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The ProboStat is a multi-purpose measurement cell suitable for various electrical and physical measurements under different atmospheres and at high temperatures. Disc and bar shaped samples are sandwiched between platinum electrodes at the top of the tubular cell. The gas tight assembly can be inserted into a furnace. Different gases can be flushed through the tube. For this study, a ProboStat was adapted to measure volume resistivity of ceramic insulators at high temperatures according to standards.
The standardized measurement of volume resistivity of ceramic insulators requires the consideration of many specifications including sample diameter, thickness, electrode design, and the proportion of these characteristics. Measurements are ideally performed in a state of dielectric equilibrium. The time-related slope of resistivity of a specific sample follows a power function. Thus, care must be taken when choosing a charge time or defining the duration of a measurement. As fringing of the guarded electrode occurs under high voltage, the effective electrode area for evaluation of the results should be corrected with respect to sample thickness and electrode design. The demands of effective standards on sample geometry and electrode design are stricter for room temperature measurements than for high temperature measurements.
To perform high temperature measurements on ceramic samples that also fulfill the demands on room temperature measurements, a ProboStat was equipped with a dedicated large sample setup for discs with diameters of up to 60 mm. The volume resistivity of different alumina samples was first measured at room temperature in a standard test fixture and then compared to results obtained with the ProboStat. All measurements were performed for at least 100 min using a 26 mm guarded electrode. High temperature measurements at 500 °C were performed using the same samples. Room temperature values obtained with the standard test fixture are in the order of 10^17 Ohm·cm. The quantitative effect of electrode area correction is presented. Practical issues related to the use of the multi-purpose cell are addressed. These include electrode material selection, application of electrodes, and compensation of leakage currents. High temperature results of volume resistivity of the different alumina samples are presented. The validity is discussed with respect to the suitability of the multi-purpose cell for such measurements.
Dielectric strength testing of ceramics can be performed with various setups and parameters. Comparisons of results from different sources are often not meaningful, because the results are strongly dependent on the actual testing procedure. The aim of this study is to quantify the influence of voltage ramp rate, electrode size, electrode conditioning, and sample thickness on the measured AC dielectric strength of a commercial alumina. Mean values, Weibull moduli, and failure probabilities determined in standardized short time tests are evaluated and related to withstand voltage tests. Dielectric strength values in the range from 21.6 to 33.2 kV/mm were obtained for the same material using different testing procedures. Short time tests resulted in small standard deviations (< 2 kV/mm) and high Weibull moduli around 30, while withstand tests at voltage levels with low and virtual zero failure probability in short time tests resulted in large scatter of withstand time and Weibull moduli < 1. The strong decrease in Weibull moduli is attributed to progressive damage from partial discharge and depolarization during AC testing. These findings emphasize the necessity of a thorough documentation of testing procedure and highlight the importance of withstand voltage tests for a comprehensive material characterization.
The 5G technology promises real time data transmission for industrial processes, autonomous driving, virtual and augmented reality, E-health applications and many more. The Low Temperature Co-fired Ceramics (LTCC) technology is well suited for the manufacturing of microelectronic components for such applications. Still, improvement of the technology such as further miniaturization is required. This study focuses on the characterization of inner metallization of LTCC multilayer modules, especially on the vertical interconnect access (VIA). Critical considerations for this characterization are delamination, pore clustering in and at the edge of the VIA, deformation, and stacking offset. A LTCC multilayer consisting of a glassy crystalline matrix with silver based VIAs was investigated by synchrotron x-ray tomography (CT). The aim of this study is to propose a multitude of structural characteristic values to maximize the information gained from the available dataset. Data analysis has been done with the open source software ImageJ as well as several additional plugins. The high-resolution CT data was evaluated through 2D slices for accessibility reasons. The segmentation of all 2000 slices to assess the different regions e.g. pores, silver and glass ceramic was done by a supervised machine learning algorithm. A quantitative evaluation of shape, deformation, and porosity of the VIA with respect to its dimensions is presented and the suitability of the characterization approach is assessed.
