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The defect chemistry of ceria has been studied using a microwave resonator. The dielectric properties of powder samples were investigated up to 600 °C and at different oxygen partial pressures pO2 (10-26 – 0.2 bar) and evaluated using the microwave cavity perturbation theory. For the ceria powder, an activation of the oxygen incorporation and release kinetics was observed from 480 °C onwards. The data suggest that the increased losses in non-stoichiometric ceria originate from the small polaron hopping mechanism, as known from literature. In addition, an increase in material polarization due to the formation of oxygen vacancies was experimentally observed. A comparison with sintered, coarse-grained samples of the same material and with literature data also demonstrated the importance of the microstructure for the defect chemistry of ceria. For the powder (ca. 80 nm grain size), significantly higher electronic conductivities (assumedly due to the higher concentration of small polarons) and lower activation energies were detected at 600 °C. On the other hand, lower conductivities, controlled by the concentration of acceptor cations in the material were reported for the sintered samples (1.1 µm) at this temperature. The findings are well in-line with existing defect chemical models of ceria. This study presents an analysis of the microwave behavior of ceria and provides insights into intrinsic and extrinsic mechanisms of oxygen exchange in ceria. The findings also contribute to a better understanding of microwave-based state diagnosis of three-way catalysts in automotive applications.

Featuring high levels of achievable oxygen non-stoichiometry δ, Ce1−xZrxO2−δ solid solutions (CZO) are crucial for application as oxygen storage materials in, for example, automotive three-way catalytic converters (TWC). The use of CZO in form of films combined with simple manufacturing methods is beneficial in view of device miniaturization and reducing of TWC manufacturing costs. In this study, a comparative microstructural and electrochemical characterization of film and conventional bulk CZO is performed using X-ray diffractometry, scanning electron microscopy, and impedance spectroscopy. The films were composed of grains with dimensions of 100 nm or less, and the bulk samples had about 1 µm large grains. The electrical behavior of nanostructured films and coarse-grained bulk CZO (x > 0) was qualitatively similar at high temperatures and under reducing atmospheres. This is explained by dominating effect of Zr addition, which masks microstructural effects on electrical conductivity, enhances the reducibility, and favors strongly electronic conductivity of CZO at temperatures even 200 K lower than those for pure ceria. The nanostructured CeO2 films had much higher electrical conductivity with different trends in dependence on temperature and reducing atmospheres than their bulk counterparts. For the latter, the conductivity was dominantly electronic, and microstructural effects were significant at T 2 films are assumed to induce their more pronounced ionic conduction at medium oxygen partial pressures and relatively low temperatures. The defect interactions in bulk and film CZO under reducing conditions are discussed in the framework of conventional defect models for ceria.

Many studies are dealing with the behavior of mixed potential sensors. However, a quantitative description of the processes leading to the sensor signals has not yet been carried out. This paper describes a first approach to address the question to what extent a mixed potential sensor can be modelled in a finite element model. In 1D geometry, the electrochemical reactions that lead to signal formation, but also the gas phase reactions at the electrodes, were taken into account. Polarization curves, taken by a novel and for such research optimized setup, are used to determine the electrochemical parameters. Those are necessary to quantify the kinetics and electrical properties of the sensor system. It will be shown how to deduce other analyte concentrations and sensor temperatures from a single data set. In addition, the geometry of the electrode can be modified. In the model, the sensor signal is calculated for the analytes propene, hydrogen and carbon monoxide and compared with measured values. In particular, it shows the limitations of the conventionally used simplified mixed potential theory, since a complete Butler-Volmer equation has to be used for each analyte, especially for small analyte concentrations. This model serves as a basis for even more detailed studies that further elucidate the mechanisms behind mixed potential formation in mixtures or by varying the electrode configuration.

Since the ammonia (NH3) based selective catalytic reduction is today’s main DeNOx strategy to meet the emission limits for Diesel cars, research focuses on sensor and control approaches for higher conversion efficiencies and system robustness. One novel approach is the catalyst state monitoring based on radio frequencies (RF). It has proven its functionality to monitor the NH3 loading on SCR catalysts directly in previous studies. This study shows the first attempt to fully describe the whole RF SCR system by simulation. A divided model into the chemical reactions and the RF response is presented and parameterized on experimental test runs. The application of the adjusted model on a test run that was not included in parameter fitting correlates well with the simulated signals and the measured catalyst behavior as well as with the RF response.

Recently, a laboratory setup for microwave-based characterization of powder samples at elevated temperatures and different gas atmospheres was presented. The setup is particularly interesting for operando investigations on typical materials for exhaust gas aftertreatment. By using the microwave cavity perturbation method, where the powder is placed inside a cavity resonator, the change of the resonant properties provides information about changes in the dielectric properties of the sample. However, determining the exact complex permittivity of the powder samples is not simple. Up to now, a simplified microwave cavity perturbation theory had been applied to estimate the bulk properties of the powders. In this study, an extended approach is presented which allows to determine the dielectric properties of the powder materials more correctly. It accounts for the electric field distribution in the resonator, the depolarization of the sample and the effect of the powder filling. The individual method combines findings from simulations and recognized analytical approaches and can be used for investigations on a wide range of materials and sample geometries. This work provides a more accurate evaluation of the dielectric powder properties and has the potential to enhance the understanding of the microwave behavior of storage materials for exhaust gas aftertreatment, especially with regard to the application of microwave-based catalyst state diagnosis.

The three-way catalytic converter (TWC) has become an integral part of the modern exhaust gas aftertreatment for gasoline engines. Some years ago, radio frequency technology has been successfully tested for monitoring and controlling ceria based TWC oxygen storage by contactless measuring the dielectric catalyst properties inside a cavity resonator. Applying the cavity perturbation method, we present results on CeO2, the oxygen storage component of three-way catalysts, and on differently prepared Pt–ceria model catalysts and combine it with in situ X-ray absorption spectroscopic (XAS) measurements conducted at the Ce L3-edge. The dielectric properties of the oxidized and reduced Pt–ceria and pure ceria powders were determined at 1.2 GHz between 250 and 550 °C. As the experiments show, the reduction of the material changes both, the polarization and the occurring dielectric losses. The complementary in situ XAS measurements, which allow monitoring variations in the redox state of ceria, gave a precise evaluation of the ceria reduction and oxidation. In addition, a substantial improvement of the low-temperature reducibility of ceria in the presence of Pt was demonstrated by both measurement techniques, in agreement with earlier studies. Furthermore, time resolved XAS data obtained during reducing/oxidizing cycles at 250 °C and 350 °C unraveled the faster and more pronounced reduction of ceria at higher temperature. Both methods proved to be valuable methods to provide insight into the oxygen storage and release process of ceria based materials, and capable to discriminate the impact of the noble metal or the ceria surface area in great detail, even under dynamic conditions, which is highly relevant for studying the function of state-of-the art three-way catalysis and giving input for OSC modeling studies.

