Impact of Microstructure on Humidity Influence on Complex Impedance of Iron Manganite

This work deals with our continued research of oxide materials for potential application in gas/humidity sensing. We have made thick film samples using iron manganite powder synthesized by a sol-gel combustion method with a net-like structure, by screen printing on alumina substrate with PdAg interdigitated electrodes. The thick film samples were fired at two temperatures 600 and 800°C. The powder morphology was retained and only the organic compounds in the paste were burnt-out at 600°C, while at 800°C the thick film surface morphology was different where grain growth and individual particles can be observed. We measured complex impedance in a temperature and humidity chamber at room temperature (25°C) in the relative humidity range 30 – 90 % and frequency 100 Hz - 1 MHz. The thick film surface morphology has a significant influence on the effect of humidity on complex impedance. The complex impedance of thick films fired at 600°C was high and did not decrease noticeably with increase in relative humidity (RH), except for RH 80 and 90%, while thick film samples fired at 800°C had an overall lower impedance that reduced more noticeably with increase in RH 40{–}70% and even more for RH 80 and 90%.