The wireless high-frequency technology requires a robust, cost-effective, and highly integrated substrate technology offering the capability for areas of tailored permittivity. The wet-chemical porosification of low temperature co-fired ceramics (LTCC) substrates offers such an approach by locally embedding air. Porosification of LTCC in both extremely acidic and alkaline media has been investigated in previous works. However, for improving the available knowledge on the porosification of LTCC with H3PO4 as a standard and a widely used etching solution, the impact of solution concentration was systematically investigated and a substantial improvement in the etching performance was achieved. Moreover, in the present study, for the first time, the intermediate pH values, and the impact of pH as a key parameter on the etching process have been investigated. For this purpose, the applicability of phosphate buffer solution (PBS) as a prospective novel etchant mixture for the porosification of a commercially available LTCC tape (Ceramtape GC) was explored. Valuable information about surface morphology, crystalline composition, and the pore structure of the etched LTCCs was gathered employing scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, and mercury porosimetry measurements. Based on these findings, the performance of PBS-based etchant systems towards the generation of porous LTCCs combining high depths of porosification with acceptable surface characteristics for subsequent metallization is demonstrated. Based on the obtained results, by application of a 0.2 mol L−1 solution of PBS, the effective relative permittivity of test samples with a thickness of approximately 600 µm and a porosification depth of 186 µm from each side, could be reduced up to 10% of its initial “as fired” value. Also, based on the measurement results and by measuring the depth of porosification, the permittivity of the etched layer was estimated to show a reduction of up to 22% compared to the initial “as fired” value.
Practical breakdown voltage calculations using dielectric breakdown strength reference values
(2020)
Dielectric breakdown is a catastrophic failure of ceramic substrates and insulators. The use of dielectric breakdown strength (DBS) reference values for the dimensioning of such components is not straightforward, as the DBS depends on sample thickness and electrode area. This fact also hampers a valid comparison of data taken from different literature sources. Based on the empirically confirmed proportionality of DBS to the reciprocal square root of sample thickness and an approach to account for the influence of electrode area on the failure probability, a practical equation is derived to calculate the breakdown voltage for arbitrary sample thickness and electrode area from one set of DBS reference data. To validate the equation, the AC DBS of commercial alumina substrates with thicknesses ranging from 0.3 mm to 1.0 mm was performed using different printed electrodes with varying areas. The breakdown voltages comprise a range from 18 kV for thick samples to 8.5 kV for thin samples, resulting in DBS values from 17 kV/mm for 1.0 mm thick samples to 29 kV/mm for 0.3 mm thin samples, all made from the same material. The influence of electrode area is comparatively smaller. The results calculated with the proposed equation are in reasonable accordance with the measured data. Thus, the equation can be applied for a proper comparison of literature DBS data measured in different setups and for a reasonable estimation of breakdown voltages in DBS tests and applications based on reference data.
Calcium cobaltite Ca3Co4O9, abbreviated Co349, is a promising thermoelectric material for high‐temperature applications in air. Its anisotropic properties can be assigned to polycrystalline parts by texturing. Tape casting and pressure‐assisted sintering (PAS) are a possible future way for a cost‐effective mass‐production of thermoelectric generators. This study examines the influence of pressure and dwell time during PAS at 900°C of tape‐cast Co349 on texture and thermoelectric properties. Tape casting aligns lentoid Co349. PAS results in a textured Co349 microstructure with the thermoelectrically favorable ab‐direction perpendicular to the pressing direction. By pressure variation during sintering, the microstructure of Co349 can be tailored either toward a maximum figure of merit as required for energy harvesting or toward a maximum power factor as required for energy harvesting. Moderate pressure of 2.5 MPa results in 25% porosity and a textured microstructure with a figure of merit of 0.13 at 700°C, two times higher than the dry‐pressed, pressureless‐sintered reference. A pressure of 7.5 MPa leads to 94% density and a high power factor of 326 µW/mK2 at 800°C, which is 11 times higher than the dry‐pressed reference (30 MPa) from the same powder.
The influence of the atomization technique on the suitability of granules for dry pressing is the focus of the presented investigations. Therefore, destabilized alumina, zirconia, and zirconia toughened alumina (ZTA) slurries were spray dried and the obtained granules were used to fabricate green and finally sintered bodies for evaluation.
