Tadashi Takada

Highly sensitive ozone sensor

Abstract A highly sensitive and reliable O3 gas sensor (below 100 ppb) has been studied using an In2O3 semiconductor thin film. The effects of various metal-oxide catalysts added to the film surface have been examined. The additives greatly affect the sensitivity characteristics of the O3 gas sensor, especially the temperature dependence of O3 sensitivity. The peak of the temperature-dependence curve for the Fe-oxide-added film is located at about 370 °C, but the peak for the Cs-oxide (or Rb-oxide)-added film is shifted up to about 500 °C. The peaks for other metal-oxide additives (Ba, Mg, Ni, Ce, Cr and Zr) are located between 400 and 460 °C. The peak for Mo-oxide (or W-oxide) is also shifted up to above 465 °C. The location of the peaks of the temperature-dependence curves is likely to relate to their standard enthalpies of formation (°H°f) of the hydroxides (or the oxyacids) from their oxides. The above phenomena suggest the effects of the surface hydroxyl groups on the O3 sensitivity. It is found that the Fe-oxide-added In2O3 semiconductor thin film has a high sensitivity and reliability to low concentrations of O3 below 100 ppb. The sensor is able to detect 8 ppb O3 with sufficient response. It also exhibits a power-law behaviour to O3 concentration between 8 ppb and 10 ppm, RoutαC0.38. It may be capable of detecting low concentrations of O3 down to 1 ppb. Thus, the present sensor has sufficiently high sensitivity and reliability to observe low concentrations of natural O3 in the atmosphere.

Aqueous ozone detector using In2O3 thin-film semiconductor gas sensor

Abstract A detector for continuous determination of a trace amount of aqueous ozone, utilizing an ozone-extraction process from liquid (water) into the gas phase and an In2O3 thin-film semiconductor ozone gas sensor, has been developed. It has many advantages, including high sensitivity to aqueous ozone and high aqueous ozone selectivity in the presence of various kinds of interfering compounds in water. It is able to detect 0.5 ppb of aqueous ozone with a sufficient response. The response to 1 ppm of free chlorine in water is smaller than that to 10 ppb of aqueous ozone. It is revealed that the high sensitivity and the high selectivity are attributed in considerable part to the higher extraction rate of ozone from water into the gas phase than those of other compounds in water. This detector is somewhat affected by a change in water temperature. The water-temperature correction has been studied. A precise determination of aqueous ozone has been performed in the water-temperature range 0–40 °C with an improved detector equipped with the correction process.

Rapid evaluation of oxidation catalysis by gas sensor system: total oxidation, oxidative dehydrogenation, and selective oxidation over metal oxide catalysts

Abstract The rapid evaluation of catalysis is an indispensable technology for the success of combinatorial chemistry. A small-sized, less expensive, easily operating screening is desirable for parallel settings which dramatically shortens the evaluation time. Recent advances in gas sensors have enabled us to use them for the rapid evaluation of oxidation catalysis. Three typical catalytic oxidations over metal oxide catalysts were evaluated by gas sensor systems optimized for each catalytic system. The first one is the total oxidation of carbon monoxide in air. Five catalytic combustion-type gas sensors were used in a parallel reactor system to shorten the evaluation time. The second one is the oxidative dehydrogenation (ODH) of ethane over the mixed oxide of nickel and iron. The evaluation of the ODH catalysis was performed by a selective olefin sensor which determines the concentration of C 2 H 4 in C 2 H 6 . The third one is the selective oxidation catalysis of propane over alkali modified Fe/SiO 2 . The effluents including CO, CO 2 , aldehydes and ketones in propane were analyzed by the CO, CO 2 and semiconductor-type gas sensors selective toward aldehydes and ketones. These evaluation results indicated that gas sensors have a good potential for the rapid evaluation of oxidation catalysts.

