Molecular modeling of the thermal poling of sodium aluminosilicate glasses

Thermal poling is a process by which network modifiers are depleted from the surfaces of oxide glasses through the application of an electric field at moderate temperatures. Several kinetic and thermodynamic questions regarding the glass structure within the depleted layers as well as the process by which they are created remain unanswered. In this study, we develop a Molecular Dynamics (MD) simulation method to create Na-depleted layers in sodium aluminosilicate glasses. We observe that as the ratio of Al ₂O ₃ to SiO ₂ concentration increases, all other conditions being equal, the thickness of the Na-depleted layer decreases. Detailed structural analysis of the Na-depleted layer reveals a decrease in the fraction of 4-coordinated Al and an increase in the fraction of higher (5 or 6)-coordinated Al. This re-distribution of Al coordination is also accompanied by a reduction in Non-Bridging Oxygen (NBO) and an increase in Oxygen Tri-Coordinated (OTC). Overall, our results agree with previous experimental studies and suggest that MD can be an effective tool to gain insights into the significant structural re-arrangements that occur in oxide glasses during thermal poling.