Aaron Landerville

Hydrostatic and uniaxial compression studies of 1,3,5-triamino- 2,4,6-trinitrobenzene using density functional theory with van der Waals correction

Hydrostatic and uniaxial compressions of 1,3,5-triamino-2,4,6-trinitrobenzene were investigated using first-principles density functional theory with an empirical van der Waals correction. The equilibrium structural and elastic properties and the hydrostatic equation of state are in good agreement with available experimental data. Physical properties such as the principal stresses, shear stresses, band gap, and the change in energy per atom as a function of compression ratio V/V0 in the directions normal to the (100), (010), (001),(110), (101), (011), and (111) crystallographic planes were calculated, showing highly anisotropic behavior under uniaxial compressions.

Reactive molecular dynamics of hypervelocity collisions of PETN molecules.

Born-Oppenheimer direct dynamics classical trajectory simulations of bimolecular collisions of PETN molecules have been performed to investigate the fundamental mechanisms of hypervelocity chemistry relevant to initiating reactions immediately behind the shock wavefront in energetic molecular crystals. The solid-state environment specifies the initial orientations of colliding molecules. The threshold velocities for initiating chemistry for a variety of crystallographic orientations were correlated with available experimental data on anisotropic shock sensitivity of PETN. Collisions normal to the planes (001) and (110) were found to be most sensitive with threshold velocities on the order of characteristic particle velocities in detonating PETN. The production of NO2 is the dominant reaction pathway in most of the reactive cases. The simulations show that the reactive chemistry, driven by dynamics rather than temperature during hypervelocity collisions, can occur at a very short time scale (10(-13) s) under highly nonequilibrium conditions.

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, trans-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.

HYDROSTATIC EQUATION OF STATE AND ANISOTROPIC CONSTITUTIVE RELATIONSHIPS IN 1,3,5‐TRIAMINO‐2,4,6‐TRINITROBENZENE (TATB)

Using first‐principles Density Functional Theory (DFT) with an empirical van der Waals (vdW) correction, we studied the equilibrium properties and hydrostatic equation of state (EOS) for TATB, and compared the results with experimental data. The equilibrium unit‐cell parameters and isothermal EOS calculated with vdW‐DFT show better agreement with experiment than standard DFT, which does not provide a proper description of long‐range dispersive interactions. Uniaxial compressions normal to the {001}, {010}, {011}, {100}, {101}, {110}, and {111} crystallographic planes were also studied. Calculated mechanical properties such as principal and shear stresses and the energy per atom show a clear anisotropy upon uniaxial compression.

PHYSICAL AND CHEMICAL PROPERTIES OF A NEW ENERGETIC MATERIAL SI‐PETN

A new energetic material Si‐PETN, having a structure similar to that of PETN, has recently been synthesized (T.M. Klapotke et al., J. Am. Chem. Soc. 129, 6908 (2007)) and shown to exhibit extreme instability that has precluded investigation of its physical and chemical properties by experiment. Although its high instability prohibits its use in munitions, Si‐PETN could provide valuable information about the nature of sensitivity in energetic materials. Because the properties for Si‐PETN are currently unknown, first‐principles van‐der‐Waals density functional theory was used to obtain the equation of state under hydrostatic compression and mechanical properties under uniaxial compressions. These properties were then compared to those of PETN.

FIRST‐PRINCIPLES REACTIVE MOLECULAR DYNAMICS OF CHEMISTRY IN DETONATING ENERGETIC MATERIALS

We investigated the initial chemistry of shock compressed energetic materials that results from inter‐molecular collisions behind the shock wave front by performing first‐principles MD simulations of bimolecular collisions for PETN and RDX with different crystallographic orientations and velocities. For each orientation, we determined the threshold collision velocity for reaction, the reaction timescales, and the products of decomposition. We find that the calculated threshold velocities lie within the range of typical particle flow velocities in detonating materials. Owing to the extremely short reaction timescales (∼10−13 s), these initial chemical events are largely driven by the direct collision dynamics, instead of temperature.

Ammonium azide under hydrostatic compression

The properties of ammonium azide NH4N3 upon compression were investigated using first-principles density functional theory. The equation of state was calculated and the mechanism of a phase transition experimentally observed at 3.3 GPa is elucidated. Novel polymorphs of NH4N3 were found using a simple structure search algorithm employing random atomic displacements upon static compression. The structures of three new polymorphs, labelled as B, C, and D, are similar to those of other metal azides.

First-principles thermodynamics of energetic materials

Using density functional theory with empirical van der Waals corrections, cold pressure curves were calculated and combined with the quasi-harmonic approximation to study thermodynamical properties of several energetic molecular solids. Vibration spectra at each compression were calculated and used for including temperature and zero-point energy contributions to the free energy. Equilibrium properties at temperatures of experiments, as well as hydrostatic equations of state, specific heat capacities, and coefficients of thermal expansion, were obtained and compared to experiment.

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 ? 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.

REACTIVE MOLECULAR DYNAMICS OF DETONATING PETN

We investigate the initial chemical events sustaining a detonation in shock‐compressed PETN resulting from intermolecular collisions behind the shock wave using first‐principles reactive molecular dynamics. The reaction dynamics of bimolecular collisions was studied as a function of collision velocities and crystallographic orientations. For each orientation, threshold collision velocities of reaction, and products of decomposition were determined. The timescale of reaction was evaluated and used to understand whether these initial chemical events are largely driven by reaction dynamics, or temperature. Bond dissociation energies were calculated and used to rationalize the outcome of the chemical events in the course of the reaction dynamics. Finally, the relationship between orientation dependent sensitivities and steric factors is discussed.