Calcium cobaltite (Ca3Co4O9) is considered as one of the most promising thermoelectric p-type oxides for energy harvesting applications at temperatures above 500 °C. It is challenging to sinter this material as its stability is limited to 920 °C. To facilitate a practicable and scalable production of Ca3Co4O9 for multilayer generators, a systematic study of the influence of powder calcination, Bi doping, reaction sintering, and pressure-assisted sintering (PAS) on microstructure and thermoelectric properties is presented. Batches of doped, undoped, calcined, and not calcined powders were prepared, tape-cast, and sintered with and without uniaxial pressure at 900 °C. The resulting phase compositions, microstructures, and thermoelectric properties were analyzed. It is shown that the beneficial effect of Bi doping observed on pressureless sintered samples cannot be transferred to PAS. Liquid phase formation induces distortions and abnormal grain growth. Although the Seebeck coefficient is increased to 139 μV/K by Bi doping, the power factor is low due to poor electrical conductivity. The best results were achieved by PAS of calcined powder. The dense and textured microstructure exhibits a high power factor of 326 μW/m K2 at 800 °C but adversely high thermal conductivity in the relevant direction. The figure of merit is higher than 0.08 at 700 °C.
Introduction The equilibrium of redox reactions between H2O and H2, CO and CO2 or water gas can be determined potentiometrically at high temperatures with a solid electrolyte cell (SEC) by establishing defined partial pressures of both partners of the redox couples according to Nernst equation [1]. The dependency of the potential on temperature allows to calculate thermodynamic parameters of the reaction from the equilibrium constant K p. The thermodynamic relations reveal that the measured cell voltage depends on partial pressures of the redox active gas components, cell temperature and the oxygen partial pressure in the reference gas. In this work, the results of experimental studies on the impact of gas convection on the equilibrium potential are presented and possible reasons for this behavior are discussed. Experimental The experimental setup is given schematically in Fig. 1. The flow rate of the redox gas is adjusted between 0 and 50 standard cm³ (sccm) by MFC. The purpose-built humidifier for establishing a constant water vapor partial pressure consists of six horizontally positioned glass tubes of around 10 cm length and 3 cm diameter sequentially connected with each other. The tubes are half filled with distilled and sterilized water and thermostated to 22.00 ± 0.01 K in a stirred water bath. The test gas pass the tubes in a laminar flow above the water filling and is saturated with water vapor avoiding aerosol formation. A downstream capacitive dew point sensor was used to verify the accuracy of the humidification. Over a wide temperature and concentration range, the water vapor dissociation is negligible so that the H2/H2O ratio set at room temperature remains practically unchanged in the heated solid electrolyte cell [2]. Different self-developed and commercial YSZ cells equipped with Pt electrodes of different morphologies were investigated by measuring redox potentials in H2/H2O and CO/CO2 mixtures with various dilution. The oxygen partial pressure of the reference gas air was monitored by continuous measurement of dew point and total pressure. Results The redox equilibrium in H2/H2O mixtures is established at catalytically highly active Pt electrodes in YSZ cells already at relatively low temperatures below 600 °C [3]. In Figure 2a the course of the cell voltage measured in such a mixture is given for different temperatures. At the adjusted H2/H2O partial pressure ratio near 1 and absolute values of component partial pressures around 3000 Pa, small oxygen leakages will not influence the measured potential [2]. The most influencing deviations on the measured voltages are related to temperature measurement and control of the YSZ cell. These are assumed to be responsible mainly for the noise visible in the curve in Fig. 2a. The systematic deviation known from thermocouple application probably causes the small difference of the measured cell voltage from values, which are expected from thermodynamic data (NIST). If the complete voltage difference Δ in Fig 2b is attributed to temperature deviation of the used thermocouple (type B), this parameter amounts to around 11 K higher than it is indicated