Nano-sized semiconducting oxide powders for thick film gas sensors: From powder processing to environmental monitoring devices

This paper reports the study of semiconducting oxides to develop gas sensors in thick-film form for use in atmospheric pollutant monitoring devices. The investigation was achieved with the following steps: selection of the suitable oxides and of their most appropriate processing method to obtain nano-sized powders, fabrication using screen-printing technology of thick-film sensors from these powders, and electrical measurements in laboratory and in the field. Chemical routes such as sol-gel techniques and thermal decomposition of heteronuclear complexes have been used to prepare nano-sized powders of n-type (TiO₂) and p-type (LaFeO₃ and SmFeO₃) semiconducting oxides. Thick-film gas sensors have been produced by screen-printing technology. Pastes have been prepared and printed on laser precut 96% alumina substrates, each 2×2 mm element being provided with a heater, comb-type Au contacts and a Pt-100 resistor for controlling the operating temperature. The firing of the films has been performed in conditions able to keep grain size at nanometer level. Electrical responses to some major polluting gases (CO, NO, NO₂ and O₃) have been tested in laboratory and in the field, and compared with results of the analytical techniques approved by the international standards.