Microwave detection with various sensitive materials for humidity sensing

Abstract Most of the humidity or gas sensing researches focus on the optimization of sensitive materials, while the studies of microwave detection focus on its sensitive-field control. It is necessary to implement the cooperative study on microwave detection with different types of sensitive materials for a better understanding of the operation mechanism as well as providing a way for performance enhancement. This study presents the double split-ring resonator as the sensitive electrode, where various sensitive materials, including polymer by pure Polyimide (PI), ceramic composite by PI-TiO2, metal composite by PI-Ag composite, are respectively tested as the sensing materials. The S-parameter is tested as the humidity indicators after coating on the sensitive regions with a strong electric-magnetic field. Owing to the improved dielectric properties by doping strong-polarity TiO2 nanoparticles into PI, the higher sensitivity with the maximum of 880.0 kHz/%RH and 0.322 dB/%RH is realized. The introduction of Ag nanoparticles facilitates the water vapor adsorption and desorption showing the sensitivity of 330.0 kHz/%RH and 0.149 dB/%RH, due to the high active-site density and surface roughness. By analyzing variations in resonant frequency and return loss, the microwave sensing mechanism of different sensitive materials is fully discussed from the perspective of polarization, dielectric property, and sensitive area.