Quantitative Study on Crystal Defects Using the Relationship between

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Quantitative Study on Crystal Defects using Relationship between Crystallization Parameter and Thermal Analysis Kyehoon Kim, and Kwang-Joo Kim Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00453 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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Crystal Growth & Design

Quantitative Study on Crystal Defects using Relationship between Crystallization Parameter and Thermal Analysis Kyehoon Kim and Kwang-Joo Kim* Crystallization Process & Engineering Laboratory, Department of Chemical Engineering Hanbat National University, Yuseong-Gu, Daejeon 305-719, South Korea *Corresponding author

ABSTRACT: Defects of cyclotrimethylene trinitramine (RDX) crystals prepared by cooling crystallization in various mixed solvents were studied. A normal RDX and a reduced sensitivity RDX were compared with recrystallized RDX crystals in terms of thermal analysis by DSC, matching refractive index, scanning electron microscopy, bulk density, and gas chromatography. Activation energy was calculated by thermal analysis using Kissinger’s plot and correlated with supersaturation, bulk density, and inclusion fraction. The quality of the RDX crystallized in various solvents was examined by the activation energy, which is expressed as a function of the supersaturation. Formation of crystals with smooth and flawed faces can be interpreted by the relationship between supersaturation and activation energy, which enables discrimination between a reduced sensitivity RDX and normal RDX. In high supersaturation, nucleation of new layers occurs at the edges and corners of the crystal, and layered growth becomes dominant. The cracks are characterized by the overlapping of layers at the crystal faces.

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INTRODUCTION Cyclotrimethylene trinitramine (RDX) is an explosive compound that is used in the mining industry and for military applications. A good performance is achieved by loading highly explosives with a low sensitivity to minimize unintentional shock. RS-RDX (produced by SNPE, France) is considerably less sensitive compared with the normal RDX grades. In order to reduce the shock sensitivity of RDX, at first studies were mainly focused on the spheroidization and smoothness of the crystal surface 1-3. It was recognized that crystal defects such as inclusions, dislocations, cavities, etc., affect the sensitivity of the explosives, and studies on the dislocation defect of RDX were performed by Halfpenny et al. 4-6. Cavities inside RDX crystals are mainly filled with solution during crystallization. They are the macroscopic part of large crystal defects.7-9 Crystals grown in solution invariably entrap the mother liquor because of the dislocation of the structure and the adhesion of crystals 10. Friction sensitivity is influenced by surface roughness, while impact sensitivity is affected by defect (i.e. inclusion). A lower amount of defects is needed in order to make the explosive more stable during storage and transport. Crystallization in solution was studied to reduce defects in RDX crystal.11,12 Horst et al. studied thermodynamically the solvent effect.13 However, kinetic parameters of crystallization such as supersaturation also affect inclusion and morphology. For the analysis of defects, studies have been carried out with qualitative methods using microscopies14 and quantitative methods such as inclusion measurement, image analysis of inclusions, and bulk density.15-17 In this study, analysis of crystal defects using thermal analysis was undertaken. Thermal analysis is a useful technique for the characterization of explosives, and thermal decomposition studies of explosives have been reported.18,19 These thermal analysis techniques have the advantage of using a small sample amount and quickness, and they frequently yield sufficient information for the accurate determination of kinetic parameters for the reaction. In this work, the DSC (differential scanning calorimetry) technique and the Kissinger dynamic method were used to determine the kinetic parameters of the thermal decomposition of RDX. Activation energy of RDX crystals produced in various crystallization conditions was measured. At various solvents and cooling rates, the supersaturation of the crystallization process and crystal growth rate were measured along with measurement of the particle size, inclusion, density, and activation energy of the RDX crystals prepared. Bulk density, inclusion fraction, and supersaturation were correlated with the activation energy. EXPERIMENTAL SECTIONS Materials. A normal grade RDX (purity of 99.9%, HW-RDX) was supplied by Hanwha Co., Korea. HW-RDX is free from byproduct such as HMX. As a raw material, HW-RDX was used without further purification. Insensitive RDX produced by SNPE (France) was used as an RS-RDX for the comparison with HW-RDX. The solvents used, N-methylpyrrolidone (NMP), dimethylsulfoxide (DMSO), and γ-butyrolactone (BL), were of analytical purity grade and purchased from Sigma-Aldrich. Redistilled deionized water was used. The tripledistilled water was produced by distillation. Crystallization Apparatus. A double-jacketed crystallizer, 80 mm internal diameter and 100 mm long, was equipped with FBRM (focused beam reflectance measurement) and temperature controller. The crystallizer was made of glass with a rounded bottom. It was equipped with an agitator, three-blade propeller with 40 mm diameter driven by multivariable speed motor. Figure 1 shows the experimental apparatus. RDX solid was dissolved in solvent 2 ACS Paragon Plus Environment

