Recently, sick-house syndrome receives public attention as a problem of indoor environment. Volatile organic compounds (VOC) such as formaldehyde (HCHO) and toluene (C₆H₅CH₃) generated from building materials are pointed out as one of the main cause of sick-house syndrome. In order to develop semiconductor gas sensor available to discriminate chemical species of these VOC, integrated thick-film gas sensors which are composed of SnO₂, WO₃, In₂O₃, ZnO, Co₃O₄ and Cr₂O₃ was fabricated on Al₂O₃ substrates. The present study was focused on examining the possibility of this sensor to discriminate 17 chemical species in VOC by pattern matching method using polar plots. First, the polar plots were constructed for each chemical species. These plots were classified into 7 groups, each of which has a characteristic shape depending on the functional group of chemical species. Next, the polar plots response time of SnO₂, WO₃ and In₂O₃ were also constructed for several chemical species. They were classified into three types of patterns. The polar plots obtained by the measurements of mixed VOC with known concentration were almost consistent with that of expected pattern The combination of the polar plots of both sensitivity and response time will assist the precise discrimination of chemical species in VOC.

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To build up VOCs identification sensor system, sensitivity and selectivity of various types of semiconductor gas sensors for VOCs (ethanol, 1-propanol, benzene, toluene, acetone, methylacetate, ethylacetate, methylethylketone and trimethylamine) were examined. By using TGS2602, TGS2600, TGS2440NF and TGS2201 gas sensors, it can be recognize the kinds of VOCs in ambient atmosphere and its concentration from sub ppm to several ten ppm levels, while the sensing characteristics for VOCs are influenced by NO₂ contamination but not by SO₂.

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Optical fiber sensors for detecting vapor pressure of alkane and gasoline gases have been studied. The sensing mechanism is based on the swelling phenomena in the rubber-type cladding polymers. The POF-type sensor heads were prepared by coating the swelling polymer, on the plastic fiber core with slightly lower refractive index than that of the cladding polymers. When it was exposed to alkane vapor or gasoline gases, the refractive index of the cladding layer decreased by swelling and the sensor head changed its structure from leaky to guided-type. In the experiment, a large change in the light intensity through the sensor head was observed for a various kind of alkane vapors and gasoline gases with good sensitivity and reproducibility.

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Pd- and Pt-loaded SnO₂ sensors were subjected to repeated surface chemical modification with diethoxydimethylsilane (DEMS) to modify the gas-sensing properties of the DEMS-modified SnO₂ sensor to H₂, CH₄ and C₃H₈ with the noble metal sensitizers. Variations in temperature programmed desorption (TPD) chromatograms of oxygen and water as well as in oxidation activities were also investigated in relation to the mechanism of the modification of the gas-sensing properties. Pt/SnO₂ was superior to Pd/SnO₂ in the H₂ sensing, and the sensitivity to H₂ increased with repeated surface modification, accompanied by a mazked increase in the H₂ oxidation activity. On the other hand, Pd/SnO₂ was superior to Pt/SnO₂ in the C₃H₈ and CH₄ sensing, and Pd/SnO₂ subjected to three times modification with DEMS exhibited the maximum sensitivity of S=180 to C₃H₈ at 350°C and S=38 to CH₄ at 400°C. Then, the response and recovery rate of the hydrocazbon sensing was satisfactorily fast at 400°C. However, the sensitivity to hydrocarbon decreased with further modification, although the catalytic activity for hydrocarbon oxidation was not so much influenced by the modification, in accordance with the almost unchanged amount of PdO species measured by TPD. The variations in gas-sensing and oxidation properties are discussed in terms of the effects of Pd- and Pt-loading and of the SiO₂ component introduced by the surface modification with DEMS.

