Nano-Sized Semiconducting Oxide Powders for Thick Film Gas Sensors: From Powder Processing to Environmental Monitoring D
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sensors [17,18] with a good reproducibility, if the starting materials are well controlled. A comparison of the results of using commercial and chemically-processed nano-sized powders has demonstrated that the performance of subsequent sensors is improved by employing the latter. In the literature, several groups have reported studies on thick-film gas sensors [19], but only a few used nano-sized powders for the thick-film processing [20-231. Our main goal is to study cheap gas sensors in environmental-monitoring applications, and therefore outdoor field tests were performed using the prepared thick-film prototype sensors [24,25]. Previous work reported in the relevant literature investigated the use of gas sensors based on semiconducting oxides only for the control of air quality indoors [26] and inside passenger cars [27,28]. EXPERIMENTAL PROCEDURE Powder Preparation The thermal decomposition at low temperatures of heteronuclear complexes is a very
interesting method for the preparation of homogeneous heterometallic perovskite-type oxides [29-32], as recently found even for YBCO [33]. A simple technique for preparing LnTO 3 perovskite-type oxides (with Ln = rare earth, and T = Fe, Co) is the thermal decomposition of hexacyanocomplexes, easily precipitated in aqueous solutions [34], as proposed by Gallagher in 1968 [35]. The complexes are single source precursors with the same stoichiometry as the desired oxides. This versatile method [36] also allows the synthesis of single-phase, trimetallic complexes, the decomposition of which may produce single-phase, trimetallic perovskite-type oxides [37-40]. This method was found to be the best for the production of high-quality powders of nano-sized perovskite-type oxides. The formation of LaFeO 3 was achieved at a temperature as low as 350'C, much lower than the temperature needed for its formation using the conventional solid-state reaction method [41]. The nano-sized LaFeO3 and SmFeO 3 powders obtained by the thermal decomposition of the corresponding complexes at 700'C for 1 h were used for the preparation of the thick-film sensors [16,42]. Nano-sized powders of TiO 2 were prepared using a sol-gel route, particularly suitable for the homogeneous doping of TiO 2 with Nb or Ta. The pure titania powders were synthesized according to a procedure described in the literature [ 10], slightly modified to prepare Ta- or Nbdoped (5 or 10 at %) powders, and using alkoxides as precursors [43,44]. The nano-sized TiO 2 -based powders obtained by calcination at 400'C for 2 h were used for the preparation of the thick-film sensors [43,44]. Fabrication of Thick Films by Screen-Printing Technology The preparation of all the thick films was performed for all the oxide powders using the screen-printing. This technology implies a processing sequence with the following major steps: (1) careful selection of the oxide powders to be deposited; (2) preparation of pastes (or inks) using an organic vehicle, normally consisting of a rheological agent mixed in a light solvent to ensure the correc
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