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Perfect Energetic Crystals with Improved Performances Obtained by Thermally-metastable Interfacial Self-assembly of Corresponding Nanocrystals Zhijian Yang, Feiyan Gong, Guansong He, Yubin Li, Ling Ding, Fude Nie, and Fenglei Huang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01604 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Crystal Growth & Design

Perfect Energetic Crystals with Improved Performances Obtained by Thermally-metastable Interfacial Self-assembly of Corresponding Nanocrystals Zhijian Yang, †,‡ Feiyan Gong, † Guansong He, † Yubin Li, † Ling Ding, † Fude Nie, *,† and Fenglei Huang *,‡ †

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

621900, China; ‡

Department of Engineering Mechanics, State Key Laboratory of Explosion Science

and Technology, Beijing Institute of Technology, Beijing 100081, China

ABSTRACT: The interfacial self-assembly of energetic nanocrystals was proposed and systematically studied in this work. Effects of the reaction temperature, grain size of nanocrystals, solvent system and addition of surfactant on the self-assembled crystals were investigated. The morphologies and crystal structures of the self-assembled products were investigated by microscopy analysis and coherence strength tests. Furthermore, the as-prepared energetic crystals by thermally-metastable self-assembly method were systematically compared with the starting raw crystalline materials and the corresponding crystals prepared by recrystallization, in terms of polymorphic transition behaviors, impact sensitivity and thermal properties. It has been shown that the energetic crystals synthesized by this novel self-assembly method was uniform with smooth surface and free of defects. These crystals also had very narrow size

*Corresponding author: Prof. Fude Nie and Prof. Fenglei Huang, Email: [email protected] (Fude Nie), [email protected] (Fenglei Huang);

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

distribution, ordered crystallographic texture and high compactness, with significant improvement in impact sensitivity. It is suggested that the polymorphic transition of energetic crystals can be favorable but not essential to reach the thermally-metastable state before nanocrystal assembly, resulting in higher thermal stability. The kinetics of the self-assembly process was found to follow the Avrami equation. The possible mechanism of this self-assembly process was also proposed, sequentially including solvent induction originated from surface solvation or localized dissolution, particle aggregation and interfacial crystal growth.

Keywords: nanocrystals; energetic materials; self-assembly; thermally-metastable; crystal growth mechanism.

1

INTRODUCTION Nanocrystals have attracted extensive attentions for their functional and distinctive

roles in fundamental science fields and technical applications in past decades [1, 2]. Thereinto, nano-sized energetic materials (nEMs), vest in organic small molecule crystals, are well-known for their high energy releasing rate, excellent combustion efficiency, tailored burning rate, improved initiation threshold and reduced sensitivity [3-5]. They have significantly different properties from their micron-sized counterparts [6]. Unfortunately, the proportion of highly reactive atoms on the surface definitely increased with the decreasing of particle size. Given appropriate external stimuli (e.g. high temperature, certain solvent medium, electronic beams, etc.) as an induction, the nanocrystal structure would gradually lose its stability, and reach a metastable state [7,


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8], causing undesired agglomeration. For a long period of time, the disordered particle agglomeration is believed to be the bottleneck problem for the application of nEMs, which should be solved, maintaining their unique properties for initiating explosives, boosters and propellants. However, the undesired metastable state may be beneficial to activate the atoms on the surface, providing noticeable possibilities in the field of nanocrystal self-assembly. To date, the reaction mechanisms and boundary conditions for such disordered agglomeration are still unclear, and few works have been published on the ordered interfacial assembly of energetic nanocrystals. For decades, self-assembly of nanocrystals has attracted considerable attention, especially for inorganic and bio-macromolecular materials [9, 10]. The basic structural unit, such as molecules, microcrystals and nano-grains were used to construct novel functional materials and structures [11]. Crystals with integrated structures could be obtained by self-assembly, which include nucleation and crystal growth. In microscopic perspective, nano-grains can also aggregate and form large crystals by consuming the raw nanoparticles without any nucleation process, generally known as Ostwald ripening [12, 13]. Apparently, current researches mainly focus on the precise control of the nucleation and crystal growth process of EMs concerning the molecular scale behaviors [14-17], whereas the assembly process based on particle agglomeration was rarely reported. Such assembly on the nano-particle scale can turn the disadvantages of disordered agglomeration into the advantages as profitable motivation for fabricating large and perfect energetic crystals. Excellent EMs should be of high detonation performances with reduced sensitivity.



