Facile, continuous and large-scale production of core–shell HMX@TATB composites with superior mechanical properties by a spray-drying process

Abstract

The increasing high-energy-density requirements of energetic materials as well as the concerns over safety problems have accelerated the development of insensitive high explosives (IHEs). Recently, studies focused on the fabrication of advanced combinations of materials such as coating a moderately powerful and extremely insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) onto the surface of a high-energy but sensitive explosive 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) have attracted a large amount of attention. However, the reported results on the construction of this core–shell structure show a low utilization of shell material due to the unconfined synthesis environment. In this report, a facile and effective spray-drying route was employed to achieve a coating of TATB nanoparticles onto pre-modified HMX crystals. The utilization of TATB shell was significantly improved due to the self-assembly in confined droplets during spray-drying process, thus leading to the decrease of shell content and further enhancement of explosive performance. Both field-emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) results confirmed the formation of the core–shell HMX@TATB composites with a uniform and compact shell layer. The influence of various experimental parameters on the core–shell structure of final products was also examined. The impact and friction sensitivity results showed that superior mechanical properties of these core–shell microparticles can be maintained. Furthermore, such a facile, continuous, and one-step synthesis strategy opens up new perspectives on the large-scale production of core–shell energetic–energetic composites.

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1. Introduction

As a significant branch of materials science, energetic materials (explosives, propellants, and pyrotechnics) represent an important class of compounds with a very large amount of stored chemical energy in their molecular structures, which can decompose rapidly to release energy and produce large quantities of hot gases, high temperature, and high pressure.¹ Since glyceryl trinitrate was originally synthesized by Alfred Nobel and has been extensively used as an explosive in civil and military areas, traditional investigations of energetic materials mainly concentrated on improving detonation performance to achieve more powerful high-energy-density materials (HEDMs).^2,3 However, high energy and safety has always been an inherent contradiction which affects the transportation, storage, and handling of these materials.⁴ Therefore, driven by growing safety concerns, considerable research efforts have been undertaken to develop insensitive high explosives (IHEs) during the past three decades.^5–8

1,3,5,7-Tetranitro-1,3,5,7-tetrazocane (HMX), firstly discovered as a byproduct in the preparation of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) in 1930, is among the most commonly used energetic materials in national defense industries today.⁹ Due to its superb detonation energy and high melting point, it has recently risen to supplant RDX as the military state-of-the-art explosive. Unfortunately, its high sensitivity towards external mechanical stimuli is a fatal drawback that limits replacement with modern explosives. Up to now, an admirable amount of work has gone into striking a balance between its unprecedented energy and safety problems. Typical strategies for reducing the sensitivity of HMX involve the control of crystal shape and quality,^10,11 exploration of cocrystal explosive,^12–14 and embedding of energetic crystallites in a polymer binder.^15,16 In recent years, it is notable that there is an alternative approach to harmonize the performance of HMX by constructing HMX@insensitive explosive core–shell structure. Systems such as HMX@2,4,6-trinitrotoluene (TNT),¹⁷ HMX@3-nitro-1,2,4-triazole-5-one (NTO),^18–20 and HMX@1,3,5-triamino-2,4,6-trinitrobenzene (TATB),^21–24 have shown a significant decrease in mechanical sensitivity. In particular, TATB, which is a moderately powerful and extremely insensitive explosive and is widely used in modern nuclear warheads,²⁵ has been considered to be the most ideal candidate for the shell component. In our previous work, a core–shell HMX@TATB composite with shell content of 15 wt% was successfully fabricated via a facile ultrasonic technique.²⁴ The construction of core–shell microparticles is highly effective in improving the mechanical sensitivity of HMX. Nevertheless, we note that TATB nanoparticles are difficult to be completely coated onto HMX crystals under an unconfined ultrasonic system, resulting in a low utilization of TATB shell. In order to construct an integral and continuous shell structure, excessive TATB nanoparticles were used. There is no doubt that this causes the explosive power of the highly energetic core component to be inevitably diluted when coated with a high-content, moderately powerful energetic shell. Based on the considerations above, it is necessary to develop a facile and effective method for decreasing the shell content of the HMX@TATB composite by increasing the utilization of shell material.

