Preparation of multi-scale FOX-7 particles and investigation of sensitivity and thermal stability

Multi-scale ultrafine 1,1-diamino-2,2-dinitroethene (FOX-7) samples with different particle size were fabricated and specifically, nano-FOX-7 was successfully prepared by a green mechanophysical milling method. All samples were characterized by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). Impact and friction sensitivities of the samples were tested and thermal analysis was performed by differential scanning calorimetry (DSC) and thermogravimetry (TG). Ultrafine particles with a mean size of 40 nm, 0.9 μm and 3.4 μm respectively showed less sensitivity than raw FOX-7, whose particles size was about 20 μm. The critical drop height H50 of ultrafine FOX-7 increased from 129 cm to 172 cm, 142 cm and 136 cm, respectively and the friction sensitivity reduced from 32% to 8%, 16% and 20%, respectively. Furthermore, the apparent activation energy of ultrafine particles increased compared with raw materials, which suggested the thermal stability of the ultrafine particles was improved.


Introduction
1,1-Diamino-2,2-dinitroethene (C 2 H 4 N 4 O 4 , also known as FOX-7) is a novel high-energy insensitive material with a symmetrical molecular component and innite 2D wave-shaped monolayer molecular packing, whose outstanding properties have attracted the attention of many scholars since it was rst reported by Latypov in 1998. 1 The special structure of FOX-7 determines its high thermal stability and low sensitivity. [2][3][4][5] A large amount of literature has reported in detail the synthesis, molecular structure, thermal behavior and reactivity of FOX-7. Some scholars have studied the preparation methods of small-sized FOX-7. FOX-7 quasi-three-dimensional (3D) grids, which are constructed from one-dimensional nanostructures $100 nm in diameter, were synthesized by a spray freeze-drying technique and had a promising high-energy-density with superior sensitivity properties. 6 Using a micellar nanoreactors, Mandal 7 prepared spherical particles of FOX-7 with diameters generally in the submicrometer to nanometer range. Nano-FOX-7 was also prepared via an ultrasonic spray-assisted electrostatic adsorption (USEA) method, with an average particle size of approximately 78 nm and nano-FOX-7 particles showed faster energy release efficiency and release more energy. 8 It was reported that when the particle size of the explosive is reduced to the nanometer size, a higher decomposition rate and a lower impact sensitivity are observed compared with the raw material. 9 Furthermore, particle size was proved to have a significant role affecting the safety of the explosive. 10 The safety of explosives is vitally important to their production, storage, transportation and use. Accordingly, the effect of FOX-7 particle size on its performance and safety is investigated in this paper.
The ultrane method of explosive particles mainly includes physical methods [11][12][13][14] and chemical methods. [15][16][17] Compared with chemical methods, mechanophysical milling, a completely physical method, will not cause environmental pollution and more importantly, is low cost and mass production can be achieved. A series of experiments have proved that mechanophysical milling is an effective method for preparing ultrane explosive particles. Liu et al. 11 prepared nano-HMX with different particle size (120.36 mm, 1.18 mm and 0.16 mm) using a bi-directional rotation mill under different drying conditions and there was a signicant sensitivity decrease for 0.16 mm HMX particles. Wang et al. 12 used a mechanical milling approach to fabricate HMX nanoparticles with a signicant proportion of nano-HMX (<100 nm) which was far less sensitive than raw HMX. In addition, mechanophysical milling method was widely adopted to produce other nanoscale explosive particles such as CL_20 (with an average size of 200 nm 13 and 73.8 nm, 14 respectively), PETN 14 (with a mean size of 267.7 nm), HNS 18 (about 89.2 nm with a narrow size distribution), HMX/TATB explosive cocrystals 19 (about 100-300 nm in size), CL_20/HMX explosive cocrystals 20 (with an average size below 200 nm) and all test results revealed that ultrane explosive particles were less sensitive than raw materials.
Herein, mechanophysical milling method was adopted to fabricate ultrane FOX-7 particles. In order to prevent possible explosion, the preparation process of the ultrane explosive particles is performed in a liquid medium, which is an aqueous suspension of explosive particles mixing with bead. Moreover, the mechanical and thermal safety of raw and ultrane FOX-7 were investigated and compared in this paper.

