DOI: 10.1002/prep.201300174

Preparation and Properties of Submicrometer-Sized LLM-105 via Spray-Crystallization MethodJuan Zhang,*[a] Peng Wu,[a] Zhijian Yang,[a] Bin Gao,[a] Jianhu Zhang,[a] Ping Wang,[a] Fude Nie,[a] andLongyu Liao[a]

1 Introduction

In recent years, micro- and nanostructured energetic mate-rials have attracted considerable interest for improvingtheir performances in energy release and ignition proper-ties, and potential applications in microscale energy-de-manding system [1–4]. It was reported that nanocrystals ofenergetic materials can not only decrease the mechanicalsensitivity, but also help for improving the combustion anddetonation properties as compared to coarse energetic ma-terials [5–8]. Various techniques, such as electrospray crys-tallization, ultrasound- and spray-assisted crystallization,direct write ink-jet, microemulsion, rapid expansion of su-percritical solutions, solvent-antisolvent crystallization, andsol-gel method, had been employed to fabricate nanocrys-tals of high explosives, including cyclotrimethylene trinitra-mine (RDX) [9] , cyclotetramethylene tetranitramine (HMX)[10,11], hexanitrohexaazaisowurtzitane (CL-20) [12], 1,1-dia-mino-2,2-dinitroethylene (FOX-7) [13], 5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one (NTO) [14], 2,2’,4,4’-,6,6’-hexanitros-tilbene (HNS) [15] , 1,3,5-triamino-2,4,6-trinitrobenzene(TATB) [16, 17] , etc.

2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is ahigh-performance energetic material, which has good per-formance and insensitivity between those of HMX andTATB, its calculated energy content is 85 % of HMX and15 % more than that of TATB. It is also thermally stable andinsensitive to shock, spark, and friction, owing to the inten-sive p conjugative system and intra- and intermolecular hy-drogen bonds between molecules [18]. The impact insensi-tivity level of LLM-105 is evaluated approaching to TATB.

These overall properties enable it to raise great potentials inseveral applications, including insensitive boosters, detona-tors, and possibly main charges in specialty munitions[19, 20]. Several publications have reported on the synthesis,scale-up and characterization techniques of LLM-105 as wellas the effects of morphologies on their physical properties,safety characteristics, and detonation performance of thePBXs based on LLM-105 [21–25]. C. W. An and co-workers[26] utilized a spray drying process to prepare nano-compo-sites of LLM-105, the nano-composite particles had a size of1–10 micro with 50–100 nm crystals in the nano-compositeshell. Moreover, LLM-105 crystals with a certain structurehave been reported in recent years, for example, Chen [27]fabricated a rectangular microtube with an average diameterof about 10 mm, which has a unique rectangular cross-sec-tion architecture. Yang [28] proposed an efficient route forthe preparation of one-molecule-thick single-crystallinenanosheets of LLM-105 by using vapor self-assemblingmethod. These controllable size and morphology LLM-105crystals may be highly desirable for microenergetic systems.

In this paper, a facile spray-recrystallization process toprepare homogeneous submicrometer-sized LLM-105 crys-tal is reported for the first time. The morphology, particle

[a] J. Zhang, P. Wu, Z. Yang, B. Gao, J. Zhang, P. Wang, F. Nie, L. LiaoInstitute of Chemical MaterialsChina Academy of Engineering PhysicsMianyang 621900, Sichuan, P. R. China*e-mail : zhijianyang@yahoo.com.cn

Abstract : Submicrometer-sized 2,6-diamino-3,5-dinitropyra-zine-1-oxide (LLM-105) crystals were prepared by spray-crystallization method with dimethyl sulfoxide (DMSO) andultra-pure water with surfactant as the solvent and anti sol-vent, respectively. Submicrometer-sized LLM-105 particleswere characterized by scanning electron microscopy (SEM),X-ray diffraction (XRD), and particle size analysis. The ther-mal stability and sensitivity properties of submicrometer-sized LLM-105 were also investigated. The results revealedthat the submicrometer-sized LLM-105 particles are sphere-

like in morphology with a narrow particle size distributionat the range of 100–600 nm. The submicrometer-sized LLM-105 has a lower exothermic peak at about 343.7 8C com-pared with the synthesized material. Sensitivity testsshowed that submicrometer-sized LLM-105 is more insensi-tive under impact stimulus with a drop height (H50) of102 cm. The submicrometer-sized LLM-105-based PBX ismore sensitive for short impulse shock wave that can be in-itiated at lower initiation current.