In recent years, research on radio frequency-based (also known as “microwave-based”) catalyst state diagnosis for diesel engines has focused mainly on SCR systems. To increase the efficiency of exhaust aftertreatment systems, SCR catalysts can be combined with NOx storage catalysts (NSC). For optimum performance, the degree of loading of the storage components has to be monitored. Up to now, in commercial application this has been done indirectly by using mathematical models. In this paper, the opportunities of a simple radio frequency (RF) based method for a direct determination are highlighted. Therefore, a commercial NSC was investigated between 210 and 390 °C. A linear dependence between the RF signals and the stored amount of NOx was found. Furthermore, the NOx and oxygen gas concentrations do not directly affect the RF parameters, but they have an indirect influence, since the stored NOx amount is NOx concentration dependent. By evaluating multiple RF signals, which are based on different material effects, a separate determination of the oxygen and NOx storage level could be realized.

Catalyst materials can be characterized with a thermoelectric gas sensor. Screen-printed thermopiles measure the temperature difference between an inert part of the planar sensor and a part that is coated with the catalyst material to be analyzed. If the overall sensor temperature is modulated, the catalytic activity of the material can be varied. Exothermic reactions that occur at the catalyst layer cause a temperature increase that can then be measured as a sensor voltage due to the Seebeck coefficient of the thermopiles. This mechanism can also be employed at stationary conditions at constant sensor temperature to measure gas concentrations. Then, the sensor signal changes linearly with the analyte concentration. Many variables influence the sensing performance, for example, the offset voltage due to asymmetric inflow and the resulting inhomogeneous temperature distributions are an issue. For even better understanding of the whole sensing principle, it is simulated in this study by a 3-D finite element model. By coupling all influencing physical effects (fluid flow, gas diffusion, heat transfer, chemical reactions, and electrical properties) a model was set up that is able to mirror the sensor behavior precisely, as the comparison with experimental data shows. A challenging task was to mesh the geometry due to scaling problems regarding the resolution of the thin catalyst layer in the much larger gas tube. Therefore, a coupling of a 3-D and a 1-D geometry is shown. This enables to calculate the overall temperature distribution, fluid flow, and gas concentration distribution in the 3-D model, while a very accurate calculation of the chemical reactions is possible in a 1-D dimension. This work does not only give insight into the results at stationary conditions for varying feed gas concentrations and used substrate materials but shows also how various exhaust gas species behave under transient temperature modulation.

Solid electrolyte gas sensors following the mixed potential sensing principle are usually operated at several hundred °C. They are often characterized using half-cell setups where one side of the sensor faces a reference gas atmosphere whereas the other side faces the gas mixture to be analyzed. Since two gas compartments are compared, a high temperature sealing is needed. Typically, such setups require indirect heating, and often, the manufacturing process is complicated and the dismantling of sensor and sealing is only possible by destroying it. To overcome these and other drawbacks of the half-cell setup, a self-heatable, stand-alone ceramic sensor device is developed in this work. We choose high temperature co-firing ceramics technology for the sensor design that bases on a self-heated yttria-stabilized zirconia disc that is hot enough for sensing in the central region and cold enough at the outer radius for contacting and plastic sealing. This work shows how problems that occurred due to the high thermal stress were overcome by employing finite element simulations. After developing and manufacturing the novel device, it is demonstrated that the sensor is capable to compare two gas mixtures electrochemically. For that purpose, the device was used to compare the gas composition up- and downstream of an oxidation catalyst. The sensor signal response correlates well with a theoretically derived dependence on the catalyst conversion.

