Crystallization kinetics of lead metasilicate

The crystallization kinetics of PbO·SiO₂ have been determined from 500°C. The thickness of the crystal layer was found to increase linearly with time; and the interface morphology was faceted at all temperatures. The heat of fusion of PbO·SiO₂ was estimated in two ways: (1) from DSC measurements of heat capacities of glassy and crystalline PbO·SiO₂ combined with tabulated ΔS₀(298) values; and (2) from direct mesurements using heat-flux DSC. The average value of ΔH_f obtained is 8.1 × 10³ cal mol^-, corresponding to an entropy of fusion of 3.9R. The growth process of PbO·SiO₂ is interface-controlled, with an interface site factor which increases with increasing undercooling. The plot of log (growth rate × viscosity) versus (T ΔT^-1 can be described by two straight lines with different slopes. The slopes correspond to edge surface energies of 113 and 51 erg cm^-2, leading to values of 0.35 and 0.16 respectively for the ratio σ_M/ΔH_fM (were σ_M is the molar surface energy and ΔH_fM is the molar heat of fusion). It is shown that the crystal growth data on PbO·SiO₂ are within an order of magnitude of the theoretically-predicted values based on the standard model of surface-nucleation controlled crystal growth, at least at small undercoolings. The data seem, however, at variance with predictions based on computer simulations of crystal growth using the measured heat of fusion. The excellent glass-formability of PbO·SiO₂, in spite of its high fluidity, may be explained by this deviation from expected behavior and by the absence of effective nucleating heterogeneities.