Anmelden

Open Access

  • Startseite
  • Suchen
  • Browsen
  • Veröffentlichen
  • Hilfe
  • Organisationseinheit der BAM

5 Werkstofftechnik

  • 5.0 Abteilungsleitung und andere (117) RSS-Feed abonnieren
  • 5.1 Materialographie, Fraktographie und Alterung technischer Werkstoffe (336) RSS-Feed abonnieren
  • 5.2 Experimentelle und modellbasierte Werkstoffmechanik (270) RSS-Feed abonnieren
  • 5.3 Mechanik der Polymerwerkstoffe (279) RSS-Feed abonnieren
  • 5.4 Keramische Prozesstechnik und Biowerkstoffe (135) RSS-Feed abonnieren
  • 5.5 Technische Keramik (137) RSS-Feed abonnieren
  • 5.6 Glas (192) RSS-Feed abonnieren

Filtern

Autor

  • Rabe, Torsten (40)
  • Mieller, Björn (34)
  • Kuchenbecker, Petra (15)
  • Heunisch, Andreas (14)
  • Bresch, Sophie (13)
  • Gemeinert, Marion (8)
  • Höhne, Patrick (8)
  • Schulz, Bärbel (8)
  • Steinborn, Gabriele (8)
  • Jakoby, R. (7)
  • Follmann, R. (6)
  • Naghib Zadeh, Hamid (5)
  • Jost, M. (4)
  • Mieller, Bjoern (4)
  • Töpfer, J. (4)
  • Bartholmai, Matthias (3)
  • Hesse, J. (3)
  • Johann, Sergej (3)
  • Moos, R. (3)
  • Quibeldey, M. (3)
  • Wäsche, Rolf (3)
  • Binder, J. R. (2)
  • Brandt, Björn (2)
  • Capraro, B. (2)
  • Hodoroaba, Vasile-Dan (2)
  • Karabey, O.H. (2)
  • Koether, D. (2)
  • Kohler, C. (2)
  • Mikolajek, M. (2)
  • Müller, Maximilian (2)
  • Naghib-zadeh, H. (2)
  • Nikfalazar, M. (2)
  • Prasetiadi, A. E. (2)
  • Rauscher, H. (2)
  • Reimann, T. (2)
  • Rethmeier, Michael (2)
  • Schmidt, Wolfram (2)
  • Strangfeld, Christoph (2)
  • Strunck, S. (2)
  • Wiens, A. (2)
  • Bachmann, Marcel (1)
  • Barth, S. (1)
  • Bartsch, H. (1)
  • Bektas, M. (1)
  • Bianchin, A. (1)
  • Bierlich, S. (1)
  • Binder, J. (1)
  • Binder, R. (1)
  • Bochmann, A. (1)
  • Bolte, J. (1)
  • Böllinghaus, Thomas (1)
  • Chen, C. (1)
  • Delorme, F. (1)
  • Dörfel, Ilona (1)
  • Feigl, Michael (1)
  • Francke, T. (1)
  • Freiberger, H. (1)
  • Gaebler, A. (1)
  • Geyler, Paul (1)
  • Ghanem, A. (1)
  • Gibson, N. (1)
  • Giovanelli, F. (1)
  • Günster, Jens (1)
  • Hoffmann, Holger (1)
  • Karkhin, Victor (1)
  • Karmazin, R. (1)
  • Kassner, J. (1)
  • Kittler, Katrin (1)
  • Koch, Matthias (1)
  • Kranzmann, Axel (1)
  • Lindemann, Franziska (1)
  • Manabe, A. (1)
  • Marzok, Ulrich (1)
  • Maul, Ronald (1)
  • Mielke, Johannes (1)
  • Mota Gassó, Berta (1)
  • Müller, Ralf (1)
  • Münchow, Marco (1)
  • Naghib-zadeh, Hamid (1)
  • Nofz, Marianne (1)
  • Oder, Gabriele (1)
  • Partsch, U. (1)
  • Pauli, Jutta (1)
  • Paulick, C. (1)
  • Pittner, Andreas (1)
  • Prasetiadi, A .E. (1)
  • Prasetiadi, A.E. (1)
  • Quibelday, M. (1)
  • Rangelov, V. (1)
  • Rasmussen, K. (1)
  • Resch-Genger, Ute (1)
  • Rohn, S. (1)
  • Schulz, Baerbel (1)
  • Schulz, T. (1)
  • Schulz, Wencke (1)
  • Schönauer-Kamin, D. (1)
  • Selleng, Christian (1)
  • Silbernagl, Dorothee (1)
  • Sohrabi, M. (1)
  • Sturm, Heinz (1)
  • Stöcker, T. (1)
  • Teichert, S. (1)
  • Vogel, A. (1)
  • Weba, Luciana (1)
  • Weickhmann, C. (1)
  • Wohlleben, W. (1)
  • Woydt, Mathias (1)
  • Wu, Chuan Song (1)
  • Zheng, Y. (1)
  • de Seauve, Victor (1)
- weniger

Erscheinungsjahr

  • 2020 (1)
  • 2019 (14)
  • 2018 (19)
  • 2017 (28)
  • 2016 (18)
  • 2015 (22)
  • 2014 (17)
  • 2013 (18)

Dokumenttyp

  • Vortrag (71)
  • Zeitschriftenartikel (29)
  • Posterpräsentation (23)
  • Beitrag zu einem Tagungsband (10)
  • Buchkapitel (3)
  • Zeitschriftenheft (Herausgeberschaft für das komplette Heft) (1)

Sprache

  • Englisch (81)
  • Deutsch (56)

Referierte Publikation

  • nein (111)
  • ja (26)

Schlagworte

  • LTCC (11)
  • Thermoelectrics (9)
  • Calcium cobaltite (7)
  • VSSA (6)
  • Multilayer (5)
  • Pressure-assisted sintering (5)
  • Slurry (5)
  • Spray drying (5)
  • Calcination (4)
  • Ceramic spring (4)
  • Ceramics (4)
  • Destabilization (4)
  • Partikelgrößenverteilung (4)
  • Sensor (4)
  • Alumina (3)
  • Dielectric strength (3)
  • Particle size (3)
  • Partikelgröße (3)
  • Pressure assisted sintering (3)
  • RFID sensors (3)
  • Sensor requirements (3)
  • Sensors in concrete (3)
  • Smart structures (3)
  • Structural health monitoring (3)
  • Thermoelectric oxide (3)
  • Ultrasound (3)
  • Analytical centrifuge (2)
  • Analytische Zentrifuge (2)
  • Atomization (2)
  • Calciumcobaltit (2)
  • Ceramic (2)
  • Ceramic springs (2)
  • Co-firing (2)
  • Gas pressure sintering (2)
  • Glass-like carbon (2)
  • Granulation (2)
  • Granules (2)
  • Hardness (2)
  • Hot-embossing (2)
  • Hybrid components (2)
  • LTCC multilayer (2)
  • LTCC technology (2)
  • Liquid crystals (2)
  • Low temperature (2)
  • Low temperature co-fired ceramics (2)
  • Microelectronic packaging (2)
  • Microfluidics (2)
  • Microstructure (2)
  • Nano particle (2)
  • Nano screening (2)
  • Nanomaterial (2)
  • Nanoparticles (2)
  • NbC (2)
  • Passive RFID (2)
  • Passive components and circuits (2)
  • Phase shifter (2)
  • RF components (2)
  • Reaction-sintering (2)
  • Shrinkage (2)
  • Sinter additive (2)
  • Sinter/sintering (2)
  • Sintering (2)
  • Solid-state-reaction (2)
  • Spring constant (2)
  • Thermoelektrika (2)
  • ceramics (2)
  • low temperature co-fired ceramics (2)
  • pressure-assisted sintering (2)
  • 5G (1)
  • Activation energy (1)
  • Adsorption (1)
  • Agglomeration (1)
  • Air oxidation (1)
  • Analysis techniques (1)
  • Analytics (1)
  • BST (1)
  • Biaxial strength (1)
  • Brazing (1)
  • Breakdown strength (1)
  • Buried thick film resistors (1)
  • Calcium Cobaltite (1)
  • Calcium maganate (1)
  • Calciummanganat (1)
  • Ceramic dispersion (1)
  • Ceramic suspension (1)
  • Ceramic wear (1)
  • Ceramic-ceramc joints (1)
  • Cermets (1)
  • Chromium (1)
  • Cofiring (1)
  • Colloidal processing (1)
  • Compsite tapes (1)
  • Cyanotoxin (1)
  • DIN EN ISO 17043 (1)
  • DP951 (1)
  • Deformation behavior (1)
  • Dialectric materials (1)
  • Dielectric breakdown (1)
  • Dielectric materials (1)
  • Diffusion (1)
  • Dilatometrie (1)
  • Drucksintern (1)
  • Druckunterstützte Sinterung (1)
  • Dry-pressing (1)
  • Electroceramics (1)
  • Embedded cavities (1)
  • Failure test (1)
  • Ferrite (1)
  • Ferrite integration (1)
  • Ferrites (1)
  • Ferroelectric (1)
  • Fertigungstechnologie (1)
  • Filters (1)
  • Fine Powder (1)
  • Folientechnik (1)
  • Force-distance diagram (1)
  • Freeze Drying (1)
  • Fully printed component (1)
  • Grain growth (1)
  • Gross slip fretting (1)
  • Grünfolieneigenschaften (1)
  • HPLC-MS/MS (1)
  • Hard machining (1)
  • Harsh environments (1)
  • Heißpressen (1)
  • High temperature laser profilometry (1)
  • High-temperature behavior (1)
  • High-temperature stability (1)
  • High-voltage testing (1)
  • Hot Press (1)
  • Hybrid laser arc welding (1)
  • Hydration (1)
  • In situ Prüfung (1)
  • Insulators (1)
  • Kalzinierung (1)
  • Keramikfeder (1)
  • Keramische Folien (1)
  • Keramische Multilayer (1)
  • Keramische Substrate (1)
  • LTCC Multilayer (1)
  • LTCC modules (1)
  • Laser beams (1)
  • Laserbearbeitung (1)
  • Laserprofilometrie (1)
  • Laserstreulichtverfahren (1)
  • Long-term requirements (1)
  • Low IR-emission ceramics (1)
  • Low pressure lamination (1)
  • Low temperature co-fired ceramics (LTCC) (1)
  • Machine Learning (1)
  • Manganese (1)
  • Manufacturing (1)
  • Master sintering curve (1)
  • Mechanical and thermal testing (1)
  • Micro fluidics (1)
  • Microwave application (1)
  • Microwave technology (1)
  • Mikrowellenkomponenten (1)
  • MnZn-ferrite (1)
  • Multilayer technology (1)
  • Nano-powder characterization (1)
  • Nanomaterial classification (1)
  • Nanomaterial screening (1)
  • Nanomaterials (1)
  • Nanopartikel (1)
  • Nanopulver (1)
  • New and emerging technologies and materials (1)
  • Ni (1)
  • Niobium carbide (NbC) (1)
  • Niobium carbide powder (1)
  • Optical centrifugation (1)
  • Optimisation (1)
  • Oxygen partial pressure (1)
  • Particle Size (1)
  • Permeability (1)
  • Phase transformation (1)
  • Phasenschieber (1)
  • Photozentrifuge (1)
  • Polycarboxylate ether (1)
  • Power factor (1)
  • Quantification (1)
  • Reduction of agglomerates (1)
  • Regulation (1)
  • Rheology (1)
  • Ringversuch (1)
  • SIDA (1)
  • Schichtverbunde (1)
  • Screen printing (1)
  • Shelf life prediction (1)
  • Shrinkage measurement (1)
  • Siebdrucken (1)
  • Silver diffusion (1)
  • Sintermechanismen (1)
  • Sintern (1)
  • Size measurement (1)
  • Slurry optimization (1)
  • Sol-gel deposition (1)
  • Solid-State-Reaction (1)
  • Solid-State-Synthesis (1)
  • Solid-state-synthesis (1)
  • Spannungs-Dehnungs-Verhalten (1)
  • Spring constant (1)
  • Stability assessment (1)
  • Stable suspensions (1)
  • Stanzen (1)
  • Strain gauge (1)
  • Structuring (1)
  • Surface water (1)
  • Suspension (1)
  • Technische Keramik (1)
  • Temperature (1)
  • Texturation (1)
  • Thermoelectric generator (1)
  • Thermoelectric oxides (1)
  • Thermoelectric properties (1)
  • Thermomechanische Eigenschaften (1)
  • Thick films (1)
  • Thick-film electrodes (1)
  • Thin-film substrate (1)
  • TiC (1)
  • Transmission electron microscopy (1)
  • Ultra sound (1)
  • Ultrasonic atomization (1)
  • Vegetable plants (1)
  • Volume specific surface area (1)
  • Wear resistant (1)
  • Welding (1)
  • Withstand voltage tests (1)
  • X20CrMoV12-1 (1)
  • Yttrium titanate (1)
  • Zirconium titanate (1)
  • Zyklische Belastung (1)
  • alumina (1)
  • centrifugal liquid sedimentation (1)
  • glass-like carbon (1)
  • granulation (1)
  • hot-embossing (1)
  • long-term requirements (1)
  • master sintering curve (1)
  • microstructure (1)
  • multilayer (1)
  • nano particles (1)
  • slurry (1)
  • spray drying (1)
  • volume resistivity (1)
- weniger

Organisationseinheit der BAM

  • 5.5 Technische Keramik (137) (entfernen)

