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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.
The sintering behavior of sub-micron Ni0.30Cu0.20Zn0.52Fe1.98O3.99 ferrite with and without Bi2O3 addition was studied. Ferrites with 0.5 wt% Bi2O3 exhibit enhanced shrinkage at T < 900 °C with significant grain growth. Additive-free ferrite powders also sinter to high density at 900 °C, however, grain growth is very limited. Both ferrites exhibit a permeability of µ = 400–450. Multilayers consisting of ferrite and low-k dielectric LTCC layers were prepared by co-firing at 900–915 °C. The shrinkage and thermal expansion characteristics of ferrite and LTCC tapes are similar. However, the permeability of integrated ferrite layers, made from ferrite tapes with Bi2O3 additive, significantly drops after co-firing with LTCC layers compared to separately fired monolithic ferrite multilayers. Contrarily, the permeability of integrated, Bi2O3-free ferrite layers, co-fired with dielectric tapes, is identical to that of monolithic ferrite multilayers. This finding is an important step toward ferrite integration into complex LTCC multilayer architectures.