compressive shear reactive molecular dynamics … shear reactive molecular dynamics studies...

Compressive Shear Reactive Molecular Dynamics Studies IndicatingThat Cocrystals of TNT/CL-20 Decrease SensitivityDezhou Guo,†,‡ Qi An,‡ William A. Goddard, III,*,‡ Sergey V. Zybin,‡ and Fenglei Huang†

†State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, People’s Republic ofChina‡Materials and Process Simulation Center, 139-74, California Institute of Technology, Pasadena, California 91125, United States

ABSTRACT: To gain an atomistic-level understanding of how compounding the TNT and CL-20energetic materials into a TNT/CL-20 cocrystal might affect the sensitivity, we carried out thecompressive−shear reactive molecular dynamics (CS-RMD) simulations. Comparing with the purecrystal of CL-20, we find that the cocrystal is much less sensitive. We find that the molecular origin ofthe energy barrier for anisotropic shear results from steric hindrance toward shearing of adjacent slipplanes during shear deformation, which is decreased for the cocrystal. To compare the sensitivity fordifferent crystals, we chose the shear slip system with lowest energy barrier as the most plausible oneunder external stresses for each crystal. Then we used the temperature rise and molecule decompositionas effective measures to distinguish sensitivities. Considering the criterion as number NO2 fragmentsproduced, we find that the cocrystal has lower shear-induced initiation sensitivity by ∼70% underatmospheric pressure and ∼46% under high pressure (∼5 GPa) than CL-20. Based on the temperatureincrease rate, the cocrystal has initiation sensitivity lower by 22% under high pressure (∼5 GPa) thanCL-20. These results are consistent with available experimental results, further validating the CS-RDmodel for distinguishing between sensitive and insensitive materials rapidly (within a few picoseconds of MD).

1. INTRODUCTION

Energetic materials, a class of compounds with high amounts ofstored chemical energy, containing both oxidizer and fuelcomponents, are widely used for both civilian and militaryapplications.1 These materials must meet several severerequirements to be viable for fielding. Although new materialsare being synthesized, this limitation prevents most of themfrom extended applications.2 The search for promisingenergetic materials has led to the discovery of a vast numberof new energetic materials with increased performance, reducedsensitivity to external stimuli, and enhanced chemical andthermal stability.3,4 One approach to improve materials’properties is to combine existing chemical entities to formeither blends and/or polymorphic crystal forms.5,6

Cocrystallization7 is already having a tremendous impact onpharmaceuticals8 and energetic materials, and it is poised tomake a significant mark on other fields such as nonlinearoptics9 and organic electronics.10 For energetic materials, theability of cocrystallization to combine two known compoundsinto a novel material with distinct properties presents anelegant means of generating improved energetic material fromexisting compounds, and several such cocrystals have recentlybeen reported.11,12

One of the most promising energetic materials today is CL-20, which is 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaiso-wurtzitane. CL-20 exhibits such novel properties as high oxygenbalance, high density, and superior performance.2,13 However,CL-20 has failed to see widespread implementation due torelatively high sensitivity to heat, friction, and shock. Table 1compares the properties of CL-20, HMX, and RDX.

Recently, a 1:1 molar ratio cocrystal of TNT and CL-20 hasbeen reported11 that displays greatly reduced sensitivitycompared to pure CL-20 with only modest reduction in theperformance due to incorporation of TNT, a relative stable butlow-power energetic material. The properties of cocrystalTNT/CL-20, pure crystal of CL-20, and TNT are listed inTable 2.However, there is little known about the origin of the

detonation sensitivity of energetic materials that is so crucial tosafe storage and transportation. Engineered energetic materialsare heterogeneous with many interfaces, impurities, and defects,making it difficult to extract information about the specificcauses of sensitivity.A breakthrough in understanding sensitivity was the

experimental demonstration by Dick15−18 that a large single

Received: September 15, 2014Revised: November 7, 2014Published: November 20, 2014

Table 1. Properties Comparison of RDX, HMX, and CL-202

properties RDX HMX CL-20(ε)

density (g/cm−3) 1.806 1.95 2.04oxygen balance (%) −21.61 −21.61 −11impact sensitivity, h50% (cm)a 71 65 16−20b

detonation pressure (GPa) 33.92 38.39 46.65ah50% is the dropping height at which there was a 50% probability ofdetonation for materials. bThe impact sensitivity here is different fromTable 2 because of different experimental conditions and materialpreparations.

