Spinel humidity sensors prepared by thermal spray direct writing

Ceramic humidity sensors consisting of MgAl₂O₄ dielectric layer and Ni-5 wt.%Al electrodes were prepared by plasma spray processing and their electrical properties were investigated as a function of the relative humidity. The complex impedance of the sensor was measured at different humidity levels, using impedance spectroscopy. The resistance and the capacitance of the sensor were determined from the impedance plots using equivalent circuit simulations. The resistance of the sensor decreased from 2.4 × 10⁹ to 9 × 10⁵ Ω with increasing the relative humidity from 11 to 98% due to the increase of surface electrical conductivity resulting from water adsorption. The capacitance increased 1-7 nF in the humidity range.