TY - CONF A1 - Stargardt, Patrick T1 - Modeling of the cooling behavior of thermoelectric multilayers N2 - Multilayered designs are an attractive approach towards cost-effective manufacturing of thermoelectric generators. Therefore, efforts are being made to co-fire two promising thermoelectric oxides, namely calcium cobaltite and calcium manganate. In this study, ceramic tapes, multilayer technology, and pressure-assisted sintering (PAS) were used. A major challenge for the PAS of low-sintered calcium manganate was cracking during cooling. A relationship between the properties of the release tape used during PAS and the cracking behavior was experimentally observed. To understand the origin of failure, reaction layers in the multilayer were analyzed and thermal stresses during cooling were estimated by finite element (FE) simulations. Thermal expansion, elastic properties, and biaxial strength of the thermoelectric oxides and selected reaction layers were determined on separately prepared bulk samples. The analysis showed that the reaction layers were not the cause for cracking of calcium manganate. Using the FE model, thermal stresses in different manganate multilayer designs with varying properties of the release tape were studied. The FEM study indicated, and a validation experiment proved that the thickness of the release tape has the main effect on thermal stresses during cooling in separately sintered calcium manganate. T2 - Keramik 2021 / Ceramics 2021 CY - Online Meeting DA - 19.04.2021 KW - Fem KW - Thermoelectric KW - Pressure assisted sintering PY - 2021 AN - OPUS4-52489 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Bresch, Sophie A1 - Mieller, Björn A1 - Moos, Ralf A1 - Rabe, Torsten T1 - Pressure-assisted sintering of tape cast calcium cobaltite Ca3Co4O9 with varied powder compositions N2 - 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. T2 - European Conference on Thermoelectrics (ECT) 2017 CY - Padua, Italy DA - 25.09.2017 KW - Thermoelectrics KW - Pressure assisted sintering KW - Grain growth PY - 2017 AN - OPUS4-42283 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Bresch, Sophie A1 - Mieller, Björn A1 - Moos, Ralf A1 - Rabe, Torsten T1 - Pressure assisted sintering of tape cast calcium cobaltite N2 - 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. T2 - 15th Conference & Exhibition of the European Ceramic Society CY - Budapest, Hungary DA - 09.07.2017 KW - Thermoelectrics KW - Pressure assisted sintering PY - 2017 AN - OPUS4-41026 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -