Measurement of the viscoelastic constants of a sol-gel material by acoustic methods

The sol-gel (SG) process leads to an oxide macromolecular network through a sol to gel transition. Apart from the determination of the gelation time, the further evolution of the matrix has been characterized by means of the temporal evolution of the resonance peaks. Very low propagation speeds inferred from resonance frequency spectra have already been reported. However, the nature of the propagation speeds (longitudinal or transverse) has not been clearly identified. The aim is to establish a link between the features of the acoustic response and the elastic modulus of the matrix. For that purpose, the unambiguous knowledge of the longitudinal and transverse propagation speeds is necessary. Thus, the measurement of the time of flight of a travelling acoustic wave has been performed. Further evidence of the existence of these low propagation speeds has been confirmed. The analysis of the time echoes series leads to the identification of the transverse and longitudinal propagation modes. Thus, one may deduce the viscoelastic constant G´, which is consistent with that deduced from rheologic methods or from quartz crystal resonator technique based measurements.