TY - CONF A1 - Mieller, Björn A1 - Rabe, Torsten T1 - Hot-embossing of low temperature co-fired ceramics during pressure-assisted sintering N2 - 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. T2 - 91. Jahrestagung der Deutschen Keramischen Gesellschaft CY - Freiberg DA - 07.03.2016 KW - Low temperature co-fired ceramics KW - Pressure-assisted sintering KW - Glass-like carbon KW - Structuring KW - Hot-embossing PY - 2016 N1 - Geburtsname von Mieller, Björn: Brandt, Björn Birth Name of Mieller, Björn: Brandt, Björn AN - OPUS4-35538 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Mieller, Björn A1 - Gemeinert, Marion A1 - Koppert, R. A1 - Bolte, J. A1 - Rabe, Torsten T1 - LTCC substrates for high performance strain gauges N2 - Recent advances in the development of high gauge factor thin-films for strain gauges prompt the research on advanced substrate materials. A glass ceramic composite has been developed in consideration of a high coefficient of thermal expansion and a low modulus of elasticity for the application as support material for thin-film sensors. Constantan foil strain gauges were fabricated from this material by tape casting, pressure-assisted sintering and subsequent lamination of the metal foil on the planar ceramic substrates. The sensors were mounted on a strain gauge beam arrangement and load curves and creep behavior were evaluated. The accuracy of the assembled load cells correspond to accuracy class C6. That qualifies the load cells for the use in automatic packaging units and confirms the applicability of the LTCC substrates for fabrication of accurate strain gauges. To facilitate the deposition of thin film sensor structures onto the LTCC substrates, the pressure-assisted sintering technology has been refined. By the use of smooth setters instead of release tapes substrates with minimal surface roughness were fabricated. Metallic thin films deposited on these substrates exhibit low surface resistances comparable to thin films on commercial alumina thin-film substrates. The presented advances in material design and manufacturing technology are important to promote the development of high performance thin-film strain gauges. T2 - CICMT 2012 - 8th International conference on ceramic interconnect & ceramic microsystems technologies CY - Erfurt, Germany DA - 16.04.2012 KW - LTCC KW - Thin-film substrate KW - Pressure-assisted sintering KW - Strain gauge PY - 2012 N1 - Geburtsname von Mieller, Björn: Brandt, B. - Birth name of Mieller, Björn: Brandt, B. SP - 000175 EP - 000180 (Session TP 4 / TP 43) AN - OPUS4-27567 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Bresch, Sophie A1 - Mieller, Björn A1 - Schönauer-Kamin, D. A1 - Moos, R. A1 - Giovanelli, F. A1 - Rabe, Torsten T1 - Influence of pressure assisted sintering and reaction sintering on microstructure and thermoelectric properties of bi-doped and undoped calcium cobaltite N2 - 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. KW - Ceramics KW - Calcium cobaltite KW - Thermoelectric properties KW - Calcination KW - Pressure-assisted sintering PY - 2019 DO - https://doi.org/10.1063/1.5107476 SN - 0021-8979 VL - 126 IS - 7 SP - 075102-1 EP - 075102-11 PB - AIP Publishing CY - Melville AN - OPUS4-48708 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Schulz, T. A1 - Reimann, T. A1 - Bochmann, A. A1 - Vogel, A. A1 - Capraro, B. A1 - Mieller, Björn A1 - Teichert, S. A1 - Töpfer, J. T1 - Sintering behavior, microstructure and thermoelectric properties of calcium cobaltite thickfilms for transversal thermoelectric multilayer generators N2 - 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. KW - Thermoelectric oxide KW - Calcium cobaltite KW - Pressure-assisted sintering KW - Multilayer PY - 2018 DO - https://doi.org/10.1016/j.jeurceramsoc.2017.11.017 SN - 0955-2219 SN - 1873-619X VL - 38 IS - 4 SP - 1600 EP - 1607 PB - Elsevier Ltd. AN - OPUS4-43983 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Mieller, Björn T1 - Sintering and interconnecting thermoelectric oxides for energy applications N2 - 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. T2 - CICMT 2018 CY - Aveiro, Portugal DA - 18.04.2018 KW - Thermoelectric oxide KW - Thermoelectric generator KW - Multilayer technology KW - Pressure-assisted sintering PY - 2018 AN - OPUS4-44740 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ibrahim, Ahmed A. S. A1 - Bochmann, Arne A1 - Löhnert, Romy A1 - Mieller, Björn A1 - Wilde, Lutz A1 - Capraro, Beate A1 - Stelter, Michael A1 - Töpfer, Jörg T1 - Performance Enhancement of Ca3Co4O9‐based Transverse Multilayer Thermoelectric Generators for Low‐Power Applications N2 - Transverse thermoelectric generators (TTEG) enable conversion of thermal into electrical energy with perpendicular directions of the applied temperature gradient and the induced thermoelectric voltage. We report on the fabrication of transverse multilayer thermoelectric generators (TMLTEG) based on p‐type Ca3Co4O9 (CCO) ceramic tapes and printed silver which were conventionally sintered (CS) in air at 920°C or using pressure‐assisted sintering (PAS) at 920°C and 1.5 MPa. The thermoelectric performance of TMLTEGs was evaluated using analytical calculations and simulations. The transverse thermoelectric power factor and thermoelectric figure‐of‐merit of an artificial layered structure composed of CCO and silver were calculated and simulated as functions of layers tilt angle φ and metal‐to‐ceramic thickness ratio. TMLTEG devices with various CCO layer thicknesses of 150 µm, 100 µm, or 33 µm were fabricated and cofired at 920°C in air, which exhibit power outputs of 2.3 mW, 3.2 mW, and 4.1 mW at ΔT = 160 K, respectively. TMLTEGs which were cofired using PAS show a higher power density of 16.4 mW/cm3 at ΔT = 225 K. This enhancement in power (≈ 80%) is crucial for thermoelectric modules comprising multiple TMLTEG devices. The device measurements were compared with 3D simulations. KW - Ceramic multilayer KW - Thermoelectric generator KW - Pressure-assisted sintering PY - 2026 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-657868 DO - https://doi.org/10.1111/jace.70656 SN - 0002-7820 VL - 109 IS - 3 SP - 1 EP - 13 PB - Wiley Periodicals LLC. AN - OPUS4-65786 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -