Publications by authors named "Ralf Moos"

50 Publications

Linking the Electrical Conductivity and Non-Stoichiometry of Thin Film CeZrO by a Resonant Nanobalance Approach.

Materials (Basel) 2021 Feb 5;14(4). Epub 2021 Feb 5.

Institute of Energy Research and Physical Technologies, Clausthal University of Technology, 38640 Goslar, Germany.

Bulk ceria-zirconia solid solutions (CeZrO, CZO) are highly suited for application as oxygen storage materials in automotive three-way catalytic converters (TWC) due to the high levels of achievable oxygen non-stoichiometry δ. In thin film CZO, the oxygen storage properties are expected to be further enhanced. The present study addresses this aspect. CZO thin films with 0 ≤ x ≤ 1 were investigated. A unique nano-thermogravimetric method for thin films that is based on the resonant nanobalance approach for high-temperature characterization of oxygen non-stoichiometry in CZO was implemented. The high-temperature electrical conductivity and the non-stoichiometry δ of CZO were measured under oxygen partial pressures O in the range of 10-0.2 bar. Markedly enhanced reducibility and electronic conductivity of CeO-ZrO as compared to CeO and ZrO were observed. A comparison of temperature- and O-dependences of the non-stoichiometry of thin films with literature data for bulk CeZrO shows enhanced reducibility in the former. The maximum conductivity was found for CeZrO, whereas CeZrO showed the highest non-stoichiometry, yielding δ = 0.16 at 900 °C and O of 10 bar. The defect interactions in CeZrO are analyzed in the framework of defect models for ceria and zirconia.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/ma14040748DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7915746PMC
February 2021

Microscopic (Dis)order and Dynamics of Cations in Mixed FA/MA Lead Halide Perovskites.

J Phys Chem C Nanomater Interfaces 2021 Jan 15;125(3):1742-1753. Epub 2021 Jan 15.

Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands.

Recent developments in the field of high efficiency perovskite solar cells are based on stabilization of the perovskite crystal structure of FAPbI while preserving its excellent optoelectronic properties. Compositional engineering of, for example, MA or Br mixed into FAPbI results in the desired effects, but detailed knowledge of local structural features, such as local (dis)order or cation interactions of formamidinium (FA) and methylammonium (MA), is still limited. This knowledge is, however, crucial for their further development. Here, we shed light on the microscopic distribution of MA and FA in mixed perovskites MA FA PbI and MAFAPbIBr by combining high-resolution double-quantum H solid-state nuclear magnetic resonance (NMR) spectroscopy with state-of-the-art near-first-principles accuracy molecular dynamics (MD) simulations using machine-learning force-fields (MLFFs). We show that on a small local scale, partial MA and FA clustering takes place over the whole MA/FA compositional range. A reasonable driving force for the clustering might be an increase of the dynamical freedom of FA cations in FA-rich regions. While MAFAPbIBr displays similar MA and FA ordering as the MA FA PbI systems, the average cation-cation interaction strength increased significantly in this double mixed material, indicating a restriction of the space accessible to the cations or their partial immobilization upon Br incorporation. Our results shed light on the heterogeneities in cation composition of mixed halide perovskites, helping to exploit their full optoelectronic potential.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpcc.0c10042DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7848893PMC
January 2021

Determination of the Dielectric Properties of Storage Materials for Exhaust Gas Aftertreatment Using the Microwave Cavity Perturbation Method.

Sensors (Basel) 2020 Oct 23;20(21). Epub 2020 Oct 23.

Bayreuth Engine Research Center (BERC), Department of Functional Materials, University of Bayreuth, 95440 Bayreuth, Germany.

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 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s20216024DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7660336PMC
October 2020

Modelling the Influence of Different Soot Types on the Radio-Frequency-Based Load Detection of Gasoline Particulate Filters.

Sensors (Basel) 2020 May 6;20(9). Epub 2020 May 6.

Bayreuth Engine Research Center (BERC), Department of Functional Materials, University of Bayreuth, 95447 Bayreuth, Germany.

Gasoline particulate filters (GPFs) are an appropriate means to meet today's emission standards. As for diesel applications, GPFs can be monitored via differential pressure sensors or using a radio-frequency approach (RF sensor). Due to largely differing soot properties and engine operating modes of gasoline compared to diesel engines (e.g., the possibility of incomplete regenerations), the behavior of both sensor systems must be investigated in detail. For this purpose, extensive measurements on engine test benches are usually required. To simplify the sensor development, a simulation model was developed using COMSOL Multiphysics that not only allowed for calculating the loading and regeneration process of GPFs under different engine operating conditions but also determined the impact on both sensor systems. To simulate the regeneration behavior of gasoline soot accurately, an oxidation model was developed. To identify the influence of different engine operating points on the sensor behavior, various samples generated at an engine test bench were examined regarding their kinetic parameters using thermogravimetric analysis. Thus, this compared the accuracy of soot mass determination using the RF sensor with the differential pressure method. By simulating a typical driving condition with incomplete regenerations, the effects of the soot kinetics on sensor accuracy was demonstrated exemplarily. Thereby, the RF sensor showed an overall smaller mass determination error, as well as a lower dependence on the soot kinetics.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s20092659DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7248741PMC
May 2020

What Happens during Thermal Post-Treatment of Powder Aerosol Deposited Functional Ceramic Films? Explanations Based on an Experiment-Enhanced Literature Survey.

