Resistive Oxygen Sensors Research Articles

Multi-gas sensor to detect simultaneously nitrogen oxides and oxygen

Abstract. Due to tightened emission limits, the efficiency of exhaust gas aftertreatment systems has to be further enhanced. Therefore, inexpensive and robust NOx sensors are required to be installed not only in automotive exhausts, but also in any other kind of combustion-based application. In this contribution, an impedimetric NOx sensor is presented. The impedance of a functional thick film (KMnO4, manufactured in a screen-printing technique on planar alumina substrates) depends selectively on the NOx concentration in the exhaust but shows a dependency on the oxygen concentration. Therefore, an additional temperature-independent resistive oxygen sensor structure was integrated on the same sensor platform. BFAT (BaFe0.74Al0.01Ta0.25O3−δ (BaFe0.74Al0.01Ta0.25O3−δ) was used for this purpose, and the measurement was conducted in the dc resistance mode. It serves not only to determine the oxygen concentration in the exhaust, but also to correct the oxygen dependency of the NOx sensor.

Microstructural, electrophysical and gas-sensing properties of CeO2–Y2O3 thin films obtained by the sol-gel process

Nanopowders and thin films of (СeO2)1-x(Y2O3)x composition (x = 0.10, 0.15 and 0.20) were obtained by the sol-gel process, using hydrolytically active complexes of the metal alkoxoacetylacetonate class [M(C5H7O2)3-y(C5H11Oi)y] (M = Ce3+ and Y3+) as precursors. The impact of the chemical composition and crystallization conditions on the microstructure, electrophysical and chemosensory characteristics of the obtained planar-type solid electrolytes was studied. The prospects of the thin-film nanostructures obtained as receptor components of resistive oxygen sensors, as well as of electrolytes of planar-type intermediate-temperature solid oxide fuel cells (SOFC) have been shown. It has been found that (CeO2)0.90(Y2O3)0.10 thin films demonstrate the maximum values of electrical conductivity (550 °C) and the highest sensory response when detecting oxygen (concentration range 1–20%, operating temperature range 300–450 °C).

Low Power Resistive Oxygen Sensor Based on Sonochemical SrTi0.6Fe0.4O2.8 (STFO40).

The current paper reports on a sonochemical synthesis method for manufacturing nanostructured (typical grain size of 50 nm) SrTi0.6Fe0.4O2.8 (Sono-STFO40) powder. This powder is characterized using X ray-diffraction (XRD), Mössbauer spectroscopy and Scanning Electron Microscopy (SEM), and results are compared with commercially available SrTi0.4Fe0.6O2.8 (STFO60) powder. In order to manufacture resistive oxygen sensors, both Sono-STFO40 and STFO60 are deposited, by dip-pen nanolithography (DPN) method, on an SOI (Silicon-on-Insulator) micro-hotplate, employing a tungsten heater embedded within a dielectric membrane. Oxygen detection tests are performed in both dry (RH = 0%) and humid (RH = 60%) nitrogen atmosphere, varying oxygen concentrations between 1% and 16% (v/v), at a constant heater temperature of 650 °C. The oxygen sensor, based on the Sono-STFO40 sensing layer, shows good sensitivity, low power consumption (80 mW), and short response time (25 s). These performance are comparable to those exhibited by state-of-the-art O2 sensors based on STFO60, thus proving Sono-STFO40 to be a material suitable for oxygen detection in harsh environments.

Elimination of flammable gas effects in cerium oxide semiconductor-type resistive oxygen sensors for monitoring low oxygen concentrations.

We have investigated the catalytic layer in zirconium-doped cerium oxide, Ce0.9Zr0.1O2 (CeZr10) resistive oxygen sensors for reducing the effects of flammable gases, namely hydrogen and carbon monoxide. When the concentration of flammable gases is comparable to that of oxygen, the resistance of CeZr10 is affected by the presence of these gases. We have developed layered thick films, which consist of an oxygen sensor layer (CeZr10), an insulation layer (Al2O3), and a catalytic layer consisting of CeZr10 with 3 wt% added platinum, which was prepared via the screen printing method. The Pt-CeZr10 catalytic layer was found to prevent the detrimental effects of the flammable gases on the resistance of the sensor layer. This effect is due to the catalytic layer promoting the oxidation of hydrogen and carbon monoxide through the consumption of ambient O2 and/or the lattice oxygen atoms of the Pt-CeZr10 catalytic layer.

