Preparation of Na2OSiO2 glasses in the metastable immiscibility region

Preparation of Na₂OSiO₂ glasses in the metastable immiscibility region has been examined by means of the sol-gel method. Gels of the binary system Na₂OSiO₂ of composition 10 Na₂O90 SiO₂ mol.% were obtained by the hydrolysis of tetramethylorthosilicate with an aqueous solution containing NaNO₃, NaOCH₃, CH₃COONa. The experiment was carried out mainly on the dense and transparent gel of composition 10 Na₂O90 SiO₂ mol.% obtained by using CH₃COONa. CH₃COO- groups contained in the gel decomposed completely at 460°C, about 50°C below the T_g of 10 Na₂O90 SiO₂ glass by the conventional method. The surface area of the gel decreased with heat-treatment at this temperature. The gel, however, became opaque while being held at the glass transition temperature of conventional glass. This opaqueness was considered to be caused by phase separation in the gel.