Electrical properties of K2O-doped Ba0.5Sr 0.5TiO3 ceramic humidity sensor

A ceramic humidity sensor using Ba_0.5Sr_0.5TiO₃ doped with a few molar percent of potassium oxide was studied. It was characterized by long life, was reversible without repeated high-temperature thermal desorption processes, and conductance against relative humidity (RH) sensitivity, as high as four orders of magnitude, was found at low frequencies. This sensor showed a good exponential relationship between the conductance, RH, and temperature at low frequencies. The plots of the relative dielectric dispersion against RH showed that it had an apparent dielectric constant increase at low frequency as a result of the electrode and water molecular polarization effects. By complex impedance plots with a non-Debye capacitor concept, an equivalent circuit model was established which could well simulate all the electrical properties of the sensor in the range of 65-95% RH for all measured temperatures (25°C-85°C) and frequencies (5 Hz-13 MHz). In the measurement of this sensor, an inductance loop over the low RH (15% and 30%) and low frequencies (5 to 40 Hz) range was observed, which might be due to the desorption of water vapor at the electrode surface for reduction reactions