Keywords: Spray-crystallization · Submicrometer-sized · Initiation threshold · Sensitivity

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size distribution, internal structure, sensitivity, and thermaldecomposition properties of submicrometer-sized LLM-105were characterized and analyzed. The uniform submicrome-ter-sized particles with sharp size distribution exhibit an ob-vious decrease in sensitivity and a desired initiation currentunder short duration pulse stimuli, compared to the rawmaterials.

2 Experimental

2.1 Materials

LLM-105 raw materials were synthesized according to thereported method [29]. All chemicals and reagents were ofanalytical grade. The dioctyl sulfosuccinate sodium salt(AOT, 96 %) used as a surfactant was purchased from AlfaAesar. Dimethyl sulfoxide (DMSO) was purchased from Tian-jin Chemical Industry Co., Ltd. The ultra-pure water wasproduced by using a Milli-Q apparatus(Millipore).

2.2 Experimental Instrument and Principle

The schematic diagram of the production device is shownin Figure 1. The solvent container with spraying nozzle waswarmed by hot oil and the target compound solution wasdriven by compressed air and atomized by spray nozzle toproduce small droplets with high speed. As the high-veloci-ty fluid jets collided with the high turbulence created by vi-olent agitation in the anti-solvent, rapid recrystallization oc-curred and fine particles formed.

2.3 Purification of LLM-105

The purification process was carried out by dissolving LLM-105 in DMSO with the concentration of 10 wt.-% at a tem-perature of 85 8C. After dissolution, the solution was collect-ed by filtration to remove the insoluble impurities, followedby a slow cooling process to room temperature under con-tinuous agitating, providing purified LLM-105 powdersafter filtration and drying.

2.4 Preparation of Submicrometer-Sized LLM-105 Particles

Purified LLM-105 powders were dissolved into DMSO at90 8C to obtain a solution with a concentration of0.05 g mL�1. It was filtered and put into the solution carry-ing container. Meanwhile, the ultra-pure water with AOTwas added into the anti-solvent container under a subzerotemperature. The anti-solvent was agitated strongly (typi-cally 1000 rpm) to form a whirlpool before spraying. Subse-quently, the high-velocity fluid jets sprayed from the nozzleinto anti-solvent container once the valve was switched on.Rapid crystallization started during the sufficient contactbetween the solvent and anti-solvent. After filtration, lava-tion, and vacuum freeze-drying, a yellow submicrometer-sized LLM-105 product was obtained.

2.5 Characterization and Properties

The morphologies and particle size distribution of submi-crometer-sized LLM-105 samples were examined by scan-ning electron microscope (SEM, Camsacn, apollo 300) andlaser diffraction particle size analyzer (Brookhaven 90 PLUS).Chemical composition was analyzed by HP1100 high per-formance liquid chromatograph (HPLC). The X-ray powderdiffraction (XRD) patterns were recorded with a Bruker D8Advance diffractometer with Cu-Ka radiation. The differen-tial scanning calorimetry/thermogravimetry (DSC/TG) analy-ses were conducted by a simultaneous thermal analyzer(NETZSCH STA 449C).

The impact sensitivity was surveyed by a 12 type drophammer apparatus according to the GJB-772A-97 standardmethod 601.2 [30]. The testing conditions were: 5.000�0.005 kg drop weight; 30�0.5 mg sample, 25–30 8C, �80 %relative humidity. The results were expressed by the dropheight of 50 % explosion probability (H50).

