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

Abstract

An Na + /Ca 2+ -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 N 2 atmosphere near its glass-transition temperature. The measurements were performed using X-ray photoelectron spectroscopy with C 60 -ion sputtering (C 60 -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 + /Ca 2+ -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 Ca 2+ is also confirmed in the dehydration layer of the glass heat treated under an N 2 atmosphere, suggesting that Na + and Ca 2+ 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 + /Ca 2+ -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 Ca 2+ ions.

Effect of heat treatment on the surface chemical structure of glass: Oxygen speciation from in situ XPS analysis

“…Presently, the cause for the reduction in sodium at 50°C is a mystery. An additional decrease in sodium is observed above 200°C . Furthermore, calcium and magnesium content is reduced by 60% and 17%, respectively .…”

“…Modifications in surface composition of the unleached glass upon heating are not immediately evident (Figure A) except for a depletion in glass modifiers such as sodium and calcium at temperatures above 300°C . The cause of the reduction in sodium content below 100°C is unknown.…”

“…Laube et al studied the subsurface composition of soda‐lime silicate glasses as a function of ex situ heating between 200°C and 400°C using Nuclear Reaction Analysis (NRA) and Rutherford Backscattering Spectrometry (RBS); but they were not able to discern changes occurring in the top surface of the glasses due to the information depth limitation of NRA and RBS . In a study conducted by Yamamoto et al, the subsurface changes in composition evident via XPS depth profiling after ex situ heating with temperature ranging from 400°C to 600°C were attributed to inward diffusion . In contrast, by utilizing in‐vacuo heating and Ion Scattering Spectroscopy on alkali silicate glasses, Kelso et al conclusively showed that the loss of alkali upon heating was due to volatilization as opposed to inward diffusion.…”

“…Consequently, a true account of surface hydroxyls at room Modifications in surface composition of the unleached glass upon heating are not immediately evident (Figure 2A) except for a depletion in glass modifiers such as sodium and calcium at temperatures above 300°C. 20,21 The cause of the reduction in sodium content below 100°C is unknown. Contrary to the unleached surface, some regions on the leached surface are observed to have an increase in sodium content beyond 200°C ( Figure 2B).…”

“…An additional decrease in sodium is observed above 200°C. 12,20,21 Furthermore, calcium and magnesium content is reduced by 60% and 17%, respectively. 20 The heterogeneity of glass surfaces does not manifest until past 200°C for both surfaces as greater variations in sodium content are observed leading to greater spread in the data.…”