Variation of the gel region with heat-treatment in the B2O3-Na2O-TiO2 system compared with the melt-quenched glass region

Abstract

Variation of the gel region with heat-treatment and variation of the melt-quenched glass region with cooling rates were compared for the B2O3-Na2O-TiO2 system. Gels were prepared by the sol-gel method without catalyst using boron tributoxide (B(OBu)3) sodium methoxide (NaOCH3) and titanium tetraisopropoxide (Ti(OPr)4). The gel region was in the composition range B2O3 = 0 to 90, Na2O = 0 to 60, and TiO2 = 0 to 100 mol % at 50 ° C. The crystalline phases precipitated were boric acid (H3BO3), sodium metaborate hydrate (NaBO22H2O), and sodium hydroxide (NaOH), or sodium carbonate (Na2CO3). A 500 °C, heat treatment for 2 h reduced the gel regions to a composition range of B2O3 = 50 to 90, Na2O = 10 to 30 and TiO2 = 0 to 20 mol %, but treatment at 800 ° C produced melts which became glassy upon cooling. On the other hand, a twin-roller method produced a wide range of glass region which includes a high concentration of TiO2 up to 70 mol %. The as-prepared gel region was found to correspond fairly well to the twin-roller glass region and the thermally most stable gel region was close to the air-quenched glass region. It was speculated that the B2O3- or Na2O-rich composition had not gelled because of the high water solubility of these gels.