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crystal of PETN (or HMX) displays dramatically anisotropicsensitivity to shock directions, allowing one to ignore manycomplicating factors involving interfaces, impurities, anddefects.In order to form a methodology to predict sensitivity, we

developed the compress-and-shear reactive dynamics (CS-RD)strategy which we used to examine the anisotropic shocksensitivity of PETN,19 RDX,20 and HMX,21 using the first-principles based ReaxFF reactive force field. The CS-RDcomputational protocol was developed to capture the essentialfeatures of sensitivities of various energetic materials at modestcomputational cost. These simulations showed dramaticallyanisotropic sensitivities for various shock directions that agreewith available experimental observations.In this paper, we use CS-RD to investigate the mechanism of

shear response of sensitivity for the TNT/CL-20 cocrystal,which we compare to that of the TNT and CL-20 crystals. Herewe consider various slip systems using the ReaxFF-lg reactiveforce field. This involves compression, followed by sheardeformation along the most plausible shear systems.

2. SIMULATION METHODS AND PROCEDURES2.1. Simulation Models. The systems examined herein are

the molecular cocrystal of CL-20/TNT, the crystal of CL-20,and the crystal of TNT. Five polymorphic modifications of CL-20 (α, β, γ, ξ, and δ) have been reported by Russell et al.22

Among these phases, the γ phase is the only thermodynamicallystable phase under ambient conditions. The α, β, and ξpolymorphs are high-temperature/high-pressure phases, butthese phases have been isolated under ambient conditions asmetastable forms. Here we consider only the γ-CL-20polymorph, which is the most stable phase at roomtemperature.22 The initial unit cell structures of cocrystalTNT/CL-20, γ-CL-20, and TNT were taken from theCambridge Structural Database available at the CambridgeCrystallographic Data Centre.We first optimized the atomic positions and cell parameters

to minimize the total energy. Then we carried out isothermal−isobaric (NPT) MD simulations until the internal stressesrelaxed to zero pressure at room temperature. Here, we used

the Berendsen thermostat (100 fs damping constant) and theBerendsen barostat (8000 fs pressure damping constant). Theequilibrium densities from ReaxFF are in reasonable agreementwith the experimental data. The lattice parameters of eachcrystal are shown in Table 3.We then considered two cases: the first one is the system at 0

GPa external pressure; the second is the system compresseduniformly by 15%, leading to initial hydrostatic stresses of ∼5.0GPa. On the basis of the temperature increase and plasticdeformation in real shock and drop weight experiments, weconsider that 15% compression is comparable to the availableexperiments.We considered six slip systems for each case. For

computational convenience, we rotated the unitcell for eachcase so that the x−z plane is the slip plane and x is the slipdirection in a Cartesian coordinate system.The unit cell was expanded to a 4 × 2 × 2 or an 8 × 2 × 1

supercell (128 CL-20 and 128 TNT molecules for a total 7296atoms) for the cocrystal; to an 8 × 2 × 2 or an 8 × 4 × 1supercell (256 CL-20 molecules or 9216 atoms) for CL-20crystal; and a 4 × 4 × 2 supercell (256 TNT molecules or 5376atoms) for TNT crystal. Then each system was equilibratedusing the NVT ensemble (constant volume, constant temper-ature, and constant number of atoms) for 10 ps at 300 K toreduce interior stresses. Finally, we carried out sheardeformation reactive dynamics (RD) on the rotated supercellsfor up to 10 ps by deforming the supercells every 10 time steps(1.0 fs) at a constant shear rate of 0.5/ps. The microcanonicalensemble was applied during the constant shear rate RD.

2.2. Compressive Shear Reactive Dynamics (CS-RD).To obtain a mechanistic understanding of anisotropic shocksensitivity and to make a comparison of sensitivity betweendifferent crystals, we developed the CS-RD model that firstcompresses and then shears at a uniform rate along the mostplausible slip systems.19 We found for PETN, HMX, andRDX19−21 that the CS-RD model captures the essentialcharacter related to sensitivity of real external force processes(mechanical shock, activated slip systems), correctly predictingthe relative sensitivity for each system. Compared with thesimulations adequate to describe hot spot formation from shearstresses,23 the CS-RD model is orders of magnitude simpler,requiring only thousands of atoms rather than millions, makingit practical on a single central processing unit (CPU). Tocompare the initiation sensitivities of crystals from physical andchemical responses during CS-RD process, we used uncom-pressed and hydrostatic compressed crystals to imitate theconditions of drop-weight experiment.

