Nano-HNS Particles: Mechanochemical Preparation and Properties Investigation

Nano-2,2,4,4,6,6-hexanitrostilbene (HNS) particles were successfully prepared by a mechanochemical (i.e., high energy milling) process without an organic solvent, which can be viewed as a green technology.The particle size, morphology, specific area, crystal phase, thermal decomposition properties, impact sensitivity, and short duration shock initiation sensitivity were characterized and tested. The diameter of milling HNS is about 89.2 nm with a narrow size distribution and without agglomeration of particles. The formation mechanism of nano-HNS can be viewed as the transformation from thin HNS sheets with a one-dimensional nanostructure to three-dimensional nanoparticles.The nano-HNS particles present a much higher Ea and lower impact sensitivity than purified HNS, revealing the outstanding safety properties. From the results of the short duration shock initiation sensitivity, 50% and 100% initiation voltages are decreased compared with those of HNS-IV, indicating the higher initiation sensitivity.


Introduction
The compound 2,2  ,4,4  ,6,6  -hexanitrostilbene, also known as HNS or JD-X, is a typical heat-resistant explosive.Due to its excellent thermal stability, HNS is widely used as an energetic ingredient in thermally stable charges (e.g., in supersonic missiles and in crude oil production) where it may encounter high temperatures [1][2][3].According to the US military specification, HNS has multiple classifications such as HNS-I, HNS-II, and HNS-IV.HNS-IV is an ultrafinesized material with a surface area of 5.0 to 25.0 m 2 /g, which is proven to be insensitive to shock, percussion, heat, and friction, but sensitive to short duration shock.Therefore, it has been acting as the main charge in exploding foil initiators (EFI), also called slapper detonators [4][5][6][7].
The US military standard MIL-E-82903 requires that HNS-IV be crash-precipitated from HNS-II that was recrystallized from HNS-I.Kaur et al. employed the ultrasonic treatment solvent/nonsolvent method to reduce the size of HNS and determined solvent contamination of HNS by NMR and GC-MS analysis [8].Wang et al. adopted prefilming twin-fluid nozzle assisted precipitation method to prepare nanocrystalline HNS, whose particle size ranges from 90 nm to 150 nm [9,10].Much organic solvent, such as dimethylformamide (DMF) and N-methyl pyrrolidone (NMP), was needed in the recrystallized process due to the relatively low solubility (∼2 g per 100 ml DMF).The large quantities of solvents were bound to increase the cost and pollution, which had been a hurdle in its processing.
Recently, a "green" method without an organic solvent, that is, a mechanochemical process, was used to prepare nanosized explosive particles, such as nano-HMX, RDX, and CL-20 [11][12][13].Herein, a planetary ball mill was used to grind HNS into nanosized explosive particles.The morphology, particle size, crystal type, impact sensitivity, thermal decomposition properties, and short duration shock initiation energy of nano-HNS particles are investigated and analyzed.

Materials.
Raw HNS was provided from Shanxi Beihua Guanlv Chemical Co., Ltd.Ethanol, AR grade, was purchased from Tianjin Hengxing Chemical Reagent Company.Dimethylformamide (CP), methyl alcohol (CP), and 1,4dioxane (CP) were purchased from Tianjin Tianda Chemicals Co., Ltd.Purified water was prepared in our own lab.

Purification and Preparation of
Nano-HNS.Firstly, raw HNS was purified according to the method referred to in our previous literatures [14,15].Secondly, 10 g of purified HNS, 100 g grinding ball (0.1-0.2 mm), and 100 g of water were added to four grinding tanks, respectively, which were fixed on the turntable of a grinding machine.Thirdly, the revolution speed of the grinding tank was set as 300 RPM and then it was put on the planetary milling machine.After five hours of milling, a yellow suspension was obtained from the grinding tank.Finally, the suspension was used for sonication to completely remove the zirconia balls from the product.After washing and freeze-drying, the dried ground HNS powders were obtained.

Characterization and Properties.
The profiles of purified HNS and milling HNS powders were characterized by TES-CAN MIRA3 LMH scanning electron microscopy (SEM).Particle size distribution was determined by Brookhaven BI-90PLUS particle size analyzer (USA), using water as dispersing media.The specific surface area (SSA) and pore size of the materials were measured, recording N 2 adsorption/desorption isotherms at 250 ∘ C for 4 h using a Quantachrome Autosorb-1 (USA).The crystal type of HNS sample was measured by a DX-2700 X-ray diffractometer with filtered 0.154 nm monochromatic Cu K radiation at 40 kV and 30 mA made by Dandong Haoyuan Instruments Co., Ltd., China.The samples were scanned from 5 ∘ to 55 ∘ in 2, with an increment of 0.02 ∘ and a scan speed of 0.5 s per step.The HNS samples were analyzed on a model 200F3 differential scanning calorimeter made by NETZSCH, Germany.The conditions of DSC were as follows: sample mass, 0.6 mg; heating rate, 5, 10, and 20 ∘ C/min; and N 2 atmosphere (20 mL/min).Impact sensitivity was surveyed by a 12-type drop hammer apparatus according to GJB-772A-97 standard method 601.2.17.The testing conditions were as follows: drop weight, 5.000±0.002kg; sample mass, 35±1 mg; temperature, 10∼35 ∘ C; and relative humidity, ⩽80%.The results were based on tests on both sides of 50% probability level using an upand-down method.They were expressed by the critical drop height of 50% explosion probability ( 50 ).The higher the value of  50 was, the lower the impact sensitivity was.
The short duration shock initiation sensitivity of the milling HNS samples was determined by the method of electrically exploded metal foil-driven flyer plate developed in our laboratory.The schematic of the experimental arrangement used to test short duration shock initiation sensitivity is shown in Figure 1.The initiation threshold, expressed as initiation voltage or initiation energy, was used to judge the initiation ability of HNS.The up-and-down method and statistical methods for sensitivity tests were used to

