Compressive Shear Reactive Molecular Dynamics Studies Indicating

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Compressive Shear Reactive Molecular Dynamics Studies Indicating That Co-Crystals of TNT-CL20 Decreases Sensitivity Dezhou Guo, Qi An, William A. Goddard III, Sergey V. Zybin, and Fenglei Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5093527 • Publication Date (Web): 20 Nov 2014 Downloaded from http://pubs.acs.org on November 26, 2014

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The Journal of Physical Chemistry

Compressive Shear Reactive Molecular Dynamics Studies Indicating that Co-crystals of TNT-CL20 Decrease Sensitivity Dezhou Guo,1,2 Qi An,2 William A. Goddard III, *2 Sergey V. Zybin,2 and Fenglei Huang1 1

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, People’s Republic of China 2

Materials and Process Simulation Center, 139-74, California Institute of Technology, Pasadena, California 91125, USA * Author to whom correspondence should be addressed. Phone: 626-395-2731. E-mail:[email protected]. Address: Caltech, Material and Process Simulation Center, Pasadena, CA 91125 USA.

ABSTRACT: To gain an atomistic-level understanding of how compounding the TNT and CL20 energetic materials into a TNT/CL-20 cocrystal might affect the sensitivity, we carried out the compressive--shear reactive molecular dynamics (CS-RMD) simulations. Comparing with the pure crystal of CL-20, we find that the co-crystal is much less sensitive. We find that the molecular origin of the energy barrier for anisotropic shear results from steric hindrance toward shearing of adjacent slip planes during shear deformation, which is decreased for the co-crystal. To compare the sensitivity for different crystals, we chose the shear slip system with lowest energy barrier as the most plausible one under external stresses for each crystal. Then we used the temperature rise and molecule decomposition as effective measures to distinguish sensitivities. Considering the criterion as number NO2 fragments produced, we find that the cocrystal has lower shear-induced initiation sensitivity by ~70% under atmospheric pressure and ~46% under high pressure (~5 GPa) than CL-20. Based on the temperature increase rate, the cocrystal has initiation sensitivity lower by 22% under high pressure (~5 GPa) than CL-20. These results are consistent with available experimental results, further validating the CS-RD model for distinguishing between sensitive and insensitive materials rapidly (within a few picoseconds of MD). Key words: energetic materials, co-crystallization, ReaxFF, initiation, dislocations 1. Introduction Energetic materials, a class of compounds with high amounts of stored chemical energy, containing both oxidizer and fuel components, are widely used for both civilian and military applications1. These materials must meet several severe requirements to be viable for fielding. Although new materials are being synthesized, this limitation prevents most of them from extended applications.2 The search for promising energetic materials has led to the discovery of a vast number of new energetic materials with increased performance, reduced sensitivity to

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external stimuli, and enhanced chemical and thermal stability3, 4. One approach to improve materials’ properties is to combine existing chemical entities to form either blends and/or polymorphic crystal forms5, 6. Crystallization7 is already having a tremendous impact on pharmaceuticals8 and energetic materials, and it is poised to make a significant mark on other fields such as non-linear optics9, and organic electronics10. For energetic materials, the ability of co-crystallization to combine two known compounds into a novel material with distinct properties presents an elegant means of generating improved energetic material from existing compounds, and several such cocrystals have recently been reported11, 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-hexaazaisowurtzitane. CL-20 exhibits such novel properties as high oxygen balance, high density, and superior performance.2, 13 However, CL-20 has failed to see widespread implementation due to relatively high sensitivity to heat, friction, and shock. Table 1 compares the properties of CL-20, HMX and RDX.

Table 1. The properties comparison of RDX, HMX and CL-202

properties

RDX

HMX

CL-20(ε)

Density(g/cm )

1.806

1.95

2.04

Oxygen balance (%)

−21.61

−21.61

−11

71

65

16–20

33.92

38.39

46.65

-3

Impact sensitivity, h50% (cm) Detonation pressure (GPa)

Recently, a 1:1 molar ratio co-crystal of TNT and CL-20 has been reported11 that displays greatly reduced sensitivity compared to pure CL-20 with only modest reduction in the performance due to incorporation of TNT, a relative stable but low-power energetic material. The properties of cocrystal TNT/CL-20, pure crystal of CL-20 and TNT are listed in the Table 2. Table 2. The properties comparison of cocrystal TNT/CL-20, pure crystal of TNT and CL-2014

properties

cocrystal

TNT

CL-20(ε)

