Structural investigation of hydrous sodium borosilicate glasses

The structural properties of a borosilicate glass with nominal 16 mol% Na2O, 10 mol% B2O3 and 74 mol% SiO2 and water contents between 0 and 8 wt% H2O (0-22 mol% H2O) were investigated with IR, Raman and B-11 MAS NMR spectroscopy. In addition to the pronounced OH stretching vibration band of weakly H-bonded species at 3580 cm(-1) the MIR spectra show a triplet at 2900, 2350 and 1750 cm(-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 5200 cm (molecular H2O), 4700 cm(-1) (B-OH), and 4500 cm(-1) (Si OH) indicate that hydroxyl groups dominate up to similar to 6 wt% total H2O. Based on the absorption coefficients known from literature for silicate and borate glasses the B-OH/Si-OH ratio is estimated to be approximate to 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 3 wt% 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 900 cm(-1) relative to the region > 1050 cm(-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). (C) 2017 Elsevier B.V. All rights reserved.