A thin-film wide-range air-fuel ratio sensor has been fabricated on a porous alumina substrate. The sensor consisted of an oxygen pump cell and a limiting-current cell on the substrate. These cells were laminated in the order, Pt/ZrO₂ electrolyte/Pt/ZrO₂ electrolyte/Pt by sputtering technique. In this sensor, the oxygen pump cell served to supply oxygen to the limiting-current cell as substitution for air introduction. The limiting-current cell with a constant applied voltage of 0.6V showed a continuous output characteristic corresponding to air ratio of 0.7-1.5. The use of the oxygen pump cell has made it possible to eliminate air introduction and the changeover of the polarity of the applied voltage to the limiting-current cell.

View full abstract

This paper represents the gas-sensing capabilities of quartz crystal microbalance (QCM) devices coated with plasma-deposited organic films for detection of environmentally problematic compounds of chlorocarbons: chloromethanes and chloroethylenes. Plasma-deposited organic films are prepared by sputtering of amino-acids and fluoropolymers. Both types of the films shows the opposite gas-sorption characteristics; the amino-acid films have high affinities for lowly chlorinated carbons, and the fluoropolymer films show large sorption capacities for highly chlorinated carbons. These tendencies can be physicochemically interpreted by Linear Solvation Energy Relationships. The 0.5-ppm tetrachloroethylene and trichloroethylene vapors can be detected by the amino-acid-film-coated QCMs, even though they have low cross sections for highly chlorinated carbons.

View full abstract

The housing environment is changing with the change in the energy situation in Japan. The house is gradually becoming an airtight structure to economize energy, and an indoor air-quality is going down. In this study, the gaseous indoor-air pollutants are recognized by a multi-gas sensory system using the OPS5 system which is one of the production systems. The sensory system has six gas sensors. Eight gases are selected as main indoor-air pollutants. They are carbon dioxide, carbon monoxide, ammonia, propane, methane, cigarette smoke, ethanol and formaldehyde gases. The concentration of some of these will be calculated by the system. The sensor outputs are input into a computer every 4 seconds. The recognition processes are done in this interval and a specific kind of pollutant is identified. About twenty rules are employed in this system.

View full abstract

Aiming at developing a semiconductor CO sensor applicable to the safety control of gas appliances, we carried out an extensive material search for promoters to In₂O₃-based elements. Among the elements added (0.5 wt%) with each of 12 transition metal oxides, the Co₃O₄-added one was outstanding in sensitivity and selectivity to CO at 200°C. Remarkably the sensing properties of this element could be improved further by the addition of Au (0.04 wt%). The doubly promoted element, Au (0.04 wt%)-Co(0.5 wt%)-In₂O₃, gave excellent characteristics in sensitivity to CO, selectivity to CO over H2 and other selected gases, and response rates at 250°C. The promoting effects of the additives were shown to originate from the proper enhancement of catalytic activity for CO oxidation.

View full abstract

Identification of primary factors responsible for indoor environments gas sensor responses is difficult due to the residual components in the room. By removing these components from observed sensor signal, we analyzed the linear components of the gas transfer route. A gas transfer routes, generally expressed by Navier Stokes equation, are non-linear fields. However, gas transfer routes can be approximated to a linear field with reducing flow velocity. We report here a model of indoor flow field generated using a linear system, and applied this model to experimental results. The results of this study indicated that over 90% of residual components of the gas sensor response were linear. Thus, the identification can be improved by reducing residual components from the sensor response using the minimum phase inverse filter.

View full abstract

Capacitive type NO sensor which is operable at high temperature was investigated in this study for monitoring NO in flue gases from gasoline engine vehicle. Although the high sensitivity is obtained at temperature lower than 400°C, it is negligibly small at temperature higher than 500°C on large part of the mixed oxide examined. On the other hand, BaSnO₃ or SrSnO₃ mixed with WO₃ exhibits the high sensitivity at temperature higher than 500°C. In particular, mixed oxide capacitor of SrSnO₃-WO₃ exhibited the high sensitivity to NO at 550°C. Since the capacitance of this oxide increased with increasing NO concentration, in the range from 10 to few 1000ppm. NO concentration can be evaluated with the capacitance of the element. Furthermore, the capacitance of SrSnO₃-WO₃ is less sensitive to CO₂, H₂O and CO comparing that to NO. Consequently, mixed oxide of SrSnO₃-WO₃ is promising for the capacitive NO sensor operable at high temperature.

