An NO2 sensor based on WO3 thin films for automotive applications in the microwave frequency range

A microwave system dedicated to the detection of nitrogen dioxide in the harsh environment of the Norway highways is proposed. An optimized transmission line type of sensor coated with a tungsten trioxide thin film that changes its electrical properties under NO2 gas exposure is developed. The sensors' response (S) is given in °/GHz and it is calculated based on wideband measurements. The advantage of wideband measurements in comparison to a single value is based on multiple measurements taken at different frequencies, which greatly suppresses noise and enables measuring low target-gas concentrations within environments of high interfering compounds. Herein, the developed system works in 1.5 GHz – 4.5 GHz, and NO2 varies in the 0–20 ppm range. The optimal thickness of the gas-sensing layer is estimated to be around 410 nm taking the advantage of the magnetron sputtering technology with the glancing angle deposition technique. The advantage of the developed sensor is the possibility to work at ambient temperature without the need to heat up the sensors. The disadvantage of the developed sensors is longer response and recovery times; however, this issue will be a subject of research in the future.