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Understanding the Desensitizing Mechanism of Olefin in Explosives versus External Mechanical Stimuli Chaoyang Zhang* Laboratory of Material Chemistry, Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P.O. Box 919-327, Mianyang, Sichuan, People’s Republic of China, 621900 ReceiVed: NoVember 16, 2009; ReVised Manuscript ReceiVed: January 27, 2010
The desensitizing mechanism of amorphous olefin in a typical explosive HMX against external mechanical stimuli was investigated by loading uniaxial compression and shear on four cells composed of pure HMX, pure olefin, HMX + 3 wt % olefin, and HMX + 9 wt % olefin, respectively. As a result, it was confirmed that the mechanism under the loading conditions is predominantly attributed to the good lubricating property of olefin, which can greatly reduce shear stress denoted by shear sliding barriers. At the same time, the addition of a little olefin in HMX, for example, below 10 wt %, cannot obviously improve the compressibility of HMX-based explosives. It therefore indicates the lowered mechanical sensitivity is not caused by improving compressibility. In addition, my simulations did not show that the more olefin added in HMX results in the more evident desensitizing effect, suggesting a critical point of the component of HMX + olefin corresponding to a lowest shear stress. 1. Introduction High energy along with low sensitivity is mostly expected in the field of explosives. However, up to the present, it still remains a challenge to break through the bottleneck of the contradiction of high energy together with high sensitivity of explosives only by synthesizing new energetic compounds.1 People have not yet found a better explosive with comprehensive performances to replace 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), which is the so-called king of explosiVes and has been the best explosive with comprehensive performances for over half a century.2,3 The very low stabilities, or the very high sensitivities, are one of the most important factors limiting the applications of some explosives even with more power than HMX. For example, the long-desired and more energetic octuplenitrocubane (ONC) has not been applied in practice since its birth in 2000,4 due to its very high sensitivities versus many kinds of stimuli. Also, this high sensitivity makes another more powerful explosive 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12- hexaazaisowurtzitane (CL-20) less applicable.5 As a matter of fact, it is just these unacceptable sensitivities that make many explosives nameless, even though they are more energetic than HMX and have been synthesized for a long time. Fortunately, people are able to overcome the contradiction successfully by reducing sensitivity without energy loss or with very little energy loss. That is to say, they can obtain explosives with high energy also and acceptable sensitivities by two ways simultaneously: one is the continuous endeavor to synthesize new explosive species and another is to look for more applicable additives to desensitize explosives. Olefin, which is an amorphous mixture of alkanes with 18-30 carbons, has long been used as a desensitizer in many explosive formulations.2 It seems that its desensitizing mechanism is attributed to its low melting point and good lubricating property. However, according to my knowledge, there is to date no molecular level understanding * Phone: 86-816-2493150. Fax: 86-816-2495856. E-mail: zcy19710915@ yahoo.com.cn.
of this. It therefore becomes a topic in this paper to explore the desensitizing mechanism of olefin in explosives on a molecular level. It can be seen from the above that the sensitivity is directly involved in the security of explosives and is therefore one of the topics of most concern in explosives. Multiscaled methods6 have been carried out to investigate these sensitivity mechanisms, due to the improvements of science and technologies. In the range of molecular simulations there are also three scales according to the sizes of the involved objects: electron, atom or molecule, and mesoscale particle. On the first scale, the relationships between molecular and crystal structures (electronic structure included) and explosive sensitivities are explored by using quantum chemistry methods or periodic quantum chemistry if necessary.1,7-16 The second is to simulate the evolution processes of explosive crystals against external mechanical forces, similar to some loading conditions.17,18 These simulations on bigger periodic structures using forcefield methods can now involve several millions of atoms or even more, closer to practice. The third is aimed at mesoscale particles, which may be a very promising method to probe the explosive sensitivity mechanism despite no related report yet. However, it has been successful in exploring the irreversible expansion of 2,4,6triamino-1,3,5-trinitrobenzene (TATB).18 Altogether, it seems that the sizes of simulated objects become bigger and bigger, and the simulation is closer and closer to practice. There are still relatively fewer research projects on the desensitizing mechanism of desensitizers in spite of much more progress in exploring detonation mechanisms. Recently, we have understood the desensitizing mechanism of some crystal desensitizers including graphite and TATB at a molecular level.20-22 This paper shows the desensitizing mechanism of olefin belonging to another kind of desensitizer, amorphous instead of crystal. I expect that it is helpful to probe the mechanism more deeply and find new more applicable desensitizers.