View full abstract

A CO₂ sensor was set up by using Na₃Zr₂Si₂PO₁₂ (NASICON) as a solid electrolyte, Li₂CO₃ as a carbonate phase. The EMF (Electromotive force) was linear to the logarithm of CO₂ concentration and hardly depended on interfering gases by using a zeolite filter. The Δ EMF (EMF_Air-EMF_Gas) was stable even after the test of unpowered exposure to a high humidity atmosphere. The CO₂ monitor in which the CO₂ concentration is calculated by the Δ EMF using a new renovating system of the standard EMF was developed as a monitor for air quality control. The output of the monitor indicated a good agreement with that of the usual NDIR analyzer at the field-test in the office.

View full abstract

Semiconductor gas sensors having a combination of p- and n-type oxides have been investigated for detecting a small amount of NO. Among 16 kinds of p-n combinations tested, the Co₃O₄-Fe₂O₃ composite sensor exhibited excellent sensing properties to NO at 300°C. When the Co₃O₄ content in the composite was set to 20wt%, the sensitivity to 50 ppm NO was the maximum as high as 2.8, while that to 50 ppm NO₂ was 1.5, indicating the highly sensitive and selective nature to NO. Further, the sensor showed the almost linear correlation between logarithm of NO sensitivity and that of NO concentration in the range of 5-100 ppm NO. These results suggest that the Co₃O₄-Fe₂O₃ composite sensor can be a candidate of NO sensor for combustion monitoring. The addition of Co₃O₄ promoted the formation of NO^- adsorbate on the composite surface, resulting in the high NO sensitivity.

View full abstract

Current (I)-voltage (V) characteristics of ZnO-based oxides have been investigated in air as well as in 100 ppm NO₂ and NO balanced with air at elevated temperatures. A pure ZnO exhibited a nonlinear I-V characteristic in air and its breakdown voltage shifted to a high electric filed upon exposure to NO₂, but to a low electric field upon exposure to NO. Grain size of ZnO was increased slightly by the Bi₂O₃-based additive, which is essential for fabricating conventional varistors used as voltage-surge suppressors. In accordance with this change the sensitivities to NO₂ and NO were decreased, especially at 300°C. The Bi₂O₃-based additive containing Y₂O₃ was effective for reducing the grain size slightly, increasing the breakdown voltage in air and enhancing sensitivities, especially to NO₂ at 400°C and to NO at 300°C. The addition of Cr₂O₃ to the Bi₂O₃-based additive containing Y₂O₃ was more effective for reducing the grain size, but led to a deterioration of sensitivities to NO₂ and NO.

View full abstract

We have developed a NO_x sensor using a multi-layered semiconducting thin-film. The first layer (current conduction region) is composed of Fe₂O₃ (91mol%), TiO₂ (5mol%) and MgO (4mol%), and the second layer (gas sensitive region) is composed of SnO₂ (96mol%) and V₂O₅ (4mol%). The resistance of the sensor increases when it is placed in an air including NO or NO₂. The sensor is thus sensitive to both NO and NO₂. By doping the SnO₂ layer with V₂O₅ (4mol%), the operating temperature decreases and the sensitivity increases for these gases. The sensor detects NO_x at a concentration as low as a few ppm at a relatively low temperature of 200°C. The response times of the sensor to NO and NO₂ are 60s and 80s at 350°C, respectively, though they are longer at lower temperatures. The sensor is also sensitive to reducing gases such as C₃H₈, i-C₄H₁₀, C₂H₄, CO, and H₂. These reducing gases, however, can be distinguished from NO_x because the resistance of the sensor decreases in these gases. The sensitivity does not degrade for at least 3 months when it is periodically cleaned by a heat treatment at 500°C for 1h.

