Compositional Trends in Surface Enhanced Diffusion in Lead Silicate Glasses

TL;DR

This study uses molecular dynamics simulations to explore how surface enhanced diffusion varies with fragility in lead silicate glasses, revealing small effects and challenging previous assumptions about their correlation.

Contribution

It provides new insights into the relationship between fragility and surface diffusion in oxide glasses, with detailed simulation data and analysis.

Findings

Surface enhanced diffusion increases with fragility but remains small.

Slower diffusing atoms like Si and O show higher enhancement.

Surface atomic arrangements are similar to bulk, indicating minimal structural changes.

Abstract

In this work, we use molecular dynamics simulations to study the enhancement of surface over bulk diffusion (surface enhanced diffusion) in (PbO)x(SiO2)1-x glasses. This work is motivated to better understand surface diffusion in glasses and its connection to fragility, and to enhance surface diffusion in silica and related glasses for greater thermodynamic stability during vapor-deposition. By adding PbO to silica, the fragility of glass increases continuously for 10% <= x <= 70% during experiments. The increase in fragility may correspond to an increase in surface enhanced diffusion, as fragility and surface diffusion are correlated. We observe that for the silicates investigated, while surface enhanced diffusion increases with fragility, the enhancement is quite small. The slower diffusing Si and O atoms have higher enhancements, which could allow for some surface stabilization effects. We demonstrate that there are only small changes in atomic arrangements, consistent with the similar diffusion rates, at the surface as compared to bulk. Finally, we examine the trend of bulk versus surface diffusion in view of previous observations in organic and metallic glasses and found that in oxides, fragility increase may not be strongly linked to enhanced surface diffusion.