Effect of Reaction Layers on Internal Stresses in Co-Fired Multilayers of Calcium Manganate and Calcium Cobaltite

A widespread recovery of waste heat requires a cost-effective production of thermoelectric generators. Thermoelectric oxides are predestined for use at high temperatures. For manufacturing reasons, a multilayer generator design will be easily scalable and cost-effective. To evaluate the potential of ceramic multilayer technology for that purpose, a multilayer of the promising thermoelectric oxides calcium cobaltite (Ca3Co4O9), calcium manganate (CMO, CaMnO3), and glass-ceramic insulation layers is fabricated. Cracks and reaction layers at the interfaces are observed in the microstructure. The compositions of these reaction layers are identified by energy-dispersive X-ray spectroscopy and X-ray diffraction. Mechanical and thermal properties of all layers are compiled from literature or determined by purposeful sample preparation and testing. Based on this data set, the internal stresses in the multilayer after co-firing are calculated numerically. It is shown that tensile stresses in the range of 50 MPa occur in the CMO layers. The reaction layers have only a minor influence on the level of these residual stresses. Herein, it is proven that the material system is basically suitable for multilayer generator production, but that the co-firing process and the layer structure must be adapted to improve densification and reduce the tensile stresses in the CMO. The co-firing of calcium manganate and calcium cobaltite in a thermoelectric prototype component results in vertical cracks in the manganate layers. Analysis of the reaction layers at the interfaces and numerical calculation of internal stress distributions shows that critical tensile stresses in the manganate layer occur. These stresses are not significantly affected by the reaction layers.image (c) 2024 WILEY-VCH GmbH