Complex Impedance Property of Humidity Sensing Material Lanthanum Orthoferrite

Resistive type humidity sensor is fabricated using nanometer lanthanum ferrite. The impedance property of the sensor is measured in relative humidity (RH) range 11% to 98%, and in frequency range 10 Hz to 100 kHz. The complex impedance plots (Cole-Cole plots) are drawn. A circuit with resistors and capacitors is used as an equivalent circuit of the conduction process of the sensor. With the changing of the resistance and capacitance in the circuit, the shape of the complex impedance curve changes gradually from one semicircle to two semicircles, and finally only the second semicircle remains. This coincides with what happens in the Cole-Cole plots of our experiment. The mechanism of the humidity sensor is analyzed in terms of the complex impedance plots and the equivalent circuit. In low RH range, the conduction process is dominated by the decomposition and polarization of the grains of the sensing material, and in high RH range, by the decomposition and polarization of the adsorbed water.