Spray-drying, which is a facile and preferred technology for the transformation of liquid solutions or suspensions into dried particulates by rapidly evaporating the solvent with a hot gas, has been widely applied in the manufacture of food and pharmaceutical products.²⁶ The process involves the formation of spherically confined droplets from atomization of an initial solution or suspension and the subsequent self-assembly of the components inside the droplet system after heating and solvent evaporation.²⁷ Due to the evaporation-driven shrinkage of the confined droplets containing the target compound, the constituent particles with defined morphology, degree of agglomeration, and resulting size can be produced via capillary forces.²⁸ Recently, this elegant one-step constructive process has also been successfully extended to the fabrication of core–shell composites.^28–30 In addition, such an effective and reliable synthesis strategy appears to be applicable for the continuous and large-scale production of core–shell heat-sensitive materials. These fascinating merits of spray-drying are additional motivations for this study.

Continuing our interest in fine-tuning the performance characteristics of core–shell HMX@TATB microparticles, we explore the preparation of this composite with higher energy while maintaining the excellent mechanical properties. In this contribution, we present a simple, continuous, and scalable method for the synthesis of core–shell HMX@TATB composites by a spray-drying means. The utilization of shell material is remarkably improved due to the self-assembly in confined droplets during spray-drying process, thus leading to the decrease of the shell content from 15 wt% to 8 wt% and further enhancement of explosive performance. The resultant core–shell HMX@TATB composites have a uniform, compact, and continuous coating layer with the degree of coverage close to 100%. Moreover, this core–shell structure with low shell content of 8 wt% also exhibits superior mechanical sensitivity. Most importantly, the accessibility, versatility, and reliability of the spray-drying methodology make the industrial large-scale production of core–shell heat-sensitive energetic composites with high performance easily feasible.

2. Experimental

2.1. Materials and chemicals

Reduced sensitivity HMX with a density of 1.90 g cm⁻³ and an average size of about 20 μm was provided by Institute of Chemical Materials, CAEP (Mianyang, China). TATB quasi-three-dimensional (3D) grids, which were constructed from zero-dimensional (0D) spherical or ellipsoidal nanoparticles with a size of about 60 nm, were prepared according to the literature.³¹ The density of nano-TATB aggregates is around 0.45 g cm⁻³. 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), RDX, and 1,1-diamino-2,2-dinitroethylene (FOX-7) were obtained from our own laboratory. Poly(ester urethane) block copolymer (Estane 5703) was purchased in pellet form from Noveon, Inc. (Cleveland, OH, USA) and was used as received without further purification. 1,2-Dichloroethane (C₂H₄Cl₂, AR grade) was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, China). Deionized water with a resistivity of 18.2 MΩ cm was produced by using a Milli-Q apparatus (Millipore, Billerica, MA).

2.2. Fabrication of core–shell HMX@TATB composites

HMX crystals were firstly modified by a surface modification agent, Estane 5703, which has been proved to remarkably improve the interfacial bonding and adhesive capacity between the HMX core and Estane as well as Estane and the TATB shell.^32,33 The details of the modification procedure have been reported in a previous publication.²⁴ A schematic diagram of the fabrication process of Estane-modified HMX@TATB core–shell microparticles by the spray-drying technique is illustrated in Fig. 1. In a typical synthesis, the nano-sized TATB grids (0.087 g) were added into 120 mL of deionized water and then irradiated with high-intensity ultrasound (240 W, 40 kHz) for 30 min at room temperature, a green-yellow TATB suspension was obtained. Subsequently, pre-modified HMX (1.00 g) microparticles were put into the above-mentioned TATB suspension and treated with ultrasound for 5 min. The resulting well-dispersed suspension was transferred at 4 mL min⁻¹ into a spray-drying apparatus (Lab Spray Dryer L-117, manufactured by Beijing Laiheng Scientific Co., Ltd., of China) with the assistance of a peristaltic pump. The operating conditions in this experiment were constant inlet temperature of 105 °C, outlet temperature of 60–70 °C, and high velocity gas with a flow rate of 6 L min⁻¹. In the two-fluid nozzle, the mixing suspension was dispersed by a preheated nitrogen carrier gas into fine droplets, which were subsequently dried in the cylinder and further separated in the cyclone. Finally, a greenish-yellow product was collected in the collector.

