Abstract: The effects of silica on the density, boron–oxygen speciation and thermal properties of glasses from the system: PbO–B2O3–SiO2 (PbO concentration: 30, 40, 50 and 60 mol% and silica concentration: 5, 10, 20 and 30 mol%) was studied by 11 B MAS NMR and DSC techniques. The incorporation of silica in the borate network steadily increases glass density, decreases the glass transition temperature and increases the thermal stability of glasses against crystallization. SiO2 at low concentrations of up to 20 mol% increases the three dimensional network connectivity by promoting the conversion of BO3 into [BO4]– units, however at higher silica contents of 30 mol%, the formation of [BO4]– was suppressed and nonbridging oxygens were rapidly generated in SiO4 and BO3 units. The average number of NBOs per BO3 unit increases with silica concentration and this was indicated in the NMR spectra of glass series with 60 mol% PbO which exhibited a shift in the centre of gravity of the BO3resonance peak towards more positive ppm values (de-shielding) at a silica concentration of 30 mol%. DSC studies indicated phase separation in the glasses which suggested that the mixing of Pb2+ ions, BO3, [BO4]– and SiO4 units was not completely random.

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