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Typical approaches to reduce the sensitivity of high explosives include decreasing the particle size [4, 5], removing the impurities and defects by recrystallization [18], preparing energetic co-crystals [19], developing efficient crystal packing [20], and using inert polymer for mitigation [21, 22]. Among these methods, techniques based on crystallization in solution were well known to prepare high-quality explosive crystals with uniform size, high density, few impurities or defects, resulting in significantly reduced shock sensitivity [23]. However, current techniques for the preparation of high-quality energetic crystals are mainly based on the solvent/non-solvent recrystallization method, focusing on the precise control of crystal morphology and growth process [24, 25], while the particle size and the preparation efficiency are still need to be improved. Generally, the architectural control of crystals with well-defined shape and size is the most challenging goal for crystal engineering [26]. Fortunately, our recent experiments showed a possible way to achieve that goal, and we found that the solvent induced thermally-metastable assembly of energetic nanoparticles could provide a novel route to fabricate high-quality explosive crystals. Traditionally,

2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazawurtzitane

(CL-20,

HNIW) and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) are two of the most widely investigated nitramine explosives, attributing to their outstanding energy output [27, 28].

In

comparison,

1,3,5-triamino-2,4,6-trinitrobenzene

(TATB)

and

2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105), with plane molecular structure, are typical insensitive EMs for their exceptional safety performances [21, 29-30]. In this work, these four EMs were selected to systematically demonstrate crystal assembly


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behaviors based on the thermally-metastable state of atoms on the surface induced by thermal and solvent treatments. Distinct from the normal assembly under a molecular scale, this reaction was accomplished under particle level without complete crystal dissolution or nucleation. Instead, large energetic crystals could be formed by the ordered agglomeration of nano-particles, since the superficial molecules in thermally-metastable state could put up the driving force for their interfacial fusion and crystal growth. A possible assembly mechanism was proposed based on the capturing and characterizations of the intermediate products. Combined with the benefits of understanding the structural evolution process and boundary conditions for the application of nano-sized explosives, the present work would also provide an alternate facile method for the fabrication of perfect micron-sized energetic crystals with improved performance and reduced sensitivity.

2

EXPERIMENT

2.1 Materials CL-20 was provided by Liaoning Qingyang Chemical Industry Co., Ltd. of China. Fine CL-20 in submicron size was obtained by mechanical grinding, and details were shown in the Supporting Information (SI). HMX nanocrystals were provided by Nanjing University of Science and Technology, China. TATB and LLM-105 were synthesized by our research group, and refined to nanoscale by spray-crystallization according to literatures reported previously [31, 32]. The other reagents in analytical grade were commercially available from Sigma-Aldrich and used without further purification.



2.2 Self-assembly of energetic nanocrystals The assembly experiments of CL-20, HMX, TATB and LLM-105 nanocrystals were conducted in stainless steel batch reactors. Typically, fine explosive powders were dispersed in a certain solvent (H2O, ethanol, ethylene glycol, etc.) with the assistance of ultrasonic treatment at frequency of 40 kHz for 5 minutes, and the mass ratio of explosive to solvent was about 1:50. After that, the suspension was transferred to a reactor and heated in an oven with precisely controlled temperature from 70 oC to 140 o