Granules made in a laboratory spray dryer with a two-fluid nozzle served as a reference. An ultrasonic atomizer was integrated into the same spray dryer and the influence on the granule properties was evaluated. Untapped bulk density, granule size distribution, and flowability are among the evaluated granule-related properties as well as the granule yield which is used as an indicator of the process efficiency.
Yield and flowability as most important granule properties are clearly improved when atomization is realized with ultrasound. The investigated sinter body properties include porosity, sinter body density, and biaxial strength and are as well positively affected by switching the atomization technique to ultrasound. Therefore, the Approach to improve the compressibility of granules by ultrasonic atomization, which leads to an improved microstructure, density, and strength of sintered bodies, has proven to be successful for single-component ceramics (alumina and zirconia) as well as for the multicomponent ceramic ZTA.
Ceramic springs combine attractive properties for applications in machinery, metrology, and sensor technology. They are electrically insulating, non-magnetic, provide a linear stress-strain behavior, and are stable at high temperatures and in corrosive environments. Generally, the precise dimensioning of a ceramic spring with respect to the spring constant is challenging. Different models are described, but many of these calculations do not match the actual spring properties. We demonstrate a reliable approach for the dimensioning and manufacturing of helical compression springs with a rectangular winding cross-section. Based on the German standard DIN 2090, which is referring to metallic springs, the spring constant can be calculated based on shear modulus, diameter, height, widths, and number of windings. Different ceramic springs were produced by milling of sintered hollow cylinders of zirconia, alumina and silicon nitride. The experimental spring constants are in very good agreement with the calculated values. Spring constants of zirconia springs were varied over three orders of magnitude between 0.02 N/mm and 5 N/mm by purposeful adaption of the spring geometry. The combination of dimensioning based on DIN 2090 and precise hard machining offers a reliable technology for the fabrication of tailored ceramic springs for special applications.
The dielectric breakdown strength of ceramics strongly depends on the test conditions. Thus, standardized test procedures and thorough documentation are indispensable. However, during dielectric strength testing the breakdown often occurs near the electrode edge or even outside the specified electrode area. This behavior is similarly observed for printed and cylindrical electrodes. The aim of the presented study was to calculate the electric field strength distribution in a ball-on-plate testing setup for metallized samples and to correlate the field distribution with the observed breakdown locations. Small misalignments in the test setup were also considered in the simulations. Furthermore, the field strength at the breakdown Location should be compared to the experimentally determined dielectric strength. Therefore, Finite Element Models of several test conditions with varying printed electrode areas and sample thicknesses were created and electrostatic calculations of the electric field Distribution were performed. The simulation results were compared to experimental data. Alumina (96 %) was used as test material. The calculations show that the electric field strength maxima match the experimentally observed locations of breakdown. Without any fitting of the model, the maximum calculated field strength is in reasonable agreement with the experimental dielectric strength. The FE analysis is a helpful tool to understand the observations in experimental dielectric strength testing.
The relationship between fracture toughness and Yttria content in modern zirconia ceramics was revised. For that purpose, we evaluated here 10 modern Y2O3-stabilized zirconia (YSZ) materials currently used in biomedical applications, namely prosthetic and implant dentistry. The most relevant range between 2-5 mol% Y2O3 was addressed by selecting from conventional opaque 3 mol% YSZ up to more translucent compositions (4-5 mol% YSZs). A technical 2YSZ was used to extend the range of our evaluation. The bulk mol% Y2O3 concentration was measured by X-Ray Fluorescence Spectroscopy. Phase quantification by Rietveld refinement considered two tetragonal phases or an additional cubic phase. A first-account of the fracture toughness (KIc) of the pre-sintered blocks is given, which amounted to 0.4 – 0.7 MPa√m. In the fully-densified state, an inverse power-law behavior was obtained between KIc and bulk mol% Y2O3 content, whether using only our measurements or including literature data, challenging some established relationships. A linear relationship between KIc and the fraction of the transformable t-phase was established within the range of 30–70 vol%.
Glass-ceramic composites as insulation material for thermoelectric oxide multilayer generators
(2021)
Thermoelectric generators can be used as energy harvesters for sensor applications.
Adapting the ceramic multilayer technology, their production can be
highly automated. In such multilayer thermoelectric generators, the electrical insulation material, which separates the thermoelectric legs, is crucial for the performance of the device. The insulationmaterial should be adapted to the thermoelectric regarding its averaged coefficient of thermal expansion α and its sintering temperature while maintaining a high resistivity.
