Boosting the Thermoelectric Performance of Calcium Cobaltite Composites through Structural Defect Engineering

Misfit-layered Ca₃Co₄O₉ as a p-type semiconductor is difficult to commercialize because of its relatively poor performance. Here, Ca_2.7-xLa_xAg_0.3Co₄O₉/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 Ca_2.61La_0.09Ag_0.3Co₄O₉/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.