Introduction In-situ monitoring of catalytic conversion with only one single (gas) sensor component represents a novel approach in gas sensor technology. The background of this idea is a sensor device that "compares" two gas atmospheres before (upstream) and after (downstream) of a catalyst. The sensor element itself separates two gas compartments from each other, but connects two (identical) electrodes in each of these gas compartments by means of an ion bridge. According to the mixed- potential principle, a half-cell potential is generated at each individual electrode. The potential difference between the two electrodes is evaluated as a sensor signal and, according to the model of mixed-potential formation, should depend directly on the ratio of the concentrations of a specific gas species between the two gas spaces, i.e., in this case directly on the conversion of the gas over the catalyst [1]. The sensor device itself is the basis for reliable and reproducible measurements in that field. It is made from oxygen ion-conducting yttria-stabilized zirconia (YSZ) and designed as a full ceramic disc (figure 1a). Furthermore, it comprises an internal heating structure. So, the sensor can be brought to an appropriate measuring temperature (up to 650 °C) locally in the middle of the disc – exactly in that region where the planar electrodes are integrated on both sides and where the ionic conductivity is necessary. In the outer region, the sensor disc remains cold enough to use simple sealing materials (e.g. Viton). More details on the sensor setup can be found in [2]. A special housing (made from PEEK-cells) acts as sensor chamber with gas feed lines connected to the up- and downstream atmospheres of the catalyst (details can be found in [3]). In the present contribution, dynamic tests concerning the catalyst temperature and the inlet concentration are discussed. In these investigations, it should be verified that the sensor signal is independent from the inlet concentration. Experimental Setup Measurements were conducted in the lab. The inlet gas was mixed with mass-flow-controllers (MFCs) from propene in lean (e.g. oxygen containing) gas atmosphere (N2 balance). The gas flows over an oxidation catalyst (Pt as precious metal component on cordierite honeycomb substrate, diameter 1”, located in a quartz tube) inside a horizontal furnace (Carbolite). The gas / catalyst temperature was controlled by a thermocouple (upstream) inside the reactor. During the experiment, the furnace was heated up and – by that – conversion over the catalyst is increased, as long as the catalyst light-off temperature is reached (exothermic conversion is visible by a small temperature increase, measured with a downstream thermocouple inside the reactor, figure 2 first graph). The sensor device (disc inside the housing) was connected to the gas atmospheres up- and downstream the catalyst – each purging one compartment of the sensor housing with contact to each electrode. Simultaneously to the catalyst heating, the inlet (upstream) concentration was changed (see figure 2, second graph), which might be typical for real applications in the automotive field. The downstream HC-concentration was measured by FID. Out of this information, the conversion was calculated and plotted as (1 – conv.) in figure 2 (third graph). Figure 2 (last graph) shows the sensor raw signal that is a voltage signal as a difference of both half-cell potentials at the up- and downstream electrodes (in this case made from zinc chromite) measured at a sensor temperature of 500 °C. Results and Conclusions If one has a look on the raw data first (figure 2), the following results can be found. Regarding the up- and downstream temperature one assumes a light-off of the catalyst at about 150 °C (t = 15000 s). Starting from here, the downstream temperature increases more than the upstream temperature. The gas data indicate a small conversion even before (lower downstream concentration @ t > 7500 s). Although the calculated conversion is still “0”, (i.e. (1 - conv.) = 1), the sensor signal shows a slight increase in that area. In the medium temperature range (15000 – 30000 s, 150 °C < T < 175 °C) the conversion depends on the inlet concentration. Independently from that fact, the sensor signal follows the calculated conversion factor. In the higher temperature range (t > 30000 s, T > 175 °C), conversion is near 100 % (i.e. (1 - conv.) approaches “0”); the sensor shows its maximum signal. Now, in a second data evaluation, the sensor signal is plotted against the calculated conversion data in a half-logarithmic representation (figure 3, thick line including all measured data points in comparison to the thin line that was obtained in stationary experiments). We found an impressing correlation between both data sets, although the experimental sequence changed dynamically two parameters (temperature and inlet concentration) simultaneously and in a wide range. Therefore, the presented novel sensor device seems to be an appropriate candidate for catalyst conversion control. References [1] G.Hagen, K.Burger, S.Wiegärtner, D.Schönauer-Kamin, R.Moos, A mixed potential based sensor that measures directly catalyst conversion – A novel approach for catalyst on-board diagnostics, Sensors and Actuators B: Chemical 217 (2015) 158–164. doi: 10.1016/j.snb.2014.10.004. [2] T.Ritter, G.Hagen, J.Kita, S.Wiegärtner, F.Schubert, R.Moos, Self-heated HTCC-based ceramic disc for mixed potential sensors and for direct conversion sensors for automotive catalysts, Sensors and Actuators B: Chemical 248 (2017) 793–802. doi: 10.1016/j.snb.2016.11.079. [3] T.Ritter, G.Hagen, J.Lattus, R.Moos, Solid state mixed-potential sensors as direct conversion sensors for automotive catalysts, Sensors and Actuators B: Chemical 255 (2018) 3025–3032. doi: 10.1016/j.snb.2017.09.126. Figure 1

Introduction Typically, gas sensors are ceramic devices. They are manufactured in ceramic techniques like tape technology and/or conventional thick-film techniques (typically screen-printing with subsequent firing) [1]. During firing, interdiffusion processes occur between substrate and gas sensitive film. At least partly, they may deteriorate the gas sensing properties of the functional oxides. Some materials can even hardly be processed without decomposition. Therefore, room temperature deposition techniques are advantageous. The Powder Aerosol Deposition Method The Powder Aerosol Deposition Method (PAD) bases on the Room Temperature Impact Consolidation (RTIC) as the film densification mechanism. This allows producing dense ceramic films without any high-temperature processes directly from an initial ceramic powder on almost any substrate material. This contribution gives examples for PAD-applications in the field of gas sensing. Although the process is rather simple, the results are very promising. Driven by a pressure difference to a vacuum deposition chamber (evacuated only to rough vacuum), the aerosol is accelerated by a slit nozzle to several hundred m/s. This aerosol jet ejects particles into the deposition chamber. Here, the particles impact on a movable substrate and get fractured into nanometer-sized fragments that are compacted by subsequently impacting particles. Fig. 1 depicts the basic principle. Further process details can be found in the reviews [2], [3], or [4]. Results and Discussion Some examples for PAD-based sensors are surveyed here. Besides conventional conductometric gas sensors based on SnO2 and other metal oxides to detect limited components, applications for temperature independent oxygen sensors are reported, e.g. of SrTi1-x Fe x O3 [5], [6] or Alumina-doped BaFe1-x Ta x O3 (BFATx) [7]. Simultaneous powder aerosol co-deposition of inert and functional oxides to fine-tune the sensing properties may be promising [6]. Figure 2 is a measurement protocol at 900 °C of the logarithm of the film conductivity of BFAT30 fabricated by PAD. The material is rapidly and reproducibly responding to stepwise changes of the oxygen partial pressure, pO2. In Figure 3, the log-log plot of the conductivity (log s vs. log pO2) of the sensor between 600 °C and 900 °C in the pO2 range from 0.01 to 1 bar is shown. Neither the sensitivity to oxygen (slope in the log-log-plot) nor the base line resistance changes with temperature. Log s depends linearly on log pO2 between 700 °C and 900 °C, showing a slope of 0.24. An improved formulation contains 1 % alumina (BFATx). Amongst the BFATx samples examined, particularly good properties were found with regard to temperature independency for BFAT25 (BaFe0.74Al0.01Ta0.25O3). Combined resistive and thermoelectric oxygen sensors with almost temperature-independent characteristics of both conductivity and Seebeck coefficient were manufactured using BFAT30 [8]. Classical tin-oxide conductometric metal oxide gas sensors were deposited by PAD at room temperature on interdigital electrodes. As expected, they show a pronounced sensitivity to nitrogen oxides at around 300 °C and to hydrogen at around 450 °C [9]. With PAD, solid electrolyte electrochemical gas sensors have also been successfully manufactured [10]. A nitrogen oxide sensor using the novel pulsed-polarization method, which attracted much attention recently with YSZ as solid electrolyte, can now be operated at lower temperatures due to the use of a Bismuth-based fast oxide ion conductors (Fig. 4). The powder aerosol deposition method is also suitable to produce thermistors with a negative temperature coefficient of resistance (NTCR) [11]. They may serve as sensitive detectors for pellistors. Conclusions It is possible to manufacture ceramic gas sensor films completely without any high-temperature process and directly from an initial ceramic powder on almost any substrate material. Even temperature sensors can be realized. Future research directions may focus on sensors on polymers or textiles by applying the powder aerosol deposition method. References [1] N. Barsan, et al., Metal oxide-based gas sensor research: How to?, Sensors and Actuators B: Chemical, 121, 18-35 (2001); doi: 10.1016/j.snb.2006.09.047 [2] J. Akedo, Aerosol Deposition of Ceramic Thick Films at Room Temperature: Densification Mechanism of Ceramic Layers, Journal of the American Ceramic Society, 89, 1834-1839 (2006); doi: 10.1111/j.1551-2916.2006.01030.x [3] D. Hanft, et al., An Overview of the Aerosol Deposition Method: Process Fundamentals and New Trends in Materials Applications, Journal of Ceramic Science and Technology, 6, 147-182 (2015); doi: 10.4416/JCST2015-00018 [4] M. Schubert, et al., Powder aerosol deposition method — novel applications in the field of sensing and energy technology, Functional Materials Letters, 12, 1930005 (2019); doi: 10.1142/S1793604719300056 [5] K. Sahner, et al., Assessment of the novel aerosol deposition method for room temperature preparation of metal oxide gas sensor films, Sensors and Actuators B: Chemical, 139, 394-399 (2009); doi: 10.1016/j.snb.2009.03.011 [6] J. Exner, et al., Tuning of the electrical conductivity of Sr(Ti,Fe)O3 oxygen sensing films by aerosol co-deposition with Al2O3, Sensors and Actuators B: Chemical, 230, 427-433 (2016); doi: 10.1016/j.snb.2016.02.033 [7] M. Bektas, et al., Aerosol-deposited BaFe0.7Ta0.3O3- δ for nitrogen monoxide and temperature-independent oxygen sensing, Journal of Sensors and Sensor Systems, 3, 223-229 (2014); doi: 10.5194/jsss-3-223-2014 [8] M. Bektas, et al., Combined resistive and thermoelectric oxygen sensor with almost temperature-independent characteristics, Journal of Sensors and Sensor Systems, 7, 289-297 (2018); doi: 10.5194/jsss-7-289-2018 [9] D. Hanft, et al., Powder pre-treatment for aerosol deposition of tin dioxide coatings for gas sensors, Materials, 11, 1342 (2018); doi: 10.3390/ma11081342 [10] J. Exner, et al., Pulsed Polarization-Based NOx Sensors of YSZ Films Produced by the Aerosol Deposition Method and by Screen-Printing, Sensors, 17, 1715 (2017); doi: 10.3390/s17081715 [11] M. Schubert, et al., Characterization of Nickel Manganite NTC thermistor films prepared by Aerosol Deposition at room temperature, Journal of the European Ceramic Society, 38, 613-619 (2018); doi: 10.1016/j.jeurceramsoc.2017.09.005 Figure 1

