Crystallization of lithium aluminosilicate and microstructure of a lithium alumino borosilicate glass designed for zero thermal expansion

Abstract

A glass with the composition 7.9 Li2O∙2.0 MgO∙1.4 ZnO∙10 Al2O3∙2.7 B2O3∙72.6 SiO2∙0.9 ZrO2∙2.1 TiO2∙0.4 Sb2O3 was thermally treated at temperatures in the range from 650 to 750 °C. During thermal treatment, at first, nearly spherical Zr1-xTixO4 crystals with sizes of 5–20 nm were formed. On these nanocrystals, elongated particles consisting of TiO2 were observed to grow. Subsequently hexagonal β-quartz solid solution was formed at temperatures ≥650 °C. In this β-quartz solid solution, Zn as well as Mg and excess Al were incorporated. At temperatures >690 °C, the formation of tetragonal β-spodumene solid solution was observed. In the microstructure of the formed glass-ceramics, a volume concentration of around 12% spherical aggregates with a diameter of 1.5–2 μm composed of β-spodumene solid solution and areas enriched in Al and Zn occur. These heterogenous structures were formed by the transformation of β-quartz solid solutions to β-spodumene solid solution. At the same time, Zn(Mg) and excess Al were expelled from the aluminosilicate phase and a cubic spinel type phase Zn(Mg)Al2O4 was formed. These aggregates did not contain a glassy phase. In-between these spherical aggregates, i.e. in around 88% of the volume, tiny Zr1-xTixO4 crystals, aluminosilicate crystals with diameters of around 200 nm, and a glass phase shoved away by these growing crystals consisting of oxides of Mg, Zn, Al, Si, and Sb happened to occur.