COMPUTATIONAL MODELING OF NANOSCALE VIBRATIONAL ENERGY TRANSFER IN CRYSTALLINE RDX
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Energetic materials appear in a wide range of industrial and defense applications such as mining, construction, rocket propellant, design of munitions, etc. Understanding the physical and chemical processes that result in phenomena leading to initiation is critical for the safe development, usage, transport, and storage of high-performance energetics. A long held belief is that shock energy induces initiation of an energetic material through an energy up-pumping mechanism involving phonon scattering through doorway modes. In Chapters 4 and 6, a 3-phonon theoretical analysis of vibrational energy up-pumping in RDX is presented that considers possible doorway pathways through which energy transfer occurs. The vibrational energy transfer is modeled via 3-phonon scattering processes based on Fermi’s Golden Rule. Our results indicate that the low frequency vibrational modes (below ~100 cm-1) scatter less than 0.5% of the vibrational energy directly to the critical high frequency intramolecular vibrations. In contrast, the mid-frequency modes between 457 and 462 cm-1 and between 831 and 1331 cm-1 are the most critical for vibrational heating of the critical intramolecular vibrations such as N-N stretching. In Chapters 3 and 5, we examine the nature of thermal transport and how bond strain and rotation carry heat in RDX. To draw the distinction between propagating and diffusive carriers of heat, we compare the thermal conductivity estimates from three microscale models: Phonon Gas Model, Cahill-Watson-Pohl formula, and Allen-Feldman harmonic theory. We observed that due to a strong crystal anharmonicity, diffusive carriers contribute to over 95% of the thermal conductivity in RDX. These results indicate that van der Waals bonded organic crystalline solids conduct heat in a manner more akin to amorphous materials than simple atomic crystals. In Chapter 7, we perform a numerical experiment to investigate the effects of stimulating different IR active vibrational modes on change in scattering rates, thermal diffusivity, and conductivity in RDX. The stimulation of the vibrational modes is performed one mode at a time using six different optical energy inputs (3 high intensity: few eV, and 3 low intensity: tens of meV). Based on the results of this study, we identify several vibrational modes stimulating which may lead to a substantial enhancement or frustration of the heat transport properties in RDX.