ARTICLE pubs.acs.org/IECR
Evaporation Crystallization of RDX by Ultrasonic Spray Jun-Woo Kim,† Moon-Soo Shin,† Jae-Kyeong Kim,† Hyoun-Soo Kim,‡ and Kee-Kahb Koo*,† † ‡
Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Korea Agency for Defense Development, Daejeon 305-600, Korea ABSTRACT: Cyclotrimethylene trinitramine (RDX) with size of 0.8 to 2.6 μm was produced by evaporation crystallization of RDX with ultrasonic spray. In the experimental range of the present work, the size of RDX crystals was found to be strongly affected by operating parameters including RDX concentration and furnace temperature. The size of RDX crystals was reduced as RDX concentration decreased and furnace temperature increased. The formation of random-shaped RDX particles due to agglomeration could be explained by coalescence between a crystal-containing droplet and a crystal or a crystal-containing droplet. In the present work, agglomeration was found to be controlled with injection of additives into starting solution: polyvinylpyrrolidone (PVP), which acted as a strong nucleation inhibitor in the crystallization of RDX from acetone, was very effective on the formation of spherical RDX particles. However, when the nucleation promoters such as polyoxyethylene 10 oleoyl ether (Brij 97) and oleylamine were added, highly agglomerated and random-shaped RDX particles were produced. Therefore, it can be concluded that the controlling onset time of nucleation is an important factor to improve crystal shape in evaporation crystallization by ultrasonic spray.
1. INTRODUCTION During last two decades, many efforts have been given into development of energetic materials with reduced sensitivity. New energetic compounds with insensitive property which is come from their molecular structures such as triamino trinitrobenzene (TATB), 1,1-diamino-2,2-dinitroethene (DADNE), and 7-amino-4,6-dinitrobenzofuroxan (ADNBF) have been synthesized. However, the high production cost and relatively weak performance of such explosives limit general applications.1,2 In parallel, the relationship between the sensitivity and the physical properties (e.g., crystal size, morphology, purity, and crystal defects, etc.) of high-energetic materials has been also extensively studied.38 In particular, crystal defects such as internal voids, dislocations, and inclusions are known to be directly related with the shock sensitivity of the explosives because they act as hot spots, which are specific regions that readily interact with the surrounding stimuli.3,5 Therefore, improving the crystal quality of widely utilized high energetic materials such as cyclotrimethylene trinitramine (RDX) and cyclotetramethylene tetranitramine (HMX) is an efficient alternative for development of explosives with reduced-sensitivity.9,10 In general, it is known that the shock sensitivity of high-energetic materials is reduced as crystal size decreases because the critical temperature to ignite an explosive increases significantly by decreasing the size of hot spots.4,6 Grinding and milling are general methods for size reduction of particles. However, those methods are not recommended for energetic materials due to the possibility of accidental ignition as well as the difficulty in manufacturing uniform submicrometersized particles.11 Damage on the crystal surface is another problem of physical grinding.1 Submicrometer crystals of highenergetic materials can be produced by the crystallization with nozzle-assisted drowning-out.12,13 However, filtration of crystal suspension is a very time-consuming process for submicrometerscaled particles. In addition, droplets violently atomized generally lead to agglomeration or coalescence. As a consequence, they r 2011 American Chemical Society
induce morphological problems such as obstruction of close packing, poor binder distribution, and bad crystal roundness. Supercritical fluids also have been applied to obtain ultrafine high-energetic materials.1,11,14,15 However, particle formation with supercritical fluids are undoubtedly uneconomical because of the requirement for ultrahigh pressure. This is a fatal problem for the production of relatively low-cost high-energetic materials such as RDX and HMX. Spray drying is widely utilized to manufacture submicrometeror nanosized particles, in part because the apparatus is simple and continuous and can be scaled easily for mass production.1620 Among various atomization techniques, ultrasonic spray is one of the favored methods because of its outstanding energy-efficiency, affordability, and the inherently low velocity of initial droplets.21 In recent years, spray drying has been employed for the particle formation of high-energetic materials. Kim et al. tried to introduce evaporation crystallization by ultrasonic spray into formation of submicrometer-sized RDX particles,22 and it was found that acetone is a suitable starting solvent. Recently, Spitzer et al. also reported that submicrometer-sized RDX crystals can be obtained by ultrasonic spray from a 1% RDX/acetone solution in a furnace at 90 °C.23 Qiu et al. reported that RDX-based nanocomposite consists of RDX crystals with size of 0.11 μm can be produced by ultrasonic spray from a RDX (5 wt %)/ binder (1 wt %)/acetone solution.2 In the present work, three different additives, polyvinylpyrrolidone (PVP), polyoxyethylene 10 oleoyl ether (Brij 97), and oleylamine, were selected. Those additives were shown to have a strong influence on onset time of nucleation of RDX and act as the habit modifiers of RDX on the contrary to general additives Received: June 20, 2011 Accepted: September 15, 2011 Revised: September 6, 2011 Published: September 15, 2011 12186
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Figure 1. Chemical structure of RDX.
