Effect of Excitation Signal Frequency on the Electrical Response of a MWCNT/HEC Composite Based Humidity Sensor

A printed flexible humidity sensor based on multi-walled carbon nanotubes (MWCNT) and hydroxyethyl cellulose (HEC) composite was fabricated for humidity sensing applications. The electrical response (resistance and impedance) of the sensor was investigated as a function of excitation signal frequency for optimizing the sensor performance, in terms of sensitivity. The MWCNT/HEC composite was gravure printed as the sensing layer on a flexible polyimide substrate. Silver based interdigitated electrodes (IDEs) were then screen printed on top of the sensing layer. The resistance and impedance responses of the sensor was measured for varying relative humidity (RH) (20% RH to 60% RH, increased in steps of 10% RH)) and for different excitation signal frequencies ranging from 0.1 kHz to 80 kHz. It was observed that the overall sensitivity for resistance and impedance response decreased by 79% and 60%, as the excitation signal frequency was increased from 0.1 kHz to 80 kHz, respectively. In addition, variations in resistance and impedance response increased by 70% and 63% as the excitation signal frequency was increased from 0.1 kHz to 80 kHz, respectively. The results demonstrated that the 1 kHz excitation signal frequency was the optimum frequency for measuring the resistive and impedance responses of the humidity sensor to obtain high sensitivity with high repeatability.