Design of SrTiO3-Based Thermoelectrics by Tungsten Substitution

Abstract

Among n-type oxide thermoelectrics, donor-substituted strontium titanates, prepared in highly reducing conditions, show a particularly attractive thermoelectric figure of merit. High electrical conductivity, combined with outstanding redox tolerance and perovskite-phase stability of these materials, also make them prospective candidates for solid oxide fuel cell (SOFC) anode components. This work represents a first attempt to process strontium titanate ceramics with significant W for Ti substitution and to assess their relevant defect chemistry-related aspects and electrical and thermal properties, seeking mainly highly performing oxide thermoelectrics. Combined XRD/XPS/SEM/EDS studies of SrTi1-xWxO3 +/-delta (x = 0.01-0.10), prepared by a conventional solid state route, demonstrated that the maximum solubility of tungsten corresponds to 3-5% mol, depending on firing conditions and other composition changes. Separation of tungsten-containing phases on a submicro- and nanoscale level and formation of core-shell microstructures were confirmed for x >= 0.06, suggesting possibilities for tuning the thermal and electrical conductivities. Titanium cations are substituted predominantly by W6+ and partially by W5+ . High electrical conductivity and the Seebeck coefficient resulted in a maximum power factor of similar to 0.5 mW x m(-1) x K-2 for SrTi0.99W0.01O3 +/-delta; maximum ZT values, observed in the case of x = 0.01-0.06, amounted to 0.18-0.24 at 1173-1273 K. Co-substitution in Sr(Ti,Nb,W)TiO3 +/-delta materials showed good prospects for boosting thermoelectric performance in titanates, predominantly by significant reduction of the thermal conductivity.