Smart Energetic Nanosized Co-Crystals: Exploring Fast Structure

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Smart Energetic Nano-Sized Co-Crystals: Exploring Fast Structure Formation and Decomposition Processes David Doblas, Martin Rosenthal, Manfred Burghammer, Dmitry Chernyshov, Denis Spitzer, and Dimitri A. Ivanov Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01425 • Publication Date (Web): 07 Dec 2015 Downloaded from http://pubs.acs.org on December 7, 2015

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Crystal Growth & Design

Smart Energetic Nano-Sized Co-Crystals: Exploring Fast Structure Formation and Decomposition David Doblas, Martin Rosenthal, Manfred Burghammer, Dmitry Chernyshov, Denis Spitzer and Dimitri A. Ivanov*

Dr. David Doblas, Dr. Dimitri A. Ivanov Institut de Sciences des Matériaux de Mulhouse, CNRS UMR 7361, 15 rue Jean Starcky, 68057 Mulhouse, France *E-mail: [email protected]

Dr. David Doblas, Dr. Denis Spitzer Institut Franco–Allemand de Recherches de Saint-Louis, Laboratoire des Nanomatériaux pour les Systèmes Sous Sollicitations Extremes, ISL/CNRS/UNISTRA, UMR 3208, 5 rue du Général Cassagnou, 68301 Saint-Louis, France

Dr. Martin Rosenthal, Dr. Manfred Burghammer, Dr. Dmitry Chernyshov European Synchrotron Radiation Facility, 6 rue Jules Horowitz, 38043 Grenoble, France

ACS Paragon Plus Environment

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KEYWORDS: energetic materials, nano-cocrystals of explosives, nanocalorimetry, nano-focus X-ray diffraction

ABSTRACT: The interest toward co-crystals of energetic materials explains by the fact that they can offer better thermodynamic stability and tunable sensitivity and detonation performance. In the present work, a combination of DSC, ultra-fast chip calorimetry, high-resolution X-ray powder diffraction and nano-focus X-ray diffraction was employed to investigate the thermal behavior and structure formation in nano-sized co-crystals of CL-20 with HMX and TNT prepared using Spray Flash Evaporation (SFE). The CL-20/HMX co-crystal does not reveal any thermal transitions up to the thermal decomposition. In contrast, CL-20/TNT exhibits an irreversible melting transition. Upon melting, it can rapidly crystallize on heating or, at a slower pace, at room temperature to form homo-crystals of γCL-20, the polymorph stable at high temperature. These observations constitute the first evidence of a CL-20 crystallization process, which occurs from the melt and not from solution. The solid-liquid phase separation occurring during heating of a CL-20/TNT melt may explain its complex thermal decomposition process as compared to that of CL-20/HMX: the main exothermic peak of decomposition can be assigned to that of a pure CL-20.


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Introduction

In the last years, there has been a continuously growing interest toward co-crystallization[1], which has become a hot topic in many fields ranging from pharmaceutics[2] to non-linear optics[3] and from ferroelectrics[4] to organic electronics.[5] In addition, co-crystallization has proven to have a significant potential for defense applications where development of insensitive highenergy explosives for modern weapon applications is targeted.[6] In this latter field, cocrystallization appears to be a particularly promising approach, as reported in a number of papers published over the last years.[7-14] It allows tuning the key properties of the materials including density, sensitivity, and detonation performance.[15] In addition, co-crystallization can offer materials with higher thermodynamic stability than the pristine compounds.[16] The development of energetic co-crystals can also significantly contribute to other research fields such as photonics and quantum dot applications, where there is a need for novel materials such as nanodiamonds.[17] As the latter are produced by detonation, they could take advantage of the intimate molecular mixtures of energetic materials in the nano-sized co-crystals to open new lines of applications. As far as the co-crystals’ fabrication methods are concerned, high throughput methods would be clearly preferential for the practical applications as compared to for example to simple cocrystallization from solution. In this respect, it is worth mentioning the development of spray drying[18] and ultrasonic spray-assisted electrostatic adsorption method[19] for preparation of binary co-crystals the sizes of which are in the submicron range. Recently, Spitzer et al. have employed the Spray Flash Evaporation (SFE) process to generate nano-co-crystals of explosive materials[20], the approach which was initially designed to elaborate nanoexplosives.[21] With the