Introduction High temperature gas sensors play an important role in monitoring or controlling energy conversion processes. Sensors containing functional materials often need to be heated to a certain operating temperature to achieve functionality. This can be realized using planar thick-film heaters, which are screen-printed on the sensor substrate. When sensors are installed in the exhaust gas flow in order to achieve a fast response behavior, high volume flow has cooling effects on the sensor and leads to an inhomogeneous temperature distribution. This might cause a changed sensor function and cross-sensitivities [1]. For this reason, protective caps are used to reduce the flow influence on the temperature distribution with sufficient response time [2]. Resulting temperature distribution can be simulated using FEM models but cannot be verified directly inside protective caps [3, 4]. The present work solves this uncertainty in two ways to directly measure the temperature homogeneity within a cap. Methods and Setup Sensors are built up on alumina substrates (Figure 1a). On the reverse side, a meander-shaped platinum structure acts as heating element. Two methods are used to determine the temperature distribution on the front side. Firstly, the temperature distribution is recorded by an IR camera (Figure 1c). For optically access, an IR-transparent glass replaces one part of the cap. The requirements for this glass are the transmission of the IR radiation (here attention must be paid to the appropriate wavelength of the camera) and a certain temperature resistance, since the cap and the glass are also heated by the heat radiation of the sensor. Secondly, the temperature distribution is directly measured by means of thermocouples (Figure 1b). Instead of a sensor structure, a matrix of planar screen-printed thermocouples (Figure 1d) is applied onto the front side of the transducer. For this purpose, Pt and Au feedlines are arranged so that five different measuring points are realized at the sensor tip, i.e., that area where a gas sensitive layer would be located. Temperature values are derived from the measured thermvoltage on basis of Seebeck coefficients for Pt/Au given in the literature. Results and Discussion In order to verify the function of the printed thermocouples, the sensor temperature was increased to 600 °C by means of the heater on the reverse side. The heat distribution was recorded by the thermocouples without the protective cap. Thermal images were taken simultaneously with the IR camera. The measured temperature of the thermocouples corresponds well to the thermal images. Afterwards the sensor was placed inside a protective cap containing the described IR-permeable glass window. After increasing the sensor temperature to 600 °C, thermal images of the sensor tip could be taken again. The transmission coefficient of the glass is required for this purpose. To check the accuracy of the transmission coefficient, the temperatures of the thermocouples were recorded simultaneously. The temperature measured by the IR camera again corresponds to the thermocouples values. Thus, the application of this method was also checked and confirmed. Both methods were used to investigate the influence of a protective cap on the temperature distribution on the front side of the sensor at 600 °C operating temperature. Figure 1e) and f) show the temperature distribution of a sensor in a horizontal mounting position recorded by an IR camera (temperature gradients in general come from the individual heater structure and several heat losses, especially heat flow along the substrate material). Figure 1e) shows the measurement without a protection cap. The thermocouple measurement shows a temperature gradient of 6.78 °C/mm between point 1 and 2. The temperature gradient derived from the thermal image is 5.88 °C/mm. Both data (thermocouple and thermal image) agree very well. Then the same measurement was carried out with a protective cap (Figure 1f). The temperature gradient, measured with the thermocouples, is 4.97 °C/mm. The thermal image shows a value of 4.43 °C/mm. The protective cap thus has a positive influence on the temperature homogeneity of a sensor even without a flow. Outlook In the next steps, the temperature distribution in an exhaust flow is investigated by means of the thermocouples. Variations in sensor orientation or mounting positions is possible. Here as well it is possible to investigate the influence of a protection cap. A high temperature setup might be realized by use of screen-printed Pt/PtRh thermopiles [5]. References [1] G. Hagen, A. Harsch, R. Moos: A pathway to eliminate the gas flow dependency of a hydrocarbon sensor for automotive exhaust applications, Journal of Sensors and Sensor Systems , 7, 79–84 (2018), doi: 10.5194/jsss-7-79-2018 [2] J. Herrmann, T. Kern, G. Hagen, R. Moos: Influence of the Gas Velocity on the Temperature Homogeneity of Transducers for Gas Sensors, SMSI 2021, Nürnberg, submitted [3] S. Wiegärtner, G. Hagen, J. Kita, W. Reitmeier, M. Hien, P. Grass, R. Moos: Thermoelectric hydrocarbon sensor in thick-film technology for on-board-diagnostics of a diesel oxidation catalyst, Sensors and Actuators B: Chemical , 214, 234–240 (2015), doi: 10.1016/j.snb.2015.02.083 [4] J. Herrmann, G. Hagen, J. Kita, F. Noack, D. Bleicker, R. Moos: Multi-gas sensor to detect simultaneously nitrogen oxides and oxygen, Journal of Sensors and Sensor Systems , 9, 327–335 (2020), doi: 10.5194/jsss-9-327-2020 [5] J. Kita, S. Wiegärtner, R. Moos, P. Weigand, A. Pliscott, M.H. LaBranche, H.D. Glicksman: Screen-printable Type S Thermocouple for Thick-film Technology, Eurosensors XXIX, September 6 - 9, 2015, Freiburg, Germany, MP-K03, Procedia Engineering, 120, p. 828-831 (2015), doi: 10.1016/j.proeng.2015.08.692 Figure 1

