Impedimetric, capacitive and resistive sensors

1 Influence of the fabrication processes in the performance of impedimetric CO2 gas sensors 

Sintering is the most widely-used technique for CO2 impedimetric devices and the most reported material is the BaliOj-CuO mixed oxide. Some examples of BaliOs-CuO sintered devices can be found in the bibliography, such as those by Ishihara et al. (1992,1995). It was their original suggestion that the sensing mechanism in this mixed oxide involves the adsorption of CO2 in the heterojunction between the n-type BaliOj and the p-type CuO, which supposes p-n junction barrier energy decrease that leads to a capacitance variation in the material. They prepared BaliOj pellets by the calcination of an equimolar mixture of BaCOj and TiOj at 1200°C. The BaTiOj was mechanically mixed with a commercial metal oxide (CuO) without further 

Wei et aL (2000) obtained the BaliOs-CuO equimolar mixture by means of CuO powders of around 50 nm and BaliOs powders of around 50-100 nm. In the next step, the material was pressed and annealed at 500°C for five hours, and a Pt wire was inserted as the contact. 

Another example of sintered resistive device is reported by Jiao et al. (2002), who mixed BaCOj andliOj and heated the mixture up to 1300°C to obtain the powder of BaliOj, which was then thoroughly mixed with CuO. The mixture was made into pellets and sintered in air. 

As seen in Table 16.2, the highest response to a level of 2% of CO2 was achieved by Ishihara when Ag was added to the BaTiOj-CuO pellet. Two different authors (Ishihara et al., 1995 Jiao et al., 2002) gave an extensive report of different additives (metals and oxides) that can improve the response of the sensing layers. Ag was found to be the additive that lead to the highest improvement (Jiao et al, 2002), but those authors do not give any theoretical explanation for this behaviour, while Ishihara et aL (1995) found the best results for La203, but at a higher temperature. 

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