Growth and Characterization of Î²-RDX Single-Crystal Particles - The

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Growth and Characterization of #-RDX Single Crystal Particles Chan Gao, Lin Yang, Yangyang Zeng, Xiangqi Wang, Chuanchao Zhang, Rucheng Dai, Zhongping Wang, Xianxu Zheng, and Zengming Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04285 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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The Journal of Physical Chemistry

Growth and Characterization of β-RDX Single Crystal Particles Chan Gao1, Lin Yang1, Yangyang Zeng2, Xiangqi Wang1, Chuanchao Zhang3, Rucheng Dai4, Zhongping Wang4, Xianxu Zheng2* and Zengming Zhang4, 5* 1. Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China; 2. Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, Sichuan, 621900,China 3. Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang, Sichuan, 621900, China 4. The Centre for Physical Experiments, University of Science and Technology of China, Hefei, Anhui 230026, China; 5. Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China Corresponding Authors：(Z.Z) [email protected]; (X. Z) [email protected].

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Abstract 1, 3, 5-trinitrohexahydro-s-triazine (RDX) single crystal particles crystallized in β phase have successfully been grown via sublimation-recrystallization. Synchrotron X-ray diffraction results confirm that the recrystallized products are β-RDX single crystals. Compared to the traditional solution deposition methods, the sublimation method used in this work breaks through the mass limitation during the growth of β phase RDX. The distribution of α- and β- RDX on different substrates reveals that the hydrophilic substrate is prone to grow the β-RDX single crystal and the hydrophobic substrate prefers α-RDX single crystal. The sublimation rate dominates the growth of pure β-RDX. The mechanisms of phase transition from β-RDX to α-RDX during the process of sublimation-recrystallization have been studied. The current results can promote the potential application of β-RDX in propellant, explosive and military fields.

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1. Introduction 1, 3, 5-trinitrohexahydro-s-triazine known as cyclonite or Research Department Explosive (RDX) is the most commonly used energetic material due to its properties of high energy, fast detonation velocity and better thermostability etc. Many researchers have focused their works on thermal decomposition of RDX.1-9 So far experimental and theoretical results have shown that RDX exhibits 5 types of phase structures including α, β, γ, δ and ε, and their molecular conformations are classified according to the positions of nitro groups with respect to triazine ring.10-22 The molecular conformation of the most stable α phase RDX at ambient condition is two of the nitro groups Axial to the triazine ring while the third in an Equatorial, which is often referred as the chair AAE conformer.10-15 Hultgren pointed out that the α-RDX crystallizes with an orthorhombic structure (space group Pbca),16 which has a Cs molecular symmetry and 8 RDX molecules including 168 atoms in the unit, as verified by single-crystal neutron-diffraction.15-17 McCrone’s pioneer work obtained the needle-like β-polymorph RDX.18 The β-RDX is a metastable phase which can be prepared either by evaporation of the high-boiling RDX solutions 19 or deposition of RDX solution on a glass substrate20-21 or by the solid-solid phase transition near the melting point with heating at very slow rates.10 β-RDX is denoted as the AAA conformer (with all NO2 groups axial to the triazine ring).10,21,23 Millar and Pulham considered that there exist two types of interpenetrating lattices in the packing of the molecules for β-RDX; these two lattices were similar to each other but exhibiting small difference with respect to the angle between the plane of the C-N-C ring atoms 3



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and the corresponding N-N bond. Molecules within each of these lattices are linked with the weak C-H…O intermolecular interactions.21 Karpowicz et.al compared the molecular structure of RDX in the vapor, solution and solid phases through infrared spectra.24 They suggested that the structural symmetry of β-RDX crystal were similar to that of RDX in vapor state. The vapor RDX molecules have a C3v symmetry, which is the same as RDX molecules in solution, but different from α-RDX in solid. However, Torres et.al reported that there exist different Raman activities and depolarizations between the β- and the vapor RDX.20 β-RDX can transform to α phase due to a very close Gibbs free energy between both structures. Some previous investigations and our own work have revealed that β-RDX can easily transform to α-RDX by touching with α-RDX, mechanical contact, temperature, concentration, specific solvent, stress and pressure.19,25 The facile transformation feature could provide the possible application in defense and military etc. The γ-RDX can be obtained from α-RDX at the pressure near 4.0 GPa and retain under 18.8 GPa then transforms to the δ-RDX with continuously uploading compression.26 ε-RDX is a high temperature and high pressure (HT-HP) phase, which can be produced at HT-HP (489 K, 4.2 GPa).11 Moreover, our recent work found other two types of phases under higher pressure, the relevant results will be published other where.

