Effects of longitudinal wave on the resonance behavior of an electrode-separated piezoelectric sensor in liquids

An electrode-separated piezoelectric sensor (ESPS) was constructed and its operation in the liquid phase was characterized. When the surface of the separated-electrode reflects the weak longitudinal wave (LW) back to the surface of the quartz disc where it was generated, the interaction of the reflected LW with the main thickness-shear wave of the quartz disc is a potential error source. In this study, the influence of the LW effect on the response of the ESPS was investigated by an impedance analysis method. It was shown that all the impedance response parameters shift periodically as a function of the distance between the separated-electrode and the quartz crystal. The LW effect depends markedly on the surface conditions, including the shape, area, position, angle and roughness, of the separated-electrode. When a rough, non-parallel or ball electrode was used, the influence of the LW could be decreased greatly. The influence of the air/liquid interface on the reflection of the LW was efficiently screened in the ESPS.