Figure 3. Schematic diagram of experimental setup for evaporation crystallization with ultrasonic spray.
Figure 2. Chemical structures of additives used in the experiments: (a) PVP; (b) Brij 97; and (c) oleylamine.
for the physical property modifiers and the sacrificial materials to introduce various nanostructures.16,2430 In the RDX/acetone system, PVP was found to act as a nucleation inhibitor and Brij 97 and oleylamine as nucleation promoters. In this context, the aims of this work were to investigate (i) the effect of additives on onset time of nucleation and (ii) the influence of onset time of nucleation on crystallization of RDX along with (iii) the effect of the typical experimental variables such as furnace temperature and solute concentration on the evaporation crystallization of RDX by ultrasonic spray.
2. EXPERIMENTAL SECTION 2.1. Materials. Conventional grade RDX (99.9%, Hanwha Co., Korea, Figure 1) were used without further purification. Commercial grade acetone (99+%, Aldrich, USA) was used as a solvent for RDX. PVP K30 (molecular weight ≈40000; Junsei Chemical, Japan), Brij 97 (Aldrich, USA), and oleylamine (70+ %, Aldrich, USA) were used as additives, and their chemical structures are depicted in Figure 2. 2.2. Experiments. 2.2.1. Evaporation Crystallization by Ultrasonic Spray. Figure 3 represents a schematic experimental setup for evaporation crystallization of RDX by ultrasonic spray. The main equipment consists of an ultrasonic nebulizer with a 1.7 MHz ultrasonic transducer for formation of microdroplets from RDX/acetone solution, an electric furnace for evaporation of acetone, a bag filter for collection of RDX particles, and a waterjet aspirator pump (Jeio Tech, VE-11, Korea) for transportation of acetone vapor and air mixture. A 100 g of starting solution was added in the ultrasonic nebulizer (a width of 5 cm, a length of 25.2 cm, and a height of
10 cm) which is made of polytetrafluoroethylene (PTFE). The height of solution was maintained at 1 cm. In this setup, the volume generation rate of RDX/acetone aerosol was found to be about 10 mL/min. The temperature of the solution was maintained at 25 °C by circulating cooling water around the container. The electric furnace was equipped with an 80 cm long quartz tube reactor with an internal diameter of 2.8 cm and an actual heating length of 50 cm. A water-jet aspirator was served to transport the aerosol by controlling the pressure in the entire apparatus. The pressure reduction by an aspirator is dependent on the vapor pressure of water, which is a function of water temperature and the residual content of acetone. Therefore, water inside the tank of the aspirator pump was continuously drained and replaced with fresh water with 25 °C for the cooling and remove of the residual acetone. To investigate the effect of RDX concentration, experiments with RDX concentrations of 0.54 wt % were carried out at the furnace temperature of 175 °C. The effect of furnace temperature was also investigated at temperature ranging from 75 to 175 °C. In those experiments, RDX concentration was fixed with 1 wt %. The effect of onset temperature of nucleation was investigated from evaporation crystallization with additives: PVP, Brij 97, and oleylamine. Solutions of 1 wt % RDX/acetone with each additive of 100 ppm were prepared, and ultrasonic spray evaporation crystallization was carried out at furnace temperatures of 175 °C. 2.2.2. Cooling Crystallization and Solubility Measurements. The cooling crystallization and solubility