Preparation
Mechanophysical milling was used to fabricate ultrane FOX-7 particles under different experimental conditions and the schematic diagram of mechanophysical milling is shown in Fig. 1. The same quality raw FOX-7 and zirconia ball were added into two ceramic tanks respectively (the mass ratio of raw FOX-7 to zirconia ball was 1 : 20). There was a difference of the liquid medium in two tanks: (1) the liquid medium was pure water which was 10 times the mass of raw FOX-7; (2) the liquid medium was a mixture of pure water and absolute ethyl alcohol and the mass ratio of pure water and ethyl alcohol to raw FOX-7 is 5 : 1 respectively. The milling conditions: 350 rpm and rotated continuously for six hours. The solid explosive particles were pulverized into small particles under the action of mechanical forces and a yellow mixed suspension of zirconia ball, liquid medium and as-prepared explosive particles was obtained. Aer washing, ltration and freeze-drying, two kinds of FOX-7 particles were obtained from two tanks (marked as FOX-7-1 and FOX-7-2, respectively). (Please pay attention to the potential danger during milling process and the corresponding safety precautions are necessary.) As a comparison, other FOX-7 particles were prepared by a recrystallization method. FOX-7 is readily dissolved in DMSO and essentially insoluble in water. 4 Therefore, we used DMSO as solvent and pure water as non-solvent. 5 g of FOX-7 was dissolved in 32 ml of DMSO to prepare FOX-7/DMSO solution. This is a prelming twin-uid nozzle-assisted precipitation method. 15,16 Pure water was pumped into a nozzle which is a venturi tube and FOX-7/DMSO solution was sucked into the nozzle under negative pressure caused by high speed ow of pure water. The explosive particles crystallize out during the mixing process of two uids which were DMSO and pure water. The mixture of explosive particles and solution was collected in a beaker and FOX-7 products was obtained aer ltration and freeze-drying (marked as FOX-7-3) ( Table 1).

Characterization
2.3.1 Field emission scanning electron microscopy (FE-SEM). The microstructure and morphology of raw FOX-7 and products were characterized by eld emission scanning electron microscopy (FE-SEM, JSM-7500F, JEOL, Japan) at an acceleration voltage of 5 kV aer gold sputtering coating. The size distribution of raw FOX-7 and FOX-7-1 was characterized using laser diffraction particle size analyzer (90 PLUS, Brookhaven Instruments Corporation, New York, America).
2.3.2 X-ray diffraction. X-ray diffraction (XRD, DX-2700, Dandong Haoyuan Corporation, Liaoning, China) analysis was used to test the crystalline data of raw FOX-7 and products with Cu-Ka radiation at 40 kV and 30 mA. And the samples were scanned from 5 to 50 in 2q, with an increment of 0.05 and a scan speed of 0.5 s per step.
2.3.5 Differential scanning calorimetry and thermogravimetry. All samples were analyzed by a differential scanning calorimetry (DSC) under the following conditions: sample mass: 0.5 mg; heating rate: 5 C min À1 , 10 C min À1 , 15 C min À1 and 20 C min À1 ; nitrogen atmosphere. Thermal gravimetric analysis (TGA1, Mettler Toledo, Switzerland) was carried out in the nitrogen atmosphere with a heating rate of 20 C min À1 .

