High thermoelectric performance of Ca3Co4O9 ceramics with duplex structure fabricated via two-step pressureless sintering

Two-step pressureless sintering processing was used to prepare high density (97.6%) Ca3Co4O9 thermoelectric ceramics. A duplex structure was observed and channel paths for charge carriers were formed between the grains. Oxygen vacancies were identified by fitting of XPS spectra, and the effect of the oxygen vacancies on thermoelectric properties was investigated. The electrical resistivity of samples decreased from 17.5 to 11.7 mΩ·cm and Seebeck coefficient decreased from 197.5 to 181.4 μV/K at 1073 K with increasing of the first-step sintering temperature. Furthermore, a low thermal conductivity of 0.94 W/(m·K) and the peak ZT of 0.30, which was 1.3 times larger than previous work, were obtained at 1073 K when the first-step sintering temperature was 1373 K.