Fig. 1 Flow chart of the core–shell HMX@TATB composites fabricated via spray-drying technique.

2.3. Characterizations

Field-emission scanning electron microscopy (FE-SEM) images of the samples were obtained by a Hitachi S-4800 cold emission field scanning electron microanalyser at an acceleration voltage of 5 kV. X-ray powder diffraction (XRD) measurements were performed on a Bruker D8 Advance diffractometer, using a Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA in the 2θ, ranging from 10° to 60° with a scanning rate of 0.02° per second. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo VG ESCALAB 250 spectrometer which consists of a monochromatic Al Kα as the X-ray exciting source (1486.6 eV). The impact sensitivity was surveyed by a WL-1 type impact sensitivity instrument according to GJB-772A-97 standard method 601.2.³⁴ A 50 mg sample was placed between steel anvils, and hit by a 5 kg drop weight. The impact sensitivity of each test sample was expressed by critical drop height with 50% explosion probability (H₅₀). The friction sensitivity was determined on a WL-1 type friction sensitivity apparatus according to GJB-772A-97 standard method 601.2.³⁴ The test conditions are as follows: pendulum weight, 1.5 kg; relative pressure, 3.92 MPa; swaying angle, 90°. The friction sensitivity of each test sample (30 mg) was expressed by explosion probability (P).

3. Results and discussion

3.1. Morphologies of the typical core–shell microstructures

The FE-SEM observation was utilized to directly analyze the morphology and size of the raw materials and as-synthesized product. Fig. 2a and b reveal that the intact HMX crystals have a fairly smooth surface, and a size ranging from 10 μm to 25 μm. After modification, the surface morphology and size of the Estane-modified HMX microparticles exhibits no noticeable change (Fig. 2c and d). The SEM images of the as-obtained core–shell sample are depicted in Fig. 2e–h. The low-magnification SEM images in Fig. 2e and f show that the monodisperse composites possess a uniform size and rough surface. The magnified FE-SEM image in Fig. 2g clearly demonstrates the formation of TATB-coated particles with a well-preserved morphology. The compact and continuous shell layer is constructed from spherical or ellipsoidal particles with a size of about 60 nm, which is in good accordance with the results of nano-TATB raw material (Fig. 2i and j). The lateral surface of composite microparticles also presents a perfect coating effect (Fig. 2h). Moreover, a close examination of the exposed profile reveals that the thickness of the outer shell is about 2 μm and appears fairly uniform (inset of Fig. 2h). The formation of such thick shell layer is mainly attributed to the use of the low-density TATB particles. The above-described analyses prove that the core–shell HMX@TATB microcomposites with low shell content of 8 wt% and high coverage can be successfully fabricated by this efficient spray-drying method.

Fig. 2 SEM images of the intact HMX crystals (a and b), Estane-modified HMX microparticles (c and d), core–shell HMX@TATB structures with a nano-TATB content of 8 wt% (e–h), and TATB nanoparticles (i and j).