Particulate matter (PM) emissions of automotive exhausts are considered to cause serious health problems. Diesel particulate filters (DPF) are in serial use to trap soot from the exhaust. For on-board diagnostics, soot sensors are applied downstream of a DPF. They follow a conductometric principle: The current signal for a constant voltage applied between two interdigital electrodes (IDE) increases with increasing soot deposition after an initial blind time, at which the sensors show no signal. In the present study, a thin insulation layer – manufactured by the aerosol-deposition-method (ADM) – covers the electrode area of a conductometric device and forms a capacitive device. During soot deposition on the surface, the sensor capacitance increases as calculated by FEM simulations. The higher the soot concentration, the faster is the capacitance increase. By applying a dc voltage between the electrodes during soot collection, the sensor performance can be further improved. Compared to the conductometric device with a similar IDE geometry, a reduced blind time at the same dc voltage is achieved. Even more, the capacitive sensor can be operated at higher collection voltages, since the electrodes are insulated towards each other. This allows for a further increase of the applied dc voltage, which reduces even more the blind time. Measurements in an engine dynamometer showed that the blind time can be reduced by a factor of three, with a high potential for a further reduction by technological means.

For the development of catalyst materials a novel sensor tool is presented. It can determine catalyst light-off temperatures. The device measures temperature changes (exothermicities) due to catalytic reactions of the material which is coated on a very small catalyst coating area directly on the sensor device. By temperature modulation via an integrated heater, light-off temperature characteristics can be determined within a few minutes. In the present contribution, the sensor setup is introduced and several experimental results for an exemplarily investigated Pt-loaded Al2O3 catalyst are shown with typical exhaust gas components and with mixtures of their components in a lean base gas atmosphere. Several typical findings are light-off hysteresis, CO poisoning, alkane light-off-behavior or acetylene poisoning.

Introduction Gas sensors provide necessary information in several processes of daily life. Air quality measurement for safety reasons, exhaust gas detection for controlling aftertreatment systems or on-board-diagnosis purposes given by law, evaluation of flue gas in biomass combustion processes for modern energy technology but also medical breath analysis require stable and reliable sensor signals. Most chemical sensors based on functional materials therefore must be operated at defined temperatures. In fact, this temperature must be homogeneous over the whole sensor area (i.e. that region of the sensing element, where the sensor mechanism takes place and functional materials are involved) and may not be influenced by external parameters such like gas velocity. The temperature and its homogeneity must be kept constant also during dynamic changes of the environmental conditions, i.e. in all possible working points. On the other hand – of course – gas contact is highly desired for fast response behavior and high sensitivity. In the present contribution, general aspects concerning sensor housing and heating are highlighted. Basically for experimental investigations, a thermoelectric sensor device is used which is an ideal candidate to identify typical problems. As well, simulations were made to verify the results. Setup: Sensor and Experiments The here used sensor transducers are derived from thermoelectric hydrocarbon sensors, developed for automotive exhaust measurements [1] and investigated for application in the flue gas of wood burning processes [2]. The sensor measures a temperature difference between two areas within the sensor tip and therefore gives a direct measure of the temperature homogeneity in the interesting region (figure 1). If one of these areas is catalytically activated and the other region is covered by an inert layer, exothermic reactions generate a temperature gradient between both areas. This gradient is measured by serially connected screen-printed thermopiles in form of a thermovoltage in the µV-range. Without test gas, temperature gradients (coming from changing flow conditions any kind of inhomogeneous heating or thermal flow) are just as well measured. In former work, it was shown that laminar and symmetric flow characteristics around the sensor tip might avoid cross sensitivities [3]. Here, the described thermoelectric sensors are operated at 600 °C. This absolute sensor temperature is adjusted with a thick-film heater, connected in four-wire technique and located on the reverse side of the substrate. The four-wire resistance is kept constant and so is also the temperature on the reverse side. To evaluate the gas flow influence on the front (sensing) side of the substrate, measurements were conducted with different flow rates (compressed air, gas flow directly facing the sensors front side) and under variation of the mounting position (rotating the sensor by a defined angle concerning the gas flow direction) as well as the use of different housings. Finite-Element-Simulations were made with COMSOLMultiphysics. The here presented data show the gas velocity distribution, regarding the effects around and inside a porous (sintered metal) protection cap (figure 2). Test gas measurements with propene (C3H6) show the influence of this protection cap concerning the gas flow but also sensitivity. Results and Conclusions In a first experiment, sensors were operated in different gas flows without a protection cap. The gas flow varied between 0 and 50 l/min in a tube of 1” diameter. Sensors were mounted from above (“hanging”) with its front side facing the gas flow. By rotating the sensor orientation +/- 45°, temperature gradients within the sensor tip occur in the range of up to 20 °C (figure 3a) coming from non-symmetric cooling effects. By using a porous protection cap around the sensor, this influence is minimized (< 0.5 °C). Simulations verify low and more homogeneous gas velocities inside the cap. Secondly, the sensor results were measured with admixing test gas (mounting in 0°-position, i.e. directly facing the gas flow, but under 20 and 40 l/min). Results were evaluated regarding the sensor sensitivity S, with is the slope in the characteristic curve (figure 4). It could be shown that without the protection cap, both parameters – the gas flow as well as the test gas concentration – influence the sensor signal in similar height. Using the cap, the gas flow influence can be avoided, but a loss in sensitivity must be taken into account. Reasons therefore could be heterogeneous catalytic effects on the cap surface or transport controlled diffusion through the porous cap. Several other types of protection caps were also tested and simulated. It is necessary to find an ideal configuration for sensor mounting, heater design and protection cap depending on the particular application. These findings should be transferred to all other kinds of chemical gas sensors where flow characteristics and heating play a role. Temperature gradients on the sensors surface influence directly all mechanistic processes in resistive, amperometric, mixed-potential or potentiometric type sensors. References [1] S.Wiegärtner, G.Hagen, J.Kita, W.Reitmeier, M.Hien, P.Grass, R.Moos, Thermoelectric hydrocarbon sensor in thick-film technology for on-board-diagnostics of a diesel oxidation catalyst, Sensors and Actuators B: Chemical, 214 (2015) 234–240. doi: 10.1016/j.snb.2015.02.083. [2] B.Ojha, G.Hagen, H.Kohler, R.Moos, Exhaust Gas Analysis of Firewood Combustion Processes: Application of a Robust Thermoelectric Gas Sensor, Proceedings 1 (2017) 457. doi: 10.3390/proceedings1040457. [3] G.Hagen, A.Harsch, R.Moos, A pathway to eliminate the gas flow dependency of a hydrocarbon sensor for automotive exhaust applications, J. Sens. Sens. Syst. 7 (2018) 79–84. doi: 10.5194/jsss-7-79-2018. Figure 1

