Improving thermoelectric properties of high-entropy (Ca_0.27Sr_0.27Ba_0.27La_0.19)TiO_3-δ ceramics through defect engineering by controlling the oxygen vacancy content

The current dependence on a singular optimization approach has severely impeded the further enhancement of the performance characteristics of strontium titanate-based thermoelectric materials, thereby significantly hindering their development and practical applications. This study integrates entropy engineering with defect engineering to explore the influence of increased oxygen vacancy concentrations and their interactions with other defects on the microstructure, electrical, and thermal transport properties of high-entropy (Ca_0.27Sr_0.27Ba_0.27La_0.19)TiO_3-δ ceramics. These ceramics were synthesized using spark plasma sintering (SPS) and subsequently subjected to varying degrees of annealing in a reducing atmosphere. The enhancement of oxygen vacancy concentration within the ceramic did not alter its intrinsic phase structure; however, it had a significant and observable effect on grain growth, porosity, and the formation of dislocation bands. Additionally, the reduction of Ti⁴⁺ to Ti³⁺ was facilitated. These modifications substantially improved the electrical conductivity of the ceramics and accelerated the reduction in thermal conductivity with temperature, thereby promoting the achievement of high ZT values at elevated temperatures. Ultimately, the thermoelectric ceramic demonstrated an impressive power factor of 492 μW/(m·K²) at 1073 K, coupled with a notably low thermal conductivity of 0.31 W/(m·K) and a ZT value of 0.2. This investigation also confirms the existence of an optimal oxygen vacancy concentration that maximizes the thermoelectric performance of the ceramic material. This research underscores the capacity of the combined application of entropy engineering and defect engineering to transcend the confines of conventional single-optimization methodologies, resulting in a marked improvement in the thermoelectric performance of the material across diverse parameters.