Influence of Dislocations on the Shock Sensitivity of RDX: Molecular

May 27, 2015 - The band gap may be further reduced by shock compression. A new model of the detonation initiation based on the electronic excitation ...
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Influence of Dislocations on the Shock Sensitivity of RDX: Molecular Dynamics Simulations by Reactive Force Field Xianggui Xue,† Yushi Wen,† Xinping Long, Jinshan Li, and Chaoyang Zhang* Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P.O. Box 919-327, Mianyang, Sichuan 621900, China S Supporting Information *

ABSTRACT: Molecular dynamics simulations of the chemical responses of shocked dislocation-contained and perfect (p) 1,3,5-trinitro-1,3,5-triazinane (RDX) crystals were performed using the ReaxFF force field combined with the multiscale shock technique. The shear dynamics of four types of dislocated RDX crystals are also modeled. The predicted mobilities of the crystals decrease in the order of (010) [001]/screw (s2) > (010) [001]/edge (e2) > (010) 1/2[100]/screw (s1) > (010)1/2[100]/edge (e1) according to their shear stress barriers, thus revealing the initial driving force required to activate a slip system. In view of the evolution of temperatures, pressures, and reactant decay rates of the shocked perfect and dislocated RDX, we confirm that shock sensitivity follows the order of e2 > e1 > s1 ≈ s2 > p. In particular, all dislocations enhance the shock sensitivity of RDX; in particular, edge dislocations enhance shock sensitivity significantly, whereas screw dislocations enhance it slightly. Shock sensitivity is not proportional to the shear stress barrier, which implies other factors influence shock initiation besides shear. heterogeneous interfaces, voids, and occlusions.18,19 Understanding the nature of defects responsible for hot spot formation is essential for explosive safety and can improve the performance and handling of explosives. Dislocations are a common type of defect believed to perform a central function in determining the responses of explosive solids against high-rate deformation. The general macroscopic properties of explosive dislocations may be understood using various experimental techniques, such as Xray diffraction topography, etch pitting, and drop-weight impact. However, the property responsible for hot spot formation remains unclear because of the complexity of the structural evolution of dislocations and the ultrafast velocity of detonation initiation. Insights into the microscopic details of the chemical and structural evolution of defects against external stimuli may be obtained through computer simulations. Molecular dynamics (MD) simulations have been employed to investigate the dislocation structure undergoing shock compression in bcc and fcc crystals.20,21 Homogenous nucleation of partial dislocation loops with Burgers vector 0.16[010] on (010) in 1,3,5-trinitro-1,3,5-triazinane (RDX) crystals behind the shock front has also been observed through large-scale MD simulations.22 Kuklja and co-workers reported the electronic structures of RDX and pentaerythritol tetranitrate (PETN) crystals with edge dislocations using ab initio

1. INTRODUCTION The sensitivity mechanism is a crucial but very complicated topic in the field of energetic materials. This mechanism depends on the multilevel structures of energetic materials, their external stimulation styles, and environmental conditions.1 For example, at the molecular level, sensitivity is believed to be governed by molecular stability. Therefore, indicators of molecular stability, such as nitro group charge, molecular surface electrostatic potential, bond order, and bond dissociation energy, have been employed to relate many types of sensitivities against shock, impact, heat, and spark, among others.2−6 At the crystal level, however, the molecular stacking mode and crystal shape, size, and defects can affect this sensitivity.7−11 At the device level, sensitivity is further influenced by shape, size, surface, and interfacial structures.12 The influence of crystal defects on the sensitivity of explosives has attracted increased research attention.13,14 To enhance explosive safety, researchers have attempted to decrease defects and promote crystal quality.15,16 However, defects may exert favorable effects if they are controllable. For example, the crystals of some secondary explosives (e.g., 1,3,5,7-tetranitro-1,3,5,7-tetrazocane, HMX) with a certain quantity of defects may be applied as primary explosives.17 In either case, clarifying the defect-dependent sensitivity mechanism is important. In practice, the nucleation and growth of explosive crystals may cause inhomogeneous and defective structures, such as polycrystals, voids, dopants, twins, and dislocations.17 According to hot spot theory, defect initiation usually originates in defective areas, such as shear bands, © XXXX American Chemical Society

Received: April 6, 2015 Revised: May 13, 2015

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Figure 1. Edge dislocation model in the RDX crystal. In the figure, Burgers vector is 1/2[100] and the slip plane is (010), as abbreviated by e1. Displacements of molecules in the dislocation core were achieved according to the Volterra displacement field.26 (a) RDX crystal without core molecules, (b) the dislocated crystal, and (c) scheme of the edge dislocation.

