DSC AND RAMAN STUDIES OF LEAD BORATE AND LEAD SILICATE-GLASSES

Several compositions in the xPbO.(1-x)B2O3 (0.20 less-than-or-equal-to x less-than-or-equal-to 0.65) and xPbO.(1-x)SiO2 (0.39 less-than-or-equal-to x less-than-or-equal-to 0.66) vitreous systems were examined by differential scanning calorimetry and by Raman scattering for the former system. Borate glasses with low PbO concentrations are built up in two and three dimensions with three- and four-coordinated boron atoms; Pb2+ acts as a charge balance in the covalent network. Glasses with high PbO content contain ring- and chain-metaborates and PbO4 entities. Differences in Raman scattering of alkali borate glasses are observed; in particular the boroxol rings still exist for x = 0.27. The heat capacities of the glasses at 200-degrees-C are an additive function of composition within the accuracy of the differential scanning calorimetry measurements. The heat capacity differences between the vitreous and supercooled liquid states are higher for lead borates than for lead silicates, indicating more profound changes in the lead borates during the glass transition. In the former glasses, the fictive temperature attains a maximum around 0.27PbO corresponding to the highest degree of polymerization. In lead silicates, the fictive temperature decreases continually upon adding PbO, and parallels the depolymerization of the silicate network.