Size and morphology
The microstructure and morphology of raw and ultrane FOX-7 were observed by FE-SEM characterization (as shown in Fig. 2) and the particle size distribution curves of raw FOX-7 and products are shown in Fig. 3. The horizontal axis in Fig. 3 indicates the particle size (nm) and the vertical axis indicates the relative percentage share of particle size (%). There are obvious differences in morphology and particles size between raw FOX-7 and products. We can see that raw FOX-7 had an irregular prismatic and massive shape with conspicuous edges and corners ( Fig. 2a and b), while FOX-7-1 had a regular ellipsoidal shape (Fig. 2c and d). The particle size of raw FOX-7 was between 2-50 mm (as seen in Fig. 3a) and FOX-7-1, as-prepared ultrane particles, had a relatively narrow size distribution and the median size was about 40 nm, ranging from 15 nm to 110 nm (Fig. 3b). Furthermore, FOX-7-2, produced by mechanophysical milling in water and absolute ethyl alcohol, had a massive and ellipsoidal shape and had a mean particle size of 0.9 mm, ranging from 0.3 mm to 3 mm (as shown in Fig. 2e and  3c). As a comparison, FOX-7-3, prepared by recrystallization, had several different shapes including cubic and rod-shaped mostly with obvious edges and corners, whose mean size was about 3.4 mm, ranging from 0.9 mm to 6.5 mm (as shown in Fig. 2f and 3d). Therefore, multi-scale FOX-7 particles with different morphologies were successfully fabricated by mechanophysical milling and recrystallization process.
It has been observed that raw FOX-7 and products were both yellow while raw FOX-7 was darker than products. Raw FOX-7 may contain trace impurities which make it darker. These impurities were dissolved in the solution during recrystallization process and were ltered out, which made the recrystallized product lighter in color. Besides, ultrane particles prepared by mechanophysical milling is slightly lighter in color than that prepared by recrystallization method. Compared with recrystallization process, the extrusion, friction, and collision between the explosive particles and the rapid rotation during the grinding cause the particles to fade in color.
During mechanophysical milling process, the edges and corners of the explosive particles are gradually smoothed by the strike generated between explosive particles and zirconia balls. Under the action of mechanical force, the explosive particles are continuously subjected to extrusion, shear, collision and friction, so that the explosive particles are pulverized into ultrane particles undergoing three stages of crack formation, expansion and fracture. So ellipsoidal and nanoscale particles were obtained through continuous grinding. Ethyl alcohol is the only difference between the fabrication process of FOX-7-1 and FOX-7-2, whose particle size obviously differed. The process of rapid grinding assisted dissolution of FOX-7 in absolute ethyl alcohol and the bigger particle size resulted from growth of crystals in absolute ethyl alcohol.
During the recrystallization process, when FOX-7/DMSO solution and the pure water are rapidly mixed, the solution suddenly reaches to a supersaturation state and fast nucleation occurs, then the explosive particles are crystallized out of the solution. Due to the high-speed uid and rapid mixing process, crystal nucleation and growth of explosive particles are not easy to control, resulting in the observation of particles with different sizes and morphologies.

XRD analysis
The XRD patterns of raw FOX-7 and products are shown in Fig. 4. Raw FOX-7 displays three typical peaks at 2q ¼ 20.654 , 26.900 , 28.066 which are assigned to the (012), (020), (021) reection lines. The XRD patterns of products in Fig. 4(b-d) are consistent with that of raw FOX-7 in Fig. 4(a). As we can see from the patterns, FOX-7 products had weaker diffraction peaks than raw FOX-7 and the diffraction peaks were broadened, which is due to the reduction of particle size. 21 The patterns of products in Fig. 4(b-d) have similar diffraction angles compared with the raw FOX-7 shown in Fig. 4(a), indicating that the ultrane particles were still FOX-7 and the crystal form didn't change during the process of mechanophysical milling and recrystallization process. 22

Impact and friction sensitivity characterization
The samples (30 AE 1 mg) were subjected to an impact of a hammer (2 AE 0.002 kg) at various heights using an up-anddown method to investigate the safety performance of the explosive particles, and the critical drop height of 50% explosion probability (H 50 ) was calculated (three tests per sample). The friction sensitivity of each product was tested under the same experimental conditions with 25 samples per group, and the explosion probability was calculated (three groups per sample). The number of explosions of each sample was As shown in Fig. 5, the H 50 of raw FOX-7 (a) is (129 AE 3) cm, which is consistent with published work 1,23 within a certain margin of error. And the H 50 of products (b, c and d) were (172 AE 3) cm, (142 AE 2) cm and (136 AE 3) cm, respectively. An obvious increase of H 50 for each product (43 cm, 13 cm and 7 cm, respectively) suggested that the ultrane explosive particles were less sensitive than raw FOX-7. The hot-spot theory is used to explain the explosion of explosive particles under impact. When impacted, the gaps between explosive particles will undergo adiabatic compression to transform into many hot spots, which can lead to burning or explosion. As the particle size decreases, the specic surface area increases. When the explosive is subjected to an external impact load, the force is rapidly transmitted along the surface of the explosive particles, and the external force is dispersed to more surfaces, and the force exerted on the unit surface is reduced. And the impact sensitivity is greatly reduced when fewer hot spots are formed.
The explosion probability of raw FOX-7 was (32 AE 4)% and that of the products were reduced to 8%, (16 AE 4)% and (20 AE 4)% respectively, corresponding to Fig. 5. Explosive particles produced by mechanophysical milling (FOX-7-1 and FOX-7-2) had an ellipsoidal shape, leading to fewer hot spots during friction process and their friction sensitivity were lower than FOX-7-3 prepared by recrystallization, which is rod-shaped and its crystal structure is not dense and easy to break, resulting in a higher friction sensitivity. It can be concluded that an efficient desensitization effect had been achieved for the ultrane FOX-7 particles.

