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 (Sr 1− x Gd x )TiO 3− 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 (Sr 1− x Gd x )TiO 3− 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 (Sr 0.9 Gd 0.1 )TiO 3− 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 κ L reduces remarkably (e.g. κ L decreases from 2.7 to 2.13   W   m −1   K −1 at ∼1000   K). Owing to the large PF and low κ L , a record value of figure of merit ZT   =   0.37 is achieved at 1006   K for (Sr 0.9 Gd 0.1 )TiO 3− 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.