Broadening the temperature range for high thermoelectric performance of bulk polycrystalline strontium titanate by controlling the electronic transport properties

Strontium titanate (SrTiO₃) is a promising n-type thermoelectric material at high temperature. However, to date, its reported high dimensional figure of merit (zT) > 0.4 has only been achieved in a narrow temperature range near 1000 K. In this study, zT values of >0.4 were achieved in the broad temperature range of 769-1009 K in bulk SrTiO₃ co-doped with La and Nb with in situ precipitation of second phases of NbC and TiO_2-δ. The electronic transport properties of the samples were optimized by adjusting the doping ratio, resulting in a large power factor of 1.82 mW m^-1 K^-2 at 622 K for 7 mol% La-7 mol% Nb-doped SrTiO₃. Notably, the power factor (PF) decreased more gradually with increasing temperature, resulting in a high PF of 1.28 mW m^-1 K^-2 even at 1009 K. In addition, precipitation of the second phases occurred during sintering of the mixture of La-Nb doped SrTiO₃ nano powder and carbon powder, which provided additional phonon scattering centers except for the phonon scattering centers of La and Nb point defects. This high thermoelectric performance achieved over a broad temperature range could be beneficial for broadening the range of application temperatures for bulk polycrystalline SrTiO₃. Furthermore, the tailoring strategy of co-doping, an in situ second phase, and oxygen vacancies applied in this study may be applicable to other oxide thermoelectric materials.