C. In some cases, mixed solvents or solvents containing surfactant (shown in SI) were

adopted to provide different reaction conditions. Then, the thermally-metastable assembled crystals were collected by filtration, washed with deionized water and dried under vacuum at 60 oC for 48h. The intermediate state products were achieved by taking samples at different reaction time and the reactors was cooled in a cold water bath immediately. 2.3 Characterizations The morphologies and structures of samples were performed by scanning electron microscopy (SEM), atomic force microscopy (AFM), optical polarizing microscope (OPM), specific surface area analysis, X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), Raman spectra and Differential scanning calorimeter (DSC). The coherence strength of energetic crystalline granules was evaluated by confined quasi-static compression method [33]. The Rietveld refinement method was used to calculate the phase content of the CL-20 crystals through the Topas Academy program according to literature [34]. The impact sensitivity measured as impact energy (EBAM)


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was tested. Detailed characterization methods and processes are shown in the SI document.

3

RESULTS AND DISCUSSION

3.1 Assembly behaviors and morphology evolutions Four of the most representative EMs, CL-20, HMX, TATB and LLM-105 were selected to investigate the solvent induced thermally-metastable assembly behaviors. Their molecular structures are shown in Fig. S1 in the SI, the multiple nitro groups bonded to nitrogen atoms of CL-20 and HMX endow their extremely high energy, while the nitro groups bonded to carbon atoms and proper amino groups in one molecular plane give TATB and LLM-105 outstanding safety performance, respectively. Fig. 1 shows the appearance of fine CL-20 particles by grinding and the assembled crystals obtained in pure H2O at 120 oC for 4h. The fine CL-20 particles prepared by planetary ball milling exhibit a relative uniform morphology, showing poorly defined crystal edges which were polished by grinding, as shown in Fig. 1a. CL-20 particles were remarkably refined, with the grain diameter ranging from 200 nm to 1 µm. It was found that large crystals were obtained after hydrothermal treatment in an autoclave, where a typical thermally-metastable self-assembly of CL-20 nanocrystals occurred, resulting in uniform large crystals with narrow size distribution, distinctly smooth crystal surfaces (Fig. 1b). The regular crystal products showed a sapphire-like octahedral appearance with the particle size of about 40 µm. It is clear that almost all CL-20 experienced the particle assembly; few small particles were left as shown the



initial independent state. Obviously, this self-assembly to form large crystals was accomplished in a particle level, instead of a molecular level. More specifically, there is no dissolution or nucleation step before the crystal growth. Additionally, the OPM test (Fig. 1c) showed that there were few impurities or cracks in the assembled CL-20 crystals, which were highly transparent, indicating that high-quality energetic crystals with few defects were obtained. To clearly monitor the surface morphology and roughness of the assembled crystals, AFM analysis was further performed. The assembled CL-20 exhibited a definitely smooth and flat surface in a selected 2×2 µm region, few wrinkles or voids were observed, which was in consistence with the surface morphology characterized by SEM and OPM. However, such self-assembly will not happen in dry solid phase without the induction of solvent even heated to high temperature. The experimental results also revealed that the amount of solvent had minor effects on the final crystal products, probably because a small amount of solvent was sufficient to infiltrate the surface and provide the inducing effect. Furthermore, it should be noted that not only solvent inducing but also heating were essential for this self-assembly process. Taking nano-sized CL-20 in H2O as an example, temperature higher than 85 oC and continuous heating for hours were necessary to complete the particle assembly.


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Fig. 1 (a) SEM image of grinded fine CL-20, and appearance of assembled CL-20 characterized by three microscopies: (b) SEM, (c) OPM and (d) AFM. The effects of different parameters, including reaction temperature, grain size of nanocrystals, different solvents and additional surfactants adopted on the thermally-metastable assembly behaviors were studied by taking CL-20 as an example. It was found that higher temperature could accelerate the assembly but had slight influence on the morphology or grain size of the obtained assembled crystals. Nevertheless, excessively high temperature (>140 oC) would cause increasing defects on surface. In addition, the assembled products were also influenced by the grain size of raw nanocrystals, solvent medium and addition of proper surfactant. By carefully controlling the reaction parameters, different CL-20 crystals with tunable size and well-defined morphology could be obtained, as shown in Fig. S2 in SI. For instance, cuboid, short rod-like, polyhedral and large block crystals with the size ranging from 4 µm to 500 µm were successfully prepared. It was praiseworthy that all the assembled



