Thick Film Microsensors Based on Nanosized Semiconducting Oxides Powders

In materials science, great emphasis is currently being given to work on structures at the nanometer scale in order to study the unique physical properties which derive from size reduction to these dimensions(1). In nanosized materials, the surface-to-bulk ratio is much greater than for coarse materials, so that the surface properties become paramount, which makes them particularly appealing in applications where such properties are exploited, as in gas sensors. Grain size reduction is one of the main factors enhancing the gas sensing properties of semiconducting oxides (Shimizu & Egashira loc cit) and indeed sharp increases in sensitivity are to be expected when the grain size becomes smaller than the space-charge depth according to currently-accepted mechanisms(2). Thus, the application of nanostructured materials, both as powders and thin films, in gas sensors is rapidly arousing the scientific community interest(3-10). Both chemical and physical methods for the production of nanosized ceramic powders are being widely investigated(11-13). Controlled, ultrafine and homogeneously-sized ceramic powders can be prepared using chemical methods(14), and the first step in keeping full control of the microstructure of a dense or porous ceramic product is to control the preparation method of the starting powders. Chemical processing is particularly important for heterometallic oxides because conventional production methods (solid-state reactions at high temperatures) have several problems involving grain growth, changes in stoichiometry, and ease of second phase formation(15). The present paper reviews our work on the use of semiconducting oxide powders for the fabrication of thick films by screen-printing technology(16-22). A comparison of the results of using commercial and chemically-processed nanosized powders has demonstrated that the performance of subsequent sensors is improved by employing the latter. This approach has been seldom reported in the relevant literature; in fact, while there are several groups working on thick-film gas sensors(23-27), only a few used nanosized powders for the processing of thick films(28-30). Moreover, the purpose of our study being for gas sensors in environmental monitoring applications, outdoor field tests were performed using the prepared thick-film prototype sensors(31,32). Previously, sensors based on semiconducting oxides have been used only for the control of indoor air quality(33), and for the control of air quality inside cars for passengers comfort(34,35).