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An impedance spectroscopic analysis has been performed on WO 3 and MoO 3 −WO 3 thick-film gas sensors, working under the flow through technique. The dielectric response of the metal oxides at typical operation temperature ͑250-300°C͒ has been analyzed in the presence of a few parts per million of NO 2 , 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 WO 3 film, as confirmed by scanning electron microscope analysis. In the case of MoO 3 −WO 3 it was also possible to determine the permittivity of the material and the depletion region width.

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