Microstructure and thermoelectric performance of La-doped (Ca_0.9Ag_0.1)₃Co₄O₉/nano-sized Ag composite ceramics

(Ca_0.9-xAg_0.1La_x)₃Co₄O₉/nano-sized Ag thermoelectric composite ceramics were fabricated by solid-state reaction method. The influences of La³⁺ dopants on the microstructure, band gaps, and thermoelectric properties were investigated. X-ray photoelectron spectroscopy fitting results showed that the reduction of oxygen vacancy and the increment of trivalent cobalt concentration could significantly improve thermoelectric performance. With the increasing of La³⁺ dopants level, the band gaps of samples enlarged. Electrical resistivity decreased from 21.3 to 17.2 mΩ·cm, while Seebeck coefficient increased from 113.3 to 182.3 μV/K. Furthermore, the peak ZT value of 0.18 was obtained at 1073 K for the x = 0.03 sample, resulting from the lower thermal conductivity of 1.05 W/(m·K). The obtained results suggest that the La³⁺ substitution, oxygen vacancy, and Ag inclusions can dramatically improve the thermoelectric performance of Ca₃Co₄O₉ composites.