Thick-films based on nanosized titania powders have been prepared via screen printing. In particular, thick films of pure TiO_2, Nb:TiO_2, Ga:TiO_2, and Ta:TiO_2 have been characterized. An high annealing temperature often results in exaggerated grain growth, which in turn makes the sensor to be poorly performing. Addition of dopant rendered the films to be more stable under annealing, hindered grain growth and conversion of anatase to rutile. Ta and Nb addition to the TiO_2 powder proved effective for stabilization of the nanophase as the average grain size was reduced from 110 to 35 and 45 nm, respectively Structural, morphological and thermal studies have been carried out in order to correlate the measurements to electrical performance. The films were first characterized electrically in terms of conduction properties and grain-to-grain energy barrier. Then, the films proved sensitive to CO and NO_2. Measurements showed that the layers were capable to detect such gases on the range of concentration useful for environmental monitoring. We have shown that the performance of a titania sensor appears to be determined by the size of the grains. Measurements indicated selectivity enhancement toward CO by Nb addition and required film preparation to stabilize the film's grain size below 50 nm.
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