10.1021/jp910883x 2010 American Chemical Society Published on Web 02/25/2010
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2. Methodologies 2.1. Theory of Mechanical Sensitivity. Mechanical sensitivities denote the sensitivity versus external stimuli including friction and impact.2 Before the chemical decomposition of explosives these stimuli can lead to the compression and shear slide of the explosives, producing internal stress. It has been confirmed that the formation of a hot spot which possibly causes the final detonation has a correlation with the internal stress of explosives: the more the stress, the higher the sensitivity.23,24 This paper studies just the desensitizing mechanism of olefin by the stress change after its addition to explosives. Two cases are considered: uniaxial compression (with a reduced volume) and shear slide (with a fixed volume). Here, the internal stress resulting from compression is calculated by using eq 1, which can be seen elsewhere.20
Pcomp,i
∆Ei Ei - E0 ) ) ∆Vi V0 - Vi
(1)
And the internal stress caused by shear slide is denoted by sliding potential and is computed by eq 2.
Pslid,i )
∆Ei Ei - Emin ) V0 V0
(2)
In above two equations, E and V represent the internal energy and the volume of the systems, respectively; the subscripts 0 and i denote the initial value and the value of ith step, respectively. In eq 2, Emin, the minimum energy when sliding, is adopted because E0 is not always Emin. This occurs in the case of pure olefin, owing to the amorphous characteristics of olefin. 2.2. Modeling and Simulation Methods. 2.2.1. Establishment of Models. The desensitizing mechanism of olefin involved in a typical explosive HMX was simulated, in view of the uniaxial compression and the shear slide along the (010) face orientation of a pure HMX cell and two HMX + olefin cells with different quantities of olefin, and a pure olefin cell for comparison purposes. Considering the rigidity of HMX and the amorphous characters of olefin, I first built a HMX cell and then other HMX + olefin cells based on the HMX cell. The detailed processes of building the four necessary cells are given below: (i) Build a HMX Cell and an Olefin Cell. I first cleaved a (010) plane from the HMX experimental crystal with a thickness 10× that of the HMX unit cell and built a slab without vacuum according to this plane, actually a HMX cell. Then I constructed a 4 × 4 × 1 supercell of the cell and relaxed it without any constraint using the COMPASS forcefield.25 The necessary HMX cell was obtained as shown in Figure 1c, with 320 HMX molecules and lattice parameters a ) 30.53 Å, b ) 25.92 Å, c ) 103.61 Å, R ) β ) 90°, and γ ) 78.34°, and was taken as a reference to establish two HMX + olefin cells. Also, an olefin amorphous cell as illustrated in Figure 1a was made by Monte Carlo and forcefield relaxation methods. The cell is composed of 100 alkane chains, each with 22 carbon atoms according to the average molecular size of the actual olefin. After 10 ns NVT dynamics at 350 K and subsequent relaxation, the necessary olefin cell was obtained. (ii) Build Two HMX + Olefin Cells. First, two olefin amorphous cells respectively containing 10 and 30 alkane chains each with 22 carbon atoms were created on the basis of the lattice parameters of the above built HMX cell, for conveniently constructing two HMX + olefin cells. That is, the amorphous cells and the HMX cell have five fixed parameters and only
Figure 1. Models for this study: (a and b) pure amorphous olefin cells, (c) a pure HMX cell, and (d) a built HMX + olefin cell by the addition of b to c. Models a, c, and d are all prepared for the following compression and shear slide simulations.
TABLE 1: Components, Structures, and Densities of Four Cellsa
olefin HMX HMX + 3 wt % olefin HMX + 9 wt % olefin
component
c, Å
density, g/cm3
100C22H24 320HMX 320HMX + 10C22H24 320HMX + 30C22H24
73.23 103.61 111.85 125.69
0.908 1.959 1.874 1.773
a Note, all cells have five fixed lattice parameters a ) 30.53 Å, b ) 25.92 Å, R ) β ) 90°, and γ ) 78.34°.