Up to now, sensor applications have rarely used materials whose dielectric properties are a function of the gas concentration. A sensor principle, by which this material effect can be utilized, is based planar radio-frequency sensors. For the first time, such a sensor was equipped with an integrated heater and successfully operated at temperatures up to 700 ∘C. This makes it possible to apply materials that show gas-dependent changes in the dielectric properties only at higher temperatures. By coating the planar resonance structure with a zeolite, ammonia could be detected. The amount of ammonia stored in the sensitive layer can thereby be determined, since the resonant frequency of the sensor shifts with its ammonia loading. Desorption measurements showed a dependence of the storage behavior of the ammonia in the gas-sensitive layer on the operating temperature of the sensor. Thus, it was possible that by operating the sensor at 300 ∘C, it only shows a gas-concentration-dependent signal. At lower operating temperatures, on the other hand, the sensor could possibly be used for dosimetric determination of very low ammonia concentrations.

The state of catalysts and filters plays a key role in automotive exhaust gas aftertreatment. The soot or ash loading of particulate filters (GPF or DPF), the oxygen loading of three-way catalysts (TWC), the amount of stored ammonia in SCR catalysts, or the NOx loading degree in NOx storage catalysts (NSC) are important parameters. Today, they are determined indirectly based on models that are calibrated by gas or pressure sensors in the exhaust pipe. This contribution overviews a novel approach that determines directly the state of the devices by a radio frequency based technique. For that purpose, the housing of the devices serves as a cavity resonator. As “sensing” element, one or two simple antennas are mounted in the catalyst canning. This contactless-obtained information correlates very well with the catalyst or filter state.

The present study is focused in two directions. In the first part, BaFe(1-x)-0.01Al0.01TaxO3−δ (BFATx) thick films with a Ta content between 0.1 and 0.4 were manufactured using the novel room temperature coating method “aerosol deposition” (ADM), and its material properties were characterized to find the best composition of BFATx for temperature-independent oxygen sensors. The material properties “Seebeck coefficient” and “conductivity” were determined between 600 and 800 ∘C at different oxygen partial pressures. BaFe0.69Al0.01Ta0.3O3−δ (BFAT30) was found out to be very promising due to the almost temperature-independent behavior of both the conductivity and the Seebeck coefficient. In the second part of this study, films of BFAT30 were prepared on a special transducer that includes a heater, equipotential layers, and special electrode structures so that a combined direct thermoelectric/resistive oxygen sensor of BFAT30 with almost temperature-independent characteristics of both measurands, Seebeck coefficient and conductance could be realized. At high oxygen partial pressures (pO2 > 10−5 bar), the electrical conductance of the sensor shows an oxygen sensitivity of m = 0.24 (with m being the slope in the logσ vs. logpO2 representation according to the behavior of σαpO2m), while the Seebeck coefficient changes with a slope of −38 µV K−1 per decade of pO2 at 700 ∘C. However, at low pO2 (pO2 −14 bar) the conductance and the Seebeck coefficient change with pO2, with a slope of m = −0.23 and −21.2 µV K−1 per decade pO2, respectively.

A manufacturing process for a planar binary lambda sensor is shown. By joining the heating and the sensing components via glass soldering with a joining temperature of 850 °C, a laboratory platform has been established that allows the manufacturing of two independent parts in high-temperature co-fired ceramics technology (HTCC) with electrodes that are post-processed at lower temperatures, as is required for mixed-potential sensors. The final device is compared to a commercial sensor with respect to its sensing performance. Important processes and possible origins of problems as well as their solutions during sensor development are shown, including heater design and joining process.

Thermoelectric hydrocarbon sensors were fully manufactured in both alumina substrate-based thick-film technique and low temperature cofired ceramics (LTCC) tape technology. Au–Pt thermocouples were screen-printed on insulating sensor substrates. Porous alumina impregnated with one weight percent platinum was used as catalytic material that covered one side of the sensor. Due to the generated thermal exothermicity, a temperature difference between the catalytically active and the catalytically inactive (reference) side occurred. Additionally, the sensor was equipped with an internal platinum heating element to operate the sensor at temperatures above the light-off temperature of the catalyst material. Owing to the lower thermal conductivity of LTCC, the sensitivity could be improved essentially. To further increase the sensor signal, the number of thermocouples per sensor was enhanced by applying thermopiles of several Au–Pt thermocouples. Therefore, the layout of the heating element was optimized. Altogether, a sensitivity of 0.57 μV/ppm propene was achieved.

