Automotive pollution monitoring by an array of thick film gas sensors from nanosized semiconducting oxides

Road traffic and utilities fuel combustion strongly influence the air quality in the urban atmosphere; Carbon Monoxide and Nitrogen Oxides polluting the atmosphere are mainly caused by the exhaust fumes released from motor vehicles. Atmospheric pollution control is today performed, in the required range, by means of traditional analytical techniques such as chromatography, infrared spectroscopy and so on, according to standardized procedures. The draw-back consists in the limited number of air quality monitoring locations due to the high cost of the equipment and to the necessity of skilled personnel. A spread of monitoring sites is greatly important for achieving a better control of the air quality and also for studying the distribution of the pollutants. Such a result could be reached through the development and exploitation of cheap, reliable and selective solid-state sensors. Recently it has been shown that gas sensor performances are improved when the particle sizes decrease to nanometric level, due to the strongly increased specific surface area. Nevertheless to achieve stable, selective and reliable ceramic sensors, the nanostructured materials need a careful control of their structural, physico-chemical and electrical properties. Correct powder preparation is a crucial point and many factors have been considered, including grain shape and size, size distribution, intragranular porosity, and surface conditions. Nanosized powders of n-type and p-type semiconducting materials obtained by chemical routes such as sol-gel techniques, laser-assisted pyrolysis and thermal decomposition of heteronuclear complexes have been used to prepare thick film gas sensors . The screen-printing technology has been adapted for the fabrication of layers composed of nanosized particles; the sensors were printed on miniaturized laser-precut 96% alumina substrates, each 2x2 mm element being provided with a heater, Au interdigitated contacts and a Pt-100 resistor for controlling the operating temperature. Despite the high firing temperature necessary to guarantee long term stability at the working temperature, control and inhibition in grain coalescence have been reached by adding appropriate transition metal ions to the nanostructured ceramic materials. In fact the usage of homogeneous nanometric powder particles, joined together with high temperature (800-1000C) treatments, leads to a great improvement in the detection properties and long-term stability. It must be highlighted once again that repetitive results can be reached only by a very careful technological control of the sensor manufacturing. Electrical, structural and optical characterization of the thick film sensors obtained by different semiconducting oxides will be presented. The correlation between nanoparticle engineering (grain size, doping, thermal treatment) and the final properties of the sensing films will be emphasized. The gas-sensitive electrical properties of the films were studied in laboratory under different gases and in the field. We have recently demonstrated the possibility of performing an environmental monitoring of CO, NOx and ozone by an array of nanostructured thick film sensors which detect the pollutant concentrations with a limited error. This result has been obtained by calibrating different sensor arrays through the pollutant concentrations obtained by internationally recognized analytical techniques.