On-line characterization of silica gels by acoustic near field

The acoustic near field technique consists in studying the features of the resonance state of an acoustic horn. The alterations of the resonance curve are investigated as a function of the evolution of the rheological properties of the medium to characterize. The contact between the sample and the horn is obtained through the immersion of the terminal section, the probe, in the medium. Two resonance peaks appears, whose impedance alteration is representative of the viscosity of the medium. We used this technique to study silica gels obtained from silicon alkoxide. The higher frequency peak, very sensitive, is used to characterize the gelation step. This peak disappears during aging, characterized by the lower frequency peak. The influence of the preparation parameters of the sol (catalyst type, hydrolysis water amount, solvent amount and temperature) on gelation, aging and final rheological properties was studied.