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Differential thermal analysis (DTA) was used to examine the effect of the particle size and morphology of cobalt (III/IV) oxide (Co3O4) on its thermal response under exposure to methane (1 vol % in dry synthetic air), which is a relevant gas for the detection of combustible gases. The DTA response results from the catalytic oxidation of methane, and its characteristics should correlate with the pellistor response. Co3O4 samples differing in particle size and morphology were produced by ball milling (top-down technique) or were synthesized from precursor molecules by precipitation (bottom-up technique). The investigations carried out in dry air and a temperature range between 250 and 450 ∘C reveal that both particle size and particle shape have a considerable effect on thermal response, since the resulting layer structures and the associated surface area available for gas interaction differ. The Co3O4 catalyst, with small particles and an irregular shape, exhibits significantly higher response than milled Co3O4 samples. Comparison of DTA with the mass spectroscopy signal of CO2 evolved by the reaction verified a certain analogy between DTA measurements and the response produced by a pellistor.

Because diesel combustion processes produce harmful detrimental nitrous oxides, the selective catalytic reduction, an after-treatment method using diesel exhaust fluid (AdBlue) to reduce these emissions, is an important part in the cycle of the combustion process. Therefore, it is crucial to continuously monitor the quality of the diesel exhaust fluid to secure the ideal selective catalytic reduction. This article presents a platinum thin-film sensor using the 3ω method which is able to characterize the diesel exhaust fluid. By means of the 3ω method, information about the concentration of urea in water can be extracted. In this investigation, a digital lock-in amplification technique is used to execute the measurements. The results show that this sensor can determine the urea content within 1 % by weight. Moreover, besides the analysis of the 3ω signal, the 1ω signal is analyzed in depth to receive additional information about the temperature. Because the same structure can measure multiple parameters, such as concentration, temperature, and flow, the sensor might be a good alternative to the state-of-the-art diesel exhaust fluid sensor.

Sensors are key elements for capturing environmental properties and are today indispensable in the industry for monitoring and control of industrial processes. Many applications are demanding for highly integrated intelligent sensors to meet the requirements on safety, clean, and energy-efficient operation, or to gain process information in the context of industry 4.0. While in many everyday objects highly integrated sensor systems are already state of the art, the situation in an industrial environment is clearly different. Frequently, the use of sensor systems is impossible due to the fact that the extreme ambient conditions of industrial processes like high operating temperatures or strong mechanical loads do not allow a reliable operation of sensitive electronic components. Eight Fraunhofer Institutes have bundled their competencies and have run the Fraunhofer Lighthouse Project “eHarsh” to overcome this situation. The project goal was to realize sensor systems for extremely harsh environments, whereby sensor systems are more than pure sensors, rather these are containing one or multiple sensing elements and integrated readout electronics. Various technologies, which are necessary for the realization of such sensor systems, have been identified, developed, and finally bundled in a technology platform. These technologies are, e.g., MEMS and ceramic-based sensors, SOI-CMOS-based integrated electronics, board assembly and laser-based joining technologies. All these developments have been accompanied by comprehensive tests, material characterization, and reliability simulations. Based on the platform, a pressure sensor for turbine applications has been realized to prove the performance of the eHarsh technology platform.

The detection of flammable gases is necessary to avoid explosive atmospheres. For this reason, low-cost pellistors are frequently used. However, such commercial pellistors require an operation temperature of 450 ∘C or more for the detection of methane and a correspondingly high power consumption. We present a novel wireless low-power catalytic gas sensor system based on non-precious metal catalyst for the detection of methane and propane operated at 350 ∘C. The combination of a microelectromechanical system (MEMS)-based sensor with a low-power radio system provides the opportunity to monitor complex infrastructure without using a power grid as power supply. The sensor system has been characterised extensively under the exposure to methane and propane at concentrations between 2000 and 8000 ppm, as these gases are the common test gases for pellistors in industry. Methane is the main component of natural gas; propane is an important component of liquified petroleum gas (LPG). In addition, the influence of changes in humidity on the sensor response to methane was examined in more detail. Due to the planned operation of the sensor and radio system in different application scenarios, short (3 s) and long (60 s) sampling rates were used for investigations.

