-crystals exhibiting preferable safety performance have been fabricated by electrospray deposition. The electrospray can provide an efficient screening to quickly distinguish whether two energetic materials can form co-crystal.


High-Yielding and Continuous Fabrication of Nanosized CL-20-Based

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