QUARTZ-CRYSTAL RESONATORS AS SENSORS IN LIQUIDS USING THE ACOUSTOELECTRIC EFFECT

Piezoelectric quartz crystal resonators (QCRs) have been investigated as detectors in liquid environments. In all the applications, mass loading and viscous coupling are the main interaction mechanisms which result in changes in the QCR resonant frequency. However, other interaction mechanisms such as the acoustoelectric interaction due to fringing fields at electrode ends arise which contribute to the total change in frequency, in particular, the parallel resonant frequency. In the present work, it is shown that by modifying the geometry of the electrode at the QCR surface in contact with the solution, a transition region can be created in which the lateral decaying acoustic field is enhanced. The electric field can then interact with an adjacent conductive/dielectric solution which will result in relatively large changes in the parallel resonance conditions of the QCR. An equivalent circuit is proposed to analyze the loaded QCR with a modified electrode geometry. It is shown that this circuit is a general circuit which can be used to analyze all cases of a loaded QCR with one side in contact with a given viscous, conductive, or dielectric liquid. Especially, expressions are obtained for the parallel resonant frequency of the loaded QCR in terms of the solution dielectric constant and conductivity. It is shown, using 11-MHz devices on AT-cut quartz, that the modified QCRs can be used as effective and reliable detectors in conductive liquid environments to detect ionic solutes and their dielectric properties. Other applications are suggested.