by the set point value of the controller. Other sources of deviations can arise from asymmetric potentials of the YSZ cell resulting in measurable voltages at equal oxygen partial pressures on both sides of the cell. In order to obtain indications of such deviations, the flow rate and the dilution of the redox mixtures were varied. The curves in Fig. 3 show that especially the flow rate has surprisingly high influence on the cell voltage. After switching off the gas flow the cell voltage decreases abruptly to values 6-10 mV lower than measured in the constant gas flow. The following drift continues this decrease. Immediately after switching on the flow, the cell voltage jumps back to values before switch-off. At flow rates below 10 sccm, this switch-off shift decreases significantly. At all investigated cells, the switch-off shift increases with decreasing dilution of the redox mixture and increasing temperature, reaching its maximum in pure H2/H2O mixtures at the investigated temperature maximum. In the contribution, the signal influencing processes like gas flow cooling of the electrode, flow induced pressure increase, convective diffusion in pores and oxygen exchange between electrode/electrolyte and gas [4] will be discussed and different scenarios will be compared. Literature R. Hartung, H.-H. Möbius, Potentiometrische Bestimmung des Wasserdampf Dissoziationsgleichgewichtes zwischen 1000 und 1300 K mit einer Festelektrolytzelle, Chemie Ingenieur Technik 40 (12), 592–600 (1968); DOI: 10.1002/cite.330401209. H.-H. Möbius, Solid-state electrochemical potentiometric sensors for gas analysis, In: Göpel, W. Hesse, J. Zemel, J.N.: Sensors. A Comprehensive Survey, Volume 3: Chemical and Biochemical Sensors Part II. Weinheim, New York: VCH-Wiley, 1104–1154 (1995). R. Hartung, H.-H. Möbius, Zur unteren Begrenzung der Arbeitstemperatur galvanischer Sauerstoff-Meßzellen mit Zirconiumdioxid-Festelektrolyten Z. Chem. 9, 197-198 (1969). M. Schelter, J. Zosel, V. Vashook, U. Guth, M. Mertig, Electrolyte related parameters of coulometric solid state devices, Solid State Ionics 288, S. 266–270 (2016); DOI: 10.1016/j.ssi.2016.01.020. Figure 1
Introduction Nitrogen oxides (NOx; NO and NO2) are limited emissions from combustion processes. They are not only harmful to human health, but also to the environment. This makes it necessary to measure and reduce nitrogen oxide emissions. Pulse polarization is a novel method for NOx concentration measurements. In contrast to existing static principles, this method utilizes the dynamic response of the sensor, similar to cyclic voltammetry or impedance spectroscopy. For pulse polarization, the sensor is polarized with a constant voltage U pol for t pol (Fig. 1). After applying the voltage, the self-discharge of the sensor is recorded over a defined time t discharge. Charging and discharging phases are repeated continuously, with alternating change of the charging voltage polarity. It has been shown that NOx selectively accelerates the discharge of the Pt|YSZ system [1]. The accelerated discharge can be used as a sensor signal by evaluating the voltages at a fixed time during the discharge phase. Due to the faster discharge, these voltages are below the values without nitrogen oxides and thus indicate the concentration of the analyte gas. A semi-log dependency between U and c NOx was found. However, the effects that lead to faster and selective discharge have not yet been fully understood. In order to investigate the effect of the catalytically active Pt electrodes in particular, they were replaced by gold electrodes, which should yield a significantly lower catalytic impact. Experimental To prepare the sensors, two rectangular gold electrodes were screen printed on both sides of an 8YSZ substrate and then fired at 850 °C. The sensors were contacted with Au wires by gap welding. The sensors were operated at 400 °C in a tube furnace. For pulse polarization, a sourcemeter was periodically connected to the sensors via relays and the voltages were recorded. A polarization voltage of U pol = 1 V, a polarization duration t pol = 1 s and a discharge time t discharge = 10 s were chosen. This lead to a total cycle time t cycle = 22 s. To determine the gas concentration dependence in the discharge phase, a mixture of 10 % O2 with 2 % H2O in nitrogen was defined as base gas. In addition, NO, NO2, and a mixture of 50 % NO and NO2 (NOx) in concentrations between 50 and 200 ppm were added to the gas flow. Results and discussion The