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 pro pagating 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 pe rturbation. If the acoustoelectric interaction between the surface wav e and the liquid is used, a surface acoustic wave sensor to detect the electrical properties of the liquid can be realized. The sensor sensi tivity is proportional to the electromechanical coupling coefficient o f the piezoelectric crystal used as the substrate. Hence, the sensitiv ity is high for the sensor made of a 36-degree rotated Y-cut X-propaga tion LiTaO3 which is a high-coupling piezoelectric crystal. In additio n, since a small amount of liquid is needed for detection and analysis of chemical reaction, this is expected to be a new chemical sensor. I n this paper, a sensor sensitivity equation is derived which can be ap plied in general to arbitrary liquid. The effectiveness of the equatio n is tested by comparison with the experimental results. A chart is pr oposed which can determine both the conductivity land the relative per mittivity of the liquid simultaneously. This chart is used for the: ev aluation of the conductometric titration. It is demonstrated for the f irst time that the permittivity also changes with the conductivity.