Sol Gel Coating Processes and Gas Sensors

The sol-gel coating processes described above have been investigated to synthesize nanocrystalline semiconductor gas sensors (typically tin oxide (SnO2)) in the last decade by several groups [5-37]. Other gas sensors, such as thin films of alumina (Al2O3) [38], iron oxide (Fe2O3) [39, 40], indium oxide (In2O3) [41-45], magnesium oxide (MgO) [46], molybdenum oxide (MoO3) [47-49], niobia (Nb2O5) [50], silicates impregnated with ruthium (Ru) complex [51], organically modified silicates [52], titania (TiO2) [53-58], barium strontium titanate ((Ba,Sr)TiO3) [59, 60], lead zirconate titanate (Pb(Zr,Ti)O3) [60, 61], tungsten oxide (WO3) [62], zinc oxide (ZnO) [63], zirconia/yttria stabilized zirconia (ZrO2/YSZ) [64-67] have also been synthesized using the sol-gel technique. The fundamental gas sensing mechanism and the reported results associated with the semiconductor metal oxides are reviewed in detail.

Environmental pollution and security in the work and domestic ambient atmospheres represent acute problems with a high social impact. The need for solid-state chemical sensors, and more specifically gas sensors, in areas such as manufacturing process control, monitoring of toxic and combustible gases in emissions, and domestic and manufacturing safety, has led to extensive research in the area of semiconductor metal oxide gas sensors. SnO2 is the most widely used commercial gas sensor today as it represents good compromise between price, stability, and reliability of material together with a relatively low sensor operating temperature (230 °C), fast response, and high mechanical strength. SnO2 has also been utilized in many other applications such as liquid crystal display, solar cells, protective coatings, and catalysts [68-70]. SnO2 gas sensors in the thin- or thick-film forms are very attractive due to their small size, simple construction, low cost, and low weight. The demand for thin-film type SnO2 gas sensor devices is also expected to grow because of their lower power consumption and their easy integration into other devices [71, 72]. Applications of SnO2 sensors have been expanded into the fuel industry, environmental protection, and biotechnology. SnO2 based semiconductor type devices have been widely used for the prototype gas sensors for detecting inflammable gases. In view of their potential applications to a high-performance micro-sensor integrated in a silicon chip [73], thin-film type SnO2 gas sensors have drawn much interest.

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