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Eingeladener Vortrag
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The development of competitive thermoelectric generators using oxide ceramics and multilayer technology requires the investigation of suitable sintering procedures. The sintering of Ca3Co4O9, which is the most promising p-type oxide regarding its figure of merit, is a special challenge, as the material starts to decompose at 926 °C. Pressure-assisted sintering (PAS), as known from large-scale production of low temperature co-fired ceramics, was used to sinter multilayers of Ca3Co4O9 green tape at 900 °C with different pressures and dwell times. In-situ shrinkage measurements, microstructural investigations and electric measurements were performed. Relative density increases from 57.4 % after free sintering for 24 h to 93.7 % after 2 h of PAS with 10 MPa axial load. The combination of tape casting and PAS induces a pronounced alignment of the anisotropic grains. In comparison to freely sintered multilayers, the electrical conductivity in the casting direction is thereby strongly increased by a factor of 5. With respect to dry-pressed bars, the improvement amounts to a factor of 15. This study shows that PAS is a proper technique to produce dense Ca3Co4O9 panels with good thermoelectric properties similar to hot-pressed tablets, even in large-scale production.
Zur Messung der Sinterschwindung keramischer Folien und daraus aufgebauter Bauteile wurden an der BAM zwei Verfahren entwickelt. Beide Verfahren erlauben die Messung von Prüfkörpern mit praxisrelevanten Abmessung (200 mm x 200 mm) bzw. Realbauteilen bis 1000 °C. Die Hochtemperatur-Laserprofilometrie ist ein berührungsloses Verfahren und eignet sich besonders zur Bewertung der Schwindungsanisotropie und zur in-situ Charakterisierung von Verwölbung. Die Sinterpresse mit in-situ Dickenmessung ist sehr gut geeignet, um Schwindungsverhalten im Drucksinterprozess unter verschiedenen Atmosphären zu untersuchen. Der Aufbau beider Messsysteme wird erläutert und Anwendungsbeispiele mit wissenschaftlichem Hintergrund und direktem Industriebezug werden vorgestellt.
Shrinkage measurements of miniaturized low temperature co-fired ceramics (LTCC) samples under load typically lead to collapsing of the samples, which hampers the characterization of shrinkage up to full densification. In this paper, a measurement setup is presented, which allows for in situ shrinkage measurements of practical, large LTCC panels during pressure-assisted sintering in a sintering press. The shrinkage behavior of two commercial LTCC systems (GreenTape 951 and Ceramtape GC) has been measured under loads of up to 1 MPa. No crushing of the specimens was observed and reproducible characterization of shrinkage up to full densification has been performed. Based on comparisons to thermomechanical analyzer measurements in this and other studies, it was found that the in situ approach is much better suited for shrinkage characterization of LTCC under load.
Reproducibility and accuracy of the method are discussed and practical as well as more academic applications are proposed.
Structural Health Monitoring (SHM) is an important part of buildings surveillance and maintenance to detect material failure as early as possible and to contribute in protection of structures and their users.
The implementation of Radio Frequency Identification (RFID) sensor systems without cable connection and battery into building components offers innovative possibilities to enable long-term in-situ SHM of addressed structures, bridges. The objectives of the presented study are complete embedding of RFID sensors systems in concrete, full passive communication with the systems, at best for the whole life span of structures. One challenge for this task is the highly alkaline environment in concrete, which requires non-degrading and robust encapsulation. Further Requirements are passive communication and energy supply, appropriate antenna design, placement and fixation in concrete, and the selection and implementation of sensors and connections. The concept is to develop and optimize a simple and robust system, which meets the requirements, as well as comprehensive validation in concrete specimen and real world applications. Two different systems were developed (HF and UHF RFID, respectively).
First tasks were the implementation of analog sensors using the superposition principle for the signal adaption. Investigation of suitable materials for robust encapsulation and sensor protection against basic environments.
Four materials were investigated in pH 13 solution for 14 days
- 3D-Printer-Polymer was completely resolved
- PVC has no noticeable decrease in weight
- (VitaPro) glass filter for the sensor protector, has weight loss 2.7 %
- The epoxy resin has increased by 1.8 % due to moisture expansion
Different concrete samples were prepared for the validation of the systems.
RFID sensors were embedded in different integration depths. Investigate the energy- and data transfer through concrete, also with varying moisture content. Additionally, signal strength data was used to optimize and validate the antenna characteristics in concrete. Next steps are to guarantee a sufficient energy supply for UHF RFID systems embedded in different concrete mixtures and further embedding the HF and UHF RFID systems in real bridges and buildings to validate the long term monitoring.