Figure 2. Screw dislocation model in the RDX crystal. In the figure, Burgers vector is 1/2[100] and the slip plane is (010), as abbreviated by s1. Displacements of molecules in the dislocation core were achieved according to the Volterra displacement field.26 (a) Perfect RDX crystal, (b) the dislocated crystal, and (c) scheme of the screw dislocation.

calculations.22−25 Edge dislocations promote dramatic changes in electronic structure, primarily a reduction of the band gap. The band gap may be further reduced by shock compression. A new model of the detonation initiation based on the electronic excitation mechanism has been proposed, and edge dislocation defects, which serve as hot spots, have been demonstrated. The Peierls stress of dislocations in RDX has recently been investigated via atomistic simulations in conjunction with the Peierls−Nabarro model; the related experimental data from nanoindentation and shock-induced plastic deformation were also discussed in this study.26 Nanoindentation of the RDX (100) crystal surface was reported by Chen et al., who revealed significant heating under the indenter caused by dislocation pileup.27 State-of-the-art computational modeling of materials has seen continuous development since the discovery of massively parallel high-performance computer systems. The simulations afforded by these systems are essential to elucidate the atomistic mechanisms of the condensed phase, microstructural evolution, and detonation, as well as their sensitivity to heat and shock. In the present work, we reveal the detailed effects of dislocation defects on the shock sensitivity of RDX through

MD simulations. In particular, the difference among various types of dislocations was elucidated. The responses of shocked dislocated and perfect RDX crystals (p) were studied using the reactive force field ReaxFF combined with the multiscale shock technique (MSST). The dislocations studied included (010)1/ 2[100]/edge (e1), (010) [001]/edge (e2), (010) 1/2[100]/ screw (s1), and (010) [001]/screw (s2). Two shock velocities (Us) of 7 and 9 km/s were assigned for the simulations; these velocities are around the detonation velocity of RDX, which is approximately 8.7 km/s. Before shock simulation, the shear dynamics of the four dislocation systems were modeled. The shear stress barrier, that is, the maximum shear stress minus the initial shear stress (τmax − τ0), was calculated based on the simulation data; this shear stress barrier represents the shear driving force required to initiate shear along a slip system.28−30 Dislocation-induced shock sensitivity variations were subsequently evaluated by comparing the evolution of temperature, pressure, and chemical decay of shocked dislocation-containing and perfect crystals. Results indicated that edge dislocations in the RDX crystal significantly enhance shock sensitivity, whereas screw dislocations mildly enhance this property. B

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tetramethylene tetranitramine (HMX),42,43 PETN,44 and TATB.45 We performed simulations with two specified shock speeds (Us) of 7 and 9 km/s perpendicular to the (100) plane; these speeds are below or above, respectively, the practical detonation velocity of RDX (8.7 km/s).46 All MD simulations with a time step of 0.05 fs were conducted using the LAMMPS software package.47 Before MD simulations, the conjugate gradient algorithm was employed to relax all RDX crystals using the COMPASS force field by the Discover tool in the Materials Studio software package (Accelrys). Thereafter, energy minimizations were performed once more by using conjugate gradient algorithm with the ReaxFF-lg reactive force field. To identify chemical species and their distributions during dynamic evolution, bond order files generated during the simulations were postprocessed by the mol_fra.c code, which was written by Sergey Zybin.

