Nanofractography of shocked RDX explosive crystals with atomic force microscopy

Sharma, J.; Armstrong, R.D.; Elban, W. L.; Coffey, Charles S.; Sandusky, H. W.

doi:10.1063/1.1342046

Cited by 29 publications

(18 citation statements)

References 9 publications

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“…Mechanically enhanced chemistry [1] is observed during indentation [2], friction [3], shock and detonation [4 -11] of energetic materials [12 -14]. A major unsolved problem in this area of mechanochemistry is how strains trigger chemical reactions that cause detonation.…”

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confidence: 99%

Dynamic Transition in the Structure of an Energetic Crystal during Chemical Reactions at Shock Front Prior to Detonation

et al. 2007

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Mechanical stimuli in energetic materials initiate chemical reactions at shock fronts prior to detonation. Shock sensitivity measurements provide widely varying results, and quantum-mechanical calculations are unable to handle systems large enough to describe shock structure. Recent developments in reactive forcefield molecular dynamics (REAXFF-MD) combined with advances in parallel computing have paved the way to accurately simulate reaction pathways along with the structure of shock fronts. Our multimillionatom REAXFF-MD simulations of l,3,5-trinitro-l,3,5-triazine (RDX) reveal that detonation is preceded by a transition from a diffuse shock front with well-ordered molecular dipoles behind it to a disordered dipole distribution behind a sharp front. [12 -14]. A major unsolved problem in this area of mechanochemistry is how strains trigger chemical reactions that cause detonation. Recently, the role of a mechanical stimulus, such as bond bending, in initiating reaction at a shock front has attracted much attention. Manna et al. have shown on the basis of density functional calculation of a TATB crystal that bond-bending of nitro group in TATB molecule closes the HOMO-LUMO gap and have speculated that reaction may proceed at supersonic speed [15]. Atomistic-level understanding of the structure of shock fronts at the onset of detonation requires multimillion-atom simulations with reaction chemistry under high strain rate deformations.We have performed a series of molecular dynamics (MD) simulations of planar shock on a slab of an l,3,5-trinitro-l,3,5-triazine (RDX) [16,17] crystal. Each RDX molecule consists of 21 atoms [ Fig. 1(a)], and the unit cell of the RDX crystal contains 8 RDX molecules [ Fig. 1(b)]. The x, y, and z axes are aligned with the [100], [010], and [001] crystallographic orientations, respectively. We apply periodic boundary conditions in all directions after adding 2 nm of vacuum layers on both sides of the slab along the x axis. We have simulated a system of N 2 322 432 atoms with dimensions 318:48 284:08 271:76 A 3 and also a system of 145 152 atoms to confirm that major results do not change with the system size. Initially, the system is relaxed for 1 ps at 5 K and then quenched to 0 K.Simulations reported here are based on a scalable implementation of a reactive force-field (REAXFF) MD [18,19] on massively parallel computers [20]. The REAXFF potential energy function consists of bonding and nonbonding interactions. Bonding interactions comprise coordination energy, two-body stretching, three-body bending and fourbody torsion energy functions. Nonbonding interactions consists of van der Waals and Coulomb energies [21], where charge transfer is included by an electronegativity FIG. 1 (color). (a) An RDX molecule with carbon (yellow), hydrogen (white), oxygen (red), and nitrogen (blue) atoms. (b) The unit cell of an RDX crystal contains 8 RDX molecules, which are colored blue and red depending on whether the NO 2 groups faces away from (group 1) or faces towards (group 2) the shock plane. Th...

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Dynamic Transition in the Structure of an Energetic Crystal during Chemical Reactions at Shock Front Prior to Detonation

et al. 2007

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“…Such observation was made for cleavage step heights on shocked RDX crystals by Sharma et al [26] Here, we suggest that such smaller step heights might be accounted for by partial dislocation Burgers vectors that penetrate the cleavage surface or by only a component of the actual unit dislocation Burgers vector being normal to the cleavage plane. The observation is important in understanding the influence of mechanical forces on the detonation of energetic materials, because such liberated energy comes from intramolecular decomposition, not from fracturing of the weaker molecular crystal bonds.…”

Section: Discussionmentioning

confidence: 67%

“…Previous studies of cyclotrimethylenetrinitramine (RDX) crystals grown during high acceleration (200,000 g) in an ultracentrifuge have been reported. [1,2] The AFM studies have shown that these RDX crystals have fewer defects than RDX crystals grown at 1 g [3,4] and therefore should be less sensitive to mechanical shock.…”

