Mechanisms of nucleation and crystallization of lithium disilicate in Li-Zn-silicate glass-ceramic fibres

Abstract

The preferred crystal orientation of Li2Si2O5 crystallites has been evaluated in several partially crystallized glass fibres with the following chemical composition in mol %: 66.75 SiO2; 23.45 Li2O; 8.00 ZnO; 1.00 K2O and 0.80 P2O5. The crystallites were shown to be grown radially with thec-axis directed from the surface towards the core of the fibre. When glass fibres have been subjected to a nucleation treatment step, this preferred orientation slightly increases in the temperature range 590 to 620° C, by increasing the crystallinity content. This behaviour was attributed to the presence of active nuclei. A kinetic study on the crystalline volume coarsening, carried out according to the Johnson-Mehl-Avrami equations, showed a mechanism of linear growth for the lithium disilicate phase with the (00/) growing planes moved inward from the surface to the core of the fibre. The small-angle X-ray scattering results were obtained by using a masking liquid with an electron density close to that of the glass fibres investigated, in order to eliminate the total external reflection and surface scattering. Results indicated that two distinct particle fractions are precipitated inside the fibres after glass-in glass phase separation and nucleation at 500° C for various periods of time. The fraction of the largest particles,13 to20 nm in size, is probably constituted with vitreous SiO2-rich droplets, whereas the smallest particles, 2.5 to 6.5 nm in size, are probably Li2O-rich clusters containing P2O5.