Gas-sensing properties of SnxWO3+x mixed oxide thick films

Screen printing has been used to fabricate SnxWO3 + x mixed oxide thick films on alumina substrates. The powders for the thick-film pastes were made by heating various mixtures of SnO and WO3 powders, corresponding to the nominal formula SnxWO3 + x with x between 0.5 and 1.72, in an argon atmosphere at 600 degrees C for 15 h. The x-SnWO4 phase was the result of heating of an equimolar mixture of SnO and WO3 powders. Tin appears in the valence state Sn2+ in alpha-SnWO4, but in the SnxWO3 + x mixed oxides the Mossbauer spectroscopy revealed both Sn2+ and Sn4+ valence states of tin. In addition to the Mossbauer experiments, X-ray diffraction (and Raman spectroscopy) were used to study the structure of the mixed oxides used in the thick films. The electrical conductivity together with the gas-response properties of the films were studied at different temperatures between room temperature and 500 degrees C. H2S H-2, CO, SO2, NO and NO2 at various concentrations in synthetic air were used as test gases in the measurements. Some thick films were very sensitive and selective to H2S and had an anomalous response to NO. (C) 1998 Elsevier Science S.A. All rights reserved.