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.