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WO₃ thin film was evaporated on substrate provided with Pt interdigitated electrodes and with a heater element. The NO₂ and NO sensing properties of the filmprepared by vacuum evaporation with post annealing were measured both in N₂ and in air. With increasing operating temperature, NO₂ sensitivity in N₂ is increased but NO₂ sensitivity in air is decreased. Oxygen gas affects the adsorption-site of NO₂, that is related to oxygen vacancy of WO₃ film, at higher temperature. NO is also adsorbed to WO₃ film with electron-attraction in N₂ only at lower temperature.
Control of oxygen composition of WO₃ film was tried by evaporation under vacuum, oxygen gas and oxygen plasma ambient. NO_x sensitivities of these WO₃ films were measured without post annealing. The film evaporated under oxygen ambient has the highest electrical resistivity and the film evaporated under oxygen plasma has the lowest resistivity. The film evaporated under oxygen plasma ambient, that is expected to have lower density of oxygen vacancy, has low sensitivity for NO_x gas. The film evaporated under oxygen ambient has high sensitivity for NO_x gas.The NO_x sensitivities depend on the injection rate of oxy gen gas. The reason why the WO₃ thin film prepared by oxygen gas-evaporation has high sensitivity for NO_x seems to be related to the nano-crystalization of WO₃.

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Thin-film-type oxide semiconductor gas sensors sensitive to acetone were explored for the diagnosis of diabetes. SnO₂ and ZnO were tested as a base sensor material. When fabricated into a thick-film-type sensor, SnO₂ was inferior to ZnO in the acetone sensitivity. However, on changing to a thin-film-type sensor, SnO₂ proved to be more sensitive to acetone than ZnO. The acetone sensitivity of the SnO₂-based thin film sensor could be improved by the addition (0.1wt%) of noble metals, such as Rh and Ru. The resulting sensor using Rh-SnO₂ could detect as dilute as 0.2ppm acetone in human expiration at 300°C without being disturbed by changes in O₂ and CO₂ concentrations.

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Acetaldehyde gas sensing properties of SnO₂ loaded with different amounts of Rh or different kinds of metal oxides have been investigated. The addition of 1.5wt% Rh resulted in a slight enhancement in sensitivity. Among the metal oxides tested, only La₂O₃ promoted the sensitivity. The sensitization effect of Rh was promoted markedly by the simultaneous addition of La₂O₃. Among the sensors tested, SnO₂ loaded with 1.0wt% La₂O₃ and 1.5wt% Rh exhibited the highest sensitivity at 300°C.

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Various types of solid electrolytes based on ammonium hydrogermanate in which the germanium site was partially replaced by phosphor were prepared by a hydrothermal method and the ammonia gas sensing characteristics were investigated by fabricating an ammonia gas concentration cell with the hydrogermanates. The EMF output of the response was greatly improved by forming a solid solution by the phosphor doping with a considerable enhancement of the relative density and also the mechanical strength of the solid electrolytes.

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An amperometric sensor for the determination of dissolved oxygen was prepared by dip-coating a platinum electrode from an aqueous dispersion of polydimethylsiloxane (PDMS). Oxygen molecules easily pass through the porous PDMS layer but the transport of hydrophilic species (e.g., hydrogen peroxide and Fe(CN)₆^3-) through the hydrophobic polymer layer was strongly restricted, which results in the high selectivity of the present oxygen sensor.

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A planar-type CO₂ sensor using NASICON and perovskite-type oxide thin-films could be prepared by novel solgel methods using aqueous complex and polymer-based precursors, respectively. NASICON thin-film could be fabricated onto alumina substates by spin-coating a tartaric acid-based gel, drying at 150°C, and sintering at 1000°C for 3h. Preparation of perovskite-type oxide thin-films at 500°C was also developed by the use of poiy (vinyl alcohol) (PVA)-acetylacetone polymer precursor. The sensor device combined with LaNiO₃ and NASICON thin-films showed high sensitivity to CO₂ at 300-400°C. The 90% response time to 1000ppm CO₂ was as short as 10-30s at 300-400°C.

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