DEVELOPMENT OF A SAW GAS SENSOR FOR MONITORING SO2 GAS

We have developed a new type of SO2 gas sensor by applying a particula r inorganic thin film on SAW (surface acoustic wave) devices. The sens or consists of twin SAW oscillators with a centre frequency of 54 MHz fabricated on LiTaO3 piezoelectric single crystals. One delay line of the sensor is coated with a CdS thin film that selectively adsorbs and desorbs SO2, while the other is uncoated for use as a stable referenc e. Deposition of CdS thin film has been carried out by the spray pyrol ysis method using an ultrasonic nozzle. Mass loading and electric-fiel d changes induced by the SO2 gas adsorbed onto the CdS film result in corresponding frequency shifts directly proportional to the gas concen tration. The relative change in the frequency of the two oscillators i s monitored with a digital signal-processing circuit. SO2 sensor prope rties investigated include sensitivity, response time and repeatabilit y. The sensor shows promising performance as a microsensing tool and i s capable of measuring concentrations in air less than 200 parts per b illion of SO2. (C) 1998 Elsevier Science S.A.