measurement were performed in a 100 mL double-jacketed glass crystallizer. The temperature of the crystallizer was controlled within (0.1 °C by a thermostat (Polyscience, model 9710, USA) and recorded every 5 s using a control software written in LabVIEW. Solution was stirred by a Teflon-coated magnetic bar (diameter: 60 mm). The agitation speed of 250 rpm was chosen to suspend crystals with no generation of bubbles due to vortex formation during crystallization. Number based chord length distribution of RDX crystals was monitored by focused beam reflective measurement (FBRM; Lasentec, S400A, USA) during the entire crystallization process. FBRM is useful to detect the onset of nucleation and 12187
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Industrial & Engineering Chemistry Research Table 1. Measured Solubilities of RDX in Acetone with Additives at 30 °C additive (100 ppm)
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Table 2. Surface Tensions of Acetone with Additives at 26 °C
solubility (g/100 g solvent)
sample
surface tension (mN/m)
no additive
9.21
pure acetone
22.68 ( 0.13
PVP Brij 97
9.01 9.15
1 wt % RDX/acetone solution 0.01 wt % PVP/acetone solution
22.83 ( 0.16 22.53 ( 0.04
oleylamine
9.08
0.01 wt % Brij 97/acetone solution
22.47 ( 0.13
0.01 wt % oleylamine/acetone solution
22.59 ( 0.18
Figure 4. Effect of additives on maximum allowable undercooling in the cooling crystallization of RDX from acetone.
measure the relative crystal population, which is expressed in number of counts per unit time.3136 For the solubility measurement, RDX/acetone suspension was injected in 100 mL crystallizer and was maintained with 30 °C for 24 h. After the stabilization of RDX suspension over 2 h, the upper clear solution was decanted from the solution. The sample solution was carefully evaporated in a vacuum drying oven without rapid boiling. RDX solubility was calculated by the weight difference between initial solution and residual mass. Solubilities of RDX in acetone with 100 ppm additives were measured by the same procedure as described above, and measured data are given in Table 1. 2.3. Measurement of Maximum Allowable Undercooling. Some polymers or oligomers have been reported that have significant effects on the deposition of certain crystallizing unit; therefore, they can influence nucleation and/or crystal growth.3740 Therefore, in the present work, the effect of some additives was investigated by cooling crystallization because it is difficult to observe directly the crystallization process in the tube reactor. At a temperature of 30 °C, saturated RDX/acetone solution with/ without 100 ppm additives were prepared by referring to the solubility data. The solutions were kept at a temperature of 10 °C higher than the saturation temperature, with a stabilization time of 30 min. The saturated solutions were cooled with a cooling rate of 0.25, 0.5, or 1.0 °C/min until primary nucleation was detected by FBRM from the rapid increase in the total number of counts. Finally, the maximum allowable undercooling, ΔTmax, which is the temperature difference between the saturation and nucleation onset temperature, was obtained. Figure 4 presents the effect of the additives and the cooling rate on ΔTmax. ΔTmax values with PVP were shown to increase by a factor of 2 compared with those without additive. Those results indicate that PVP acts as a strong nucleation
Figure 5. Example of an advanced image analysis technique for equivalent diameter estimation of agglomerated RDX particles.