This paper presents a platinum thin film sensor able to investigate the thermal properties of a fluid whereas the sensing element is protected against corrosion by an intermediate stainless-steel wall. Progress claims include: 1) the 3-omega method is applied to investigate fluids which are not directly adjacent to the sensitive layer, 2) the 3-omega-method is applied for a meander structure, 3) a theoretical model is developed and agrees well with the experiment. These findings can have a key role for future stand-alone sensors measuring thermal parameters, or for sensors which combine several measurement principles, e.g. the 3-omega method for thermal parameter detection and the thermal flow meters for flow speed measurements.

This article presents a platinum thin-film sensor to distinguish fluids via their thermal properties by applying the 3 ω -method. The thin film element is attached to a stainless steel tube piping the fluid. The fluids of interest in this article are gas (air), water, and alcohol (isopropanol). The progress claims include: First, the 3 ω -method is applied to distinguish fluids which are not directly in contact with the sensitive structure. Second, the 3 ω -method is applied on a meander structure instead of the commonly used straight strip. These two features should lead to high flexibility, high corrosion resistance and high hygiene and safety standards in applications. Third, because of the complexity of the sensor structure, a simplified analytical model is elaborated and a numerical simulation is conducted to obtain a profound understanding of the sensor's behavior. Based on these results, the sensor is designed and manufactured by standard thin film technology. The agreement between experiment and simulation results is within 5%. These findings can play a key role for future in-line sensors in process control systems e.g. in the food industry. Moreover, a deep understanding of the sensor's physical behavior with respect to its thermal properties can lead to a feasible combination of the 3 ω -method and thermal anemometry to achieve a robust thermal flow sensor, which is able to compensate the dependence on the thermal properties of different fluids.

Low temperature thermoelectric (TE) materials are in demand for more efficient cooling and power generation applications. Iron antimonide (FeSb2) draws great attention over the past few years because of its enhanced power factor values. Polycrystalline bulk FeSb2 nanopowder was prepared via a low-temperature molten salts approach followed by subsequent thermal treatment in synthetic air and hydrogen gas for calcination and reduction reactions, respectively. Structural analysis confirms the desired final phase with submicrometer grain size and high compaction density after consolidation using spark plasma sintering (SPS). TE transport properties revealed that the material is n-type below 150 K and p-type above this temperature; this suggests antimony vacancies in FeSb2. The electrical conductivity increased significantly, and the highest conductivity achieved was 6000 S/cm at 100 K. The maximum figure-of-merit, ZT, of 0.04 is achieved at 500 K, which is about 6 times higher than the earlier reported state-of-the art ZT value for the same material.