2.3. ReaxFF and Bond Fragment Analysis. ReaxFFretains nearly the accuracy of quantum mechanics (QM) butenables reactive molecular dynamics (RMD) for computational

Table 2. Properties Comparison of Cocrystal TNT/CL-20,Pure Crystal of TNT, and CL-2011,14

properties cocrystal TNT CL-20(ε)

density (g/cm−3) 1.908 1.63 2.04melting point (°C) 133.8 80.9 210detonation velocity (m/s) 8600 6900 9500detonation pressure (GPa) 35 21 43impact sensitivity, h50% (cm) 99 >275 47

Table 3. Lattice Parameters and Volume (V) of the Cocrystal of TNT/CL-20, Compared to the Pure Crystals of CL-20 andTNT

parameters of cocrystal

comp method a (Å) b (Å) c (Å) V (Å) ρ (g/cm3)

cocrystal ReaxFF-lg 9.98 19.67 24.91 4890.69 1.81experiment 9.67 19.37 24.69 4626.22 1.91

CL-20 ReaxFF-lg 13.30 7.88 15.37 1522.61 1.912experiment 13.23 8.17 14.89 1519.99 1.915

TNT ReaxFF-lg 22.10 15.23 5.7 1729.31 1.71experiment 21.41 15.02 6.09 1828.86 1.65

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costs nearly as low as for simple force fields. This has enabledRMD computational studies on larger systems containing up to3.7 million atoms for periods of 50 ns, which has providedvaluable information on the atomistic mechanism of hot spotformation and chemical reactions during decomposition and forsubsequent reactions under extreme conditions. Applicationswith ReaxFF for studying high-temperature and high-pressurethermal and shock-induced decompositions had been reportedfor many systems, such as RDX, HMX, TATB, PBX, etc.24−28

ReaxFF-lg29 is an extension of ReaxFF, in which anadditional term is added to account for London dispersion(van der Waals attraction). This provides a more accuratedescription of cell parameters for molecular crystals at lowpressure. ReaxFF-lg has been tested for several energeticmaterials,29 including crystals of RDX, PETN, TATB, andnitromethane (CH3NO2). The calculated crystal structures andequations of state are in good agreement with experimentalresults.We used the BondFrag method to analyze complex reactions

that occur during ReaxFF simulations in systematic criterion.Here we use the bond order values defined in ReaxFF, rangingfrom 0 to 1. After optimizing the bond order cutoff values fromsimulations of several energetic material systems, we choose aset of values which is able to have a good description offragments during chemical reactions. These cutoff values aretabulated in Table 4 for various atom pairs. In order that

instantaneous fluctuations in bond orders are not confused withtrue bond breaking or formation, we required that the newlycreated (or annihilated) bonds exist over a time window of 1ps. BondFrag assigns identification number to each molecularfragment to trace the reaction pathways and to calculatemolecular properties.

3. RESULTS AND DISCUSSION3.1. Shear under Normal Pressure. To determine the

most likely shear direction and plane for each crystal, weconsidered the six most likely slip systems. For each of thethree crystals we used the Dick concept of steric hindrance asshown in Figure 1. We consider the preferred slip systems asthe one with the minimum shear stress barrier for sheardeformation.Time evolutions of the shear stress and temperature for each

of these six slip systems for each of the three crystals during theCS-RD simulations are shown in Figure 2. Here the shear rateis 0.5/ps.Figure 2 shows that the shear stresses reach a maximum

within the first 1 or 2 ps, followed by a downtrend for another 1or 2 ps until reaching a constant value. Based on the maximumshear stress, the most plausible shear systems are (i) forcocrystal, (011)⟨100⟩ with a shear stress barrier of 1.13 ± 0.10GPa and (1−10)<110 > with a shear stress barrier 1.16 ± 0.07