inhibitor in the RDX/acetone crystallization system. However, ΔTmax values in the experiments with Brij 97 and oleylamine are shown to decrease. 2.4. Measurement of Surface Tension. Surface tension of starting solution is closely related to the size of droplet generated by ultrasonic spray.21,23,41,42 The surface tensions of RDX/ acetone solution with additives were measured 10 times by the € Du No€uy Ring method-based surface tensiometer (KRUSS, K100, Germany) at a temperature of 26 °C. There is no significant difference in surface tension of solutions with additives as listed in Table 2. 2.5. Product Characterization. 2.5.1. Estimation of Equivalent Diameter of Agglomerated RDX. RDX particles were characterized by scanning electron microscopy (SEM; FEI, Nova NanoSEM 230, USA). As submicrometer-sized RDX particles were very difficult to observe by SEM due to the decomposition of RDX crystals by the strong electron beam. In this work, an accelerating voltage of 2 kV and a magnification of 5000 were employed to prevent decomposition of RDX. However, it is extremely difficult to characterize the individual size and growth habit of crsytals in the original SEM images with 5000 magnification. Therefore, SEM images were digitally zoomed to 2, i.e., the magnification of the modified image files was 10000. The size of isolated spherical particles from the SEM image can be readily determined. However, it is very difficult to estimate the equivalent diameter of agglomerated crystals. In most studies, only elementary particles have been used for estimation of crystal size without consideration of agglomeration and coalescence.1,23 In the present study, an advanced image analysis technique was 12188
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solute concentration, and the coalescence frequency among droplets and crystals. The mean size of droplet produced by an ultrasonic nebulizer may be expressed by Lang’s equation,41 which represents the droplet size as a function of ultrasonic frequency, surface tension, and solution density !1=3 8πγ ð1Þ Ddroplet ¼ 0:34 Fsolution f 2
Figure 6. PXRD patterns of (a) α-RDX simulated and (b) conventional grade RDX. PXRD patterns of RDX prepared from 1.0 wt % RDX/ acetone solution (c) without additive and with 100 ppm (d) PVP, (e) oleylamine, and (f) Brij 97 at furnace temperature of 175 °C.
employed to resolve those problems. For example, let us take a look at a random-shaped RDX crystal consisting of four elementary spherical crystals like the SEM image shown in the center of Figure 5(a). The image of the crystal is subjected to image processing combined with an automatic thresholding technique and precise manual matching of contour to yield a monocolored image (Figure 5(b)). If each elementary spherical particle is oriented along a single axis, its volume, mass, and equivalent diameter can be estimated. Therefore, manual reconstruction of the object is carried out as shown in Figure 5(c). In this example, one small elementary spherical object is moved to the end of the other three elementary spherical objects, which are already oriented approximately along a single axis. Finally, a uniaxially oriented single object, which assumes a group of several 1-pixelhigh cylinders, is obtained. Thus, the equivalent diameter of a RDX crystal can be measured from the volume of the modified object, which is a sum of the volumes of the 1-pixel-high cylinders. At least 100 crystals were analyzed for determining average crystal size of RDX. 2.5.2. Polymorphism of RDX. In the solid state, RDX is known to have at least 5 polymorphs: α, β, γ, δ, and ε.4347 The α-form (orthorhombic, a = 13.182 Å, b = 11.574 Å, c = 10.709 Å, Z = 8, space group Pbca) is a stable form at ambient conditions.43 Therefore, the crystal structure of RDX obtained in the present experiment was closely investigated by using an X-ray diffractometer (Rigaku, MiniFlex, Japan) operated at 30 kV and 15 mA with graphite-monochromatized Cu Kα radiation (λ = 1.5418 Å). Powder X-ray diffraction (PXRD) data were collected using a rotating flat-plate sample holder over the 2θ range from 10° to 50° with step size of 0.02° at ambient conditions and scanning rate of 1.0 deg/min. As shown in Figure 6, all samples are found to be α-forms and additives have no influence on crystal structure of RDX. Here, the simulated XRD pattern of α-RDX was the one generated by using a software, the Reflex module, which is implemented in Materials Studio (Accelrys, version 5.5, USA), with crystallographic parameters. 2.6. Determination of Coalescence Frequency. The main factors determining the final crystal size are the droplet size, the