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at above saturation temperature. The solution was cooled from the saturated temperature to 293.0 K at a constant cooling rate. Temperature was controlled by using a circulator (HTRC30, Jeio Tech.) with a programmable temperature controller. The stirring speed was 380 rpm. It was confirmed that the slurry was well mixed during crystallization. After crystallization, the crystals were filtered and then were analyzed by using optical microscopy with matching reflective index liquid, DSC, and SEM (scanning electron microscopy). The quantity of entrapped solvent was analyzed by gas chromatography (DS6200, Donam Instruments) after dissolution of the crystals in acetone. Bulk Density Measurement. The crystal apparent density measurement was performed using a flotation method. Crystals were immersed into a mixture of toluene and methylene iodide (CH2I2), and the mixture concentration was adjusted until the particles floated. The density of mixture can be tuned between 0.9 g/cm3 and 3.3 g/cm3. The accuracy for the mixture density measurements was very high at 0.0001 g/cm3, as confirmed using a PAAR densimeter using the vibrating tube principle. Microscopy using Refractive Index Liquid. Optical microscopy with matching refractive index allows observation of refractive index variations inside the particles. The principle of the technique is to immerse the particles in a liquid of matching refractive index to reduce the contrast on the particles and reveal the inside of the particles. Inclusion Fraction. The inclusion fraction was analyzed using gas chromatography. The composition was analyzed by an FID gas chromatograph (GC 6200, Donam Instrument) equipped with capillary columns (GC-8A capillary column ULBON HR-1, Shinwa Chemical Industries). Gas chromatography was calibrated for all solvents investigated in this study. During the crystallization, inclusions were entrapped in the crystal. The crystals were separated from solution by filtration, dried in an oven for removal of the solvent adhered to their surfaces, and then stored at a constant temperature of about 25 oC for over 5 h. Thus, inclusion composition is equilibrated at 25 oC. The inclusion fraction is defined as the ratio of inclusion mass to crystal mass. It was calculated by the ratio of the impurity content of crystals to the impurity content of inclusions in equilibrium at 25 oC.11 Determination of Heat of Decomposition. The determination of heats of decomposition of RDX was realized by means of a differential scanning calorimeter (DSC 50, Shimadzu) in the temperature range of 298–723 K under a dynamic nitrogen atmosphere (ca. 50 mL min-1). Sample masses were about 1.5 mg, and each sample was heated in hermetically sealed aluminum pans. Five different heating rates ranging from 2 to 20 K min-1 were established (Figure 2a). The DSC system was calibrated with indium (m.p.= 429.6 K; ∆Hfus=28.54 J/g) and zinc (m.p.= 692.6 K). The kinetic parameters of decomposition for RDX were determined using Kissinger’s method. The relevant equation is:18,19

ln = −

(1)

where φ is the heating rate and T is the peak temperature of a DSC scan at the rate. Values of ln(φ/T) were plotted against values of 1/T. A straight line through the data points 3 ACS Paragon Plus Environment