Adv Mater 2020 May 16;32(19):e1908104. Epub 2020 Mar 16.

Department of Functional Materials, University of Bayreuth, Universitätsstraße 30, 95440, Bayreuth, Germany.

Powder aerosol deposition (PAD) is a unique ceramic spray coating method that produces dense and well-adhering thick-films directly at room temperature, without requiring any heating or sintering. After the successful film formation, mechanical film properties like hardness or plasma resistance are remarkably good. However, when it comes to electrical properties like permittivity or electrical conductivity, the nanocrystalline structure of PAD films combined with high internal strains deteriorates partly the characteristic properties. The electrical conductivity may already be present within the as-deposited films. However, it is mostly lowered by several orders of magnitude. Therefore, a thermal post-deposition annealing is oftentimes required. In this work, electrically conducting films produced by powder aerosol deposition are surveyed based on published data. Their microstructural and electrical behavior during the post-deposition annealing treatment is summarized and reasons for the lowered electrical conductivity are identified. Additionally, the processes taking place during annealing, which eventually allow to regain bulk-like functional properties, are examined. A universal annealing behavior is found that leads to a quantitative recommendation for the suitable film annealing temperatures to regain the electrical conductivities.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/adma.201908104DOI Listing
May 2020

Novel Operation Strategy to Obtain a Fast Gas Sensor for Continuous ppb-Level NO Detection at Room Temperature Using ZnO-A Concept Study with Experimental Proof.

Sensors (Basel) 2019 Sep 23;19(19). Epub 2019 Sep 23.

Department of Functional Materials, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany.

A novel sensor operation concept for detecting ppb-level NO concentrations at room temperature is introduced. Today's research efforts are directed to make the sensors (low response and recovery times). Nevertheless, hourly mean values can hardly be precisely calculated, as the sensors are still too slow and show baseline drifts. Therefore, the integration error becomes too large. The suggested concept follows exactly the opposite path. The sensors should be made and operated as resistive gas dosimeters. The adsorption/desorption equilibrium should be completely shifted to the adsorption side during a sorption phase. The gas-sensitive material adsorbs each NO molecule (dose) impinging and the sensor signal increases linearly with the NO dose. The actual concentration value results from the time derivative, which makes the response very fast. When the NO adsorption capacity of the sensor material is exhausted, it is regenerated with ultraviolet (UV) light and the baseline is reached again. Since the baseline is newly redefined after each regeneration step, no baseline drift occurs. Because each NO molecule that reaches the sensor material contributes to the sensor signal, a high sensitivity results. The sensor behavior of ZnO known so far indicates that ZnO may be suitable to be applied as a room-temperature chemiresistive NO dosimeter. Because UV enhances desorption of sorbed gas species from the ZnO surface, regeneration by UV light should be feasible. An experimental proof demonstrating that the sensor concept works at room temperature for ppb-level NO concentrations and low doses is given.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s19194104DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6806225PMC
September 2019

Operando Determination of the Thermal Decomposition of Supported Ionic Liquids by a Radio-Frequency-Based Method.

ACS Omega 2019 Feb 14;4(2):3351-3360. Epub 2019 Feb 14.

Department of Chemical Engineering and Department of Functional Materials, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany.

The analysis of the thermal stability of supported ionic liquids (ILs) is of great interest for their application in catalysis. However, thermogravimetric (TG) measurements are very time-consuming, destructive, and cannot be conducted operando. Therefore, a new radio-frequency (RF)-based method is presented that analyzes the electrical properties of supported ILs in the microwave range and can detect a possible IL mass loss caused by evaporation or decomposition. In this study, the decomposition of supported 1-butyl-3-methylimidazolium dimethylphosphate ([BMIM][DMP]) with and without palladium (as an active metal) is investigated operando during the selective hydrogenation of 1,3-butadiene. In addition to volatile decomposition products, solid products are formed, which remain on the carrier. These solid products impair the activity of the Pd catalyst. Using the RF-based method, a distinction can be made between "intact" IL and the solid decomposition products because the electrical properties of both substances differ substantially. In contrast, the destructive TG analysis only measures the mass loss by the formation of gaseous decomposition products and thus cannot distinguish between "intact" IL and the solid decomposition products of [BMIM][DMP]. In addition, a model of the thermal decomposition which depicts the measured mass losses well is presented.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsomega.8b02421DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6648664PMC
February 2019

Catalyst State Diagnosis of Three-Way Catalytic Converters Using Different Resonance Parameters-A Microwave Cavity Perturbation Study.

Sensors (Basel) 2019 Aug 15;19(16). Epub 2019 Aug 15.

Bayreuth Engine Research Center (BERC), Department of Functional Materials, University of Bayreuth, 95447 Bayreuth, Germany.

