Inward Migration of Glass‐Modifier Cations During Heat Treatment Under an N2 Atmosphere

Abstract

An Na+/Ca2+‐deficient layer is observed to form on the glass surface region up to a depth of hundreds of nanometers when a soda‐lime‐silicate glass is heat treated under an N2 atmosphere near its glass‐transition temperature. The measurements were performed using X‐ray photoelectron spectroscopy with C60‐ion sputtering (C60‐XPS) and dynamic secondary‐ion mass spectrometry (D‐SIMS) with consideration of the mass and charge balances. The increase in the amount of hydrogen is substantially less than the decrease in the total charge due to the loss of modifier cations in the Na+/Ca2+‐deficient layer; furthermore, the oxygen concentration in this layer is lower than the bulk value, suggesting that the silanol groups in the surface layer of the glass are dehydrated. A high‐concentration layer of Ca2+ is also confirmed in the dehydration layer of the glass heat treated under an N2 atmosphere, suggesting that Na+ and Ca2+ ions migrate inward into the glass via an ion‐exchange reaction with protons, which migrate toward the surface from the bulk. We also confirmed that a thicker Na+/Ca2+‐deficient layer is formed on glass surfaces with higher water content. Our results suggest that the dehydration of the silanol groups is the driving force of the inward migration of Na+ and Ca2+ ions.