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.