Recently, radio frequency (RF) technology was introduced as a tool to determine the oxygen storage level of a three-way catalyst (TWC) for gasoline vehicles. Previous studies on the investigation of commercial catalysts mostly use only the resonant frequency to describe the correlation of oxygen storage level and RF signal. For the first time this study presents a comparison under defined laboratory conditions considering both, resonance frequency and also the quality factor as measurands. Furthermore, various advantages over the sole use of the resonant frequency in the technical application are discussed. Experiments with Ø4.66'' catalysts and Ø1.66'' catalyst cores with alternating (rich/lean) gas compositions showed that the relative change in signal amplitude due to a change in oxygen storage is about 100 times higher for the inverse quality factor compared to the resonant frequency. In addition, the quality factor reacts more sensitively to the onset of the oxygen-storage ability, and delivers precise information about the necessary temperature, which is not possible when evaluating the resonant frequency due to the low signal amplitude. As investigations on aged catalysts confirm, the quality factor also provides a new approach to determine the ageing state of a TWC.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s19163559DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6719166PMC
August 2019

High Versatility and Stability of Mechanochemically Synthesized Halide Perovskite Powders for Optoelectronic Devices.

ACS Appl Mater Interfaces 2019 Aug 7;11(33):30259-30268. Epub 2019 Aug 7.

IPVF, Institut Photovoltaïque d'Ile de France (IPVF) , 30 route départementale 128 , 91120 Palaiseau , France.

We show that mechanochemically synthesized halide perovskite powders from a ball milling approach can be employed to fabricate a variety of lead halide perovskites with exceptional intrinsic stability. Our MAPbI powder exhibits higher thermal stability than conventionally processed thin films, without degradation after more than two and a half years of storage and only negligible degradation after heat treatment at 220 °C for 14 h. We further show facile recovery strategies of nonphase-pure powders by simple remilling or mild heat treatment. Moreover, we demonstrate the mechanochemical synthesis of phase-pure mixed perovskite powders, such as (CsFAPbI)(MAPbBr), from either the individual metal and organic halides or from readily prepared ternary perovskites, regardless of the precursor phase purity. Adding potassium iodide (KI) to the milling process successfully passivated the powders. We also succeeded in preparing a precursor solution on the basis of the powders and obtained uniform thin films for integration into efficient perovskite solar cells from spin-coating this solution. We find the KI passivation remains in the devices, leading to improved performance and significantly reduced hysteresis. Our work thus demonstrates the potential of mechanochemically synthesized halide perovskite powders for long-time storage and upscaling, further paving the way toward commercialization of perovskite-based optoelectronic devices.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.9b09160DOI Listing
August 2019

Novel Method for NTC Thermistor Production by Aerosol Co-Deposition and Combined Sintering.

Sensors (Basel) 2019 Apr 5;19(7). Epub 2019 Apr 5.

Department of Functional Materials, Universität Bayreuth, 95440 Bayreuth, Germany.

A novel three-stage process to produce NTCR sensors is presented. In this process, an uncalcined powder mixture of NiO and Mn₂O₃ was deposited onto an alumina substrate via aerosol co-deposition (AcD). Then, an electrode structure was screen-printed onto the surface and the composite film was sintered in a multifunctional temperature treatment. Thereby, the sintering of the electrode, the formation of the NiMn₂O₄ spinel and the removal of film strains took place simultaneously. This enabled a significant reduction in energy demand and workload. The manufactured sensors, both as first prototypes, as well as miniaturized chip components, were characterized by a single-phase cubic NiMn₂O₄ spinel structure, mechanical stability and electrical properties that were similar to those of classical NiMn₂O₄ bulk ceramics or tempered aerosol deposited (AD) NiMn₂O₄ films. Particularly noteworthy was the high reproducibility and low variation of the NTCR parameters, such as the specific resistivity at 25 °C , the electrical resistance at 25 °C and the thermistor constant . The NTCR parameters were as aging-stable as for NiMn₂O₄ bulk ceramics or tempered NiMn₂O₄ AD-films and could even be further improved by thermal post-treatment.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s19071632DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6480331PMC
April 2019

Manufacturing Dense Thick Films of Lunar Regolith Simulant EAC-1 at Room Temperature.

Materials (Basel) 2019 Feb 5;12(3). Epub 2019 Feb 5.

Department of Functional Materials, University of Bayreuth, 95440 Bayreuth, Germany.

The Aerosol Deposition (AD, also known as gas kinetic spraying or vacuum deposition) method is a rather novel coating process to produce dense thick films directly from dry ceramic (or metal) powders on a variety of substrates without any heat treatment. Because of the similarity of the up to now used powders and lunar regolith, it is imaginable to use AD systems for future in situ resource utilization missions on the Moon planned by several space agencies. To test the feasibility of such an endeavor, the processability of lunar mare simulant EAC-1 by the AD method has been examined in this study. Three regolith films with an area of 25 × 10 mm², and thicknesses between 2.50 µm and 5.36 µm have been deposited on steel substrates using a standard AD setup. Deposited films have been investigated by Laser Scanning Microscopy (LSM) and Scanning Electron Microscopy (SEM). Moreover, the roughness and Vickers hardness of the deposited films and the underlying substrates have been measured. It has been shown that dense consolidated films of regolith simulant can be produced within minutes by AD. The deposited films show a higher roughness and, on average, a higher hardness than the steel substrates. Since on the Moon, naturally available regolith powders are abundant and very dry, and since the required process vacuum is available, AD appears to be a very promising method for producing dense coatings in future Moon exploration and utilization missions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/ma12030487DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6384728PMC
February 2019

Mechanistic Understanding of Cu-CHA Catalyst as Sensor for Direct NH-SCR Monitoring: The Role of Cu Mobility.