Introduction The early detection of explosive gas atmospheres is highly relevant for industrial process measurement technology and avoids the endangerment of people. In the field of safety technology, catalytic combustion sensors, so-called »pellistors«, are used to detect flammable gases such as hydrocarbons or hydrogen. Pellistor sensors measure the heat produced from catalytic oxidation of the gas by detecting the resistance alteration of a Pt-based heater induced by temperature changes using a Wheatstone bridge circuit. The heat generated is related to the kind and concentration of gas by specific combustion enthalpy [1]. The major advantages of catalytic sensors are simple operation principle, easy installation and calibration. However, state-of-the-art pellistors have some disadvantages, such as high operation temperatures of over 400°C, high-power consumption and high susceptibility to catalyst poisons. High-power consumption limits the usage of pellistors in mobile applications because of the short battery lifetime. Reducing operating temperature will contribute to decrease the power consumption. The primary gas recognition element of the pellistor is the catalytic layer. To reduce the operation temperature high active catalysts are required. Especially for detection of methane, which is one of the most inert combustible gases, catalysts of high activity or particularly high working temperatures of at least 450°C are mandatory. For this reason, a reliable detection of methane is the most important challenge. Catalysts implemented in nowadays pellistors were developed already in 1960s and contain Pd or/and Pt nanoparticles stabilized on a porous metal oxide support in its active chemical and physical state. Alumina and, to a lesser extent, zirconia are commonly used due to its high surface area and thermal stability [2]. However, some reports show that the application of metal oxides like CeO2 as support can lead to an improved sensor performance due to high oxygen storage and release ability [3]. The researches in the field of catalytic combustion provided many further evidences that especially metal oxides with spinel structures as Co3O4, NixCo3-xO4, Co3-xCuxO4, Co3-xZnxO4 etc., can contribute to catalytic combustion of methane and therefore considerably decrease the temperature for catalytic reaction [4]. New catalysts have the potential to considerably improve the performance of pellistors. This research focuses on the development of alternative, highly active catalysts based on spinels for catalytic detection of methane in the low-temperature range. Catalytic activity was investigated by Simultaneous Thermogravimetry-Differential Thermal Analysis (TG-DTA, termed as STA) coupled with Quadrupole Mass Spectrometer (STA-QMS). Method STA (NETZSCH, STA 409 CD-QMS 403/5 SKIMMER) is a calorimetric method to monitor the heat consumption (endothermic) or release (exothermic DTA signal) occurring by different processes. Heat release during exothermic oxidation reaction is measured by comparison the temperature of sample placed in an alumina crucible with that of reference crucible after calibration and signal normalization. In contrast to a pellistor, in which each element of the sensor including electronics, contacts and catalytic layer, etc. influences the formation of the sensing signal, STA shows unaffected thermal signal originating from catalytic reaction. Additionally, the complex preparation steps as in case of pellistor can be avoided. Thermal analysis like DSC [5] and DTA ensure comparable conditions for appropriate testing and investigation of different catalytic materials. QMS allows the analysis of gaseous residues and reaction products by ionization. For catalyst investigation, Co3O4 spinel doped with nickel (synth. NiCo2O4) and palladium (4wt% Pd/synth. Co3O4) were synthesized according to [4]. For comparison, commercial Co3O4 (comm. Co3O4) was used. Results and Conclusions Co3O4 spinel and his derivatives have been chosen for the investigation of catalytic activity. The thermal behavior of investigated oxides in synthetic air (reference) and in 1 vol% CH4 is shown in Fig. 1a. The mass spectrum presented for mass ion m/z of 44 for NiCo2O4 (Fig. 1b) confirms the CO2 evolution by CH4 injection into the chamber as a result of catalytic reaction. Figure 1: (a) DTA signal of four investigated catalysts at four temperatures (red line) measured successively in synthetic air (gray background) and 1vol% CH4 (green background); (b) simultaneously recorded mass spectrum (STA-QMS) of mass ion 44 from the NiCo2O4 sample. Spinel NiCo2O4 and 4wt% Pd/synth. Co3O4 (Fig. 1a) reveal first traces of activity already at 250°C, while undoped commercial Co3O4 shows a minor signal at 300°C first. Certainly, with temperature increasing the behavior changes. Consequently, NiCo2O4 is a preferred catalyst for low operation temperature. It shows higher activity at lower temperatures, whereas at higher temperatures (450°C) Pd doped Co3O4 catalysts prevail in catalytic activity. Especially doped NiCo2O4 spinel exhibited excellent catalytic activity towards methane oxidation. The structure-properties relation will be presented and discussed. References [1] J.B.Miller, Catalytic Sensors for Monitoring Explosive Atmospheres. IEEE Sens J. 1 (2001) 88–93. doi:10.1109/JSEN.2001.923591 [2] G.Korotcenkov, Handbook of Gas Sensor Materials; Springer, Vol. 1 (2014). doi.org:10.1007/978-1-4614-7165-3 [3] Y.Wang, M.M.Tong, D.Zhang, Z.Gao, Improving the Performance of Catalytic Combustion Type Methane Gas Sensors Using Nanostructure Elements Doped with Rare Earth Cocatalysts. Sensors. 11 (2011) 19–31. doi:10.3390/s110100019. [4] F.Tao, J.Shan, L.Nguyen, et al. Understanding complete oxidation of methane on spinel oxides at a molecular level. Nat Commun. 6 (2015) 7798. doi:10.1038/ncomms8798 [5] J.Loskyll, W.F.Maier, K.Stoewe. Application of a Simultaneous TGA-DSC Thermal Analysis System for High-Throughput Screening of Catalytic Activity. ACS Comb. Sci. 14 (2012) 600−604. doi:10.1021/co3000659. Figure 1