GPa; (ii) for CL-20, (001)⟨1−10⟩ with a maximum shear stressof 1.39 ± 0.09 GPa and (1−10)⟨110⟩ with a maximum shearstress of 2.15 ± 0.06 GPa; and (iii) for TNT, (010) ⟨001⟩ thewith shear stress barrier a of 1.49 ± 0.11 GPa barrier and (100)⟨001⟩ with shear stress barrier of 1.48 ± 0.06 GPa. We considerthat the temperature increases during the shear deformationprovide a measure of shear-induced initiation sensitivity forthese crystals. Thus, the range of temperature for the six shearsystems for the cocrystal is from 873 to 1002 K at 6 ps, wherethe more sensitive CL-20 has much higher temperaturesranging from 1069 to 1264 K at 6 ps. For insensitive TNTcrystal, the range of temperature among the six systems is quitesmall, from 811 to 836 K at 6 ps.The process of shear deformation is described as follows.

First, molecules on adjacent slip planes are pushed into eachother as they shear along a given slip direction, which isindicated by the increased shear stress. After passing throughthe first energy barrier (about 1 ps), there are few additionalbarriers to overcome. After about 3 ps, the system becomesamorphous. The shear work done to overcome molecularinteractions increases the temperature until it is high enough tobreak the N−N bonds for the CL-20 molecule or to break theC−N bonds or have proton transfer for TNT molecule.After the system becomes amorphous, the shear stresses

reach constant values, leading to the shear viscosityτ ηγ=xy xy (1)

Here, τxy is the converged shear stress (GPa) for theamorphous crystals, γxy is the shear rate 0.5 ps−1, and η isviscosity in the units of poise. We calculate shear viscosities of(i) 1.72 cP for the cocrystal TNT/CL-20 (2.3 GPa pressureafter the shear stress converges), (ii) 2.12 cP for CL-20 (2.9GPa pressure after the shear stress converges), and (iii) 1.52 cPfor TNT (2.7 GPa pressure after the shear stress converges).The temperature and fragments of NO2 per unit volume of

the two plausible slip systems for each crystal are shown inFigure 3. To track the chemical processes during the RMD, weanalyzed the molecular fragments from the correspondingtrajectories. The temperature and NO2 release, used tocharacterize the sensitivity under mechanical stress, arepartitioned into three groups to more clearly indicate thesensitivities. (i) For the CL-20 crystal, the temperature

Table 4. Bond Order Cutoff Values for Different AtomPairsa

C H O N

C 0.55 0.40 0.80 0.30H 0.55 0.40 0.55O 0.65 0.55N 0.45

aThe BondFrag program uses these values as default parameter sets(can be adjusted by the user) to determine molecular fragments.

Figure 1. Unit cells of the cocrystal TNT/CL-20 (left), CL-20 crystal(upper right), and TNT crystal (lower righ) including schematicillustrations of molecule contacts during shear deformation.



increased significantly from 300 to 1750 K within 10 ps, leadingto large amount of NO2 dissociation, more than 3 per nm3.Here the first NO2 fragments appeared above 750 K, indicatingthe breaking of N−N bond of CL-20 molecules. (ii) For thecocrystal, the temperature increased moderately to about 1300K, resulting in about 0.9 NO2 per nm

3 within 10 ps. (iii) ForTNT, the temperature reached only about 1200 K with no NO2products within 10 ps. Even after reaching temperatures as highas 1200 K, the TNT molecules did not decompose at allbecause breaking the C−N bond requires higher energy thanthe N−N bond.30 This shows that CL-20 is much moresensitive than the cocrystal, which in turn is much moresensitive that TNT. The TNT molecules dilute the thickness ofCL-20, giving CL-20 molecules in the cocrystal fewer

opportunities to contact with other CL-20 molecules incocrystal, making cocrystal material less sensitive.

3.2. Shear under High Pressure. The sensitivity data ofthe real experiments involve drop weight experiments, in whicha drop hammer produces significant compression to thematerials. In order to connect the simulation results withthese measurements, we compressed each of the three crystalsuniformly 15% to simulate the reaction processes and thephysical and chemical response of materials under highpressures. This is a more relevant measure of shock sensitivity.The temperature and shear stress changes during the shearprocess are shown in Figure 4.For compressed situations, the final temperatures at the end

of simulation and shear stress barriers increased significantly.