where Ddroplet is the mean diameter of a droplet, γ is the surface tension of the solution, Fsolution is the solution density, and f is the ultrasonic frequency. If the solvent is completely evaporated, the mean mass of the solute can be estimated by the following equation 1 3 πDdroplet Fsolution Csolute msolute, 0 ¼ 6 8πγ 3 1 ¼ 0:34 3 π ð2Þ Csolute 6 f2 where msolute,0 is the mean mass of the solute without coalescence, and Csolute is the concentration of the solute. Here only one crystal is assumed to be generated in each droplet.4850 Therefore, the mean equivalent diameter can be estimated from the mean mass of solute by the following equation !1=3 1 Fsolute π ð3Þ Dsolute, 0 ¼ 6 msolute, 0 where Dsolute,0 is the mean equivalent diameter of the crystal generated without coalescence, and Fsolute is the solute density. In the present study, the ultrasonic frequency was fixed, and the surface tensions of acetone, RDX/acetone solution, and PVP/ acetone solution were roughly the same as shown in Table 2. Therefore, according to eq 1, the initial droplet size is almost the same for all experimental conditions. Therefore, if there is no coalescence, the solute concentration is the most significant factor that affects the crystal size. However, it is obvious that coalescence occurs in the present experiments, and thus the final particle size is increased. When the coalescence occurs once, the mass of solute in a single droplet will be doubled. In this way, if droplets are continuously combined, the mean mass and equivalent diameter of the solute may be estimated by the following equations ! msolute, n ð4Þ n ¼ log2 msolute, 0
n ¼ log2
Dsolute, n Dsolute, 0
!3 ð5Þ
where n is the mean frequency of coalescence, msolute,n is the mean mass of the solute after n coalescences, and Dsolute,n is the mean equivalent diameter of the solute after n coalescences. Here mean mass and equivalent diameters of the solute are determined by image analysis as described in Section 2.5.1. The number of coalescence increases with increasing residence time of the liquid phase inside the droplet. Therefore, the evaporation rate, which is dependent on the pressure in the apparatus and furnace temperature, is a major factor that affects the frequency of 12189
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Figure 7. SEM images of RDX particles prepared from (a) 0.5 wt % and (b) 4.0 wt % RDX/acetone solution (the furnace temperature: 175 °C).
Figure 8. Effect of RDX concentration on (a) the crystal size and (b) the number of coalescences (the furnace temperature: 175 °C).
coalescence. Furthermore, because evaporation rate of the liquid phase is directly proportional to the furnace temperature, the furnace temperature seems to be the major variable that controls the frequency of coalescence. Detailed interpretation on those phenomena has been reported by Reuge et al.51
3. RESULTS AND DISCUSSION 3.1. Effect of RDX Concentration on Crystal Size and Habit. Figure 7 shows SEM images of RDX crystals produced from 0.5 and 4.0 wt % RDX/acetone solutions at furnace temperature of 175 °C. This figure clearly shows that the crystal size increases with increasing RDX concentration. The mean equivalent diameter of RDX crystals produced from 0.5 wt % RDX/acetone (Figure 7(a)) is 0.8 ( 0.3 μm, whereas that from 4.0 wt % RDX/ acetone (Figure 7(b)) is 2.6 ( 0.4 μm, which is much larger than the former. Figure 8 shows the influence of the RDX concentration on the crystal size (Figure 8(a)) and the frequency of coalescence (Figure 8(b)). The dotted, dashed, and dash-dotted curves indicate the predicted mean equivalent diameter of RDX with coalescence frequency of zero (DRDX,0), 5 (DRDX,5), and 10 (DRDX,10), respectively. Those values were calculated using eqs 2 to 5 with an ultrasonic frequency (f) of 1.7 MHz, a surface
tension (γ) of 22.7 mN/m, and a solute density (Fsolute) of 1.82 g/cm3. The mean frequency of coalescence was shown to increase with increasing RDX concentration as can be seen from Figure 8(b). This result indicates that there are some factors influencing on coalescence, except the furnace temperature as mentioned in Section 2.6. Reuge et al. have adequately described the origin of the coalescence of droplets suspected in ultrasonic spray.51 They concluded that gravity and drag forces, which depend on droplet mass and diameter, are significant factors influencing on coalescence in a tube reactor. Therefore, a heavier droplet due to a higher RDX concentration seems to increase the mean frequency of coalescence. 3.2. Effect of Furnace Temperature on Crystal Size and Habit. Figure 9 shows SEM images of RDX particles produced from 1.0 wt % RDX/acetone at furnace temperatures of 75 and 175 °C. The mean equivalent diameter and coalescence frequency for the former (Figure 9(a)) are 2.6 ( 0.4 μm and 8.6 ( 0.9, respectively, whereas those of the latter (Figure 9(b)) are 1.1 ( 0.3 μm and 5.0 ( 0.8, respectively. Figure 10 shows the effect of the furnace temperature on the crystal size (Figure 10(a)) and coalescence frequency (Figure 10(b)). These results confirm that a low furnace temperature increases coalescence frequency and subsequently crystal size. On the other hand, as the furnace 12190
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Figure 9. SEM images of RDX particles prepared from 1.0 wt % RDX/acetone solution at furnace temperatures of (a) 75 °C and (b) 175 °C.
