Influence of liquid−phase sintering on microstructure and thermoelectric properties of Ca3Co4O9−based ceramics with Bi2O3 additive

Secondary phase Bi2O3, which could promote the sintering densification of ceramics, was used to prepare (Ca0.85Ag0.1La0.05)3Co4O9 thermoelectric ceramics. The mechanism of liquid−phase sintering process revealed that the diffusion rate of particles originated from the dissolved–precipitated processing for samples and suppressed the coarsening of grain growth. The effects of Bi2O3 on the microstructure and thermoelectric properties were investigated. The results showed that the relative density of samples increased from 92.1% to 95.5% through the liquid−phase sintering mechanism. The band gaps were tuned and it had the profound impact on the transport of charge carriers. Electrical resistivity decreased while Seebeck coefficient increased from 110 μV/K to 190 μV/K with increasing Bi2O3. Furthermore, the peak ZT value of 0.25 for 6 wt% Bi2O3 sample at 1073 K was obtained, resulting from low thermal conductivity of 0.92 W/(m·K). It suggests that Bi2O3 additive can dramatically improve the thermoelectric properties of Ca3Co4O9.