A high sensitivity and flexibility capacitance-type sensor using the polyacrylamide/MWCNT /ZnO and Ni-foam combination for humidity detection

Capacitance-type humidity sensors, known for their broad testing range, high linearity, fast response time, and long-term stability, have garnered significant attention over the past years. However, the present sensitivity materials and structures limit the improvement of detection accuracy and the expansion of its application range. This paper introduces a novel moisture-sensitive material composed of polyacrylamide (PAM), multi-walled carbon nanotube (MWCNT), and zinc oxide (ZnO), (named PAM/MWCNT/ZnO, PMZ). A capacitance-type humidity sensor is fabricated using PMZ as the sensitive layer and foam metal nickel (Ni-foam) as the electrode layer, benefiting from its excellent electrical conductivity and the mesh structure that facilitates the passage of water molecules. Finally, a packaged sandwich structure humidity detection sensor based on PMZ/Ni-foam is developed. Through thorough characterization of mechanical, physical, and radar schematic properties, the optimal mixing ratio of PAM, MWCNT, and ZnO was determined as 10:0.1:0.1. Experimental results demonstrate that the PMZ humidity sensor exhibits a sensitivity of 1.33 pF/%RH within the humidity range of 20–90 %RH. Notably, it achieves a sensitivity of 3.4 pF/%RH in the relative humidity range of 70–90 %RH, indicating higher sensitivity in high humidity environments. Furthermore, applications involving humidity monitoring in diapers for the elderly (60 –90 %RH), humidity detection of plant leaves, and humidity monitoring in grain silos to prevent wheat germination due to increased Gibberellin (GA) content were conducted. The proposed PMZ/Ni-foam sandwich structure humidity sensor holds potential for implementation in various fields, including food storage and production, agriculture, forestry, and wearables.