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help of this method, different types of mixtures such as entirely crystalline composites, semicrystalline mixtures and sub-micron and nano-sized co-crystals have been produced, depending on the type of molecular interaction between the initial compounds. In this work, we focus on binary energetic co-crystals based on 2,4,6,8,10,12-hexanitro2,4,6,8,10,12-hexaazaisowurtzitane (CL-20). These materials were shown to have a high potential for lowering sensitivity and thereby enhancing safety of the resulting energetic materials.[13] In particular, we address an equimolar co-crystal of 2,4,6- trinitrotoluene (TNT) with CL-20, CL-20/TNT, and another with 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), CL-20/HMX, at a molar ratio of 2:1. A combination of advanced methods of physical characterization such as in-situ nanocalorimetry and synchrotron-based nano-focus X-ray diffraction are used among others to explore the fast structure formation processes in the cocrystals under conditions close to those of detonation, which can shed light on the decomposition mechanisms in these materials.

Experimental Section Preparation of co-crystals: The co-crystals were prepared by means of the Spray Flash Evaporation technique.[21,22] The process makes use of a solution where the pristine compounds are dissolved. By applying an overpressure to the solution, the superheated liquids are directed to an atomization chamber where the pressure is much lower, causing the “flash evaporation” of the solvent which induces crystallization of the solute.


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The pressure drop is accompanied also by a temperature drop, from the hot nozzle (160 ºC) to the cold spray outlet (-30 ºC) which prevents growth of nano-cocrystals. The final products are separated in a continuous way using a setup of parallel axial cyclones. The yield (%) of the process is 43.8 and 46.1 for CL-20/TNT and CL-20/HMX, respectively.

Combined Nanocalorimetry- Nano-focus X-ray scattering. The home-built nanocalorimetric accessory has been designed compatible with the sample environment at the synchrotron beamlines employing nano-focused X-ray beams. The nanocalorimetric experiments can be realized, thanks to the home-built controller capable of applying custom temperature programs to the heaters of the nanocalorimetric sensor and performing fast readout of the thermopile signal of the sensor. The controller is operational from a computer running under Windows. For experiments at a synchrotron, the controller operation can be performed distantly (e.g., via a Remote Desktop using an Ethernet connection). Importantly, in the setup, the sensor holder is physically separated from the controller. This feature allows making the X-ray scattering experiments in transmission with our setup. For the in-situ X-ray scattering measurements, the sensor holder with the nanocalorimetric sensor plugged in is precisely positioned with the help of a special sample stage, including a hexapod and an array of piezo-actuators (Cube). The sample position can be monitored using an on-axis optical microscope, exchangeable with an X-ray detector. Nano-beam X-ray scattering experiments have been carried out at the ID13 beamline of the European Synchrotron Radiation Facility (ESRF). Focusing of the X-ray beam down to below


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100nm is achieved with a Fresnel zone plate placed 98m downstream the undulator source. Sample positioning and rastering is achieved with a miniature hexapod for coarse sample positioning and alignment, while the high-resolution scanning of the sample was performed with the Cube. The nanobeam experiments were carried out using X-ray photons with an energy of 14.9keV. During high-resolution mapping in the nanobeam X-ray experiments, the 2D diffraction data were collected using a large-area integrating FreLon CCD detector. Several diffraction peaks of silver behenate and corundum were used to calibrate the norm of the scattering vector s (|s| = 2 sin(θ)/λ, where θ is the Bragg angle and λ- the wavelength) in the small and wide angle regions. The acquired data were reduced and corrected using protocols provided by the beamline. The data analysis including two dimensional integration and visualization was done using home build routines developed in the Igor Pro software package.