Preparation of a porous ceria coating for a resistive oxygen sensor

Oxygen sensors play important roles in areas such as determining the oxygen content in exhaust gas and optimizing combustion. Resistive oxygen sensors made of an n-type semiconductor, doped ceria (CeO2), have recently received much attention due to their relatively compact and simple structures. In the present study, alumina substrate was first coated with electrodes by screen printing. Porous ceria films were then coated on the electrodes using different mixtures of two undoped ceria powders synthesized by spray-pyrolysis and precipitation, respectively. The ratios of the two powders were varied to produce differences in the heat-treated porosity of the ceria films. The resistive oxygen sensors exhibited a 3-D interconnected pore structure that could be optimized by varying the powder ratio for sensing properties. The dynamic response to the change in oxygen partial pressure of the optimized porous structure at 1073 and 1123K was >1.8 times faster than those of non-optimized structures. Details on the processing, optimization of compositions, and responses to variation in oxygen partial pressure are provided. The mechanisms for the formations of the powders and the porous coatings are also described.

BaFe1-xTaxO3-δ – A material for temperature independent resistive oxygen sensors

The sensing properties for resistive oxygen sensors of BaFe1-xTaxO3-δ (BFT) were investigated as a function of the Tantalum content x (0.2≤x≤0.7). Bulk samples were produced by using the mixed-oxide route. X-ray diffraction pattern showed that the perovskite structure was obtained and no second phases were found up to x=0.5. The electrical conductivity of sintered samples was determined in an oxygen concentration between 1% and 100% in a wide temperature range between 400 and 900°C. Samples between x=0.2 and 0.4 are p-type conducting perovskite oxides with almost zero thermal activation. The range, at which the temperature dependency is absolutely zero, shifts with x from 450°C for x=0.2 to about 800 to 900°C for x=0.4. Specimens of BaFe0.7Ta0.3O3-δ revealed the lowest overall temperature dependency in the entire temperature range. All samples up to x=0.4 responded fast and reversibly to oxygen concentration changes, even at 400°C.

Temperature independent resistive oxygen sensor prepared using zirconia-doped ceria powders

A resistive oxygen sensor with extremely good temperature independence within a temperature range of 923–1123K was fabricated from precipitated ceria–zirconia solid solution powders. Experimental results identified the as-precipitated powders as a nanocrystalline ZrO2-doped CeO2 (ZDC) solid solution phase. The amount of Ce3+ in CeO2 film was increased by doping with ZrO2, identified by XPS. The electrical resistance of ZDC film showed a more significant decreasing rate than that of the undoped CeO2 at lower oxygen partial pressure. An n-type semiconductance of ZDC films was obtained under oxygen partial pressures from 1.0 to <10−15atm. We selected two compositions of the ZDC films whose activation energies were approximately equal (∼1.69eV at 1.0atm and ∼1.66eV at 0.01atm) for the fabrication of temperature independent resistive oxygen sensors. ZDC films containing 10at% and 5at% ZrO2 (10ZDC and 5ZDC) were used as OMM (oxygen partial pressure measurement material) and TCM (temperature compensating material), respectively. This study confirms the conductive behaviors of ZDC at various oxygen partial pressures as a function of temperature, as well as the suitability of ZDC films for application as temperature independent oxygen sensors. The dynamic response of the resistive oxygen sensor with TCM also revealed that the sensor possessed a relatively good sensibility and stability.

Resistive Oxygen Gas Sensors for Harsh Environments

Resistive oxygen sensors are an inexpensive alternative to the classical potentiometric zirconia oxygen sensor, especially for use in harsh environments and at temperatures of several hundred °C or even higher. This device-oriented paper gives a historical overview on the development of these sensor materials. It focuses especially on approaches to obtain a temperature independent behavior. It is shown that although in the past 40 years there have always been several research groups working concurrently with resistive oxygen sensors, novel ideas continue to emerge today with respect to improvements of the sensor response time, the temperature dependence, the long-term stability or the manufacture of the devices themselves using novel techniques for the sensitive films. Materials that are the focus of this review are metal oxides; especially titania, titanates, and ceria-based formulations.