crystals were well-proportioned with regular appearance. Similarly, the assembly behaviors of the other three nEMs HMX, TATB and LLM-105 were also investigated, with the initial size of about 200 nm, 60 nm and 150 nm for these three nanocrystals (Fig. 2a, 2d, 2g), respectively. After hydrothermal treatment in pure water at 120 oC for 4h, ordered assembly was achieved (Fig. 2b, 2e, 2h). Specifically, both HMX and LLM-105 formed polyhedral crystal products with particle diameter around 5 µm, whereas the assembled TATB was presented as a small hexagonal plate. Obviously, the native molecular structures and their interactions with solvents had critical effects on the morphology as obtained. Nevertheless, the assembled crystals exhibited smooth crystal surface and highly homogeneous particle sizes. By prolonging the processing time excessively to 240 h at 120 oC, larger crystals in regular morphology would be produced (Fig. 2c, 2f, 2i). The mean grain size of HMX, TATB and LLM-105 was increased to 25 µm, 40 µm and 20 µm, respectively. For TATB, such metastable self-assembly based on solvent induction can bring particular significance to prepare large crystals. Owing to its poor solubility in common solvents, large TATB crystals were difficult to be prepared by traditional recrystallization. Apparently, the assembly technique reported herein can provide an alternative route to stride over this obstacle, since the dissolution of TATB is not essential for such assembly process at a particle level.


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Fig. 2 SEM images of (a) nano-sized HMX, (b, c) assembled HMX crystals; (d) nano-sized TATB, (e, f) assembled TATB crystals; (g) nano-sized LLM-105 and (h, i) assembled LLM-105 crystals. To further study the properties of energetic crystals after assembly, the size distribution of different samples was measured, and the coherence strength of energetic crystalline granules was evaluated by taking CL-20 as an example. As shown in Fig. 3a, typical CL-20, HMX, TATB and LLM-105 after assembly showed mean grain size of 39.6 µm, 9.2 µm, 31.2 µm and 14.7 µm, respectively, corresponding to the SEM results as demonstrated in Fig. 1 and Fig. 2. Note that all the assembled crystals have narrow size-distribution, especially when it is compared with the CL-20 crystals prepared by conventional recrystallization method (Fig. S3a in SI). Although the assembled crystals exhibited integrated morphologies and smooth surfaces, the crystal strength regarding internal structure should be evaluated since such crystals were prepared from nanocrystals. Therefore, the coherence strength of CL-20



was determined by a confined quasi-static compression method, and the results of uniaxial stress versus compressive rate are shown in Fig. 3b. The curves marked as assembled CL-20-1# and CL-20-2# refers to CL-20 crystals of 4 µm and 40 µm, corresponding to the samples shown in Fig. S2a in SI and Fig. 1b, respectively. Clearly, assembled and recrystallized CL-20 showed similar compression behaviors, particularly compared with the grinded fine particles. The scheme of this compressive stiffness test setup and the characteristic stages of the compaction curve for typical assembled CL-20 crystals were provided in Fig. S4 in SI [33], and the coherence strength expressed by initial secant modulus (ISM) was calculated in Table S1 in SI. The ISM of the grinded fine CL-20 was 39.3 MPa, while the assembled one around 40 µm showed an ISM value of 78.6 MPa, close to that of recrystallized sample of 50 µm (84.4 MPa), indicating a compact internal structure for the assembled crystals. It should be mentioned that the particle size was influential for the coherence strength. For example, assembled CL-20 of 4 µm showed an ISM value as 56.7 MPa, which was lower than that of large crystals. Moreover, the specific surface was also determined by Brunauer, Emmett and Teller (BET) method and the results are shown in Table S1 in SI. For grinded fine CL-20, the value of specific surface area reached 3.814 m2/g, which was visibly decreased to 0.596 m2/g and 0.377 m2/g for the assembled crystals in small and large size. This value was approximately equivalent compared with that of the recrystallized sample, further indicating compact internal crystal structure. In general, based on the morphology characterizations and assembly behaviors, it was obvious that energetic crystals in high-quality and tunable size can be fabricated