X-ray powder diffraction. Single-crystal diffraction was measured at the Swiss−Norwegian Beamlines of the ESRF. The Xray patterns were collected with a PILATUS 2M pixel area detector using a monochromatic beam at a wavelength of λ = 0.69411 Å, which was slit-collimated down to 100 × 100 µm2. The sample-to-detector distance and parameters of the detector were calibrated using a LaB6 NIST standard. The detector images were recorded by phi-scans in shutter-free mode with a 0.1 deg angular step. The data were pre-processed by SNBL Tool Box

[23]

and then by CrysAlis Pro

(Agilent Technologies, version 171.36.24). The data was refined with SHELX.[24]


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DSC measurements. A heat-flux DSC, Mettler-Toledo DSC 1, and a power compensated DSC, Perkin Elmer 8500, were used to characterize the energetic materials used in this paper. Heating rates employed were 5 and 10 K min−1, respectively. High-pressure crucibles DSC-M20 gold-plated crucibles of 20 microliters from Swissi were used in the Mettler-Toledo experiments due to the high pressure generated during the decomposition of the materials (the crucibles can stand up to 217 bars). The measurements at lower temperatures (i.e. below the decomposition) have been performed in a Perkin Elmer DSC using standard aluminum sample pans.

Polarized Optical Microscopy: The observations have been carried out using a Leica DM2500M microscope equipped with a Sony DXC-390P camera in reflection geometry. The camera employs a 1/3” type DSP 3-chip charge-coupled device (CCD) sensor to display the images.

Results and Discussion Morphological and structural characterization of the energetic materials. The samples of εCL-20, TNT and βHMX were firstly characterized with solid-state NMR and Raman spectroscopy. The acquired spectra confirmed the expected structure and purity of the


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materials; they are provided in Supporting Information. The morphology of the energetic materials was further examined with SEM (Figure 1).

Figure 1. SEM micrographs of pure explosives and co-crystals, (a) TNT, (b) εCL-20, (c) βHMX, (d) CL-20/TNT 1:1 and (e) CL-20/HMX 2:1. Particle size distribution of co-crystals (f) CL20/TNT and (g) CL-20/HMX. Notice the different mean sizes of co-crystal particles (100 and 58 nm for CL-20/TNT and CL-20/HMX, respectively) compared with pure explosives (in the range of microns). The micro-particles of TNT and εCL-20 clearly reveal the facets characteristic of single crystals whereas the ones of βHMX appear more heterogeneous in shape and size. However, it is clear that the shape of crystals per se is not always sufficient to conclude on their texture.


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Therefore, additional analyses have been performed on the synthesized micro-crystals using nano-focus X-ray diffraction. The substrates used to deposit the particles are the nanocalorimetric sensors that will be used further to explore fast structure formation processes in the co-crystals with nanocalorimetry and nano-focus X-ray diffraction. The home-built setup for performing combined nanocalorimetry / nano-focus X-ray scattering (cf. Figure 2a) developed in our group[25,26,27] allows obtaining simultaneously the thermodynamic[28] and structural information on nano-gram-sized samples such as micro-crystals of explosives. To make the home-built nanocalorimeter compatible with the environment of the nano-focus X-ray beamline, several severe constraints had to be met (cf. Figure 2b). More technical details on the combined measuring setup can be found in the experimental section of the paper.

Figure 2. (a) Front view of the experimental set-up for combined Nanocalorimetry and nanofocus X-ray scattering. (b) Side view of the nanocalorimetric sensor holder positioned at the working distance with an on-axis optical microscope and illuminated aperture shield put in place. The photo shows the geometrical constraints imposed on the nanocalorimetric sensor holder by the beamline environment.


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Figure 3 shows the so-called mesh scans that were acquired by scanning micro-particles of the pristine explosive materials in 2D with a nano-focus X-ray beam. To appreciate the crystal texture, selected regions of the diffractograms containing only one strong peak have been stripped out of the patterns and overlaid with the micrograph of the sample. It can be seen that the selected reflection exhibits invariable intensity over a significant fraction of the particles’ surface. This testifies that the studied particles have a pronounced single-crystalline texture.


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Figure 3. 2D scans performed with a nano-focus X-ray beam on micro-particles of CL-20 (top) and HMX (bottom). To create these images, the diffraction intensity corresponding to a selected reflection of the sample such as 110 peak of the εCL-20 phase and 120 reflection of the βHMX phase was chosen (cf. the insets of the top and bottom panels, respectively). The zone of interest with the selected reflection was cut out of each of the 2D nano-focus diffractograms and plotted at the corresponding position of the sample, which was scanned with an X-ray beam with a step of 2 µm. The micrographs of the samples deposited on XEN-39392 nanocalorimetric sensors are overlaid with the corresponding X-ray mesh scans. The uniformity of the X-ray intensity reveals the single-crystalline nature of the particle under study.

