Establishing and interpreting an electrical circuit representing a TiO2–WO3 series of humidity thick film sensors

This paper reports the establishment and interpreting of an equivalent electrical circuit representing the humidity-sensitive electrical properties of TiO2–WO3 thick films. Prototype sensors have been prepared by depositing an emulsion of TiO2 and WO3 powders in acetone with cellulous glue onto an alumina substrate, by a spin coating technique using a low spreading speed (250 rpm). Films were prepared with five different atom proportions of Ti and W: 100:0, 100:1, 100:6, 100:18 and 100:36. The oxides thick films were successively fired at the temperatures of 600, 700, 900, 1100 and 1300 °C, for 2 h at each of these temperatures. The variation of the electrical signal with humidity in ceramic sensors is originated by the chemical and physical sorptions of water molecules existing in the atmosphere.For all the sensors, the same type of circuit was established based both on circuit theory and on the physical mechanisms underlying conduction and polarization. The best fitting for the Nyquist plots obtained at the temperature of 25 °C and various relative humidities (RHs), in the frequency range 0.1 Hz to 40 MHz, was achieved with two R, C parallel circuits in series with two parallels of constant-phase elements (CPEs) and capacitances.As examples, for two of the sensors, TiW2 600 and TiW2 700, the values of the electrical components are presented in tables and graphs. The evaluated values of the components are interpreted and compared, and conclusions could be drawn about the mechanisms of conduction and polarization taking place in the adsorbed water.