The short duration pulse initiation sensitivity of the ini-tiating explosive based on submicrometer-sized LLM-105was tested by means of a slapper detonator (Figure 2). Theinitiation threshold, expressed as initiation voltage or initia-tion current of the capacitor discharge unit, was used tojudge the initiation ability of submicrometer-sized LLM-105.The up-and-down method and statistical methods for sensi-

Figure 1. Schematic diagram of the experimental device.

Figure 2. Diagram of slapper detonator (1-plug, 2-cable, 3-board,4-shell, 5-barrel, 6-flyer, 7-explosive, 8-bridge foil, 9-bolt).

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tivity tests were used to determine the 50 % or 99.9 % prob-ability initiation current and initiation voltage values.

3 Results and Discussion

3.1 Morphology and Particle Size

The morphologies of synthesized LLM-105 and submicrom-eter-sized LLM-105 were measured by SEM as shown inFigure 3. The synthesized LLM-105 (Figure 3a) has an “X-shaped” morphology with a length of about 100 mm, whileFigure 3b reveals that the submicrometer-sized LLM-105particles are spherical in shape with smooth surface. Theslight agglomeration of such submicrometer-sized LLM-105was observed because of its high surface energy. Particlesize analysis data of submicrometer-sized LLM-105 by laserdiffraction is shown in Figure 4. The median particle size ofsubmicrometer-sized LLM-105 is 337 nm, and its particlesize distribution is relatively narrow, with a size range of100–600 nm.

3.2 Chemical Composition

HPLC analysis was adopted to determine the chemicalcomposition of submicrometer-sized LLM-105. It is reported

that the residual solvent and other impurities are harmfulto the stability over time of ultrafine/nano-explosives [31] .The synthesized LLM-105 has a purity of 97.00 wt.-%(Table 1), and the main impurity is 2,6-diamino-3,5-dinitro-pyrazine (ANPZ). After the purification process, the puritywas improved to 99.85 wt.-%. However, the purity of sub-micrometer-sized LLM-105 is reduced to 98.60 wt.-%, prob-ably due to 0.3 wt.-% of DMSO residual solvent introducedinto the product in the process of spray-crystallization.

3.3 X-ray Diffraction

X-ray powder diffraction was used to study the submicrom-eter-sized LLM-105 crystal structure. The characteristic dif-fraction of submicrometer-sized LLM-105 shows completelythe same diffraction peaks as those of synthesized materials(Figure 5), implying the crystal structure of submicrometer-sized LLM-105 was maintained during the refining process.The sharp peaks in pattern (b) prove that the submicrome-ter-sized LLM-105 is highly crystalline. However, as the sizeof LLM-105 particles is in the submicrometer range, the dif-fraction peaks become slightly broadened (e.g. the peaks (02 0), (0 4 0) and (�1 �4 1) at 2q of 11.18, 22.48, 28.48), at-tributing to the fact that the X-ray yield strength decreaseswith decreasing grain size.

Figure 3. SEM images of (a) synthesized LLM-105, (b) submicrome-ter-sized LLM-105 particles.

Figure 4. Particle size distribution of submicrometer-sized LLM-105.

Table 1. Chemical composition of synthesized and submicrometer-sized LLM-105 particles.

Purity[wt.-%]

Residual DMSO[wt.-%]

Synthesized LLM-105 97.00 –Purified LLM-105 99.85 <0.10Submicrometer-sized LLM-105 98.60 0.30

Figure 5. XRD patterns of (a) synthesized LLM-105, and (b) submi-crometer-sized LLM-105 particles.

