Investigation of Piezoelectric Quartz Crystal Resonators As Conductivity Sensors for Chemical Liquids

Quartz crystal resonators(QCRs) have been recently employed as mass and/or viscosity detectors in liquid environments. Some work have been done on detection of conductive solutions with the grounded side of the QCR in contact with the liquid. However, because of the absence of the surface potential on the grounded electrode of the QCR used in the work, only fringing fields can interact with the conductive solution, which have resulted in a lower sensitivity. This thesis presents an experimental investigation of the QCRs with a modified grounded electrode as conductivity detectors in liquid environments as well as an analysis of the electrical equivalent circuit of the loaded QCR. In the study, the grounded electrode is in contact with the solution. Several 11-MHz AT-cut QCRs with electrodes of different geometries and configurations are used. The electric field related to the surface potential in the unelectroded effective area is enhanced. This field contributes to the acoustoelectric interaction and results in a considerable changes in the parallel resonance conditions. A number of experiments are performed with both aqueous and non-aqueous dilute conductive solutions with the QCR in a Miller-type oscillator circuit. The equivalent circuit of the loaded QCR is also studied by analyzing the admittance/impedance characteristics of the circuit. The effects of the electrodes are taken into account. As a result, the parallel resonant frequency shift is obtained in terms of the liquid electrical parameters, the quartz crystal parameters and the electrode geometry and sizes. The experimental results which show good reproducibility indicate good agreement with the theoretical results. This work indicates that the modified electrode QCRs can be used as effective and reliable conductivity detectors in liquid-phase based environments