A capacitive soot sensor was built up on basis of a known resistive device by covering the electrode area with a thin but dense alumina layer manufactured by the aerosol-deposition-method (ADM). Deposited soot cannot short-circuit the electrodes but leads to an increasing capacitance of the sensor. Experiments were conducted in real exhaust. Increasing soot concentrations lead to a more fast and sharp increase of the capacitance and the blind time of the sensor decreased to almost zero. The performance of such sensors could be enhanced when a voltage is applied at the electrodes during soot collection.

A planar microstrip ring resonator structure on alumina was developed using the commercial FEM software COMSOL. Design parameters were evaluated, eventually leading to an optimized design of a miniaturized microwave gas sensor. The sensor was covered with a zeolite film. The device was successfully operated at around 8.5 GHz at room temperature as a humidity sensor. In the next step, an additional planar heater will be included on the reverse side of the resonator structure to allow for testing of gas-sensitive materials under sensor conditions.

The quality of firewood combustion processes can be considerably improved by automated control of the combustion air streams. For this purpose, continuous flue gas analysis using in-situ sensors is essential. In order to monitor the unburned exhaust gas components like carbon monoxide (CO) and hydrocarbons (CxHy), for the first time a novel thermoelectric gas sensor was applied directly in the exhaust gas. The sensor is made of high temperature stable materials and promises the needed robustness. Here, the sensor signal is compared to flue gas analysis data from FTIR measurements during a wood-log batch firing experiment. An impressive correlation between both data was found.

This contribution presents the integration of glass-ceramic tapes on alumina substrates to increase the sensitivity of thermoelectric hydrocarbon gas sensors. Both ceramic materials have different thermal conductivity. Their combination into one sensor structure significantly improves the sensitivity by at the same time maintaining the excellent mechanical stability at high temperatures. Furthermore, this special technology allows for an easy integration of additional functional elements such as screen-printed thermocouples for temperature control purposes.

Monitoring automotive exhaust gas aftertreatment components is required by law as part of the on-board diagnostics (OBD). For this purpose, a novel sensor device that determines directly the catalyst conversion could be used. It consists of a single, self-heated yttria stabilized ZrO2-based disc, separating two gas atmospheres. Two identical mixed-potential electrodes yield a voltage signal by comparing a certain trace gas concentration up- and downstream of the catalyst. Measurements in synthetic gas flow verify the theoretical assumption that this voltage signal only depends on the ratio of both concentrations, respectively on the conversion of the catalyst.

A planar microstrip ring resonator structure on alumina was developed. It was covered with a zeolite film. The device was successfully operated at around 8.5 GHz at room temperature as a humidity sensor. In the next step, an additional planar heater will be included on the reverse side of the resonator structure to allow for testing of gas sensitive materials under sensor conditions.

The gas sensing properties of resistive gas sensors of BaFe0.7Ta0.3O3-δ (BFT30) prepared by the so-called aerosol deposition method, a method to manufacture dense ceramic films at room temperature, were investigated. The electrical response of the films was investigated first under various oxygen concentrations and in a wide temperature range between 350 and 900 °C. Between 700 and 900 °C, the conductivity of BaFe0.7Ta0.3O3-δ (BFT30) depends on the oxygen concentration with a slope of almost 1/4 in the double-logarithmic plot vs. oxygen partial pressure. In addition, the sensor response is temperature independent. BFT30 responds fast and reproducibly to changing oxygen partial pressures even at 350 °C. The cross-sensitivity has been investigated in environments with various gases (C3H8, NO, NO2, H2, CO, CO2, and H2O) in synthetic air between 350 and 800 °C. BFT30 exhibits good sensing properties to NO between 350 and 400 °C in the range from 1.5 to 2000 ppm with a high selectivity to the other investigated gas species. Thus this semiconducting ceramic material is a good candidate for a temperature-independent oxygen sensor at high temperatures with the application in exhausts and for a selective nitrogen monoxide (NO) sensor at low temperatures for air quality monitoring.

Different amounts of Ru were implanted into thermally evaporated WO3 thin films by ion implantation. The films were subsequently annealed at 600oC for 2 hours in air to remove defects generated during the ion implantation. The Ru concentrations of four samples have been quantified by Rutherford Backscattering Spectrometry as 0.8, 5.5, 9 and 11.5 at%. The un-implanted WO3 films were highly porous but the porosity decreased significantly after ion implantation as observed by Transmission Electron Microscopy and Scanning Electron Microscopy. The thickness of the films also decreased with increasing Ru-ion dose, which is mainly due to densification of the porous films during ion implantation. From Raman spectroscopy two peaks at 408 and 451 cm-1 (in addition to the typical vibrational peaks of the monoclinic WO3 phase) associated with Ru were observed. Their intensity increased with increasing Ru concentration. X-Ray Photoelectron Spectroscopy showed a metallic state of Ru with binding energy of Ru 3d5/2 at 280.1 eV. This peak position remained almost unchanged with increasing Ru concentration. The resistances of the Ru-implanted films were found to increase in the presence of NO2 and NO with higher sensor response to NO2. The effect of Ru concentration on the sensing performance of the films was not explicitly observed due to reduced film thickness and porosity with increasing Ru concentration. However, the results indicate that the implantation of Ru into WO3 films with sufficient film porosity and film thickness can be beneficial for NO2 sensing at temperatures in the range of 250°C to 350°C.

For the radio frequency-based catalyst state monitoring of SCR catalysts, the dielectric properties of the used SCR catalyst materials are essential to design the measurement system and to adjust it to the given conditions. This work focuses on the experimental determination of the effective complex dielectric properties of zeolite coatings on cordierite substrates and the approximation of the latter to describe and transfer these systems in an easier manner. Therefore, different influences were investigated to approximate the real and the imaginary part of the complex permittivity of zeolite coatings on cordierite substrates.

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.

