Investigations on Humidity Sensing Properties of Thick Films of the TiO₂:WO₃ System

This paper reports the study of 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 (125 r.p.m.). Films were prepared with five different atoms 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, during 2 hours at each of these temperatures. Measurements were obtained at a constant temperature of 25 °C and at various relative humidities (RHs) in the frequency range of 0.01 Hz – 40 MHz. It was thought that the humidity sensing of the TiO2 matrix would be influenced by the substitution of the Ti4+ ions for higher valence ions, W6+ in the present study. In the measuring frequency range, samples impedance varies of three-four orders of magnitude over the RH range of 10-100 %. It is found to be the sensitivity highly dependent on both the measuring frequency and the sintering temperature. A parameter called characteristic humidity is defined to represent the sensitive response of sensors. This parameter is used to evaluate the influence of the W6+ ions on the sensitivity behaviour of the sensors.