Design of Ceramic Materials for Chemical Sensors: SmFeO3 Thick Films Sensitive to NO2

Ultrafine SmFeO₃ powders were prepared by the thermal decomposition at 700°C of the corresponding hexacyanocomplex, SmFe(CN)₆]·4H₂O. These powders were used for the preparation of pastes which were deposited as thick films on alumina substrates with comb-type Au electrodes. The films were fired at different temperatures in the 800-1000°C range. The content of α-terpineol, a component of the organic vehicle, was varied in the range 0.0046-4 wt%. The microstructure, the chemical composition at the surface, the electrical conductivity, and the NO₂ sensing properties of the films were investigated. The content of α-terpineol strongly influenced the electrical conductivity and its activation energy. A significant reduction in the NO₂ response was observed for the films containing smaller amounts of α-terpineol, together with an increase in conductivity. On the other hand, the largest NO₂ response was observed for the films fired at 1000°C when 4 wt% of α-terpineol was used. Such increase in conductivity is attributed to a different oxygen surface layer on the SmFeO₃ surface, which is induced by the decomposition reaction of α-terpineol during sintering. The materials processing parameters are thus of primary concern for the NO₂ sensing properties of the SmFeO₃ thick films. The correlations found between activation energy, NO₂ sensitivity, and materials characteristics (influenced by the preparation parameters) are reported. These correlations can be used to design the gas-sensing properties of SmFeO₃ thick films for the optimization of their sensing characteristics.