Figure 2. Time evolution of temperature and shear stress for the six slip systems predicted for the cocrystal TNT/CL-20, CL-20 crystal, and TNTcrystal at 1 atm pressure and a shear rate of 0.5/ps.

Figure 3. Time evolution of temperature and number of NO2 fragments per nm3 for the chosen slip systems for cocrystal TNT/CL-20, pure crystal

of CL-20, and TNT under 1 atm pressure.



For the compressed pure crystal CL-20, the temperatureincreased sharply from 300 K to over 2700 K (from 2707 to2964 K for different cases) at the end of this simulation, whichis 1000 K higher than for the uncompressed situation. This isbecause the external compression increases the overlaps in thecrystals leading to more intensive contacts, resulting in higherstress barriers to be overcome to slip for adjacent layers ofmolecules.Because of the low shear barrier rules, we choose (1−

10)⟨110⟩ and (011)<1−10> systems to be the easy slip ways.We found the easy slip system changed for some cases fromuncompressed systems. This is because when we compressed

the monoclinic CL-20 crystal, there would be some changes ofmolecular arrangement, leading to different contacts andoverlaps during shear slipping. The phenomena were alsoobserved during our uniaxial compression simulation of PETN,RDX, and HMX crystals.19−21 For the cocrystal, the temper-ature increased sharply from 300 to over 2100 K (from 2180 to2380 K for different cases) at the end of this simulation, 600−700 K higher than uncompressed situation. Here, thetemperatures are much lower than for CL-20. The reason isthat for the pure CL-20 crystal, all active molecules orfragments that dissociate from CL-20 molecules can interactwith other CL-20 molecules, leading to an autocatalytically

Figure 4. Time evolution of temperature and shear stress for the preferred slip systems predicted for cocrystal TNT/CL-20, pure crystal of CL-20,and TNT under high pressure.

Figure 5. Time evolution of temperature and number of NO2 fragments per nm3 for the chosen slip systems for cocrystal TNT/CL-20, pure crystal

of CL-20, and TNT under high pressure.



accelerated decomposition.31 In contract for the cocrystal,many of these molecules interact with the much less reactiveTNT. This leads to much less intensive chemical reactions. Wechose (011)⟨100⟩ and (1−10)⟨110⟩ slip systems as the easiestshear systems. This is the same as for 1 atm pressure, but shearbarrier increases to 2.3 ± 0.11 and 2.2 ± 0.12 GPa, respectively.For the TNT systems, the compressed case also leads to

higher temperatures and shear stress barriers than theuncompressed case. Under high pressure, TNT moleculesbegan to decompose and start to produce NO2 fragments,indicating that the C−N bond in TNT molecules has started tobreak apart. In addition, we found a proton transferphenomenon in the TNT dissociation process, indicating thatC−N bond breaking and proton transfer are the two mostpossible way for TNT to decompose. The (010)⟨001⟩ and(101)⟨−101⟩ slip system are the preferred systems to shear dueto the lowest shear stress barrier.Also, we calculate a viscosity of (i) 3.78 cP (9 GPa pressure

after the shear stress converges) for the TNT/CL-20 cocrystal,(ii) 4.58 cP for CL-20 (12 GPa pressure after the shear stressconverges), and (iii) 3.2 cP for TNT (10 GPa pressure after theshear stress converges). The temperature and fragments ofNO2 per unit volume for each crystal under high pressure shearare shown in Figure 5. The temperature and NO2fragmentation analyses are partitioned into three groups tobetter represent the material’s sensitivity.Under high pressure, for the CL-20 crystal, the temperature

rises most quickly from the initial 300 K to 2750 K within 10ps, with production of more than 4.5 per nm3 of NO2fragments. Here the amount of NO2 reached a maximum ofabout 9 ps and then started to decrease, indicating that somesecondary reactions are already occurring in the CL-20 system.After this point one can expect a quick consumption of NO2molecules with a faster production of stable reaction products,such as N2, H2O, and CO2, as shown in our previous cook-offsimulations.26,32

For the cocrystal, the temperature increases moderately toabout 2200 K within 10 ps, leading to 2.6 NO2 per nm

