Sensitivity and complex impedance of nanometer zirconia thick film humidity sensors

Nanometer zirconia (ZrO₂) with grain size 20 nm was used to make thick film humidity sensor on silicon substrate. The impedance of the sensor changed from 10⁶ Ω to 10² Ω when the relative humidity (RH) varied from 11% to 98%. The curve of impedance vs frequency showed good linearity when the measured frequency was in between 100 Hz and 1 kHz. The temperature influenced the impedance of the sensor. Complex impedance (Nyquist) diagrams of the sensor at different relative humidities and temperatures were drawn. An equivalent circuit with resistors and capacitors was built to explain the conduction process of the sensor. In low RH range, the conduction process was dominated mainly by conduction and polarization of the grains of nanometer zirconia, while in high RH range, by decomposition and polarization of the absorbed water.