Latest nanotechnology concepts applied in thermoelectric (TE) research have opened many new avenues to improve the ZT value. Low dimensional structures can improve the ZT value as compared to bulk materials by substantial reduction in the lattice thermal conductivity, κL. However, the materials were not feasible for the industrial scale production of macroscopic devices because of complicated and costly manufacturing processes involved. Bulk nanostructured (NS) TEs are normally fabricated using a bulk process rather than a nano-fabrication process, which has the important advantage of producing in large quantities and in a form that is compatible with commercially available TE devices.We developed fabrication strategies for bulk nanostructured skutterudite materials based on FexCo1-xSb3. The process is based on precipitation of a precursor material with the desired metal atom composition, which is then exposed to thermochemical processing of calcination followed by reduction. The resultant material thus formed maintains nanostructured particles which are then compacted using Spark Plasma Sintering (SPS) by utilizing previously optimized process parameters. Microstructure, crystallinity, phase composition, thermal stability and temperature dependent transport property evaluation has been performed for compacted NS FexCo1-xSb3. Evaluation results are presented in detail, suggesting the feasibility of devised strategy for bulk quantities of doped TE nanopowder fabrication.

Skutterudites are known to be efficient thermoelectric (TE) materials in the temperature range from 600 K to 900 K. Dimensionless figure of merit (ZT) for filled skutterudite TE materials have been reported as ca. 1 at 800 K. Novel nano- engineering approaches and filling of the skutterudites crystal can further improve the transport properties and ultimately the ZT. Although classified among the promising TE materials, research on their large-scale production via bottom up synthetic routes is rather limited. In this work, large quantity of cobalt antimonide (CoSb3) based skutterudites nanopowder (NP) was fabricated through a room temperature co-precipitation precursor method. Dried precipitates were process by thermo-chemical treatment steps including calcination (in air) and reduction (in hydrogen). CoSb3 NPs were then mixed with silver (Ag) nanoparticles at different weight percentages (1%, 5% and 10% by wt) to form nanocomposites. Skutterudite NP was then consolidated by Spark Plasma Sintering (SPS) technique to produce highly dense compacts while maintaining the nanostructure. Temperature dependent TE characteristics of SPS’d CoSb3 and Ag containing nanocomposite samples were evaluated for transport properties, including thermal conductivity, electrical conductivity and Seebeck coefficient over the temperature range of 300 - 900 K. Physicochemical, structural and microstructural evaluation results are presented in detail.

(Bi,Sb)2Te3 + 4 mol%PbTe was quenched in water and on a rotating copper wheel (melt spinning). It was found that PbTe was immiscible in (Bi,Sb)2Te3 when the material is quenched in water and that the thermoelectric figure of merit increases by annealing. Natural nanostructures (nns) were found in melt-spun (Bi,Sb)2Te3, whereas they were hard to detect in (Bi,Sb)2Te3 alloyed with PbTe. There is a correlation between the orientation of the strain field and the nns. Within the grains of melt-spun (Bi,Sb)2Te3 alloyed with PbTe, the chemical composition was homogeneous. An enrichment of Pb was found at the grain boundaries. Quenched (Bi,Sb)2Te3 alloyed with 0.3 wt%PbTe have been spark plasma sintered (SPS). After optimization, the Seebeck coefficients of the melt-spun SPS (MS-SPS) materials were larger than for materials quenched in water and sintered (QW-SPS) materials. In addition, the mobility increases with the carrier concentration in MS-SPS materials, whereas it decreases in QW-SPS materials.