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via the solvent induced thermally-metastable assembly. They exhibited well-defined morphology and size, smooth surface, high strength and few defects, even compared with those crystals after careful solvent/non-solvent recrystallization (as shown in Fig. S3 in SI), which were fairly favorable to provide improved safety and detonation performances for energetic materials [23].

Fig. 3 (a) Particle size distribution of different assembled crystals and (b) curves of uniaxial stress versus compressive rate for CL-20. 3.2 Assembly kinetics and proposed mechanism Further investigations were provided for deeper insight into the nature of the assembly of energetic nanocrystals by studying CL-20 as a typical example. The intermediate crystal could be observed by real-time sampling, as shown in Fig. 4. It was found that the refined CL-20 firstly witnessed a particle packing and agglomeration process under the inducement of solvent (Fig. 4a~c), followed by the surface integration and crystal growth (Fig. 4d~f). Fortunately, it was successful to find the intermediate crystal state from which the inner structure of a typical assembled particle could be detected distinctly, as shown in Fig. 4d, revealing an interfacial assembly process. Finally, integrated crystals were obtained after 4 h.



Fig. 4 SEM images of transition-state crystals for the assembly process at 100 oC: (a) 5min, (b) 20min, (c) 1h, (d) 1.5h, (e) 2h, and (f) 4h. As mentioned in the assembly behavior study, the temperature should reach a certain value to promote the occurrence of such thermally-metastable assembly. For CL-20, the nanocrystals were proved difficult to assemble under a temperature below 85 oC due to the energy barrier for creating a thermally-metastable state. Nevertheless, this energy barrier can be reduced by adding more active solvent. Fox example, the assembly of CL-20 could be achieved under 75 oC in a 10 wt% ethanol/H2O solution (Fig. S5a in SI), probably attributing to the surface activation originated by the inducing of ethanol molecules. In addition, similar result was found for TATB. As previously described, the assembly of TATB was more difficult to occur due to its weak interaction with solvents, thermal treatment in H2O at 120 oC for 4h can only fabricate small hexagonal plate less than 3 µm, and negligible assembly would be obtained below 105 oC. However, in a dimethyl sulphoxide (DMSO) aqueous solution at mass concentration of 10 wt%, the assembly could be conducted under 100 oC, and larger crystals were obtained, as shown


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in Fig. S5b in SI. The induction of DMSO on TATB surface was beneficial to reach the metastable state, facilitating assembly to obtain large crystals. With regard to the four EMs studied in this work, CL-20 and HMX are well known as polymorphic compounds, the common crystalline forms of CL-20 are α, β, γ and ε, whereas there are α, β, γ and δ forms for HMX [16]. The most stable phase with highest density and energy for CL-20 and HMX is ε and β phase, respectively. For TATB and LLM-105, only one crystal phase was reported. Herein, XRD analysis was introduced to determine the polymorph of CL-20 during the assembly process. For ε-CL-20, the space group is P21/a and the lattice is monoclinic. Fig. 5 shows the XRD patterns of assembled CL-20 at different times in the 100 oC hydrothermal system. The characteristic diffraction peaks (1 1 -1), (2 0 0), (0 2 2) and (2 0 -3) of the virgin CL-20 nanocrystals appeared at 2θ of 12.6o, 13.8o, 25.8o and 30.3o, respectively, corresponding to the ε form. It is clear that the crystal phase of CL-20 was gradually transferred from ε to α (Pbca lattice), which is consistent with the particle assembly process as determined by SEM (Fig. 4). The characteristic diffraction peaks (1 1 1), (1 1 2), (2 1 4) and (1 2 6) of the α-CL-20 as obtained appear at 2θ of 12.1o, 13.8o, 24.9o and 28.0o, respectively, as marked in Fig. 5. Therefore, it can be concluded that once the CL-20 nanocrystals encountered the thermally-metastable assembly using H2O or aqueous solution as the solvent system, the polymorphic transition from ε to α would happen at the same time. Assembled CL-20 in γ form could also be obtained in other organic solvent (e.g. γ-CL-20 obtained in ethylene glycol, Fig. S2d in SI) because γ form is the most stable phase at high temperature in different solvents. However, once