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was obtained by linear regression (Figure 2b). The activation energy, E , was determined from the slope of the curve using the following equation: E = Rdln⁄ !$dT "#

(2)

The method used in the analysis was based on DSC experiments in which the temperatures of the extrapolated onset of the thermal decomposition process and the temperatures of maximum heat flow were determined from the resulting measured curves for exothermic reactions. Characteristics of HW-RDX and RS-RDX Crystals. SEM photos, optical microscopy images, and Kissinger plot of HW-RDX and RS-RDX are provided in Figures 3 and 4. RDX particles of Run 2 (Figure 4) exhibit a typical crystal habit of RDX. Usually a solvent inclusion looks like a black point or a black area inside the RDX particle on an optical microscopy image with matching refractive index. The optical microscopy images with matching refractive index for RS-RDX and HW-RDX, respectively, are shown in Figures 3c and 3f. From these figures, it is difficult to conclude that RS-RDX crystals have smaller inclusions then HW-RDX crystals. However, HW-RDX particles exhibit more heterogeneities than RS-RDX particles. These heterogeneities result in an increase of roughness on the surface. These black areas look like small crystals with a refractive index differing from the RDX refractive index. Some of these small crystals seem to be located on the RDX particle surface. SEM photos were used to observe the particle surface of the RDX particles (Figures 3b and 3e) and confirm the high quality of the RDX crystals (Figure 3b) with the RS-RDX particles having more regular shapes than the HW-RDX particles. Kissinger plots of ln(ϕ/Tp2) against 1/Tp for RS-RDX and HW-RDX are shown in Figures 3a and 3d, respectively. Compared with HW-RDX, RS-RDX has larger activation energy indicating a high degree of thermal stability. The decomposition temperature measured for HW-RDX is relatively lower. From these measurements, the activation energies of HW-RDX and RS-RDX were 34 kcal/mol and 53 kcal/mol, respectively (Figures. 3a and 3d). Compared with SEM and micrographs, the activation energy is the most useful from a quantitative point of view for understanding defects. RESULTS AND DISCUSSION Crystallization Results. Cooling crystallization in three mixed solvents was carried out at various cooling rates. In this study, cooling rate was in the range of 0.2 to 2 K/min and the temperature was in the range of 293.0 to 348.0 K. Mass fraction of RDX feed was 0.25. The solvent/water ratio was set as 5.0. The crystals obtained in these runs were measured in terms of bulk density, inclusion fraction, and activation energy. The crystallization parameters such as supersaturation and growth rate were measured, and the operating conditions and characterization data for 19 runs are listed in Table 1. Average crystal size was measured as square-weighted mean chord length (L) according to time (t) every 5 seconds using FBRM. Crystal growth rate was determined by dL/dt (m/s). SEM photos and microscopy of RDX crystals obtained for various solvents and cooling rates are shown in Figure 4. At the same cooling rate (Runs 6, 15, and 19), the inclusion fraction was highest for the water-NMP solvent (Table 1). The macrosteps, which were mainly caused by crystal defects, mainly occurred in Runs 18 and 19, in contrast with the water-BL solvent of Runs 5 and 6 (Figure 4). All crystals were found to have the same octahedral shape, but the most pores were found on the surface of crystals in Run 19. This means that the solvent 4 ACS Paragon Plus Environment

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affects defects inside the crystals at the same cooling rate. The RDX particles of Runs 1-15 have more regular shapes than the RDX particles of Runs 16-19. Edges and crystal faces can be observed at all runs. The RDX particles of Run 10 exhibit a new feature on their surface with small crystals embedded on the surface of the RDX particles. The crystal habit of these small crystals is different from that of the RDX particles of Run 1. Especially, big pores on the surface of the crystals are exhibited in Run 19. To understand the solvent effect, the solubility of RDX in three solvent mixtures was measured, namely the solubilities of RDX in BL-water, NMP-water, and DMSO-water solvents20 (Figure 5). To procure the dissolution enthalpy of RDX crystals in the solvents, the ideal solution theory was used20, where x1 is the mole fraction of solute; ∆Hsol is the enthalpy of dissolution of RDX; and Tm is the melting temperature of RDX. ∆Hsol is equal to ∆Hfus for an ideal system, and ∆Hfus + ∆Hmix for nonideal systems. ln%# =

