SHEAR SURFACE-ACOUSTIC-WAVE LIQUID SENSOR-BASED ON ACOUSTOELECTRIC INTERACTION

The piezoelectric potential generated along with the surface acoustic wave (SAW) propagating on a piezoelectric crystal, penetrates into the liquid loading the crystal unless there is a metallic film on the propagating surface. Therefore, the variation of the electrical property of the liquid (such as the permittivity and conductivity) affects the piezoelectric potential and, as a result, the surface wave receives perturbation. If the acoustoelectric interaction between the surface wave and the liquid is used, a surface acoustic wave sensor to detect the electrical properties of the liquid can be realized. The sensor sensitivity is proportional to the electromechanical coupling coefficient of the piezoelectric crystal used as the substrate. Hence, the sensitivity is high for the sensor made of a 36-degree rotated Y-cut X-propagation LiTaO3 which is a high-coupling piezoelectric crystal. In addition, since a small amount of liquid is needed for detection and analysis of chemical reaction, this is expected to be a new chemical sensor. In this paper, a sensor sensitivity equation is derived which can be applied in general to arbitrary liquid. The effectiveness of the equation is tested by comparison with the experimental results. A chart is proposed which can determine both the conductivity land the relative permittivity of the liquid simultaneously. This chart is used for the: evaluation of the conductometric titration. It is demonstrated for the first time that the permittivity also changes with the conductivity.