The binary co-crystals prepared by the Spray Flash Evaporation technique are shown in the middle row of Figure 1. They are much smaller than particles of the pure energetic materials


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studied here, and do not reveal any crystal habit. The formation of such morphology can be accounted for by harsh conditions of the co-crystals preparation during SFE; these conditions are obviously very different from solution crystallization used to produce such materials previously.[8,9] The average particle size was measured by non-invasive back scatter provided by the Zetasizer Nano ZSP from Malvern. The experiments were realized by measuring the light scattered after shining the sample with a He-Ne laser of 633 nm and applying an electric field in order to mobilize the particles. The co-crystals’ size histograms shown in the bottom row of Figure 1 allow concluding that the particles are in the nanometer range and that the CL-20/TNT particles (the most probable particle size is 100 nm) are somewhat bigger than those of the CL20/HMX (58 nm). X-ray Powder Diffraction. The structure of the co-crystals synthesized by the Spray Flash Evaporation process was characterized by high-resolution X-ray powder diffraction. Figure 4 displays the experimental diffraction curves of the compounds together with the results of modelling.


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Figure 4. X-Ray Powder Diffractograms of CL-20/TNT 1:1 (top) and CL-20/HMX 2:1 (bottom). In black solid lines are plotted the diffraction patterns described in the literature. Red solid lines show the experimental data measured at BM01A (ESRF). Blue solid lines show the difference between literature and experimental data. Black sticks represent the peak position of the pure compounds. In the case of CL-20/TNT, the peak position of εCL-20 are also displayed (red sticks).


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Interestingly, although for the case of CL-20/HMX 2:1 the experimental diffraction data agrees very nicely with the literature report given in ref.9, modelling the structure of CL-20/TNT was more problematic. Thus, it was found that in order to match the peak positions, the unit cell of the co-crystal has to be deformed noticeably along the a- and c-directions. The resulting unit cell parameters are the following: a= 9.6778±0.0007 Å, b=19.8053 ± 0.0015 Å and c=24.5140 ± 0.0019 Å. By comparing these parameters with the ones reported for a room-temperature measurement in ref.8, it can be seen that in our case the a- and c-parameters are contracted by 0.9 and 0.8%, respectively, with the overall crystal density being higher than the literature value by about 2%. According to the interpretation given in ref.8, packing along the b-parameter involves interactions between the molecules of CL-20 and TNT while interactions along the c-direction occur essentially between the adjacent CL-20 molecules. Therefore, it can be deduced that the Spray Flash Evaporation technique results in a structure distortion involving both the interactions between CL-20 and TNT molecules and those between the neighbouring CL-20 molecules. The temperature-dependent structural data provided in ref.8 show that the b-parameter is the most labile when one compares the measurement at the liquid-nitrogen and room temperatures as it exhibits the highest thermal expansion (2.53%). Therefore, the distortion of b due to the harsh preparation conditions would not be very surprising. However, the difference in the c-parameter is more striking as this parameter proved to be the most temperature independent. More experiments will be required to understand what could be the associated variation in the mutual orientation of the CL-20 molecules in the co-crystal unit cell. In addition to this finding, the analysis of the diffraction curve shows that it cannot be accounted by the co-crystal structure alone and requires the presence of a small fraction of a pure εCL-20 phase (3.11±0.82%). We speculate that despite the rapid pressure drop, the fast crystallization of εCL-20 cannot be fully


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avoided in a superheated jet directed to the atomization chamber resulting in formation of a small fraction of the CL-20 homo-crystals. Summarizing, the phase analysis performed by highresolution X-ray powder diffraction confirms that the high-throughput technique of SFE employed here is capable of producing almost pure phases of co-crystals. The tiny fraction of the εCL-20 phase observed in the CL-20/TNT co-crystal will not affect the main conclusions of the paper.

Thermal behavior of the co-crystals. The thermal properties of the co-crystals were studied with conventional DSC and nanocalorimetry using the heating rates of 10 ºC/min and 1000 ºC/s, respectively (Figure 5).


