Masaharu Hasei

Stabilized zirconia-based sensor using oxide electrode for detection of NOx in high-temperature combustion-exhausts

Abstract New type NOx sensors were newly designed by using stabilized zirconia and an oxide sensing electrode. Among the various oxides examined for the sensing electrode, CdMn2O4 was found to be the most excellent material. Both planar and tubular devices attached with a sputtered CdMn2O4 layer could respond well to NO2 as well as NO in air at higher temperature, such as 500 °C. The EMF values of the devices were almost linear to the logarithm of NOx concentration with a positive slope for NO2 and a negative slope for NO. A NOx sensing mechanism involving mixed potential was proposed based on the measurements of polarization curves.

Mixed potential type NO{sub x} sensor based on stabilized zirconia and oxide electrode

A new solid-state NO{sub x} sensor operative at high temperature was designed using stabilized zirconia and an oxide sensing electrode. Among the various oxides examined, CdMn{sub 2}O{sub 4} was found to be the most excellent material for NO{sub x} sensing at 500 C. The EMF values of the sensor were almost linear to the logarithm of NO{sub x} concentration with a positive slope for NO{sub 2} and a negative slope for NO. A nitrogen oxide sensing mechanism involving mixed potential was proposed based on the measurements of polarization curves.

Sensing Characteristics of YSZ-Based Mixed-Potential-Type Planar NO x Sensors Using NiO Sensing Electrodes Sintered at Different Temperatures

The correlation between the sintering temperature and the NO 2 sensing performances of the planar mixed-potential-type yttria-stabilized zirconia (YSZ)-based NO x sensor attached with NiO sensing electrode (SE) was studied by materials characterization techniques, such as impedance analysis, polarization analysis, scanning electron microscopy, and transmission electron microscopy. The sensing properties of the planar YSZ-based NO x sensors were investigated in the operating temperature range of 700-900°C. The NO 2 sensitivity of the present sensor was found to be modulated by changing the sintering temperature of NiO-SE. Rather large sensitivity (43 mV) to 100 ppm NO 2 was obtained even at 800°C for the sensor using the NiO-SE sintered at 1400°C. A sensing mechanism that involves mixed potential at SE as well as catalysis in gas-phase reaction was discussed based on the results obtained.

Improving NO2 sensitivity by adding WO3 during processing of NiO sensing-electrode of mixed-potential-type zirconia-based sensor

A planar NO 2 sensor was fabricated by using an yttria-stabilized zirconia plate and NiO-based sensing electrode (SE). The improvement in sensing characteristic of mixed-potential-type planar NO 2 sensor was examined by the addition of various metal oxides to NiO-SE. Among the metal-oxide additives tested, WO 3 was found to give a significant enhancement in NO 2 * sensitivity. In addition, this enhancement was maximum when the initial WO 3 content in the NiO-based paste before sintering was 10 wt %. The sensitivity (A electromotive force) to 50 ppm NO 2 was as high as about 70 mV for the sensor using the 10 wt % WO 3 -added NiO-SE at 800°C under the wet condition (containing 5 vol % H 2 O). The sensitivity of this sensor was still 8 mV even down to 1 ppm NO 2 at 800°C. The NO 2 sensitivity was found to be invariant to different concentrations of H 2 O and CO 2 in the examined range of 5-15 and 5-20 vol %, respectively. It is speculated that the modification of morphology of the NiO-SE layer by the addition of WO 3 is the main contributing factor for the improved NO 2 sensitivity of the sensor.

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.

Preparation of a high-Jc Y-Ba-Cu-O film at 700 °C by thermal chemical vapor deposition

A high‐Jc Y‐Ba‐Cu‐O superconducting film was successfully prepared at 700 °C on a SrTiO3(100) single‐crystal substrate without postannealing by thermal chemical vapor deposition under a low‐oxygen partial pressure of 0.036 Torr in the total gas introduced into a hot‐wall‐type reactor (total gas pressure: 10 Torr), using 1% O2 balanced with Ar as a reactant gas. The sources for the elements of Ba, Y, and Cu were β‐diketone metal chelates. The film mainly consisted of YBa2Cu3Ox with c‐axis orientation perpendicular to the substrate plane and included small amounts of a‐axis oriented and randomly oriented grains. The film showed the superconducting transition temperature defined by zero resistivity at 89 K. The critical current density based on a 2 μV/cm criterion was 2.2 × 106 A/cm2 at 77.3 K and 0 T.

Low-Temperature Deposition of Y-Ba-Cu-O Superconducting Films by Thermal Chemical Vapor Deposition

Y-Ba-Cu-O superconducting films having a zero-resistance temperature of 84-85 K were prepared at 650°C on SrTiO3(100) single-crystal substrates without postannealing by thermal chemical vapor deposition using β-diketone metal chelates as sources and O2 with low partial pressure as a reactant gas. The films mainly consisted of YBa2Cu3Ox with a-axis and c-axis orientations normal to the film planes.

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.

Low-temperature formation of YBaCuO superconducting films by thermal CVD and their Jc in high magnetic fields

Y-Ba-Cu-O superconducting films were prepared at 700 and 720°C on SrTiO 3 (1 0 0) single crystal substrates by thermal chemical vapor deposition (CVD) using β-diketone metal chelates as sources. Total gas pressure in a hot-wall type CVD reactor used was 10 Torr. Oxygen partial pressure ( P (O 2 )) was 0.036 Torr. The films mostly consisted of YBa 2 Cu 3 O x with c -axis orientation perpendicular to the substrate plane and included small amounts of grains with a -axis orientation. These films showed zero resistivity at 90 K. The critical current densities ( J c , criterion: 2 μV/cm) of the films obtained at 700 and 720°C were 2.2×10 6 and 2.9×10 6 A/cm 2 at 77.3 K and 0 T, respectively. A J c of 2.1 × 10 4 A/cm 2 was measured at 77.3 K in a magnetic field of 23 T for the film prepared at 720°C.

Preparation of a-Axis and c-Axis Oriented Y-Ba-Cu-O Superconducting Oxide Films by CVD

Y-Ba-Cu-O (YBCO) superconducting films were prepared on SrTiO3(100) at 850°C by chemical vapor deposition using β-diketone metal chelates as sources. The films mainly composed of YBa2Cu3Ox(123 compound) with c-axis orientation and a-axis orientation were obtained on the substrates on which Ba-O was deposited in advance. The ratio of the a-axis oriented 123 compound and the c-axis oriented 123 compound depended upon the amount of Ba-O. The films having the matrix of the c-axis orientated 123 compound showed superconducting transition temperature defined by zero resistance at 90 K.