In this study, starting from theoretical calculations, a glass-ceramic Composite material adapted for multilayer generators fromcalciummanganate and Calcium cobaltite is developed. The material is optimized towards an α of 11 × 10−6 K−1 (20–500◦C), a sintering temperature of 900◦C, and a high resistivity up to 800◦C.
Calculated and measured α are in good agreement. The chosen glass-ceramic composite with 45 vol.% quartz has a resistivity of 1 × 107 Ωcm and an open porosity of <3%. Sintered multilayer samples from tape-cast thermoelectric oxides and screen-printed insulation show only small reaction layers. It can be concluded that glass-ceramic composites are a well-suited material class for insulation layers as their physical properties can be tuned by varying glass composition or dispersion phases.
Der Vortrag gibt einen Überblick über die Materialklasse Keramik, insbesondere Technische Keramik. Nach einem kurzen Überblick über typische Anwendungen und Werkstoffe wird die keramische Prozesskette erläutert. Am Beispiel von Festigkeit wird der für Keramik typische, enge Zusammenhang zwischen Technologie, Mikrostruktur und Eigenschaften herausgearbeitet. Daraus werden Anforderungen für eine erfolgreiche Digitalisierung abgeleitet.
Aufgrund ihres spezifischen Eigenschaftsprofils sind keramische Federn attraktiv für Spezialanwendungen in Maschinenanlagen, Metrologie und Sensortechnik. Durch Hartbearbeitung keramischer Hohlzylinder können Spiralfedern mit rechteckigem Windungsquerschnitt präzise gefertigt werden. Dabei kann durch gezielte Auslegung der Federgeometrie die Federkonstante über mehrere Größenordnungen variiert werden. Der Vortrag gibt einen Überblick über den Herstellungsprozess, verschiedene Eigenschaften keramischer Federn und Anwendungsbeispiele.
Dielectric breakdown of insulators is a combined electrical, thermal, and mechanical failure. The exact breakdown mechanism in ceramics and the formulation of useful models are still subject of investigation. Recent studies highlighted that several experimental aspects of dielectric breakdown strength testing affect the test results, and thus impede the recognition of fundamental principles.
Excess field strength near the electrode can lead to premature breakdown in the insulating liquid. This would cause superficial damage to the test specimen and thus falsify the measurement results.
The field strength distribution is influenced by the ratio of permittivity of the sample and the surrounding insulating liquid. Premature breakdown depends on the breakdown strength of the liquid and the actual test voltage. The test voltage again depends on the specimen thickness.
To systematically investigate these relations, a numerical simulation study (FEM) of the electric field distribution in a typical testing rig with cylindrical electrodes was performed. The permittivity of the sample and the insulating liquid was parameterized, as well as the sample thickness. The electric field distribution was calculated for increasing test voltage. Field strength maxima are compared to experimental breakdown strength of typical insulating liquids and experimental breakdown locations on alumina. Strategies are discussed to adjust the insulation liquid and the sample thickness to reduce the influence of the testing setup on the dielectric breakdown strength results.
Roughening of zirconia dental implants is a common clinical practice to improve ingrowth behavior. It depends on the manufacturer of the implant at which stage of the manufacturing process and by which method the surface is roughened. Systematic studies on this topic are rarely found in the literature. Therefore, the influence of surface treatment on the strength of a dental zirconia was investigated as part of a research project on the development of ceramic implants. The material under test was a commercial zirconia consisting of a Y-TZP matrix and Ce-TZP inclusions in the sintered state. This material is characterized by a slightly higher fracture toughness and slightly reduced strength compared to typical 3Y-TZP. Sets of samples were sandblasted in the white-fired or sintered condition. The ball-on-three-ball-strength of these samples was measured and compared to the strength of as-fired samples and polished samples. The complete study was performed two times for validation of the results. It is found that the average strength of TZP ceramics differs by almost 500 MPa depending on the surface treatment. Conventionally sintered specimens with as-fired surface exhibit a strength of 880 MPa. Sandblasting in the white fired state reduces the strength to 690 MPa. Both polishing and sandblasting in the sintered condition result in an increase in strength to about 1180 MPa. Comparative microstructural investigations, roughness measurements and X-ray phase analyses were carried out to determine the causes of these huge differences in strength. These findings may challenge the practice of white body surface treatment and give reason for further investigations on other commercial dental TZP materials.