H2O exists, α phase will become more stable, leading to α-CL-20 crystal products [35]. The polymorphic transition of CL-20 in water or other organic solvents can be profitable to reduce the energy barrier to gain the metastable state easier for assembly.

Fig. 5 XRD patterns of CL-20 during the assembly process. The crystal structures of assembled HMX, TATB and LLM-105 were also tested by XRD, compared with the spectra of the nano-sized and recrystallized ones, as shown in Fig. S6 in SI. Obviously, there is no polymorphic transition for TATB, LLM-105 and especially for HMX. All the nano-sized, recrystallized and assembled HMX showed representative β phase, with characteristic diffraction peaks of (-1 0 2), (-1 2 0) and (-1 3 2) at 20.5o, 23.0o, 31.9o, respectively. TATB displayed a primary diffraction peak of (0 0 2) at 28.4o, and LLM-105 crystals demonstrated characteristic peaks of (1 3 1), (-1 -4 1) and (-1 -5 1) at 2θ of 26.8o, 28.5o, 33.2o before and after assembly. These examples indicate that the polymorphic transition is favorable but not essential to reach the thermally-metastable state before nanocrystal assembly. Furthermore, from the XRD


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patterns, the diffraction peaks for raw energetic nanocrystals were broadened owing to their nano structures. The diffraction peaks with strong intensity were observed for recrystallized ones, implying high degree of crystallinity, corresponding to the characterization results in Fig. 1. FT-IR and Raman spectra were also adopted to determine the crystal form of CL-20 and HMX, and the results are shown in Fig. S7 in SI. As can be seen in the FT-IR curves, the nano-sized and recrystallized CL-20 showed characteristic quartet absorption peaks around 750 cm-1 and 1600 cm-1[16, 27], indicating a typical ε phase. The characteristic FT-IR peaks of ε-CL20 were identifiable from those of CL-20 in α phase, as observed for the assembled sample, suggesting a polymorphic transition during the assembly process. For HMX, similar absorption peaks representing β phase were observed for the nano-sized, recrystallized and assembled samples. In Raman results, CL-20 in ε and α phase for nano-sized and assembled crystals showed different characteristic absorption peaks at the regions from 3070 cm-1 to 3000 cm-1 and 850 cm-1 to 800 cm-1, while the constant HMX absorption peaks suggested pure β phase maintained during the assembly process. To sum up, the FT-IR and Raman results were well consistent with XRD analysis, further validating the polymorphic transition of CL-20 excluding HMX. Since the polymorphic transition was well correlated with the assembly process observed by SEM analysis, combined with the advantage of quantitative determination of different crystal form by XRD Rietveld refinement method, the assembly kinetics of CL-20 could be obtained by calculating the degree of polymorphic transition versus reaction time at different temperatures, as shown in Fig. 6. It was found that such



thermally-metastable experienced a three-step assembly in the kinetic curves, including an induction, rapid growth and stable periods. It is visible that the assembly rate increased with the increasing reaction temperature. The kinetic data could be well fitted and expressed by an Avrami model [36, 37], as shown in Equation (1).