137 Treffer

  • 1 bis 100
  • CSV
  • RIS
  • 10
  • 20
  • 50
  • 100

Sortieren nach

  • Jahr
  • Jahr
  • Titel
  • Titel
  • Autor
  • Autor
Alternative Verfahren zur Strukturierung von Grünfolien und Laminaten (2019)
Rabe, Torsten
Die Strukturierung keramischer Grünfolien und Laminate ist ein wesentlicher Prozessschritt in der keramischen Multilayer-Technologie und begründet die funktionelle Vielfalt keramischer Multilayer. Benötigt wird die Grünbearbeitung für die Herstellung elektrischer und thermischer Vias sowie innerer Kavitäten und Kanäle für Transport und Lagerung von Gasen und Flüssigkeiten in hochintegrierten Schaltungsträgern, Sensoren und Reaktoren. Standardverfahren in der industriellen Fertigung sind Heißschneiden, Stanzen und Laserbearbeitung. Darüber hinaus werden auch Bohren, Fräsen, Sägen und Heißprägen verwendet. Über die Erprobung weiterer Verfahren wie Wasserstrahlschneiden, Dampfstrahlätzen und Powder Blasting wird in der Literatur berichtet. Im Vortrag werden spezifische Vor- und Nachteile der eingesetzten Verfahren beleuchtet. Abschließend werden die Kriterien für die Auswahl des optimalen Strukturierungsverfahrens diskutiert. Zusammensetzung und Gefüge sowie die dadurch bedingten mechanischen und thermomechanischen Eigenschaften sowie das Absorptionsverhalten von Grünfolien sind sehr unterschiedlich. Daraus resultieren unterschiedliche optimale Bearbeitbarkeitsparameter.
Volume-specific surface area by gas adsorption analysis with the BET method (2020)
Gibson, N. ; Kuchenbecker, Petra ; Rasmussen, K. ; Hodoroaba, Vasile-Dan ; Rauscher, H.
This chapter first gives an introduction to the concepts of SSA and volume-specific surface area (VSSA) and an outline of the BET method. It continues with a discussion of the relationship between particle size, shape, and the VSSA, followed by an overview of instrumentation, experimental methods, and standards. Finally, sections on the use of the VSSA as a tool to identify nanomaterials and non-nanomaterials and its role in a regulatory context provide some insight on the importance of VSSA in the current Regulation of nanomaterials.
Design and fabrication of ceramic springs (2019)
Mieller, Björn
Ceramic springs offer versatile possibilities for load bearing or sensor applications in challenging environments. Although it may appear unexpected, a wide range of spring constants can be implemented by material selection and especially by the design of the spring. Based on a rectangular cross-section of the windings, it is possible to design a spring geometry that generates the desired spring constant simply by choosing appropriate diameter, height, widths, and number of windings. In a recent research project the calculation of helical compression springs made of rectangular steel (German standard DIN 2090) was applied for the design of ceramic springs. A manufacturing technology has been worked out to fabricate such springs from hollow cylinders of several highly dense technical ceramics by milling. Ceramic springs with precise rectangular section, without edge damage, and mean surface roughness smaller than 0.2 µm were produced after parameter optimization. Tolerances of less than 10 µm were achieved regarding spring diameter, height, and width of cross section. It is shown that the calculations outlined in the standard are valid for a variety of ceramic materials as well. Demonstrator springs with a wide range of spring constants have been fabricated, including zirconia springs with 0.02 N/mm, alumina springs with 1 N/mm and Si3N4 springs with 5 N/mm. A reproducibility study of six zirconia springs with a constant of 0.3 N/mm showed a relative difference in spring constants of less than +/- 1 %. This combination of a valid calculation approach for spring geometry and a reliable manufacturing technology allows for purposeful development and fabrication of ceramic springs with precise mechanical properties and superior chemical stability.
Influence of pressure assisted sintering and reaction sintering on microstructure and thermoelectric properties of bi-doped and undoped calcium cobaltite (2019)
Bresch, Sophie ; Mieller, Björn ; Schönauer-Kamin, D. ; Moos, R. ; Giovanelli, F. ; Rabe, Torsten
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 analysed. 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/mK² at 800 °C but adversely high thermal conductivity in the relevant direction. The figure of merit is higher than 0.08 at 700 °C.
Development of ceramic helical springs for sensor applications (2019)
Rabe, Torsten
At high temperatures and in harsh environments ceramic springs are often superior to metallic ones and allow for innovative solutions. A further application was proposed by using ceramic springs as capacitive force sensor. Lower and upper coil surfaces are coated by electrically conducting layers. Deformation of such spring results in a change of capacity. Sensor application calls for helical springs with rectangular cross-section, a linear stress-strain characteristic over entire deformation range and low manufacturing tolerances relating to inner and outer diameter, coil cross section and spring pitch. Furthermore, complex spring design with integrated connecting elements has to be realized. Alumina, zirconia (Y-TZP) and silicon nitride springs were produced by hard machining starting from sintered hollow cylinders. After external and internal cylindrical grinding the hollow cylinders were filled with hard wax, followed by multi-stage cutting of spring coils with custom-made cutting discs. Finally, hard wax was removed by melting and burnout. Best surface and edge qualities of springs were reached using Y-TZP material and hot isostatic pressed alumina. Y-TZP springs produced with material-specifically selected cutting discs and optimized process parameters show sharp coil edges without spallings and mean roughness values of inner surfaces < 0.2 μm. Manufacturing tolerances of spring diameters, spring pitch, height and width of coil cross section are in the range of ± 10 microns. Good reproducibility of spring geometry by optimized hard machining technology allows for production of Y-TZP springs with spring constants differing less than ± 1 % within a series. According to DIN 2090 spring constant for rectangular coil cross section is proportional to the square of height and width of coil cross section and indirectly proportional to number of active coils and to the cube of the mean spring diameter. Hence, spring constants can be tailored over a range of many orders of magnitude by changing the spring dimensions. Good agreement was reached between calculated target spring constants and measured values on produced springs. Alumina and zirconia springs were characterized relating to deformation behavior under dynamic compression load with various deformation speeds and under static tensile loads over long periods of time. Contrary to alumina springs, a non-linear stress-strain behavior of TZP springs was proved in both test series. It is supposed, that pseudoelasticity caused by stress-induced transformation of tetragonal to monoclinic phase is responsible for this special feature of TZP springs. Therefore, TZP material cannot be used for capacitive spring sensors.
Influence of test procedure on dielectric breakdown strength of alumina (2019)
Mieller, Björn
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.
Small batch preparation of ready-to-press powder for systematic studies (2019)
Höhne, Patrick
Efficient studies of scarce or expensive materials require material saving processes. Therefore, a high yield concept for small batch preparation of ready-to-press powder is exemplarily presented for yttria stabilized nano-zirconia (d50 < 50 nm). The concept involves small batch preparation in an ultrasound resonator, dispersant selection based on zeta potential measurements, evaluation of slurry stability using an analytical centrifuge, and preparation of ready-to-press powder by freeze drying. Freeze drying offers key advantages. Process efficiency and high yield above 95 % are independent of sample size. The dried product does not require further mechanical treatment like milling or grinding. Side effects like migration of additives are avoided. An optimized freeze drying process tolerates slurries with moderate stability. Thus, efforts for slurry development can be reduced. Generally, identifying a suitable dispersing agent requires only 3-5 zeta potential measurements. Slurry stability is rechecked using an analytical centrifuge, which also accounts for steric stabilization. An ultrasound resonator is used to disperse the powder without contamination, which becomes critical for small batches. The described route is exemplarily presented for the development of an additive recipe for nano-sized zirconia powder, targeting for good pressing behavior and high green density. Therefore, a variety of binding and lubricating agents were tested. Following the presented route, 80 g zirconia powder were sufficient to conduct a study including slurry development and five sample sets with varying composition, each set comprising five discs (d = 20 mm and h = 2 mm).
Superior granule properties by spray drying controlled destabilized slurries with ultrasound (2019)
Höhne, Patrick
Homogeneous introduction of organic additives is a key of ceramic powder processing. Addition of organics to ceramic slurries holds advantages compared to dry processing like organic content reduction and a more homogeneous additive distribution on the particle surface. Investigations of the alumina slurries were primarily based on zeta potential measurements and sedimentation analysis by optical centrifugation. Both methods were combined to determine a suitable additive type, amount and composition, whereas the spray drying suitability has been ensured by viscosity measurements. Granules, yielded by spray drying of such ideally dispersed alumina slurries, are mostly hollow and possess a hard shell. Those granules cannot easily be processed and can only hardly be destroyed in the following shaping step, leading to sinter bodies with many defects and poor strength and density. The precise slurry destabilization, carried out after ideally dispersing the ceramic powder, shows a strong influence on the drying behavior of the granules and hence on the granule properties. A promising degree of destabilization and partial flocculation was quantified by optical centrifugation and resulted in improved granule properties. Spray drying the destabilized alumina slurries yielded homogeneous “non-hollow” granules without the above mentioned hard shell. Sample bodies produced of these granules exhibited a reduction of defect size and number, leading to better results for sinter body density and strength. The positive effect of the slurry destabilization has been further improved, by exchanging the atomizing unit from a two-fluid one to an ultrasound atomizer with only minor slurry adjustments necessary. The controlled destabilization and ultrasound atomization of the ceramic slurry show excellent transferability for zirconia and even ZTA (zirconia toughened alumina) composite materials.
Machine learning assisted evaluation of the shape of VIAs in a LTCC multilayer (2019)
Geyler, Paul
The introduction of the 5G technology and automotive radar applications moving into higher frequency ranges trigger further miniaturization of LTCC technology (low temperature co-fired ceramics). To assess dimensional tolerances of inner metal structures of an industrially produced LTCC multilayer, computer tomography (CT) scans were evaluated by machine learning segmentation. The tested multilayer consists of several layers of a glass ceramic substrate with low resistance silver-based vertical interconnect access (VIA). The VIAs are punched into the LTCC green tape and then filled with silver-based pastes before stacking and sintering. These geometries must abide by strict tolerance requirements to ensure the high frequency properties. This poster presents a method to extract shape and size specific data from these VIAs. For this purpose, 4 measurements, each containing 3 to 4 samples, were segmented using the trainable WEKA segmentation, a non-commercial machine learning tool. The dimensional stability of the VIA can be evaluated regarding the edge-displacement as well as the cross-sectional area. Deviation from the ideal tubular shape is best measured by aspect ratio of each individual layer. The herein described method allows for a fast and semi-automatic analysis of considerable amount of structural data. This data can then be quantified by shape descriptors to illustrate 3-dimensional information in a concise manner. Inter alia, a 45 % periodical change of cross-sectional area is demonstrated.
Bestimmung der Partikelgrößenverteilung mittels Zentrifugen-Sedimentationsverfahren nach ISO 13318-2 (Küvette) (2019)
Kuchenbecker, Petra
Im Vortrag werden das Messprinzip des Zentrifugen-Sedimentationsverfahrens erläutert und die Anforderungen der zugrundeliegenden Normen diskutiert. Es schließen sich umfangreiche Ausführungen zur praktischen Durchführung der Messung, insbesondere auch zu den vorbereitenden Arbeiten, sowie zur Auswertung der Rohdaten an. Nach einem Vergleich der Ergebnisse mit anderen Messverfahren, wird das Verfahren in einer Zusammenfassung bewertet.
Evaluation of a multi-purpose measurement cell for standardized volume resistivity measurements at high temperatures (2019)
Mieller, Björn
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.
Deformation behavior of alumina and zirconia springs at room temperature (2019)
Rabe, Torsten
At high temperatures and in harsh environments ceramic springs are often superior to metallic springs and allow for innovative solutions. A recently proposed application involves ceramic springs with metallized surfaces as capacitive force sensor. A strictly linear stress-strain characteristic of the spring is a precondition for such a sensor. Helical ceramic springs with rectangular cross-section have been produced from sintered hollow cylinders. Alumina, ATZ, Y-TZP, and Ce-TZP springs with identical dimensions were characterized and compared regarding deformation behavior. Spring deformation was investigated under various load scenarios. Dynamic compression was performed with deformation speeds from 0.3 to 30 mm/min. Spring constants of alumina springs are strain rate independent. By contrast, Y-TZP spring constant increases by approximately 3 % within the experimental framework. A high-precision test facility was developed to characterize spring displacement in nm range under static tensile load over long periods of time. Spring elongation with asymptotic course was observed for zirconia containing materials at room temperature. This effect is particularly strong in the case of Y-TZP. Up to 0.3 % time-dependent elongation was measured after 24 h under constant load. Deformation is completely reversible after unloading. Alumina springs do not show any time-dependent deformation under identical test conditions. Contrary to alumina springs, a non-linear stress-strain behavior of TZP springs at room temperature was proved in both test series. It is supposed, that pseudo-elasticity caused by stress-induced phase transformation from tetragonal to monoclinic is responsible for this special behavior of TZP springs.
Electrode area dependence of dielectric breakdown strength (2019)
Mieller, Björn
Dielectric breakdown of ceramics is widely believed to originate from microstructural defects. Still, there is no commonly accepted model for the origin and process of dielectric failure that covers all observed phenomena and dependencies. In analogy to mechanical strength, the Weibull distribution is commonly used to evaluate dielectric strength data. This works well for a given group of specimens with constant geometry. But unlike mechanical strength, dielectric strength scales with the inverse square root of sample thickness. This cannot be explained by the classic Weibull concept. The Griffith type energy release rate model of dielectric breakdown proposed by Schneider is based on space charge injection and conducting filaments from the sample surface. This model incorporates the distinct thickness dependence and the pronounced influence of surface defects. Based on this model and the classic Weibull probability of failure, Schneider’s group theoretically derived a probability of breakdown that predicts an increase of failure probability with increasing electrode area. In our study we tested this model with dielectric strength data measured on dense alumina samples using different electrode areas. Weibull modulus and characteristic dielectric strength (scale parameter) were determined for a set of measurements using small electrodes. These values were used to calculate the failure probability under large electrodes according to the model. The calculated data excellently fits the measured values. Thus, our experiments substantiate the assumptions made in the breakdown model and the significance of surface defects for dielectric failure.