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Figure 5. (a) Conventional DSC heating ramp performed on CL-20/HMX. (b) Fast heating ramp of a CL-20/HMX micro-particle using fast chip calorimetry (c,d) State of the micro-particle before and after the experiment performed in (b), (e) Conventional DSC heating and cooling curves performed in CL-20/TNT co-crystal. (1) initial heating up to 160 ºC shows melting of the co-crystal structure immediately followed by crystallization of CL-20, (2) cooling from 160 ºC to room temperature giving no evidence of crystallization. (3) Second heating to the same


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temperature showing melting of TNT and the absence of the co-crystal melting, The inset (4) shows the decomposition peaks that can be assigned to that of CL-20 and TNT. (right) (f) Nanocalorimetric curve of a CL-20/TNT co-crystal and the corresponding morphology of the micro-particle before (g) and after (h) the experiment.

The CL-20/HMX co-crystal reveals only one strong exothermic peak corresponding to the thermal decomposition (cf. Figure 5a and Figure 5b). A close-up of the conventional DSC curve up to the decomposition peak is given in the inset in panel a) to show the absence of any noticeable thermal transitions. In the nanocalorimetric measurement, the curve is qualitatively similar to the DSC trace showing somehow a small instability of the baseline just before the decomposition. This can be explained by some wiggling of the micro-particle on the nanocalorimetric sensor due to the intensifying sample sublimation. The comparison of the conventional DSC and nanocalorimetric measurements reveals a displacement of the decomposition peak toward high temperatures, which can be accounted for by a difference of the heating rates by approximately 5,000 times. The absence of the melting point of the co-crystal before decomposition is in agreement with the data reported in ref.9. Such behavior is reminiscent of the pure CL-20 and therefore can be attributed to the composition of this cocrystal that is rich in CL-20. In addition, the structure can be further stabilized by strong hydrogen bonds reported previously.[9] The change of the particle morphology due to the thermal decomposition can be appreciated from the micrographs given in panels c) and d) of Figure 5. It can be seen that the decomposition is accompanied by a dramatic change of the micro-particle shape and that a solid carbonaceous residue stays on the sensor upon cooling to room temperature. The latter is typical of the CL-20 decomposition.[29]


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The CL-20/TNT co-crystal has a richer thermal behavior: it exhibits a main melting transition with an onset at 130.0 ºC (cf. curve 1 in panel e of Figure 5). This transition is immediately followed by an exothermic peak, the nature of which will be clarified further in the text. Apart from the main melting transition, one can clearly see the peak characteristic of the pure TNT melting, the presence of which can be explained by incomplete mixing of the TNT and CL-20 during the co-crystal fabrication. Once molten, the co-crystal does not recrystallize in the same form again upon cooling to 0 ºC (not shown here) (cf. curve 2 in panel e). Instead, one can observe only the enhanced melting peak characteristic of the pure TNT crystal (cf. curve 3 in panel e) showing that more TNT crystals have been formed from the melt during the lowtemperature isothermal segment of the DSC profile. The subsequent heating ramp (cf. curve 4 in panel e) exhibits again only melting of TNT (not shown on the figure) followed by a decomposition peak. The latter exhibits a complex shape, which may be ascribed to the decomposition processes of pure CL-20 (stronger peak) and that of TNT (weaker peak). The melting transition of the co-crystal is also nicely visible in the nanocalorimetric curve (cf. panel f of Figure 5). The shape of the decomposition peak is however different in the nanocalorimetric curve, which is probably due to fast evaporation of TNT at these high temperatures. The absence of TNT at the moment of decomposition on the nanocalorimetric sensor can also be inferred from a more compact morphology of the carbonaceous residue (cf. panel h of Figure 5) and also from the comparison of the micro-RAMAN spectra measured on the residues left by decomposition of the co-crystal and pure εCL-20 (Figure S2).

Crystallization behavior of CL-20/TNT.


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The interesting thermal behavior of the co-crystal in question prompted us to investigate it in some more detail. Figure 6 displays the micrographs of a droplet of the co-crystal brought to room temperature after a short-term heating to 150 ºC, i.e. above the melting point.

Figure 6. (top) Kinetics of the crystallization process of a molten single particle of CL-20/TNT co-crystal after quenching from 150°C to R.T. Experimental data have been fitted using Avrami equation. (bottom) POM micrographs at different states of the crystallization process.

The images recorded in polarized light allow visualizing the increase of the sample birefringence with time, which is caused by room-temperature crystallization. It can be seen that the


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crystallization is almost complete after about 2 hrs. The overall kinetics of crystallization can be quantified by modelling the integral light intensity with Avrami equation:

(

)

I (t ) = I (t0 ) + ( I (t ) − I (t0 ) ) 1 − exp ( − K (t − t0 ) n ) (1)

The results of the fit with Equation 1 give n=1.9 and K=5.2.10-4 (s-n). The value of the Avrami coefficient, which is close to two, can be interpreted in the case of athermal nucleation as a 2Dconfined growth, which may correspond to formation of flat lamellar-like crystals. Since it was found that the co-crystal does not recrystallize after melting and that TNT can crystallize only at much lower temperatures, the described kinetics should reflect exclusively formation of the CL20 crystals. Interestingly, some birefringence is present in the droplet at the very beginning of the process and therefore cannot be studied with this method due to its insufficient time resolution. As was mentioned before, in the DSC curves the melting peak of the co-crystal is immediately followed by a small exothermic peak, which can be due to the CL-20 crystallization.

Study of the fast stage of the structure formation in CL-20/TNT. In order to clarify what happens during the very early stage of the structure formation in the CL-20/TNT melt, a combination of nanocalorimetry and nano-focus X-ray diffraction has been employed. The methodology of the experiment and the results obtained are depicted in Figure 7.


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Figure 7. (top) Selected 1D-reduced WAXS profiles probing the structure of the CL-20/TNT cocrystal and its morphological evolution during heating. The two first patterns recorded after heating to 115 and 135 °C, respectively, show the characteristic crystalline phase of the cocrystal, while the following contain mainly crystals of γCL-20. (bottom) Selected 2D-WAXS patterns corresponding to the 1D-reduced profiles shown above. At the lowest temperature, the patterns show a quasi-powder diffraction distribution which evolves toward a pronounced grainy texture upon annealing to a progressively higher temperature.


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In this case, a small amount of co-crystal was heated from room temperature to an increasingly higher annealing temperature (Tann) at a rate of 2000 ºC/s. Upon dwelling for 10 milliseconds at Tann, the sample was cooled back to room temperature at the same rate and analyzed with nanofocus X-ray diffraction. The representative 2D X-ray patterns are given in the bottom row of Figure 7. It can be observed that for the annealing temperatures up to 140 ºC the 112 and 004 peaks of the co-crystal located at s= 0.139 and 0.161 Å-1, respectively, are visible in the diffractograms showing that the co-crystal is still present in the sample. However, when the annealing temperature increases, the co-crystal’ peaks start to disappear while some new peaks grow up. The analysis of the peaks’ position allows assigning the new peaks to the γ-phase of CL-20. In particular, in the chosen angular region one can see an increase in the amplitude of the 11-1 peak of the γ-phase, which is for example absent in εCL-20, the most stable polymorph at room temperature.[30] Also, another diffraction peak pertinent to γCL-20 is visible at the s-value of ca. 0.17 Å-1 (i.e., 111 peak). Therefore, one can suggest that during annealing of the co-crystal melt, the nuclei of the γCL-20 are generated and that they give rise to the growth of this polymorph, the formation of which is unusual at room-temperature.[31] Importantly, if there would be εCL-20 crystals forming in the melt they will not be converted to the γCL-20 at the annealing temperatures used in the experiment

[32]

and will therefore be observable. This shows

that even a very short dwelling time in the melt is sufficient to nucleate the γ-phase of CL-20 to the extent that overtakes the crystal growth at room temperature, as was shown above with the optical microscopy experiments. These observations probably constitute the first case when crystallization of CL-20 is observed from the melt and not from solution. The solid-liquid phase separation occurring during heating the melt of the CL-20/TNT cocrystal may explain its complex thermal decomposition process exhibiting two strong exothermic


