Akira Kunimoto

Potentiometric NOx sensor based on stabilized zirconia and NiCr2O4 sensing electrode operating at high temperatures

Abstract The two types of electrochemical sensors using stabilized zirconia and the oxide sensing electrode (SE) were developed for NOx detection at high temperatures. For the mixed-potential-type sensor, NiCr2O4 was found to give fairly excellent NOx sensing characteristics in air among several spinel-type oxides tested. This NOx sensor provided a linear correlation between EMF and the logarithm of NO or NO2 concentration in the range 25–436 ppm and in the temperature range 550–650°C. With fixed bias voltage being applied between the SE (oxide) and the counter (Pt) electrode (CE), the EMF between SE and the reference (Pt) electrode (RE) was measured as a sensing signal. The NiCr2O4-attached tubular device was found to provide selective response to NO over NO2 if SE was polarized at +175 mV versus RE. It was also found that this device gave selective response to NO2 over NO, if SE was polarized at −250 mV versus CE. The new design of the planar device was proposed to avoid the cross-sensitivities to the others gases usually coexisting in car exhausts.

Performance of the NOx sensor based on mixed potential for automobiles in exhaust gases

The NOx sensor based on mixed potential was made by laminating YSZ green sheets, on which electrodes including an NOx sensing electrode, an NOx conversion electrode, a Pt heater and thermocouple were printed and sintered. The output signal of this sensor was fairly independent of gas temperatures and the velocity of gas flows in the test gases. The engine test for the exhaust gases at around λ=1 showed that the sensor outputs changed corresponding to NOx concentrations from the gas analyzer. It is expected that this sensor based on mixed potential can be utilized for automobiles.

Highly sensitive semiconductor NOx gas sensor operating at room temperature

Abstract The novel semiconductor NOx gas sensor based on a heterojunction structure workable at room temperature has been investigated. The sensor consists of Pt/Tin-dioxide(SnO2)/n-Si/p+-Si/Al in which vertical direction current between Pt and Al electrode was measured with applying reverse direction bias voltage on Pt electrode. All the films including SnO2 with the thickness of 50–200 nm were deposited on an epitaxial layer of n-Si over p+-Si substrate by RF sputtering method. As a result, the current in the sensor decreased when the gas flow was switched from dry air to mixed gas of air and NOx. Clear response was obtained at the NOx gas concentration as low as 1 ppm at room temperature, while the almost no response was observed for the n-Si, p-Si, and p-Si/n+-Si substrate. The generation of large change in current for the sensor was considered that the barrier height change or conductivity change of SnO2 gas sensitive layer may cause the modulation of the depletion layer at the n/p+-junction of Si substrate.

Total-NOx Sensor Based on Mixed-Potential for Detecting of Low NOx Concentrations

We have been developing the mixed-potential type NOx sensor which can detect the total-NO x concentration directly in exhausts for automobiles. It has been confirmed that the sensor is capable of detecting wide concentrations of total-NOx from 20 to 1000 ppm under the condition from rich-burn (A/F=12) to lean-burn without any interference from reducing gases, such as HC and CO. In addition, it has been confirmed that the sensor output is correlated fairly well to NOx concentrations from the analyzer in the engine test at any rotations. The results obtained here indicate that the present sensor has great possibility of being utilized as an on-board NOx sensor for practical use.

High Temperature NOx Sensor Based on Stabilized Zirconia and ZnFe 2 O 4 Electrode

Electrochemical sensors using tubular yttria-stabilized zirconia (YSZ) and the spinel-type oxide sensing-electrode (SE) were fabricated and examined for NOx detection at high temperatures. The ZnFe2O4-attached device gave a linear correlation between EMF and the logarithm of NO2 (NO) concentration from 50 ppm to 436 ppm in the temperature range 550–700°C. This sensor was found to give the highest sensitivity to NO2 in air among the spinel-type oxides tested and reported to date. The sensing mechanism of the sensor was discussed on the basis of the catalytic activity and the TPD data for the oxides examined.