2. COMPUTATIONAL DETAILS 2.1. Modeling. In our simulations, we focused on the α phase of RDX; this phase is the most stable morphology of the crystals at ambient conditions. The initial crystal structure was obtained from the Cambridge Structure Database. RDX possesses an orthorhombic Pbca space group containing eight molecules in a unit cell with lattice vectors of a = 13.182 Å, b = 11.574 Å, and c = 10.709 Å.31 This crystal structure was relaxed at room temperature and pressure (300 K, 105 Pa) by the ReaxFF-lg force field32 and MD simulations. In contrast to the previous version of ReaxFF,33−35 an additional term of London dispersion, which provides a more accurate description of cell parameters for molecular crystals at low pressure, was added to ReaxFF-lg. This force field has been tested for several explosives, such as RDX, PETN, 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), and nitromethane.32 The relaxed lattice vectors are a = 13.272 Å, b = 11.653 Å, and c = 10.782 Å, in good agreement with the experimental values. Then an RDX supercell was built by enlarging the unit cell 10, 5, and 2 times along the a, b, and c axes, respectively. The evolution of this supercell against shock is regarded as a basis for evaluating the influences of dislocations on shock sensitivity. Edge and screw dislocations were then accounted for to establish dislocated RDX crystals. The primary plane of α-RDX has been proposed to be its (010) face.36,37 Therefore, two directions of the Burgers vectors, namely, [100] and [001], were considered in the (010) plane; each of these vectors features screw and edge dislocations. A dislocation dipole was created by imposing the Volterra displacement field on the centers of mass of the molecules, according to the method proposed in ref 26. Modeling details of the dislocation of RDX crystals are provided in the Supporting Information. Figures 1 and 2 illustrate the models of edge and screw dislocations, each with two slip systems: (010)1/2[100]/edge (e1) and (010)[001]/ edge (e2) and (010) 1/2[100]/screw (s1) and (010)[001]/ screw (s2). The dislocations were selected particularly because they feature maximum or minimum Peierls stresses in edge or screw dislocations, as determined by Mathew et al.26 The model information on these dislocated systems, as well as that of the perfect RDX crystal, is listed in Table 1.

3. RESULTS AND DISCUSSION Given that the focus of this work is the influence of dislocations on shock sensitivities, we compare various evolutions of dislocated and perfect crystals resulting from shock, including shear stress barrier, temperature, pressure, and reactant decay. From these comparisons, we confirm the influences of various dislocations. 3.1. Shear Stress Barriers. The resolved shear stress reflecting the driving force of shear deformation was calculated for the four dislocated RDX crystals. As demonstrated in Figure 3, the shear stresses reach their maxima after approximately 0.5

Table 1. Some Information of the Simulation Systems systems

no. of atoms

supercell size

density, g/cm3

e1 e2 s1 s2 p

16 632 16 464 16 800 16 800 16 800

10 × 5 × 2 2 × 5 × 10 2 × 5 × 10 10 × 5 × 2 10 × 5 × 2

1.787 1.770 1.806 1.806 1.806

Figure 3. Shear stress evolution of four dislocated RDX crystals.

ps and then relax to constant values thereafter. The shear stress barrier (τmax − τ0) is usually employed to correlate with the initial driving force required to activate a slip system, that is, lower barriers suggest easier slips. Table 2 reveals the decreasing order of the barriers of s2, e2, s1, and e1, which is in good agreement with the Peierls stress ranking reported recently.26 3.2. Evolution of Temperature, Pressure, and Volume. The temperature, pressure, and volume evolutions of the five systems during MSST simulations with steady-state shock velocities of 7 and 9 km/s are shown in Figures 4, 5, and 6, respectively. With respect to temperature, we can conclude by comparing Figure 4a and 4b that stronger shock results in faster temperature increases. When Us = 7 km/s (Figure 4a), the

2.2. Simulations. Two types of simulations were performed in this work. The first involved shear dynamics for the four slip systems. This simulation was performed in the direction of the Burgers vector with a constant shear rate of 0.25 ps−1. Then MD simulations combined with MSST proposed by Reed et al.38,39 were applied to study the effects of shock acting on dislocated and perfect RDX crystals. MSST follows a Lagrangian point through the shock wave and enables simulation of a system experiencing a shock wave with far fewer atoms than normally required by the direct method of inducing a shock wave in a very large scale simulation cell. MSST has been successfully applied to studies of several shocked energetic materials, including nitromethane,40,41 cycloC

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a

dislocated crystals

e1

e2

s1

s2

τmax − τ0a τPNb

1.58 0.201

1.97 0.782

1.68 0.280

2.41 0.831

From the present work, and. bFrom ref 26.