The relationship between the core–shell structure and the content of nano-TATB shell was studied. As illustrated in Fig. 3a and b, when the content of TATB shell was 2 wt% and other conditions were kept unchanged, it was found that TATB nanoparticles were not continuously distributed over the surface of pre-modified HMX core. When the content of TATB was increased to 5 wt%, the SEM image in Fig. 3c clearly shows a TATB shell layer with relatively uniform thickness was deposited onto individual modified HMX microparticles. However, the enlarged SEM image in Fig. 3d reveals that a few of areas could not be covered. With further increase of TATB content to 15 wt%, all bare surface of pre-modified HMX disappeared, and the surface of the core–shell microcomposites showed significantly increased roughness due to the grainy structure of the outer TATB layer (Fig. 3e). In addition, a few aggregates composed of the 60 nm nano-sized particles can be observed (Fig. 3f). Accordingly, the appropriate amount of sheath layer is a key factor for the formation of a uniform and integral core–shell structure.

Fig. 3 SEM images of the HMX@TATB composites synthesized at a sheath content of 2 wt% (a and b), 5 wt% (c and d), and 15 wt% (e and f).

3.2. Phase structure and elemental composition of the core–shell composites

To determine the overall phase and crystallinity of the as-synthesized core–shell samples with different shell content, XRD measurements were carried out. As shown in Fig. 4a, all characteristic diffraction peaks from the core layer can be indexed to the standard diffraction pattern of monoclinic β phase of HMX (JCPDS no. 45-1539). When the surface of the HMX core was decorated with the amorphous copolymer Estane, the XRD pattern remained in accordance with a monoclinic β-HMX phase. After coated with TATB nanoparticles, no obvious difference was found in the patterns of the core–shell HMX@TATB composites because of the relatively low diffraction intensity of nano-TATB (JCPDS no. 44-1627). From the XRD results, we conclude that the mild coating conditions during the spray-drying process had no influence on the crystalline phase of HMX.

Fig. 4 XRD patterns (a) and N1s XPS spectra (b) for the intact HMX crystals, modified HMX microparticles, core–shell HMX@TATB composites with a sheath content of 2 wt%, 5 wt%, and 8 wt%, physical mixture of modified HMX and nano-TATB with 92 : 8 weight ratios, and TATB nanoparticles.

Surface chemical composition and degree of coverage of the core–shell composites can be verified by the XPS analysis in comparison with the uncoated HMX crystals, pre-modified HMX, and TATB nanoparticles. It is well-known that both HMX and TATB contain the element C, H, O, and N; however, there is an obvious difference between their N1s spectra, as depicted in Fig. 4b. For HMX and pre-modified HMX, the N1s spectrum was resolved into two peaks with binding energies of 407.1 eV and 401.6 eV, which can be typically ascribed to the nitrogen in –NO₂ group and nitrogen in the framework of HMX, respectively. For TATB, the N1s spectrum can be deconvoluted into a secondary peak at 408.2 eV, and two main peaks at 405.2 eV and 399.6 eV. The secondary peak is a satellite one, which is in good accordance with results reported previously.³⁵ The other two peaks at 405.2 eV and 399.6 eV originate from the nitro–nitrogen bonds and amine–nitrogen bonds, respectively. The pre-modified HMX coated with a low sheath content at 2 wt% and 5 wt% displayed a combination of the characteristic peaks of HMX and TATB, implying an imperfect coating effect. With the increase of nano-TATB content (8 wt%), the two N1s peaks of HMX disappeared completely, and only the characteristic peaks of TATB were detected, indicating that the surface of HMX crystals were compactly coated by TATB nanoparticles and the degree of coverage is close to 100%. These results are consistent with those of SEM as shown in Fig. 2 and 3. Therefore, the successful fabrication of the desired core–shell architecture prepared by this spray-drying route was further confirmed.

