Disadvantages of Nanocomposites for Application in Gas Sensors
This short chapter analyzes disadvantages of nanocomposites for application in gas sensors. It is shown that complication the gas sensing matrix composition can be accompanied by deterioration of sensor parameters’ reproducibility. Incompatibility of mate
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Disadvantages of Nanocomposites for Application in Gas Sensors
Our short review shows that the use of nanocomposites can lead to an improvement in gas sensors parameters. However, one should remember that the complicated nature of the composition of gas-sensing matrix is always accompanied by the deterioration of sensor parameters’ reproducibility. Many additional factors, which can have an effect on gas-sensing properties of materials, appear in nanocomposites. It is especially typical for nanocomposites used as sensing materials in metal oxide chemiresistors. It has been established that the change of additional component concentration has an equally powerful influence on the gas-sensing materials’ parameters as does the change of deposition temperature (synthesis) or technological route used. All the main parameters of gas-sensing materials such as grain size, porosity, interphase interaction, surface and interfacial energy, catalytic activity, chemical reactivity, texture, stress, and strain, which control sensor response, significantly depend on the type and the concentration of additives (Gas’kov and Rumyantseva 2001, 2009; Korotcenkov 2005, 2007; Zhang et al. 2003a, b). Therefore, even small additives of the second phase could change parameters of metal oxide gas-sensing matrix fundamentally. However, it was established that for achievement of optimizing effect we have to find specific composition of nanocomposites, because as a rule the optimizing effect is being observed at certain concentration of one of the components only. Moreover, as one can see in Fig. 17.1, the range of doping concentration, which could be accompanied by sensor response improvement, is narrow, and deviation from this optimal concentration could lead to not an improvement but a sharp drop of sensitivity or lead to fast sensor degradation (see Fig. 17.1). For example, it was established for metal oxide-based nanocomposites that if additives do not form a conductive phase with good sensing properties, the optimizing concentrations of additives in the metal oxide matrix would not exceed 1–3 % (Tricoli et al. 2008; Liu et al. 2010). Only in the case of composites which are formed by two gas-sensing conductive materials could optimal content of the second phase reach 10–20 % (see Chap. 14 (Vol. 2)). The same effect was also observed in gas sensors based on polymer–black carbon (metal, NCTs, or metal oxide) nanocomposites, where, due to the swelling effect, the change of conductivity near the “percolation threshold” was being used. In mathematics, percolation theory describes the behavior of clusters connected in a random graph. In our case the percolation threshold corresponds to the concentration of conductive phase, at which in composite consisted from conductive and insulating phases, the transition from insulating state to conductive one takes place, i.e., the transition from a high-resistance to a low-resistance state. Maximal sensor response, conditioned by the swelling effect, is possible only if the nanocomposite composition corr
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