Improving High –Temperature Energy Conversion Materials: The Role of Reducing Conditions and Thermal Cycling in Rare-Earth Doped Strontium Titanate.

Influence of a strongly reducing conditions and periodic elevated temperature treatment (thermal cycling) on the thermoelectric performance of 20 mol. % La+Sm rare earth doped strontium titanate (Sr_0.7La_0.1Sm_0.1TiO₃) ceramic materials was investigated. The ceramics were synthesized via conventional solid state reaction route. For comparison, 1300A samples (20 mol. % of VSTO-A, Sr_0.7La_0.1Sm_0.1TiO₃ air calcined) exhibited relatively high thermal conductivity, which is detrimental to thermoelectric performance. In contrast, powders calcined at 1300 ^oC and sintered at 1500^oC in a 5 % H₂/N₂ atmosphere produced the sample designated 1300H, which exhibited a reduced thermal conductivity of 2.67 Wm^-1k^-1 and a maximum dimensionless figure of merit (ZT) of 0.30 at 973 K. To investigate the effect of periodic temperature increase under reducing conditions, portions of the 1300H sample were subsequently recalcined at 1300 ^oC (codenamed 1350H) and 1400 ^oC (codenamed 1400H) in 5 % H₂/N₂ and sintered at 1500^oC. A progressive enhancement in thermoelectric performance was observed with increasing recalcination temperature. The 1400H sample exhibited the lowest thermal conductivity of 2.50 Wm^-1k^-1 and achieved the highest ZT of 0.35 at 973 K. the improved performance is attributed to enhanced reduction induced defect formation and increased phonon scattering, which effectively suppressed heat transport while preserving favorable electrical transport characteristics. These findings demonstrate that periodic elevated-temperature treatment in a strongly reducing atmosphere provides an effective route for reducing thermal conductivity and enhancing the thermoelectric performance of rare-earth co-doped SrTiO₃ ceramics, thereby highlighting their potential for high temperature waste recovery applications.