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Thermoelectric generators can be used for energy harvesting by directly transforming a temperature gradient into a voltage. Multilayer generators based on ceramic multilayer technology are an interesting alternative to conventional π-type generators. They exhibit several advantages like high filling factor, possibility of texturing, co-firing of all materials in one single-step, and reduction of production costs due to the high possible degree of automation. But, co-firing of promising oxide thermoelectric materials, Ca3Co4O9 (p-type) and CaMnO3 (n-type), is very challenging due to the large difference in sintering temperature (300 K).
In this work we show the material development of Ca3Co4O9, CaMnO3, and insulation for multilayer generators co-fired under uniaxial pressure at 900 °C. The materials are tailored regarding their sintering behavior, electrical performance and coefficients of thermal expansion. Tape-casting and pressure assisted sintering are applied to fabricate textured Ca3Co4O9. Compared to conventional sintering, pressure assisted sintering increases the strength by the factor 10 and the power factor by the factor of 20. The combination of sintering additives and uniaxial pressure is used to decrease the sintering temperature of CaMnO3 to 900 °C while maintaining acceptable thermoelectric properties.
Different generator designs (unileg and pn-type) were fabricated and analyzed regarding microstructure and thermoelectric performance. A lower level of complexity is beneficial for co-firing and performance. The unileg demonstrators reach 80% of the simulated output power and the power output is highly reproducible between the different demonstrators (99%).
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.