Nano Powder - a Challenge for Granulometry (2019)
Kuchenbecker, Petra
If the particle size decreases, the ratio of surface area to volume increases considerably. This provides benefits for all surface-driven processes that run faster or at lower temperatures than larger particles. However, handling and characterization of the nanopowders are much more difficult. Particularly polydisperse powders with irregular shape, as grinding products, represent a challenge. Granulometry in the submicron and nanoscale often leads to incorrect results without knowledge of particle morphology. This presentation demonstrates potentials of using the volume-specific surface area (SV or VSSA) in the granulometric characterization of nanopowders, for instance, correlations between the volume-specific surface area and the median particle size are discussed considering the particle morphology and the model of the logarithmic normal distribution. Moreover, the presentation deals with the optimal dispersion of nanopowders during sample preparation. Indirect ultrasound device with defined cooling was developed to prevent both contamination by sonotrode abrasion and sample changes by heat. Successful granulometric characterization of nanopowders demands both improved dispersion technology and very often an effective combination of two or more measurement methods.
Considerations for nanomaterial identification of powders using volume-specific surface area method (2019)
Kuchenbecker, Petra
The EC’s recommendation for a definition of nanomaterial (2011/696/EU) should allow the identification of a particulate nanomaterial based on the number-based metric criterion according to which at least 50% of the constituent particles have the smallest dimension between 1 and 100 nm. However, it has been recently demonstrated that the implementation of this definition for regulatory purposes is conditioned by the large deviations between the results obtained by different sizing methods or due to practical reasons such as high costs and time-consuming. For most measurement methods for particle size determination it is necessary to initially disperse the particles in a suitable liquid. However, as the particle size decreases, the adhesion forces increase strongly, making it more difficult to deagglomerate the particles and to assess accurately the result of this process. Therefore, the success of the deagglomeration process substantially determines the measurement uncertainty and hence, the comparability between different methods. Many common methods such as dynamic light scattering (DLS), centrifugal liquid sedimentation (CLS) or ultrasound attenuation spectroscopy (US) can give good comparable results for the size of nanoparticles, if they are properly separated and stabilized (e.g. in reference suspensions). In order to avoid the use of hardly available and expensive methods such as SEM / TEM for all powders, an agglomeration-tolerant screening method is useful. One of the measurement methods well suited to probe the size of particulate powder is the determination of the volume-specific surface area (VSSA) by means of gas adsorption as well as skeletal density. The value of 60 m2/cm3 corresponding to spherical, monodisperse particles with a diameter of 100 nm constitutes the threshold for decisioning if the material is a nano- or non-nanomaterial. The identification of a nanomaterial by VSSA method is accepted by the EU recommendation. However, the application of the VSSA method was associated also with some limitations. The threshold of 60 m2/cm3 is dependent on the particle shape, so that it changes considerably with the number of nano-dimensions, but also with the degree of sphericity of the particles. For particles containing micro-pores or having a microporous coating, false positive results are induced. Furthermore, broad particle size distributions made necessary to additionally correct the threshold. Based on examples of commercially available ceramic powders, the applicability of the VSSA approach was tested in relation with SEM and TEM measurements. The introduction of a correction term for deviations from sphericity and further additions improved the applicability of VSSA as a screening method.
Liquid-crystal-based amplitude tuner and tunable SIW filter fabricated in LTCC technology (2018)
Prasetiadi, A. E. ; Jost, M. ; Schulz, Baerbel ; Quibeldey, M. ; Rabe, Torsten ; Follmann, R. ; Jakoby, R.
Tunable microwave devices will play an important role in future wireless systems, in which high-frequency bands, e.g. millimeter waves, will become promising, due to its huge spectrum availability. At such frequency bands, liquid crystals (LCs) exhibit low microwave loss, which is excellent compared with the other tuning elements. In this paper, LC-based microwave components are fabricated by using low temperature co-fired ceramic technology, allowing the integration of the LC into microwave structures. The first component, the Amplitude tuner, controls the signal’s amplitude by using the interference concept, which exhibits a tunable attenuation range from 11 dB to 30 dB at 30 GHz. The second component is a 3-pole tunable bandpass filter, which is realized by using a substrate integrated waveguide topology, enabling a device with comparatively high-quality factors (Q-factors). The measurement results show Q-factors in the range of 68 to 100 for a frequency tuning of 29.4–30.1 GHz, i.e. a tuning range of 700 MHz, accompanied by an insertion loss 2 dB to 4 dB.
Keramikfedern – Herstellung, Eigenschaften und Applikation (2018)
Rabe, Torsten
Entwickelt wurde eine mehrstufige Fertigungstechnologie (Schruppen, Schlichten, Trennschnitt) zur Herstellung keramischer Federn aus gesinterten Hohlzylindern. Durch Optimierung von Maschinenparametern, Schleifscheiben sowie Werkstück- und Werkzeugaufnahmen ist es gelungen, Federn aus Hochleistungskeramik (Aluminiumoxid und Zirkonoxid) mit hoher Kanten- und Oberflächenqualität reproduzierbar herzustellen. Eine hohe Variabilität bezüglich Außen- und Innendurchmesser, Steigung, Windungsquerschnitt und Abstand zwischen den Windungen ermöglicht es, die Federkonstante über drei Größenordnungen zu variieren. Untersucht wurden Federstabilität und Spannungs-Dehnungs-Verhalten unter konstanter und zyklischer Druckbelastung sowie die thermomechanische Stabilität der Keramikfedern. Aluminiumoxid-Federn können bis etwa 800°C, Zirkonoxid-Federn bis etwa 600°C ohne bleibende geometrische Verformung eingesetzt werden. Unter konstanter Spannung zeigen Federn aus Y-stabilisiertem TZP (Zirkonoxid)-Werkstoffen, von anderen Keramikwerkstoffen abweichend, bei Raumtemperatur eine zeitabhängige, elastische, Verformung (Superelastizität), die nach Entlastung über einen Zeitraum von mehreren Stunden reversibel verläuft. Als Ursache wird eine spannungsinduzierte reversible Phasenumwandlung zwischen austenitischer (tetragonales ZrO2) und martensitischer (monoklines ZrO2) Phase postuliert. Diskutiert wird das Anwendungspotenzial der entwickelten Federn für kapazitive keramische Federsensoren für Gravimeter und Wägetechnik.
Slurry development for spray granulation of ceramic multicomponent batches (2018)
Höhne, Patrick ; Mieller, Björn ; Rabe, Torsten
The granules commonly yielded by spray drying procedures exhibit a hard shell and an irregular, dimpled shape, which is often described as donut-like morphology. Sintered parts produced from such granules suffer from microstructural defects and reduced mechanical properties resulting from these disadvantageous granule properties. Using the example of alumina, zirconia and zirconia-toughened alumina (ZTA) batches, this paper shows that the morphology of the granules can be tuned by adjusting slurry stability. High zeta potential is essential to optimally disperse the particles. But to achieve spherical and soft granules the electrostatic repulsion forces between the particles should be reduced before spray granulation. Electrostatic repulsion forces were changed with the addition of nitric acid. Measurements of zeta potential and viscosity, as well as sedimentation investigations with an optical centrifuge were used for precise slurry assessment as a major precondition for optimal and reproducible adjustment of slurries before spray drying. Sedimentation analysis using an optical centrifuge was performed to investigate different influences like that of additive composition, solids content or pH-value on the sedimentation behavior. Adequately flocculated slurries lead to homogeneous, soft granules that can be easily deformed and pressed. The fraction of donut-shaped particles and the rigidity of granules were reduced. Consequently, the sintered parts produced from these granulates show improvements regarding porosity, pore size distribution, sintered density and biaxial strength.
Influence of Reaction-Sintering and Calcination Conditions on Thermoelectric Properties of Sm-doped Calcium Manganate CaMnO3 (2018)
Bresch, Sophie ; Mieller, Björn ; Delorme, F. ; Chen, C. ; Bektas, M. ; Moos, R. ; Rabe, Torsten
A wide range of solid-state synthesis routes for calcium manganate is reported in the literature, but there is no systematic study about the influence of the solid-state synthesis conditions on thermoelectric properties. Therefore, this study examined the influence of calcination temperature and calcination cycles on the Seebeck coefficient, electrical conductivity, and thermal conductivity. Higher calcination temperatures and repeated calcination cycles minimized the driving force for sintering of the synthesized powder, leading to smaller shrinkage and lower densities of the sintered specimens. As the electrical conductivity increased monotonously with increasing density, a higher energy input during calcination caused deterioration of electrical conductivity. Phase composition and Seebeck coefficient of sintered calcium manganate were not influenced by the calcination procedure. The highest thermoelectric properties with the highest power factors and figures of merit were obtained by means of reaction-sintering of uncalcined powder.
Weiterentwicklung der VSSA-Screening-Methode zur Identifizierung von NanoPulvern (2018)
Kuchenbecker, Petra
Der Vortrag zeigt zwei Möglichkeiten zur Verbesserung der VSSA-Screening-Methode zur Identifizierung von Nanopulvern auf. Bisher ist das Verfahren nur für monodisperse Partikel mit idealer Kugelform valide. Die Verteilungsbreite der Partikelgröße soll durch die Nutzung des Modells einer logarithmischen Normalverteilung implementiert werden. Die Abweichung der gemessenen Partikel von einer idealen Kugel in Sphärizität, Rundheit und Rauigkeit sind über einen Morphologiefaktor MF zu berücksichtigen. An einem konkreten Beispiel werden Auswirkungen der Implementierungen rechnerisch dargestellt und mit dem bisherigen Verfahren verglichen.
Pressure-assisted sintering of tape cast calcium cobaltite for thermoelectric applications (2018)
Bresch, Sophie
Thermoelectric materials can convert waste heat directly into electrical power by using the Seebeck effect. Calcium cobaltite Ca3Co4O9 is a promising p-type oxide thermoelectric material for applications between 600 °C and 900 °C in air. The properties and morphology of Ca3Co4O9 are strongly anisotropic because of its crystal structure of alternating layers of CoO2 and Ca2CoO3. By aligning the plate-like grains, the anisotropic properties can be assigned to the component. 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. Pressure-less sintered multilayers have a 2.5 times higher electrical conductivity at room temperature than dry pressed test bars with randomly oriented particles. The combination of tape casting and PAS induces a pronounced alignment of the anisotropic grains. Relative density increases from 57 % after free sintering for 24 h to 94 % after 2 h of PAS with 10 MPa axial load. By applying a uniaxial pressure of 10 MPa during sintering, the electrical conductivity (at 25°C) improves by a factor of 15 compared to test bars with randomly oriented particles. The high temperature thermoelectric properties show the same dependencies. The smaller the applied axial load, the lower the relative densities, and the lower the electrical conductivity. Longer dwell times may increase the density and the electrical conductivity significantly if the microstructure is less densified as in the case of a small axial load like 2 MPa. At higher applied pressures the dwell time has no significant influence on the thermoelectric properties. 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.
Niedrigsinterndes CaMnO3 für thermoelektrische Anwendungen (2018)
Bresch, Sophie
Thermoelektrische Materialien können durch die Nutzung des Seebeckeffektes einen Temperaturunterschied direkt in eine Spannung umwandeln. Calciumcobaltit (p-typ) und Calciummanagant (n-typ) sind 2 der vielversprechendsten oxidischen thermoelektrischen Materialien. Für die Entwicklung von kostengünstigen Multilayergeneratoren ist das Co-sintern dieser beiden Materialien notwendig und deshalb eine Anpassung der Sintertemperatur nötig. Calciummangant wird herkömmlicherweise zwischen 1200°C und 1350°C gesintert. Calciumcobaltit erfährt einen ungewünschte Phasenumwandlung bei 926°C, es kann allerding bei 900°C unter 7.5MPa zu 95% dicht gesintert werden. Demzufolge, ist eine Co-sintertemperatur von 900°C anzustreben. Aus diesem Grund wurden mehrere Strategien zur Absenkung der Sintertemperatur von Calciummanaganat untersucht. Zum einen die Zugabe niedrigschmelzender Additive, zum anderen die Zugabe von Additiven, die eine eutektische Schmelze bilden. Es konnte gezeigt werden, dass für Calciummanganat die Verwendung von eutektischen Schmelzen besser geeignet ist als die Verwendung von niedrigschmelzenden Additiven um die Sintertemperatur zu senken.“
Bestimmung der Partikelgrößenverteilung mittels Zentrifugen-Sedimentationsverfahren nach ISO 13318-2 (Küvette) (2018)
Kuchenbecker, Petra
Im Vortrag werden das Messprinzip des Zentrifugen-Sedimentationsverfahrens erläutert und die Anforderungen der zugrundeliegenden Normen diskutiert. Es schließen sich umfangreiche Ausführungen zur praktischen Durchführung der Messung, insbesondere auch zu den vorbereitenden Arbeiten, sowie zur Auswertung der Rohdaten an. Nach einem Vergleich der Ergebnisse mit anderen Messverfahren, wird das Verfahren in einer Zusammenfassung bewertet.
Advanced screening method using volume-specific surface area (VSSA) for nanomaterial identification of powders (2018)
Kuchenbecker, Petra
The EC’s recommendation for a definition of nanomaterial (2011/696/EU) should allow the identification of a particulate nanomaterial based on the number-based metric criterion according to which at least 50% of the constituent particles have the smallest dimension between 1 and 100 nm. However, it has been recently demonstrated that the implementation of this definition for regulatory purposes is conditioned by the large deviations between the results obtained by different sizing methods or due to practical reasons such as high costs and time-consuming (SEM, TEM). Within the European project NanoDefine (www.nanodefine.eu) a two-tier approach has been developed, whereby firstly a screening method is applied for the rough classification as a nano-/non-nanomaterial, and for borderline cases a confirmatory method (imaging methods or field flow fractionation) must be considered. One of the measurement methods well suited to particulate powder is the determination of volume-specific surface area (VSSA) by means of gas adsorption as well as skeletal density. The value of 60 m2/cm3 corresponding to spherical, monodisperse particles with a diameter of 100 nm constitutes the threshold for decisioning if the material is a nano- or non-nanomaterial. The correct identification of a nanomaterial by VSSA method is accepted by the EU recommendation. However, the application of the VSSA method is associated also by some limitations. The threshold of 60 m2/cm3 is dependent on the particle shape, so that it changes considerably with the number of nano-dimensions of the particles. For particles containing micro-pores or having a microporous coating false positive results will be produced. Furthermore, broad particle size distributions make necessary to adjust the threshold. Based on examples of commercially available ceramic powders, the applicability of the VSSA approach will be tested (in relation with SEM and TEM measurements) in order to expand the actual knowledge and to improve this good available and agglomeration tolerant method.