ACS Appl Mater Interfaces 2019 Feb 13;11(8):8097-8105. Epub 2019 Feb 13.

Institute of Inorganic Chemistry , RWTH Aachen University , Landoltweg 1 , Aachen 52074 , Germany.

The concept to utilize a catalyst directly as a sensor is fundamentally and technically attractive for a number of catalytic applications, in particular, for the catalytic abatement of automotive emission. Here, we explore the potential of microporous copper-exchanged chabazite (Cu-CHA, including Cu-SSZ-13 and Cu-SAPO-34) zeolite catalysts, which are used commercially in the selective catalytic reduction of automotive nitrogen oxide emission by NH (NH-SCR), as impedance sensor elements to monitor directly the NH-SCR process. The NH-SCR sensing behavior of commercial Cu-SSZ-13 and Cu-SAPO-34 catalysts at typical reaction temperatures (i.e., 200 and 350 °C) was evaluated according to the change of ionic conductivity and was mechanistically investigated by complex impedance-based in situ modulus spectroscopy. Short-range (local) movement of Cu ions within the zeolite structure was found to determine largely the NH-SCR sensing behavior of both catalysts. Formation of NH-solvated, highly mobile Cu species showed a predominant influence on the ionic conductivity of both catalysts and, consequently, hindered NH-SCR sensing at 200 °C. Density functional theory calculations over a model Cu-SAPO-34 system revealed that Cu reduction to Cu by coadsorbed NH and NO weakened significantly the coordination of the Cu site to the CHA framework, enabling high mobility of Cu species that influences substantially the NH-SCR sensing. The in situ spectroscopic and theoretical investigations not only unveil the mechanisms of Cu-CHA catalyst as sensor elements for direct NH-SCR monitoring but also allow us to get insights into the speciation of active Cu sites in NH-SCR under different reaction conditions with varied temperatures and gas compositions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.8b22104DOI Listing
February 2019

Thermal Treatment of Aerosol Deposited NiMn₂O₄ NTC Thermistors for Improved Aging Stability.

Sensors (Basel) 2018 Nov 15;18(11). Epub 2018 Nov 15.

Department of Functional Materials, Universität Bayreuth, 95440 Bayreuth, Germany.

This paper examines the influence of a short-term thermal treatment of aerosol deposited negative temperature coefficient (NTC) thermistor films on the NTCR characteristics and their long-term stability with different electrode materials. An aerosol deposition of a spinel-based NiMn₂O₄ powder on alumina substrates with screen-printed AgPd and Au interdigital electrode structures was performed. The manufactured components of the typical size of 1206 were tempered in a moderate temperature range of 200 °C to 800 °C and aged for 1000 h at 125 °C in air. Based on - measurements in a high-precision silicone oil thermostat bath and high temperature XRD analyses, the influence of the thermal treatment was analyzed and discussed. A 60-min tempering at 400 °C proved to be optimal, as both the NTCR parameters and their ageing stability could be significantly improved. The findings are explained.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s18113982DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6263728PMC
November 2018

Annealing of Gadolinium-Doped Ceria (GDC) Films Produced by the Aerosol Deposition Method.

Materials (Basel) 2018 Oct 23;11(11). Epub 2018 Oct 23.

Department of Functional Materials, University of Bayreuth, Universitätsstraße 30, 95440 Bayreuth, Germany.

Solid oxide fuel cells need a diffusion barrier layer to protect the zirconia-based electrolyte if a cobalt-containing cathode material like lanthanum strontium cobalt ferrite (LSCF) is used. This protective layer must prevent the direct contact and interdiffusion of both components while still retaining the oxygen ion transport. Gadolinium-doped ceria (GDC) meets these requirements. However, for a favorable cell performance, oxide ion conducting films that are thin yet dense are required. Films with a thickness in the sub-micrometer to micrometer range were produced by the dry room temperature spray-coating technique, aerosol deposition. Since commercially available GDC powders are usually optimized for the sintering of screen printed films or pressed bulk samples, their particle morphology is nanocrystalline with a high surface area that is not suitable for aerosol deposition. Therefore, different thermal and mechanical powder pretreatment procedures were investigated and linked to the morphology and integrity of the sprayed films. Only if a suitable pretreatment was conducted, dense and well-adhering GDC films were deposited. Otherwise, low-strength films were formed. The ionic conductivity of the resulting dense films was characterized by impedance spectroscopy between 300 °C and 1000 °C upon heating and cooling. A mild annealing occurred up to 900 °C during first heating that slightly increased the electric conductivity of GDC films formed by aerosol deposition.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/ma11112072DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6266497PMC
October 2018

Conductometric Soot Sensors: Internally Caused Thermophoresis as an Important Undesired Side Effect.

Sensors (Basel) 2018 Oct 19;18(10). Epub 2018 Oct 19.