First-Principles Modeling of Oxygen Interaction with SrTiO3(001) Surface: Comparative Density-Functional LCAO and Plane-Wave Study

Large scale first-principles calculations based on density functional theory (DFT) employing two different methods (atomic orbitals and plane wave basis sets) were used to study the energetics, geometry, the electronic charge redistribution and migration for adsorbed atomic and molecular oxygen on defect-free SrTiO3(001) surfaces (both SrO- and TiO2-terminated), which serves as a prototype for many ABO3-type perovskites. Both methods predict substantial binding energies for atomic O adsorption at the bridge position between the oxygen surface ions and an adjacent metal ion. A strong chemisorption is caused by formation of a surface molecular peroxide ion. In contrast, the neutral molecular adsorption energy is much smaller, ∼0.25 eV. Dissociative molecular adsorption is energetically unfavorable, even at 0 K. Adsorbed O atoms migrate along the (001) direction with an activation energy of ∼1 eV which is much larger than that for surface oxygen vacancies (0.14 eV). Therefore, the surface O vacancies control encounter with the adsorbed O atoms and oxygen penetration to the surface which is the limiting step for many applications of ABO3-type perovskites, including resistive oxygen sensors, permeation ceramic membranes and fuel cell technology.

Pt catalytic effects on a resistive oxygen sensor using Ce0.9Zr0.1O2 thick film in rich conditions

In this study, we investigated the Pt catalyst effects on two structures through loading a Pt catalyst onto a resistive oxygen sensor using Ce0.9Zr0.1O2 thick film. We used (1) Ce0.9Zr0.1O2 thick film with a Pt/alumina layer, and (2) Pt/Ce0.9Zr0.1O2 thick film, in which Pt catalysts were prepared by heating platinum chloride. In the case of sensor structure (1), it became clear that the Pt/alumina layer with a Pt concentration of 40 wt% was the most effective in the Pt/alumina layers fabricated in this study, where R(λ = 0.6)/R(λ = 1.4) was 0.0099, and R(λ = 0.6)/R(λ = 1.4) of the 40 wt% Pt/alumina layer decreased with decreasing temperature. In the case of sensor structure (2), the minimum values of R(λ = 0.6)/R(λ = 1.4) at 600 and 700°C were the same at 0.035. The ultrasonic treatment in the Pt loading process improved R(λ = 0.6)/R(λ = 1.4) to 0.02. The Pt particle size in this study was in a range from 20 to 100 nm because of the high temperature annealing in the Pt loading process.

Application of Nano-Sized CeO&lt;sub&gt;2&lt;/sub&gt; Powder for Fast Response Oxygen Sensors

For resistive oxygen sensor elements of Ce1-xZrxO2 (x = 0.0, 0.05, 0.1, 0.2), grain diameter was varied in the range of 86 – 300 nm by changing sintering temperature or changing Zr content. The grain diameter decreased with increasing Zr content. The response time was approximately proportional to the square of the grain diameter. In the relationship between the amplitude of sensor output, An and the frequency, f of sine wave of variation in oxygen partial pressure, the gradient in the high-frequency region of a plot of log An vs. log f in was approximately –0.5. From these results, it was concluded that the sensor response was determined by the oxygen vacancy diffusion rate. The grain diameter of Ce0.8Zr0.2O2 element was 86 nm and the response time at 1073 K was 9 ms, which result opens the door to the technological development of independent control of engine cylinders.

Resistive Oxygen Sensor Using Ceria-Zirconia Sensor Material and Ceria-Yttria Temperature Compensating Material for Lean-Burn Engine

Temperature compensating materials were investigated for a resistive oxygen sensor using Ce0.9Zr0.1O2 as a sensor material for lean-burn engines. The temperature dependence of a temperature compensating material should be the same as the sensor material; therefore, the Y concentration in CeO2-Y2O3 was optimized. The resistance of Ce0.5Y0.5O2-δ was independent of the air-to-fuel ratio (oxygen partial pressure), so that it was confirmed to function as a temperature compensating material. Sensor elements comprised of Ce0.9Zr0.1O2 and Ce0.5Y0.5O2-δ were fabricated and the output was determined to be approximately independent of the temperature in the wide range from 773 to 1,073 K.