Advanced screening method using volume-specific surface area (VSSA) for nanomaterial classification of powders (2018)
Kuchenbecker, Petra
The EU recommendation for a definition of nanomaterial (2011/696/EU) should allow the identification of a particulate nanomaterial based on the number-based metric criterion according to which at least 50% of the constituent particles have the smallest dimension between 1 and 100 nm. Within the European Project NanoDefine (www.nanodefine.eu) a two-tier approach has been developed, whereby firstly a screening method is applied for the rough classification as a nanomaterial or non-nanomaterial, and for borderline cases a confirmatory method (imaging methods or field flow fractionation) must be considered. One of the measurement methods well suited to particulate powder is the determination of volume-specific surface area (VSSA) by means of gas adsorption as well as skeletal density. The value of 60 m2/cm3 corresponding to spherical, monodisperse particles with a diameter of 100 nm constitutes the threshold for decisioning if the material is a nanomaterial or non-nanomaterial. The correct identification of a nanomaterial by VSSA method (positive test) is accepted by the EU recommendation. However, the application of the VSSA method is associated also by some limitations. The threshold of 60 m2/cm3 is dependent on the particle shape. For particles containing micro-pores or having a microporous coating, false positive results will be produced. Furthermore, broad particle size distributions – as typically for ceramic materials – as well as multi-modal size distributions make necessary to adjust the threshold. Based on examples of commercially available ceramic powders, the applicability of the VSSA approach will be tested (in relation with SEM and TEM measurements) in order to expand the actual knowledge and improve the method.
Lowering the sintering temperature of calcium manganate CaMnO3 for thermoelectric applications (2018)
Bresch, Sophie
Thermoelectric materials can convert waste heat directly into electrical power by utilizing the Seebeck effect. Calcium cobaltite (p-type) and calcium manganate (n-type) are two of the most promising oxide thermoelectric materials. The development of cost-effective multilayer thermoelectric generators requires the co-firing of these materials and therefore the adjustment of sintering temperatures. Calcium manganate is conventionally sintered between 1200 °C and 1350 °C. Calcium cobaltite exhibits an undesired phase transition at 926 °C but can be sintered to high relative density of 95 % at 900 °C under axial pressure of 7.5 MPa. Hence, co-firing at 900 °C would be favourable. Therefore, strategies for lowering the sintering temperature of calcium manganate have been investigated. Basically, two approaches are common: i) addition of low melting additives like Bi2O3-ZnO-B2O3-SiO2 (BBSZ) glass or Bi2O3, and ii) addition of additives that form low-melting eutectics with the base material, for example CuO. In this study, several low melting additives including BBSZ glass and Bi2O3, as well as CuO were tested regarding their effect on calcium manganate densification. Bi2O3 did not improve the densification, whereas BBSZ glass led to 10 % higher relative density at 1200 °C. An addition of 4 wt% CuO decreases the temperature of maximum sinter rate from above 1200 °C to 1040 °C. By reducing the particle size of the raw materials from 2 μm to 0.7 μm the maximum sinter rate could be further shifted 20 K towards lower temperatures and the sinter begin decreased from 920 °C to 740 °C. It is shown that eutectic phase formation is more effective in lowering sintering temperature and accelerating densification than low-melting additives.
Liquid-Crystal-Based Amplitude Tuner Fabricated (2017)
Prasetiadi, A. E. ; Jost, M. ; Schulz, Bärbel ; Quibeldey, M. ; Rabe, Torsten ; Follmann, R. ; Jakoby, R.
A 30GHz liquid-crystal-based amplitude tuner is proposed for the first time. The amplitude of a signal can be controlled by using the interference principle. An input signal is divided into a tunable liquid crystal phase shifter and a fixed transmission line. Later, the divided signals are combined together at the output port. The output amplitude depends on the phase difference between the tunable and the fixed line. The low temperature co-fired ceramic technology is utilized to fabricate the device. The measurement shows an attenuation range of 11 dB to 30 dB with a maximum biasing voltage of 100V.
Liquid-Crystal-Based Amplitude Tuner Fabricated (2017)
Prasetiadi, A .E. ; Jost, M. ; Schulz, Bärbel ; Quibelday, M. ; Rabe, Torsten ; Follmann, R. ; Jakoby, R.
30GHz liquid-crystal-based amplitude tuner is proposed for the first time. The amplitude of a signal can be controlled by using the interference principle. An input Signal is divided into a tunable liquid crystal phase shifter and a fixed transmission line. Later, the divided signals are combined together at the output port. The output amplitude depends on the phase difference between the tunable and the fixed line. The low temperature co-fired ceramic technology is utilized to fabricate the device. The measurement shows an attenuation range of 11 dB to 30 dB with a maximum biasing voltage of 100V.
Sintering and interconnecting thermoelectric oxides for energy applications (2018)
Mieller, Bjoern
Today, more than 12% of the primary energy is lost in the form of waste heat. Thermoelectric generators (TEGs) can convert waste heat directly into electrical power by utilizing the Seebeck effect. The performance of such a generator is defined by a dimensionless figure of merit ZT of the thermoelectric pairs and the resistance R of the metallic contacts between these pairs. The figure of merit of thermoelectric oxides is considerably smaller compared to semiconductors. Still, thermoelectric oxides like calcium cobaltite (Ca3Co4O9) are attractive for applications at elevated temperatures in air. In contrast to the established π-type architecture of common TEGs, tape casting and multilayer technology may be applied for cost-effective manufacturing of oxide TEGs. Promising demonstrations of multilayer TEGs have been published in the last years. Still, the development of reliable and scalable manufacturing processes and proper material combinations is necessary. The aim of our project is to evaluate the feasibility of low temperature co-fired ceramics (LTCC) technology for a practical manufacturing of oxide multilayer TEGs of Ca3Co4O9 (p-type) and calcium manganate (CaMnO3, n-type). Ca3Co4O9 exhibits an undesired phase decomposition at 926 °C. Because of that, the application of sintering strategies and interconnect concepts well known from LTCC technology is a promising approach. We present results of pressure-assisted sintering of Ca3Co4O9 multilayer at 900 °C and axial pressures of up to 7.5 MPa. Ca3Co4O9 was produced by solid state reaction of CaCO3 and cobalt(II,III)oxide at 900 °C. Green tapes were prepared by a doctor-blade process, manually stacked and laminated by uniaxial thermocompression. Sintering was conducted in a LTCC sintering press between SiC setter plates. The thickness shrinkage was recorded by an in-situ technique. After sintering under 7.5 MPa, the microstructure of the single phase material shows a high density of 95 % and an advantageous alignment of the platelet grains. This results in good electrical conductivity and a comparatively high ZT of 0.018 at room temperature. However, the lowering of CaMnO3 sintering temperature from above 1200 °C to below 920 °C remains a challenge. To select a proper metal paste for interconnections of an oxide TEG, several pastes have been investigated regarding contact resistance of internal and external (soldered) connections in a preliminary study. Commercial pastes containing Ag, Au, Au/Pt, Ag/Pd, and Ag/Pd/Bi were manually applied and post-fired on sintered test bars of Ca3Co4O9 and CaMnO3 at 900 °C for 2 h. All tested pastes formed mechanically stable metallization after firing. For resistance measurement, 4-wire method and a custom-made probe head were used. The contacts on Ca3Co4O9 exhibit significantly (2-sample t-test, α = 5%) higher resistance compared to contacts on CaMnO3. Pure silver paste exhibits the lowest resistance for internal contacts on both materials, lower than 5 mΩ on CaMnO3. Ag/Pd/Bi paste resulted in conspicuously high variance of resistance. EDX analyses clarified an enrichment of Bi in the thermoelectric material near the interface and thereby the formation of an oxide layer with probably high electrical resistance. The thickness of that layer varies with the thickness of metallization. In conclusion, the use of Bi containing pastes is not advisable. Pure Ag paste shows the best results regarding resistance and solderability.
Helical zirconia (TZP) springs manufacturing and testing under mechanical and thermal load (2018)
Rabe, Torsten
Helical springs with a rectangular cross-section have been machined from sintered and grinded hollow cylinders with high geometrical precision and good reproducibility. Such springs made from tetragonal zirconia polycrystal (TZP) ceramic show excellent edge quality because of high fracture toughness and bending strength of the starting material. Hence, springs with desired geometric dimension and tailored spring constant can be manufactured for highly demanding applications at high temperatures and in harsh environments. Prior to any practical use, application limits of springs under mechanical and thermal load have to be analyzed. Therefore, different displacement experiments were carried out on the helical TZP springs. - Dynamic displacement tests at various temperatures from -15°C to +60°C using a piezo actor to load/unload springs with frequencies between 1 and 40 Hz: Springs remained undamaged and the spring constants were not altered, even after more than one million cycles of compression loading. - Long-time displacement measurements under static tensile loading at room temperature with a high-precision interferometer test facility: Significant spring elongation under constant strain was surprisingly proved over a period of many hours already at room temperature. - Creeping experiments for 48 h under static compression load at different temperatures up to 1000 °C: After cooling down and load removing no permanent length reduction of springs was observed for test temperatures up to 700 °C. However, reshaping of TZP springs by plastic deformation is possible at higher temperatures and opens up additional possibilities for spring design and manufacturing.
Strategies to improve spray dried multi-component granules (2018)
Höhne, Patrick
Dry pressing of ceramic materials requires homogeneously soft granules with good flowability to allow rapid die filling and to avoid packing defects. Spray-drying granulation appears to be the best method for obtaining granules with high flowability in industrial scale. But, strength reducing internal microstructural defects caused by spray-dried granules with hollow and hard shells are often observed using nano and/or multi-component starting powders. Using the example of a ZTA composite, the potential of slurry optimization, ultrasound atomization and infrared drying for better granule properties and compaction behavior were investigated. Starting granules produced in a conventional spray dryer (Niro, Denmark) with a two fluid nozzle showed typical defects like large central pores and dimples. The early step of slurry preparation already possesses an essential optimization possibility in the form of stability adjustments. Granule compaction was clearly improved upon a specific reduction in slurry stability. The second optimization opportunity to improve the granule quality was the atomization step. Implementation of an ultrasound atomizing unit into the conventional spray dryer positively affected granule size distribution and therefor flowability and as well granule yield. But, a combination of both process optimizations delivered the best sinter bodies with highest density and strength due to further reduction in maximum size and fraction of pores. As last step of a spray drying process, the drying is the focus of further investigations. A current setup implying a spray dryer prototype utilizes stacked infrared heater in a countercurrent setup delivering a further increase in granule yield and enduring spraying process stability.
Influence of testing conditions on dielectric strength of alumina (2018)
Mieller, Bjoern
Dielectric strength testing of ceramics is simple and yet challenging. The execution of a breakthrough voltage measurement of a given sample is fast and straightforward. ASTM D149 describes the standardized procedure. But, there are versatile effects of test conditions and sample properties that affect the result of such a measurement. As one example, ASTM D149 allows different sizes of test electrodes and does not unambiguously prescribe the condition of the electrodes. Thus, different electrode configurations are used in the field. We conducted several test series on alumina samples to comprehensively quantify the effect of test conditions and sample properties on dielectric strength results. In our study, testing of alumina substrates using different electrode configurations resulted in differences of mean values of up to 20%. Further test series on alumina focused on the effect of voltage ramp rate. The results are complemented by calculations of failure probability at different voltage levels and corresponding withstand voltage tests. We conclude that a communication and comparison of single dielectric strength values is insufficient and may be misleading. A meaningful comparison of dielectric strength studies from different sources requires a thorough consideration of test conditions.
Influence of the calcination procedure on the thermoelectric properties of calcium cobaltite Ca3Co4O9 (2018)
Bresch, Sophie ; Mieller, Björn ; Selleng, Christian ; Stöcker, T. ; Moos, R. ; Rabe, Torsten
Calcium cobaltite is one of the most promising oxide p-type thermoelectric materials. The solid-state reaction (or calcination, respectively), which is well known for large-scale powder synthesis of functional materials, can also be used for the synthesis of thermoelectric oxides. There are various calcination routines in literature for Ca3Co4O9 powder synthesis, but no systematic study has been done on the influence of calcination procedure on thermoelectric properties. Therefore, the influence of calcination conditions on the Seebeck coefficient and the electrical conductivity was studied by modifying calcination temperature, dwell time, particle size of raw materials and number of calcination cycles. This study shows that elevated temperatures, longer dwell times, or repeated calcinations during powder synthesis do not improve but deteriorate the thermoelectric properties of calcium cobaltite. Diffusion during calcination leads to idiomorphic grain growth, which lowers the driving force for sintering of the calcined powder. A lower driving force for sintering reduces the densification. The electrical conductivity increases linearly with densification. The calcination procedure barely influences the Seebeck coefficient. The calcination procedure has no influence on the phase formation of the sintered specimens.
Sintering behavior, microstructure and thermoelectric properties of calcium cobaltite thickfilms for transversal thermoelectric multilayer generators (2018)
Schulz, T. ; Reimann, T. ; Bochmann, A. ; Vogel, A. ; Capraro, B. ; Mieller, Björn ; Teichert, S. ; Töpfer, J.
The sintering behavior and the thermoelectric performance of Ca3Co4O9 multilayer laminates were studied, and a multilayer thermoelectric generator was fabricated. Compacts and multilayer samples with anisotropic microstructure and residual porosity were obtained after conventional sintering at 920 °C, whereas dense and isotropic multilayer samples were prepared by firing at 1200 °C and reoxidation at 900 °C. A hot-pressed sample has a dense and anisotropic microstructure. Samples sintered at 920 °C exhibit low electrical conductivity due to the low density, whereas the Seebeck coefficient is not sensitive to preparation conditions. However, thermal conductivity of multilayers is very low, and, hence acceptable ZT values are obtained. A ransversal multilayer thermoelectric generator (TMLTEG) was fabricated by stacking layers of Ca3Co4O9 green tapes, AgPd conductor printing, and co-firing at 920 °C. The TMLTEG has a power output of 3 mW at ΔT =200 K in the temperature interval of 25 °C to 300 °C.
