Session F2: Energetic and Reactive Materials III: Static and Ramp Loading

Experimental and theoretical investigation of pressure-dependent Raman spectra of triaminotrinitrobenzene (TATB) at high pressures

The experimental pressure dependent Raman spectra of triamino-trinitrobenzene (TATB) are determined up to 27 GPa, and compared with those obtained using density functional theory (DFT). The density functional perturbation theory calculations include the Grimme empirical van der Waals correction, as well as corrections for both thermal and zero-point energy contributions to pressure. DFT calculations of the crystal structure of TATB at ambient conditions, the equation of state, and Raman spectra up to 25 GPa are in good agreement with experiment. Pressure-dependence of specific vibrational modes is discussed in detail. Further, the comparison of experimental and calculated Raman spectra of TATB offers evidence that no first-order polymorphic phase transition occurs at least up to 25 GPa.   

High-Pressure Polymorphism of 1,1-Diamino-2,2-dinitroethene (FOX-7)

Understanding the polymorphic response of energetic crystals is important for understanding their initiation and reactive behavior. Here, we report on the high-pressure polymorphism of the energetic crystal FOX-7 [C$_{2}$(NO$_{2}$)$_{2}$(NH$_{2}$)$_{2}$]; the low sensitivity to initiation has attracted considerable research interest in this crystal. Micro-Raman spectroscopy and synchrotron x-ray diffraction measurements were used to gain insight into the mechanisms of polymorphic transformations, and their role in the high-pressure structural stability of FOX-7. Experiments were performed on single crystals compressed statically to 40 GPa (Raman) and to 12 GPa (x-ray). Two instances of spectral changes were detected at 2 and 4.5 GPa with Raman spectroscopy. Different experimental approaches, including isotope substitution (H/D), nonhydrostatic compression and laser radiation were used to understand the molecular processes associated with the observed spectral changes. The x-ray diffraction results demonstrated that the same space group, P\textit{2}$_{\mathrm{1}}$/n, is maintained to 4.5 GPa, the $\beta $ angle reduces to almost 90$^{\mathrm{0}}$, and the crystal shows anisotropic compression. Preliminary structure refinement results indicate that changes at 2 GPa can result from the amino groups twist out of the molecular plane. The structural changes at 4.5 GPa indicate the reconstructive character of the phase transition at this pressure.   

Phase Diagram and Decomposition of 1,1-Diamino-2,2-Dinitroethene (FOX-7)

To understand the reactive behavior of 1,1-diamino-2,2-dinitroethene (FOX-7) at the thermo-mechanical conditions relevant to shock-wave initiation, Raman and FTIR measurements were performed at high-pressures (HP) and high-temperatures (HT). Experiments were performed on single crystals of FOX-7 in a diamond anvil cell to 10 GPa and 800 K to provide the phase diagram and to gain insight into the HP decomposition mechanisms. Previous studies have demonstrated that the ambient structure of FOX-7 (alpha) transforms to beta and gamma phases at higher temperatures, and phase I (2 GPa) and II (4.5 GPa) at higher pressures. In this work, we determined the boundaries between these phases and the decomposition/melting curve. In particular, we found that: (i) both beta and gamma phases exist in a limited P-T domain (\textgreater 386 K and \textless 1 GPa), (ii) the transition between phase-I and phase-II takes place along the isobar, (iii) the decomposition temperature increases significantly with pressure ($\sim$ 25 K / GPa), and (iv) pressure inhibits the decomposition. Using FTIR spectroscopy, we observed that CO$_{2}$ is the first dominating decomposition product, followed by N$_{2}$O, NO$_{2}$, HCN, and HNCO. Pressure effects on reaction kinetics will be presented along with the possible mechanisms of decomposition.   

Disk Acceleration Experiment Utilizing Minimal Material (DAXUMM)

A venture between the US Army Research Laboratory (ARL) and Lawrence Livermore National Laboratory (LLNL) is currently underway in an effort to characterize novel energetic material performance properties using a single, high-precision, gram-range charge. A nearly all-inclusive characterization experiment is proposed by combing LLNL's disk acceleration experiment (DAX) with the ARL explosive evaluation utilizing minimal material (AXEUMM) experiment. Spherical-cap charges fitted with a flat circular metal disk are centrally initiated using an exploding bridgewire detonator while photonic doppler velocimetry is used to probe the metal disk surface velocity and measure its temporal history. The metal disk's jump-off-velocity measurement is combined with conservation equations, material Hugoniots, and select empirical relationships to determine performance properties of the detonation wave (i.e., velocity, pressure, particle velocity, and density). Using the temporal velocity history with the numerical hydrocode CTH, a determination of the energetic material's equation of state and material expansion energy is possible. Initial experimental and computational results for the plastic-bonded energetic formulation PBXN-5 are presented.