Highly sensitive odour sensors using various SnO2 thick films

Abstract To improve the sensitivity to odour and the odour selectivity, the effect of calcination temperature, crystallite size and surface area have been examined for SnO2 thick films. The sensitivity and the selectivity depended strongly on the crystallite size (or calcination temperature), and the optimum crystallite size (or calcination temperature) for the odour sensor is 53 nm (or 1200 °C). The odour selectivity to odourless gases is improved by lead-oxide addition to an SnO2 thick film calcined at 1200 °C. The effects of various metal-oxide coating layers on lead-oxide-added SnO2 thick films have also been examined. The layers are effective in characterization of each odour sensor. It seems that the metal oxide functions as a filter through the coating layer and as a catalyst at the interface to the films. The present group of sensors might facilitate the construction of an odour-discrimination system utilizing the different odour selectivities of plural sensors.

Identification of odors using a sensor array with kinetic working temperature and Fourier spectrum analysis

A method for the identification of odors using a dynamically driven sensor array has been developed, in which transient responses of the sensor array were used to recognize target odors. Fourier transformation was used to transform the transient response curves of the sensor array into Fourier spectra, and patterns composed of some of the magnitudes in the spectra were used in the pattern recognition to identify the odors. Identification of odors was performed using this method with three kinds of 10% ethanol solution of tea extract and three kinds of Japanese soy sauce. In consequence, the sample odors could be correctly recognized without any pretreatment device for separation of minor components from the main component.

Temperature drop of semiconductor gas sensor when exposed to reducing gases — simultaneous measurement of changes in sensor temperature and in resistance

Abstract Changes in sensor temperature, Δ T , and in semiconductor resistance, Δ R , were simultaneously measured when various semiconductor gas sensors were exposed to reducing gases. The drop of the sensor temperature was often observed, together with a decrease in semiconductor resistance, despite the exothermic reaction of the gases with the surface oxygen adsorbates. As a plausible explanation for this phenomenon, it was considered that the temperature drop was caused by a change in thermal conductivity of the sensors, as a result of an increase in the amount of conduction electrons due to consumption of negatively charged oxygen ad-ions.

A temperature drop on exposure to reducing gases for various metal oxide thin films

Abstract The temperature drop on exposure to C 2 H 5 OH was examined for various metal oxide thin films of In 2 O 3 , SnO 2 , CuO, WO 3 , ZnO, CeO 2 (W), CeO 2 and TiO 2 to study the mechanism. The temperature drop was observed for films of In 2 O 3 , SnO 2 and WO 3 , of which the change in electric conductivity Δ G was comparatively large, and the magnitude of the drop increased with Δ G . On the other hand, the temperature drop was not observed clearly for films of ZnO, CeO 2 (W), CeO 2 and TiO 2 , of which the Δ G was small. The dependence of the temperature drop on sensor temperature was also measured in the range of 300–500°C of sensor temperature for films of In 2 O 3 , SnO 2 and WO 3 . It was found that the dependence was nearly expressed as A ( T 4 − T 0 4 )×Δ G ( T ), where T and T 0 denote a sensor temperature and a room temperature in Kelvin scale, respectively. The mechanism of the temperature drop was discussed.

Aqueous Ozone Detector Using In2O3 Thin Film Semiconductor Gas Sensor

Abstract A detector for continuous detennination of a trace amount of aqueous ozone, utilizing an ozone extraction process from liquid (water) into the gas phase and an In2O3 thin film semiconductor ozone gas sensor, has been developed. It offers many advantages including high sensitivity to aqueous ozone and high aqueous ozone selectivity in the presence of various kinds of interfering compounds in water. It is able to detect 0.5 ppb of aqueous ozone with a sufficient response. The response to 1 ppm of free chlorine in water was smaller than that to 10 ppb of aqueous ozone. Long term monitoring aqueous ozone over 100 days was performed in an experimental plant for ozone treatment in water supply with the present aqueous ozone detector. The readings of the detector agreed well with the results of iodometry during the monitoring.