This study combines three different approaches to lower the sintering temperature of Sm-doped CaMnO3 to save energy in production and facilitate co-firing with other low-firing oxides or metallization. The surface energy of the powder was increased by fine milling, sintering kinetics were enhanced by additives, and uniaxial pressure during sintering was applied. The shrinkage, density, microstructure, and thermoelectric properties were evaluated. Compared to micro-sized powder, the use of finely ground powder allows us to lower the sintering temperature by 150 K without reduction of the power factor. By screening the effect of various common additives on linear shrinkage of CaMnO3 after sintering at 1100 ○C for 2 h, CuO is identified as the most effective additive. Densification at sintering temperatures below 1000 ○C can be significantly increased by pressure-assisted sintering. The power factor at room temperature of CaMnO3 nano-powder sintered at 1250 ○C was 445 μW/(m K2). Sintering at 1100 ○C reduced the power factor to 130 μW/(m K2) for CaMnO3 nano-powder, while addition of 4 wt.% CuO to the same powder led to ∼290 μW/(m K2). The combination of fine milling, CuO addition, and pressureassisted sintering at 950 ○C resulted in a power factor of ∼130 μW/(m K2). These results show that nano-sized powder and CuO addition are successful and recommendable strategies to produce CaMnO3 with competitive properties at significantly reduced temperatures and dwell times.
Als elektrische Durchschlagfestigkeit bezeichnet man die elektrische Feldstärke, bei der es zur Entladung durch ein isolierendes Medium kommt. Im Falle von Festkörpern ist dies ein zerstörender Prozess. Bei der messtechnischen Bestimmung der Durchschlagfestigkeit haben neben den verwendeten Messgeräten auch die eingesetzten Elektroden, die Form des Prüfkörpers und das Isoliermedium, in dem die Prüfung stattfindet, einen signifikanten Einfluss auf die ermittelten Messwerte. Im Vortrag werden die Einflüsse erläutert und anhand von Messreihen aus der Literatur und der eigenen Forschung quantifiziert. Aufgrund der vorgestellten Effekte wird klar, dass es sich bei elektrischer Durchschlagfestigkeit nicht um absolute Materialkennwerte handelt, sondern vielmehr um systemabhängige Größen.
Zirconium vanadate (ZrV2O7) is a well-known negative thermal expansion (NTE) material which exhibits significant isotropic contraction over a broad temperature range (~150°C < T < 800°C). The linear thermal expansion coefficient of ZrV2O7 is −7.1×10-6 K-. Therefore, it can be used to create composites with controllable expansion coefficients and prevent destruction by thermal shock.
Material characterization, leading to application, requires pure, homogenous samples of high crystallinity via a reliable synthesis route. While there is a selection of described syntheses in the literature, it still needs to be addressed which synthesis route leads to truly pure and homogenous samples. Here, we study the influence of the synthesis methods (solid-state, sol-gel, solvothermal) and their parameters on the sample's purity, crystallinity, and homogeneity. The reproducibility of results and data obtained with scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry, and thermogravimetric analysis (DSC/TGA) were analyzed extensively. The sol-gel method proves superior to the solid-state method and produces higher-quality samples over varying parameters. Sample purity also plays an important role in NTE micro and macro-scale characterizations that explain the impact of porosity versus structural changes.
Moreover, we implement ab-initio-based vibrational computations with partially treated anharmonicity (quasi-harmonic approximation, temperature-dependent effective harmonic potentials) in combination with experimental methods to follow and rationalize the negative thermal expansion in this material, including the influence of the local structure disorder, microstructure, and defects. Khosrovani et al. and Korthuis et al., in a series of diffraction experiments, attributed the thermal contraction of ZrV2O7 to the transverse thermal motion of oxygen atoms in V-O-V linkages. In addition to previous explanations, we hypothesize that local disorder develops in ZrV2O7 crystals during heating.
