Abstract
Perovskite-type SrTiO3-delta ceramics are multifunctional materials with significant potential as n-type thermoelectric (TE) materials. The electronic and thermal transport properties of spark plasma sintered polycrystalline Sr1-xYxTiO3-delta (x = 0.05, 0.075, 0.1) ceramics are systematically investigated from (15-800) K. The Sr0.9Y0.1TiO3-delta simultaneously exhibits a large Seebeck coefficient, alpha > -80 mu V/K and moderately high electrical resistivity, rho similar to 0.8 m Omega-cm at a carrier concentration of similar to 10(21) cm(-3) at 300K resulting in a high TE power factor defined herein as (alpha(2)sigma T) similar to 0.84 W/m-K at 760 K. Despite the similar atomic masses of Sr (87.6 g/mol) and Y (88.9 g/mol), the lattice thermal conductivity (kappa L) of Sr1-xYxTiO3-delta is significantly reduced with increased Y-doping, owing to the smaller ionic radii of Y3+ (similar to 1.23 angstrom, coordination number 12) compared to Sr2+ (similar to 1.44 angstrom, coordination number 12) ions. In order to understand the thermal conductivity reduction mechanism, the kappa(L) in the Sr1-xYxTiO3-delta series are phenomenologically modeled with a modified Callaway's equation from 30-600K. Phonon scattering by elastic strain field due to ionic radii mismatch is found to be the prominent scattering mechanism in reducing kappa(L) of these materials. In addition, the effect of Y-doping on the elastic moduli of Sr1-xYxTiO3-delta (x = 0, 0.1) is investigated using resonant ultrasound spectroscopy, which exhibits an anomaly in x = 0.1 in the temperature range 300-600 K. As a result, the phonon mean free path is found to be further reduced in the Sr0.9Y0.1TiO3-delta compared to that of SrTiO3-delta, resulting in a considerably low thermal conductivity kappa similar to 2.7 W/m-K at 760 K. Finally, we report a thermoelectric figure of merit (ZT) similar to 0.3 at 760K in the Sr0.9Y0.1TiO3-delta, the highest ZT value reported in the Y-doped SrTiO3 ceramics thus far. (C) 2014 AIP Publishing LLC.