Computational and Phenomenological Modeling of Atomic-Scale Transport Processes in Glasses and Glass-Forming Liquids

Abstract

This work focuses on the computational investigation and phenomenological model development of atomic-scale transport properties in glasses and glass forming liquids. Specifically, we study ion diffusion and viscous processes as they arise from phenomenologically similar elementary mechanisms. A greater understanding of ionic mobility in glasses can elucidate important materials design criteria for the development of solid-state electrolyte batteries while improvements in viscosity modeling of glass formers is vital to advancing the development and manufacturing of novel glasses. These endeavors work toward providing insight into essential characteristics of the glass transition phenomenon. We begin with a study of diffusivity in a simple two-component model solid electrolyte using molecular dynamics (MD) simulations. This model system is composed of lattice elements, forming a covalent support structure for solute particles that exhibit weaker non-bonding interactions with the network. The solute species size is systematically varied, while the network atoms are unchanged and a constant system volume is maintained. The atomic mobility increases by orders of magnitude in conjunction with cohesive rupture between solute and network and enhanced anharmonicity, as revealed by analyzing the internal pressure, compressibility, and vibrational spectra as a function of solute size. This finding inspires using ion-exchange to enhance ionic mobility in oxide glasses. Replacing cesium in a MD simulation-generated melt-quench cesium silicate glass with the smaller sodium cation results in a 4.5 to 6-fold increase in diffusivity compared to the melt-quenched sodium silicate control. This increase results from a greater free volume of the ion-exchanged glass. Similarly, subjecting the control glass to isotropic volumetric strains, a sodium mobility increase is observed, peaking at 25% strain, which corresponds to the tensile limit of the simulated glass. Expansion causes the potential energy topography to flatten, allowing sodium to readily access transition pathways between neighboring sites. Greater strain causes cavitation of the silica network, creating non-traversable gaps for sodium migration. A hallmark of thermally activated transport processes in glass forming materials is the non-Arrhenius temperature dependence above Tg. This applies to ionic conductivity and viscoelasticity. Conjecturing that this behavior is rooted in a variable free energy topography associated with structural changes occurring in a system upon traversing the glass transition regime, we juxtapose the complex mechanical modulus of a sodium borate melt measured at GHz frequencies and its zero Hz viscosity. Modifying the Maxwell-Wiechert model to account for a temperature dependent activation free energy, the high-frequency and steady-state quantities are perfectly reconcilable with one another, thus validating the underlying atomic scale mechanisms. Expanding on this framework, we develop a workflow for analyzing steady-state viscosity data of 847 oxide glass formers using our new variable activation free energy (VAFE) model in case the adiabatic complex mechanical modulus is not available. We compare the performance of the VAFE model with those of the established VFT and MYEGA equations and find our model to be more robust to extrapolation and possessing more reasonable behavior in the infinite-temperature. Furthermore, our model encodes a relationship between fragility and the temperature-dependent change in the ground-state potential energy associated with structural changes in glass formers between the glassy and liquid states. It also allows one to estimate the number of atoms onto which the activation energy is imparted per elementary viscous dissipation event.