α (t ) = 1 − e− kt

n

(1),

where the parameter k represents the rate constant of polymorphic transition or assembly, and the value of n stands for the assembly probability and dimensionality. Fitted parameters for this kinetic equation were summarized in Table S2 in SI. The value of parameter k gradually increased from 1.234×10-12 to 3.478×10-10 with the temperature increased from 100 oC to 140 oC, indicating an increasing assembly rate in kinetics. By contrast, the parameter n showed a stable value around 6, indicating a constant assembly mode at various temperatures.

Fig. 6 Assembly kinetics of CL-20 at different temperatures in H2O. It is common that the self-assembly mechanism for both organic and inorganic nanoparticles usually contains a two-step procedure: nucleation and crystal growth. For


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instance, Huang [14] raised a possible mechanism for the surfactant-assisted self-assembly of dendritic micro/nano 2,6-diamino-3,5-dinitropyrazine (ANPZ), indicating that the formation of dendritic ANPZ experienced a process of nucleation, initial growth, aggregation and further growth. Herein, the thermally-metastable assembly property as mentioned in this work belongs to the behaviors in a particle scale, thus the most obvious difference is that there would be no nucleation process during the assembly of energetic nanocrystals. On the basis of the facts found in this study, a comprehensive mechanism was proposed, as described in Fig. 7. For a typical assembly process, the crystal surface experiences an interaction (e.g. slight dissolution or surface solvation) with the solvent, then it is induced and activated to reach a metastable state with the assistance of heating. The molecules on the surface layer become more reactive, which is so called thermally-metastable state. With the promotion of thermal effect, the nanocrystals tend to agglomerate gradually, followed by the interaction and fusion on the surface of each particle. Subsequently, the molecules at thermally-metastable state on the surface will be used as crucial resource for the crystal growth in order to fill the spaces between different particles. More generally, it can be regarded as a “big eating the small” process, which is similar to the Ostwald ripening phenomenon commonly seen for inorganic nanocrystals [12, 13]. Consequently, large high-quality crystals with regular morphology and clear crystal edges can be obtained. In this way, the obtainment of different assembled crystals (Fig. 1 and Fig. S2 in SI) might be explained by the difference of the relative velocity between the particle agglomeration and crystal growth process, which is determined



by the solvent and thermal effects. Specifically, if the particle agglomeration is dominant, the products are likely to gain a larger size, and vice versa. For the induction process, polymorphic transition of energetic nanocrystals (e.g. CL-20) can be beneficial to reach the thermally-metastable state more easily in kinetics, but it is not essential, as confirmed by the study of HMX, TATB and LLM-105.

Fig. 7 Proposed schematic mechanism for the thermally-metastable assembly process. In order to further research the internal structure of assembled crystals, CL-20 slices of an intermediate and final crystal product were prepared by curing and cutting in epoxy resin. The cross-sectional SEM images of intermediate and final CL-20 crystals are shown in Fig. 8. As for the intermediate crystal, evident interfaces and voids inside were presented, revealing an incomplete interfacial crystal growth. On the contrary, the CL-20 slice of the final product demonstrated integrated internal crystal structure in high dense, no cracks or voids were found, corresponding to the high coherence strength as displayed in Fig. 3b. In this case, the proposed schematic mechanism in three steps was further confirmed.


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Fig. 8 Cross-sectional SEM images of assembled CL-20 after curing and cutting in epoxy resin: (a) intermediate crystal at 1h, (b) final product at 4h. 3.3 Thermal and sensitivity properties For energetic materials, thermal stability is widely considered as a key performance. In this work, DSC analysis was adopted to study the thermal properties of the resultant crystals after assembly compared with nano-sized and recrystallized samples, and the results are displayed in Fig. 9, with the peak data marked out. Thermal decomposition of these four energetic materials witnessed similar changes, especially for CL-20, TATB and LLM-105. The nano-sized samples exhibited apparent exothermic peak in advance, resulting from the high reactivity for nanocrystals. Specifically, the decomposition temperature reduced compared with those recrystallized crystals for CL-20, HMX, TATB and LLM-105 was 9.0 oC, 2.0 oC, 8.8 oC and 11.9 oC, respectively. However, the decomposition peaks of assembled samples displayed reasonably close value to those of recrystallized ones by careful solvent/non-solvent recrystallization. These results suggested that the crystals after assembly possess similar features of recrystallized samples in high-quality rather than those nano-sized particles, corresponding to the SEM of slice and coherence strength results.



