Thermoelectric enhancement in triple-doped strontium titanate with multi-scale microstructure

Strontium titanate (SrTiO₃) is a thermoelectric material with large Seebeck coefficient that has potential applications in high-temperature power generators. To simultaneously achieve a low thermal conductivity and high electrical conductivity, polycrystalline SrTiO₃ with a multi-scale architecture was designed by the co-doping with lanthanum, cerium, and niobium. High-quality nano-powders were synthesized via a hydrothermal method. Nano-inclusions and a nano/micro-sized second phase precipitated during sintering to form mosaic crystal-like and epitaxial-like structures, which decreased the thermal conductivity. Substituting trivalent Ce and/or La with divalent Sr and substituting pentavalent Nb with tetravalent Ti enhanced the electrical conductivity without decreasing the Seebeck coefficient. By optimizing the dopant type and ratio, a low thermal conductivity of 2.77 W·m^-1·K^-1 and high PF of 1.1 mW·m^-1·K^-2 at 1000 K were obtained in the sample co-doped with 5-mol% La, 5-mol% Ce, and 5-mol% Nb, which induced a large ZT of 0.38 at 1000 K.