J. Phys. Chem. 1993,97, 6193-6196
6193
Orientation-Dependent Explosion Sensitivity of Solid Nitromethane J. J. Dick Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received: December 2, 1992; In Final Form: March 9, 1993
A new model for explosive sensitivity has been applied to solid nitromethane. This model based on steric hindrance to shear flow gives a plausible explanation for the explosive behavior reported when nitromethane crystals of certain orientations were subjected to rapid loading in the diamond-anvil, high-pressure cell.
Introduction Catastrophic reactions or explosions have been observed when energetic materials are compressed in diamond cells (DAC) in a nonhydrostatic manner. This has been observed by G. J. Piemarini, S. Block, and co-workers for single crystals of protonated and deuteriated nitromethane1.2as well as for powders of 1,4-dinitrocubane and cubane.2 In nitromethane crystals, the explosive behavior was dependent on crystal orientation. For nitromethane violent explosions occurred in certain orientations when a rapid change in stress was applied from 0.5 to 3.0 GPa. In deuteriated nitromethane the explosions occurred when the stress was raised from 0.5 to 9.0 GPa. This orientation-dependent behavior is akin to that observed in the shock initiation sensitivity of crystals of pentaerythritol tetranitrate (PETN).3 This strong dependence on crystal orientation relative to the shock direction was explained in terms of the orientation-dependent steric hindrance to the shear flow at the molecular level which occurs in the uniaxial strain of a shock wave. The hindered shear generates a localized, endothermic, nonequilibrium excitation process leading to exothermic decomposition reactions. One model for this process based on preferential excitation of optical phonon modes by the sterically hindered shear process with their concomitant strong phonon-vibron coupling was presented by R. D. Bardo in ref 4. In this paper we present the results of applying the steric hindrance model to nitromethane.
Steric Hindrance Analysis When nonhydrostatic stresses are present in the DAC with shear stresses exceeding the critical shear stress for plastic flow, irreversible shear deformation of the crystal occurs. This is analogous to the shear in the uniaxial strain of a plane shock wave. If compression in the DAC involved only a decrease in the separation of the diamond faces, a state of uniaxial strain would be induced here also. However, since compression in the DAC involves a reduction in gasket diameter in addition to a decrease in anvil separation, the total strain deformation is multidimensional. Nevertheless it seems worthwhile to apply the steric hindrance analysis to nitromethane in order to see if an orientation effect exists. Shear deformation occurs mainly by multiplication and motion of dislocations and twinning. If the strength of the perfect crystal is exceeded, deformation can proceed by simultaneous shear without the aid of crystal defects.5 The steric hindrance modeling is performed by analyzing the shear at the level of the unit cell by considering the motion of rigid molecules on one side of the shear plane over stationary molecules on the other side. For shocks, the shear will occur on the slip system (plane and direction) which has the maximum resolved shear stress on it from the applied uniaxial strain. For isotropic materials this is a t 45' to the applied strain (shock propagation direction). For anisotropic crystals it will be on some crystallographic plane and direction near 45O. The shear stress is determined by tensor transformation
of the longitudinal stress in the shock direction onto a slip system via the elastic constants. Provided that the critical shear stress for plastic flow is exceeded, the shear will take place on the slip system with the maximum resolved shear stress. A view of the unit cell of nitromethane at 3.5 GPa is shown in Figure 1 based on data in refs 6 and 7. There are four molecules per unit cell in the orthorhombic system with space group P2121 and lattice parameters of 4.89,5.88, and 8.12 8,. The molecules are stacked in columns parallel to the c axis. Adjacent molecules in the columns have nearly parallel C-N bonds perpendicular to the c axis with the methyl and nitro groups alternating ends. In adjacent columns the C-N bonds are nearly perpendicular to one another. Since the elastic constants of solid nitromethane are not known, it is not possible to calculate which slip system has the maximum resolved shear stress for a given applied stress. Also, the preferred slip system has not been determined experimentally. Therefore it will be necessary to do a general assessment of possible slip systems. A computational analysis is performed on a model in which rigid molecules on one side of the shear plane are moved incrementally past stationary molecules on the other side of the shear plane. Atom-atom approaches closer than 1.70 8, were taken as close approaches. This is longer than most covalent bond lengths but shorter than most van der Waals distances. The shear displacement across the unit cell (Burgers vector) was divided into 100 increments. At each position the intermolecular atom-atom distances across the shear plane were computed by a series of nested DO loops in a FORTRAN program constructed for this purpose. Any distances less than 1.70 8, were selected out, identifying the atomic pair for the purpose of plotting the curve of interatomic distance vs shear displacement. This