High thermoelectric performance in donor-substituted strontium titanate-based composites processed by laser floating zone

Abstract

Oxide thermoelectrics, made from abundant, eco-friendly materials, can withstand high-temperature gradients, making them highly promising for high-temperature waste heat harvesting. Recent advancements in the efficiency of thermoelectric material families have been driven by composite approaches, fostering synergistic effects between materials, further enhanced by advanced processing techniques. Accordingly, this study explores the design of oxide-based thermoelectric composites, involving the redistribution of a common substituting cation between composite phases, driven by laser floating zone (LFZ) processing. Niobium substituted strontium titanate/rutile composite material with a nominal composition Sr_0.97Ti_0.8Nb_0.2O₃ / 0.15Ti_0.95Nb_0.05O₂ was processed by LFZ at various pulling rates (50, 100, and 200 mm/h), followed by post-thermal treatment under highly reducing conditions. The obtained samples showed inhomogeneous niobium distribution between perovskite and rutile phases shaped by strongly non-equilibrium conditions inherent to the LFZ processing, in contrast to the conventional solid-state route. Adjusting the pulling rate enabled a certain degree of control over niobium incorporation into both phases. Primarily driven by an enhanced Seebeck coefficient, the LFZ-processed and thermally treated samples demonstrated high power factors, reaching 1350–820 μW·K⁻²·m⁻¹ at 473–1173 K, respectively. An appealing ZT of 0.52 at 1173 K was achieved for the composite sample processed at 100 mm/h and subjected to thermal treatment. This performance is attributed to a combination of a high power factor and low thermal conductivity (∼2 W·m⁻¹·K⁻¹ at 1173 K), enabled by phase and compositional inhomogeneities, as well as residual porosity introduced by LFZ processing. The LFZ technique offers considerable potential for optimisation and has proven to be a powerful tool for designing ceramic composite thermoelectric materials.