Instability of Metal Oxide Parameters and Approaches to Their Stabilization
Metal oxides are the most stable materials used for gas sensor design. However, the problem of parameters’ instability still remains for metal oxides. Present chapter analyzes in detail this problem. In particular, in this chapter one can find a descripti
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Instability of Metal Oxide Parameters and Approaches to Their Stabilization
As shown earlier, metal oxides are the most stable materials used for gas sensor design. However, one should recognize that the problem of parameters’ instability also remains for metal oxides. It is known that the temporal drift of operating characteristics of conductometric gas sensors based on metal oxides (MOX), along with the low selectivity of sensor response, is considered to be a major disadvantage of these devices (Massok et al. 1995; Sayago et al. 1995; Barsan et al. 1999; Ozaki et al. 2000; Haugen et al. 2000; Romain and Nicolas 2010). According to Meixner and Lampe (1996) and Korotcenkov and Cho (2011), the main reasons for long-term instability of conductometric gas sensors are the change of the metal oxide parameters caused by structural and phase transformations taking place in these materials.
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Role of Structural Transformation of Metal Oxides in Instability of Gas-Sensing Characteristics
As mentioned before, structural change in gas-sensing material, caused by the grain growth due to their coalescence, is one of the most important reasons for observed changes in the metal oxide gas sensor parameters during their fabrication and exploitation (Meixner and Lampe 1996; Pijolat et al. 1999; Korotcenkov 2007a, b; Korotcenkov and Cho 2011). For example, Nakamura (1989) established that the grain size in polycrystalline structures fabricated by SnO2 powder sintering changed during 3 years of operation, from 5–15 to 20–40 nm. Matsuura and Takahata (1991) then observed a growth in grain size, even after 20 days of use. As shown by Korotcenkov and co-workers (Korotcenkov 2008; Korotcenkov et al. 2005a, 2009; Korotcenkov and Cho 2011), the above-mentioned changes in sensor parameters, caused by structure transformation of gas-sensing material, can be conditioned by the following processes. Changing geometric sizes in the network of grains, forming gas-sensing matrix. This process is an unavoidable consequence of the grain growth. Due to coalescence and mass transfer from the smaller grain to the bigger one, we can observe the increase of the contact area between crystallites and the forming of necks between grains (Korotcenkov et al. 2005b; Korotcenkov 2005, 2008). One of the variants of possible transformation of intergrain contacts is shown in Fig. 20.1. One can see in images shown in Fig. 20.2 how strong the change of morphology can be in a real situation. AFM and SEM images of the In2O3 films subjected to various thermal treatments are presented in Fig. 20.2. These results were presented by Korotcenkov et al. (2005a). It is seen that, depending on both temperature of thermal treatment and the crystallite size, the shape of the In2O3 G. Korotcenkov, Handbook of Gas Sensor Materials, Integrated Analytical Systems, DOI 10.1007/978-1-4614-7388-6_20, © Springer Science+Business Media New York 2014
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Instability of Metal Oxide Parameters and Approaches to Their Stabilization after sintering
before sintering Metal o
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