geometric analysis appears to correlate with the physical steric hindrance to shear.' Nitromethane has some slip systems which are unhindered (no close approaches). Some can be seen in Figures 2 and 3. They are ( 100)(001), (010)(001), and (001) ( 100). When shear occurs across those planes in those directions, there are no close atomatom approaches. A set of stresses which causes slip on these planes should not initiate decomposition chemistry or explosion. The (1 10)(001) slip system has a pair of close 0-H approaches down to 1.5 8, (Figure 2). The (01 l)(TOO) system has a pair of 0-H approaches down to 0.97 8, (Figure 3). When the Burgers vector (slip direction) involves diagonal components with disparate lattice parameters, there are cases of severe hindrance. A shock in the [OOl] direction has no unhindered slip systems available toit. Insteadthestrongly hindered (lOl)(TOl) and(lOlj(T11) slip systems are available (Figure 4). The first has 31 and the second has 27 close approaches (Table I). The curves of atomatom approaches during the shear across the unit cell for the (101)( 111) slip system are displayed in Figures 5-7. Especially in this case there are many 0-H interactions. These close approaches can generate hydrogen bonding which in turn will
This article not subject to U.S.Copyright. Published 1993 by the American Chemical Society
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The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
t P a
1
Figure 1. Unit cell of nitromethane at 3.5 GPa. The ( 110) diagonal axis is toward the viewer. Note the two types of columns of molecules with C-N bonds nearly normal to one another. In each stack the methyl and nitro groups alternate ends from molecule to molecule. There are 4 molecules per unit cell, 2 in each stack. Atoms are shown at full covalent radii. C, H, N, and 0 are shown in black, white, blue, and red, respectively.
Figure 2. Unit cell of nitromethane at 3.5 GPa. The positive c axis is toward the viewer. Note the two types of columns of molecules with C-N bonds nearly normal to one another. In each stack the methyl and nitro groups alternate ends from mqlecule to molecule. There are 4 molecules per unit cell, 2 in each stack. In this view one can visualize steric hindrance to shear for { 100)( 001 ),{O 10)(00 1) ,and { 110)(00 1) slip systems. Visualize molecules on or on one side of a particular plane moving toward the viewer past stationary molecules on the other side of the shear plane. Atoms are shown at half of the covalent radii. C, H, N, and 0 are shown in black, white, blue, and red, respectively.
d t 1 I
.
. .
Figure 3. Stereoviewof some relatively unhindered slip systems of nitromethane. The positive a axis is toward the viewer. The positive c axis is upward. One can visualize steric hindrance to shear for the {OOl)( 100) and (01I)( TOO) systems. Visualize molecules on or on one side of a particular plane moving toward the viewer past stationary molecules on the other side of the shear plane. The {OOl)( 100) system has no interactions (approaches closer than 1.7 A), while {01I)( 100) has two 0-H approaches down to 0.97 A. Atoms are shown at half of the covalent radii. C, H, N, and 0 are shown in black, white, blue, and red, respectively.
weaken the C-N bond and lead to the formation of HONO fragments, for instance, if local temperatures are high due to the dissipative processes in the shear bands. This is an example of a shear-induced decomposition reaction. Based on these results we predict that for a shock along the c axis the shear will be stericallyhindered and will lead to direct decomposition reactions and detonation for moderate strength shocks. The [OOl]
orientation will be much more shock sensitive than most other orientations. It is interesting to note that an analysis from a different theoretical point of view found that the [OOl]direction in nitromethane is more favorable for detonation.8 Testing the prediction of increased shock sensitivity for the [OOl] direction will be difficult since nitromethane solidifiesat -29 O C at ambient pressure. Doing large-scale impact experiments on crystals of
Explosion Sensitivity of Solid Nitromethane
The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6195
Figure 4. Stereoview of a hindered slip system. The (101 ) direction is toward the viewer. The (101) plane is normal to the view and transects the cell across the middle of the view. Visualize molecules above the { 1011plane moving toward the viewer over molecules below the plane. Per unit cell there are 8 0 - H interactions and a total of 31 interactions. The separation between atoms as seen in this view represents their closest approach. Atoms are shown at half of the covalent radii. CyH, N, and 0 are shown in black, white, blue, and red, respectively.
TABLE I: Atom-Atom Interactions for Nitromethane shocka slip direction system [loll {lOO)(OOl)
shock/slip angleb (deg)
[loll [Oli]
{001)(100) {010)(001)
58.9 31.1 54.1
[iio] [OOl]
{oii)(ioo) {lOl)(iOl)
50.2 58.9
atom pair 0-H 0-H 0-H O-H 0-H H-H C-H N-0
c-c
0-0 N-H C-0 N-N
no. of interactions 0
closest approach
(A) 21.70
0 0
2 1.70 2 1.70
2 8 7 4
0.97 1.16 0.27 0.36 0.89 1.14 0.39 1.15 1.11 1.53
4
1 3 2 1 -1
Ee log
+ v)
m
a
5
7
E 0 %
5
*,
v)
-
€
0 +
4
3
Y 2 E
0
'
3
0 0.0
0.2
0.4
0.6
0.8
1.0
Burgers displacement Figure 5. Intermolecular 0-H close approaches for a [OOl] shock as the shear displacement occurs across the {loll plane in the direction of the (111) Burgers vector. Burgers displacement is 11.15 A.
