Photoacoustically Measured Speeds of Sound of Liquid HBO₂: Semi-Empirical Modeling of Boron-Containing Explosives

Elucidation of geodynamic, geochemical, and shock-induced processes is often limited by challenges to determine accurately molecular fluid equations of state (EOS). High-pressure liquid-state reactions of carbon species underlie physicochemical mechanisms such as differentiation of planetary interiors, deep carbon sequestration, propellant deflagration, and shock chemistry. Here we introduce a versatile photoacoustic technique developed to measure precise adiabatic speeds of sound of high-pressure molecular fluids and fluid mixtures. Metaboric acid, HBO₂ speeds of sound are measured up to 0.5 GPa along the 277 °C isotherm. A polarized Exponential-6 interatomic potential form, parametrized using our data, enables EOS determinations and corresponding semiempirical evaluations of >2000 °C thermodynamic states including energy release from bororganic formulations. Our thermochemical model predictions of boronated hydrocarbon shock Hugoniot states differ from experiment by <3%

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

Effects of Plume Hydrodynamics and Oxidation on the Composition of a Condensing Laser-Induced Plasma

High-temperature chemistry in laser ablation plumes leads to vapor-phase speciation, which can induce chemical fractionation during condensation. Using emission spectroscopy acquired after ablation of a SrZrO₃ target, we have experimentally observed the formation of multiple molecular species (ZrO and SrO) as a function of time as the laser ablation plume evolves. Although the stable oxides SrO and ZrO₂ are both refractory, we observed emission from the ZrO intermediate at earlier times than SrO. We deduced the time-scale of oxygen entrainment into the laser ablation plume using an ¹⁸O₂ environment by observing the in-growth of Zr¹⁸O in the emission spectra relative to Zr¹⁶O, which was formed by reaction of Zr with ¹⁶O from the target itself. Using temporally resolved plume-imaging, we determined that ZrO formed more readily at early times, volumetrically in the plume, while SrO formed later in time, around the periphery. Using a simple temperature-dependent reaction model, we have illustrated that the formation sequence of these oxides subsequent to ablation is predictable to first order