Bayreuth Engine Research Center (BERC), University of Bayreuth, Department of Functional Materials, 95440 Bayreuth, Germany.

Particulate matter sensors are of interest for application in the exhaust of any combustion processes, especially for automotive aftertreatment systems. Conductometric soot sensors have been serialized recently. They comprise planar interdigital electrodes (IDE) on an insulating substrate. Between the IDEs, a voltage is applied. Soot deposition is accelerated by the resulting electric field due to electrophoresis. With increasing soot deposition, the conductance between the IDE increases. The timely derivative of the conductance can serve as a sensor signal, being a function of the deposition rate. An increasing voltage between the IDE would be useful for detecting low particle exhausts. In the present study, the influence of the applied voltage and the sensor temperature on the soot deposition is investigated. It turned out that the maximum voltage is limited, since the soot film is heated by the resulting current. An internally caused thermophoresis that reduces the rate of soot deposition on the substrate follows. It reduces both the linearity of the response and the sensitivity. These findings may be helpful for the further development of conductometric soot sensors for automotive exhausts, probably also to determine real driving emissions of particulate matter.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s18103531DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6210793PMC
October 2018

Effect of Oxygen Partial Pressure on the Phase Stability of Copper⁻Iron Delafossites at Elevated Temperatures.

Materials (Basel) 2018 Oct 2;11(10). Epub 2018 Oct 2.

Department of Functional Materials, Zentrum für Energietechnik (ZET), University of Bayreuth, 95440 Bayreuth, Germany.

Oxide-based materials are promising candidates for use in high temperature thermoelectric generators. While their thermoelectric performance is inferior to commonly used thermoelectrics, oxides are environmentally friendly and cost-effective. In this study, Cu-based delafossites (CuFeO₂), a material class with promising thermoelectric properties at high temperatures, were investigated. This work focuses on the phase stability of CuFeO₂ with respect to the temperature and the oxygen partial pressure. For this reason, classical material characterization methods, such as scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction, were combined in order to elucidate the phase composition of delafossites at 900 °C at various oxygen partial pressures. The experimentally obtained results are supported by the theoretical calculation of the Ellingham diagram of the copper⁻oxygen system. In addition, hot-stage X-ray diffraction and long-term annealing tests of CuFeO₂ were performed in order to obtain a holistic review of the phase stability of delafossites at high temperatures and varying oxygen partial pressure. The results support the thermoelectric measurements in previous publications and provide a process window for the use of CuFeO₂ in thermoelectric generators.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/ma11101888DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6213647PMC
October 2018

The Aerosol Deposition Method: A Modified Aerosol Generation Unit to Improve Coating Quality.

Materials (Basel) 2018 Sep 1;11(9). Epub 2018 Sep 1.

Department of Functional Materials, University of Bayreuth, 95447 Bayreuth, Germany.

Owing to its ability to produce dense thick-films at room temperature directly from a ceramic powder, the Aerosol Deposition Method (AD) possesses a unique feature in ceramics processing. For this technology, the aerosol generation of particles is a decisive part of reliable process control. However, there has only been a small amount of work published addressing this topic. In this work, we compare the aerosolization and deposition behavior of a fluidized bed generator with an aerosol generator with the rotary brush principle. While film properties very much depend on deposition time for the fluidized bed generator, films produced with the brush generator show a constant film profile, and their film thickness correlates with the controllable aerosol concentration and the duration of deposition. This type of aerosol generation may improve the setup towards a more reliable AD process.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/ma11091572DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6165444PMC
September 2018

Powder Pre-Treatment for Aerosol Deposition of Tin Dioxide Coatings for Gas Sensors.

Materials (Basel) 2018 Aug 2;11(8). Epub 2018 Aug 2.

Department of Functional Materials, University of Bayreuth, 95447 Bayreuth, Germany.

The Aerosol Deposition (AD) method has the unique property to allow for manufacturing dense ceramic films at room temperature. As found in many publications, the deposition process is very sensitive to powder properties. In particular, powders of nano-sized particles and grains, e.g., from precipitation, are usually beyond the conventional size range of AD processability, yielding chalk-like films of low mechanical stability. Thus, the conventional AD process is limited in applicability. In this study, we try to overcome this problem by adapting the standard milling treatment of powders for improved deposition by additional temperature pre-treatment. Using commercial tin dioxide and including a temperature treatment for grain growth, makes the powder accessible to deposition. In this way, we achieve optically translucent and conductive SnO₂ thick films. With the application such as a gas sensitive film as one of many possible applications for SnO₂ thick-films, the sensors show excellent response to various reducing gases. This study shows one exemplary way of extending the range of adequate powder and applications for the AD method.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/ma11081342DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6119988PMC
August 2018

Towards an Electrochemical Immunosensor System with Temperature Control for Cytokine Detection.

Sensors (Basel) 2018 Apr 24;18(5). Epub 2018 Apr 24.

Department of Functional Materials, University of Bayreuth, Universitätsstraße 30, D-95440 Bayreuth, Germany.