Morphology, Particle Size, and Structural Characterization.
The morphologies of purified HNS and the milling HNS samples were observed using SEM and the results are shown in Figure 2. The SEM image in Figure 2(a) indicates that purified HNS powders are composed of many thin sheetlike structures with a length and width of several tens of microns.From the magnified SEM images in Figure 2(b), it can be seen that the layer thickness is relatively uniform and ranges from about 100 to 200 nm.It can be interpreted that some crystal faces of HNS are inhibited from growth when HNS crystals are recrystallized from DMF solvent.By mechanical milling process, the HNS particles in Figures 2(c) and 2(d) became many tiny polyhedral granules, which are distributed evenly and sized at around 100 nm.Due to the fact that the thickness of the purified HNS sheet is lower than 200 nm, the milling balls cannot grind and reduce the particle size on the thickness dimension further.Thus, the shape of milling HNS particles is different from the other spherical explosive particles, which reduce the particle size on three dimensions at the same time.
The particle size distribution curves of the milling HNS powders are shown in Figure 3(a), where () is the relative percentage contribution of the size range and () is the cumulative percentage contribution.It can be found from Figure 3(a) that the median diameter is 89.2 nm and ranges from 40 to 190 nm.The grinding HNS powders assume a unimodal pattern with a narrow distribution.The results are in good agreement with that of SEM analysis.
The crystal structures of purified and milling HNS powders were tested by X-ray powder diffraction (XRD) and the results are presented in Figure 3 (2, 0, 0)  However, the diffraction peaks of milling HNS are weaker and wider than those of purified HNS, which is attributed to the fact that the diffraction peaks weaken and disappear with the decrease of particle size [16,17].Meanwhile, it also can be observed that the diffraction intensity of milling HNS at 17.90 ∘ , 19.91 ∘ , 23.63 ∘ , and 28.50 ∘ is visibly increased after milling.This is probably because of the preferred orientation caused by the effect of ball milling.From these variations, it can be concluded that the crystal morphology of HNS has been changed in the mechanochemical milling process.

Fragmentation Mechanism.
Combining the results of SEM and XRD, the fragmentation mechanism can be inferred, which is diagrammed in Figure 4.The rotation of the jar will provide the inner substances with kinetic energy, which can make the explosive particles and milling balls collide with each other.From the morphologies in Figure 2, it can be seen that the thin HNS sheets with onedimensional nanostructure are milled into many threedimensional nanoparticles.Through the mechanochemical milling process, the size in the  and  direction of HNS sheets is reduced from several tens of microns to ∼100 nm.However, the thickness in the  direction reaches the limitation of fragmentation in such milling condition.Therefore, vertical rupture enables HNS fragmentation leading to the changes on crystal faces.The crystal faces (paralleled  surface) are divided into various nanosized facets, which can decrease the diffraction intensity of these faces.At the same time, the flank areas of HNS crystals are increased evidently owing to the increase in the number of nanoparticles.Thus, the diffraction intensity at this side increases.These facts fit perfectly the changes in XRD spectrum (Figure 3

Specific Surface Area (SSA) and Pore Diameter Analysis.
The adsorption and desorption curves of the milling HNS particles are given in Figure 5.The BET and Langmuir SSA of milling HNS were determined to be 14.75 m 2 /g and 21.62 m 2 /g, respectively, which are lower than the SSA reported [9].Commonly, ultrafine HNS particles are recrystallized from explosive solutions.In the recrystallization process, the residual solvents and bubbles are easily trapped in the crystal to form the crystal defect, which can increase the SSA of HNS particles.In the process of ball milling, the crystal defects are crushed and the pore diameter is determined as 1.464 nm.According to the crystal data of HNS in PDF Card in Jade 9, the values of a, b, and c for HNS crystal are 2.2326 nm, 0.5571 nm, and 1.4667 nm, respectively.Thus, the pore diameter is the same as a single unit cell.There is no pore or there is a single cell pore.

