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 ΔHf 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 σM/ΔHfM (were σM is the molar surface energy and ΔHfM 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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Materials Chemistry