Einfluss der Pulversynthese auf die Eigenschaften thermoelektrischer Oxide (2018)
Bresch, Sophie
Calciumcobaltit und Calciummanganat gehören zu den vielversprechendsten thermoelektrischen Oxiden im Temperaturbereich zwischen 600 °C und 800 °C an Luft. Mittels thermoelektrischer Generatoren kann ein Temperaturgradient direkt in elektrische Leistung umgewandelt werden. Für die kostengünstige Pulverherstellung von Funktionsmaterialien wird im industriellen Maßstab meist die Festphasenreaktion (bzw. Kalzinierung) verwendet. Da es sich dabei um einen Hochtemperaturprozess handelt, ist diese Kalzinierung sehr energieintensiv. In der Literatur werden sehr unterschiedliche Prozessbedingungen zur Pulversynthese thermoelektrischer Oxide genutzt. Soweit dem Autor bekannt, ist keine systematische Untersuchung des Einflusses der Pulversynthesebedingungen auf die thermoelektrischen Eigenschaften publiziert. Deshalb wurde eine systematische Untersuchung des Einflusses der Pulversynthesebedingungen (Temperatur, Haltezeit, Partikelgröße, Wiederholungen) auf die thermoelektrischen Eigenschaften von Calciumcobaltit und Calciummanganat durchgeführt. Es konnte gezeigt werden, dass sich ein höherer Energieeintrag während der Kalzinierung negativ auf die thermoelektrischen Eigenschaften auswirkt.
Sintern (2018)
Mieller, Björn
Unter Sintern versteht man allgemein die Überführung eines aus Pulver geformten Rohlings in ein Formteil mit angestrebter Mikrostruktur bzw. gewünschten Gebrauchseigenschaften durch thermische Prozesse. In diesem Kapitel werden die Grundlagen zu Triebkräften und Kinetik sowie die prinzipiellen Mechanismen für Stofftransport und Verdichtung vorgestellt. Die verschiedenen Sintermechanismen Festphasensintern, Flüssigphasensintern und Reaktionssintern werden erläutert und mit einem Überblick über Drucksinterverfahren ergänzt. Abschließend wird ein Überblick über technologische Einflussfaktoren auf die Sinterung gegeben.
Schwindungsmessung an keramischen Folien (2017)
Mieller, Björn
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.
Pressure-assisted sintering of Ca3Co4O9 multilayers for thermoelectric applications (2017)
Mieller, Bjoern
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.
Improved microstructure and strength of dry-pressed advanced ceramics by controlled slurry destabilization and ultrasonic atomization (2017)
Rabe, Torsten
Spray drying of ceramic slurries aims for soft and free-flowing granules with homogenous microstructure neither containing voids nor hard shells suitable for uniaxial and isostatic pressing. The controlled destabilization of the slurries was introduced, since the unwanted formation of hard granules with voids donut-like shape could be drastically reduced. Spray drying of such modified slurries resulted in soft granules and finally leaded to sintered bodies with improved microstructure, density and strength. Optimized slurries were atomized in a laboratory spray dryer (Hi-Tec, Niro, Denmark) both with a commercial two stream nozzle and an adapted ultra sound atomization unit. Potential advantages of the ultra sound nebulization are investigated for model systems of alumina, zirconia and a ZTA composite while focusing on solids content of slurries, yield, and granule properties (size distribution, flowability, shape, microstructure, pressability) as well as the final sinter body properties (density, microstructure, strength). Using ultrasound nozzles in spray drying of identical slurries, as before tested and optimized for a two stream nozzle atomization process, resulted in a more suitable size distribution for dry pressing (less particles below 20 µm), a better flowability and a higher yield. Furthermore the sinter bodies produced of the ultra sound granules have higher sintered density, more homogenous microstructure without large pores and higher strength.
Prozessbegleitende Prüfung beim Sintern keramischer Schichtverbunde (2017)
Rabe, Torsten
Die zuverlässige Herstellung von Bauteilen aus Hochleistungskeramik erfordert den Einsatz von prozessbegleitenden Prüfverfahren in allen Fertigungsstufen. Prozessbegleitende Prüfung umfasst sowohl die Bewertung von Ausgangs- und Zwischenprodukten (Pulver, Suspensionen, Grünkörper) als auch die in-situ Überwachung kritischer Fertigungsstufen (Aufbereitung, Formgebung, Entbinderung, Sinterung). Der Arbeitskreis „Prozessbegleitende Prüfverfahren“ im DKG/DGM-Gemeinschaftsausschuss „Hochleistungskeramik“ bietet eine Plattform zum Informationsaustausch über neue Messverfahren und zur Initiierung von Ringversuchen und Referenzmaterialentwicklung. Als aktuelle Beispiele werden zwei applikationsnahe Verfahren zur in-situ Charakterisierung von Geometrieveränderungen (Schwindung, Verformung) an großformatigen keramischen Schichtverbunden vorgestellt. Die Hochtemperatur-3D-Laserprofilometrie ermöglicht die Visualisierung und Quantifizierung der lokalen Verformung keramischer Multilayer während der drucklosen Sinterung. Beobachtet wurden an LTCC-Multilayern starke Verformungen bereits während der Entbinderung und in der Abkühlphase. Das Wissen um zwischenzeitliche Verformungen erweitert das Verständnis über die Ursachen von Defekten in Multilayer-Bauteilen. Die in-situ Messung der Dickenschwindung an realen, großformatigen LTCC-Bauteilen während der druckunterstützten Sinterung wurde erstmals durch die Entwicklung eines LTCC-kompatiblen Drucksinteraggregat mit integriertem Dilatometer ermöglicht. Zusätzlich wurde das Drucksinteraggregat mit einer prozessbegleitende Mess- und Regelungseinrichtung für den Sauerstoffpartialdruck ausgestattet. Durch ein optimiertes Sauerstoffpartialdruckregime konnte an LTCC-Modulen eine qualitätsgefährdende Silberdiffusion aus den Leiterbahnen in das umgebende keramische Dielektrikum drastisch reduziert werden. Das mit einem industriellen Partner entwickelte Drucksinteraggregat eröffnet unikale Möglichkeiten zur Optimierung von Aufheizgeschwindigkeit, Druck und Sinteratmosphäre in allen Phasen des Sinterprozesses.
Pressure-assisted sintering of tape cast calcium cobaltite Ca3Co4O9 with varied powder compositions (2017)
Bresch, Sophie
Calcium cobaltite Ca3Co4O9 is a promising p-type oxide thermoelectric material for applications between 600 °C and 900 °C in air. The properties and the morphology of Ca3Co4O9 are strongly anisotropic because of its layered crystal structure. By aligning the plate-like grains, the anisotropic properties can be assigned to the component. Hot-pressing of tablets is a well-known technology for grain alignment of Ca3Co4O9 and increases the thermoelectric properties in a/b-direction remarkably. However, hot-pressing of tablets is limited by the tablet size. An interesting alternative for larger components is the pressure assisted sintering of panels from tape casted layers. Tape casting already leads to grain orientation during green body forming. By combining tape casting and pressure assisted sintering (50 kN maximum force) of Ca3Co4O9, high densities and high thermoelectric properties can be reached for large components up to 200 mm edge length. The morphology of Ca3Co4O9-grains can be designed by doping as well as by varying the powder synthesis conditions. For example Bi-doping increases the anisotropy of the grains, and reaction sintering of uncalcined powder leads to a fine grained microstructure and increases the electrical conductivity for pressure-less sintered specimens. Doped and undoped Ca3Co4O9 powders were successfully tape cast with the doctor blade technique. Several layers of tape were stacked and laminated to 7 cm x 7 cm panels. These panels were sintered in a LTCC sintering press with combined in-situ shrinkage measurement. Pressure-less sintered panels from undoped powder have a 2.5 times higher electrical conductivity at room temperature than dry-pressed test bars with randomly orientated particles. By applying a uniaxial pressure of 10 MPa during sintering, the electrical conductivity (σ25°C = 15000 S/m) increases by the factor of 6 compared to the pressure-less sintered panels, which is in good accordance to the values reported in literature for conventional hot pressing [1, 3]. It is not possible to assign the increased anisotropy of Ca2.7Bi0.3Co4O9 to the pressure-assisted sintered panels, as Bi leads to an abnormal grain growth (up to 500 µm) with randomly oriented grains. This decreases the electrical conductivity (σ25°C = 5000 S/m). Such an abnormal grain-growth is reported for Bi over-doped Ca3Co4O9 but not because of hot-pressing.
RFID sensor systems embedded in concrete – Validation experiments for long-term monitoring (2017)
Johann, Sergej ; Strangfeld, Christoph ; Müller, Maximilian ; Mieller, Björn ; Bartholmai, Matthias
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.
Integration of screen-printed tunable BST thick films in LTCC modules for microwave applications (2017)
Binder, J. R.
Reconfigurable microwave components play an important role in modern communication systems to meet the increasing demands for functionality and flexibility of the systems. Various technologies, such as semiconductor technology, microelectromechanical systems (MEMS) or ferroelectric thin and thick films, are suitable for the realization of tunable microwave components. Ferroelectric materials are characterized by high tuning speeds, negligible power consumption and low manufacturing costs. In particular, ferroelectric thick films based on barium strontium titanate (BaxSr1-xTiO3, BST) are promising systems for use in the frequency range up to approx. 12 GHz. However, due to the high sintering temperatures above 1100°C for pure BST thick films, the fabrication of tunable components based on silver or gold electrodes is limited to planar structures, which are applied after the sintering process. In this presentation, it will be shown the development of BST-ZnO-B2O3 composite materials for reducing the sintering temperature to 850-900°C. Furthermore, the material and component properties of these composites or rather corresponding MIM (metal-insulator-metal) varactors are presented and compared to planar-structured varactors. The LTCC integration potential of such fully screen-printed MIM varactors based on the developed low sintering BST composites are investigated by the fabrication of phase shifters embedded in LTCC modules.
Ringversuch: Partikelgrößenbestimmung von keramischen Pulvern mittels Laserstreulichtverfahren nach ISO 13320 (2017)
Kuchenbecker, Petra
Der Vortrag gibt anhand eines konkreten Beispiels einen Überblick über die notwendigen, umfangreichen Schritte zur Vorbereitung, Durchführung und Auswertung von Ringversuchen nach DIN EN ISO 17043.
Dicplacement Behavior of Ceramic Springs (2017)
Paulick, C.
Due to the promising combination of chemical, thermal and mechanical properties, springs made of advanced ceramics have attracted much attention as a replacement for metal springs in highly demanding applications, operating at high temperatures and in harsh environments when hardened metals can no longer be used. A further application was recently proposed by using ceramic springs with metalized surfaces as capacitive force sensors. Prior to any design of an instrument, application-specific static and/or dynamic loading experiments are necessary to investigate the stability of spring properties under the given conditions. These experiments can also be used to determine seldom measured material properties like the shear modulus of ceramics. Helical Springs with a rectangular cross-section have been machined from straight tubes of alumina (99.99% α-AI203,) and zirconia (Y/Ce-TZP). The sintered density of both materials was above 99% of the theoretical density. The stress/displacement curves turned out to be extremely linear and the spring constants were not altered, even after more than one million cycles of compression loading at various temperatures from -15°C to +60°C. This means that such springs can be a far more reproducible and reliable source of an elastic response to applied forces than handmade springs from molten quartz wires. The behavior found for in-house fabricated springs contrasts to the behavior found for a commercial ceramic spring, which was produced by injection molding and exhibits a less linear response. Furthermore, high-temperature displacement behavior of fabricated alumina and zirconia springs was tested under static loading conditions in different atmospheres (air, N2 and H2) at temperatures up to 1000 °C.
Implementation of Ba0.6Sr0.4TiO3-ZnO-B2O3 based tunable microwave phase shifters in LTCC technology (2017)
Kohler, C. ; Nikfalazar, M. ; Heunisch, Andreas ; Schulz, Bärbel ; Mikolajek, M. ; Rabe, Torsten ; Jakoby, R. ; Binder, J.
Tunable dielectric Ba0.6Sr0.4TiO3-ZnO-B2O3 thick-films were analyzed regarding their integration potential into the LTCC technology. Therefore, tunable loaded line phase shifters based on metal-insulator-metal varactors with single- and double- printed BST thick-films were fabricated and co-sintered inside a four layer LTCC module. Microstructural and chemical investigations showed a sufficient compatibility and adhesion between the silver, BST composite and LTCC layers and a resulting morphology depending on the processing route. The microwave characterization of the LTCC-embedded phase shifters revealed comparable results to phase shifters with the same design on alumina substrates.
Pressure assisted sintering of tape cast calcium cobaltite (2017)
Bresch, Sophie
Thermoelectric materials can convert waste heat directly into electrical power by utilizing the Seebeck effect. Calcium cobaltite Ca3Co4O9 is a promising p-type oxide thermoelectric material for applications between 600 °C and 900 °C in air. The properties and morphology of Ca3Co4O9 are strongly anisotropic because of its crystal structure of alternating layers of CoO2 and Ca2CoO3. The electrical conductivity is for example 13.5 times higher in a/b-direction than in c-direction. By aligning the plate-like grains, the anisotropic properties can be assigned to the component. Hot-pressing of tablets is a well-known technology for grain alignment of Ca3Co4O9 and increases the thermoelectric properties in a/b-direction remarkably. It also increases the relative density. However, hot-pressing of tablets is limited by the tablet size. An interesting alternative for larger components is the pressure assisted sintering of panels from tape casted layers. Tape casting already leads to a grain alignment during green body forming. By combining tape casting and pressure assisted sintering (50 kN maximum force) of Ca3Co4O9, high densities and high thermoelectric properties can be reached for large components up to 200 mm edge length. Ca3Co4O9 was successfully tape casted with the doctor blade technique (binder: polyvinyl butyral, organic solvent). Several layers of tape were stacked and laminated to 5 cm x 5 cm panels. These panels were sintered with different applied pressures in a LTCC sintering press with combined in-situ shrinkage measurement. Pressure-less sintered panels have a 2.5 times higher electrical conductivity at room temperature than test bars with randomly orientated particles. By applying a uniaxial pressure of 10 MPa during sintering, the electrical conductivity increases by the factor of 6 compared to the pressure-less sintered panels. About 40 % linear shrinkage are reached in pressing direction. The Seebeck coefficient (S25 °C=146 μV/K) and the electrical conductivity (σ25 °C=15100 S/m) are in good agreement with the values published in literature for hot-pressed tablets.
Effect of electrode configuration on dielectric strength testing (2017)
Mieller, Bjoern
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.
Colloidal processing of metal bonded niobium carbide (2017)
Wäsche, Rolf ; Steinborn, Gabriele ; Woydt, Mathias
Manufacturing of Niobium carbide cermets with nickel binder and different additions of titanium carbide has been realized by colloidal processing in aqueous environment.
Colloidal processing of metal bonded niobium carbide (NbC-Ni) (2017)
Wäsche, Rolf
The manufacturing of NbC with Ni binder with addition of titanium carbide by using a colloidal process for blending the different powders without a milling step were investigated. The the sintering process and formation of the resulting microstructures, the phase relations and the hardness of the produced cermet materials are characterized.
Druckunterstützte Sinterung von Calciumcobaltitfolien (2017)
