Thermodynamics and kinetics of phase changes in energetic materials

Applications of explosives have shaped the lives of society for centuries. From guns to fireworks, airbags to tunnels, construction to destruction, and peace to war, the infamous association to terrorism and militaristic conquest undermines our awareness of the positive impact that these materials have on everyday life. Commonly known as energetic materials, the very definition of an explosive is related to the uniqueness of its extremely rapid phase change to a gaseous mixture upon ignition. Furthermore, different types of phase changes in explosives serve important roles in their production, modification, detection, storage, (de)sensitization, and detonation.
The most widely employed method for detecting explosives is through “swabbing” (e.g., a suitcase at an airport). The main issue with this approach is that detection does not occur in real time or at a stand-off distance such that a more suitable approach would be to detect explosive particles that are airborne (e.g., optically or with a chemical sensor). The vapor pressure can be summarized as being the total number of residual gas phase particles in equilibrium with its respective condensed phase for a specific volume, temperature and pressure, and the basis for the analytical detection of any volatile airborne compound, outside of direct observation, is pursuant to the vapor pressure. Methods that allow for the simultaneous detection of the respective partial pressures within an environment containing a mixture of chemical species are exceptionally rare. In the present thesis, commercial UV/Vis spectroscopy is demonstrated as a method that allows for the direct and simultaneous analysis of the partial pressures in a binary mixture of ferrocene and benzoic acid despite overlapping spectral interferences.
The nature of the crystal growth process in explosives can dramatically impact their safety and performance. Thin films are a commonly used tool for investigating the fundamental crystallization behavior in many systems, and the interfacial physics between a crystallizing species and the substrate can significantly affect the crystal growth process. Atmospheric vapor deposition was used to prepare thin films of supercooled droplets of pentaerythritol tetranitrate (PETN) on copper, gold, and silicon surfaces. Crystallization of the supercooled PETN droplets was induced by scanning the surface of each substrate type with an atomic force microscope upon which crystallization occurred in the form of compact, circularly shaped dendrites. The surface roughness and surface energy of all three substrates were characterized, and high-resolution, time-lapse images of the crystal growth process were acquired on each surface to determine the crystal growth rates by measurements of the distance of “ring” features in the crystal growth patterns that appeared upon changing the substrate temperature. 2,4,6-trinitrotoluene (TNT) is known to predominantly exist as one of two polymorphs – monoclinic or orthorhombic – with the former being more thermodynamically stable. The production of different polymorphs is commonly achieved via crystal growth from solution in which the outcome is highly dependent on the absolute solubility. Effects related to both the nature and temperature of different solvents used for TNT crystallization were investigated via two separate crystal growth techniques: solvent evaporation and solvent/antisolvent precipitation. An investigation of TNT crystals produced from solutions at temperature in excess of 25 °C is reported for the first time and shows that TNT exhibits unique behaviors related to the liquid phase. These TNT crystals were grown from a multitude of different solvents in which the results were remarkably consistent with respect to the solubility. At the same time, TNT crystals grown from aniline produced a unique and intriguing result: the formation of a red cocrystal solvate. The crystallographic and thermodynamic properties of this new complex were investigated and detailed. The mechanism of the red color was attributed to the formation of a charge-transfer complex. Furthermore, red to yellow discoloration of the crystals was observed to occur upon exposure to an open atmosphere. It was determined that aniline leaves the crystal’s interior, which becomes pure TNT. This process, called desolvation, was further investigated in situ.