one different c which are 6.66, 19.97, and 103.61 Å, respectively. The densities of two olefin amorphous cells are designated as 1 g/cm3, a little above its experimental value of 0.9 g/cm3, which is useful to saturate olefin in the HMX surface through a squeeze-relaxation mode. Then, two HMX + olefin cells (demonstrated as Figure 1d) were obtained by the respective addition of olefin cells (Figure 1b) to a HMX (Figure 1c) cell. After 5 ns NVT dynamics at 350 K with fixed HMX molecules, and subsequent relaxation with the five fixed lattice parameters excluding c, the two necessary HMX + olefin cells were obtained. The four necessary cells are summarized in Table 1 and will be taken as the initial structures for following compression and shear slide simulations. 2.2.2. Calculating Methods. A COMPASS forcefeild, based on first principle calculations and experiments, was adopted here for geometry relaxations and molecular dynamics. The validity of the forcefield to the HMX crystal and the HMX-based plastic bonded explosives (PBX) already has been established.26 Here, the density of relaxed HMX shown in Table 1 is 1.959 g/cm3, 3.1% higher than its experimental value. However, this reduces to 1.925 g/cm3 after a NPT dynamic equilibrium at 298 K, and under atmospheric pressure, it is only 1.3% higher than the experimental value. This therefore shows again the applicability of COMPASS to HMX. Also, from the above calculations in building models, we can find the density of relaxed olefin in pure olefin and HMX + olefin mixed systems is consistent with the experimental value, about 0.9 g/cm3. This verifies the reliability of COMPASS to the interested systems according to these densities, or it is at least qualitatively adequate. In all energy summation calculations, Ewald and atom based methods were employed for electrostatic and van de Waals interactions, respectively. Andersen thermostat27,28 and Berendsen barostat29 methods were respectively used for NVT and NPT dynamic simulations. All simulations were carried out with use of Accelrys’ Material Studio code.30 2.2.3. Simulations for Compression and Shear Slide. A series of step-by-step calculations were performed to simulate
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Figure 4. Plot showing the difference between the shear slide of a cell and that of a molecule: (a) a shear-slide HMX unit cell; (b) the original cell; and (c) a shear-slide HMX molecule, the upper molecule in the cell. Figure 2. Plot showing the uniaxial compression of an HMX + olefin cell.
Figure 3. Plot showing the shear slide of an HMX + olefin cell parallel to AOB plane and along OA orientation. The arrows point to the slide orientation. Figure 5. Internal stress induced by compression as a function of V/V0.
four representative cells against external mechanical stimuli, namely, the compression and shear slide. This step-by-step static process is to a certain extent close to a dynamic process, or a quasidynamic process, which may be closer to practice. That is to say, in these successive relaxation calculations, the relaxed structure of a loaded cell in one step was used as the initial structure for the next step in which the compression or shear slide was loaded again. In all relaxations, the cells remained fixed. The uniaxial compression along the c axis of the built cells shown in Figure 2 was considered. Namely, all lattice parameters except c were fixed during the compression. Note that the c axis here is not that of the original HMX unit crystal because the HMX cell illustrated in Figure 1c was rebuilt from the original HMX unit crystal as mentioned in Section 2.2.1. The cell was compressed up to 40% with a step of V/V0 ) 0.02. As illustrated in Figure 3, the shear slide is parallel to the AOB plane and along the OA orientation. It should be emphasized that the shear slide is of the cell and not of molecules. The difference between the two kinds of slides can be seen in Figure 4. In the case of the horizontal shear slide of an upper HMX molecule in the unit cell (Figure 4c), the upper molecule is translated while both the intraatomic distances in the upper and the bottom molecules remain fixed. This implies that there may be a very short distance between two atoms respectively belonging to two different HMX molecules and even their superposition, which increases great ly the repulsion after sthe lide. In the case of the shear slide of a cell (Figure 4a), all atomic fractional coordinates do not change, and the interatomic distances change according to the changes of the lattice parameters. Obviously, no atomic superposition can take place in this case. During the simulation, the cell angle β was
successively reduced from 90° to 50° with a step of 2°, and the cell length OC was simultaneously elongated to maintain a fixed cell volume. Also note, as indicated in Figures 2, 3, and 4a, after loading a compression or a shear slide, atomic fractional coordinates in cells do not change, which can in a certain range reflect the practical loading process. 3. Results and Discussion The simulation results of compression indicated in Figure 5 are discussed first. It appears that the internal stress induced by compression decreases in an order HMX > HMX + 3 wt % olefin > HMX + 9 wt % olefin > olefin at a given V/V0 in the full range, suggesting the more olefin in the cell, the less induced internal stress. Furthermore, it seems that there exists an approximate simple algebraic relationship between the stress and the components of cells, which can be denoted as eq 3
Pmix ) PHMXXHMX + PolefinXolefin
(3)
in which Pmix, PHMX, and Polefin represent the stress of the HMX + olefin mixture, pure HMX, and pure olefin, respecitively, at a given V/V0, and X represents the mass percentage of HMX or olefin. However, it does not allow too much olefin in practical explosive formulations; otherwise it will lead to too much energy loss and little satisfaction of the energy requirement. In most olefin-containing explosives, its mass percentage is usually lower than 5%, showing that their compressibility is very close to that of pure HMX according to the data in Figure 5. I therefore
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Figure 6. Molecular configurations of HMX under the different compression conditions. The dashes denote hydrogen bonds, to which a criteria is that the maximum hydrogen-acceptor distance is 2.2 Å and the minimum donor-hydrogen-acceptor angle is 130°. Red, blue, and gray represent oxygen, nitrogen, and carbon atoms, respectively.