Electroceramic functional materials often need to be characterized in a sensor-like setup close to application. We present a novel sensor transducer that allows four-wire impedance spectroscopy of planar thick-film materials. Fast and easy temperature adjustment is possible by an integrated heater element. The tool can be manufactured either with Au or with Pt electrodes. The measurement of the transducers idle capacitance and calibration measurements with water covering allowed us to define a geometry factor, which is useful to determine specific materials parameters directly from the four-wire measurements. Characterization of three different kinds of materials (an n-type semiconductor, an oxygen ion-conductor and a sodium ion-conductor material) demonstrates the applicability of planar four-wire impedance spectroscopy as a powerful tool to investigate functional materials close to application.

Diesel particulate filters (DPF) are indispensable parts of modern automotive exhaust gas aftertreatment systems due to the stringent emissions legislation. For a fuel-efficient control strategy, it would be beneficial to determine directly and in-operation their actual trapped soot mass. Two novel approaches—based on the electrical conductivity of trapped soot particles—emerged recently. By measuring the electrical resistance between different single walls inside the filter, the soot load is determined with local resolution. The microwave-based technique is a contactless approach that gives an integral value depending on the soot mass in the DPF. We present investigations on loading and regeneration of DPFs in a dynamometer test bench applying both methods. The results are compared with each other and correlated with the differential pressure and the soot mass. Especially the microwave-based technique has a potential for serial application.

The oxygen loading degree in TWCs, the amount of stored ammonia in SCR catalysts, the NOx loading degree in LNTs, or the soot loading of DPFs play a key role in automotive exhaust gas aftertreatment. Today’s methods determine the catalyst state indirectly. They utilize gas sensors installed up- or downstream of the catalysts and the catalyst state is inferred from the sensor signals. This overview reports on the status of an alternative approach based on the interaction of electromagnetic microwaves with the catalyst material. Since the catalyst state is strongly correlated with the electrical properties of the catalyst material itself, this concept shows a great potential.

Multilayer ceramics technology, as LTCC, offers several advantages for the fabrication of miniaturized three-dimensional structures, e.g. for microsystem applications, where electrical, mechanical and fluidic functions can be combined in robust and compact packages. 3D features, required by those applications, are e.g. vias for electrical and thermal interconnection, cavities for chip integration and channels for fluidic functions. They can be realized, in principle, in the sintered, as well as in the green state, but structuring in the green state dominates due to the then far better machinability of the material. Ceramic green tape structuring is usually accomplished by mechanical or laser machining. Punching is the standard process for realizing vias or cavities in LTCC. By nature, only through tape features can be realized, but 3D features can be realized by stacking of several layers. Embossing can be used for the realization of quite complex 3D structures with high resolution. It can be carried out either at elevated temperature, at which the binder of the tape is softened (hot embossing) or at room temperature (cold embossing). Laser structuring is a quite flexible method, which allows both through-cutting and engraving without any specific tool. However, a certain roughness of ablated areas cannot be avoided, and depth control and uniformity of laser engraved features remain challenging. In the present paper, the different techniques will be compared regarding their appropriateness for different structuring tasks. A combined use of punching, embossing and laser processing is described, which has been made possible by a novel machine and tool concept.

Ceramic multilayer technologies such as LTCC (low temperature cofired ceramics) or HTCC (high temperature cofired ceramics) are applied for the fabrication of highly integrated ceramic microelectronic packages. Furthermore, ceramic multilayer technologies offer the possibility of additionally integrating 3D structures for multilayer-based microsystems. This paper presents a new machine for tape/multilayer structuring that combines micro punching tools and micro UV-laser ablation/cutting. The application for the production of different multilayer-based components is described (e.g., LTCC-based PEM fuel cell system, LTCC-based pressure sensors). Aerosol jet printing is a new technology, for example, for rapid prototyping for LTCC multilayer and 3D deposition of functional layers on LTCC. Advantages and limitations of the technology are discussed.

A study on the application of V(2)O(5)/WO(3)/TiO(2) (VWT) as the sensitive material for resistive-type SO(2) sensor was conducted, based on the fact that VWT is a well-known catalyst material for good selective catalytic nitrogen oxide reduction with a proven excellent durability in exhaust gases. The sensors fabricated in this study are planar ones with interdigitated electrodes of Au or Pt. The vanadium content of the utilized VWT is 1.5 or 3.0 wt%. The resistance of VWT decreases with an increasing SO(2) concentration in the range from 20 ppm to 5,000 ppm. The best sensor response to SO(2) occurs at 400 °C using Au electrodes. The sensor response value is independent on the amount of added vanadium but dependent on the electrode materials at 400 °C. These results are discussed and a sensing mechanism is discussed.

Recently, a novel approach for the characterization of the state of automotive catalysts has been proposed, viz., the direct electrical characterization of the catalyst material itself. This direct approach has the potential to do away with the complex models needed by indirect measurement methods to infer the catalyst state, for instance from the oxygen partial pressure outside of the catalyst determined by lambda probes. In this work, we compare two ways of directly observing the catalyst state. Firstly, the electrical conductivity of the catalyst material is determined by wirebound sensors mounted inside of the catalyst and fabricated from catalyst-formulation coated planar interdigital electrodes. Secondly, we employ a contactless method based on the perturbation of the partially filled microwave cavity resonator formed by the catalyst and its metallic housing. For a better understanding of the observed effects, the data obtained at microwave frequencies from engine tests are compared with the local electrical conductivity measurements. It is shown that both methods give a coherent picture of the oxygen loading state of an automotive three-way catalyst and that both methods enable one to follow the progress of reaction fronts inside the catalyst. Furthermore, it is demonstrated that the resonance frequency is strongly correlated with the oxygen load.

For high temperature application, a planar SO2 sensor using NASICON as an electrolyte and V2O5/WO3/TiO2 + Au or Pt as a sensing electrode was fabricated and investigated. In the case of Pt electrodes, the sensor showed the strange response in temperature range from 300–600°C. In the case of Au electrodes, the sensor showed a strange behavior at 300 and 400°C, while at 600°C the sensor showed good response to SO2. The highest sensitivity with 80–83 mV/decade in the SO2 range from 20 to 200 ppm was observed at 600°C, although offset shifting was observed in repeated measurements. The sensors showed the potential for good selectivity at 600°C in the case of Au electrodes. The fact that the electric potential difference of the sensor is strongly affected by the electrode materials may intend that the mechanism of the sensor is a mixed potential type.