Structural relaxation of glass and its influence on other dynamic processes

Abstract

The physics of the glassy state is governed by two fundamental phenomena. The first is the liquid-to-glass transition, characterized by a kinetic constraint that restricts molecular rearrangement at a low enough temperature (or high enough viscosity), leading to the formation of a glass (unstable). The second is the spontaneous structural relaxation of glass back towards the metastable supercooled liquid state, wherein this constraint is overcome. This thesis investigates the mechanisms and effects of the spontaneous structural relaxation on the atomic configurations and macroscopic properties of single- and mixed-modifier oxide glasses. The research is organized into chapters that focus on: (i) the interplay between cooling rates and structural relaxation, studied using the flash-DSC technique; (ii) the kinetics of structural relaxation induced by physical aging, analyzed through changes in refractive index and ionic conductivity at various temperatures; (iii) the response of enthalpy recovery and refractive index of glasses to up- and down-jumps in fictive temperature, evaluated through the Kohlrausch–Williams–Watts function and the Tool–Narayanaswamy–Moynihan model; (iv) the network connectivity speciation and atomic rearrangements during structural relaxation in silicate glasses, analyzed using Raman and Nuclear Magnetic Resonance spectroscopy; and (v) the impact of slow structural changes due to the phenomenon of structural relaxation on ionic diffusion, which significantly influences the faster process of ionic conductivity in different glasses. The findings highlight the importance of understanding and controlling structural relaxation to design new glasses with optimized properties and ensure the long-term stability of these properties.