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In this microwave study, the defect chemistry of ceria–zirconia solid solutions (CZO, Ce1−yZryO2−δ) was investigated at high temperatures by a resonant microwave method. Specifically, the effects of temperature and Zr content on the dielectric properties and defect chemistry mechanisms in CZO were analyzed. Experiments were performed on a series of different CZO powders (y = 0.2, 0.33, 0.50, 0.67). Measurements at 600°C and different oxygen partial pressures (pO2 = 10−26–0.2 bar) confirm a dominant n-type conduction of small-polarons in CZO due to the preferred formation of oxygen vacancies, which is also supported by a multimodal analysis. Polarization losses were found to be negligible in the GHz range. Furthermore, an increased relative permittivity was observed in CZO, which correlates with the concentration of oxygen vacancies in CZO. Our microwave study is the first to provide a comprehensive data set for the dielectric properties of CZO powder sample in a wide range of different conditions. In addition, the connection of dielectric properties to CZO defect chemistry mechanisms is presented. The results are in good agreement with findings in the literature and may contribute to a better understanding of microwave-based state diagnosis of CZO-based materials, as it discussed for three-way catalysts.

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.

The Seebeck-coefficient (aka thermopower) is a less wide-spread parameter to determine constants for defect chemical models of semiconductor oxides. It is a measure for the charge carrier concentration. In contrast to the often investigated electrical conductivity, it has the benefit of being independent of the material geometry. BaFe0.7Ta0.3O3‐δ is known as a temperature-independent conductometric oxygen sensor material with perovskite crystal structure. The present work considers the Seebeck-coefficient as well as the electrical conductivity of bulk and thick-film BaFe0.89Al0.01Ta0.1O3‐ δ (BFAT10). Seebeck-coefficient and electrical conductivity were measured simultaneously between 600 and 950 °C under varying oxygen partial pressures from 10− 24 to 1 bar. BFAT10 thick films have been successfully deposited by the novel Aerosol Deposition Method (ADM) at room temperature on a special transducer. The electrical conductivity of thick-film sensors shows almost no temperature dependency but depends strongly on the oxygen partial pressure in the temperature range from 600 to 800 °C. An n-to-p-type transition was observed in the investigated oxygen partial pressure and temperature range. The defect parameters were derived from the experimental results and an initial defect model for BFAT10 is discussed. A hopping-type conduction mechanism is assumed due to the very low charge carrier mobility.

In order to determine directly the quantity “conversion of a catalyst”, we developed a new YSZ solid-electrolyte based mixed-potential sensor that enables to compare electrochemically two gas compartments. Core of the sensor is a self-heated YSZ disc that provides in the center sufficiently high temperatures for sensing. At the sensor rim, the temperatures are low enough to allow for applying polymer sealings to separate both gas chambers gas tightly. In this study, the YSZ sensor disc compares two gas mixtures emulating the propene concentrations that occur up- and downstream of a diesel oxidation catalyst (DOC). At a temperature of 500 °C at the center of the disc, the changing propene concentrations on one side of the sensor, which emulate a changing propene conversion of the catalyst, are reflected by the sensor signal. Up to a conversion of 90 %, the sensor signals follow a theoretical equation that is derived from mixed-potential theory. For higher propene conversions, the signal differs from that theory. An initial explanation for this may be the catalytic activity of the platinum electrode at these high operation temperatures. This is supported by the fact that when reducing the catalytic activity, which is achieved by reducing the sensing temperature, the sensor signal is meeting more and more the theoretically derived equation. At 425 °C and below, there is no more dependency on the feed gas concentration, and all measured data fit exactly with the above-derived theory, i.e., the sensor signal depends only on the “conversion of the catalyst” and not on the propene concentration. As result, a sensor that measures directly the quantity “conversion” is obtained.

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.

In Folge der Verscharfung der gesetzlichen Abgasnormen wurden fur direkt-einspritzende Benzinmotoren Partikelfilter notwendig. Zur Beladungsuberwachung konnen aufgrund stark unterschiedlicher Rahmenbedingungen die aus Dieselmotoren bekannten Systeme, wie dem Differenzdrucksensor, nur eingeschrankt ubernommen werden. Ein hochfrequenzbasiertes Verfahren koppelt mittels Antennen elektromagnetische Wellen in das Filtergehause ein, deren Ausbreitungsverhalten durch die dielektrischen Eigenschaften des eingelagerten Ruses beeinflusst wird. Hierdurch kann bei Auswertung von Transmissionsdampfung oder Resonanzfrequenzen die Rusbeladung direkt detektiert werden.

