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 hem. The alterations of the resonance curve are investigated as a function of the evolution of the theological properti es of the medium to characterize. The contact between the sample and the ho rn is obtained through the immersion of the terminal section, the probe, in the medium. Two resonance peaks appears, whose impedance alteration is rep resentative of the viscosity of the medium. We used this technique to study silica gels obtained from silicon alkoxide. The higher frequency peak, ver y sensitive, is used to characterize the gelation step. This peak disappear s during aging, characterized by the lower frequency peak. The influence of the preparation parameters of the sol (catalyst type, hydrolysis water amo unt, solvent amount and temperature) on gelation, aging and final theologic al properties was studied. (C) 2001 Elsevier Science B.V. All rights reserv ed.