Structural Control on the Viscoelastic Behavior and Shear Relaxation in Chalcogenide Glass-Forming Liquids
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Structural Control on the Viscoelastic Behavior and Shear Relaxation in Chalcogenide Glass-Forming Liquids


Chalcogenide glasses are an important class of materials that consist of sulfides, selenides or tellurides of group IV and/or V elements, namely Ge, As, P and Si with minor concentrations of other elements such as Ga, Sb, In. Because of their infrared transparency that can be tuned by changing chemistry and can be actively altered by exposure to band gap irradiation, chalcogenide glasses find use in passive and active optical devices. Chalcogenide glass infrared lenses have found applications in the auto industry and in night vision security systems for both civil and military use. Infrared transmission in several key wavelength windows has enabled their use as optical fibers, lasers and fiber-amplifiers for telecommunication and as infrared laser power delivery systems for medical applications. Moreover, their large optical nonlinearity and high refractive index make them attractive candidate materials for all-optical switches in optical circuits. Finally, in thin film form, chalcogenide phase-change materials are of extraordinary technological importance in rewritable non-volatile memory storage applications (e.g., CD, DVD and Blu-Ray). A complete knowledge of the atomic structure of any material is of key importance in understanding and formulating accurate predictive models for its properties. Besides structure, the dynamical processes associated with structural relaxation and annealing near the glass transition controls the technological utility of inorganic glasses. In this dissertation Raman and one- and two- dimensional multi-nuclear (31P, 77Se, 125Te) nuclear magnetic resonance (NMR) spectroscopic techniques are utilized to elucidate the compositional evolution of the atomic structure at the short- and intermediate- range and chemical order in a series of binary selenide glass-forming systems, namely: SxSe100-x, Se100-xIx, TexSe100-x, and PxSe100-x. These spectroscopic results yield a rather detailed picture of the compositional evolution of the atomic structure of these glasses, where the constituent structural moieties range from 0-dimensional molecules to 1-dimensional linear chains all the way to a 3-dimensional network. The corresponding physical properties such as the glass transition temperature and the molar volume are shown to be intimately linked to and consistent with the compositional evolution of the atomic structure in each system. On the other hand, the shear-mechanical response of all compositions in the supercooled liquid state are investigated via a combination of small-amplitude oscillatory and steady shear parallel plate rheometry to build an atomic-scale understanding of the frequency dependent viscoelasticity and structural relaxation processes. These results reveal the mechanistic connection between the structure including its connectivity and dimensionality and the shear relaxation processes, such as the rapid segmental chain motion associated with polymeric chains, the slow bond scission/renewal dynamics caused by interconversions between the structural moieties and the strong secondary bonding interactions, as well as the fast rotational motion and cooperative dynamics resulted from molecules. Depending on the structural moieties presented in a liquid, one or more shear relaxation processes may exist and contribute to the viscous flow. In addition, the shear-mechanical response evolves systematically with the structural transformation of these liquids as a function of composition within each system. When taken together, these results enable us to establish fundamental connections between the “microscopic” and the “macroscopic” aspects of viscoelasticity and relaxational phenomena in chalcogenide glass-forming liquids and to develop robust predictive atomistic models of structure-dynamics-relaxation for all studied systems.  

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