Density(g/cm-3)

1.908

1.63

2.04

Melting point ( C)

133.8

80.9

210

Detonation velocity (m/s)

8600

6900

9500

Detonation pressure (GPa)

35

21

43

Impact sensitivity, h50% (cm)

99

>275

47

O


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However, there is little known about the origin of the detonation sensitivity of energetic materials that is so crucial to safe storage and transportation. Engineered energetic materials are heterogeneous with many interfaces, impurities and defects, making it difficult to extract information about the specific causes of sensitivity. A breakthrough in understanding sensitivity was the experimental demonstration by Dick15-18 that a large single crystal of PETN (or HMX) displays dramatically anisotropic sensitivity to shock directions, allowing one to ignore many complicating factors involving interfaces, impurities, and defects. 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 shock sensitivity of PETN19, RDX20, and HMX21, using the first principles based ReaxFF reactive force field. The CS-RD computational protocol was developed to capture the essential features of sensitivities of various energetic materials at modest computational cost. These simulations showed dramatically anisotropic sensitivities for various shock directions that agree with available experimental observations. In this paper, we use CS-RD to investigate the mechanism of shear response of sensitivity for the TNT/CL20 cocrystal, which we compare to that of the TNT and CL-20 crystals. Here we consider various slip systems using the ReaxFF-lg reactive force field. This involves compression, followed by shear deformation along the most plausible shear planes. 2. SIMULATION METHODS AND PROCEDURES 2.1. 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 thermodynamically stable phase under ambient conditions. The α, β, and ξ polymorphs are high-temperature/high-pressure phases, but these phases have been isolated under ambient conditions as metastable forms. Here we consider only the γ-CL-20 polymorph, which is the most stable phase at room temperature22. The initial unit cell structures of cocrystal TNT/CL-20, γ-CL-20 and TNT were taken from the Cambridge Structural Database available at the Cambridge Crystallographic 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 stresses relaxed to zero pressure at room temperature. Here, we used the Berendsen thermostat (100 fs damping constant) and the Berendsen barostat (8000fs pressure damping constant). The equilibrium densities from ReaxFF are in reasonable agreement with the experimental data. The lattice parameters of each crystal are shown in Table 3.



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Table 3. The lattice parameters and volume (V) of the cocrystal of TNT/CL-20, compared to the pure crystals of CL-20 and TNT

Parameters of cocrystal Comp.

cocrystal

CL-20

TNT

Method

o

o

o

o

a( Α )

b( Α )

c( Α )

V( Α )

ρ(g/cm3)

ReaxFF-lg

9.98

19.67

24.91

4890.69

1.81

Experiment

9.67

19.37

24.69

4626.22

1.91

ReaxFF-lg

13.30

7.88

15.37

1522.61

1.912

Experiment

13.23

8.17

14.89

1519.99

1.915

ReaxFF-lg

22.10

15.23

5.7

1729.31

1.71

Experiment

21.41

15.02

6.09

1828.86

1.65

We then considered two cases: the first one is the system at 0 GPa external pressure; the second is the system compressed uniformly by 15%, leading to initial hydrostatic stresses of ∼5.0 GPa. Based on the temperature increase and plastic deformation in real shock and drop weight experiments, we consider that 15% compression is comparable to the available experiments. We considered six low index planes as shear directions for each case. For computational convenience, we rotated the compressed unitcell for each case so that the x−z plane is the slip plane and x is the slip direction 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 7296 atoms) for the cocrystal; to an 8 × 2 × 2 or an 8 × 4 × 1 supercell (256 CL-20 molecules or 9216 atoms) for CL-20 crystal; and a 4 × 4 × 2supercell (256 TNT molecules or 5376 atoms) for TNT crystal. Then each system was equilibrated for the NVT ensemble (constant volume, constant temperature, and constant number of atoms) for 10 ps at 300 K to reduce interior stresses. Finally, we carried out shear deformation reactive dynamics (RD) on the rotated supercells for up to 10ps by deforming the supercells every 10 time steps (1.0 fs) at a constant shear rate of 0.5/ps. The Microcanonical ensemble was applied during the constant shear rate RD. 2.3. Compressive Shear Reactive Dynamics (CS-RD). To obtain a mechanistic understanding of anisotropic shock sensitivity and to make a comparison of sensitivity between different crystals, we developed the CS-RD model that first compresses and then shears at a uniform rate along the most plausible slip systems.19 We found for PETN, HMX and RDX19-21 that the CS-RD model captures the essential character related to sensitivity of real external force processes (mechanical shock, activated slip systems), correctly predicting the relative sensitivity for each system. Compared with the simulations adequate to describe hot spot formation from


