The relationship between structure and viscosity-temperature behavior in silicate and other oxide melts

Carol Ann Scamehorn, Purdue University

Abstract

The molecular dynamics simulation method has been used to simulate the structures of several glass-forming oxide melts. The local environments of noble gas atoms dissolved in a jadeite melt and aluminum ions in densified albite were examined in order to determine the ability of this method to produce acceptable structures under unusual conditions. These preliminary studies gave results that were in accord with experimental results obtained by other investigators thus making it possible to continue on to three more detailed studies. In the first of these studies, a series of fully polymerized aluminosilicate melts with varying aluminum content and countercation type was examined. These compositions are interesting because their basic structure is similar yet their viscosity behavior is different. It was determined that the difference in Si-Al ordering at high and low temperatures, the difference in the T-O-T angular distribution and differences in the local environment about the aluminum and silicon ions are significant influences on the strength or fragility of these melts. The second major study was performed on SiO$\sb2$ compressed to reduced volumes ranging from V/Vo = 1.0 to 0.55. The Si-O-Si angular distribution function for the most highly compressed configuration is similar to that seen in the high-pressure crystalline silica phase, stishovite. From this information and other simulation results, it was shown that the simulated high-pressure amorphous phase of silica is related to stishovite just as amorphous silica created at ambient pressure is related to crystobalite. It was also noted that the diffusion of the silicon and oxygen ions decouples at high pressures with the oxygen ions diffusing more rapidly than the silicon ions. The final study consists of both experimental measurements and theoretical calculations on a lanthanum triborate glass. High-temperature viscosity studies confirmed the highly fragile nature of this substance. The Kauzmann temperature of this glass was calculated using thermodynamic information from DTA measurements. The structure of this material was examined by molecular dynamics simulations. The formation of eight-membered borate rings like those found in the crystalline state of this material were observed at low temperatures but not at elevated temperatures under some simulation conditions.

Degree

Ph.D.

Advisors

Angell, Purdue University.

Subject Area

Chemistry

COinS