Section: Introductionmentioning

confidence: 99%

Atomic force microscopy studies of fracture surfaces of composition B energetic materials

Lanzerotti

Sharma

Armstrong

2004

Metall Mater Trans A

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The characteristics of trinitrotoluene (TNT) crystals in composition B have been studied using atomic force microscopy (AFM). The size of TNT crystals has been examined by analyzing the surface structure that is exhibited after mechanical failure of the composition B. The mechanical failure occurs when the material is subjected to high acceleration (high g) in an ultracentrifuge and the shear or tensile strength is exceeded. The AFM examination of the topography of the composition B fracture surface reveals fracture across columnar grains of the TNT. The width of the columnar TNT grains ranges in size from ϳ1 to ϳ2 m. Their height ranges in size from ϳ50 to ϳ300 nm. Flat TNT columns alternate with TNT columns containing river patterns that identify the direction of crack growth. Steps in the river patterns are a few nanometers in depth. The TNT constituent fracture surface morphology is shown to occur on such fine scale, beginning from adjacent columnar crystals only 1 to 2 m in width and including river marking step heights of only a few nanometers, that AFM-type resolution is required even to begin to make clear what has happened.

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“…This may cause even stronger distortion of some of the atomic bonds located in this region and the appearance of a significant number of additional bonding and antibonding states in the RDX band gap. An extensive body of dislocation analysis in a variety of materials has been performed over the years by Armstrong and Elban [14,103,104]; in particular, they demonstrated an interesting contrast in the mechanical properties of molecularly bonded energetic crystals containing dislocations: the crystals are elastically soft, plastically hard, and brittle. They also show that the generation of dislocation clusters at large shock pressures plays an important role similar to the role of anharmonic lattice strains at the dislocation coresthey generate either "in situ" hot spot heating or direct electronic excitation [105].…”

Section: The Electronic Structure Of Rdx Crystals Containing Edge Dismentioning

confidence: 99%

Modeling Defect-Induced Phenomena

Kuklja

Rashkeev

2009

Static Compression of Energetic Materials

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Elucidation of dissociation mechanisms, energy localization, and transfer phenomena in the course of explosive decomposition of energetic materials (EMs) are central for understanding, controlling, and enhancing the performance of these materials as fuels, propellants, and explosives. Quality of energetic materials is often judged using two main parameters: sensitivity to detonation and its performance. Low sensitivity is desired to make the material relatively stable to external stimuli, i.e., controllable and able of triggering rapid dissociation only when needed and not accidentally. Performance, on the other hand, is to be high to provide larger heat of the explosive reaction. These parameters do not necessarily correlate with each other and depend on many variables such as molecular and crystalline structures, history of samples, the particle size, crystal hardness and orientation, external stimuli, aging, storage conditions, and others. Mechanisms governing performance are fairly well understood whereas mechanisms of sensitivity are poorly known and need to be much more extensively studied. It is widely accepted though that the thermal decomposition reactions of the materials play a significant role in their sensitivity to mechanical stimuli and their explosive properties [1].The decomposition of energetic materials can be initiated with a mechanical impact, thermal heating, a shock wave, or a spark. Such events in solids generate molecules in highly excited vibronic and/or electronic states. Clearly, the decomposition of solid explosives under shock, spark, laser, or plasma ignition must include contributions from both ground and excited electronic states. Excitation in the UV can markedly reduce the power requirements for detonation of some secondary explosives. Therefore, establishing the initial steps of high explosive (HE) decomposition is an important goal to pursue. Decomposition triggered by excited electronic states seems appealing because electronic excitation of the system to an unstable potential energy surface can result in rapid dissociation of a molecule and consequent chain reaction.

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Nanofractography of shocked RDX explosive crystals with atomic force microscopy

Cited by 29 publications

References 9 publications

Dynamic Transition in the Structure of an Energetic Crystal during Chemical Reactions at Shock Front Prior to Detonation

Dynamic Transition in the Structure of an Energetic Crystal during Chemical Reactions at Shock Front Prior to Detonation

Atomic force microscopy studies of fracture surfaces of composition B energetic materials

Modeling Defect-Induced Phenomena

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