The impact of the epoxy thin-film layer on microwave-based SnO2 gas sensor for NO2 detection

A microwave gas sensor with tin dioxide as a gas-sensitive layer and epoxy thin film for enhanced detection of nitrogen dioxide in a harsh environment with high relative humidity content is proposed. An optimized transmission line type of sensors operated in the 1.5 GHz - 4.5 GHz was tested under exposure to: NO2 in the 0-100 ppm range, operating temperature in the range of 22-60 degrees C and relative humidity 0-85 % range. The crosssensitivity was tested under exposure to common volatile organic compounds such as acetone, and ethanol. The sensors' response (S) is given in phase changes based on wideband measurements. The advantage of wideband measurements compared to a single value is that they are based on multiple measurements taken at different frequencies. This greatly suppresses noise and enables measuring low target-gas concentrations within environments of high interfering compounds. The experimental results confirmed that using an additional epoxy thin film layer can reduce the impact of relative humidity on the gas-sensing properties of nitrogen dioxide in the microwave frequency range. The results should be considered a starting point for further investigation and pave the way for implementing microwave-gas sensors in harsh environments.