Figure 10. Effect of the furnace temperature on (a) the crystal size and (b) the number of coalescences (1.0 wt % RDX/acetone solution).
temperature increases, residence time of liquid phase decreases due to that evaporation rate of liquid phase increases. Therefore, coalescence frequency decreases because solid particles cannot be combined by their collision. In addition, a large number of coalescence has an adverse effect on the roundness of the crystals. Figure 11 presents the schematic representations of the effect of coalescence on crystal size and habit. As represented in Figure 11(a), when a crystalcontaining droplet was merged with a crystal or a crystal-containing droplet, nonspherical particles seems to be formed. On the other hand, coalescence between droplets may be not responsible for the formation of nonspherical shaped crystals (Figure 11(b)). 3.3. Influence of Onset Time of Nucleation on Crystal Size and Habit. Figure 12 shows SEM images of RDX particles produced from 1.0 wt % RDX/acetone solution with 100 ppm PVP, Brij 97, and oleylamine at furnace temperature of 175 °C. When Brij 97 and oleylamine, which act as a nucleation promoter, were added in starting solution, the mean equivalent diameter and coalescence frequency with Brij 97 (Figure 12(a)) are 2.9 ( 0.5 μm and 9.4 ( 1.2, respectively, and those with oleylamine (Figure 12(b)) are 1.5 ( 0.5 μm and 5.7 ( 0.7, respectively. Compared with the habit of RDX particles without additive (Figure 9(b)), Brij 97 and oleylamine have an adverse influence on roundness of RDX crystals.
Figure 11. Schematic representations of production of (a) randomshaped crystal and (b) spherical crystal due to difference of onset temperature of nucleation. 12191
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Figure 12. SEM images of RDX particles prepared from 1.0 wt % RDX/acetone solution with 100 ppm (a) Brij 97, (b) oleylamine, and (c) PVP at furnace temperature of 175 °C.
The mean equivalent diameter and coalescence frequency for RDX crystals produced with PVP, which acts as a nucleation inhibitor, are 1.2 ( 0.2 μm and 5.2 ( 0.5, respectively (Figure 12(c)). It is interesting to note that roundness of RDX was improved as can be seen from Figure 12(c). If the residence time of the liquid phase in the droplet is the same, the onset time of nucleation, which is strongly influenced by additives, is an important factor affecting the roundness of forming crystals. As mentioned in Section 3.2, random-shaped crystals are formed by coalescences of a crystal-containing droplet with RDX crystals or other crystal-containing droplets. However, the number of coalescences between droplets of solution seems to increase only when a PVP is employed, because the nucleation of RDX is largely delayed. As a result, the shape of RDX crystals was found to be greatly improved as shown in Figure 12(c).
