ULTRAFAST BROADBAND MIDINFRARED ABSORPTION SPECTROSCOPY ON SHOCKED ENERGETIC MATERIALS
thesisposted on 16.04.2020, 17:03 by Michael S Powell
Balancing increased safety against detonation performance is paramount for new explosive energetic materials in the development process. Often these two requirements are in opposition to each other. Sensitivity tests to external stimuli are used to determine how safe an energetic material is to phenomena such as impact, heat, or friction. Meanwhile, detonation performance is assessed by the maximum pressure and shock velocity induced from chemical reactions. Tailoring the performance while maintaining safety of the explosive would be possible with knowledge of the chemical reactions that functional groups provide during detonation. Current knowledge of the chemical reactions that occur during detonation is limited. Several mechanisms have been suggested for first step reactions throughout the detonation process for energetic molecules; however, no single chemical pathway has been irrefutably substantiated by experiments. Alternatively, models can provide insight into the types of reactions that may transpire, but lack direct experimental comparisons. If experiments and models could be compared at the equivalent time and length scales, then measurements could guide the physics and chemistry assumptions present in models. Experiments presented in this document bridge that gap by using an ultrafast laser system to generate shocks in samples and spectroscopically probe vibrational and electronic absorption changes that occur during shock compression. A review of how to turn a benchtop chirped pulse amplifier into a shock physics and chemistry laboratory is first presented. Applications of the spectroscopic techniques developed were then applied to trinitrotoluene (TNT) and pentaerythritol tetranitrate (PETN) during shock compression. Mid-infrared absorption results for shock compressed TNT and PETN were compared to current suggestions on chemical pathways and inconsistencies were present for both materials. It is suggested that a carbon-carbon bond breaking mechanism is present for PETN, and a hydrogenic stretch like hydroxyl or amide bond formation mechanism is suggested for TNT based on the MIR absorption measurements. Recommendations for future experimental thrusts are also provided. The results provided in this document could be directly compared to simulations to refine the assumptions present in models.
PC@Xtreme - Predictive Chemistry & Physics at Extreme Temperature and Pressure: Molecules, Crystals and Microstructures
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