Bresch, Sophie
Calciumcobaltit ist eines der vielversprechendsten thermoelektrischen Oxide, welche zur direkten Wärmerückgewinnung genutzt werden können. Calciumcobaltit weist sowohl eine anisotrope Partikelform als auch anisotrope thermoelektrische Eigenschaften auf. Durch gezielte Ausrichtung der Partikel mittels Foliengießens und druckunterstützter Sinterung kann diese Anisotropie gezielt auf ein Bauteil übertragen werden. Die Partikelmorphologie kann durch Dotierung und Kalzinierung beeinflusst werden. Verschiedene Calciumcobaltitpulver wurden druckgesintert und das Gefüge sowie die thermoelektrischen Eigenschaften untersucht. Die Dotierung mit Bismut führte zu einer Verschlechterung der elektrischen Leitfähigkeit bei druckgesinterten Proben im Gegensatz zu undotierten Pulvern.
RFID sensor systems embedded in concrete – validation experiments for long-term monitoring (2017)
Johann, Sergej ; Strangfeld, Christoph ; Müller, Maximilian ; Mieller, Björn ; Bartholmai, Matthias
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.
RFID sensor systems embedded in concrete – validation experiments for long-term monitoring (2017)
Bartholmai, Matthias ; Johann, Sergej
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.
High-temperature stability of ceramic springs (2017)
Schulz, Bärbel
Metallic springs undergo oxidation and creep at elevated temperatures and their use is limited to temperature of about 650°C. Therefore, there is a need for stable materials which can easily withstand temperatures up to 1000°C for long periods in different atmospheres. Ceramic materials have been drawing attention due to their excellent properties. This work aimed at investigating the high-temperature stability of zirconia and alumina ceramic springs at elevated temperatures under different atmospheres (air, N2 and H2) in order to determine the limitation of use of these ceramic springs.
Improved wear resistant ceramics of metal incorporated NbC processed with gas pressure sintering (2017)
Steinborn, Gabriele
Tungsten carbide (WC) dominates wear protection and machining since more than 90 years due to its hardness with an associated good toughness and high melting point. It was recently demonstrated that Niobium Carbides are wear resistant and show a better performing under cutting than WC grades. In order to profit of the good properties of Niobium Carbide, specific processing techniques need to be developed. The density of NbC is half of WC and offers colloidal processing for perfectly mixing NbC with metallic binder (up to 15 vol.-% Ni) and secondary carbide-particles. The used NbC-powder has an average particle size d50 of 3 micron measured by laser granulometry analysis. The true density was 7.68 g/cm3. An Oxygen content of the used NbC of 0.4 % is caused by residual slag content. It is possible to increase the stability of the NbC-suspensions and to reduce their viscosity by adding a specific dispersing agent. This surface-modified NbC powders (containing Ni)were uniaxially pressed and then gas pressure sintered in vacuum or Argon atmosphere. All samples reached more then 98 % of there theoretical density. The micrographs showed a homogenius distribution of the Ni-binder in the product. The tribological behaviour under dry sliding and oscillating up to 600 °C and the mechanical properties were studied.
Bestimmung der Partikelgrößenverteilung mittels Zentrifugen-Sedimentationsverfahren nach ISO 13318-2 (Küvette) (2017)
Kuchenbecker, Petra
Der Vortrag stellt ein Messverfahren zur Bestimmung der Partikelgrößenverteilung an Suspensionen vor, behandelt die Anforderungen der zugrundeliegenden ISO 13318 und gibt praktische Hinweise zur Durchführung und Auswertung. Der Messbereich umfasst Partikel mit Durchmessern von wenigen nm bis etwa 5 µm; die Partikelkonzentration muss kleiner 0,2 Ma.-% sein. Enthalten sind ergänzend Vergleichsmessungen mit anderen volumenbasierten Verfahren.
Structuring of LTCC substrates by a combination of pressure-assisted sintering and hot-embossing (2015)
Mieller, Björn ; Rabe, Torsten
A novel technology for the structuring of LTCC surfaces is introduced. The material is shaped in a zero-shrinkage process by embossing a glassy carbon mold into the softened LTCC directly after termination of the shrinkage. Three commercially available LTCC compositions (Ceramtape GC, Heratape CT707, and DP951) were tested. Diverse raised and lowered structures including rings, grids, and characters were fabricated. Different material behavior was observed for the tested compositions. Promising results were achieved with Ceramtape GC. Embossing of precise, 40 µm deep circular cavities and 50 µm high raised characters is demonstrated. Processing of 100 × 100 mm² substrates is possible. DP951 showed very good moldability, but also unwanted material displacement due to evaporating lead. A high displacement capacity but uneven heights of embossed structures were observed on CT707 samples. SEM investigations proved the precise transfer of surface contours from the mold to the LTCC. Thereby, the high potential of the hot-embossing process for micro-patterning of LTCC is illustrated.
Utilization of an ultra sound atomizer for spray granulation of oxide ceramic fine powder (2017)
Höhne, Patrick
Spray drying based granulation processes aim for flowable granules neither containing voids nor hard shells thus leading to a homogenous microstructure in the green and sinter bodies without strength reducing large pores. The increase of the specific surface area due to the utilization of finer powders as raw materials makes the production of granules of demanded quality more sophisticated. Innovations regarding additives as well as process engineering are therefore required. While conventional spray granulation processes of ceramic materials are based on rotational, one stream or two stream nozzles for nebulization, the investigations in the ZIM project concentrate on the applicability of an ultra sound atomizer unit. A spray dryer comprised of the aforementioned ultra sound atomization unit implemented in a commercial spray dryer (Niro, Denmark) was used as test system. Potential advantages of the ultra sound nebulization are investigated for model systems of alumina, zirconia and a ZTA composite while focusing on solids content, yield, pressability and granule properties (size, size distribution, flowability, shape and microstructure) as well as the final sinter body properties (density, microstructure and flexural strength). First ultra sound spray drying experiments yielded granules with excellent processability. Spray drying of identical slurries, as before tested and optimized for a two stream nozzle atomization process, resulted in a more suitable size distribution for dry pressing (less particles below 20 µm) and a higher yield. Furthermore, sinter bodies produced of ultra sound granules seem to have less large pores and a more homogenous microstructure
Advanced spray drying process by controlled slurry destabilization and ultrasonic atomization (2017)
Höhne, Patrick
Spray drying of ceramic slurries aims for soft and free-flowing granules with homogenous microstructure suitable for uniaxial and isostatic pressing. A stable and continuous spray drying process of slurries with maximized solids content is a further development target, since a reduction of the energy intensive drying procedure is desirable. First part of the investigation focuses on the development of an appropriate zirconia slurry for spray drying with optimized organic additive contents (dispersant + binder + pressing and lubricating agents). Characterization and improvement of slurries are based on zeta potential measurements and investigations of the sedimentation behavior in an optical centrifuge. Therefor the slurries were spray dried with a conventional spray dryer with a two stream nozzle run in fountain mode. The controlled destabilization of the slurries was introduced, since the unwanted formation of hard granules with donut-like shape could be circumvented. Spray drying of such modified slurries resulted in soft granules without voids and finally leaded to sintered bodies with improved microstructure, density and bending strength. Expectedly, the destabilization process causes a significant increase in viscosity of the ceramic slurry. Hence, an alternative spraying concept utilizing an ultrasonic nozzle was tested. Indeed, the ultrasonic nozzle seems able to atomize slurries with viscosities beyond the capability of the regular two stream nozzles due to the slurry’s shear thinning behavior. In second part of the investigation the integration of the ultrasonic nozzle in a commercial spray dryer is shown and first results of the spraying tests are discussed. Continuous spraying processes of highly viscous alumina, zirconia and ZTA slurries were realized over an extended production period.
In-situ measurement of thickness shrinkage during pressure-assisted sintering of low temperature co-fired ceramics (2017)
Mieller, Björn
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.
Influence of solid-state-synthesis conditions on properties of oxide thermoelectric materials (2017)
Bresch, Sophie
Calcium cobaltite and calcium manganate are promising oxide thermoelectric materials for applications between 600 °C and 900 °C in air to convert waste heat directly into electrical power. The solid state reaction, well known for large scale powder synthesis of functional materials, is used for the production of thermoelectric oxides. As a high temperature process, the powder synthesis consumes a lot of energy. In different studies, different synthesis conditions were used for the preparation of calcium cobaltite and calcium manganate powder. To the author’s knowledge, a systematic study of the synthesis conditions of calcium cobaltite and calcium manganate has not yet been published. Therefore, the synthesis conditions for calcium cobaltite (temperature, dwell time, and particle size of raw materials) were studied with a statistical design of experiments (2³) and investigated regarding phase composition (XRD), densification and thermoelectric properties. The gained knowledge was used to optimize the solid state reaction of calcium manganate. This study showed that a higher energy input (elevated temperatures, longer dwell times, or repeated calcinations) during powder synthesis does not improve but deteriorate the thermoelectric properties of calcium cobaltite. The same correlation was determined for the densification. As a higher energy input during powder synthesis leads to a larger grain size and therefore to a reduced sinter activity the densification at a given sinter profile is minimize as well as the thermoelectric properties. These results can be used to minimize the energy demand for the powder synthesis of oxide thermoelectric materials.
In situ shrinkage measurement during pressure-assisted sintering of low temperature co-fired ceramic panels (2017)
Mieller, Björn ; Rabe, Torsten ; Rangelov, V.
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.
Reliable nanomaterial classification of powders using the volume-specific surface area method (2017)
Wohlleben, W. ; Mielke, Johannes ; Bianchin, A. ; Ghanem, A. ; Freiberger, H. ; Rauscher, H. ; Gemeinert, Marion ; Hodoroaba, Vasile-Dan
The volume-specific surface area (VSSA) of a particulate material is one of two apparently very different metrics recommended by the European Commission for a definition of "nanomaterial" for regulatory purposes: specifically, the VSSA metric may classify nanomaterials and non-nanomaterials differently than the median size in number metrics, depending on the chemical composition, size, polydispersity, shape, porosity, and aggregation of the particles in the powder. Here we evaluate the extent of agreement between classification by electron microscopy (EM) and classification by VSSA on a large set of diverse particulate substances that represent all the anticipated challenges except mixtures of different substances. EM and VSSA are determined in multiple labs to assess also the level of reproducibility. Based on the results obtained on highly characterized benchmark materials from the NanoDefine EU FP7 project, we derive a tiered screening strategy for the purpose of implementing the definition of nanomaterials. We finally apply the Screening strategy to further industrial materials, which were classified correctly and left only borderline cases for EM. On platelet-shaped nanomaterials, VSSA is essential to prevent false-negative classification by EM. On porous materials, approaches involving extended Adsorption isotherms prevent false positive classification by VSSA. We find no false negatives by VSSA, neither in Tier 1 nor in Tier 2, despite real-world industrial polydispersity and diverse composition, shape, and coatings. The VSSA screening strategy is recommended for inclusion in a technical guidance for the implementation of the definition.
Effect of oxygen partial pressure on co-firing behavior and magnetic properties of LTCC modules with integrated NiCuZn ferrite layers (2016)
Naghib-zadeh, H. ; Oder, Gabriele ; Hesse, J. ; Reimann, T. ; Töpfer, J. ; Rabe, Torsten
Low-κ dielectric LTCC was developed, to realize successful co-firing with NiCuZn ferrite tapes. A critical high-temperature process in the production of highly integrated LTCC modules is the migration of silver from inner conductors into the LTCC glass phase. Intensive silver migration causes strong deformation of LTCC multilayers during firing in air. Silver migration into the LTCC glass phase depends on oxygen content of the sintering atmosphere and can be minimized by sintering in nitrogen atmosphere. However, partial decomposition of NiCuZn-ferrite and formation of cuprite was observed during sintering in nitrogen and, consequently, the permeability of the ferrite decreases. As shown by a combined XRD/thermogravimetric study the co-firing of LTCC modules with silver metallization and integrated ferrite layer demands precise adjustment of oxygen partial pressure.
Integration von gedruckten, steuerbaren Mikrowellenkomponenten in LTCC Module (2016)
Binder, J. R.
Rekonfigurierbare Mikrowellenkomponenten spielen in modernen Kommunikationssystemen eine wichtige Rolle, um den zunehmenden Anforderungen in Bezug auf Funktionalität und Flexibilität der Systeme gerecht zu werden. Für die Realisierung steuerbarer Mikrowellenbauteile eignen sich verschiedene Technologien, wie z. B. die Halbleitertechnik, mikroelektromechanische Systeme (MEMS) oder ferroelektrische Dünn- und Dickschichten. Prinzipiell zeichnen sich ferroelektrische Materialien durch hohe Schaltgeschwindigkeiten, einen vernachlässigbaren Leistungsverbrauch und geringe Prozesskosten aus. Insbesondere ferroelektrische Dickschichten auf Basis von Barium-Strontium-Titanat (BaxSri-xTiCb, BST) stellen aussichtsreiche Systeme für den Einsatz im Frequenzbereich bis ca. 12 GHz dar. Allerdings unterliegt die Herstellung dieser BST-Dickschichtvaraktoren einer Einschränkung: Aufgrund der erforderlichen hohen Sintertemperatur von über 1100°C ist die Anordnung der Varaktoren auf planare Strukturen begrenzt. In diesem Beitrag wird zum einen die Entwicklung von BST-ZnO-B203 Komposit-Dickschichten zur Verringerung der Sintertemperatur auf 850-900°C präsentiert und die Material- und Bauteileigenschaften der Komposite bzw. entsprechender MIM-(metal-insulator-metal) Varaktoren mit planar strukturierten Varaktoren auf Basis von BST-Dickschichten verglichen. Zum anderen wird die Integration solcher MIM-Varaktoren auf Basis der niedrigsinternden BST-Komposite in LTCC Module aufgezeigt.
Drucksintern von hochintegrierten LTCC-Schichtverbunden - Möglichkeiten und Grenzen (2016)
Rabe, Torsten
In der Keramikfertigung werden verschiedene druckunterstützte Sintertechnologien wie heißisostatisches Pressen, Gasdrucksintern und uniaxiales Heißpressen eingesetzt. Eine Variante des uniaxialen Heißpressens stellt das druckunterstützte Sintern von LTCC-Modulen dar. Es wird mit einem gegenüber dem klassischen Heißpressen deutlich reduzierten Druck von nur etwa 1 MPa operiert. Mit der Einführung der Drucksintertechnik wurde es möglich, großflächige, ebene LTCC-Schichtverbunde ohne laterale Schwindung dicht zu sintern und so eine kostengünstige Fertigung im Nutzen zu realisieren. Mit dem Trend zur Miniaturisierung und Funktionserweiterung von Schaltungsträgern sind Applikationskonzepte entwickelt worden, die eine Integration von Mikrowellen-, Kondensator-, Piezo- und Ferritmaterialien durch Co-Sinterung beinhalten. Kavitäten und Kanäle werden benötigt, um LTCC-Schichtverbunde auch in der Sensorik und Mikrofluidik einzusetzen. Die Umsetzung dieser Konzepte erfordert häufig diffizile Sinterprogramme mit temperaturgesteuerter Einstellung von uniaxialem Druck und Sauerstoffpartialdruck. Basierend auf den Ergebnissen zahlreicher F+E-Vorhaben werden Möglichkeiten und Grenzen der Drucksintertechnologie für die Herstellung dieser LTCC-Schichtverbunde illustriert. Vorgestellt wird ein weiterentwickeltes Drucksinteraggregat. Durch Integration von in-situ Schwindungsmessung sowie Mess- und Regeleinrichtungen für den Sauerstoffpartialdruck stehen erweiterte Möglichkeiten für die Optimierung des Drucksinterprozesses von LTCC-Schichtverbunden zur Verfügung.