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shear stresses23, the CS-RD model is orders of magnitude simpler, requiring only thousands of atoms rather than millions, making it practical on a single central processing unit (CPU). To compare the initiation sensitivities of crystals from physical and chemical responses during CS-RD process, we used uncompressed and hydrostatic compressed crystals to imitate the conditions of drop-weight experiment. 2.4. ReaxFF and Bond Fragment Analysis. ReaxFF retains nearly the accuracy of quantum mechanics (QM) but enables reactive molecular dynamics (RMD) for computational costs nearly as low as for simple force fields. This has enabled RMD computational studies on larger systems containing up to 3.7 million atoms for periods of 50 nanoseconds, which has provided valuable information on the atomistic mechanism of hot spot formation and chemical reactions during decomposition and for subsequent reactions under extreme conditions. Applications with ReaxFF for studying high-temperature and high pressure thermal and shock-induced decompositions had been reported for many systems, such as RDX, HMX, TATB, PBX and etc24-28. ReaxFF-lg29 is an extension of ReaxFF, in which an additional term is added to account for London dispersion (van der Waals attraction). This provides a more accurate description of cell parameters for molecular crystals at low pressure. ReaxFF-lg has been tested for several energetic materials29, including crystals of RDX, PETN, TATB, and nitromethane (CH3NO2). The calculated crystal structures and equations of state are in good agreements with experimental results. 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, ranging from 0 to 1. After optimizing the bond order cutoff values from simulations of several energetic material systems, we choose a set of values which is able to have a good description of fragments during chemical reactions. These cutoff values are tabulated in Table 4 for various atom pairs. In order that instantaneous fluctuations in bond orders are not confused with true bond breaking or formation, we required that the newly created (or annihilated) bonds exist over a time window of 1 ps. BondFrag assigns identification number to each molecular fragment to trace the reaction pathways and to calculate molecular properties. Table 4. Bond order cut-off values for different atom pairs. BondFrag program uses these values as default parameter sets (can be adjusted by the user) to determine molecular fragments.

C

H

O

N

C

0.55 0.40 0.80 0.30

H

0.55 0.40 0.55

O

0.65 0.55

N

0.45



3. RESULTS AND DISCUSSION 3.1. Shear under normal pressure To determine the most likely shear direction and plane for each crystal, we considered the six most likely slip systems. For each of the three crystals we used the Dick concept of Steric Hindrance as shown in figure 1. We consider the preferred slip systems as the one with the minimum shear stress barrier for shear deformation.

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

Time evolutions of the shear stress and temperature for each of these six slip systems for each of the three crystals during the CS-RD simulations are shown in Fig.2. Here the shear rate is 0.5/ps.

0.5

cocrystal{001} cocrystal{010} cocrystal{011} cocrystal{100} cocrystal{1-10} cocrystal{101}

1200

cocrystal 0.0

(101)

900

Shear stress (GPa)

1500

Temperature (K)

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

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(1-10)

600

(1-10) 300

(011)

-0.5 -1.0 -1.5

(1-10)


2

4

-2.0

cocrystal 0

0

2

4

6

8

10

-2.5

0

Time (ps)


Time (ps)

6

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2100

CL-20{001} CL-20{010} CL-20{100} CL-20{011} CL-20{101} CL-20{1-10}

Temperature (K)

1500

(1-10)

1200

(1-10) 900

(1-10)

600 300

0

2

4

6

8

-1

(001)

-2

(1-10) CL-20{001} CL-20{010} CL-20{100} CL-20{011} CL-20{101} CL-20{1-10}

-3

-4

CL-20 0

CL-20

0

(1-10)

Shear stress (GPa)