Zusammenfassung Der Einsatz energieautarker Sensorik bietet die Möglichkeit, wartungsfrei Sensoren über einen sehr langen Zeitraum an unzugänglichen Orten zu betreiben. Durch den Einsatz thermoelektrischer Generatoren wird die Verwendung energieautarker Systeme überall da ermöglicht, wo Temperaturgradienten mit wenigen Kelvin existieren. In diesem Artikel wird der Einsatz eines durch Thermogeneratoren versorgten energieautarken Systems für den Einsatz im Flugzeug vorgestellt und die Minimierung thermischer Widerstände als wesentlicher Punkt für eine maximale Leistungsausbeute diskutiert.

Thermoelectric device performance is determined by not only the properties of the thermoelectric material but also the geometrical design and thermal matching of the materials. Leg length and contact quality strongly influence thermoelectric generator efficiency. Experimental results for contact properties are compared with the latest performance measurements on modules manufactured from Bi2Te3 compounds. Module performance is related to the obtained contact resistance and thermoelectric material properties. The different influences are studied using thermoelectric multiphysics finite-element modeling of examples where, in addition to the thermoelectric field equations, further effects such as convection and radiation as well as the temperature dependency of the material properties are taken into account. Extensive thermoelectric device modeling is used to understand the experimental findings with respect to contact properties and geometry.

Besides half-Heusler compounds, skutterudites, and lead salts, silicides (especially higher manganese silicides and magnesium silicides) are very promising thermoelectric materials regarding medium temperature applications. Higher manganese silicides and magnesium silicides offer suitable ZT values and are synthesized of non-toxic, cheap and abundant raw materials. In this paper, different contact fabrication strategies for silicides and progress on the development of modules based on silicides for medium temperature application up to 550 °C are discussed. Therefore, several contact materials, brazing solders as well as brazing and soldering methods were investigated. Thermoelectric modules with one to 31 unicouples and a maximum size of 50 mm × 50 mm were assembled. The thermoelectric modules for automotive applications were characterized and compared. Their performances are presented as well as cycling and stability test results.

A novel synthesis route for the fabrication of p-type nanostructured skutterudite, FexCo1-xSb3 in large quantity is reported. This scalable synthesis route provides nano-engineered material with less impact on the environment compared to conventional synthesis procedures. Several Fe substituted compositions have been synthesized to confirm the feasibility of the process. The process consists of a nano-sized precursor fabrication of iron and cobalt oxalate, and antimony oxides by chemical co-precipitation. Further thermochemical processes result in the formation of iron substituted skutterudites. The nanopowders are compacted by Spark Plasma Sintering (SPS) technique in order to maintain nanostructure. Detailed physicochemical as well as thermoelectric transport properties are evaluated. Results reveal strongly reduced thermal conductivity values compared to conventionally prepared counterparts, due to nanostructuring. P-type characteristic was observed from the Seebeck measurements while electrical conductivity is high and shows metallic behavior. The highest TE figure of merit of 0.25 at 800 K has been achieved, which is strongly enhanced with respect to the mother compound CoSb3. This suggests the promise of the utilized method of fabrication and processing for TE applications with improved performance.

This paper describes the first thermoelectric devices based on the V-VI-compounds Bi/sub 2/Te/sub 3/ and (Bi,Sb)/sub 2/Te/sub 3/ which can be manufactured by means of regular thin film technology in combination with microsystem technology. Fabrication concept, material deposition for some 10-/spl mu/m-thick layers and the properties of the deposited thermoelectric materials will be reported. First device properties for Peltier-coolers and thermogenerators will be shown as well as investigations on long term and cycling stability. Data on metal/semiconductor contact resistance were extracted form device data. Device characteristics like response time for a Peltier-cooler and power output for a thermogenerator will be compared to commercial devices.

