INFLUENCES OF ZIRCONIA AND SILICON NUCLEATING-AGENTS ON THE DEVITRIFICATION OF LI2O.AL2O3.6SIO2.GLASSES

The effects of Si and ZrO2 dopants on the crystallization and phase transformation process in Li2O.Al2O3.6SiO2 glasses were investigated using differential thermal analysis, X-ray powder diffractometry (XRD), and high-resolution transmission electron microscopy (TEM) interactively. Phase separation was observed in the studied glasses prior to substantial crystallization. Elemental Si (1 mol%) significantly aided in glass devitrification. Dropletlike phase-separated regions in the as-quenched or heat-treated glass devitrified at approximately 760-degrees-C, which in turn provided sites for the heterogeneous nucleation and growth of beta-quartz(ss) (solid solution), which transformed to beta-spodumene(ss) at higher temperature. Low-temperature surface crystallization in these glasses occurred as low as 760-degrees-C. ZrO2 has limited solubility in this glass system. Small ZrO2 crystallites (approximately 5 nm) in the as-quenched glass acted as sites for the heterogeneous nucleation and subsequent growth of large ( > 5-mu-m) beta-quartz(ss) crystals in glasses containing 1.0 mol% or more ZrO2. The transformation from beta-quartz(ss) to beta-spodumene(ss) was increasingly inhibited with ZrO2 additions. The nucreating efficiency of Si was significantly greater than that of ZrO2 in this glass system.