1800

10

0

2

4

6

0.5 TNT{001} TNT{010} TNT{100} TNT{101} TNT{110} TNT{011}

900

TNT

TNT 0.0

(010)

Shear stress (GPa)

1200

Temperature (K)

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


(010)

600

300

0

-0.5 -1.0

(010)

TNT{001} TNT{010} TNT{100} TNT{101} TNT{110} TNT{011}

-1.5

(100) -2.0

0

2

4

6

8

10

-2.5

0

2

Time (ps)

4

6

Time (ps)

Figure 2. Time evolution of temperature and shear stress for the 6slip systems predicted for the cocrystal TNT/CL-20, CL-20 crystal, and TNT crystal at 1atmpressure and a shear rate of 0.5/ps.

Figure 2 shows that the shear stresses reach a maximum within the first one or two picoseconds, followed by a downtrend for another one or two picoseconds until reaching a constant value. Based on the maximum shear stress, the most plausible shear systems are • for cocrystal, (011) with a shear stress barrier of 1.13 ± 0.10 GPa and (1-10)with a shear stress barrier1.16±0.07 GPa; • for CL-20, (001) with a maximum shear stress of 1.39±0.09 GPa and (1-10) with a maximum shear stress of 2.15±0.06 GPa; • for TNT, (010) the with shear stress barrier a of 1.49±0.11 GPa barrier and (100) with shear stress barrier of 1.48±0.06 GPa. We consider that the temperatures increase during the shear deformation provide a measure of shear-induced initiation sensitivity for these crystals. Thus, the range of temperature for the six shear systems for the cocrystal is from 873 to 1002 K at 6 ps, where the more sensitive CL-20 has much higher temperatures ranging from 1069 to 1264 K at 6 ps. For insensitive TNT crystal, the range of temperature among the six systems is quite small, 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 each other as they shear along a given slip direction, which is indicated by the increased shear stress. After passing through the first energy barrier (about 1ps), there are few additional barriers to overcome. After about 3ps, the system becomes amorphous. The shear work done to overcome molecular interactions increases the temperature until it is high enough



to break the N−N bonds for CL-20 molecule, or to break the C-N bonds or have proton transfer for TNT molecule. After the system becomes amorphous, the shear stresses reach constant values, leading to the shear viscosity τxy= ηγxy(1) Here, τxy is the converged shear stress (GPa) for the amorphous crystals, γxy is the shear rate 0.5 ps−1, and η is viscosity in the units of poise. We calculate shear viscosities of • 1.72 cP for the cocrystal TNT/CL-20 (2.3 GPa pressure after the shear stress converges), • 2.12 cP for CL-20 (2.9 GPa pressure after the shear stress converges), • 1.52 cP for TNT (2.7 GPa pressure after the shear stress converges). 2000

4

1200

800

400

0

0

2

4

cocrystal{1-10} cocrystal{011} CL-20{1-10} CL-20{001} TNT{010} TNT{100}

-3

NO2 fragments/volume (nm )


1600

Temperature (K)

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

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6

8

10

3

2

1

0

0

2

4

Time (ps)

6

8

10

Time (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.

The temperature and fragments of NO2 per unit volume of the two plausible slip systems for each crystal are shown in Figure 3. To track the chemical processes during the RMD, we analyzed the molecular fragments from the corresponding trajectories. The temperature and NO2 release, used to characterize the sensitivity under mechanical stress, are partitioned into three groups to more clearly indicate the sensitivities. •

For CL-20 crystal, the temperature increased significantly from 300K to 1750K within 10 ps, leading to large amount of NO2 dissociation, more than 3 per nm3. Here the first NO2 fragments appeared above 750K indicating the breaking of N-N bond of CL-20 molecules.

•

For the cocrystal, the temperature increased moderately to about 1300K, resulting in about 0.9 NO2 per nm3 within 10ps.

•

For TNT, the temperature reached only about 1200K with no NO2 products within 10 ps. Evan, after reaching temperatures as high as 1200K, the TNT molecules did not decompose at all because breaking the C-N bond requires higher energy than N-N bond30.