We are working on the experimental ZrV2O7 development and discuss difficulties one might face in the process as well as high-quality sample significance in further investigation. The obtained samples are currently used in the ongoing research of structure analysis and the negative thermal expansion mechanism.
Zirconium vanadate (ZrV2O7) is a well-known negative thermal expansion (NTE) material which exhibits significant isotropic contraction over a broad temperature range (~150°C < T < 800°C). The linear thermal expansion coefficient of ZrV2O7 is −7.1×10-6 K-. Therefore, it can be used to create composites with controllable expansion coefficients and prevent destruction by thermal shock.
Material characterization, leading to application, requires pure, homogenous samples of high crystallinity via a reliable synthesis route. While there is a selection of described syntheses in the literature, it still needs to be addressed which synthesis route leads to truly pure and homogenous samples. Here, we study the influence of the synthesis methods (solid-state, sol-gel, solvothermal) and their parameters on the sample's purity, crystallinity, and homogeneity. The reproducibility of results and data obtained with scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry, and thermogravimetric analysis (DSC/TGA) were analyzed extensively. The sol-gel method proves superior to the solid-state method and produces higher-quality samples over varying parameters. Sample purity also plays an important role in NTE micro and macro-scale characterizations that explain the impact of porosity versus structural changes.
Moreover, we implement ab-initio-based vibrational computations with partially treated anharmonicity (quasi-harmonic approximation, temperature-dependent effective harmonic potentials) in combination with experimental methods to follow and rationalize the negative thermal expansion in this material, including the influence of the local structure disorder, microstructure, and defects. Khosrovani et al. and Korthuis et al., in a series of diffraction experiments, attributed the thermal contraction of ZrV2O7 to the transverse thermal motion of oxygen atoms in V-O-V linkages. In addition to previous explanations, we hypothesize that local disorder develops in ZrV2O7 crystals during heating.
We are working on the experimental ZrV2O7 development and discuss difficulties one might face in the process as well as high-quality sample significance in further investigation. The obtained samples are currently used in the ongoing research of structure analysis and the negative thermal expansion mechanism.
Die Nutzung von Sensortechnologien, insbesondere im Bereich der Gasdetektion mit einem Schwerpunkt auf Wasserstoff, spielt eine entscheidende Rolle in verschiedenen Anwendungsbereichen der Wasserstofftechnologie. Sicherheitsüberwachung, Leckdetektion und Prozesskontrolle gehören zu den prominenten Anwendungsgebieten dieser Sensortechnologien. Ein zentrales Ziel ist die Erkennung von freigesetztem Wasserstoff sowie die genaue Bestimmung des Wasserstoff-Luftverhältnisses mithilfe von Gassensoren. Dies ist von entscheidender Bedeutung, um potenzielle Gefahren frühzeitig zu erkennen und angemessene Maßnahmen zu ergreifen.
Ein weiterer Schwerpunkt dieses Beitrags liegt auf der Analytik und der Verwendung zertifizierter Referenzmaterialien in Verbindung mit Metrologie für die Wasserstoffspeicherung. Dies gewährleistet eine präzise und zuverlässige Charakterisierung von Wasserstoff und unterstützt die Entwicklung sicherer Speichertechnologien. Im Rahmen des Euramet-Vorhabens Metrology for Advanced Hydrogen Storage Solutions (MefHySto) wird eine Kurzvorstellung präsentiert.
Der Vortrag stellt zwei zerstörungsfreie Prüfverfahren zum strukturellen Zustandsüberwachung (Structural Health Monitoring, SHM) für Wasserstofftechnologien vor. Insbesondere die Fehlstellenerkennung mittels geführter Ultraschallwellen spielt eine bedeutende Rolle bei der Lebensdauerüberwachung von Wasserstoffspeichern. Ein weiterer Aspekt ist die Anwendung faseroptischer Sensorik zur Schadensfrüherkennung von Wasserstoffspeichern. Diese zerstörungsfreien Prüfverfahren ermöglichen eine präzise und frühzeitige Identifizierung von Schäden, was die Sicherheit und Effizienz von Wasserstoffspeichersystemen entscheidend verbessert.