"∆'()* #

#

− +

(3)

The enthalpy of mixing, ∆Hmix, is a measure for the solute-solvent interaction, whereas the enthalpy of fusion ∆Hfus is solvent independent. Through the plot of ln x1 vs 1/T, the enthalpy of dissolution was calculated using its slope, d ln x1/dT-1, and its values are listed in Table 2. The heat of dissolution of RDX in all solvents studied was exothermic. It was found that ∆Hmix increases in the order: NMP > DMSO > BL. A large ∆Hmix shows a large solventsolute interaction, and this could have a large effect on the inclusions inside the crystals, which generate the defects inside the crystals. This supports previous results that the crystals obtained in solvent BL have less defects than those obtained in solvents NMP and DMSO.11,12 Effect of Cooling Rate. There are two reasons why the crystals have defects. One is the voids formed inside crystals, and the other is the structural defects during crystal growth. Those can include mother liquor or gas inside the crystals. The dislocation during growth is an important reason that the inclusions are formed. However, the formation of inclusions inside crystals can easily occur because of the impurity gradient in the surface during crystallization, which is a supersaturation effect induced by the concentration gradient in the boundary layer. As the cooling rate is a factor for determining supersaturation in cooling crystallization, it affects the inclusions. The effect of cooling rate on the activation energy (Figure 6) along with the SEM photos and optical microscopy images with matching refractive index of RDX crystals obtained by cooling crystallization at various cooling rates for various solvent mixtures (Figures 3 and 4) suggest that the activation energy of some RDX obtained in these experiments might be better than that of RS-RDX. In the microscope photos using the refraction liquid, the black spots indicate the points of solution inclusion. It can be seen that the black spots increase as the cooling rate increases. The activation energy decreases with cooling rate and can be affected by the kind of solvent. Activation energy values were in the order of BL> DMSO> NMP. Cracks and defects can be found inside the crystal, especially at the high cooling rate of DMSO and at all of the cooling rate range of NMP. The effect of the solvent is due to the different RDX solubility for the three solvents used. The heat of mixing of the three solvents is NMP> DMSO> BL as listed in Table 2. Therefore, the defects inside the crystals are determined by a kinetic property such as the cooling rate and a thermodynamic property such 5 ACS Paragon Plus Environment


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as the slope of the solubility curve. As the supersaturation can be controlled by adjusting the cooling rate, it is an important parameter for the control of defects in crystallization of RDX. Previous works have predicted that crystallization in DMSO and BL produced crystals with lower defects than in NMP through morphology predictions from calculations of attachment energies of solvents and RDX.21,22 But these could not explain defect difference of RDX crystals obtained in DMSO and BL. One possibility to affect the defect is to enhance face growth rate by causing a reduction in the interfacial tension. The interfacial energy effect is related to the influence of the solvent on the surface roughening which under certain circumstances may induce a change in the growth mechanism. RDX were crystallized by cooling. The cooling rates were controlled linearly as 0.2 to 2 K/min at a solvent/water ratio of 5.0. At the higher cooling rate, crystals had a tendency to form more pores and an irregular surface. The crystals should show growth dislocations during crystallization, and large sized pores were found inside the crystals. The pores formed in the crystal faces were grown in layered structures, which provides support that dislocations of the crystal lead to the formation of defects. Effect of Supersaturation. Supersaturation is the decisive driving force with respect to the kinetics of crystallization. Optimal supersaturation is a prerequisite for the economical production of crystals. Control of the actual supersaturation is mandatory for being able to exert a targeted influence on nucleation and growth processes. The relationship between ∆cmax and activation energy is shown in Figure 7. Activation energy decreased with increasing ∆cmax. The maximum supersaturation (△cmax) was calculated from the maximum metastable zone width (△Tmax) measured at a cooling rate.

