Abstract
Sol-gel process in the ternary system of tetraethylorthosilicate (TEOS)-H_2O-ethanol was subjected to a power ultrasound of 20kHz-15W/cm^2. Sonication abruptly reduced the gelling time, and enhanced polycondensation to form Si-O-Si bonds. The ultrasonic effect increased with the increasing time up to 〜2 hours, and decreased with the increasing temperature. The latter is due to the "sonochemical effect" which prefers lower pressure of solvent vapor in the bubbles at lower temperature. On the other hand, argon-substituted atmospehre which should have higher effect of sonochemistry did have a lower effect than unsubstituted atmosphere (air). Comparing the sol-gel processes in N_2, O_2, Ar, and air, we found that oxygen has a positive effect on emulsification between TEOS and water. Ultrasonic-derived xerogels have higher density and specific surface area, and smaller pore size than normal ones. Incorporation of ammonia in the ternary causes the catalytic enhancement of polycondensation, resulting in the formation of spherical particles at a certain concentration condition. Ultrasonication to the system induced the polymerization at the surface of particles. The spherical shape was preserved when the particle size is as large as 300nm, while it was not when as small as 50nm. Ultrasonic effect on the sol-gel process might involve different mechanisms for homogeneous and heterogeneou systems.
