Humidity sensing and electrical properties of Na- and K-montmorillonite

Cationic clays, Na- and K-montmorillonite, were used to make a humidity sensor. The impedances of the Na- and K-montmorillonite were measured at various humidities and temperatures in a frequency range of 60 Hz–100 kHz. The results indicated that the counterions played a dominant role in conductance mechanism of the montmorillonite films. Moreover, the complex impedance spectra were adopted to elucidate the transport process by counterions in the mechanism for the conductance variation with %RH. For practical use, the K-montmorillonite film was chosen for further studying the other humidity sensing properties such as sensitivity, hysteresis, influences of applied frequency and ambient temperature, response and recovery times, and long-term stability.