Improvement in response of resistive oxygen sensor based on ceria–zirconia thick film with Pt catalyst on surface

A resistive oxygen sensor based on ceria doped with 10 mol% zirconia can distinguish between fuel-rich and lean conditions in the case of a model exhaust gas in an equilibrium state. However, the response is unpredictable in a model exhaust gas in a nonequilibrium state. In this study, we investigated the response of an oxygen sensor in a model exhaust gas in a nonequilibrium state. The following problems were identified in relation to the ceria–zirconia sensor. First, in a nonequilibrium state, the resistive oxygen sensor indicated a propane lean condition when in fact the system was in a propane rich condition; under the same conditions, an electromotive force sensor based on an oxide ion conductor indicated a propane rich condition. Second, in an equilibrium state, the response of the resistive sensor, which was heated by a self-heater at 600 °C, was very slow in indicating a change from lean to rich conditions. In order to solve these problems, we fabricated a resistive oxygen sensor including a Pt catalyst on the surface by a simple new process and investigated its response. Through optimization of the Pt content, we were able to resolve the first response problem. That is, the sensor could distinguish between rich and lean conditions in a nonequilibrium state. Furthermore, the response speed was also improved by the Pt catalyst.

FeSrTiO3-based resistive oxygen sensors for application in diesel engines

SrTi0.4Fe0.6O2.8 (STFO60) powders were prepared by self-propagating high-temperature synthesis (SHS) and subsequent ball-milling (BM) treatment, and then deposited on an interdigited alumina substrate by screen-printing to fabricate a planar resistive oxygen sensor for direct injection (DI) diesel engines. The electrical characteristics and response to oxygen of the STFO60 sensor have been carefully examined first in a bench system with synthetic gases thus simulating diesel exhaust composition. The results obtained highlighted the promising performances of the sensor in terms of the temperature resistance independence as well as sensor response at the high oxygen concentrations typical of diesel engines. Cross-sensitivity tests also indicated that the sensor response is not influenced by main components in the exhausts, such as HC, CO2, NOx, CO. The STFO60 sensor was then tested as an oxygen probe on a diesel engine car under real driving conditions. Road tests confirmed that the performances of the resistive sensor are comparable with those of a commercial lambda probe.

Evaluation of response characteristics of resistive oxygen sensors using Ce0.9Zr0.1O2 thick film by pressure modulation method

Abstract Response properties of resistive oxygen sensors using zirconia-doped ceria (Ce 0.9 Zr 0.1 O 2 ) thick film were investigated using the pressure modulation method (PMM). This method measures the amplitude of the output of an oxygen sensor under periodic changes in oxygen partial pressure. The PMM results revealed that the resistive oxygen sensor using our Ce 0.9 Zr 0.1 O 2 thick film responds precisely when the oxygen partial pressure periodically changes at a period of 50 ms or more. The rate-limiting step of the response of the Ce 0.9 Zr 0.1 O 2 thick film was the diffusion of oxygen vacancies. The diffusion coefficients ( D ) of the oxygen vacancies were calculated from the PMM results, and were determined almost the same as those for CeO 2 thick films.

Response properties of resistive oxygen sensors using Ce1−xZrxO2 (x=0.05, 0.10) thick films in propane combustion gas

Abstract Sensing properties of resistive oxygen sensors using zirconia-doped ceria were investigated in a model gas of the exhaust gas prepared by propane combustion. Resistance of the thick film of the sensor was measured as a function of excess oxygen factor ( λ = C (O 2 )/{5 C (C 3 H 8 )}, C (O 2 ): oxygen concentration and C (C 3 H 8 ): propane concentration) and temperature. The resistance of the sensor dramatically changed on λ = 1 by 1.5–2.5 orders of magnitude, and the resistance showed high and low in the lean and rich region, respectively. The temperature dependence of the resistance at λ = 0.9 (rich region) was smaller than that at λ = 1.1 (lean region). It was revealed that the sensor could detect whether exhaust gas is rich or lean in the temperature range from 723 to 1073 K without temperature compensation. When the atmosphere was alternately changed between λ = 0.9 and λ = 1.1, the resistances in rich or lean were almost the same, respectively. The results of the experiment for the sample heated by the self-heater were in accord with that heated by the furnace. It was demonstrated that the sensor using a CeO 2 –ZrO 2 thick film could be used as a λ sensor, which can detect whether the exhaust gas is rich or lean region.

β‐Gallium Oxide as Oxygen Gas Sensors at a High Temperature

Resistive oxygen sensors based on gallium oxide were fabricated in order to analyze their sensing performances (as sensitivity, response, and recovery time) in an oxygen atmosphere at 1000°C. We prepared three types of sensors using a β‐Ga2O3 single crystal in a sandwich structure with Pt pad electrodes and β‐Ga2O3 polycrystalline thin films deposited by using both the sputtering technique and the chemical solution deposition method. For thin‐film sensors, Pt interdigital electrodes were deposited on the surface of the films using the lift‐off method. X‐ray diffraction and atomic force microscopy investigations were performed to compare the structure and surface morphology of the samples. We achieved a response time of 10 s at 1000°C, while the sensitivity was 1.03 for the single crystal and 1.35–1.45 for thin films. The sensing properties depend on the preparation condition of Ga2O3 devices.