TABLE 2: Some Dihedral Angles (in deg) on the Ring of HMX under Different Compressiona V/V0
D1234
D2345
D3456
D4567
0.70 0.72 0.74 0.76
-76.8 -77.9 -112.8 -113.5
124.1 124.1 116.0 117.6
-82.9 -81.3 -37.9 -34.4
3.5 2.3 -28.8 -30.9
a
The atomic numbering is showed in the first cell in Figure 6.
TABLE 3: β Corresponding to the Maximum or the Minimum of Sliding Potentials β, deg
HMX
olefin
HMX + 3 wt % olefin
HMX + 9 wt % olefin
maximum minimum
68 90
62 86
52 90
74 90
cannot think that the desensitizing mechanism of olefin is attributed to the better compressibility of olefin than that of HMX. At the same time, the phase transition of olefin during compression is not found due to a smooth line of olefin demonstrated in Figure 5. Or, the phase transition will not lead to changed compressibility. Additionally, there is an obvious extra bump around V/V0 ) 0.74 on the stress line of HMX. Considering only the case of the unit cell of HMX (Figure 6) instead of analyzing the complicated configurations of 320 relaxed HMX molecules in a cell (Figure 1c) under different compression conditions, it was confirmed that the bump is mainly attributed to the abrupt changes of molecular configurations of HMX (Table 2) and the formation of hydrogen bonds (the dashed bonds in Figure 6) during the compression between V/V0 ) 0.72 and 0.74. There is possibly a structural transition in the stacking of the HMX in this case. Next, we discuss the case of shear slide. I did not find a monotonous relationship between the sliding potentials (SP) induced by shear and the cell angle, β. For example, the minimum or maximum SP in Table 3 does not always correspond to the minimum or maximum β, 90° or 50°. These nonmonotonicities are probably due to the amorphous property of olefin, the molecular configuration changes of HMX in the shear slide, and the interaction among the related molecules in the cells. The first and the third points may be easily understood,
Figure 7. Molecular configurations of the shear slide unit cell of HMX.
and the second point can be explained by some shear slide unit cells of HMX demonstrated in Figure 7 instead of complicated configurations of 320 relaxed HMX molecules in a cell in Figure 1c. In contrast to the slide cell with γ ) 70°, this one with γ ) 50° has more variation of molecular configuration, which is helpful to decrease the total energy, relative energy (RE) 36 versus 128 kJ/mol, suggesting the maximum SP does not necessarily correspond to the maximum sliding angle. As a matter of fact, the maximum SP, or the sliding barrier (SB), should be the most concerned. As indicated in Figure 8, 41 SP data of each cell are therefore arranged as a hill to clearly show the SB. From the figure, we can first find a huge SP difference between HMX and others containing olefin. That is, HMX has more than 20 even 30 times SB than olefin or HMX + olefin mixtures. This should be the main reason for the olefin’s desensitizing effect. Second, different from the above case of uniaxial compression, there is no obvious relationship between the SB and the components of four cells. Comparing the SB of three cells containing olefin, it is surprising that the less olefin, the lower the SB. Combining this point with the very huge SB of pure HMX, it is proposed that there is a critical point corresponding to the lowest SB as illustrated in Figure 9, showing a dependence of the SB and the components of cells. The sliding behaviors of HMX + olefin cells when loading a shear are determined by the interactions in the bulk phases of HMX and olefin, and on the interface between them. Apparently, these three kinds of interactions correspond to three kinds of SB belonging to two bulk phases and an interface, in which the smallest one determines the SB of the whole material. It can be deduced from Figure 8 that the SB of pure HMX or
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Zhang more refer to the rather weak intermolecular interactions on the interface between olefin and HMX instead of those in their bulks. Acknowledgment. Financial support from the CAEP’s fund (2009B0302032), the China National Project (973-61383), and the China National Natural foundation (10972055/A020601) is greatly appreciated. References and Notes
Figure 8. Sliding potential induced by shear of four cells.
Figure 9. A proposed plot showing a dependence of sliding barriers and components.
pure olefin is higher than that of their interface between them. This is the reason for the possible critical point. 4. Conclusions A series of simulations with COMPASS forcefield were carried out for understanding the desensitizing mechanism of olefin in typical HMX-based explosives versus external mechanical stimuli, in which two cases including uniaxial compression and shear slide were considered. I think that this desensitizing mechanism greatly depends on the enormous reduce of shear stress of explosives after the addition of a little olefin. It is just the so-called good lubricating property of olefin. In addition, it has not been found that more added olefin leads to a more reduced sliding barrier, suggesting there is a critical point of explosive components corresponding to a lowest barrier. Therefore, the so-call good lubricating property of olefin may
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