Enhanced thermoelectric performance of highly dense and fine-grained (Sr1−xGdx)TiO3−δ ceramics synthesized by sol-gel process and spark plasma sintering

Highly dense and fine-grained (Sr1−xGdx)TiO3−δ ceramics (x = 0.05-0.12) were fabricated by a sol-gel process plus spark plasma sintering, and their thermoelectric properties were investigated from ∼300 K to 1100 K. The results show that the electrical resistivity of the (Sr1−xGdx)TiO3−δ ceramics decreases greatly while maintaining large absolute value of the Seebeck coefficient |S| upon Gd doping. As a result, a large power factor (PF) greater than 10 μW cm−1 K−2 is obtained in (Sr0.9Gd0.1)TiO3−δ in the temperature range from 400 K to 1000 K, and peak PF reaches 16 μW cm−1 K−2 at ∼570 K. Simultaneously, the lattice thermal conductivity κL reduces remarkably (e.g. κL decreases from 2.7 to 2.13 W m−1 K−1 at ∼1000 K). Owing to the large PF and low κL, a record value of figure of merit ZT = 0.37 is achieved at 1006 K for (Sr0.9Gd0.1)TiO3−δ, indicating that appropriate combinations of fine-grained microstructure with both high densities and proper doping as well as oxygen deficiency are essential to the effective enhancement of thermoelectric performance of STO-based ceramics.