Figure 5. Pressure evolution of dislocated and perfect RDX crystals undergoing shock.

Figure 4. Temperature evolution of dislocated and perfect RDX crystals undergoing shock.

temperatures of e1, e2, s1, s2, and p, respectively, increase to approximately 4080, 4160, 3950, 4000, and 3950 K after 60 ps; when Us = 9 km/s, the temperatures of these samples significantly increase to 5110, 5210, 4980, 4990, and 4980 K, respectively. The temperature increase in the case of Us = 9 km/s smoothens after 6 ps, different from the case of Us = 7 km/s, where the increase remains steep. Given that fast initial temperature increase usually means high sensitivity to shock, the shock sensitivity of the five systems can be ranked as e2 > e1 > s2, s1 > p at Us = 7 km/s. Meanwhile, at Us = 9 km/s, the shock sensitivity of edge-dislocated RDX (e1 and e2) is higher than that of screw-dislocated (s1 and s2) and perfect RDX (p). The temperature increase rate of e2 is higher than that of e1, which shows its increased shock sensitivity. Moreover, the shock sensitivity of s1, s2, and p must be close to one another. We can determine from the preceding discussion that the shock sensitivity of screw-dislocated and perfect RDX can be distinguished at Us = 7 km/s but not at 9 km/s, thereby suggesting that shock sensitivity can depend on shock velocity. Nonetheless, edge dislocation evidently increases shock sensitivity, whereas screw dislocation increases it mildly. In this work, we only focused on pressure increases within the initial 5 ps for the two cases with different Us because (1)

Figure 6. Volume evolution of dislocated and perfect RDX crystals undergoing shock.

D

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Figure 7. Evolution of the number of fragments of dislocated and perfect RDX crystals undergoing shock.

volume is completely consistent with those of increasing temperature and pressure, as illustrated in Figure 6. In fact, this order is related to the densities listed in Table 1, i.e., looser RDX corresponds to faster temperature and pressure increases. Given that the density of RDX represents its free volume, dislocation-induced enhancement of shock sensitivity can be attributed to the free volume mechanism:43 free volume supplies space for molecular vibrations to decompose RDX molecules, thereby leading to easier final detonation. We can also determine from Figure 6 that the volume decreases rapidly during compression at the first 2.5 ps, after which stronger shock results in increased volume compression (1 − v/v0). After compression to the minimum volume, the volume expands slowly because of chemical reactions, as discussed in the next section. 3.3. Evolution of Chemical Species. The evolution of chemical species of the shocked perfect RDX crystal has previously been detailed.34 Therefore, we only pay attention to the evolution of the main chemical species, such as NO2, HONO, NO, CO, CO2, HO, H2O, and N2. In Figure 7a, we show the evolution of the number of the main fragments in all systems undergoing shock at 7 km/s. Compared with other subsequent products, the primary dissociation species NO2,

the pressure increase rate can also describe shock sensitivity, i.e., faster increases suggest high sensitivity. Second, differences among the final equilibrium pressures of the five systems at a given Us can be negligible (Figure S1, Supporting Information). Similar to the case of temperature evolution, stronger shock leads to faster pressure increases and higher equilibrium pressures, as illustrated in Figure S1, Supporting Information: shocks with velocities of 7 and 9 km/s can lead to final pressures of 43 and 78 GPa, respectively, regardless of the presence of dislocations. According to the pressure enhancement rate displayed in Figure 5a, sensitivity to shock at 7 km/s decreases from e2 to e1, s2, s1, and p, thereby showing an order identical to that of the sample with temperature increases, as shown in Figure 4. At Us = 9 km/s, the sensitivity to shock of e2 is higher than that of e1. The pressures of s1, s2, and p cannot be adequately ranked, and the pressure increase rates of e2 and e1 are higher than those of s2, s1, and p. These findings suggest that the shock sensitivity of e2 and e1 is higher than that of s2, s1, and p in this case. Similar to the discussion on temperature increase, shock sensitivity induced by edge dislocations is greater than that induced by screw dislocations. The temperature and pressure evolution discussed earlier can also be reflected by the volume evolution. The order of shrink E

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where t, c0, and c(t) are time, initial molecular amount, and molecular amount at t of RDX, respectively. Equation 1 can be converted into eq 2 c0 1 k = ln t c0 − c(t ) (2)

HONO, and NO clearly appear early. This observation reveals that rupture of the N−NO2 bond accompanied by HONO elimination occurs at the initial stage of RDX decay. NO2 is dominant in all systems at earlier stages of decay, which suggests that breakage of N−NO2 bonds dominates decay. Such a result is consistent with the simulation results of Strachan et al.34 Both of the initial reactions of RDX have been observed through experiments.48−52 Over time, the primary species NO2 and HONO disappear and some new secondary products, such as N2, CO2, and H2O, appear and increase in concentration. NO2 and HONO disappear approximately at 35, 30, 40, 40, and 42 ps in e1, e2, s1, s2, and p, respectively. The time required for NO2 and HONO to disappear can help predict differences in the reaction rates of these shocked crystals. In particular, e2, e1, s1(s2), and p are decayed in decreasing order, in accordance with the previous temperature and pressure results. Overall, our simulations reproduce the previous results of earlier stages of RDX decay and provide a possible order of decay rate. When the shock velocity increases to 9 km/s, as indicted in Figure 7b, the decay rates of RDX are much faster. The initial decay mechanism is identical to that of Us = 7 km/s. Here, N−NO2 bonds rupture and HONO elimination occurs at very early stages (during compression). NO2 and HONO then disappear followed by an increase in the secondary products N2, CO2, and H2O. We noted that the numbers of secondary small molecular products at Us = 9 km/s are lower than those at Us = 7 km/s. This observation may be attributed to the large numbers of clusters formed by higher shock pressures at Us = 9 km/s. Cluster formation by shock has been observed through previous simulations on shocked twinned and perfect β-HMX, which is an analogue of RDX.43 To evaluate the response to shock quantitatively, the decay rate of the reactant, which is usually used to assess the shock sensitivities of related materials, was calculated. When Us = 7 km/s, as demonstrated in Figure 8, the decay of all crystals was

The rate constants of five types of crystals were fitted using eq 2 and are listed in Table 3. Table 3 shows that e2, e1, s1(s2), Table 3. Decay Rates of Perfect and Dislocated RDX Crystals at Us = 7 km/s e1