Zirconium vanadate (ZrV2O7) is a well-known negative thermal expansion (NTE) material which exhibits significant isotropic contraction over a broad temperature range (~150°C < T < 800°C). The linear thermal expansion coefficient of ZrV2O7 is −7.1×10-6 K-1. Therefore, it can be used to create composites with controllable expansion coefficients and prevent destruction by thermal shock.
Material characterization, leading to application, requires pure, homogenous samples of high crystallinity via a reliable synthesis route. While there is a selection of described syntheses in the literature, it still needs to be addressed which synthesis route leads to truly pure and homogenous samples. Here, we study the influence of the synthesis methods (solid-state, sol-gel, solvothermal) and their parameters on the sample's purity, crystallinity, and homogeneity. The reproducibility of results and data obtained with scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry, and thermogravimetric analysis (DSC/TGA) were analyzed extensively. The sol-gel method proves superior to the solid-state method and produces higher-quality samples over varying parameters. Sample purity also plays an important role in NTE micro and macro-scale characterizations that explain the impact of porosity versus structural changes.
Moreover, we implement ab-initio-based vibrational computations with partially treated anharmonicity (quasi-harmonic approximation, temperature-dependent effective harmonic potentials) in combination with experimental methods to follow and rationalize the negative thermal expansion in this material, including the influence of the local structure disorder, microstructure, and defects. Khosrovani et al. and Korthuis et al., in a series of diffraction experiments, attributed the thermal contraction of ZrV2O7 to the transverse thermal motion of oxygen atoms in V-O-V linkages. In addition to previous explanations, we hypothesize that local disorder develops in ZrV2O7 crystals during heating.
We are working on the experimental ZrV2O7 development and discuss difficulties one might face in the process as well as high-quality sample significance in further investigation. The obtained samples are currently used in the ongoing research of structure analysis and the negative thermal expansion mechanism.
Zirconium vanadate (ZrV2O7) is a well-known negative thermal expansion (NTE) material that exhibits significant isotropic contraction over a broad temperature range (~150°C < T < 800°C). Therefore, it can be used to create composites with controllable expansion coefficients and prevent thermal stress, fatigue, cracking, and deformation at interfaces. We implement interdisciplinary research to analyze such material. We study the influence of the synthesis methods and their parameters on the sample's purity, crystallinity, and homogeneity. Moreover, we implement ab initio-based vibrational computations with partially treated anharmonicity in combination with experimental methods to follow temperature-induced structural changes and rationalize the negative thermal expansion in this material, including the influence of the local structure disorder.
Destructive and non-destructive 3D-characterization of inner metal structures in ceramic packages
(2023)
Ceramic multilayer packages provide successful solutions for manifold
applications in telecommunication, microsystem, and sensor technology. In such packages, three-dimensional circuitry is generated by combination of structured and metallized ceramic layers by means of tape casting and multilayer technology. During development and for quality assurance in manufacturing, characterization of integrity, deformation, and positioning of the inner metal features is necessary.
Visualization with high resolution and material contrast is needed.
Robot-assisted 3D-materialography is a useful technique to characterize such multimaterial structures. In that, many sections of the specimen are polished and imaged automatically. A three-dimensional representation of the structure is created by digital combination of the image stack. A quasi non-destructive approach is to perform X-ray computer tomography (CT) with different beam energies. The energies are chosen to achieve a good imaging of either the metal features, or the ceramic matrix of the structure. The combination of the respective tomograms results in a high contrast representation of the entire structure. Both methods were tested to characterize Ag and Ag/Pd conductors in a ceramic multilayer package. The results were compared in terms of information content, effort, and applicability of the methods.
Thermoelectric generators are very attractive devices for waste heat energy harvesting as they transform a temperature difference into electrical power. However, commercially available generators show poor power density and limited operation temperatures. Research focuses on high‐temperature materials and innovative generator designs. Finding the optimal design for a given material system is challenging. Here, a theoretical framework is provided that allows appropriate generator design selection based on the particular material properties. For high‐temperature thermoelectric oxides, it can be clearly deduced that unileg multilayer generators have the highest potential for effective energy harvesting. Based on these considerations, prototype unileg multilayer generators from the currently best thermoelectric oxide Ca3Co4O9 are manufactured for the first time by industrially established ceramic multilayer technology. These generators exhibit a power density of 2.2 mW/cm² at a temperature difference of 260 K, matching simulated values and confirming the suitability of the technology. Further design improvements increase the power density by a factor of 22 to facilitate practicable power output at temperature differences as low as 7 K. This work demonstrates that reasonable energy harvesting at elevated temperatures is possible with oxide materials and appropriate multilayer design.