∆cmax

 dc*   = ∆Tmax   dT 

(4)

where dc*/dT is the slope of the solubility curve. The metastable supersaturation was controlled by cooling rate in cooling crystallization. From a representation of the activation energy of RS-RDX and HW-RDX (Figure 7, dashed lines), it can be seen that, as the activation energy of RS-RDX was 53 kcal/mol, the maximum supersaturation of desensitized RDX should be 0.00204 or less; while, as the activation energy of HW-RDX was 34 kcal/mol, it can be produced at a maximum supersaturation of 0.0443, which is about 20 times higher than that of RS-RDX. The activation energy decreased with increasing supersaturation regardless of the type of solvent (Figure 7). In our study on cooling crystallization with a mixed solvent, the inclusion fraction depends mainly on cooling rate, which affects the supersaturation. Therefore, the defect formation can be characterized by means of controlling the supersaturation during the whole crystallization process.23 Despite a little scattering, a good correlation between supersaturation and activation energy was obtained. Inclusion Fraction and Activation Energy. From the effect of inclusion fraction on activation energy for RDX crystals (Figure 8), it can be concluded that inclusions are generated not only on the impacted surface or the adhered surface but also inside crystals. Inclusions in RDX crystals produced in the cooling crystallization were mostly layer inclusions, which are seen in crystals in parallel with crystal surfaces, as shown typically in Run 19 of Figure 4. The isolated inclusions, which are considered to be caused by direct 6 ACS Paragon Plus Environment

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mechanisms, were relatively few, but they were found in high supersaturation. Therefore, the inclusions could be mainly caused by adhesion and contact induced mechanisms. Previous research showed that the fraction of inclusions per crystal can be correlated with a single equation with supersaturation, regardless of the solvent.11 This suggests a general mechanism working on the formation of liquid inclusions, and a non-direct mechanism could be a possible mechanism. The probability of inclusion formation increases with an increase in growth rate, when the probability of attachment of small crystals and contact will be high. Bulk Density and Activation Energy. Defects inside crystals lead to low density of crystals. The relationship between bulk density and activation energy (Figure 9) shows that activation energy increased in accordance with increasing bulk density, which was decreased with increasing inclusion fraction. The contact points between bulk density and activation energy of HW-RDX and RS-RDX are shown in Figure 9, and the experimental results were found to be on the same line as these contacts. From these results, it is shown that the results of this study show high reliability and that crystal defects can be analyzed by activation energy. It was reported that a lower density led to a higher shock sensitivity.24 For samples with relatively high porosity, additional desensitization can be anticipated by decreasing porosity. Previous results indicate that activation energy could served as a criterion to determine sensitivity.25 Especially, it was reported that shock sensitivity was proportional to activation energy, and a good linear relationship between the calculated activation energies and the experimental impact sensitivity was found. By comparing the sensitivity data with the corresponding activation energy, it was shown that the smaller the activation energy, the more sensitive the explosives. Here it was found that supersaturation is related to activation energy, which indicates that supersaturation can be correlated to the sensitivity of explosives. The sensitivity difference for the RDX crystals in this study was fairly large, as was the calculated activation energy. It was found that the activation energies of RDX produced in BL and DMSO were higher than the activation energy of RS-RDX. The semi-logarithm relationship between the calculated activation energy and the experimental supersaturation shows that the relative magnitude of sensitivity is determined mainly by the activation energy of the thermal decomposition. To a certain extent, it can be used to predict the relative order of sensitivity. Some structural defects were believed to be generated somewhere on the crystal surfaces by adhesion or mechanical contacts.26 From these defects new micro-steps originate successively followed by the development of macro-steps by bunching. Although these micro-steps could not be seen, macro-steps could be observed. Faster, active macro-steps were frequently observed to overlap the preceding macro-steps, which were proceeding slowly. At this moment, the overlapping mother liquid was observed to be trapped under the overlapping step. Thus, many liquid inclusions were trapped two-dimensionally in parallel with crystal surfaces. Further, we must add another important experimental fact that every adhesion or contact did not always bring about the formation of inclusions. These results are in agreement with the relationship between supersaturation and activation energy (Figure 9). Roughness of Crystal Faces. SEM photos and microscopy images with refractive index liquid for defect and non-defect RDX crystals, which were prepared in various solvents and cooling rates, are shown in Figure 10. RDX crystals were produced by BL (a), DMSO (c), and NMP (e) at a cooling rate of 0.15 K/min and by BL (b), DMSO (d) and NMP (f) at a cooling rate of 15 K/min. 7 ACS Paragon Plus Environment


Many parameters affect the defect formation of RDX crystallization. As discussed previously, varying the process parameters can influence the shape of the crystals as well as the defect. The shape is mainly determined by the choice of solvent, but also the cooling rate at the same nonsolvent/solvent ratio has an effect on shape. The SEM observations of the crack surfaces have confirmed that the overall crack surface is along the 010 plane. However, the fracture surfaces of b, d, and f in Figure 10 are much rougher than those of a, c, and e, and many secondary cracks and subsurfaces are formed during the whole crack growth process. These secondary cracks as well as the subsurfaces appear as layered crystal growth. Most of these subsurfaces have been determined as other octahedral slip planes, as shown in Figure 10(b), (d), and (f). It seems as if crack formation is a competitive mechanism among those octahedral slip surfaces. In high supersaturation, crack increment is in the 011 plane for BL and NMP, but it is found in the 001 plane for DMSO. However, as the crack increment is along the 011 plane, the overall crack surfaces remain in the 011 plane. The above discussion shows that crystals with flat, smooth faces are grown when supersaturation is kept low. Because such growth proceeds slowly, the accidental formation of various defects can be minimized. The condition of low supersaturation can be achieved by the slow cooling of the solution, but real growth processes often involve interactions of different mechanisms and it is always difficult to predict the exact outcome of a given experiment. In general, crystals grow faster under higher supersaturation. When supersaturation is very low, surface nucleation is not possible and only spiral growth can take place. However, in high supersaturation, nucleation of new layers occurs at the edges and corners of the crystal and layer growth becomes dominant. Both spiral growth and layer growth are characterized by the spreading of layers at the crystal surface. CONCLUSIONS Quantitative formation on crystal defects was grasped using thermal analysis. The DSC technique and the Kissinger dynamic method were used to determine the kinetic parameters of the thermal decomposition of RDX. Activation energy of RDX crystals produced in various crystallization conditions was measured. At various solvents and cooling rates, the supersaturation of the crystallization process and crystal growth rate were measured along with measurement of the particle size, inclusion, density, and activation energy of the RDX crystals prepared. Bulk density, inclusion fraction, and supersaturation were correlated with the activation energy. Activation energy increased with increasing bulk density, which was decreased with increasing inclusion fraction. A good correlation between parameters and activation energy was obtained. Formation of crystals with smooth surfaces and flaws can be interpreted by the relationship between supersaturation and activation energy and enables discrimination between a reduced sensitivity RDX and a normal RDX. It was found that the activation energies of RDX produced in BL and DMSO were higher than the activation energy of RS-RDX. In high supersaturation, nucleation of new layers occurs at the edges and corners of the crystal and layer growth becomes dominant. The cracks are characterized by the overlapping of layers at the crystal surface. In very high supersaturation, crack increment is in the 011 plane for BL and DMSO, but it is found in the 001 plane for NMP. As the crack increment is along the 011 plane, the overall crack surfaces remain in the 011 plane. The formation of smooth faces and cracked faces can be predicted by activation energy, which can be controlled by supersaturation.

REFERENCES 8 ACS Paragon Plus Environment

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Table 1. Summary of experimental results of cooling crystallization of RDX in various solvent mixtures at a solvent/water ratio of 5.0 Run Solvent Saturation Cooling Growth Bulk Activation ∆cmax Inclusion temperature rate rate fraction density energy (g/g) (K/min) (m/s) (g/g) (g/cm3) (kcal/mol) (K) 1 BL 346 0.3 1.41x10-8 0.0007 0.0008 1.8226 61.15 2 BL 346 0.5 6.63x10-8 0.0011 0.0009 1.8208 58.80 3 BL 346 0.5 1.85x10-8 0.0009 0.0012 1.8018 55.16 -7 4 BL 346 1 2.12x10 0.0098 0.0152 1.8005 50.27 -7 5 BL 346 1 1.98x10 0.0108 0.0140 1.7990 49.47 -6 6 BL 346 2 2.37x10 0.0306 0.0317 1.7958 41.78 -8 7 DMSO 348 0.2 5.90x10 0.0024 0.0023 1.8009 55.09 -8 8 DMSO 348 0.2 3.89x10 0.0022 0.0008 1.8013 55.63 -8 9 DMSO 348 0.3 3.25x10 0.0022 0.0007 1.8008 55.04 -8 10 DMSO 348 0.3 9.23x10 0.0043 0.0038 1.8022 54.41 -8 11 DMSO 348 0.5 2.50x10 0.0013 0.0009 1.8009 54.52 -7 12 DMSO 348 0.5 6.60x10 0.0032 0.0035 1.7999 52.58 -7 13 DMSO 348 1 6.04x10 0.0098 0.0152 1.7942 43.90 -7 14 DMSO 348 1 7.73x10 0.0109 0.0117 1.7965 44.63 -6 15 DMSO 348 2 1.21x10 0.0114 0.0336 1.7939 37.42 -8 16 NMP 335 0.05 1.10x10 0.0692 0.0411 1.7912 34.89 -8 17 NMP 335 0.2 1.25x10 0.0684 0.0358 1.7876 31.84 -8 18 NMP 335 1 4.05x10 0.1151 0.0562 1.7865 25.28 -8 19 NMP 335 2 8.08x10 0.1374 0.0615 1.7875 24.40



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Table 2. Thermodynamic data for various solvent mixtures at a solvent/water ratio of 5.0. Solvent

∆Hsol(kJ/mol)

∆Hmix(kJ/mol)

d(ln x)/d(T-1)

γ-Butyrolactone+Water

30.4

-5.1

-3665.9

DMSO+Water

27.3

-8.2

-3284.5

N-Methyl Pyrrolidone(g) Water(g)

21.5

-14.0

-2592.7


Page 13 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. A batch crystallizer 4. Temperature recorder 7. Filtering apparatus 10. Image analysis system

2. Thermostated bath 5. Thermocouple 8. Aspirator 11. FBRM and PVM

3. Temperature controller 6. Agitator 9. Gas chromatography

Figure 1. Schematic diagram of the apparatus for crystallization.


2K/min 4K/min 6K/min 8K/min 10K/min

110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60

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TC1-BL2 Linear Fit of ln(f/Tm^2)

-10.0 -10.2

Equation

y = a + b*x

Adj. R-Square

0.98968 Value

-10.4 -10.6

ln(f/Tm^2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Heat Flow (mW)


ln(f/Tm^2)

Intercept

ln(f/Tm^2)

Slope

Standard Error

49.77768

3.08819

-30439.3155

1551.98795

-10.8 -11.0 -11.2 -11.4 -11.6 -11.8

0

50

100

150

200

250

300

0.00196

0.00197

o

0.00198

0.00199

0.00200

0.00201

0.00202

0.00203

-1

Temperature ( C)

1/Tm (K )

(a) (b) Figure 2. DSC curves of thermal decomposition of Run 2 samples at various heating rates (a), and Kissinger plot (b)