e2

s1

s2

p

k

0.16

0.21

0.12

0.12

0.10

and p possess a lower k, thereby suggesting the slower and slower decay or the lower and lower shock sensitivity. This observation shows that, at a Us of 7 km/s, RDX decay enhances in the order of p, s1, s2, e1, and e2, consistent with previous analyses of other properties. We can confirm from the preceding discussion that edge dislocations (e1 and e2) can cause evident shock sensitivity enhancement, whereas screw dislocations (s1 and s2) cause mild increases in sensitivity. We believe that the different influences of these two types of dislocations on sensitivity are related to the free space volume of the crystal, which mainly induces sensitivity enhancement in void56−59 and twincontaining explosive crystals.43 Our results are consistent with the work by Politzer and co-workers, who concluded that measured impact sensitivities show an overall tendency to increase as free volume increases.60,61 Our work is also in good agreement with the results reported by Kuklja et al., who determined that the dissociation barriers of N−NO2 bonds are low in molecules close to the free surface of the crystal.14,62 As listed in Table 1, the densities of e1 and e2 are lower than those of s1, s2, and p, which suggests that edge dislocations cause larger free space volumes. Thus, e2 possesses the lowest density and the largest free space volume, and therefore, e2 is the most sensitive to shock among the crystals tested. Considering that higher free space volumes lead to high sensitivity, large free space volumes may suggest strong molecular vibrations and easier molecular dissociation, subsequent hot formation, and final combustion and detonation. As s1, s2, and p possess the same density, they also possess similar sensitivity. Slight sensitivity differences between these crystals may be attributed to other factors except free space volume, such as stress. Our simulation results are consistent with the ab initio calculations reported by Kuklja and co-workers for RDX containing edge dislocations.23−25 These researchers demonstrated that lattice defects, particularly edge dislocations, can serve as hot spots, which are characterized by a local internal stress and reduced band gap. Compression introduced by shock strongly increases stress and reduces the band gap further. In this work, we demonstrated that edge dislocations contribute much to increasing the shock sensitivity of RDX to detonation initiation, which can serve as hot spots. The order of sensitivity in terms of temperature, pressure, and rate constants (increases as p, s1[s2], e1, and e2) is not consistent with that of the shear stress barrier or Peierls stress (increases as e1, s1, e2, and s2). This finding suggests the dominant influence of free space volume on enhancing the sensitivity and complexity of explosives against shock, although many other factors can influence sensitivity besides free space volume and shear stress.

Figure 8. Evolution of RDX decay under a shock of 7 km/s.

completed within about 20 ps. We can obtain the decay kinetics of the simulation under a time limit. As the heat- and shockinduced decay of explosives or fossil fuel is usually assumed to follow first-order kinetics,43,53−55 the same assumption was also applied in the present work. The RDX decay rate k can be evaluated from the first-order expression c(t ) = c0(1 − exp(−kt ))

