WO3 thick-film gas sensors

Tungsten trioxide is a promising material for NOx gas sensors. This paper describes the preparation of tungsten trioxide thick films by the use of the traditional screen-printing technology. We present the characterization of both their structural properties by means of XRD measurements and the film morphology by the SEM microscope studies, and the electrical response of the films of various thicknesses to NO. The NO-sensing properties of WO3 thick films doped with Bi2O3 are also investigated in this paper. The thick films show a good sensitivity to NO over a wide temperature range and much better results are obtained by the doped films. It is demonstrated that a bottom thick-film layer of about 15 μm thick determines the sensing characteristics of the WO3-based sensors. We also present the fabrication of planar-type WO3-based thick-film sensors for on-line monitoring of NO concentrations. The sensor with the sensing film doped with Bi2O3 shows a good sensitivity to low NO concentrations (2-300 ppm) at 300 °C.