ANALYSIS OF ELECTRICAL EQUIVALENT-CIRCUIT OF QUARTZ-CRYSTAL RESONATOR LOADED WITH VISCOUS CONDUCTIVE LIQUIDS

An electrical equivalent circuit of quartz crystal resonator (QCR), with modified electrodes, loaded with a viscous conductive liquid is proposed. The electrode in contact with the liquid is modified so as to create an unmetallized crystal surface in contact with the liquid and thus to introduce an acoustoelectric interaction at the crystal/liquid interface. The admittance-impedance characteristics of the equivalent circuit of the loaded QCR are analyzed to obtain expressions for the changes in the series and parallel resonant frequencies in terms of the liquid viscosity eta(L), density rho(L), dielectric constant epsilon(L), and conductivity sigma, as well as the quartz parameters and electrode area geometry. Using a network analyzer, the S parameters of the equivalent circuit of various conductive liquid-loaded QCRs are measured and analyzed. It is shown that in addition to being a well-known detector of mechanical properties of liquids, a QCR, when properly designed, can be used to monitor the electrical properties of dilute ionic solutions.