crystals

(1) F

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(8) Zhang, C.; Cao, X.; Xiang, B. Sandwich Complex of TATB/ Graphene: An Approach to Molecular Monolayers of Explosives. J. Phys. Chem. C 2010, 114, 22684−22687. (9) Ma, Y.; Zhang, A.; Zhang, C.; Jiang, D.; Zhu, Y.; Zhang, C. Crystal Packing of Low-Sensitivity and High-Energy Explosives. Cryst. Growth Des. 2014, 14, 4703−4713. (10) Kuklja, M. M.; Rashkeev, S. N. Shear-strain-induced chemical reactivity of layered molecular crystals. Appl. Phys. Lett. 2007, 90, 151913:1−3. (11) Kuklja, M.; Rashkeev, S. Shear-strain-induced structural and electronic modifications of the molecular crystal 1,1-diamino-2,2dinitroethylene: Slip-plane flow and band gap relaxation. Phys. Rev. B 2007, 75, 104111:1−10. (12) Feng, C. G. Theory of themal explosion; Science Press: Beijing, 1988. (13) Li, H. Z.; Xu, R.; Kang, B.; Li, J. S.; Zhou, X. Q.; Zhang, C. Y.; Nie, F. D. Influence of crystal characteristics on the shock sensitivities of cyclotrimethylene trinitramine, cyclotetramethylene tetranitramine, and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetra-cyclo[5,5,0,03,1105,9]dodecane immersed in liquid. J. Appl. Phys. 2013, 113, 203519:1−6. (14) Sharia, O.; Kuklja, M. M. Rapid Materials Degradation Induced by Surfaces and Voids: Ab InitioModeling of β-Octatetramethylene Tetranitramine. J. Am. Chem. Soc. 2012, 134, 11815−8365. (15) Xu, R.; Kang, B.; Huang, H.; Li, J.; Huang, H. Quantitative Characterization of HMX Particle Sphericity. Chin. J. Energ. Mater. 2006, 14, 280−282. (16) Huang, H.; Dong, H.; Shu, Y.; Hao, Y.; Wang, X. The Preparation of HMX Crystals with Defects and the Influences of Crystal Defects on Thermal Sensitivity and Stability. Chin. J. Energ. Mater. 2003, 11, 123−126. (17) Teipel, U. Energetic Materials; WILEY-VCH Verlage GmbH & Co. KGaA: Weiheim, 2005. (18) Bowden, F. P.; Yoffe, A. D. Initiation and growth of explosion in liquids and solids; Cambridge University Press: Cambridge, 1952. (19) Field, J. E. Hot spot ignition mechanisms for explosives. Acc. Chem. Res. 1992, 25, 489−496. (20) Bandak, F. A.; Tsai, D. H.; Armstrong, R. W.; Douglas, A. S. Formation of nanodislocation dipoles in shock-compressed crystals. Phys. Rev. B 1993, 47, 11681−11687. (21) Germann, T. C.; Holian, B. L.; Lomdahl, P. S.; Tanguy, D.; Mareschal, M.; Ravelo, R. Metall. Dislocation structure behind a shock front in fcc perfect crystals: Atomistic simulation results. Mater. Trans. A 2004, 35, 2609−2615. (22) Cawkwell, M. J.; Ramos, K. J.; Hooks, D. E.; Sewell, T. D. Homogeneous dislocation nucleation in cyclotrimethylene trinitramine under shock loading. J. Appl. Phys. 2010, 107, 063512:1−11. (23) Kuklja, M. M.; Stefanovich, E. V.; Kunz, A. B. An excitonic mechanism of detonation initiation in explosives. J. Chem. Phys. 2000, 112, 3417−3423. (24) Kuklja, M. M.; Kunz, A. B. Compression-induced effect on the electronic structure of cyclotrimethylene trinitramine containing an edge dislocation. J. Appl. Phys. 2000, 87, 2215−2218. (25) Kuklja, M. M.; Kunz, A. B. Electronic structure of molecular crystals containing edge dislocations. J. Appl. Phys. 2001, 89, 4962− 4970. (26) Mathew, N.; Picu, C. R.; Chung, P. W. Peierls Stress of Dislocations in Molecular Crystal Cyclotrimethylene Trinitramine. J. Phys. Chem. A 2013, 117, 5326−5334. (27) Chen, Y. C.; Nomura, K. I.; Kalia, R. K.; Nakano, A.; Vashishta, P. Molecular dynamics nanoindentation simulation of an energetic materia. Appl. Phys. Lett. 2008, 93, 171908:1−2. (28) An, Q.; Liu, Y.; Zybin, S. V.; Kim, H.; Goddard, W. A., III Anisotropic Shock Sensitivity of Cyclotrimethylene Trinitramine (RDX) from Compress-and-Shear Reactive Dynamics. J. Phys. Chem. C 2012, 116, 10198−10206. (29) Zybin, S. V.; Goddard, W. A., III; Xu, P.; van Duin, A. C. T.; Thompson, A. P. Physical mechanism of anisotropic sensitivity in

4. CONCLUSIONS Dislocation-induced enhancement of shock sensitivity is verified in the present work through MD simulations in combination with MSST and the molecular reactive force field ReaxFF-lg; here, the evolutions of temperature, pressure, and reactant decay of dislocated and perfect RDX crystals were compared. In the simulations, two types of edge dislocations and two kinds of screw dislocations are considered. Edge dislocations evidently enhance sensitivity as a result of increases in free space volume relative to the perfect crystal, whereas screw dislocations only mildly enhance sensitivity because of the lack of free space volume change relative to the perfect crystal. We determined that the function of free volume change (or density) in determining shock sensitivity is more important than that of shear stress, which is also a crucial factor influencing sensitivity. This finding suggests the complexity of the sensitivity mechanism. To date, many of these complexities remain unclear.

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ASSOCIATED CONTENT

* Supporting Information S

Construction of the models of edge and screw dislocations in the RDX crystals, and pressure evolution of dislocated and perfect RDX crystals undergoing shock. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03298.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

Xianggui Xue and Yushi Wen are equal contributors.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the Science and Technology Innovation Fund of ICM (KJCX-201305) and the National Natural Science Foundation of China (21173199).

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b03298 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b03298 J. Phys. Chem. C XXXX, XXX, XXX−XXX