Hot spot formation and chemical reaction initiation in shocked HMX crystals with nanovoids: a large-scale reactive molecular dynamics study

Abstract

We report million-atom reactive molecular dynamic simulations of shock initiation of β-cyclotetramethylene tetranitramine (β-HMX) single crystals containing nanometer-scale spherical voids. Shock induced void collapse and subsequent hot spot formation as well as chemical reaction initiation are observed which depend on the void size and impact strength. For an impact velocity of 1 km s(-1) and a void radius of 4 nm, the void collapse process includes three stages; the dominant mechanism is the convergence of upstream molecules toward the centerline and the downstream surface of the void forming flowing molecules. Hot spot formation also undergoes three stages, and the principal mechanism is kinetic energy transforming to thermal energy due to the collision of flowing molecules on the downstream surface. The high temperature of the hot spot initiates a local chemical reaction, and the breakage of the N-NO2 bond plays the key role in the initial reaction mechanism. The impact strength and void size have noticeable effects on the shock dynamical process, resulting in a variation of the predominant mechanisms leading to void collapse and hot spot formation. Larger voids or stronger shocks result in more intense hot spots and, thus, more violent chemical reactions, promoting more reaction channels and generating more reaction products in a shorter duration. The reaction products are mainly concentrated in the developed hot spot, indicating that the chemical reactivity of the hmx crystal is greatly enhanced by void collapse. The detailed information derived from this study can aid a thorough understanding of the role of void collapse in hot spot formation and the chemical reaction initiation of explosives.

Reaction mechanisms and sensitivity of silicon nitrocarbamate and related systems from quantum mechanics reaction dynamics

“…At crystal scale, sensitivity is related to molecular packing mode, anisotropy, and crystal quality such as defect, shape, and size. [16][17][18][19][20][21][22] At molecular scale, sensitivity is strongly dependent on molecular composition and geometry configuration. 1,[23][24][25][26][27][28] For example, sila-pentaerythritol tetranitrate (Si-PETN)…”

X‐ray Phase Contrast Imaging of the Impact of Multiple HMX Particles in a Polymeric Matrix

“…These debated interactions are wide-ranging including defect and pore interactions [6-9], particle-particle interactions [10,11], energy dissipation [12], and frictional heating [13][14]; however as this paper focuses on PBX samples with two or more HMX particles, the most relevant factor is the particle-particle interactions and frictional heating. Zhou et al looked into the surface roughness and found that there is a "statistically significant" correlation between the surface quality and the shock sensitivity [1]. They discussed that shock sensitivity could be lessened by a smooth surface finish, changing the initiation pressure from 3.3 GPa to 3.9 GPa by merely changing the surface texture [1].…”

“…These HE particles would be a production-grade, thus polycrystalline containing possible defects and voids. These defects and voids are prime locations for the formation of hot spots and increase the sensitivity of PBX [21][22][23][24][25].…”

X‐Ray Phase Contrast Imaging of the Impact of a Single HMX Particle in a Polymeric Matrix

“…Production HE particles are polycrystalline and inherently possess intraparticle defects, mostly in the form of voids and cracks ( Figure 1) making them candidates for the observation of hot spot dynamics [19,20]. Defects in the form of voids, grain boundaries, or cracks are considered to be nucleation sites for hot spots and therefore increase the sensitivity of energetic samples [21][22][23].…”