Pressure-assisted sintering of tape cast calcium cobaltite for thermoelectric applications

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... [ view full abstract ]

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.