Single crystals with high quality possess the inherent physical properties of the materials.25,27,28 The quality and morphology of explosive crystals determine the properties of energetic materials, such as explosion, combustion, safety, sensitivity and so on.29 Single crystal explosives have usually less defects compared to 4


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polycrystalline explosives30-32, and are hence desirable for many applications, for example, for studying their intrinsic physical properties. An effective method to grow explosive single crystals is thus important. Karpowicz et.al obtained the needle-like β-RDX along with a few plates of α-RDX from certain high-boiling solvents. In their work, however, the samples are too small and dispersed to be detected by Raman or X-ray diffraction methods.19 Goldberg et al. investigated the crystal growth of β-RDX by drop cast crystallization method from a broad range of solvents, such as acetone, tetrahydrofuran (THF), nitromethane and dimethylsulfoxide (DMSO). However, there is no more evidence that the β-RDX obtained by this method is single crystal.25 Torres et al. also prepared β-RDX using the acetonitrile solution deposition method, they found that the transition from β-RDX to α-RDX is induced by the amount of RDX deposited. The morphology of β-RDX resembles as an island, as well as scattered particles.20 Infante-Castillo et al. demonstrated that α-RDX can undergo a solid-solid phase transition to form β-RDX by heating, but without detail information of β-RDX.10 Recently, Figueroa-Navedo et al. used spin coating method to grow α- and β-RDX polymorphs. Although they can grow RDX films predominated by the β phase by controlling the spin coating speed, the pure β-RDX film is difficult to obtain.14

In general, it is necessary and important to find a safe, simple and effective method to generate high-quality β-RDX crystals. In this work, a sublimation-recrystallization process has been employed, which is a new, effective, low-cost, facile and secure method to grow pure β-RDX single crystal particles. The growth mechanism and influence from substrates are also studied. 5



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2. Experimental Section RDX crystals（99.9% purity）from China Academy of Engineering Physics were used without any further purification. Quartz plates, silicon wafers, polytetrafluoroethylene and stainless steel plates were used as substrates for the growth of RDX crystals. The substrates were cleaned by ethanol and acetone. Two RDX particles (about 0.12mg) were mounted on the heating stage (TS1500) and then heated from 25 °C to 200 °C at a heating rate of 20 °C/min. The console of the heating stage is composed by a DC power and a solid heater (TMS94/1500). A confocal microscope Raman spectrometer system (equipped with Princeton Instruments Acton SP2750 monochromator and Princeton Instruments Pixis 100-BR multichannel CCD) was used for in situ analysis. Argon ions laser of 514.5 nm was utilized to excite the sample at a power of 40 mW, and the Raman spectra were collected in a reﬂection geometry. Morphology of α- and β-RDX was observed by using a field emission scanning electron microscope (SEM, Hitachi SU8100). The thermal properties of RDX were studied by Discovery series Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analyzer (TGA). Synchrotron X-ray diffraction measurements were performed at beam line 14 B1 with the X-ray beams (1.2438Å, 10keV) and a focus spot size of 200 µm × 200 µm, and beam line 15 U1 with the X-ray beams (0.6199 Å, 20 keV ) and a focus spot size of 3-4µm of Shanghai Synchrotron Radiation Facility (SSRF) in China, respectively.

3. Results and discussion 3.1 Sublimation of α-RDX Below Melting Temperature 6


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Figure 1 shows the thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of α-RDX. RDX initially loses its weight at 160 °C while there is no any endothermic or exothermic peak below the melting temperature of 206 °C in the corresponding DSC, Fig. 2b; with increasing temperature, the weight loss becomes significant and a nearly 100% weight loss has been observed at about 234 °C, which is related to the decomposition of RDX. Time-dependent TGA (inset of Fig. 1a) shows about 3.5% weight loss during a continuous heating at 160 °C for 7 hours, indicating the sublimation process of RDX before the decomposition reaction.

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Figure 1 Thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) curves of RDX crystals.

Figure 2 presents the typical temperature dependent Raman spectra of RDX. The Raman peaks gradually lose their intensities and broaden and merge with increasing temperature as shown in Fig. 2(a). The temperature dependence of some selected Raman modes is shown in Fig. 2(b). Obviously, all vibration amplitudes decrease from 120 °C to 140 °C. There exists a larger attenuation rate for the intensities from molecular bending mode at 108 cm-1, C-N stretching modes at 853 and 890 cm-1, and N-N stretching mode at 1279 cm-1 between120 °C and 140 °C in Fig. 2(b). The largest attenuation rate for molecular bending mode is attributed to the weakened the van der Walls force between molecules and the C-H…O intermolecular interaction due to the strong thermal motion of molecules at high temperature. With further increase of temperature, the top layer molecules become active enough to break the binding force to sublimate. The weaken force indicates the decrease of force constant and induces the smaller vibration frequency relative to the molecular bending mode as described 8


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in Eq.(1). The result is consistent with the red shifts of Raman peaks with increasing temperature as seen in Fig.S1 of Supporting Information (SI). ω

(1)

where ω is the vibration frequency, k is the force constant and m is the effective mass of oscillator.

Figure 2 (a) The temperature dependent Raman spectra of RDX. Top left: photomicrographs of α-RDX at different temperatures. Top right: fluorescence

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background of RDX at 200 °C for 3 and 4 minutes, respectively. (b) Dependence of intensities of Raman modes on heating temperatures for RDX.

The top right inset of Fig. 2 shows the fluorescence background of RDX after retaining 3 minutes and 4 minutes at 200°C, respectively. The inset reveals that Raman vibrations (from 515 nm to 620 nm) decrease dramatically with increasing holding time at this temperature, which can be attributed to the melting and decomposition of RDX at a higher rate. The Raman scattering background may be due to decomposition-induced fluorescence, as discussed previously by Castillo.10 It can also be seen that some holes appear at the surface of RDX particle with the elevating temperature. This is in agreement with the TGA results, Fig.1. It can be concluded that there exists a sublimation process of RDX particles before the melting and decomposition.

3.2 Recrystallization of β-RDX from Thermal Sublimation Deposition

Figure 3 shows schematically the sublimation-recrystallization process used in this work for the growth of β-RDX crystals. Obviously, a rational control of heating temperature is very important, which should be lower than the melting or thermal decomposition temperature, but high enough to facilitate the sublimation process. The RDX molecules in gas phase can be deposited on the substrate to crystallize into high quality β-RDX crystals. Moving the substrate along one direction enables the formation of pure β-RDX crystals within large area. Figure 4 shows the real-time Raman spectra of the recrystallized products on the quartz substrate. β-RDX is 10


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produced at 160 °C accompanied with the appearance of the fingerprint C-H vibration modes at 2991 cm-1 and 3080 cm-1 in its Raman spectrum. After maintaining temperature at 190 °C for 10 minutes, β-RDX transforms to α-RDX although it remains in β phase before 5 minutes.

Figure 3 Schematic diagram of recrystallization from sublimating RDX.

Figure 4 Raman spectra of deposition products with different holding time at different heating temperature.

As for the deposition from sublimating RDX molecules on the quartz substrate, β-RDX nucleation is easily generated due to the temperature difference between the heating sample and the substrate. A lower sublimating rate is required to 11



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grow larger β-RDX single crystal particle at lower evaporation temperature. Time-dependent Raman spectra are employed to monitor the phase structures of deposited products at different heating temperatures as shown in Fig. S2 of SI. It indicates that the recrystallization of β-RDX from the sublimation process at 160 °C always keeps its structure stability within 240 minutes persistent growth. Because of the low sublimation rate, the dispersion single-molecule in gas state touches the nucleation and grows in AAA conformation. Vladimiroff’s work verified that RDX molecular conformations in gas phase are consist of AAE of α phase and AAA of β phase

20,33-35

Moreover, some works also revealed that the β-RDX facilely transits to

α-RDX by touching with α-RDX or contacting with a tip of needle.19, 21, 25

With the further increase of temperature, the kinetic energy of gas molecule increases, consequently the sublimation rate is also enhanced. The stronger collisions between gas molecules and the β-RDX nuclei may make the β-RDX convert to α-RDX. On the other side, some clusters of RDX could be sublimated at higher temperature. The results demonstrate that the β-RDX transforms into α-RDX at higher heating temperatures with longer time, as shown in Fig.4. Therefore, the temperature and the depositing time are important for controlling the phase purity of β-RDX during the sublimation-recrystallization process.

A comprehensive analysis of the above-mentioned results indicates that, there are three stages in the process of sublimation-crystallization of RDX: (1) At lower temperature, such as: 160 °C，170 °C and 180 °C，β-RDX is generated at a slower 12


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sublimation rate and a lower substrate temperature, since the nucleation of metastable β-RDX is mainly taken place at the condensation process;10 (2) With further increasing temperature, such as at 190 °C, the strong molecular and cluster collision leads to β-RDX transition to α-RDX; (3) As the RDX on the thermal stage significantly reduced with long time sublimation, the sublimation rate is also slow. The products with same deposition duration (e.g. 5 minutes) can grow in β-RDX again at the final step with a little of residual RDX on the heating stage. It can be concluded that the sublimation rate plays a key role for the growth of pure β-RDX crystal particles.

3.3 Substrate Dependent Recrystallization of β-RDX

The structure of RDX from the sublimation-recrystallization process depends on not only the heating temperature and deposition time, but also the substrate. Table 1 shows the different amount of β-RDX and α-RDX on the different substrates at a deposition time of 5 minutes at 160 °C. The results are obtained by using Raman spectra along two perpendicular diameters of the sample region as shown in Fig.5. About 100 data points were collected to calculate the ratio of α and β phases on different substrates. The results indicate that the polytetrafluoroethylene (PTFE) substrate prefers the formation of α-RDX, while the silicon, quartz glass and stainless steel substrates favor the crystallization of β-RDX. The Raman scattering images provide similar results, as shown in Fig. S3 of SI. Fig.S4 of SI shows the water contact angle with different substrate surfaces. PTFE substrate is hydrophobic due to 13



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no active groups. When the sublimated molecular contacts with the surface of PTFE, the surface hydrophobic groups may reject the nitro groups of the vapor RDX, and leads one nitro group to turn a little angle to generate the AAE conformer, instead of AAA conformer of β-RDX. The conformers of α- and β-RDX are displayed in Fig. S5 of SI. On the contrary, the hydrophilic material such as silicon, quartz glass and stainless substrate may prefer the growth of the AAA conformer and generate the nucleation of β-RDX. The interaction between the hydrophilic groups of substrate and stronger polarity nitro groups promotes the third equatorial nitro group rotating an angle to generate the structure symmetric conformer of AAA.34, 36

Table 1 Content of α and β phase RDX on different substrates

Substrate

α phase (%)

β phase (%)

Quartz Plate (100)

0

100

Stainless Steel

2

98

Silicon Wafers (111)

5

95

Polytetrafluoroethylene

100

0

14


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Figure 5 Schematic diagram of content measuring for α- and β-RDX in the products.

3.4 Comparison of Raman Spectra of α and β Phases RDX

There exists some differences for Raman mode assignment for α- and β-RDX among different research groups.10, 20, 25, 37 Figure 6 shows Raman spectra of α- and β-RDX crystals in ambient conditions. As for the strong ring breathing mode, the Raman peak of β-RDX centered at 881cm-1 bluely shifts to 887 cm-1 in α-RDX. β-RDX has only one Raman peak at 1580 cm-1 assigned to the N-O stretching vibrations, while α-RDX exhibits three Raman peaks centered at 1543 cm-1,1575 cm-1 and 1599 cm-1, respectively. The AAA conformer of β-RDX indicates that β-RDX possesses a higher symmetry and three nitro groups are indistinguishable. The identical nitro groups result in one N-O stretching mode not three in α-RDX. These results demonstrate that the high quality β-RDX crystals are obtained by sublimation-recrystallization in current work. Three bands at 345 cm-1, 605 cm-1 and 669 cm-1 of α-RDX are 15



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associated with ring vibrations and assigned to twisting, rocking and bending of the ring, respectively. Moreover, the bands at 791 cm-1,860 cm-1 and 1034 cm-1 are assigned to C-N stretching vibrations; modes at 1391 cm-1 and 1437 cm-1 are related to CH2 group’s wagging and scissor vibrations; Raman peaks at 2909 cm-1 and 2950 cm-1 are corresponded to C-H stretching vibration along the axial orientation. All of the above Raman modes did not appeared in the β-RDX, which confirms that β-RDX owns high symmetry in the AAA conformation again.38 The vibrational modes and modes assignment of α- and β-RDX are listed in table 2. Compared with Dreger and Gupta’s works, some combination or overtone peaks in their works do not occur in this work. It is worth pointing out that the peak at 1507 cm-1 in β-RDX should be assigned to CH2 group scissor vibration not N-O stretching as proposed in ref. 37 by referring the vibration mode with same frequency in α-RDX.

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Figure 6 Raman spectra of α-RDX and β-RDX with region of (a) from 50 to 1100 cm-1, (b) from 1100 to 1680 cm-1 and (c) from 2700 to 3200 cm-1 at ambient conditions. 17



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Table 2 Frequencies of Raman Modes of α- and β-RDX mode

α-RDX this work

β-RDX

Ref. 39

this work

Ref. 37

freq.(cm-1)

freq.(cm-1) assign

freq.(cm-1) freq.(cm-1) assign

v1

3077

3076

CH st(eq)

3080

3081

C-H st

v2

3067

3067

HCH st

3071

3070

C-H st

v3

3002

3003

HCH st

2991

2992

C-H st

v4

2950

2949

CH st(ax)

v5

2907

2906

CH st(ax)or

1597

N-O st

1578

N-O st

1559

N-O st

1509

N-O st

1446

C-H skl

comb v6 v7

1594

1600

comb/OT

1595

O-N-O st(ax)

v8

1572

1573

O-N-O

1578

st(ax) v9

1541

1542

O-N-O st(eq)

v10

1535

comb/OT

v11

1516

comb

1507(CH2 sci)

v12

1508

1508

CH2 sci or

1444

comb 18


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v13

1475

comb/OT

1420

1421

C-H skl

1376

CH2 sci

v14

1458

1460

CH2 sci

v15

1435

1436

CH2 sci

1374

1427

comb

1344

v16

N-NO2(ax) st 10

v17

1422

1422

CH2 wag or

1314

N-NO2 st 10

comb v18

1388

1388

CH2 wag

1278

1273

N-O st

v19

1377

1377

CH2 tw

1265

1263

N-N st

v20

1346

1346

comb

1351

CH2 wag or

v21

1226 1214

1211

CH2 wag

1001

999

ring

comb v22 v23

1312

1334

comb

1309

CH2 tw,N-N st

v24

1277

1273

st/N-N st

CH2 tw,N-N

935

st(ax) v24'

1273

N-N st(ax),ONO st

v25

1254

1249

N-N st(ax)

v26

1237

1232

CH2 tw or 19


933/940

ring st


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comb v27

1220

1215

N-C st

v28

1033

1031

N-C st

1023

CH2 r

945

N-N st(eq)

v29 v30

947

881

880

ring breath

850

849

CH2 r

789

ring st/NO def

v31

920

CH2

r

or

comb v32

886

885

C-N st

756

755

v33

851

858

C-N st

734

733

848

N-N st,NO2

655

656

591

589

v34

C-N-C def

sci(ax) v35

791

788

C-N st,NO2 sci

ring

in

plane bending10

v36

759

757

ring

b,NO2

sci,or comb v37

741

739

N-NO2 u(ax)

502

504

478

480/473

or

comb v38

672

670

ring b

ring plane

20


in

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bending 10 v39

608

607

ring r

423

425

ring breath

v40

592

590

ring b

376

376

C-N-C def

588

ring tw

488

ring tw,NO2

v41 v42

490

248 229

228

sci

ring out of plane bending 10

v43

465

464

ring

217

b(fold),N-N st(eq) v44 v45

414

429

ring b(f)

415

ring

153

mode 10

b(flattening) v46

364

N-NC(2) u(ax)

v47

348

347

ring tw

v48

302

301

molecular st or

112

102

overtone v49

226

226

ring rot

96

92

v50

208

207

N-NC(2)

81

85

nmbrella(eq) v51

148

149

external

NO2 tor 40 21


76


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v52

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131

NO2 tor 40

52

v53

108

107

molecular b

40

v54

93

90

NO2 tor(ax)

30

Abbreviations:

st=stretch,

tw=twist,

r=rock,

b=bend,

u=umbrella,

wag=wag,

rot=rotation, sci=scissor, f=fold, tor=torsion, comb/OT=combination or overtone, ax=axial, eq=equatorial, skl=skeletal, def=deformation.

3.5 XRD of β Phase RDX Figure 7 presents the X-ray diffraction patterns of α- and β-RDX single crystals measured at beamlines 14 B1 and 15 U1 of SSRF. The focus spot size of X-ray (200µm × 200µm) in 14 B1 is bigger than the particle size of single crystal RDX (about 50µm), thus the X-ray diffraction pattern is contributed by three or four crystal particles as shown in Fig.7(a). Furthermore, with the smaller size of X-ray beam (3-4µm) in 15 U1, the diffraction patterns from single α-RDX particle and β-RDX are shown in Fig.7 (b) and (c), respectively. The bright rings are from the quartz substrate which means the β-RDX is single crystal particle. The β-RDX single crystal has an orthorhombic structure with the lattice parameter: a=16.6577 Å, b=7.1171 Å, c=13.5849 Å, V=1610.586 Å3. No combined Raman vibration modes are found in the Raman spectra of β-RDX and there are also no 2nd XRD diffraction peaks of β-RDX in the XRD, which indicates that the β-RDX belongs to well single crystal particles with high purity. Few XRD results of β-RDX have been reported so far. Hunter and Pulham et al. reported a simulated powder X-ray diffraction of β-RDX at 150 K.41 22


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The traditional preparing methods make the crystal nucleus growth too fast and lead to

crystal

defects

and

crystal

aggregation

growth

for

β-RDX.

So,

sublimation-recrystallization is an efficient and safe way to get β-RDX single crystal.

23



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Figure 7 (a) X-Ray diffraction patterns of single crystals of RDX with spot size of X-ray of 200µm in 14 B1 of SSRF; X-Ray diffraction patterns from (b) single α-RDX particle and (c) single β-RDX particle with spot size of X-ray about 4µm in 15 U1 of SSRF.

3.6 The morphology of β-RDX and α-RDX

Figure 8 shows the SEM images of α- and β-RDX single crystal particles. All samples are prepared by sublimation-recrystallization of RDX. The α-RDX crystals display as uniform rod shape and bamboo joint shape as seen in Figs. 8(a) and (b). The high temperature sublimation and longtime deposition favor the growth of α-RDX crystal particles with bamboo joint morphology. β-RDX crystals are usually diamond-shaped particles and short rods with two branches as shown in Figs. 8(c) and (d). Because of the higher temperature, the diamond-shaped particles are thicker than the others. Considering the growth conditions, the heating temperature and deposition time determine the shapes of crystal particles. These SEM images demonstrate that the 24


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α-RDX is denser and bigger than the β-RDX, and the α-RDX crystals tend to get together while β-RDX crystals are randomly dispersed. There are few evidences from literatures about the morphology of β-RDX crystals. Karpowicz et.al distinguished the dendritic β-RDX crystals from the plates of α-RDX by their opposite birefringence.19 Goldberg et.al obtained the different morphology of β-RDX using drop cast crystallization. The obtained α-RDX crystal surfaces are smooth with well-defined edges while the surfaces of β-RDX crystals are rough and stepped.25

Figure 8 SEM images of RDX crystal particles by recrystallized from sublimation with depositing time (a) 15 minutes at 190 °C, (b) 10 minutes at 190 °C for α phase; (c) 5 minutes at 180 °C and (d) 5 minutes at 170 °C for β phase.

Conclusion Single crystals of α-RDX and β-RDX were obtained by sublimation-recrystallization 25



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at a lower temperature below its melting point. This method breaks through the reported mass growth limitation and easily and safely generates pure β-RDX single crystal particles. Synchrotron X-ray diffraction confirmed the single crystal structure of β-RDX.

The sublimation temperature and deposition time can be modified to control the distribution of α-RDX and β-RDX in the recrystallized products. The lower sublimation temperature benefits the growth of β-RDX nucleation. With further increasing temperature and prolonging the depositing time the strong molecular and cluster collision leads to β-RDX transition to α-RDX. The results showed that the sublimation rate plays the predominating role for the growth of pure β-RDX crystal particles. The nucleation of β-RDX strongly depends on the hydrophilicity of substrate. The hydrophobic material, such as Polytetrafluoroethylene, is prone to produce α-RDX, on the contrary, the hydrophilic materials prefers to generate β-RDX.

Supporting Information The Supporting Information includes dependence of frequency shift of Raman modes on heating temperatures of Fig. S1, monitoring Raman spectra with increasing deposition time at 160 °C of Fig. S2, Raman imaging of α- and β-RDX of Fig. S3, the water contact angles on different substrate surfaces and the schematic diagram of molecular structure of α-RDX and β-RDX of Fig. S4.

Acknowledgements 26


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This

work

is

supported

by

Science

Challenging

Program

(Grant

No:

JCKY2016212A501), the National Natural Science Foundation of China (Grant Nos. 11304300 and 11404320), and 909 project of the China Academy of Engineering Physics (Grant No: 991912) and beamlines 14 B1 and 15 U1 of Shanghai Synchrotron Radiation Facility (SSRF).

27



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TOC Graphic

35



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Thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) of RDX crystals. 73x65mm (300 x 300 DPI)


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Thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) of RDX crystals. 71x66mm (300 x 300 DPI)



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The temperature dependent Raman spectra of RDX. Top left inset is photomicrographs of α-RDX at different temperatures. Top right inset is flurescence background of RDX after holding 3 and 4 minutes at 200 °C. 63x49mm (300 x 300 DPI)


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Dependence of Intensities of Raman modes on heating temperatures for RDX. 66x52mm (300 x 300 DPI)



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Schematic diagram of recrystallization from sublimating RDX. 82x38mm (96 x 96 DPI)


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Raman spectra of deposition products with different holding time at different heating temperature. 63x49mm (300 x 300 DPI)



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Schematic diagram of content measuring for α- and β-RDX in the products. 82x80mm (300 x 300 DPI)


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Raman spectra of α-RDX and β-RDX with region of (a) from 50 to 1100 cm-1, (b) from 1100 to 1680 cm-1 and (c) from 2700 to 3200 cm-1 at ambient conditions. 202x498mm (300 x 300 DPI)



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X-Ray diffraction patterns of single crystals of RDX with spot size of X-ray of 200µm in 14 B1 of SSRF 73x65mm (300 x 300 DPI)


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X-Ray diffraction patterns from single α-RDX particle with spot size of X-ray about 4µm in 15 U1 of SSRF. 71x72mm (150 x 150 DPI)



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X-Ray diffraction patterns from single β-RDX particle with spot size of X-ray about 4µm in 15 U1 of SSRF. 73x68mm (150 x 150 DPI)


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SEM images of RDX crystal particles by recrystallized from sublimation with depositing time (a) 15 minutes at 190 °C, (b) 10 minutes at 190 °C for α phase; (c) 5 minutes at 180 °C and (d) 5 minutes at 170 °C for β phase. 119x89mm (300 x 300 DPI)



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Table of Contents Image 33x13mm (300 x 300 DPI)


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Growth and Characterization of Î²-RDX Single-Crystal Particles - The

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