Synthesis, structure, and properties of glasses under extreme conditions

Abstract

Pressure-quenching from the non-rigid state near the glass transition temperature imparts structural signatures to densified silica glass that cannot be accomplished through cold compression at room temperature. The unique structures of pressure-quenched silica glass are reflected in decreased anomalous response of silica glass to external stimuli of high temperature or high pressure, and therefore greater thermo-mechanical stability. The nonlinear elastic behavior of silica glass has been directly probed from the compressive to the tensile side of silica fibers in bend by using in-situ Brillouin light scattering. This provides a convenient way to determine the neutral axis shift, which in turn allows more accurate stress calculations in bent fibers. The corresponding structure changes in compressive and tensile regions relative to the neutral axis were studied by Raman spectroscopy. A fundamental understanding of the structure and elastic properties of silica glass under extreme conditions was gained, which is of critical importance for developing strong and stable silicate glasses.

Anomalous mechanical properties of silica glass include stiffening upon heating, initially softening under pressure, and non-linear elastic response to strains. Through understanding structural changes in silica glass under a broad range of temperature, pressure, and strain conditions and how they influence the mechanical properties, insight was gained for how to change the silica glass network to better suit specific uses in extreme conditions.

In this dissertation, pressure-quenching routes were used to effectively change the glass atomic packing and to make densified glass. Applied in the non-rigid state near the glass transition temperature, quench pressures up to 8 GPa have been used to achieve density increase of 25% in silica glass. The resulting structure and properties of as-quenched samples have been investigated using XRD, Raman and Brillouin spectroscopy. In-situ Raman and Brillouin light scattering techniques were developed to study the structure, elastic and dynamic properties of silica glass under high temperature, high pressure and high strain conditions. High temperature measurements were carried out in an optical furnace up to 1500 °C, a diamond anvil cell was used to carry out high pressure experiments up to 25 GPa, and a two-point bender was used for measuring glasses in excess of 6% strain in both tensile and compressive regions.