Thermal Analysis.
Probing the kinetic and thermodynamic parameters was very important to master the thermolysis properties of explosives [18,19].Herein, DSC curves of purified and milling HNS collected at different heating rates were obtained and used to calculate the parameters shown in Figure 6.From Figure 6, the DSC curves are presented for 150∼400 ∘ C for better visualization.Both HNS samples are exothermic in the decomposition.These three DSC graphs show that the decomposition temperature and the heat output keep rising with the increase in heating rate from 5 to 20 ∘ C/min.The tendency is consistent with the equation presented [20].By comparing the curves in Figures 6(a) and 6(b), it can be observed that the variation of the peak temperature is not obvious in each curve at the same heating rate.Before the strong exothermic decomposition peak, a weak endothermic peak is found in each DSC curve which corresponds to the melting point of milling HNS and purified HNS, respectively.The maximal endothermic peak of milling HNS is 319.31 ∘ C lower than purified HNS (319.91 ∘ C) at a heating rate of 5 ∘ C/min.The main reason for this phenomenon is that the solid surface energy could reach the solid-liquid equilibrium at a lower temperature, and the smaller particle with lower lattice energy would lose the crystal structure at lower temperature.Thus, the milling HNS with fine particles has a lower melting point than purified HNS with big particles.The Kissinger equation [21] was enlisted to calculate the activation energy () and the fitting curve and results are shown in Figure 6(c).
From Figure 6(c),  of purified HNS and milling HNS is 179.61 kJ/mol and 190.04 kJ/mol, respectively.Compared with that of purified HNS,  of milling HNS increases by 10.43 kJ/mol, suggesting that the decomposition reaction of HNS after ball milling becomes difficult and the thermal stability of HNS is improved.

Impact and Short Duration Shock Initiation Sensitivity.
To investigate the safety performance of purified HNS and  milling HNS, tests of impact sensitivity and short duration shock initiation sensitivity were performed, and the results are presented in Table 1 and Figure 7. Table 1 shows experimental groups (EG) where the drop height ( 50 ) of milling HNS is higher than that of purified HNS.This occurs because the nanosize and fewer defects of particles can reduce the impact sensitivity.
In the in-line secured ignition system, booster charges were initiated by a flyer directly.When the flyer impacts the charge surface, a shock wave with high pressure and narrow pulse can detonate the charge.Thus, short duration shock initiation sensitivity is an important performance index of the explosive used in the slapper detonator.Herein, the input voltage and energy at 50% initiation or 100% initiation probability are tested and calculated, and the values are summarized in Figure 7.The test results of HNS-IV, which are obtained from solvent/nonsolvent recrystallization process, are also included for comparison.
From Figure 7, it can be found that the 50% and 100% initiation voltages of milling HNS are 1205 V and 1407 V, respectively, which are lower by over 100 V than that of HNS-IV.Based on the energy function of capacitance (see (1)), the 100% initiation energy can be calculated.The 100% initiation energy of milling HNS samples is about 20% lower than HNS-IV, or the 100% initiation energy of milling HNS samples has a lower energy by about 20% than HNS-IV, indicating the higher short duration shock initiation sensitivity.
where  is the initiation energy, J;  is the capacitance, pF;  is the voltage, V.The flyer initiation of secondary explosive can be demonstrated by a shock-detonation transition.The initiation mechanism can be interpreted from the "hot-spot" theory [22].Under the shock of flyer, the voids among HNS particles are subjected to adiabatic compression to increase the temperature.When the temperature of the "hot spot" exceeds the critical temperature, the "hot spot" forms.As is known, both the size of the "hot spot" and the reaction rate of explosive particles are dependent on the particle size of explosives.In this research, the milling HNS particles are sized at about 100 nm, and the nanovoids among particles can easily form the high temperature "hot spot" under the shock compression.Moreover, the nanoparticles have more surface atoms than micron and submicron particles, which can increase the chemical reaction rate.Thus, the milling HNS samples have higher short duration shock initiation sensitivity.

Conclusion
With the mechanochemical technology help, the thin HNS sheets with one-dimensional nanostructure have been changed into three-dimensional nanoparticles with a median diameter of 89.2 nm.Compared with the recrystallized process, this technology is better for the environment and has a lower cost due to the no organic solvent used in the milling process.The as-prepared HNS particles are insensitive under the thermal and impact stimulus ( 50 is 10.5 cm higher), but sensitive to short duration shock (the 100% initiation energy is about 20% lower).This means that the milling HNS with higher security is very suitable as the main ingredient in the booster in the EFI initiation system.

Figure 1 :
Figure 1: Schematic of the experimental arrangement used to test short duration shock initiation sensitivity.

Figure 2 :
Figure 2: SEM images of purified (a, b) and ground HNS (c, d) samples.

Figure 3 :
Figure 3: (a) Particle size distribution of milling HNS and (b) XRD spectrum of purified and milling HNS.

Figure 4 :
Figure 4: Schematic of the formation of milling HNS.

Figure 5 :
Figure 5: Adsorption and desorption curve of HNS prepared by ball milling.

Figure 7 :
Figure 7: Short duration shock initiation sensitivity of HNS samples.

Table 1 :
Impact sensitivity of purified HNS and milling HNS.