Crystal impedance: A new technique for monitoring the sol-gel process

Crystal impedance measurements have been made during HCl-catalysed sol-gel reactions of tetramethoxysilane in aqueous ethanol solutions. Admittance spectra for thin films show responses that evolve from those characteristic of a liquid, to those of a viscoelastic system, and ultimately to those of a rigid film. The time course of these responses allows one to monitor the sol-gel process. In particular, the response undergoes dramatic changes upon gelation. Analysis of the impedance in terms of an equivalent circuit model provides a means of distinguishing predominantly viscoelastic effects from those where significant mass changes are involved.