3.3. Effect of experimental parameters on the structure of the core–shell composites

By a series of controllable experiments, it was found that the as-described spray-drying method provides a facile strategy to tailor the structures and assembly of the core–shell HMX@TATB microcomposites. This can be done by varying the experimental parameters, such as the size of core particle, volume of deionized water, as well as the component material and core shape. Fig. 5 shows the surface morphology evolution of HMX@TATB composites with different core size. When the average size of the core was decreased from 20 μm to 1–2 μm, pre-modified HMX and nano-TATB were observed in almost completely isolated states (Fig. 5a and b). This can be ascribed to the fact that the significant decrease in core size increases the ratio of surface atoms to interior atoms, which is disadvantageous for the formation of a core–shell structure when the content of nano-TATB was kept constant. When the HMX crystals with size of about 45–75 μm was used, it was clearly observed from Fig. 5c and d that the pre-modified HMX microparticles are highly encapsulated in the nano-TATB matrix and the TATB shell presents a typical coarse and continuous morphology. However, further increases in the size of core material (125–150 μm) led to the generation of TATB three-dimensional porous aggregates, and no micro-sized polyhedrons were obtained (Fig. 5e and f). This can be explained that the modified HMX microparticles fall directly into the bottom of spray chamber after passing through the nozzle of spray dryer due to their high weight. This surface morphology evolution process validates that the size of the core material plays an important role in the formation of a perfect core–shell structure.

Fig. 5 SEM images of the products prepared with different sizes of HMX: 1–2 μm (a and b), 45–75 μm (c and d), and 125–150 μm (e and f).

It was found that the volume of deionized water also has a significant effect on the surface morphology of the final product. Fig. 6 shows the SEM images of the samples obtained with different volumes of 360 mL and 40 mL. When the volume of deionized water was increased from 120 mL to 360 mL, the as-prepared product was composed of incompletely coated microparticles, as shown in Fig. 6a and b. After decreasing the volume to 40 mL, the SEM images in Fig. 6c and d clearly display that the core surface is then entirely covered by the nano-sized particles. Results using even less amount of water were not easily obtained, as the decrease of the volume also leads to increased viscosity of the suspension system, and it is difficult to prevent frequent clogging of the spray nozzle.

Fig. 6 SEM images of the samples obtained with different volumes of deionized water: 360 mL (a and b), and 40 mL (c and d).

Besides the experimental parameters discussed in the two preceding subsections, the relationship between the final core–shell structure and the component material and core shape was studied. As shown in Fig. 7a, c and e, when HMX was replaced by other core component CL-20, RDX, or FOX-7, a rough and continuous surface coating was acquired, indicating that micro-sized crystals were well-preserved by numerous TATB nanoparticles. The magnified FE-SEM images of individual composite more clearly display the detail of the surface (Fig. 7b, d and f). Noteworthy is that when microrods were used instead of polyhedrons, TATB nanoparticles were interconnected and fully covered the entire rod-like core (Fig. 7e and f). These results demonstrate that this simple spray-drying strategy may represent a universal approach for the fabrication of core–shell energetic–energetic composites regardless of specific properties of the core material chosen.

Fig. 7 SEM images of the core–shell CL-20@TATB (a and b), RDX@TATB (c and d), and FOX-7@TATB (e and f) composites.

3.4. Formation mechanism of the core–shell HMX@TATB composites

A possible formation mechanism is proposed for the core–shell structured HMX@TATB composites through spray-drying process, as schematically illustrated in Fig. 8. A good aqueous dispersion which contains TATB nanoparticles (Fig. 8a) and pre-modified HMX microparticles (Fig. 8b) is firstly achieved. Then, the mixture is pumped into a spray-drying apparatus and nebulized to form much smaller droplets through a nozzle by a preheated nitrogen carrier gas (Fig. 8c). Once droplets contact hot gas, an equilibrium of temperature and vapor partial pressure would be established between liquid phase and gas phase.³⁶ Therefore, heat transfer is carried out from the nitrogen gas toward each droplet, leading to an increase in the droplet temperature up to a constant value. Subsequently, the rapid evaporation of water within the droplet is achieved at a constant temperature and water vapor partial pressure. As a result, the intensive removal of water induces isotropic shrinkage of the droplet surface and an increase in nano-TATB concentration at the surface; after that, suspended TATB nanoparticles diffuse toward the center of the droplets and spontaneously form an outer shell which is able to encapsulate the pre-modified HMX microparticles (Fig. 8d).^36,37 Further evaporation removes moisture from inside the droplet and promotes compressive capillary stresses on the shell, which could facilitate the formation of the core–shell HMX@TATB composites with a uniform and compact crust (Fig. 8e).³⁸ Based on the above-mentioned analysis, almost all TATB nanoparticles are forced to coat on the surface of core material, thus leading to remarkable improvement of the utilization of TATB shell. Finally, the core–shell microparticles are separated and collected through a cyclone placed outside the dryer. The effect of this proposed formation mechanism is supported by FE-SEM images as discussed previously (Fig. 2).