View full abstract

The influence of RF-sputtering conditions on the micro-structures of SnO₂ thin films were investigated and the mechanism of gas sensing in the film was studied. As a result, the micro-structure (morphologic and crystallografic) of SnO₂ thin films was well affected by sputtering gas pressures. In this case, the granular size of SnO₂ and distance between granules increased as the pressure was increased. Moreover, post-annealing for the films enhanced these tendencies. For the gas sensitivity, the more porous the film structure was, the higher the sensitivity was. On the other hand, the gas response time was just opposite to that. From these results, the sensitivity of SnO₂ thin films remarkably depends on the film porosity and gas diffusivity into the films.

View full abstract

A new planar-type microsensor in which a gas-sensing layer was surrounded by a platinum heater horizontally on the same plane of a diaphragm was fabricated by a simple silicon micromachining technique using only 3 masks for photolithography. The gas sensing layer of SnO₂ or Sb-doped SnO₂ was spin-coated from each sol solution and trimmed into the designated dimensions by chemical etching with an aqueous HI solution. Computer simulation of thermal properties showed that the uniformity of temperature over the sensing layer was fairly good as expected, although there were some discrepancies between the temperatures of the sensing layer simulated and observed under given heater power conditions because of the simplified simulation model used. The fabricated microsensor proved a fairly good performance for sensing CO gas in air at 250°C.

View full abstract

A method of realizing a non-contact atmospheric temperature monitoring system using a sound probe is described. The sound probe consists of iterate burst sound of an audible frequency. The burst sound flights from an audio speaker to an electrostatic plane condenser microphone. Time of flight of the sound increases in inverse proportion to a sound propagation velocity which is dependent on the air temperature. The time of flight is acquired by correlation result between a launching burst signal and its detecting signal. This present system has a high-speed time response. Measurement error was within 0.5 degrees at the base line of 3.0m. Experimental results on this temperature monitoring system using the sound probe were in good agreement with the operation principle. This technique has the advantage of having a real-time measurement and a non-contact sensing in atmospheric monitoring.

View full abstract

In order to make clear the mechanism of the variation in the conductivity of La₂CuO₄ with a small amount of NO_x (x=1, 2) in oxygen atmosphere, measurements were made on the conductivity of La₂CuO₄ in the stream of NO_x-0₂-N₂ gas mixtures at temperatures 773-1023K and the outlet gas was analyzed by a zirconia sensor. The conductivity and the EMF of the zirconia sensor were analyzed in relation to P(NO_x). P(O₂) and T. When NO₂ was applied to La₂CuO₄, catalytic reaction of NO₂→NO+(1/2)O₂ was found to take place. In the catalytic reaction process, excess oxygen was considered to be incorporated into La₂CuO₄. Under the steady state. the oxygen potential in La₂CuO₄ was kept much higher than that in the gas phase. resulting in the increase in the p-type conduction of La₂CuO₄. Numerical calculations were made for the oxygen potential enhanced in La₂CuO₄ as a function of P(NO₂) and T to make clear the detection limits of La₂CuO₄-type NO₂ sensors in relation to the gas phase P(O₂) and T. Under the condition studied, NO affects very little to reduce the conductivity of La₂CuO₄. The reason was discussed based on the catalytic reaction kinetics to show the direction to enhance the NO effect in La₂CuO₄.

View full abstract

The surface chemical binding states of NOx-adsorbed La_2xBa_xSrCu₂O₆_??_ were investigated by X-ray photoelectron spectroscopy (XPS) analysis. The chemical shift of Cu2p XPS peak for LaBaSrCu₂O₆_??_indicated the increasing hole concentrations at the level of 02p with NO and NO₂ adsorption. On the other hand, the chemical shift of Cu2p XPS peak for La₁. ₅Ba₀. ₅SrCu₂O₆_??_indicated the decreasing hole concentrations at the level of 02p with NO adsorption. These results suggest that the Ba content affects the NOx gas-sensing properties of the layered-cuprates.

View full abstract