Sol-gel processing of lithium disilicate

Abstract

Lithium silicate powders were prepared by several sol-gel routes. Starting solutions contained equimolar amounts of lithium and silicon, but single-phase lithium disilicate (Li₂Si₂O₅) formed only when local stoichiometry was maintained through gelation and drying. Gels prepared from solutions containing LiNO₃, tetraethylorthosilicate (TEOS), water and ethanol were visibly homogeneous, but on drying the local stoichiometry was upset by the recrystallization of LiNO₃. Consequently, a lithium-rich phase (Li₂SiO₃) was the first to crystallize on heating with a lesser amount of Li₂Si₂O₅ forming at a higher temperature. Solutions containing TEOS and lithium methoxide formed a precipitate when combined with a water/ethanol hydrolysis solution. The precipitate dissolved before gelation, but the resulting powders crystallized into a mixture of Li₂SiO₃ and Li₂Si₂O₅. The relative amount of Li₂Si₂O₅ could be increased by adding HNO₃ to the hydrolysis solution and using lower water contents. Precipitation was avoided by partially hydrolysing TEOS before adding the lithium alkoxide; these powders crystallized directly into Li₂Si₂O₅ after heating at 550 °C. Gel-derived powders prepared using an Li-Si methoxyethoxide solution also crystallized directly into Li₂Si₂O₅.