31 [ooi]
{ioi)(iii)
0-H 0-0 N-0 N-H N-N H-H C-H
46.7
c-0 C-N
9 3 5 2 1 3 2 1 1 27
0.53 0.62 0.58 1.16 1.13 0.67 1.23 0.59 1.21
*
a Or direction of uniaxial strain. Angle between shock propagation direction or direction of uniaxial strain and the slip direction (Burgers vector).
known orientation will be difficult. Perhaps -~ experiments with laser-driven plane shocks would be appropriate. The steric hindrance analysis for uniaxial strain conditions predicts an orientation effect for the sensitivity of nitromethane. This corroborates the effect seen in diamond anvil cells.' They reported explosions when either the (01 1) or the {llO) crystal faces were parallel to the anvil faces. The {Ol1) crystal has the unhindered (OlO)(OOi) slip system at 54O to the applied anvil stresses. The {110)crystal has the (01 1)( 100) slip system at 40' to the anvil stresses. This system has just two 0-H interactions withclosest approaches of 0.97 A (Table I). Therefore, sensitivity would not be expected if the strain were uniaxial. However, the compression in the gasketed DAC is not a state of uniaxial strain. As stated earlier, the system of nonhydrostatic stresses depends on the details of the reduction in gasket diameter as well as the decrease in the separation of the anvil faces. If conditions are suchthatslipisforcedina (lOl), (Oll),or (111) typeofdirection,
0.0
0.2
0.4
0.6
0.8
1.0
Burgers displacement Figure 6. Intermolecular atom-atom close approaches for a [OOl] shock as the shear displacement occurs across the {lOl) plane in the direction of the (111) Burgers vector. Burgers displacement is 11.15 A. Solid lines are for 0-0,chain-dotted lines are for N-O, chain-dashed lines are for N-N, and dashed lines are for N-H.
the slip will be sterically hindered and reactions will be induced. For example, the longitudinal stress on a (011) plane has the (111) slip direction at 31.3' to it and the (101) at 59.8'. The longitudinal stress on a { 110) plane has the ( 111) slip direction at 46.7' to it. The explosion effect in nitromethane during rapid compression in the DAC was not completely reproducible? The effect appeared to depend on the details of the uncontrolled, asymmetric contraction of the gasket interior circumference. The sample radius was not decreasing at the same rate through all 360O. The state of deformation was being governed by details of the
6196 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
Dick system leading to runaway decomposition reactions, in our interpretation. Acknowledgment. Helpful conversations with Choong-Shik
Yoo are appreciated. The stereoview figures were generated by Melvin Prueitt. The work was performed under the auspices of the U.S.Department of Energy and partially supported by the Office of Munitions Memo of Understanding between the Department of Energy and the Department of Defense. References and Notes I
0.0
I
0.2
0.4
I
0.6
I
0.8
I
1.0
Burgers displacement
Figure 7. Intermolecular atom-atom close approaches for a [OOl]shock as the shear displacement occurs across the (101)plane in the direction of the ( 1 1 1 ) Burgers vector. Burgers displacement is 11.15 A. Solid lines are for H-H, chain-dotted lines are for C-H, chain-dashed lines are for C-N, and dashed lines are for C-O. multidimensional strain field engendered by these conditions. This deformational state in turn governed on which slip system theshear deformation occurred. For certain conditions the (011) and ( 110) oriented crystals were forced to slip on a hindered
(1)
Piermarini, G.J.; Block, S.; Miller, P. J. J . Phys. Chem. 1989, 93,
451. ( 2 ) Piermarini, G. J.; Block, S.; Damavarapu, R.; Iyer, S. Propellants, Explos. Pyrotech. 1991, 16, 188. (3) Dick, J . J.; Mulford, R. N.; Spencer, W. J.; Pettit, D. R.; Garcia, E.; Shaw, D. C. J. Appl. Phys. 1991, 70, 3512. (4) Bardo, R. D. Proc. 9th Symp. (In?.)on Detonation 1989, 235. (5) Dick, J. J. Appl. Phys. Lett. 1992, 60, 2494. (6) Cromer, D. T.; Ryan, R. R.; Schiferl, D. J . Phys. Chem. 1985,89, 2315. (7) Trevino, S. F.;Prince, E.; Hubbard, C. R. J . Chem. Phys. 1980,73, 2996. (8) Rudel, P.; Odiot, S.; Mutin, J. C.; Peyrard, M. J. Chim. Phys. 1990, 87, 1307. (9) Piermarini, G. J. Private communication, 1991.