Manufacturing and deformation behavior of alumina and zirconia helical springs at room temperature
(2023)
Ceramic helical springs with identical dimensions were produced by hard machining from alumina, alumina toughened zirconia (ATZ), and tetragonal zirconia polycrystals (TZP) stabilized with different oxides. According to the results of the spring constant determination under deformation rates of 3 mm/min, the deformation behavior of all ceramic springs obeys to Hook’s law. However, variation of the deformation rate, tests under constant load, and spring recovery behavior revealed differences in the deformation behavior of alumina, TZP, and ATZ springs. Alumina springs exhibited time-independent deformation in all tests. In contrast, anelastic deformation at room temperature was demonstrated in all springs containing TZP. This deformation is completely reversible over a period of several days. Anelastic behavior is particularly pronounced in Y-TZP springs, whereas Ce-TZP springs exhibit comparatively very low but still reliably detectable anelasticity. Oxygen vacancies in the TZP ceramic are considered the most likely explanation for the anelastic behavior of TZP springs at room temperature.
Automated computational workflows are a powerful concept that can improve the usability and reproducibility of simulation and data processing approaches. Although used very successfully in bioinformatics, workflow environments in materials science are currently commonly applied in the field of atomistic simulations. This work showcases the integration of a discrete element method (DEM) simulation of powder pressing in the convenient SimStack workflow environment. For this purpose, a Workflow active Node (WaNo) was developed to generate input scripts for the DEM solver using LIGGGHTS Open Source Discrete Element Method Particle Simulation code. Combining different WaNos in the SimStack framework makes it possible to build workflows and loop over different simulation or evaluation conditions. The functionality of the workflows is explained, and the added user value is discussed. The procedure presented here is an example and template for many other simulation methods and issues in materials science and engineering.
Modern dry pressing of ceramic powders using spray‐dried granulates cannot avoid the occurrence of defects related to persisting inter‐ and intra‐granulate interstitial voids. These constitute the parent defect size population limiting the application of polycrystalline ceramics in high‐stress conditions. The mitigation of such defects could widen the range of application in technical and biomedical engineering, reduce the safety range for design, and extend the lifetime of components. Herein, the Weibull size‐effect on strength in size‐partitioned Yttria‐stabilized zirconias (YSZ) feedstocks is used to explore the viability of changing the density distribution of granulate sizes as an effective strategy to obtain a denser particle packing that could reduce the size distribution of strength‐limiting pressing defects. In a direct assessment of critical defect size using multiscale strength testing with a dataset of ≈1300 values, the success of such an approach in increasing the strength reliability for small volume components is demonstrated, along with its ultimate failure in altering the defect size distribution in sintered YSZ ceramics across several length scales. Finally, it is shown that granule morphology (spherical or dimpled) fails to affect the defect density and size distribution in YSZ ceramics.
A widespread recovery of waste heat requires a cost‐effective production of thermoelectric generators. Thermoelectric oxides are predestined for use at high temperatures. For manufacturing reasons, a multilayer generator design will be easily scalable and cost‐effective. To evaluate the potential of ceramic multilayer technology for that purpose, a multilayer of the promising thermoelectric oxides calcium cobaltite (Ca3Co4O9), calcium manganate (CMO, CaMnO3), and glass–ceramic insulation layers is fabricated. Cracks and reaction layers at the interfaces are observed in the microstructure. The compositions of these reaction layers are identified by energy‐dispersive X‐ray spectroscopy and X‐ray diffraction. Mechanical and thermal properties of all layers are compiled from literature or determined by purposeful sample preparation and testing. Based on this data set, the internal stresses in the multilayer after co‐firing are calculated numerically. It is shown that tensile stresses in the range of 50 MPa occur in the CMO layers. The reaction layers have only a minor influence on the level of these residual stresses. Herein, it is proven that the material system is basically suitable for multilayer generator production, but that the co‐firing process and the layer structure must be adapted to improve densification and reduce the tensile stresses in the CMO.