LOW-TEMPERATURE SYNTHESIS OF CAO-SIO2 GLASSES HAVING STABLE LIQUID LIQUID IMMISCIBILITY BY THE SOL-GEL PROCESS

Calcium silicate glass compositions lying within the liquid-liquid immiscibility dome of the phase diagram, which could not have been prepared by the conventional melting method, have been synthesized by the sol-gel process. Hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) solutions containing up to 20 mol% calcium nitrate, resulted in the formation of clear and transparent gels. The gel formation time decreased with increase in water:TEOS mole ratio, calcium content and the reaction temperature. Smaller values of gel times in the presence of calcium nitrate are probably caused by lowering of the ionic charge on the sol particles by the salt present. The gelation activation energy, E(gel), was evaluated from the temperature dependence of the gel time. Neither the presence of Ca2+ ions nor the water:TEOS mole ratio had any appreciable effect on the value of E(gel). The presence of glycerol in the solution helped in the formation of crack-free monolithic gel specimens. Chemical and structural changes occurring in the gels, as a function of the heat treatments, have been monitored using differential thermal analysis, thermogravimetric analysis, infrared spectroscopy, X-ray diffraction, surface-area and pore-size distribution measurements.