Heißprägen im Grünzustand und Sinterprägen - Zwei Verfahren zur Strukturierung keramischer Folien und Laminate (2016)
Mieller, Björn
Prägeverfahren bieten eine interessante Alternative zur Strukturierung keramischer Folien und Laminate. Im Gegensatz zur etablierten Stanz- oder Laserbearbeitung von Einzelfolien sind damit einerseits Strukturtiefen realisierbar, die nicht einem Vielfachen der Einzelfoliendicke entsprechen, andererseits können Strukturierungsgrade erzielt werden, bei denen perforierte Einzelfolien nicht mehr handhabbar sind. Heißprägen grüner Folien und Laminate erfolgt bei Temperaturen um 130 °C mit Metallstempeln. Die plastische Verformung erfolgt zeitabhängig. Dabei sind die Fließeigenschaften des polymeren Binders der Grünfolie maßgeblich für die Konturtreue der Prägung. Randeinzug und Gründichtegradienten um den Eindruck können zu Defekten im Sintergefüge führen. Je nach Sinterverfahren muss die Schwindung der geprägten Struktur berücksichtigt werden, eine Kombination dieses Verfahrens mit zero-shrinkage Techniken ist aber möglich. Sinterprägen stellt eine Erweiterung der Drucksintertechnologie für glaskeramische Komposite dar. Dabei wird ein Pressstempel aus glasartigem Kohlenstoff während des Brandes in die erweichte Glasphase des Komposits gedrückt. Die Abformung erfolgt durch viskosen Fluss der Glasphase. Mit Kenntnis des Dichte- und Viskositätsverlaufs des Werkstoffs kann das Verfahren so gestaltet werden, dass die Pressform konturtreu abgeformt wird. Mittels Sinterprägen lassen sich auch erhabene Strukturen realisieren, wobei diese häufig eine erhöhte Porosität aufweisen.
Entwicklung oxidkeramischer Werkstoffe und Folien für thermoelektrische Multilayergeneratoren (2016)
Bresch, Sophie
Calcium cobaltite is a promising oxide thermoelectric materials for applications between 600 °C and 900 °C in air to convert waste heat directly into electrical power. The solid-state reaction, well known for large scale powder synthesis of functional materials, is used for the production of thermoelectric oxides. As a high temperature process, the powder synthesis consumes a lot of energy. In different studies, different synthesis conditions were used for the preparation of calcium cobaltite. To the author’s knowledge, a systematic study of the synthesis conditions of calcium cobaltite and calcium manganate has not yet been published. Therefore, the synthesis conditions for calcium cobaltite (temperature, dwell time, and particle size of raw materials) were studied with a statistical design of experiments (2³) and investigated regarding phase composition (XRD), densification, and thermoelectric properties. This study showed that a higher energy input (elevated temperatures, longer dwell times, or repeated calcinations) during powder synthesis does not improve but deteriorate the thermoelectric properties of calcium cobaltite. The same correlation was determined for the shrinkage. As a higher energy input during powder synthesis leads to a larger grain size and therefore to a reduced sinter activity the shrinkage at a given sinter profile is minimize as well as the thermoelectric properties. These results can be used to minimize the energy demand for the powder synthesis of oxide thermoelectric materials. In addition an increase of power factor by factor 10 can be achieved by applying pressure assisted sintering.
Optimization of ceramic slurries for spray drying yielding non-hollow granules (2016)
Höhne, Patrick
Economic industrial spray drying of ceramic slurries aims for as high as possible solids content. Investigated slurries of up to 80 wt% solids content were analyzed regarding stability while staying processable for granule production via spray drying. Preliminary stability examinations were carried out on the one hand via zeta potential measurements and on the other hand by optical centrifuge analysis for determination of suitable additive type, quantity and composition while even allowing the detection of potential side effects. The processability of the slurry for spraying has primarily been quantified by viscosity measurements. Early spray dried granules turned out to have internal voids and/or hard shells leading to defective sinter bodies and low density. Focusing on the root of these voids, the “hollow hard granules”, a controlled destabilization and flocculation was initiated by weakening electrostatic repulsion and approaching the isoelectric point. Destabilization, quantifiable by optical centrifugation, leaded to a change in speed of clarification as well as packing density, influencing movement speed of the phase boundary and the final height of the sediment, respectively. For sufficient destabilization, the solids content needed to be reduced in order to keep the viscosity suitable for the following spray drying procedure. The versatile controlled destabilization of the ceramic slurry finally leaded to a significantly reduced fraction of hollow granules featuring a sinter body of higher density with smaller pores and a narrower pore size distribution, additionally this destabilization approach has shown to be transferrable with excellent results to zirconia and even ZTA (zirconia toughened alumina) composite materials.
Optimization of Solid-State-Reactions of Calcium Cobaltite Ca3Co4O9 (2016)
Bresch, Sophie
Calcium cobaltite is a promising p-type oxide thermoelectric material for high temperature applications due to its high figure of merit between 600 °C and 900 °C in air. The solid-state-reaction is well known for large scale powder synthesis of functional materials. As a high temperature process, the powder synthesis consumes a lot of energy. In different studies, different synthesis conditions were used for the preparation of calcium cobaltite powder. This study showed that a higher energy input (elevated temperatures, longer dwell times, or repeated calcinations) during powder synthesis does not increase but decrease the Seebeck coefficient and the electrical conductivity. The same correlation was determined for the densification. As a higher energy input leads to a larger grain size and therefore to a reduced sinter activity, it can be concluded that the thermoelectric properties are correlated with the sinter activity of the powder. These results can be used to minimize the energy demand for the powder synthesis of Ca₃Co₄ O₉.
Advancements in pressure-assisted sintering technology for low temperature co-fired ceramics (LTCC) (2016)
Mieller, Björn
Steadily increasing demands on design and dimensional accuracy of ceramic multilayer modules, as well as the processing of new materials, require continuous improvements of manufacturing technology, especially thermal processes. The capabilities of pressure-assisted sintering (PAS) for the manufacturing of highly integrated low temperature co-fired ceramics (LTCC) multilayer have been considerably extended in the last years by procedural and device-related advancements. A lambda probe has been integrated in a sintering press prototype to monitor and control the process atmosphere. Thereby, the development of oxygen partial pressure during binder burnout of real modules can be observed. On the other hand, the oxygen partial pressure can be regulated during densification, for example to prevent diffusion of silver from circuit paths into the surrounding LTCC. Thin-film capable surfaces can be produced without post-processing by using setter plates made of glass-like carbon in an advanced PAS process under nitrogen. A newly developed advancement of this approach enables in-situ hot-embossing of LTCC during PAS by using structured glass-like carbon molds. The prototype press is further extended by a sensitive displacement transducer for monitoring the thickness shrinkage of real modules with an edge length of up to 8 inch.
Vergleich verschiedener Messverfahren zur Partikelgrößenanalyse am Beispiel von nanodispersem ZrO2-Pulver (2016)
Steinborn, Gabriele ; Gemeinert, Marion ; Schmidt, Wolfram
Fünf verschiedene Messverfahren wurden zur Partikelgrößenanalyse von nanodispersen ZrO₂-Pulver verglichen. Mit der Laserstreulichtanalyse, der dynamischen Lichtstreuung (heterodyne DLS und homodyne DLS-PCS), der Sedimentationsanalyse im Zentrifugalfeld und der Ultraschallspektrometrie wurden wässrige ZrO₂-Suspensionen mit verschiedenen Feststoffkonzentrationen hinsichtlich ihrer Partikelgrößenverteilung analysiert. Als Referenz diente die REM-Analyse zur Ermittlung der Primärpartikelgröße (ca. 40 nm). Mit den hier vorgestellten Messverfahren konnten in den entsprechenden Suspensionen nur Sekundärpartikel im Bereich von 105 nm bis 224 nm detektiert werden, die somit auf das Vorhandensein von Aggregaten bzw. harten Agglomeraten hinweisen.
Influences of nano effects on the flow phenomena of self-compacting concrete (2016)
Schmidt, Wolfram ; Weba, Luciana ; Silbernagl, Dorothee ; Mota Gassó, Berta ; Höhne, Patrick ; Sturm, Heinz ; Pauli, Jutta ; Resch-Genger, Ute ; Steinborn, Gabriele
Chemical admixtures like superplasticisers or stabilising agents are of ever increasing importance for modern concrete technology. They liberate the workability of concrete from its dependency on water content, and thus, open the gate towards innovative and future oriented concrete technologies such as selfcompacting concrete. Meanwhile admixtures have become common practice in concrete technology, but the understanding of these highly complex polymers in the entire concrete system lags far behind their application. Due to its complex time-dependent, multi-phase and multi-scale behaviour, flowable concrete systems are highly complicated and cannot be described comprehensively by simple models. It is therefore extremely challenging to identify the relevant parameters that predominantly control flow phenomena on different size scales, since these may occur on any scale between the nano scale (e.g. superplasticizer adsorption) and macro scale (e.g. grading of the aggregates). The present study discusses fundamental mechanisms at the interface between particle or hydrate surfaces and the fluid phase at a very early stage of concrete formation, and links these effects to macroscopic flow phenomena. Methods are discussed that appear promising interdisciplinary tools for enhancement of the understanding of the relevant interactions that are responsible for the macroscopic flow of flowable concrete.
Stability assessment of ceramic high dielectric (2016)
Gemeinert, Marion
Resolution of capacitive sensors can be improved enormously by replacement of the dielectric material between the capacitor plates (e.g. air-dielectric) by a dielectric fluid with high permittivity. High dielectric liquid dispersions of ceramic micro and nano powders should be qualified as dielectric fluid with longtime shelf life. For this purpose it was necessary to produce stabilized ceramic suspensions with high particle concentration and to investigate sedimentation processes of the particles. Characterization of particles was done by use of zeta potential measurement, gas adsorption measurements (BET), density measurement with gas pycnometer as well as particle sizing by ultrasound spectroscopy and by use of an optical centrifuge. Shelf life of optimized electrostatic and steric stabilized ceramic suspension was investigated by use of an optical centrifuge, a LUMISizer 651 MW (LUM Ltd.) with STEP technology and front tracking analysis. Two different wave lengths – NIR (865 nm) and blue light (470 nm) were available for examination. Centrifugation measurements with different rotation speed were used to study the rheological behavior and the sedimentation process. By this way it was possible to achieve accelerated stability determination. Measured values could be used to simulate the sedimentation process under gravity acceleration and to predict shelf life for suspensions with different dispersants.
The Analytical centrifuge – an additional tool to optimize ceramic slurries for spray drying (2016)
Kuchenbecker, Petra
Spray drying is a widely used process step in ceramic technology to convert fine grained raw powders to free-flowing granules. Type and amount of the organic additives (dispersants, binders and plasticizers) define viscosity and stability of the slurry, the granulate structure and the compaction behaviour. Alumina and Zirconia powders were characterized with regard to particle size, shape and specific surface area (BET). Zeta-potential measurements by Stabino® were used to investigate the effect of different dispersants on slurry stability. Unfortunately, this method is restricted to suspensions with less than about 30% by weight of solid content. The multi-sample analytical centrifuge LUMiSizer® overcomes this problem. This device enables analysis of the separation behaviour of suspensions over a wide range of solid content. On the basis of position and time resolved photometric detection of transmitted light, it is possible to determine the instability index, the sedimentation velocity, and the sediment height in dependence of type and amount of organic additives at various centrifugal accelerations. Additional information is obtained from the shape of the transmission profiles. Based on LUMiSizer® measurements, suitable additive concentrations were derived. Thereby, the number of investigations by viscometry and spray drying experiments can be significantly reduced. Finally, it was possible to produce alumina, zirconia and ZTA aqueous slurries with up to 80 weight-% of solid. The optimized suspensions were successfully spray dried and the resulted granules show good compaction behaviour.
High-strength and gas-tight ceramic-ceramic joints by RAB composite tapes (2016)
Naghib-zadeh, Hamid
Ceramic components with complex shape cannot be produced frequently by usual ceramic forming and sintering processes. Therefore, numerous joining methods were developed and introduced in industrial scale. Nowadays, multi-stage Mo-Mn-process and active brazing are preferentially used, if temperature-stable and gastight joints are required. Unfortunately, both processes involve cost-intensive thermal processes: hydrogenous atmosphere is essential for metallization in Mo-Mn-process and active brazing takes place under vacuum. Thermal processes can be drastically simplified by using Reactive Air Brazing (RAB). Joining under air atmosphere is an interesting alternative, especially to join oxide ceramic components among themselves. So far, main disadvantage of RAB is low strength of join connections. Aim of this investigation was the development of high-strength, thermal shock resistant and gastight ceramic-ceramic joints by RAB. Therefore, - commercial, silver and copper oxide containing RAB soldering composition was modified by addition of ceramic particles with low thermal expansion coefficients (TEC). Hence, thermal misfit between TEC of solder and ceramic components was significantly reduced. - RAB soldering paste was replaced with newly developed RAB composite tapes, produced by ceramic “doctor blade” technology. Thereby, improved potential exist to tailor the brazing layer relating to composition, thickness and thickness uniformity. Gastight alumina-alumina, alumina-zirconia and zirconia-zirconia joints with strongly improved strength were produced by novel composite tapes. No strength degradation of joints was observed after thermal cycling up to 700°C.
Integration of additive-free Ni–Cu–Zn ferrite layers into LTCC multilayer modules (2016)
Hesse, J. ; Naghib-zadeh, H. ; Rabe, Torsten ; Töpfer, J.
The sintering behavior of sub-micron Ni0.30Cu0.20Zn0.52Fe1.98O3.99 ferrite with and without Bi2O3 addition was studied. Ferrites with 0.5 wt% Bi2O3 exhibit enhanced shrinkage at T < 900 °C with significant grain growth. Additive-free ferrite powders also sinter to high density at 900 °C, however, grain growth is very limited. Both ferrites exhibit a permeability of µ = 400–450. Multilayers consisting of ferrite and low-k dielectric LTCC layers were prepared by co-firing at 900–915 °C. The shrinkage and thermal expansion characteristics of ferrite and LTCC tapes are similar. However, the permeability of integrated ferrite layers, made from ferrite tapes with Bi2O3 additive, significantly drops after co-firing with LTCC layers compared to separately fired monolithic ferrite multilayers. Contrarily, the permeability of integrated, Bi2O3-free ferrite layers, co-fired with dielectric tapes, is identical to that of monolithic ferrite multilayers. This finding is an important step toward ferrite integration into complex LTCC multilayer architectures.
