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 m Omega cm, Seebeck coefficient was 192.08 mu V/K and the power factor was 0.08 mW m(-1) K (-2) at 800 degrees C. Moreover, a maximum power factor of 0.21 mW m(-1) K-2 was obtained at the temperature of 500 degrees C. The resistance of the module was approximately 3.92 Omega, which is 60% lower than that of the unheated module.

Misfit-layered Ca3Co4O9 as a p-type semiconductor is difficult to commercialize because of its relatively poor performance. Here, Ca2.7-xLaxAg0.3Co4O9/Ag composites prepared by spark plasma sintering were systematically investigated in terms of La3+ dopant levels and nano-sized Ag compacts. Multiscale microstructures of stacking fault, dislocation, and oxygen vacancy-linked defects could be recognized as an effective strategy for tuning the transport of charge carriers and phonon scattering. An increasing concentration of charge carriers was caused by the introduction of nano-sized Ag particles at the grain boundary. The multiscale structural defects served as phonon scattering centers to reduce the thermal conductivity. Finally, the Ca2.61La0.09Ag0.3Co4O9/Ag sample exhibited a maximum ZT of 0.35 at 1073 K. The results suggest that the interplay of structural defects provides an impetus for a huge improvement in thermoelectric performance.