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This shows that CL-20 is much more sensitive than the cocrystal which in turn is much more sensitive that TNT. The TNT molecules dilute the thickness of CL-20, giving CL-20 molecules in the cocrystal fewer opportunities to contact with other CL-20 molecules in cocrystal, making cocrystal material less sensitive. 3.2 Shear under high pressure The sensitivity data of the real experiments involve drop weight experiments, in which a drop hammer produces significant compression to the materials. In order to connect the simulation results with these measurements, we compressed each of the three crystals uniformly 15% to simulate the reaction processes and the physical and chemical response of materials under high pressures. This is a more relevant measure of shock sensitivity. The temperature and shear stress changes during the shear process are shown in Fig 4. For compressed situations, the final temperatures at the end of simulation and shear stress barriers increased significantly. For the compressed pure crystal CL-20, the temperature increased sharply from 300K to over 2700 K (from 2707K to 2964K for different cases) at the end of this simulation, which is 1000K higher than for the uncompressed situation. This is because the external compression increases the overlaps in the crystals leading to more intensive contacts, resulting in higher stress barriers to be overcome to slip for adjacent layers of molecules. Due to the low shear barrier rules, we choose (1-10) and (011) system to be the easy slip ways. We found the easy slip system changed for some cases from uncompressed systems. This is because when we compressed the monoclinic CL-20 crystal, there would be some changes of molecular arrangement, leading to different contacts and overlaps during shear slipping. The phenomena were also observed during our uniaxial compression simulation of PETN, RDX and HMX crystals.19-21 For the cocrystal, the temperature increased sharply from 300K to over 2100K (from 2180K to 2380K for different cases) at the end of this simulation, 600 to 700K higher than uncompressed situation. Here, the temperatures are much lower than for CL-20. The reason is that for the pure CL-20 crystal, all active molecules or fragments that dissociate from CL-20 molecules can interact with other CL-20 molecules, leading to an autocatalytically accelerated decomposition31. In contract for the cocrystal, many of these molecules interact with the much less reactive TNT. This leads to much less intensive chemical reactions. We chose (011) and (1-10) slip systems as the easiest shear systems. This is the same as for 1 atm pressure but shear barrier increases to 2.3±0.11 GPa and 2.2±0.12 GPa, respectively. For the TNT systems, the compressed case also leads to higher temperatures and shear stress barriers than the uncompressed case. Under high pressure, TNT molecules began to decompose and start to produce NO2 fragments, indicating that the C-N bond in TNT molecules has started to break apart. In addition, we found a proton transfer phenomenon in the TNT dissociation process, indicating that C-N bond breaking and proton transfer are the two most possible way for TNT to decompose. The (010) and (101)slip system are the preferred systems to



shear due to the lowest shear stress barrier. 2500

1

Temperature (K)

1500

cocrystal

(010)

1000

500

0

cocrystal

0

(101) Shear stress (GPa)


2000

(011)

-1 -2 -3

(1-10)

-4

0

2

4

6

8

-5

10

0

2

cocrystal{001} cocrystal{010} cocrystal{011} cocrystal{100} cocrystal{1-10} cocrystal{101} 4

6

Time (ps) 3000

1

CL-20

2000

(101)

(100) (011)

1500

CL-20{001} CL-20{010} CL-20{100} CL-20{011} CL-20{101} CL-20{1-10}

1000 500 0

0

2

4

6

CL-20{001} CL-20{010} CL-20{100} CL-20{011} CL-20{101} CL-20{1-10}

0

8

10

Shear stress (GPa)

Temperature (K)

2500

-1

(1-10)

(001)

-2 -3 -4

CL-20 -5

0

2

4

6

1

TNT{001} TNT{010} TNT{100} TNT{101} TNT{110} TNT{011}

1500

TNT

TNT 0

(110) Shear stress (GPa)

2000

Temperature (K)

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

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(101) (100)

1000

(010) 500

0

0

2

4

6

8

10

-1 -2 -3

(010)

-4

(101)

-5

0

2

Time (ps)

TNT{001} TNT{010} TNT{100} TNT{101} TNT{110} TNT{011} 4

6

Time (ps)

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.

Also, we calculate a viscosity of • 3.78 cP (9 GPa pressure after the shear stress converges) for the TNT/CL-20cocrystal, • 4.58 cP for CL-20 (12 GPa pressure after the shear stress converges)and • 3.2 cP for TNT (10 GPa pressure after the shear stress converges). The temperature and fragments of NO2 per unit volume for each crystal under high pressure shear are shown in Figure 5. The temperature and NO2 fragmentation analyses are partitioned into three groups to better represent the material’s sensitivity.