sensor signals are shown in Fig. 2. The voltages U 4s_neg shown were measured during self-discharging 4 s after each negative polarization. The long measuring time of 18 h and the cycle duration of 22 s result in over 2900 cycles during the measurement. The baseline of the voltage curve shows that the cycles are very stable during the entire measuring period. It is also noticeable that NO and NO2 have an opposite effect on the self-discharge of the sensor. While NO2 as well as a 50/50 mixture of NO and NO2 accelerate the discharge, NO gas decelerates it. This means, the resulting absolute value of U 4s_neg is lower than that in NOx-free gas in case of accelerated discharge and higher at decelerated discharge. The mixture of 50% NO and 50% NO2 has an accelerating effect. The accelerated self-discharge in the presence of NO2 was already observed for platinum electrodes. In contrast to this, the discharge decelerated by NO as found here for gold electrodes has not yet been observed on Pt electrodes. We attribute the behavior on platinum electrodes to the gas phase reactions that occur during diffusion through the electrode and to the assumption that the gases are almost in thermodynamic equilibrium at the three-phase contact between electrode, electrolyte and gas phase [2]. At a temperature of 400 °C and an oxygen content of 10 %, this equilibrium is about 50 % NO and 50 % NO2. This would result in conditions similar to those for NOx to be dosed, which also accelerates the discharge. The equilibrium thus explains the same signal for NO and NO2. The catalytically less active gold electrodes make it possible to separate the influence of NO and NO2. The strong oxidizing effect of NO2 [3] is expected to play a key role in the sensor effect. By applying the voltages, oxygen is pumped from the cathode to the anode. This leads to a lack of oxygen at the cathode and an excess at the anode. After polarization, NO2 probably supplies the electrode, which is depleted of oxygen due to polarization, with additional oxide. This helps to reduce the oxygen gradient and thus leads to an accelerated discharge. In contrast, NO seems to slow down the oxygen supply at the oxygen depleted electrode and thus decelerate the discharge. The removal of oxygen at the oxygen-rich electrode seems to play a minor role. If both the oxygen supply on one side and the oxygen removal on the other were equally important, NOx would provide the strongest acceleration of the discharge. References [1] N. Donker, et al., Influence of Pt paste and the firing temperature of screen-printed electrodes on the NO detection by pulsed polarization, J. Sens. Sens. Syst., 293–300 (2020); doi: 10.5194/jsss-9-293-2020. [2] T. Ritter, et al., On the influence of the NOx equilibrium reaction on mixed potential sensor signals: A comparison between FE modelling and experimental data, Sens. Actuators B, 126627 (2019); doi: 10.1016/j.snb.2019.126627. [3] D. Bhatia, et al., Experimental and kinetic study of NO oxidation on model Pt catalysts, J. Catal., 106–119 (2009); doi: 10.1016/j.jcat.2009.05.020. Figure 1
Introduction Gas chromatography (GC) is an important tool for the analysis of trace gases. In a chromatography column, the components of the sample are completely separated from a mixture of substances. Individual gas pulses occur at the retention times of the respective gas species [1]. These are converted afterwards by a GC detector into an electrical sensor signal and a chromatogram originates It shows the detector signal, means the analyte-depending concentration peaks, versus analysis time. To achieve the total amount of each substance, i.e., for quantitative analysis, the detector signal must be timely integrated. For small peaks that are difficult to distinguish from background noise and that are in the range of the detection limit, peak integration is often subject to errors [2]. A concept for a novel kind of detector is presented in this study, where the timely integral of the concentration can be determined directly, without mathematical integration. The concept of an impedimetric gas dosimeter, that measures directly the amount of a certain gas species is evaluated as a GC detector [3]. The concept is validated with cancerogenic epichlorohydrin serving as a model substance. Dosimeter concept The gas dosimeter concept consists of two phases, the sorption and the regeneration phase (Fig. 1). During sorption, the analyte is sorbed on the functional material and simultaneously the electrical