Structural investigation of hydrous sodium borosilicate glasses

Abstract

The structural properties of a borosilicate glass with nominal 16mol% Na2O, 10mol% B2O3 and 74mol% SiO2 and water contents between 0 and 8wt% H2O (0–22mol% H2O) were investigated with IR, Raman and 11B MAS NMR spectroscopy. In addition to the pronounced OH stretching vibration band of weakly H-bonded species at 3580cm−1 the MIR spectra show a triplet at 2900, 2350 and 1750cm−1, similar as observed in water-bearing silicate glasses. These bands are assigned to OH groups and water molecules which are strongly H-bonded, to non-bridging oxygen. Water species contents determined from absorption bands in the NIR at 5200cm−1 (molecular H2O), 4700cm−1 (BOH), and 4500cm−1 (SiOH) indicate that hydroxyl groups dominate up to ~6wt% total H2O. Based on the absorption coefficients known from literature for silicate and borate glasses the BOH/SiOH ratio is estimated to be ≈0.8.As indicated by density, Raman and NMR data the incorporation of water has strong structural impacts in particular at low water contents up to 3wt% H2O. While the nominally dry glasses still contain a significant fraction (12%) of three-fold coordinated boron, almost all boron is four-fold coordinated in hydrous glasses.The increase of band components in the Raman spectra near 900cm−1 relative to the region>1050cm−1 gives evidence for depolymerization of the network upon hydration. Fitting of the spectra with Gaussians implies that silica tetrahedra with two non-bridging oxygen (Q2) are preferentially formed by reaction with water on expense of tetrahedra linked to four tetrahedra (Q4).