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2500 2000

-3

1500 1000 500 0

0

2

4


5

NO2 fragments/volume (nm )

3000

Temperature (K)

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


6

8

10

4

3

2

1

0

0

2

4

Time (ps)

6

8

10

Time (ps)

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.

Under high pressure, for CL-20 crystal, the temperature rises most quickly from the initial 300K to 2750 K within 10 ps, with production of more than 4.5 per nm3 of NO2 fragments. Here the amount of NO2 reached a maximum of about 9 ps and then started to decrease, indicating that some secondary reactions are already occurring in the CL-20 system. After this point one can expect, a quick consumption of NO2 molecules with a faster production of stable reaction products, such as N2, H2O, and CO2, as shown in our previous cook-off simulations26, 32. For the cocrystal, the temperature increases moderately to about 2200K within 10 ps, leading to 2.6 NO2 per nm3. Thus the temperature is lower and fewer NO2 are produced than for CL-20 system, suggesting lower initiation sensitivity of the cocrystal TNT/CL-20 than for the pure crystal of CL-20. For TNT, the temperatures reach about 1750K within 10 ps and the molecules start to decompose slowly under high pressure. In particular, OH and NO2 fragments were observed almost at the same time, indicating that the decomposition mechanism for TNT different from CL-20. Unlike nitramines with easy breaking bond of the N-N bond, for TNT both breaking the hydrogen bond and the C-N bond are possible initial chemical reaction pathways for TNT molecules.30, 33 The comparison of sensitivity for TNT/CL-20 cocrystal, CL-20 crystal and TNT crystal are listed in the Table 5. The predicted relative sensitivities are consistent with the drop weight experiments. According to the number of NO2 fragments, the rations of initiation sensitivity between the cocrystal and CL-20 are 30% under 1 atm and 54% under high pressure (~5 GPa) from our simulation results, comparing to the drop weight experimental value 47%. According to the temperature increase rate, the cocrystal has a lower initiation sensitivity of 31% under atmospheric pressure and 22% under high pressure (~5 GPa) than CL-20; but 11% higher under 1 atm and 31% higher under external high pressure than TNT.



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Table 5. The sensitivity comparison of TNT/CL-20 cocrystal, TNT crystal and CL-20 crystal

Temperature increase rate(K/s) under 1 atm NO2 fragments (nm-3) under 1 atm at 10 ps Temperature increase rate (K/s) under high pressure (~5 GPa) NO2 fragments (nm-3) under high pressure (~5 GPa) at 9 ps Experimental Impact sensitivity h50% (cm)

cocrystal

TNT

CL-20

100.2±0.8

89.2±0.6

141.9±1.5

0.9±0.04

0

3.2±0.1

191.8±1.3

148.6±1.0

241.7±1.7

2.86±0.07

0.02±0.01

4.8±0.1

99

>275

47

4. CONCLUSIONS We used the CS-RD protocol to predict the sensitivity of the TNT/CL-20 cocrystal. As expected, it is less sensitive than CL-20 and more sensitive than TNT. The predicted relative sensitivities are consistent with the drop weight experiments. According to the number of NO2 fragments, the cocrystal has a lower shear-induced initiation sensitivity about 70% under atmosphere 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% under atmospheric pressure and 22% under high pressure (~5 GPa) than CL-20; but 11% higher under 1 atm and 31% higher under external 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 examine the new TNT/CL-20 cocrystal energetic systems to distinguish the 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 shear rates used here already lead to a continuously shearing fluid system by 10ps, and all slip systems considered here would rapidly detonate under usual conditions. Even so, we find dramatic differences between sensitive and insensitive slip systems, as reflected in the temperature rise and molecule decomposition. Our reactive dynamics studies for PETN, HMX, RDX, CL-20, TNT and cocrystal of TNT/CL20 support the concept that molecular steric hindrance dominate the origin of anisotropic sensitivity of energetic materials. Thus consideration of molecular steric hindrance is critical to the development of energetic materials with reduced sensitivity.


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TOC


Compressive Shear Reactive Molecular Dynamics Studies Indicating

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