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RS-RDX(SNPE) Linear Fit of ln(f/Tm^2)

-9.6 -9.8

Equation

y = a + b*x

Adj. R-Square

0.98179 Value

-10.0

ln(f/Tm^2)

Intercept

ln(f/Tm^2)

Slope

Standard Error 47.0108

3.90876

-27087.29944

1976.20929

ln(f/Tm^2)

-10.2 -10.4 -10.6 -10.8 -11.0 -11.2 -11.4 0.00195

0.00196

0.00197

0.00198

0.00199

0.00200

0.00201

1/Tm (K^-1)

(a)

(b)

(c)

-9.4 Equation

-9.6

Adj. R-Squar

y = a + b* 0.99173 Value

-9.8

ln(¥‫ص‬/Tm^2) Intercept ln(¥‫ص‬/Tm^2) Slope

Standard Erro

16.84159

1.24642

-13958.7349

636.65107

-10.0 -10.2 2

ln (ϕ/Tm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-10.4 -10.6 -10.8 -11.0 -11.2 -11.4 0.00188

0.00192

0.00196

0.00200

0.00204

1/Tm

(d)

(e) (f) Figure 3. Kissinger plots((a) and (d)), SEM photos((b) and (e), and microscopy images using refractive index liquid((c) abd (f)) for RS-RDX and HW-RDX, respectively. 15 ACS Paragon Plus Environment


Figure 4. SEM photos and microscopy images for RDX prepared.


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0.40 0.35

BL NMP DMSO

0.30

x(mass fraction)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.25 0.20 0.15 0.10 0.05 0.00 260

280

300

320

340

360

380

T(K)

Figure 5. Solubility of RDX in BL-water, DMSO-water and NMP-water20


400


70

60

Activation energy (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50

40

30

20 DMSO BL

10

NMP 0 0.0

0.5

1.0

1.5

2.0

Cooling rate (K/min) Figure 6. Effect of cooling rate on activation energy in various solvents


2.5

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70

60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RS-RDX 50

40 HW-RDX 30

20

10

0 0.0001

DMSO BL NMP

0.001

0.01

0.1

Supersaruration (g/g)

Figure 7. Relationship between supersaturation and activation energy


1


70

60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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RS-RDX 50

40

HW-RDX

30

20

10

DMSO BL NMP

0 0.00

0.01

0.02

0.03

0.04

0.05

Inclusion fraction (g/g) Figure 8. Effect of inclusion fraction on activation energy


0.06

0.07

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70

60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RS-RDX 50

40

HW-RDX

30

20

10

DMSO BL NMP

RS-RDX

HW-RDX

0 1.780

1.785

1.790

1.795

1.800

3

Bulk density (g/cm )

Figure 9. Relationship between activation energy and bulk density


1.805


Figure 10. SEM photos and microscopy images with refractive index liquid for defect and non-defect RDX crystals: (a) and (b) for BL-water, (c) and (d) for DMSO-water, and (e) and (f) for NMP-water


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For Table of Contents Use Only

Quantitative Study on Crystal Defects using Relationship between Crystallization Parameter and Thermal Analysis Kyehoon Kim, Kwang-Joo Kim*

TOC graphic 70

60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RS-RDX 50

40

HW-RDX

30

20

10

0 1.780

DMSO BL NMP

1.785

RS-RDX

HW-RDX

1.790

1.795

1.800

1.805

3

Bulk density (g/cm )

Synopsis Activation energy of cyclotrimethylene trinitramine (RDX) crystals produced in various crystallization conditions was measured by Kissinger’s plot. At various solvents and cooling rates, the supersaturation and crystal growth rate were measured along with measurement of the particle size, inclusion, density, and activation energy of the RDX crystals prepared. Bulk density, inclusion fraction, and supersaturation were correlated with the activation energy.


Quantitative Study on Crystal Defects Using the Relationship between

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