properties change linearly. If the concentration of the model substance is zero, the signal remains constant and no desorption should occur. The sensor signal increases as soon as the substance is available. Then, the change of the sensor signal corresponds directly to the amount of the analyte and the time derivative is determined by the concentration of the substance. After a certain sorption state is reached, a regeneration, e.g. thermally or by UV light, is necessary to reset the signal [3, 4]. The dosimeter concept itself is already proven for detection of NOx and NO2 at high working temperatures and even for devices operated at room temperature [3, 4]. Materials and Methods For the purpose of detection of epichlorohydrin and to proof the concept of gas dosimeters as GC detectors, we focused on room temperature operation. It has been reported in [5] that copper-containing zeolites change their electrical properties during epichlorohydrin sorption. We investigated different MFI-type zeolites, e.g. one with M = 200 and 0.27 wt-% Cu. They were applied by screen-printing on alumina substrates equipped with interdigitated Au electrodes and fired at 650 °C. The sensors were mounted in a gas purgeable tube and were exposed to dry synthetic air. Epichlorohydrin was added in the ppm-range by special experimental setups. The electrical properties are measured by impedance spectroscopy (f = 1 MHz, U eff = 1 V). Since zeolites are poorly conducting in dry atmospheres, the capacitance C was determined. As sensor signal, the relative capacitance change (C-C 0)/C 0 was calculated, with C 0 being the capacitance in base gas and C the capacitance during epichlorohydrin sorption. Results and Discussion Initial results of zeolite MFI 200 with 0.27 wt-% Cu are shown in Fig. 2. The relative capacitance change (C-C 0)/C 0 (given in ppm) during exposure to 16.5 ppm epichlorohydrin is plotted versus time. The sensor signal shows dosimeter-type behavior, meaning that the signal increases linearly during epichlorohydrin exposure and remains constant after the dosing pulse. No desorption effects are visible. In real application behind a GC column, a typical concentration peak is plotted schematically in Fig. 3. The expected sensor signal of a gas dosimeter shows, that the sensor signal increases directly depending on the current concentration (= slope of the curve) and remains at a constant sensor signal after the analyte peak, which corresponds directly to the total amount of the analyte. The dosimeter signal of MFI 200 with 0.27 wt-% Cu was determined in this operation mode, too. Epichlorohydrin was sorbed at an activated carbon filter for a certain loading time (40 min) and was pulse-like thermally desorbed (P = 250 W) afterwards. The dosimeter results, the sensor signal (C-C 0)/C 0, and the corresponding time derivative, are shown in Fig. 4. The filter was unloaded twice (100 and 400 s) and the dosimeter shows a clear signal change which depends on the desorbed amount of epichlorohydrin. The shape of the time derivative corresponds approximately to a typical concentration peak after a GC column. Conclusions This study shows, that a sensitive material for detection of epichlorohydrin with dosimeter-type characteristics at room temperature was found. Therefore, the potential exists to develop a detector for gas chromatography, whereby the sensor detects directly the total amount of an analyte peak downstream of a GC column. References [1] C.A. Cramers et al., High-speed gas chromatography: an overview of various concepts, J. Chromatography. A 856 (1999) 315–329. [2] K. Dettmer-Wilde et al., Quantitative Analysis, in: K. Dettmer-Wilde, W. Engewald (Eds.), Practical Gas Chromatography, Springer Berlin Heidelberg, Berlin, Heidelberg, 2014, pp. 271–302. [3] I. Marr et al., Resistive NOx dosimeter to detect very low NOx concentrations - Proof-of-principle and comparison with classical sensing devices, Sens. Act. B: Chem., 248, 848–855 (2017) [4] R. Wagner et al., Novel Operation Strategy to Obtain a Fast Gas Sensor for Continuous ppb-Level NO2 Detection at Room Temperature Using ZnO-A Concept Study with Experimental Proof, Sensors 19 (2019) 4104. [5] D. Kalló, Applications of Natural Zeolites in Water and Wastewater Treatment, in: D.L. Bish (Ed.), Natural zeolites: Occurence, properties, applications, Mineralogical Society of America, Washington, DC, 2001, pp. 519–550. 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
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