Microstructural characterization of a titanium-tungsten oxide gas sensor
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Microstructural characterization of a titanium-tungsten oxide gas sensor Matteo Ferroni, Vincenzo Guidi, and Giuliano Martinelli INFM, Department of Physics, University of Ferrara, Via Paradiso, 12-44100 Ferrara, Italy
Giorgio Sberveglieri INFM, Department of Chemistry and Physics for Materials, University of Brescia, Via Valotti, 9-25133 Brescia, Italy (Received 14 March 1996; accepted 9 September 1996)
Thin films of Ti–W–O were prepared from a W–Ti alloy target by rf magnetron sputtering in reactive atmosphere. Analysis devoted to investigate the microstructural properties of this material was carried out in order to explain the origin for the high sensing performance of a W–Ti-oxide gas sensor. Scanning and transmission electron microscopy techniques showed that after annealing the film consists of a polycrystalline layer, isostructural to tetragonal WO3 , over which crystallites of pure WO3 are dispersed. The WO3 crystallites are insulated from each other and do not enter into the process of conduction of the layer. It was shown that Ti is soluted in the tetragonal WO3 lattice of the underlying layer. This layer exhibits fine granularity, which is an optimal feature for materials suited to gas sensing.
I. INTRODUCTION
Research on novel materials with unusual features takes an interest in wide and feasible applications. In particular, the research on chemical sensors has gained great interest because of its prompt applications in the fields of exhaust-gas pollution,1 environmental monitoring,2 domestic safety,3 and food industry.4 Present research preferentially addresses those materials that are both suitable for gas sensing and compatible with the promising techniques of silicon microsystems. Thus, a sensor’s performance could be enhanced by readout and signalprocessing electronics, which should be developed over a Si substrate.5 Solid-state chemical sensors are the best candidates for the development of low-cost devices. Both structural stability and microgranularity of the material should be accomplished by relatively simple and cheap processing. Presently, there is a reasonably good understanding of binary oxides as thin-film gas sensors.6 Refined techniques for preparation of sensing materials with the desired features have been developed7 ; also, the possibility to dope and catalyze these materials has been explored.8 This led to significant results in terms of both sensitivity and selectivity of the sensor.9 A new research scheme should now focus upon two strategies: preparation of multilayers of binary compounds,10 and research on multicomponent materials. The main effort should be to seek out new materials. This plan must be accompanied by a thorough study of the microstructural properties of the material under examination, aimed at correlating these features to the sensor’s electrical J. Mater. Res., Vol. 12, No. 3, Mar 1997
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response. This knowledge can drive the research toward unexpected extents
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