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 One of the most important key elements for the exhaust gas aftertreatment of modern gasoline vehicles is the three-way catalytic converter (TWC). Its combination of oxygen storage (high-surface ceria) and precious metal loading (Pt, Pd, Rh) ensures the effective reduction of nitrogen oxides (NO x ) as well as the oxidation of unburnt hydrocarbons (HC) and carbon monoxide (CO) during stoichiometric operation. For optimal performance of the TWC, the determination of the current oxygen storage level is crucial. Today’s systems using oxygen sensors can only estimate the catalyst state by means of balancing approaches together with numerical models. Some time ago, however, a radio frequency-based measurement system was introduced that allows to determine directly the oxygen storage level [1]. In non-contact measurement, the catalyst housing serves as a cavity resonator in which standing electromagnetic waves propagate (GHz range). This article sheds light on the basic behavior of the radio frequency-based state diagnosis of three-way catalytic converters, deals with cross-sensitivities of the measuring principle, and provides an orientation for vehicle application. The advantages of different resonance parameters are considered. Methods and Setup In order to determine the oxygen storage level using RF technology, the metallic catalyst housing forms a cavity resonator. Its cylindrical geometry is limited by two inserted perforated plates, as shown in Fig. 1 [2]. In this case, the three-way catalyst is located in the center of the arrangement. Thermocouples and ceramic oxygen sensors (binary and wideband) are placed upstream and downstream of the resonator to analyze exhaust gas temperature and stoichiometry. For a transmission measurement, two coupling elements, also called antennas, are inserted upstream and downstream of the catalyst. At discrete frequencies, so-called resonance modes propagate within the canning, which are characterized by their resonance frequency and quality factor. The exact resonance properties are also a function of the dielectric properties of the catalyst and can be determined with the microwave cavity perturbation theory [3]. For the TWC, the dielectric properties (polarization and dielectric losses) change with the oxygen stoichiometry within the oxygen storage component. Therefore, reduction and re-oxidation of the oxygen storage material can be detected with the resonance frequency as well as with the quality factor [4]. To evaluate the method for the state diagnosis of three-way catalysts, both full-catalysts (Ø4.66'') at lower (gas hourly) space velocities (GHSV = 1000 h-1) and smaller cores (Ø1.66'') at application-typical space velocities (GHSV = 32.000 h-1) were investigated. The TE111 mode was used for the radio frequency method as the electric field can be assumed to be almost constant over the entire length of the catalyst (Figure 1) and therefore a steady RF sensitivity can be assumed. Results In principle, both, resonance frequency and quality factor, are suitable to determine the current oxygen storage level, however, both signals also feature individual characteristics. As an example, Fig. 2 shows the relative signal amplitude of the resonance frequency as a function of the oxygen storage level of a TWC [2]. The signal amplitude is very low at temperatures just above the light-off (≈ 300°C) and increases noticeably with higher temperatures. Although the determination of the actual oxidation level is theoretically possible at low temperatures, the resonance frequency is more suitable for higher exhaust temperatures. An opposite behavior was found for the quality factor. Here, the relative signal amplitudes are high especially at low temperatures, but decrease at higher signal amplitudes at higher catalyst temperatures due to increasing resonator losses. Therefore, the quality factor is particularly suitable for the state diagnosis at low temperatures. It was shown that these resonance parameters are strongly dependent on the catalyst temperature, which can be attributed on the one hand to the thermal expansion of the catalyst housing and on the other hand to a temperature-related change in the dielectric TWC properties. A reliable diagnosis of the catalyst state therefore requires knowledge of the TWC temperature. In contrast, the effect of the water concentration can be neglected. The RF method also provides information on the aging condition of the catalyst. An increase of the temperature necessary for the activation of the oxygen storage component with proceeding catalyst ageing was clearly observed with the quality factor, due to its high sensitivity at temperatures close to the catalyst light-off. The quality factor therefore provides a new approach to determine TWC aging. Considering the different characteristics of resonance frequency and quality factor, a combined analysis of both resonance parameters does provide detailed information about the catalyst state and opens promising possibilities for technical application. Literature Reiß, S.; Spörl, M.; Hagen, G.; Fischerauer, G.; Moos, R.: Combination of Wirebound and Microwave Measurements for In Situ Characterization of Automotive Three-Way Catalysts. IEEE Sens. J. 2011, 11, 434–438, doi: 1109/JSEN.2010.2058798 Steiner, C.; Malashchuk, V.; Kubinski, D.; Hagen, G.; Moos, R.: Catalyst State Diagnosis of Three-Way Catalytic Converters Using Different Resonant Parameters — A Microwave Cavity Perturbation Study. Sensors 2019, 19, 3559, doi: 3390/s19163559 Chen, L.; Ong, C.K.; Neo, C.P.; Varadan, V.V.; Varadan, V.K.: Microwave Electronics: Measurement and Materials Characterization. John Wiley & Sons, Ltd, Chichester, 2006, doi: 10.1002/0470020466 Steiner, C.; Gänzler, A.M.; Zehentbauer, M.; Hagen, G.; Casapu, M.; Müller, S.; Grunwaldt, J.-D.; Moos, R.: Oxidation State and Dielectric Properties of Ceria-Based Catalysts by Complementary Microwave Cavity Perturbation and X-Ray Absorption Spectroscopy Measurements. Top. Catal. 2019, 62, 227–236, doi: 1007/s11244-018-1110-3 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.

Diesel Particulate Filters (DPF) are an essential part of today’s diesel exhaust gas aftertreatment systems. For an effective filter regeneration strategy, the precise knowledge of the actual trapped soot mass is essential. Besides the state-of-the-art technology of determining the soot load via pressure drop and/or model-based, a microwave-based method enables direct and in situ soot load detection. Thereby, an electric field is impressed into the filter housing and the power transmission is measured, which correlates with the soot load. In this study, influencing parameters, especially temperature and humidity, are examined and compared to the sensitivity towards soot accumulation. Measurements were conducted in a laboratory test bench with a filter-core, which had been previously loaded with soot in an engine dynamometer. While humidity does not have a notable effect, the influence of temperature needs to be considered for real world application. Finally, a complete filter regeneration of the DPF-core in the laboratory test bench could be monitored with the microwave-based system. The carbon balance-derived burned soot mass coincides very well with the microwave derived transmission parameter.

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.