Improving the thermoelectric properties of SrTiO₃-based ceramics with metallic inclusions

High quality Sr_0.8La_0.067Ti_0.8Nb_0.2O_3-δ ceramics containing inclusions of Cu or Fe were prepared by the mixed oxide route and sintered under reducing conditions at 1700 K. Products were high density; Fe enhanced density (to 98% theoretical) and grain size (to 15 µm) whilst Cu tended to reduce both slightly. XRD and SEM analyses showed that the material is mainly composed of cubic SrTiO₃ phase, with rutile-structured minor phases and metallic inclusions along the grain boundaries. The inclusions increased carrier concentration and carrier mobility, leading to a reduction in electrical resistivity by a factor of 5 and increase in power factor by 75%. Both types of metallic inclusions were beneficial, increasing the thermoelectric figure of merit (ZT) from 0.25 at 1000 K for the control sample, to ZT of 0.36 at 900 K for the Cu-containing samples and to 0.38 at 1000 K for the Fe-containing samples.