Shrinkage measurement of miniaturized low temperature co-fired
ceramics (LTCC) samples under load typically leads to a collapsing of
the sample due to the softening of the glassy phase, which hampers
the characterization of shrinkage up to full densification. In practical
sintering processes in contrast, LTCC panels are sintered under axial
loads of up to 1 MPa without lateral shrinkage or collapsing of the
structure. To characterize the shrinkage behavior during such
processes, a measurement setup was developed, which allows for in-situ
thickness shrinkage measurements of practical, large LTCC panels
during pressure-assisted sintering in a sintering press. Using this setup,
the shrinkage behavior of two commercial LTCC tapes (GreenTape 951
and Ceramtape GC) has been measured under loads of up to 1 MPa.
No crushing of the specimens was observed and reproducible
characterization of shrinkage up to full densification has been
performed. Based on comparisons to thermomechanical analyzer
measurements in this and other studies, it is concluded that the in-situ
approach is much better suited for shrinkage characterization of LTCC
under load.
Structural Health Monitoring (SHM) is an important topic for Industry 4.0. More and more systems are embedded in different materials and are connected to each other. To embed sensors for a long time in concrete, an exact preparation is required. In this paper, we identify Radio-Frequency Identification (RFID) as promising technology for monitoring of concrete structures. This paper present the concept for long term monitoring, defines the requirements and shows first development steps, for example, the system design, the possibilities of sensor connection, and an encapsulation for embedding in concrete.
Dielectric strength is a critical property for materials used as electrical insulators. The measurement of dielectric strength is well established and straightforward, but the values determined in a measurement are strongly dependent on the measurement setup and the specimen characteristics. For example, the size of the electrodes has a significant influence on the results. ASTM D149 covers a range of electrode sizes and does not unambiguously prescribe the quality of the electrodes. Thus, different test setups and procedures are used in the field. Consequently, a comparison of dielectric strength values of different origins, for example material supplier and customer, is often not meaningful.
To quantify the influence of the test procedure on the dielectric strength values, a batch of industrial thick-film substrates has been tested with different electrode configurations under AC conditions. Opposing cylinders with diameters of 6.4 mm and 25 mm, and a thick-film metallization with a diameter of 25 mm were used in the study. At least 20 specimens were tested with each type of electrodes. The results range from 21.7 ± 0.7 kV/mm measured with printed electrodes to 26.7 ±1.2 kV/mm measured with reused 6.4 mm cylinders. This means a difference of 23 %. Measurements performed with 6.4 mm cylinder electrodes produce significantly lower values (ANOVA, α = 0.01) if a new set of electrodes is used for each measurement instead of reusing the same pair of electrodes for the entire batch. The dielectric strength measured with new 25 mm cylinders is 11.2 % lower than the values determined with new 6.4 mm cylinders. No significant difference (ANOVA, α = 0.01) was found for measurements with printed electrodes and opposing 25 mm cylinders. Weibull evaluation of the data showed that all tested electrode configurations result in a similar reliability of the specimen (Weibull modulus) but in a significantly different characteristic dielectric strength (scale factor, Bonferroni, α = 0.05).
This study emphasizes the importance of a thorough and comprehensive documentation and communication of the test procedure for dielectric strength measurements. It further helps to evaluate the significance of differences in dielectric strength data provided by different sources.
Structural Health Monitoring (SHM) is an important part of buildings surveillance and maintenance to detect material failure as early as possible and to contribute in protection of structures and their users.
The implementation of Radio Frequency Identification (RFID) sensor systems without cable connection and battery into building components offers innovative possibilities to enable long-term in-situ SHM of addressed structures, bridges. The objectives of the presented study are complete embedding of RFID sensors systems in concrete, full passive communication with the systems, at best for the whole life span of structures. One challenge for this task is the highly alkaline environment in concrete, which requires non-degrading and robust encapsulation. Further Requirements are passive communication and energy supply, appropriate antenna design, placement and fixation in concrete, and the selection and implementation of sensors and connections. The concept is to develop and optimize a simple and robust system, which meets the requirements, as well as comprehensive validation in concrete specimen and real world applications. Two different systems were developed (HF and UHF RFID, respectively).
First tasks were the implementation of analog sensors using the superposition principle for the signal adaption. Investigation of suitable materials for robust encapsulation and sensor protection against basic environments.
Four materials were investigated in pH13 solution for 14 days - 3D-Printer-Polymer was completely resolved - PVC has no noticeable decrease in weight - (VitaPro) glass filter for the sensor protector, has weight loss 2.7% - The epoxy resin has increased by 1.8% due to moisture expansion Different concrete samples were prepared for the validation of the systems. RFID sensors were embedded in different integration depths. Investigate the energy- and data transfer through concrete, also with varying moisture content. Additionally, signal strength data was used to optimize and validate the antenna characteristics in concrete. Next steps are to guarantee a sufficient energy supply for UHF RFID systems embedded in different concrete mixtures and further embedding the HF and UHF RFID systems in real bridges and buildings to validate the long term monitoring.