Fig. 9 DSC curves of the nano-sized, recrystallized and assembled energetic crystals: (a) CL-20, (b) HMX, (c) TATB, (d) LLM-105. The impact sensitivity of nano-sized, recrystallized and assembled samples for these four energetic materials was tested by BAM method, the results are listed in Table 1. As TATB was too insensitive to determine its impact energy, for the other three explosives CL-20, HMX and LLM-105, it can be concluded that an efficient desensitization effect had been achieved for the assembled crystals, particularly compared with the samples after careful recrystallization. For CL-20, the impact energy of nano-sized sample was 8.5 J, which was higher than that of recrystallized one of 6.0 J, ascribing to the desensitization effect caused by the refinement to nanoscale. By contrast, the safety performance regarding impact sensitivity can be improved dramatically to 11.0 J after


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the solvent induced assembly. Despite the fact that polymorphic transition from ε to α phase was happened during assembly, the impact energy can be greatly enhanced, probably due to the preferably well-defined morphology and few impurities or defects in the assembled crystals. The cases for HMX and LLM-105 showed similar changes in sensitivity. In a word, the impact safety can be markedly enhanced through such crystal assembly in this work, which can provide appreciable benefits for their applications. Furthermore, it is worth to point out that the solvent induced thermally-metastable assembly in this work was finished in a static treatment, where no external agitation was needed. Therefore, this novel route to prepare large energetic crystals in high-quality can be easily scaled up. Uniform crystal products can be obtained in large scale since the assembly process will not be affected by mass or heat transfer in a static reaction state, thus further enhancing the application value. Table 1 Impact sensitivity of the energetic crystals by BAM method Materials

CL-20

HMX

Status

Mean size/µm

EBAM/J

nano-sized

0.5

6.5

recrystallized

50

4.5

assembled

40

11.0

nano-sized

0.2

7.5

recrystallized

70

6.0

assembled

25

11.5

nano-sized

0.06

>100

recrystallized

15

>100

TATB



LLM-105

4

Page 24 of 30

assembled

2

>100

nano-sized

0.15

19.0

recrystallized

10

16.5

assembled

20

24.5

CONCLUSIONS In summary, ordered interfacial assembly of energetic nanocrystals was first

achieved through a thermally-metastable state induced by solvent. A brand new transforming perspective was raised as to the disordered agglomeration which should be avoided traditionally. The assembled large HMX, CL-20, LLM-105 and TATB energetic crystals assembled from their nanocrystals evidently exhibit regular morphology, uniform size, smooth surface, few impurities or internal defects, resulting in high crystal strength as well as reduced impact sensitivity. The following conclusions can be made: (1) The assembly process includes three typical steps: surface induction by solvent, particle agglomeration and interfacial crystal growth. (2) The assembly of CL-20 nanocrystals correlated well with the accompanied polymorphic transition process from ε to α, which followed the Avrami equation. (3) The thermally-metastable self-assembly of energetic materials provided a facile, general and promising method for the fabrication of any other non-polymeric organic crystals.


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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (11502243, 11402238, 11502245, 51703211).

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2357-2364. (37) Dill, E. D.; Folmer, J. C. W.; Martin, J. D. Crystal Growth Simulations to Establish Physically

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Table of Contents Graphic and Synopsis Solvent induced thermally-metastable assembly of energetic nanocrystals was studied, including morphology evolution, growth mechanism and properties. Through this novel assembly at particle scale, perfect crystals with improved performances can be achieved.


Perfect Energetic Crystals with Improved ... - ACS Publications

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