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peaks (cf. curve 4 in panel e of Figure 5). The low-temperature peak is positioned rather close the decomposition peak of pure CL-20. Given that the weight fraction of CL-20 in the co-crystal is ca. 65.9%, the enthalpy of the first peak (3,500 J/g) is similar to what is expected for the pure CL-20 decomposition. In contrast, the presence of the second peak in the decomposition process is not typical of a pure CL-20. It can be thought of as being due to decomposition of TNT, however it occurs at a somewhat lower temperature than what is documented for the decomposition peak of pure TNT. This could mean that CL-20 triggers the decomposition process of TNT. The comparison of the decomposition processes in the two co-crystals studied in this work emphasizes the role of the solid-liquid phase separation occurring in the CL-20/TNT co-crystal melt on heating. The experiments show that in-situ exploration of the structure formation processes can be helpful in understanding the mechanisms of the decomposition behavior of energetic materials and therefore be used to optimize their performance.

Conclusions In summary, the structure and thermal behavior of binary nano-sized co-crystals of CL-20 with HMX and TNT prepared using Spray Flash Evaporation (SFE) was investigated. For the CL-20/TNT co-crystal, the SFE fabrication technique results in deformation of the crystalline lattice of the compound when compared to the same material crystallized from solution. A small fraction of εCL-20 phase is found to be present in the sample, which can result from rapid crystallization of the superheated jets. As far as the thermal behavior is concerned, the CL20/HMX co-crystal does not reveal any thermal transitions up to the thermal decomposition. In


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contrast, CL-20/TNT exhibits an irreversible melting transition. Upon melting, a fraction of CL20 rapidly crystallizes from the melt. This process occurs at a somewhat lower pace at room temperature to form homo-crystals of CL-20. To address the very onset of the crystallization process, a combination of in-situ nanocalorimetry and nano-focus X-ray diffraction was employed. It was observed that the rapidly-formed crystals correspond to the γCL-20 modification, which is stable at high temperature. This observation is probably the first study of crystallization of CL-20, which occurs from the melt and not from solution. The solid-liquid phase separation operating during heating of CL-20/TNT may explain its complex thermal decomposition process, the main exothermic peak of which can be assigned to that of a pure CL20 and the secondary peak- to that of a pure TNT.

Supporting Information This material is available free of charge at http://pubs.acs.org. 1

H NMR spectra of the pristine compounds and their chemical shifts (S1). Raman spectra of the

pristine compounds (S2). Raman spectra of the residues formed by decomposition (S3). Nanocalorimetric curve of a micron-sized CL-20 particle (S4). Conventional DSC curves of the pristine compounds (S5).


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AUTHOR INFORMATION Corresponding Author * Dr. Dimitri A. Ivanov E-mail: [email protected]

Present Addresses † Dr. David Doblas Faculty of Fundamental Physical and Chemical Engineering, Lomonosov Moscow State University (MSU), GSP-1, Leninskie gory - 1, Moscow, 119991, Russia

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style).

Acknowledgments D. D. acknowledges the PhD bursary granted by the EADS Foundation. The authors acknowledge F. Snell for the help with SEM imaging and Dr. Han for the NMR experiments.


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“For Table of Contents Use Only” Smart Energetic Nano-Sized Co-Crystals: Exploring Fast Structure Formation and Decomposition David Doblas, Martin Rosenthal, Manfred Burghammer, Dmitry Chernyshov, Denis Spitzer and Dimitri A. Ivanov

Synopsis: During heating, nano-sized CL-20/TNT co-crystals exhibit an irreversible melting transition, and a fraction of CL-20 rapidly crystallizes from the forming melt. This process occurs at a somewhat lower pace at room temperature to form homo-crystals of CL-20. To address the very onset of the crystallization process during rapid heating at 2000 ºC/s, a combination of in-situ nanocalorimetry and nano-focus X-ray diffraction was employed. It was observed that the rapidly-formed crystals correspond to the γCL-20 modification. This observation probably constitutes the first example of crystallization of CL-20, which occurs from the melt and not from solution. The solid-liquid phase separation operating during heating of CL20/TNT may explain its complex thermal decomposition process, the main exothermic peak of which can be assigned to that of a pure CL-20 and the secondary one- to that of a pure TNT.


31

Smart Energetic Nanosized Co-Crystals: Exploring Fast Structure

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