Die vorliegende Publikation fasst die Ergebnisse des bmb+f-Pilotprojekts NanoCal zusammen. Ziel war die Erforschung, der Aufbau und der Test eines hochempfindlichen, schnellen Differential-Multiparameter-Nanokalorimeters beispielsweise für Untersuchungen der Faltungsdynamik von Proteinen und von Relaxationszeiten in Lipidmembranen. Dabei wurden die Grundlagen für die Entwicklung eines mikrostrukturierten Kalorimeterchips gelegt, welcher ähnlich empfindlich wie moderne Kalorimeter ist, durch die Miniaturisierung jedoch nicht nur deutlich kleinere Analytmengen zur Vermessung benötigt, sondern auch eine geringere Ansprechzeit aufweisen soll. Es wurden erste Demonstratorkalorimeter auf 4"-Wafer-Basis hergestellt. Die durch das thermische Rauschen begrenzte Temperaturauflösung lag bei ca. 75 μK. Dies entspricht einer Leistungsauflösung von ungefähr 2 μW. Aus den gewonnenen Daten lässt sich, da der technologische Weg nun vorgezeichnet ist, über ein Redesign der Kalorimeterlayouts die Leistungsfähigkeit der Kalorimeterchips weiter verbessern.

Antimony telluride, (Sb2Te3), and its doped derivatives are considered to be among the best p-type thermoelectric (TE) materials for room temperature (300–400 K) applications. However, it is still desirable to develop rapid and economical routes for large-scale synthesis of Sb2Te3 nanostructures. We report herein a high yield, simple and easily scalable synthetic method for polycrystalline Sb2Te3 nanostructures. Prepared samples were compacted into dense pellets by use of spark plasma sintering. The products were characterized by x-ray diffraction and scanning electron microscopy. To investigate the anisotropic behavior of Sb2Te3 TE transport property measurements were performed along and perpendicular to the direction of compaction. Thermal conductivity, electrical conductivity, and Seebeck coefficient measurement over the temperature range 350–525 K showed that the anisotropy of the material had a large effect on TE performance.

Sensors are the eyes and ears in the service of people - especially in inaccessible areas where regular maintenance or battery replacement is extremely difficult. By using thermoelectric generators, which are capable of directly converting heat flux into electrical energy, self-powered sensor systems can be established wherever temperature differences of a few Kelvin exist. After installation of the sensors, they collect and transmit their data without any need for further maintenance like battery replacement. Intelligent building automation for instance is the key for significant energy reduction in buildings. Through precise control of sun blinds and set temperature for thermostats and air conditioning, radio signal sensors help to increase a building's efficiency massively. Thermoelectric self-powered sensors have the additional potential to introduce more flexibility in the area of building technology as complex wiring is avoided. Buildings can easier be adapted to altered utilization. Structural health monitoring is another field where energy autarkic sensors could be of vital use. In-situ measurements of e.g. temperature, humidity, strain and cracks of buildings are essential in order to determine the condition of construction materials. Respective sensors are hardly accessible, wiring or battery replacement is costly or even impossible. Sensors that are driven by thermoelectric generators are maintenance-free and can help enhancing the longevity of buildings as well as reducing the maintenance costs. Furthermore, leakages in water transport systems can be reduced by in-situ monitoring by self-powered sensors and thus, reduce unnecessary water losses. The large progress in the development of low-power sensors, power management and radio communication combined with the availability of high efficiency thermoelectric generators opens the possibility to run a self-powered sensor node with temperature gradients as low as 0.8K. This potential will be presented with respect to selected fields of application.

A novel gas sensor device is presented. Its configuration resembles a “normally on” n-type thin film transistor (TFT) with a gas sensitive metal oxide as its channel. Without a gate voltage, the device works as a conventional metal oxide gas sensor. Applied gate voltages affect the channel carrier concentration and surface potential of the metal oxide, thus causing a change in sensitivity. With a gate-voltage-pulse operation a novel measurement technique is introduced.