TEMPERATURE CHARACTERISTICS OF ELECTRICAL-PROPERTIES OF (BA,SR)TIO3 THICK-FILM HUMIDITY SENSORS

Ceramic materials based on perovskite-type oxides can be used as humidity sensors. Thick film sensors based on (Ba0.5Sr0.5)TiO3 material doped with MgO were prepared by printing and firing thick film pastes on 96% alumina substrates. Fritless platinum paste was used as the electrode material. The a.c. response of the thick film humidity sensors at 20-80% relative humidity and 10-60 degrees C was measured. The conductance and susceptance of the sensors versus relative humidity were measured at 400 Hz. The capacitive component of the sensors impedance at low humidity (20-40% relative humidity) has negative temperature characteristics and obeys the Curie-Weiss law. This indicates the dielectrical properties of the adsorbed water on the perovskite sensor surface. The conductance is logarithmically linear. At high humidity a positive temperature characteristic of the capacitive component was observed. An inversion between these two characteristics was detected at about 40% relative humidity for all measured temperatures.