Understanding the Link between Elasticity, Viscosity and Cracking in Glass-forming Systems

摘要

Oxide glasses hold paramount engineering interest today considering the increasing demand for optical-fiber-based communication, active displays for electronics and automobiles, etc. It is, therefore, critical to develop a fundamental understanding of structure-property relationships, property-property correlations and their underlying mechanisms. Such an understanding will enable the prediction and tuning of properties in a controllable manner for specific applications. In this dissertation, in-situ characterization techniques were integrated to study the response of glass to temperature or pressure. Insights gained from this work improved the current understanding on the link between structure, elasticity, viscosity and deformation of glass.;Since glass is a non-equilibrium material, its structure and properties depend on both composition and thermal history. Intermediate glasses which show nearly constant elastic moduli with increasing temperature up to the glass transition temperature were identified using in-situ high-temperature Brillouin light scattering (BLS) by varying composition in the Na2O-SiO 2, Na2O-Al2O3-SiO2 and Na 2O-TiO2-SiO2 glass systems. Insights gained from in-situ Raman spectroscopy combined with molecular dynamics simulations reveal that the intermediate elastic behaviors come from a delicate balance between the stiffening effect associated with conformation changes in the medium range flexible rings and the softening effect due to the weakening of short range chemical bonds with temperature. Thermal-history effects were investigated using the Na2O-B2O3 glass system where elastic moduli were found to increase anomalously with temperature just below the glass transition temperature in fast-cooled, low-Na2O containing glasses but not in corresponding annealed glasses. These differences were explained by different structural relaxation mechanisms in the glass transition range using Raman spectroscopy.;High-temperature BLS experiments aided in the development of a modified elastic model for viscosity which considers configurational entropy as a factor controlling the activation energy for viscous flow in addition to the high-frequency shear modulus. The modified model works much better than the original elastic shoving model in fitting equilibrium viscosity for both strong and fragile systems. It also has the capability to estimate the non-equilibrium isostructural viscosity of glass from the equilibrium viscosity and the temperature-dependent elasticity of the glassy state.;Deformation of glass was studied from its elastic and densification responses to hydrostatic compression and decompression by implementing BLS and optical microscopy experiments in-situ in a diamond anvil cell (DAC). A few multicomponent glasses with vastly different indentation cracking behaviors and varying B2O3 contents were studied in this work. Our study showed that glass networks with a high ability to undergo reversible structure changes in response to compression and decompression exhibit a high cracking resistance under indentation due to the reduced residual stress build-up after unloading.;By using structure and elasticity as a bridge, this thesis work substantially improved the understanding of viscosity and deformation in glass-forming systems, which is of critical importance for both fundamental glass science and glass technology (e.g., manufacturing and end use of glass).