The effect of P2O5 on the crystallization and microstructure of glass-ceramics in the SiO2-Li2O-K2O-ZnO-P2O5 system

Six glasses in the SiO2-Li2O-K2O-ZnO-P2O5 system were prepared, varying the P2O5 content from 0.0 to 2.5 mol% with the molar ratios of the other components being invariant, Crystallization processes of these glasses were investigated using high temperature XRD and differential scanning calorimetry, Morphology and microstructure of the glass-ceramics were studied using scanning electron microscopy. Crystallization of these glasses occurred in the temperature range 500-750 degrees C. The main crystalline phase was lithium disilicate (Li2Si2O3). Lithium metasilicate, lithium orthophosphate and cristobalite were secondary phases. In glasses with a P2O5 content of more than 1.0 mol%, precipitation of cristobalite took place prior to the main crystallization of lithium disilicate. Surface crystallization occurred in glasses with a P2O5 content of 0.0 and 0.5 mol%. Furthermore, a spherulitic morphology of lithium disilicate crystals in the interior of the samples was observed. Lithium disilicate crystals in glasses with a P2O5 content of 1.0-2.5 mol% had an elongated shape along the c-axis, a crystal length ranging from 1 to 5 mu m and aspect ratios between 2 and 6. Rod-shaped crystals were randomly oriented in the volume of the glass-ceramics and were responsible for the mechanical strength of up to 440 MPa. (C) 2000 Elsevier Science B.V. All rights reserved.