Stable aqeous niobium carbide suspensions for production of homogeneous hard materials (2016)
Steinborn, Gabriele
Hard materials consist of a hard phase embedded in a metallic binder. In order to achieve high toughness and strength, it is necessary to have a perfect mixing of hard phase and binder, which is mainly achieved by ball milling. Niobium carbide (NbC) has a high potential to substitute tungsten carbide as hard material. The publication presents the development of stable homogeneous and de-agglomerated NbC-dispersions. To prevent agglomeration of the powder, stable suspensions were achieved by surface treatments with the dispersants (PD and HD), which resulted in a charge reversal from a negative to a positive zeta potential. This surface-modified powder guaranteed a stable re-dispersion in the binder suspension. Nickel powder was added as metallic binder. This suspension was suited for 3D-printing. The green samples could be sintered in vacuum or Argon atmosphere.
Hot-embossing of low temperature co-fired ceramics during pressure-assisted sintering (2016)
Mieller, Björn ; Rabe, Torsten
A novel process to structure the surfaces of low temperature co-fired ceramics (LTCC) is presented. Lowered and raised structures are formed by hot-embossing with glass-like carbon molds during pressure-assisted sintering. Molding is driven by viscous flow of the LTCC glassy phase above the glass transition temperature. For accurate molding of embossments on the LTCC surface, proper filling of cavities in the glass-like carbon mold is necessary. Therefore, de-airing of the mold cavity has to be assured. Two strategies have been investigated: (i) hot-embossing at 850 °C after termination of LTCC shrinkage with de-airing through vent holes in the mold; and (ii) hot-embossing of open porous LTCC at 775 °C with dense molds, de-airing through pore channels in the LTCC, and subsequent densification by further heating to 850 °C. Circular embossments with 10 mm diameter were molded on a commercially available LTCC (Ceramtape GC, CeramTec GmbH, Marktredwitz, Germany). The sintered height was measured using optical profilometry. Image processing was used to evaluate porosity distributions in the sintered structures. The influence of embossing temperature on LTCC viscosity and mold filling behavior is discussed. Successful molding of 47 µm high raised grids and characters by hot embossing with 0.41 MPa at 775 °C and further heating to 850 °C under constant load is demonstrated. Thereby, the high potential of hot-embossing for precise structuring of LTCC surfaces is illustrated.
Structuring of LTCC Substrates by a Combination of Pressure-Assisted Sintering and Hot Embossing (2015)
Brandt, Björn ; Rabe, Torsten
A novel technology for the structuring of low temperature co-fired ceramic (LTCC) surfaces is introduced. The commercial LTCC Ceramtape GC is shaped in a zero-shrinkage process by embossing a glass-like carbon mold into the softened LTCC during pressure-assisted sintering. Diverse raised and lowered structures including rings, grids, and characters were fabricated. It was found that de-airing of mold cavities is crucial for the molding of embossments. De-airing is possible through pore channels in the LTCC if embossing is performed at intermediate temperatures. The influence of LTCC viscosity on the mold filling behavior during the formation of raised structures is discussed. For accurate molding and proper densification of the LTCC, hot embossing with 0.41 MPa at 775 °C and subsequent heating under load to 850 °C is proposed. Embossing of precise, 40 µm deep circular cavities and 50 µm high raised bars and characters is demonstrated. Thereby, the high potential of the hot-embossing process for micro-patterning of LTCC is illustrated.
Glass-ceramic composite multilayer structures - modeling of densification and shrinkage mismatch (2016)
Mieller, Björn
Low temperature co-fired ceramics (LTCC) are glass-ceramic composites that are processed by tape casting and multilayer technology to create multilayer circuit boards and electronic packages for high frequency applications, microsystems, and sensors. Pressure-assisted sintering (PAS) enables the densification of LTCC without lateral shrinkage, and thereby the manufacturing of miniaturized modules with minimal shrinkage tolerances. Co-firing of LTCC with other functional ceramics facilitates the implementation of innovative module concepts. The master sintering curve model is a useful tool to supplement the empirical optimization of such sintering processes. Two applications of the model are presented. Firstly, an algorithm is deduced that allows the simulation of shrinkage mismatch in a combined multilayer structure. Based on this simulation, an optimized process for the co-firing of LTCC and ferrite is derived. Secondly, the pressure-assisted sintering of LTCC is modeled. A comprehensive description of the densification behavior of LTCC during PAS is thereby possible.
Sensitivity analysis of the residual stress state in friction stir welding of high strength aluminium alloy (2016)
Bachmann, Marcel ; Rethmeier, Michael ; Wu, Chuan Song
In this paper, the friction stir welding process was numerically investigated for 6 mm thick aluminum alloy AA2024-T3. The finite element software COMSOL Multiphysics was used to calculate the transient thermal field during welding and the mechanical reaction depending on different mechanical clamping conditions and hardening models subsequently. A thermal pseudo-mechanical (TPM) heat source was implemented. Softening effects of the material due to precipitation hardening dissolution caused by the frictional heat were accounted for. The transient temperature evolution measured by thermocouple elements at various locations was compared to the numerical results. A good agreement was found for the thermal field. A sensitivity study of the mechanical models showed the strong influence of the clamping conditions and the softening model.
Continuously tuneable liquid crystal based stripline phase shifter realised in LTCC technology (2015)
Jost, M. ; Strunck, S. ; Heunisch, Andreas ; Wiens, A. ; Prasetiadi, A.E. ; Weickhmann, C. ; Schulz, Bärbel ; Quibeldey, M. ; Karabey, O.H. ; Rabe, Torsten ; Follmann, R. ; Koether, D. ; Jakoby, R.
This work presents the design, fabrication and measurements of a low temperature cofired ceramic (LTCC) integrated liquid crystal (LC) phase shifter. The effective permittivity of the phase shifter and therefore its differential phase shift can be tuned continuously by orienting the LC directors with electric fields. The phase-shift demonstrator is designed for Ka-band frequencies around 30 GHz and represents a stripline filled with LC, embedded inside an LTCC multilayer structure, which provides a space-qualified and hermetically-sealed LC cavity within an RF-capable material system. The total length of the device is 37 mm while the phase shifting LC section has a length of 14.6 mm. At 30 GHz it exhibits a differential phase shift of 60° with an insertion loss around 6 dB, resulting in a figure of merit around 10°/dB. An advantage is that the insertion loss is nearly independent of the tuning state of the LC. The response time of the phase shifter, depending on the desired LC orientation, is between 62 ms and 37 s.
Foreword to topical issue "Low temperature co-fired ceramics -LTCC" (2015)
Rabe, Torsten ; Partsch, U.
Journal of ceramic science and technology - Topical issue "Low temperature co-fired ceramics - LTCC" (2015)
Pressure-assisted sintering of buried thick film resistors in LTCC (2015)
Heunisch, Andreas ; de Seauve, Victor ; Rabe, Torsten
In this work, the effect of the pressure-assisted sintering process on buried thick film resistors integrated in LTCC multilayer has been studied. Four commercial resistor pastes with sheet resistivities between 10 kΩ and 10 MΩ/cm were analyzed. First they were characterized by SEM/EDX, XRD and Laser diffraction to determine composition and particle distribution. The pastes consist of isolating particles and of Ruthenium based particles that are supposed to build the conductive phase. The pastes were screen printed on LTCC green tape (DP 951) and buried in four layer laminates. Sintering was done in two ways, pressureless (PLS) and also pressure-assisted (PAS). The pressureless sintered resistors showed electrical resistance values roughly in the range of the nominal sheet resistivity and only relatively small fluctuation within one sample. The PAS samples on the other hand showed significantly higher resistances and larger deviations. The microstructure of the sintered resistors was again investigated by SEM and XRD. It seems that the resistivity is determined by the ratio of the two Ruthenium phases RuO2 and Pb2Ru2O6.5, where RuO2 has the higher conductivity. Buried resistors cannot be trimmed by a laser to adjust the resistance. But we discovered that a refiring step will reduce and normalize the resistivity of the PAS resistors significantly.
In situ characterization of the sintering behavior of LTCC laminates with embedded cavities by high temperature laser profilometry (2013)
Heunisch, Andreas ; Marzok, Ulrich ; Münchow, Marco ; Müller, Ralf ; Rabe, Torsten
LTCC phase shifters based on tunable ferroelectric composite thick films (2015)
Nikfalazar, M. ; Kohler, C. ; Heunisch, Andreas ; Wiens, A. ; Zheng, Y. ; Schulz, Bärbel ; Mikolajek, M. ; Sohrabi, M. ; Rabe, Torsten ; Binder, R. ; Jakoby, R.
This paper presents, the investigation of tunable components based on LTCC technology, implementing ferroelectric tunable thick-film dielectric. The tunable loaded line phase shifters are fabricated with metal-insulator-metal (MIM) varactors to demonstrate the capabilities of this method for packaging of the tunable components. The MIM varactors consist of one tunable dielectric paste layer that is printed between two silver layers. The tunable ferroelectric paste is optimized for LTCC sintering temperature around 850°C. The phase shifters are fabricated in two different process. They were achieved a figure of merit of 24°/dB (phase shift 192°) at 3 GHz and 18°/dB (phase shift 98°) at 4.4 GHz by using seven unit cells that each unit cell consisting of two MIM varactors.
Compilation and evaluation of the pressure-assisted master sintering surface for low-temperature cofired ceramics (2015)
Brandt, Björn ; Rabe, Torsten
Pressure-assisted sintering (PAS) is an established procedure for the production of low-temperature cofired ceramics (LTCC) without lateral shrinkage and minimal shrinkage tolerances for automotive and high-frequency applications. To develop a feasible model for the prediction of densification during that process, master sintering curves (MSCs) for the commercial LTCC DP951 were generated from thermomechanical analysis (TMA) data in the pressure regime from 2 to 500 kPa. Strain mainly related to creep deformation of the LTCC was identified by evaluation of the strain rate and was discarded for the determination of MSC parameters. It was found that no creep occurred at any pressure up to a relative density of 0.9. Different pressure levels can be modeled with the same activation energy of 400 kJ/mol. Densification curves predicted by the model were in good agreement with experimental data. Based on MSCs, the pressure-assisted master sintering surface was compiled to illustrate the influence of pressure on densification. The results show that the MSC approach is a suitable method to feasibly predict the densification of LTCC during PAS.
Bursting Tensile Strength Testing of brazed joints formed by Reactive Air Brazing with Novel Composite Tapes (2015)
Mieller, Björn
Reconstruction of 3D transient temperature field for fusion welding processes on basis of discrete experimental data (2015)
Pittner, Andreas ; Karkhin, Victor ; Rethmeier, Michael
This paper presents an approach to reconstruct the three-dimensional transient temperature field for fusion welding processes as input data for computational weld mechanics. The methodology to solve this inverse heat conduction problem fast and automatically focuses on analytical temperature field models for volumetric heat sources and application of global optimisation. The important issue addressed here is the question which experimental data is needed to guarantee a unique reconstruction of the experimental temperature field. Different computational-experimental test cases are executed to determine the influence of various sets of discrete experimental data on the solvability of the optimisation problem. The application of energy distributions utilised for laser beam welding allows reconstructing the temperature field efficiently. Furthermore, the heat input into the workpiece determined by the simulation contributes to the evaluation of the thermal efficiency of the welding process.
Das Laserstreulichtverfahren in der Rohstoffcharakterisierung - Erfahrungen aus Ringversuchen zur Präszision der Messergebnisse (2015)
Kuchenbecker, Petra ; Gemeinert, Marion ; Rabe, Torsten
Die Bestimmung der Partikelgrößenverteilung mittels Laserstreulichtverfahren hat sich bei den verschiedensten Pulvern im Mikrometer- und Submikrometerbereich insbesondere wegen der kurzen Messdauer und des breiten Messbereichs zu einem der führenden Verfahren entwickelt. In den letzten Jahren gab es eine Reihe von gerätetechnischen Weiterentwicklungen durch die Hersteller, die der Tatsache Rechnung tragen, dass die Anwender immer feinere Ausgangsprodukte einsetzen und damit auch charakterisieren wollen. Die aktualisierte Fassung der ISO 13320 von 2009, welche die normative Basis der Methode bildet, gilt grundsätzlich im Größenbereich von 3000 gm bis hinunter zu 0,1 gm. Es sind aber zusätzliche Ausstattungsmerkmale beschrieben, die diesen Bereich unter Umständen erweitern können. Weltweit gibt es etwa zehn etablierte Hersteller von Lasergranulometern, deren modernste Modelle jeweils über derartige Zusatzeinrichtungen verfügen. Die eingesetzten Geräte unterscheiden sich in ihrem Aufbau damit nicht nur zwischen den einzelnen Herstellern, sondern auch innerhalb verschiedener Gerätegenerationen ein und desselben Herstellers. Hinzu kommt der mathematische Prozess der Konvertierung der gewonnenen Streulichtdaten in eine Partikelgrößenverteilung (Dekonvolution), bei dem die Hersteller verschiedene Ansätze und Glättungsmethoden nutzen. Seit 2004 hat die Bundesanstalt für Materialforschung und -prüfung drei Ringversuche zur Bestimmung der Partikelgrößenverteilung mittels Laserstreulichtverfahren initiiert. Neben dem eigentlichen Zweck - der Eignungsbewertung der teilnehmenden Labore - wurden die erhobenen Daten genutzt, um Aussagen zur Präzision der Ergebnisse zu gewinnen und Ursachen für Abweichungen zu finden. Die Wiederholbarkeit der Messergebnisse innerhalb der Labore war unabhängig vom eingesetzten Gerätetyp sehr gut und erfüllte die Anforderungen der ISO 13320. Geringe Abweichungen bei der Reproduzierbarkeit wurden nur zwischen Laboren gefunden, die identische Gerätetypen desselben Herstellers verwendeten. Über alle Labore betrachtet, ergaben sich Vergleichstandardabweichungen, die in Abhängigkeit von der Größe der gemessenen
New Setter Materials for the Pressure-Assisted Sintering of Low Temperature Co-Fired Ceramics (2015)
Mieller, Björn
LTCC multilayer with integrated cavities for tunable microwave components (2015)
Heunisch, Andreas
Pressure-Assisted Sintering of Buried Thick Film Resistors in LTCC (2015)
Heunisch, Andreas
Structuring of LTCC Substrates by a Combination of Pressure-Assisted Sintering and Hot-Embossing (2015)
Mieller, Björn
Vergleich verschiedener Messverfahren zur Partikelgrößenanalyse am Beispiel von nanodispersen ZrO2-Pulvern (2015)
Steinborn, Gabriele
LIQUIDA-Sky Project: Electronically Tunable Filters for K-Band Satellites based on Microwave Liquid Crystal Technology (2015)
Heunisch, Andreas ; Francke, T.
LTCC mit integrierten Widerstandselektroden für ein steuerbares Hochfrequenz-Filter (2015)
Heunisch, Andreas
Druckunterstützendes Sintern glaskeramischer Komposite mit Opferfolie aus hexagonalem Bornitrid (2015)
Mieller, Björn
  • 1 bis 100

OPUS4 Logo

  • Kontakt
  • Impressum
  • Sitelinks