Resistive λ-sensors based on ball milled Fe-doped SrTiO 3 nanopowders obtained by self-propagating high-temperature synthesis (SHS)

A series of SrTi 1− x Fe x O 3− δ (STO or STFO) powders, with x ranging from 0 to 0.6, were prepared by self-propagating high-temperature synthesis (SHS) starting from SrO 2, Ti, TiO 2 and Fe. A ball-milling (BM) treatment was subsequently carried out for further structure refinement and size reduction. Morphological and microstructural characteristics of both untreated and ball-milled SHS samples were investigated by SEM and XRD. Screen-printed SrTi 1− x Fe x O 3− δ SHS powders on alumina substrate were investigated for resistive oxygen sensors operating at high operating temperature (450–650 °C). STFO powders have shown better performances with respect to the corresponding undoped STO ones. Moreover, when STFO powders were subjected to BM treatment, both temperature-independent resistance characteristics and sensor response greatly improved. On the basis of the reported data, it can be suggested that the ball milling treatment likely acts: (i) stabilizing the formation of non-equilibrium structures; (ii) decreasing the particle size, increasing surface defects and hence surface reactivity; (iii) favouring the substitution of titanium by iron in the SrTiO 3 perovskite structure.

Output Evaluation of Resistive Oxygen Sensor having Ce0.9Zr0.1O2 Sensing Material and Zr0.8Y0.2O2-.DELTA. Temperature Compensating Material in Model Exhaust Gas

We fabricated resistive-type oxygen sensors having a Ce0.9Zr0.1O2 thick film as a sensing material and a Zr0.8Y0.2O2-δ thick film as a temperature compensating material for the first time, and measured the resistance of the two thick films and the output of the sensors in model exhaust gas which was a combustion gas of propane. Then, we considered whether the sensor could be used as a “lambda sensor,” which can identify whether the air-to-fuel ratio of exhaust gas is rich or lean region. As a result, we confirmed that the resistance of the Ce0.9Zr0.1O2 thick film was low and high in the case of rich and lean regions, respectively. The activation energy of the resistance of the Ce0.9Zr0.1O2 film was 0.41 and 1.20 eV in rich and lean regions, respectively, and the mean value of them was 0.8 eV. The resistance of the Zr0.8Y0.2O2-δ thick film in rich was almost the same as that in lean. The activation energy was 0.92 and 0.98 eV in rich and lean, respectively, and similar to the mean value of the activation energies of the Ce0.9Zr0.1O2 film in rich and lean. This result shows that the combination of the Ce0.9Zr0.1O2 and Zr0.8Y0.2O2-δ films is better for the “lambda sensor.” The electrode structures of the sensor were optimized so that the mean value of the resistances of the Ce0.9Zr0.1O2 film in rich and lean was the same as the resistance of the Zr0.8Y0.2O2-δ film. The difference in output of the resistive oxygen sensor having the optimized electrode between rich and lean regions was more than 0.7 V, when the source voltage was 1.0 V. The output dramatically changed on the stoichiometric air-fuel ratio. From above-mentioned results, we conclude that the resistive oxygen sensors fabricated in this study are able to be used as “lambda sensors.”

The effect of hafnia doping on the resistance of ceria for use in resistive oxygen sensors

In order to investigate hafnia concentration dependence of resistance of resistive oxygen sensors using the ceria (CeO 2)–hafnia (HfO 2) system, the sensors were fabricated using ceria thick films doped with various hafnia concentrations. The resistance of the thick films with a hafnia concentration of 3–20 mol% was lower than that of the non-doped ceria thick film and was measurable more easily than that of 30 or 50 mol%. The activation energy of the thick films with a hafnia concentration of 7 mol% or more was similar to that without hafnia. This result showed that a hafnia concentration of 7 mol% or more was preferable for the thick films from the perspective of small temperature dependence of resistance. A hafnia concentration of 2 mol% or more was preferable for the thick films from the perspective of large oxygen partial pressure dependence of resistance, which means high sensitivity. These results revealed that the resistive oxygen sensor using the ceria–hafnia system showed more excellent sensing properties in the range of 7–20 mol% than that using non-doped ceria in perspective of low resistance, good sensitivity, and small temperature dependence of resistance.