Interfacial reaction and thermoelectric properties of Ca₃Co₄O₉ ceramic diffusion bonding joints with different electrode intermediate layers

p-Ca₃Co₄O₉ ceramic and Al₂O₃ substrate were welded by vacuum diffusion bonding via different interlayers. Metal oxides could be observed at the interface when copper foil and nickel foil were applied. There was no obvious reaction layer at the interface when silver foil was applied. In the diffusion bonding process, Ca₃Co₄O₉ thermoelectric ceramic decomposed into CaO and CoO. CaO and CoO reacted again to form Ca₃Co₄O₉ after oxygen enriched treatment at 850 °C for 10 h. The joints fabricated using silver foil exhibited the highest shear strength of 10.8 MPa. The electrical resistivity of p-type Ca₃Co₄O₉ ceramic was 44.57 mΩ cm, Seebeck coefficient was 192.08 μV/K and the power factor was 0.08 mW m⁻¹ K⁻² at 800 °C. Moreover, a maximum power factor of 0.21 mW m⁻¹ K⁻² was obtained at the temperature of 500 °C. The resistance of the module was approximately 3.92 Ω, which is 60% lower than that of the unheated module.