Morphological differences affecting the dielectric response of MoO3-WO3 and WO3 thick films

An impedance spectroscopic analysis has been performed on WO3 and MoO3-WO3 thick-film gas sensors, working under the flow through technique. The dielectric response of the metal oxides at typical operation temperature (250-300 degrees C) has been analyzed in the presence of a few parts per million of NO2, in dry air. The experimental data were processed to get the best equivalent circuit with known circuital elements such as resistance, capacitance, and constant phase element. The sensors have been modeled taking into account the contribution to the total impedance of the intergranular contacts and of the substrate. As a result, a bimodal distribution of grains was foreseen for the WO3 film, as confirmed by scanning electron microscope analysis. In the case of MoO3-WO3 it was also possible to determine the permittivity of the material and the depletion region width. (c) 2007 American Institute of Physics.