Fig. 8 Schematic showing the evolution of a droplet containing both pre-modified HMX and TATB nanoparticles during the spray-drying process.

3.5. Mechanical sensitivity

The impact and friction sensitivity results of the intact HMX crystals, coated HMX samples, and TATB nanoparticles are listed in Table 1. It can be seen that the H₅₀ value and explosion probability P of the raw HMX crystals are 54 cm and 72%, respectively. For pre-modified HMX microparticles, the H₅₀ value decreases to 33 cm and P has a slight increase (from 72% to 80%), indicating that the pre-modified HMX microparticles possess a higher impact and friction sensitivity. The reason for this is likely that modification of the HMX core by copolymer Estane results in the presence of loose product. The increase of the size of inter-granular voids can sensitize HMX to mechanical stimuli by aggravating the formation of critical hot-spots.³⁹ When a core–shell structure is successfully fabricated, the mechanical properties of the designed HMX@TATB microparticles are remarkably improved. Particularly, the impact and friction sensitivity of the perfect core–shell HMX@TATB composites obtained with shell content of 8 wt% are higher than 112 cm and 0%, respectively, which is significantly better than the physical mixture and is also comparable to that of a pure sample of the extremely insensitive TATB compound. Such superior mechanical properties can be explained by viewing the graphite-like structured TATB shell as an outstanding buffer and lubricant. Hence, the mechanical sensitivity is improved from the reduction of hot spots within the HMX crystals when the composite undergoes external mechanical stimuli.^24,40 The above-mentioned results prove that this spray-drying technique can be effectively employed to improve the utilization of shell material by allowing a minimal, uniform coating to adequately shield the core while maintaining excellent mechanical properties.

Table 1 Impact and friction sensitivity of raw and coated HMX samples, and TATB nanoparticles

Sample	Formation method	TATB wt%	Impact sensitivity H50/cm	Friction sensitivity P/%
Raw HMX crystals	—	0	54	72
Pre-modified HMX	—	0	33	80
HMX@TATB	Spray-drying	5	75	20
HMX@TATB	Spray-drying	8	>112	0
HMX@TATB	Spray-drying	15	>112	0
HMX/TATB	Physical mixing	8	42	32
TATB nanoparticles	—	100	>112	0

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4. Conclusions

In summary, we adopted a simple but effective spray-drying route to fabricate TATB-wrapped HMX composites. The resultant HMX@TATB microparticles possess a compact and continuous shell structure, in which micro-sized HMX particles are uniformly embedded into the TATB encapsulation. With the assistance of confined droplets during the spray-drying process, the utilization of TATB shell was significantly improved with shell content being decreased from the previously reported 15 wt% to 8 wt%, thus leading to further enhancement of explosive performance. More importantly, the impact and friction sensitivity results showed that superior mechanical properties of the core–shell composites with low shell content can be maintained. A possible formation mechanism was proposed for the core–shell HMX@TATB structure, and results suggest this technique is a very promising and scalable method for production of various core–shell energetic–energetic composites with high performance.

Acknowledgements

We thank the financial support from the National Natural Science Foundation of China (NSFC, nos 11202193, 11272292, 11172275, 11172276, 21172203, 11372288, 11372289, and 11372290), and the Key Foundation of China Academy of Engineering Physics (no. 2012A0302013).

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