Polymer Resistive Thin Film Humidity Sensor--Investigation On Device Construction,Aging Mechanism And Humidity Sensing Mechanism

Thin film resistive-type polymer humidity sensors have been prepared on small interdigital gold electrodes, using sodium polystyrenesulfonate (NàPSS) and sodium salt of hydrated poly(styrene-alt-maleic anhydride) (Na-SMA) as sensing materials.The effect of sensing materials on humidity response of sensors has been investigated. Humidity sensor based on Na-SAM exhibits fast response and small hysteresis, and sensors with high sensitivity, small hysteresis and wide sensing range can be prepared using composite of Na-SMA and NaPSS. Applying suitable protecting layer is proved to be helpful for improving the stability of sensor at high humidities. The effect of composition of coating solution, dipping time and other technical parameters for preparation of sensors on sensing behaviour has also been described and discussed.It was found sensors with porous substrate shows lower impedance and better stability. The effect of the structure of interdigital array on impedance of sensor can be determined using the parameter A (A = mln[2.55(1±lWeIWg)I) , and the increase in the value of A leads to reduced impedance. Sensors with wider distance between electrode tracks exhibit improved stability in aging process.The aging mechanism of the humidity sensors has been studied. and it ~vas proposed that dissolution effect, tunnel effect and ion-destroying effect are the main factors affecting the aging of sensors. The improvement in stability of sensors by aging can be explained by irreversible tunnel effect. The optimal aging conditions for the sensors investigated are: U 200 mV, f1 KHz, t24 -48 h, RH 81-87%, T 3 80CIIThe sensing mechanism of the humidity sensors has been explored by means of complex impedance analysis. It is suggested the conductivity in the sensing film is ionic in nature, and its change with humidity can be described with modified Onsager equation.The humidity sensors so prepared enjoy the advantages of linear response and wider working humidity range, etc. compared with imported commercial sensors. A portable smart instrument for accurate measurement of humidity and temperature has been developed by cooperation research.