A-Site deficiency- and redox-driven functional performance of Laser Floating Zone-processed SrTiO3-based thermoelectric ceramics

High-temperature waste heat harvesting using thermoelectric technology requires the development of materials that combine high thermoelectric performance with stability under large thermal gradients. In this context, oxide-based thermoelectrics have emerged as promising candidates for high-temperature operations. Among these, SrTiO 3 stands out as one of the most promising oxide system, combining high phase stability with tunable electrical and thermal transport properties. In this work, Sr 1-x Ti 0.9 Nb 0.1 O 3 ceramics with varied nominal A-site deficiency (x = 0-0.07) were successfully processed using the laser floating zone (LFZ) technique under both air and H 2 -containing atmospheres. The LFZ process enabled rapid, crucible-free growth of mechanically robust samples while allowing controlled phases formation and cations distribution as a function of processing conditions and Sr deficiency. X-ray diffraction studies confirmed the predominance of the cubic perovskite phase in all compositions, while energy-dispersive X-ray spectroscopy revealed the presence of secondary TiO 2 and Nb-rich phases depending on the growth atmosphere. The best thermoelectric performance was achieved for the sample Sr 0.95 Ti 0.9 Nb 0.1 O 3 grown in H 2 /N 2 atmosphere and heat treated in reducing conditions (10% H 2 /90% N 2 ) atmosphere (0.95STNO_h2+tt) sample, which exhibited a high-power factor and a peak ZT of 0.36 at 1173 K. This enhancement is attributed to the combined effects of improved electrical conductivity and reduced lattice thermal conductivity, influenced by A-site deficiency, phonon scattering from point defects, and secondary-phase interfaces. A four-leg UNILEG-type thermoelectric module fabricated from this composition delivered a maximum power output of approximately 10 mW under a 248 K thermal gradient, with a hot-side temperature of 675 K. Although contact resistance remains a major limitation, this study highlights the potential of LFZ processing for tailoring microstructure, defect chemistry, and thermoelectric performance in SrTiO 3 -based materials.