The cytokine interleukin-13 (IL-13) plays a major role in airway inflammation and is a target of new anti-asthmatic drugs. Hence, IL-13 determination could be interesting in assessing therapy success. Thus, in this work an electrochemical immunosensor for IL-13 was developed and integrated into a fluidic system with temperature control for read-out. Therefore, two sets of results are presented. First, the sensor was set up in sandwich format on single-walled carbon nanotube electrodes and was read out by applying the hydrogen peroxide⁻hydroquinone⁻horseradish peroxidase (HRP) system. Second, a fluidic system was built up with an integrated heating function realized by Peltier elements that allowed a temperature-controlled read-out of the immunosensor in order to study the influence of temperature on the amperometric read-out. The sensor was characterized at the temperature optimum of HRP at 30 °C and at 12 °C as a reference for lower performance. These results were compared to a measurement without temperature control. At the optimum operation temperature of 30 °C, the highest sensitivity (slope) was obtained compared to lower temperatures and a limit of detection of 5.4 ng/mL of IL-13 was calculated. Taken together, this approach is a first step towards an automated electrochemical immunosensor platform and shows the potential of a temperature-controlled read-out.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s18051309DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5982244PMC
April 2018

Radio-Frequency-Controlled Urea Dosing for NH₃-SCR Catalysts: NH₃ Storage Influence to Catalyst Performance under Transient Conditions.

Sensors (Basel) 2017 Nov 28;17(12). Epub 2017 Nov 28.

Bayreuth Engine Research Center (BERC), Department of Functional Materials, University of Bayreuth, 95447 Bayreuth, Germany.

Current developments in exhaust gas aftertreatment led to a huge mistrust in diesel driven passenger cars due to their NO emissions being too high. The selective catalytic reduction (SCR) with ammonia (NH₃) as reducing agent is the only approach today with the capability to meet upcoming emission limits. Therefore, the radio-frequency-based (RF) catalyst state determination to monitor the NH₃ loading on SCR catalysts has a huge potential in emission reduction. Recent work on this topic proved the basic capability of this technique under realistic conditions on an engine test bench. In these studies, an RF system calibration for the serial type SCR catalyst Cu-SSZ-13 was developed and different approaches for a temperature dependent NH₃ storage were determined. This paper continues this work and uses a fully calibrated RF-SCR system under transient conditions to compare different directly measured and controlled NH₃ storage levels, and NH₃ target curves. It could be clearly demonstrated that the right NH₃ target curve, together with a direct control on the desired level by the RF system, is able to operate the SCR system with the maximum possible NO conversion efficiency and without NH₃ slip.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s17122746DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5751734PMC
November 2017

Planar Microstrip Ring Resonators for Microwave-Based Gas Sensing: Design Aspects and Initial Transducers for Humidity and Ammonia Sensing.

Sensors (Basel) 2017 Oct 24;17(10). Epub 2017 Oct 24.

Department of Functional Materials, University of Bayreuth, 95447 Bayreuth, Germany.

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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s17102422DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5677052PMC
October 2017

Pulsed Polarization-Based NO Sensors of YSZ Films Produced by the Aerosol Deposition Method and by Screen-Printing.

Sensors (Basel) 2017 Jul 26;17(8). Epub 2017 Jul 26.

Department of Functional Materials, University of Bayreuth, Universitätsstraße 30, 95440 Bayreuth, Germany.

The pulsed polarization technique on solid electrolytes is based on alternating potential pulses interrupted by self-discharge pauses. Since even small concentrations of nitrogen oxides (NO) in the ppm range significantly change the polarization and discharge behavior, pulsed polarization sensors are well suited to measure low amounts of NO. In contrast to all previous investigations, planar pulsed polarization sensors were built using an electrolyte thick film and platinum interdigital electrodes on alumina substrates. Two different sensor layouts were investigated, the first with buried Pt electrodes under the electrolyte and the second one with conventional overlying Pt electrodes. Electrolyte thick films were either formed by aerosol deposition or by screen-printing, therefore exhibiting a dense or porous microstructure, respectively. For screen-printed electrolytes, the influence of the electrolyte resistance on the NO sensing ability was investigated as well. Sensors with buried electrodes showed little to no response even at higher NO concentrations, in good agreement with the intended sensor mechanism. Electrolyte films with overlying electrodes, however, allowed the quantitative detection of NO. In particular, aerosol deposited electrolytes exhibited high sensitivities with a sensor output signal Δ of 50 mV and 75 mV for 3 ppm of NO and NO₂, respectively. For screen-printed electrolytes, a clear trend indicated a decrease in sensitivity with increased electrolyte resistance.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s17081715DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5579518PMC
July 2017

Radio-Frequency-Based NH₃-Selective Catalytic Reduction Catalyst Control: Studies on Temperature Dependency and Humidity Influences.

Sensors (Basel) 2017 Jul 12;17(7). Epub 2017 Jul 12.

Bayreuth Engine Research Center (BERC), Department of Functional Materials, University of Bayreuth, 95447 Bayreuth, Germany.