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Submicrometer-Sized LLM-105 via Spray-Crystallization Method

3.4 Thermal Properties

The thermal properties of the submicrometer-sized LLM-105 and synthesized LLM-105 were investigated by DSC/TG, as shown in Figure 6. The submicrometer-sized LLM-105 has two obviously exothermic peaks at 343.7 8C and356.9 8C, respectively, corresponding to the thermal decom-position occurred in the region of 333.3–362.4 8C. The twoexothermic peaks for the submicrometer-sized LLM-105shift to a lower temperature by approximately 11 8C and8 8C, respectively, compared with the synthesized LLM-105.Furthermore, the distinct weight loss of submicrometer-sized LLM-105 takes place at 338.3 8C or so, displayingabout 4.5 8C lower than that of synthesized LLM-105. Thiscan be explained by the fact that the decreases in averageparticle size makes submicrometer-sized LLM-105 decom-pose at lower temperature. It is well-known that the ratioof surface atoms to interior atoms increases visibly as theparticle size decreases and thus, the high surface energy ofnanometer materials may significantly change a variety ofphysical properties including thermal behavior. Therefore,the submicrometer-sized LLM-105 crystals possess a highersurface energy and are easier to be stimulated under ther-mal initiation than the coarse counterpart.

3.5 Impact Sensitivity

The impact sensitivities of synthesized LLM-105 and submi-crometer-sized LLM-105 are listed in Table 2 (the first twolines). The drop height of submicrometer-sized LLM-105and synthesized LLM-105 are 102 cm and 80 cm, respec-tively. The H50 value of submicrometer-sized LLM-105 ismuch higher than that of synthesized LLM-105, revealingthat submicrometer-sized LLM-105 achieved a visible de-sensitization under impact stimulus. This phenomenon can

be interpreted by the “hot spots” theory. Due to the relation-ship of sensitivity and inclusions content of energetic materi-als, crystals inclusions (crystal defects like dislocations, thevolume of void among particles) are the main causes to theformation of “hot spots”, submicrometer-sized crystals arebelieved to contain a smaller amount of inclusions, causinga reduction of the probability to form “hot spots”. Addition-ally, nano- or submicrometer-sized crystals of explosiveshave a very large surface area and better capacity for heattransmission and therefore they are more insensitive to stim-ulate “hot spots” formations due to impact [9].

3.6 Initiation Sensitivity of Slapper

The short-duration pulse initiation sensitivity of the PBXbased on submicrometer-sized LLM-105 was also tested, asshown in Table 2. The 50 % and 99.9 % initiation currents ofPBX based on submicrometer-sized LLM-105 are 3.11 kAand 3.46 kA, respectively, and the corresponding 50 % and99.9 % initiation voltages are 4.41 kV and 4.90 kV, respec-tively. The results are similar to PBX based on nano-TATB(Table 2), suggesting that the submicrometer-sized LLM-105is sensitive for short impulse shock waves, thus the PBXbased on submicrometer-sized LLM-105 exhibits great per-formance and potential usage for military applications asa promising initiating explosive, especially for the applica-tion in slapper detonator.

4 Conclusions

A spray-recrystallization method to prepare submicrometer-sized LLM-105 crystals was explored. Spherical submicrome-ter-sized LLM-105 with a narrow particle size distributionand a mean diameter of 337 nm was obtained. Submicrom-eter-sized LLM-105 matches the XRD patterns as those ofsynthesized materials. Compared with synthesized LLM-105,the two main exothermic peaks of submicrometer-sizedLLM-105 particles shifted about 11 8C and 8 8C towardlower temperature, respectively. In addition, submicrome-ter-sized LLM-105 is not only evidently insensitive toimpact stimulus, but also sensitive to short impulse shockwaves with a 50 % initiation current of 3.11 kA. This indi-cates that the submicrometer-sized LLM-105 described inthis work is definitely promising to become a potential ini-tiating explosive applied in the slapper detonator.

Figure 6. Thermal analysis results of synthesized and submicrome-ter-sized LLM-105 particles: (a) DSC curve; (b) TG curve.

Table 2. Sensitivity test results.

H50 [cm] Initiation voltage [kV] Initiation current [kA] Probability percent [%]

Synthesized LLM-105 80 – – –Submicrometer-sized LLM-105 102 – – –PBX based on submicrometer-sized LLM-105 – 4.41 3.11 50

– 4.90 3.46 99.9PBX based on nano-TATB [32] 4.35–4.73 3.32–3.8 50

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Received: November 14, 2013Revised: April 4, 2014

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Submicrometer-Sized LLM-105 via Spray-Crystallization Method