Thermal analysis
The exothermal reaction is another important property. Differential scanning calorimetry (DSC) was used to investigate the thermal properties of raw FOX-7 and products. The DSC curves at different heating rates and TG curves of raw FOX-7 and products are shown in Fig. 6 and 7 respectively.
As we can see in Fig. 6, there are three distinct stages in the decomposition process of both raw FOX-7 and products: (1) endothermic melting stage (at 110-125 C), (2) the rst exothermic decomposition stage (at 210-230 C) and (3) the second exothermic decomposition stage (at 290-310 C). The rst decomposition peak is caused by the rearrangement of nitro-tonitrite in the FOX-7 molecule, bringing about the destruction of the conjugated system and hydrogen bonds, 24 and the break of the carbon skeleton in the FOX-7 molecule leads to the second decomposition peak. 8 It can also be seen from Fig. 6 that the decomposition peak temperature of the ultrane FOX-7 was lowered compared with raw FOX-7, which can be attributed to an easier decomposition of ultrane molecules at lower temperature because of more ability of atomic vibration, surface energy and the capacity of heat transmission. 25 For each sample, the decomposition peak temperature and the heating rate were substantially positively correlated. And the second exothermic peak was higher than the rst one when the heating rate was faster (15 C min À1 and 20 C min À1 ). Otherwise the rst exothermic peak was higher than the second one (when the heating rate was slower such as 5 C min À1 and 10 C min À1 ). The advance of the highest exothermic peak may be due to a slower heating rate. Fig. 7 showed the TG curves of raw FOX-7 and products. Two obvious stages of weight loss resulted from exothermic decomposition can be observed and the position of the decomposition peak was consistent with the DSC curves. Moreover, there was no obvious inexion point in the TG curves of ultrane particles, and as-prepared ultrane particles had higher decomposition rates compared to raw materials. These results are in good agreement with former studies. 6,8 The Kissinger equation (eqn (1)) was enlisted to calculate the E a (apparent activation energy) and A (pre-exponential factor) of raw FOX-7 and products, using the exothermic peak temperatures at different heating rates, respectively.
where b i is the heating rate (in kelvin per minute); T pi is the exothermic peak temperature in the DSC curve (in kelvin); A is the pre-exponential factor; R is the gas constant (8.314 J mol À1 K À1 ); E a is the apparent activation energy (in J mol À1 ).
The activation energy (E a ) and the pre-exponential factor (A) can be calculated by eqn (1), using the exothermic peak temperature (T pi ) at different heating rates (b i ), respectively. All the results are presented in Table 2. Fig. 6(e) shows the Kissinger's plots of all samples and the results presented good linear relationships (R 2 > 0.99) and the close plots may mean that the samples undergo similar This journal is © The Royal Society of Chemistry 2019 decomposition reaction. 13 Table 2 shows that the activation energy of raw FOX-7 and products were 275.53 kJ mol À1 , 324.62 kJ mol À1 , 291.18 kJ mol À1 and 280.29 kJ mol À1 , respectively. The physical meaning of apparent activation energy is the difficulty of the initiation of decomposition or deagration. Thus, the higher apparent activation energy of ultrane FOX-7 suggested that the decomposition reaction of ultrane FOX-7 particles become difficult and had a better thermal stability. In this respect, the thermal stability of nano-FOX-7 particles (FOX-7-1) was most signicantly improved.

Conclusion
In this paper, nano-FOX-7 particles were successfully prepared by mechanophysical milling and the safety of multi-scale FOX-7 particles were investigated. Particles with different sizes and morphologies were observed by FE-SEM. Impact and friction sensitivity of raw FOX-7 (20 mm) and ultrane FOX-7 particles with different mean size (40 nm, 0.9 mm and 3.4 mm) were tested and thermal analysis of all samples was studied by DSC and TG. The results of sensitivity test showed that ultrane particles were less sensitive and the apparent activation energy of ultra-ne particles increased, suggesting their mechanical sensitivity and thermal stability were improved signicantly.

Conflicts of interest
There are no conicts to declare.