The upcoming more stringent automotive emission legislations and current developments have promoted new technologies for more precise and reliable catalyst control. For this purpose, radio-frequency-based (RF) catalyst state determination offers the only approach for directly measuring the NH₃ loading on selective catalytic reduction (SCR) catalysts and the state of other catalysts and filter systems. Recently, the ability of this technique to directly control the urea dosing on a current NH₃ storing zeolite catalyst has been demonstrated on an engine dynamometer for the first time and this paper continues that work. Therefore, a well-known serial-type and zeolite-based SCR catalyst (Cu-SSZ-13) was investigated under deliberately chosen high space velocities. At first, the full functionality of the RF system with Cu-SSZ-13 as sample was tested successfully. By direct RF-based NH₃ storage control, the influence of the storage degree on the catalyst performance, i.e., on NO conversion and NH₃ slip, was investigated in a temperature range between 250 and 400 °C. For each operation point, an ideal and a critical NH₃ storage degree was found and analyzed in the whole temperature range. Based on the data of all experimental runs, temperature dependent calibration functions were developed as a basis for upcoming tests under transient conditions. Additionally, the influence of exhaust humidity was observed with special focus on cold start water and its effects to the RF signals.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s17071615DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5539605PMC
July 2017

Comparative Study of Different Methods for Soot Sensing and Filter Monitoring in Diesel Exhausts.

Sensors (Basel) 2017 Feb 18;17(2). Epub 2017 Feb 18.

Bayreuth Engine Research Center (BERC), Department of Functional Materials, University of Bayreuth, 95447 Bayreuth, Germany.

Due to increasingly tighter emission limits for diesel and gasoline engines, especially concerning particulate matter emissions, particulate filters are becoming indispensable devices for exhaust gas after treatment. Thereby, for an efficient engine and filter control strategy and a cost-efficient filter design, reliable technologies to determine the soot load of the filters and to measure particulate matter concentrations in the exhaust gas during vehicle operation are highly needed. In this study, different approaches for soot sensing are compared. Measurements were conducted on a dynamometer diesel engine test bench with a diesel particulate filter (DPF). The DPF was monitored by a relatively new microwave-based approach. Simultaneously, a resistive type soot sensor and a Pegasor soot sensing device as a reference system measured the soot concentration exhaust upstream of the DPF. By changing engine parameters, different engine out soot emission rates were set. It was found that the microwave-based signal may not only indicate directly the filter loading, but by a time derivative, the engine out soot emission rate can be deduced. Furthermore, by integrating the measured particulate mass in the exhaust, the soot load of the filter can be determined. In summary, all systems coincide well within certain boundaries and the filter itself can act as a soot sensor.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s17020400DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5336101PMC
February 2017

Compact Layers of Hybrid Halide Perovskites Fabricated via the Aerosol Deposition Process-Uncoupling Material Synthesis and Layer Formation.

Materials (Basel) 2016 Apr 8;9(4). Epub 2016 Apr 8.

Department of Functional Materials, University of Bayreuth, Bayreuth 95440, Germany.

We present the successful fabrication of CH₃NH₃PbI₃ perovskite layers by the aerosol deposition method (ADM). The layers show high structural purity and compactness, thus making them suitable for application in perovskite-based optoelectronic devices. By using the aerosol deposition method we are able to decouple material synthesis from layer processing. Our results therefore allow for enhanced and easy control over the fabrication of perovskite-based devices, further paving the way for their commercialization.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/ma9040277DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5502970PMC
April 2016

Influence of Oxygen Partial Pressure during Processing on the Thermoelectric Properties of Aerosol-Deposited CuFeO₂.

Materials (Basel) 2016 Mar 24;9(4). Epub 2016 Mar 24.

Department of Functional Materials, Zentrum für Energietechnik (ZET), University of Bayreuth, Bayreuth 95440, Germany.

In the field of thermoelectric energy conversion, oxide materials show promising potential due to their good stability in oxidizing environments. Hence, the influence of oxygen partial pressure during synthesis on the thermoelectric properties of Cu-Delafossites at high temperatures was investigated in this study. For these purposes, CuFeO₂ powders were synthetized using a conventional mixed-oxide technique. X-ray diffraction (XRD) studies were conducted to determine the crystal structures of the delafossites associated with the oxygen content during the synthesis. Out of these powders, films with a thickness of about 25 µm were prepared by the relatively new aerosol-deposition (AD) coating technique. It is based on a room temperature impact consolidation process (RTIC) to deposit dense solid films of ceramic materials on various substrates without using a high-temperature step during the coating process. On these dense CuFeO₂ films deposited on alumina substrates with electrode structures, the Seebeck coefficient and the electrical conductivity were measured as a function of temperature and oxygen partial pressure. We compared the thermoelectric properties of both standard processed and aerosol deposited CuFeO₂ up to 900 °C and investigated the influence of oxygen partial pressure on the electrical conductivity, on the Seebeck coefficient and on the high temperature stability of CuFeO₂. These studies may not only help to improve the thermoelectric material in the high-temperature case, but may also serve as an initial basis to establish a defect chemical model.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/ma9040227DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5502800PMC
March 2016

Correlating the Integral Sensing Properties of Zeolites with Molecular Processes by Combining Broadband Impedance and DRIFT Spectroscopy--A New Approach for Bridging the Scales.

Sensors (Basel) 2015 Nov 13;15(11):28915-41. Epub 2015 Nov 13.

Institute of Inorganic Chemistry (IAC) and Center for Automotive Catalytic Systems Aachen (ACA), RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany.

