Investigations of the mechanical relaxation of glasses at high temperatures

Abstract

Glasses are in daily use by virtually all humanity and have dramatically expanded the frontiers of industry and science as well. Owing to the developments of newest technologies, especially for fifth-generation (5G) wireless communication and artificial intelligence (AI), the demand for glass-based optical devices and structural components has sharply increased. This leads to new challenges to large-scale precision manufacturing as the mechanical behaviors of glass are complex during its thermal history. In this thesis, I attempt to explore the mechanical relaxation of glass at high temperatures, which not only benefits important industrial demands but also helps understand essential questions associated with glass transition. This thesis summarizes my efforts in performing mechanical experiments and theoretical modeling to promote the understanding of glass relaxation and transition. (1) Primary (α) relaxation was studied experimentally using the impulse excitation technique (IET) in borosilicate and chalcogenide glasses. The glass transition point (Tg) determined from temperature-dependent Young's modulus was found to be close to that determined by viscosity in borosilicate glasses. A non-destructive and instantaneous measurement method for determining viscosity is proposed for borosilicate glasses that have little non-exponentiality. This method can be explained by the implicit features in the Burgers model and a physics-based minimal model that considers the solid-like to liquid-like behavior transition. For chalcogenide glass, a striking non-exponential relaxation was found using the Cole-Davidson (CD) function, and the non-exponential estimate agrees well with previous research. (2) A new mechanism for the mechanical secondary (β) relaxation was established based on the normal mode analysis of a potential energy landscape and experimentally confirmed by the predicted double-peak phenomenon in the Fourier spectrum of a fluorosilicate glass beam. This leads to a new method for probing β relaxation. Using the proposed model, the β relaxation in the fluorosilicate glass is found to be negatively temperature-dependent and can be explained based on the picture of fragmented oxide-network patches in liquid-like regions, which broadens the understanding of β relaxation. (3) A long-expected phenomenon of non-zero to zero transition around Tg was firstly observed in structural glass, and the Kovacs' paradox was firstly confirmed in inorganic glasses by monitoring the relaxation of Young's modulus of an As2Se3 glass in two-step aging experiments. The effective relaxation time and Young's modulus at a quasi-equilibrium state obtained from long-term aging are both found to be dependent on the thermal history. The effect of thermal history is found to be related to the survival parts of glass after aging (i.e., the persistent memory). An elasticity-based relaxation model is proposed to explain the relationship behaviors, and a Mori-Tanaka (MT) analysis is used to determine the volume fraction of the local survival parts. The obtained memory persistence from either analysis agrees well with each other. With a series of experiments that change the aging temperature, it is found that structural memory persists below a critical temperature Tp ~ Tg and becomes zero above Tp. (4) A birth-death model is proposed to reveal the coupling effects between elastic modulus relaxation and local heterogeneity, of which the analytical solution can be obtained. A preliminary examination shows that the model can capture the normal and anomalous relaxation of different glasses. Based on the birth-death model, a non-Gaussian distribution of microscopic elastic modulus can be obtained which was found in previous molecular dynamic simulations. With novel experiments and theoretical inspections, an in-depth understanding of glass relaxation and transition is explored. The findings of this thesis could help industrial glass communities that need reliable but facile mechanical relaxation models. This investigation also provides new ideas on probing the glass transition, which can benefit the basic science in the future.