4. CONCLUSIONS In the present work, RDX crystals with size ranging from 0.8 to 2.6 μm were produced using evaporation crystallization by ultrasonic spray. The crystal size of RDX crystals was found to reduce as RDX concentration decreased and furnace temperature increased. The number of random-shaped crystals, due to coalescence between a crystal-containing droplet and a crystal or a crystal-containing droplet, was found to be reduced with increasing the furnace temperature. The number of random-shaped crystals also slightly decreased with decreasing RDX concentration because of low gravity and drag force. It was found that PVP acts as a strong nucleation inhibitor in the cooling crystallization of RDX from acetone and shown that PVP obstructs effectively the formation of irregular-shaped crystals by delaying the onset time of nucleation. Those results suggest that a nucleation inhibitor could be utilized as a habit modifier in evaporation crystallization or pyrolysis by ultrasonic spray. On the other hand, additives act as nucleation promoters such as Brij 97 and oleylamine have an inverse influence on shape of RDX crystals. ’ AUTHOR INFORMATION Corresponding Author
*Phone: +82-2-705-8680. Fax: +82-2-711-0439. E-mail: koo@ sogang.ac.kr.
’ ACKNOWLEDGMENT This work was supported by Defense Acquisition Program Administration and Agency for Defense Development, a grant (20114010203090) from the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and a grant (M2009010025) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea. ’ REFERENCES (1) Stepanov, V.; Krasnoperov, L. N.; Elkina, I. B. Production of Nanocrystalline RDX by Rapid Expansion of Supercritical Solutions. Propellants, Explos., Pyrotech. 2005, 30, 178–183. (2) Qiu, H.; Stepanov, V.; Di Stasio, A. R.; Chou, T.; Lee, W. Y. RDX-based Nanocomposite Microparticles for Significantly Reduced Shock Sensitivity. J. Hazard. Mater. 2011, 185, 489–493. (3) Bowden, F. P.; Yoffe, Y. D. Initiation and Growth of Explosion in Liquids and Solids; Cambridge University Press: London, U.K., 1952. (4) Armstrong, R. W.; Coffey, C. S.; DeVost, V. F.; Elban, W. L. Crystal Size Dependence for Impact Initiation of Cyclotrimethylenetrinitramine Explosive. J. Appl. Phys. 1990, 68, 979–984. (5) Field, J. E. Hot Spot Ignition Mechanisms for Explosives. Acc. Chem. Res. 1992, 25, 489–496. (6) Tarver, C. M.; Chidester, S. K.; Nichols, A. L., III Critical Conditions for Impact- and Shock-Induced Hot Spots in Solid Explosives. J. Phys. Chem. 1996, 100, 5794–5799. (7) Doherty, R. M.; Watt, D. S. Relationship between RDX Properties and Sensitivity. Propellants, Explos., Pyrotech. 2008, 33, 4–13. (8) Shi, Y.; Brenner, D. W. Molecular Simulation of the Influence of Interface Faceting on the Shock Sensitivity of a Model Plastic Bonded Explosive. J. Phys. Chem. B 2008, 112, 14898–14904. (9) Kim, J.-W.; Kim, J.-K.; Kim, H.-S.; Koo, K.-K. Characterization of Liquid Inclusion of RDX Crystals with a Cooling Crystallization. Cryst. Growth Des. 2009, 9, 2700–2706. (10) Kim, J.-W.; Kim, J.-K.; Kim, H.-S.; Koo, K.-K. Application of Internal Seeding and Temperature Cycling for Reduction of Liquid Inclusion in the Crystallization of RDX. Org. Process Res. Dev. 2011, 15, 602–609. (11) Lee, B.-M.; Jeong, J.-S.; Lee, Y.-H.; Lee, B. C.; Kim, H.-S.; Kim, H.; Lee, Y.-W. Supercritical Antisolvent Micronization of Cyclotrimethylenetrinitramin: Influence of the Organic Solvent. Ind. Eng. Chem. Res. 2009, 48, 11162–11167. (12) Yang, G.; Nie, F.; Huang, H.; Zhao, L.; Pang, W. Preparation and Characterization of Nano-TATB Explosive. Propellants, Explos., Pyrotech. 2005, 31, 390–394. 12192
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’ NOTE ADDED AFTER ASAP PUBLICATION The version of this paper that was published online September 29, 2011, contained an error in Figure 2. The revised version was published October 7, 2011.
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