Zeolites have been found to be promising sensor materials for a variety of gas molecules such as NH₃, NOx, hydrocarbons, etc. The sensing effect results from the interaction of the adsorbed gas molecules with mobile cations, which are non-covalently bound to the zeolite lattice. The mobility of the cations can be accessed by electrical low-frequency (LF; mHz to MHz) and high-frequency (HF; GHz) impedance measurements. Recent developments allow in situ monitoring of catalytic reactions on proton-conducting zeolites used as catalysts. The combination of such in situ impedance measurements with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), which was applied to monitor the selective catalytic reduction of nitrogen oxides (DeNOx-SCR), not only improves our understanding of the sensing properties of zeolite catalysts from integral electric signal to molecular processes, but also bridges the length scales being studied, from centimeters to nanometers. In this work, recent developments of zeolite-based, impedimetric sensors for automotive exhaust gases, in particular NH₃, are summarized. The electrical response to NH₃ obtained from LF impedance measurements will be compared with that from HF impedance measurements, and correlated with the infrared spectroscopic characteristics obtained from the DRIFTS studies of molecules involved in the catalytic conversion. The future perspectives, which arise from the combination of these methods, will be discussed.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s151128915DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4701314PMC
November 2015

Conductometric Sensor for Soot Mass Flow Detection in Exhausts of Internal Combustion Engines.

Sensors (Basel) 2015 Nov 13;15(11):28796-806. Epub 2015 Nov 13.

Department of Functional Materials, Bayreuth Engine Research Center (BERC), University of Bayreuth, 95440 Bayreuth, Germany.

Soot sensors are required for on-board diagnostics (OBD) of automotive diesel particulate filters (DPF) to detect filter failures. Widely used for this purpose are conductometric sensors, measuring an electrical current or resistance between two electrodes. Soot particles deposit on the electrodes, which leads to an increase in current or decrease in resistance. If installed upstream of a DPF, the "engine-out" soot emissions can also be determined directly by soot sensors. Sensors were characterized in diesel engine real exhausts under varying operation conditions and with two different kinds of diesel fuel. The sensor signal was correlated to the actual soot mass and particle number, measured with an SMPS. Sensor data and soot analytics (SMPS) agreed very well, an impressing linear correlation in a double logarithmic representation was found. This behavior was even independent of the used engine settings or of the biodiesel content.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s151128796DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4701308PMC
November 2015

Why does the Conductivity of a Nickel Catalyst Increase during Sulfidation? An Exemplary Study Using an In Operando Sensor Device.

Sensors (Basel) 2015 Oct 23;15(10):27021-34. Epub 2015 Oct 23.

Department of Functional Materials, Zentrum für Energietechnik (ZET), University of Bayreuth, 95440 Bayreuth, Germany.

In order to study the sulfidation of a catalyst fixed bed, an in operando single pellet sensor was designed. A catalyst pellet from the fixed bed was electrically contacted and its electrical response was correlated with the catalyst behavior. For the sulfidation tests, a nickel catalyst was used and was sulfidized with H₂S. This catalyst had a very low conductivity in the reduced state. During sulfidation, the conductivity of the catalyst increased by decades. A reaction from nickel to nickel sulfide occurred. This conductivity increase by decades during sulfidation had not been expected since both nickel and nickel sulfides behave metallic. Only by assuming a percolation phenomenon that originates from a volume increase of the nickel contacts when reacting to nickel sulfides, this effect can be explained. This assumption was supported by sulfidation tests with differently nickel loaded catalysts and it was quantitatively estimated by a general effective media theory. The single pellet sensor device for in operando investigation of sulfidation can be considered as a valuable tool to get further insights into catalysts under reaction conditions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s151027021DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4634427PMC
October 2015

Microwave-Based Oxidation State and Soot Loading Determination on Gasoline Particulate Filters with Three-Way Catalyst Coating for Homogenously Operated Gasoline Engines.

Sensors (Basel) 2015 Sep 2;15(9):21971-88. Epub 2015 Sep 2.

Department of Functional Materials, Bayreuth Engine Research Center (BERC), Zentrum für Energietechnik (ZET), University of Bayreuth, 95440 Bayreuth, Germany.

Recently, a novel method emerged to determine the oxygen storage degree of three way catalysts (TWC) by a microwave-based method. Up to now, this method has been investigated only in lab-scale reactors or under steady state conditions. This work expands those initial studies. A TWC-coated gasoline particulate filter was investigated in a dynamic engine test bench simulating a typical European driving cycle (NEDC). It could be shown that both the oxygen storage degree and the soot loading can be monitored directly, but not simultaneously due to their competitive effects. Under normal driving conditions, no soot accumulation was observed, related to the low raw emissions and the catalytic coating of the filter. For the first time, the quality factor of the cavity resonator in addition to the resonance frequency was used, with the benefit of less cross sensitivity to inconstant temperature and water. Therefore, a temperature dependent calibration of the microwave signal was created and applied to monitor the oxidation state in transient driving cycles. The microwave measurement mirrors the oxidation state determined by lambda probes and can be highly beneficial in start-stop phases (where lambda-probes do not work) and to determine the oxygen storage capacity (OSC) without unnecessary emissions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/s150921971DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4610536PMC
September 2015