3. Thus,the temperature is lower and fewer NO2 are produced than forCL-20 system, suggesting lower initiation sensitivity of thecocrystal TNT/CL-20 than for the pure crystal of CL-20.For TNT, the temperatures reach about 1750 K within 10 ps,

and the molecules start to decompose slowly under highpressure. In particular, OH and NO2 fragments were observedalmost at the same time, indicating that the decompositionmechanism for TNT is different from CL-20. Unlike nitramineswith easy breaking bond of the N−N bond, for TNT bothbreaking the hydrogen bond and breaking the C−N bond arepossible initial chemical reaction pathways for TNT mole-cules.30,33

The comparison of sensitivity for TNT/CL-20 cocrystal, CL-20 crystal, and TNT crystal are listed in Table 5. The predictedrelative sensitivities are consistent with the drop weightexperiments. According to the number of NO2 fragments, therations of initiation sensitivity between the cocrystal and CL-20are 30% under 1 atm and 54% under high pressure (∼5 GPa)from our simulation results, comparing to the drop weightexperimental value 47%. According to the temperature increaserate, the cocrystal has a lower initiation sensitivity of 31% underatmospheric pressure and 22% under high pressure (∼5 GPa)than CL-20, but 11% higher under 1 atm and 31% higher underexternal high pressure than TNT.

4. CONCLUSIONSWe used the CS-RD protocol to predict the sensitivity of theTNT/CL-20 cocrystal. As expected, it is less sensitive than CL-20 and more sensitive than TNT. The predicted relativesensitivities are consistent with the drop weight experiments.According to the number of NO2 fragments, the cocrystal has alower shear-induced initiation sensitivity about 70% underatmosphere pressure and 46% under high pressure (∼5 GPa)than that of CL-20. According to the temperature increase rate,the cocrystal has a lower initiation sensitivity of 31% underatmospheric pressure and 22% under high pressure (∼5 GPa)than CL-20, but 11% higher under 1 atm and 31% higher underexternal high pressure than TNT.The CS-RD protocol was previously validated for PETN,

HMX, and RDX. Here, we show that it can be used to examinethe new TNT/CL-20 cocrystal energetic systems to distinguishthe sensitivity of crystals. We should emphasize here that CS-RD was developed to provide a rapid assessment of shear-induced sensitivity for new energetic materials. The high shearrates used here already lead to a continuously shearing fluidsystem by 10 ps, and all slip systems considered here wouldrapidly detonate under usual conditions. Even so, we finddramatic differences between sensitive and insensitive slipsystems, as reflected in the temperature rise and moleculedecomposition.Our reactive dynamics studies for PETN, HMX, RDX, CL-

20, TNT, and cocrystal of TNT/CL-20 support the conceptthat molecular steric hindrance dominate the origin ofanisotropic sensitivity of energetic materials. Thus, consid-eration of molecular steric hindrance is critical to thedevelopment of energetic materials with reduced sensitivity.

■ AUTHOR INFORMATIONCorresponding Author*Phone 626-395-2731; e-mail [email protected] (W.A.G.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the U.S. ONR (N00014-09-1-0634). It was also supported by the National Natural ScienceFoundation of China (Grants 11172044 and 11221202).

■ REFERENCES(1) Becuwe, A.; Delclos, A. Low-Sensitivity Explosive Compoundsfor Low Vulnerability Warheads. Propellants, Explos., Pyrotech. 1993,18, 1−10.

Table 5. Sensitivity Comparison of TNT/CL-20 Cocrystal,TNT Crystal, and CL-20 Crystal

cocrystal TNT CL-20

temp increase rate (K/s)under 1 atm

100.2 ± 0.8 89.2 ± 0.6 141.9 ± 1.5

NO2 fragments (nm−3)under 1 atm at 10 ps

0.9 ± 0.04 0 3.2 ± 0.1

temp increase rate (K/s)under high press(∼5 GPa)

191.8 ± 1.3 148.6 ± 1.0 241.7 ± 1.7

NO2 fragments (nm−3)under high press (∼5GPa) at 9 ps

2.86 ± 0.07 0.02 ± 0.01 4.8 ± 0.1

exptl impact sensitivity h50%(cm)

99 >275 47



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■ NOTE ADDED AFTER ASAP PUBLICATIONThis article was published ASAP on December 8, 2014. Severaltext corrections have been made, and Figures 2 and 5 have beenreplaced. The correct version was published on December 9,2014.



compressive shear reactive molecular dynamics … shear reactive molecular dynamics studies...

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