The Role of Temporal and Thermal Stability in Sensing Material Selection
The problems of stability and reliability of gas sensors operation are dominant while designing devices for sensor market, regardless of used sensing materials. Therefore, sensing materials and conditions of their operation should be selected according to
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The Role of Temporal and Thermal Stability in Sensing Material Selection
The problems of stability and reliability of gas sensors operation remain dominant while designing devices for the sensor market. Devices designed for this market should provide a stable and reproducible signal for a period of at least 2–3 years (typically 17,000–26,000 h of operation). Therefore, sensing materials and conditions of their operation should be selected in consideration of the above-mentioned requirements (Korotcenkov 2007; Korotcenkov and Cho 2011). For example, the organic polymer Nafion may retain working capacity in electrochemical gas sensors for a period of up to about 1 year. To achieve this result, however, the Nafion should be wetted by a wick system connected to a reservoir (Pasierb et al. 2004). This means that dry atmosphere does not facilitate a long lifetime for polymer-based electrochemical gas sensors. The same situation takes place with exploitation of polymer-based sensors in ozone-containing atmosphere. It has been reported that ozone and other oxidizing components in the polluted atmospheres of industrial centers can either initiate or accelerate the photochemical destruction of polymers (Razumovskii and Zaikov 1982). Thus their lifetime in sensors may be limited by the presence of atmospheric ozone. Polymer sensors used for environmental control also have a significant disadvantage in terms of their sensitivity to UV radiation. Moreover, it was found that polymer degradation is almost always faster in the presence of oxygen (air) and moisture. It was established that, as a rule, a polymer-based conductometric gas sensor can present a high sensitivity in its first post-produced day, but it can also exhibit important signal response reduction on the next few days. Experiment has shown that this effect is especially strong for chemiresistors based on doped conducting polymers. Results obtained by Lima and de Andrade (2009) for polymer-based chemiresistors and presented in Table 18.1 are a good illustration of this effect. Table 18.1 shows the normalized sensitivity loss of each sensor for different analytes. The sensors were produced by spin-coating and layer-by-layer techniques using the different conducting polymers indicated in Table 18.1. All sensors were submitted to the same different analytes during 5 consecutive days. It is seen that the sensitivity loss is different for each sensor, but these changes are considerable. As a result of such temporal instability, in spite of the wide range of chemical sensor prototypes based on polymer films, very few have found their way onto the market. Though they may have excellent analytical qualities, the devices are often unsuitable for industrial fabrication, either because of low technological effectiveness of the fabrication process or insufficient reliability and stability. This means that to utilize the advantages of some polymers, such as a rare combination of electrical, electrochemical, and physical properties, it is necessary to increase their processabilit
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