Formation and crystallization of gels in the Na2O-SiO2-TiO2-ZrO2 system

Abstract

5Na2O·95(SiO2 + TiO2 + ZrO2) gels together with Na2O-MO2 gels (MO2 = SiO2, TiO2, ZrO2) were synthesized and effect of Na2O addition to the gel formation and crystallization behavior were studied. From Si(OC2H5)4, Ti(iso-OC3H7)4, Zr(n-OC3H7)4 and NaOCH3 solutions in EtOH without H2O, transparent, opaque and heterogeneous gels were obtained. By SEM observation, it was found that the opague bulk gels, rich in TiO2 or ZrO2 component, contain agglomerated spherical particles of less than 10 μm diameter. Crystallization temperature (Tc) measured by DTA decreased sharply from 1258°C (SiO2) to 613°C (30Na2O·70SiO2) due to the increasing non-bridging oxygens in the Na2O-SiO2 system. But in the Na2O-TiO2 and Na2O-ZrO2 systems, Tc increased as Na2O content was increased up to 10 mol%. The thermal stability, defined by Tc/TL ratio, decreased from Tc/TL = 0.77 (SiO2) to 0.57 (5Na2O·95SiO2) or increased from Tc/TL = 0.30 (TiO2) to 0.67 (30Na2O·70TiO2) with Na2O addition. In the 5Na2O·95(SiO2 + TiO2 + ZrO2) system maximum Tc was 891°C at 5Na2O·76SiO2·19ZrO2 composition and decreased as TiO2 or ZrO2 contents increased. Compared with the SiO2-TiO2-ZrO2 gels, Tc of the 5 mol%Na2O containing gels (Na2O-SiO2-TiO2-ZrO2) was lower in SiO2 rich composition and higher in TiO2 or ZrO2 rich compositions. It is assumed that addition of Na2O changed the structure of amorphous TiO2 and ZrO2 someway to increase their thermal stability or amorphous forming ability. After DTA run up to 1000°C, cystalline phases precipitated in the 5Na2O·95(SiO2 + TiO2 + ZrO2) gels were determined by XRD analysis.