Density, Surface Tension, and Viscosity of PbO‐B ₂ O ₃ ‐SiO ₂ Glass Melts

The density, surface tension, and viscosity of the melts from the PbO‐B 2 O 3 ‐SiO 2 system have been measured at temperatures in the range 1073–1473 K. The effect of composition on these properties was also investigated. The density of the melt was found to increase linearly with increasing PbO content. Molar volume was derived from the density data, and its deviation from the additivity of partial molar volumes was calculated. These deviations in molar volume from those obtained from additivity rules have been used along with the ratio of various coordination numbers of boron (as reported by Bray) to discuss the structure of the melts. The surface tension was found to decrease with decreasing SiO 2 /B 2 O 3 ratio, and to increase in the range of the PbO content between 30 and 60 mol%, showing a maximum at ∼60 mol% PbO, and then decreased with further additions. This result suggested that the surface tension would be affected primarily by the B 2 O 3 content in the range of the PbO content between 30–60 mol%, and mainly by the PbO content in the range of the PbO content >60 mol%, respectively. The viscosity of the melt was found to decrease linearly with increasing PbO content. The results obtained indicate that the increase in viscosity with B 2 O 3 was half that of SiO 2 (on a molar basis), and an empirical equation has been proposed for the viscosity as a function of mole fraction.