Shear stresses in shock-compressed diamond from density functional theory

We report density functional theory (DFT) results for the shear stresses of uniaxially compressed diamond under conditions corresponding to strong shock wave compression. A nonmonotonic dependence of shear stresses on uniaxial strain was discovered in all three low-index crystallographic directions: , , and . For compression the shear stress even becomes negative in the region near the minimum of the shear stress-strain curve. The DFT results suggest that anomalous elastic regime observed in recent molecular dynamics shock simulations is a real phenomenon caused by a significant delay or even freezing of the plastic response

Density Functional Theory Investigation of Sodium Azide at High Pressure

High pressure experiments utilizing Raman spectroscopy indicate that the a phase of sodium azide undergoes a polymeric phase transition at high pressure. In this work, the structural and vibrational properties, including the first order Raman and infrared spectra, of the a phase of sodium azide are calculated using first-principles density functional theory up to 92 GPa. The equation of state of α NaN3 is obtained within the quasi-harmonic approximation at various temperatures. Each Raman-active mode blue shifts under compression whereas the doubly degenerate IR-active azide bending mode red-shifts under compression. However, at 70 GPa, the intensity of the Bu IR-active bending mode decreases substantially, and a new distorted azide bending lattice mode appears in the IR spectrum. In contrast to the bending mode, this new mode blue-shifts under compression. No new modes appear in the Raman spectra at high pressure, indicating that the changes in the Raman spectrum seen in experiment at high pressure are signs of new high nitrogen content structures, but not due to sodium azide

From Laminar to Turbulent Detonations in Energetic Materials from Molecular Dynamics Simulations

The structure of a self-sustained detonation wave in solid energetic materials was studied using molecular dynamics simulations. Energetic materials are described by the AB model with parameters modified to investigate the detonation-wave structures. It is found that depending on the reaction barrier for the exothermic reactions driving the detonation and the boundary conditions of the sample this simple model exhibits a detonation structure that can range from a planar to a complex turbulent detonation. The different regimes of condensed-phase detonation seen are similar to those observed in gases and diluted liquids

Nano-Scale Spinning Detonation in a Condensed Phase Energetic Material

A single-headed spinning detonation wave is observed in molecular dynamics simulations of a condensed phase detonation of an energetic material confined to a round tube. The EM is modeled using a modified AB reactive empirical bond order (REBO) potential. The simulated spinning detonation is similar to those observed in the gas phase. However, in addition to the incident, oblique, and transverse shock waves well known from gas-phase spinning detonations, a contact shock wave generated by a contact discontinuity is uncovered in our MD simulations

Prediction of Isothermal Equation of State of an Explosive Nitrate Ester by van der Waals Density Functional Theory

A new energetic material, nitrate ester 1 (NEST-1), has shown promise as a powerful, technologically attractive explosive. Its physical properties under compression, however, are currently unknown. Accurate density functional calculations together with a reliable empirical van der Waals correction are employed to predict the isothermal hydrostatic equation of state for this material prior to any known experimental results. The accuracy of results obtained from this approach was tested against experimentally known NEST-1 equilibrium properties and found to be excellent

Ammonium Azide under High Pressure: A Combined Theoretical and Experimental Study

Efforts to synthesize, characterize, and recover novel polynitrogen energetic materials have driven attempts to subject high nitrogen content precursor materials (in particular, metal and nonmetal azides) to elevated pressures. Here we present a combined theoretical and experimental study of the high-pressure behavior of ammonium azide (NH₄N₃). Using density functional theory, we have considered the relative thermodynamic stability of the material with respect to two other crystal phases, namely, <i>trans</i>-tetrazene (TTZ), and also a novel hydronitrogen solid (HNS) of the form (NH)₄, that was recently predicted to become relatively stable under high pressure. Experimentally, we have measured the Raman spectra of NH₄N₃ up to 71 GPa at room temperature. Our calculations demonstrate that the HNS becomes stable only at pressures much higher (89.4 GPa) than previously predicted (36 GPa). Our Raman spectra are consistent with previous reports up to lower pressures and at higher pressures, while some additional subtle behavior is observed (e.g., mode splitting), there is again no evidence of a phase transition to either TTZ or the HNS