Lowering the thermal conductivity of Sr(Ti0.8Nb0.2)O3 by SrO and CaO doping: microstructure and thermoelectric properties

Excess SrO and CaO were added to the Sr(Ti0.8Nb0.2)O-3 thermoelectric material, which was structurally compensated by the formation of Ruddlesden-Poppertype planar faults with the compositions SrO and/or (Sr,Ca)O. Both types of doping significantly changed the original isotropic Sr(Ti0.8Nb0.2)O-3 microstructure and resulted in the formation of lamellar Ruddlesden-Popper-type phases within the Sr(Ti0.8Nb0.2)O-3 grains. Three-dimensional networks of single Ruddlesden-Popper-type faults were also observed in the Sr(Ti0.8Nb0.2)O-3 for both types of doping. The combination of both structural features significantly lowered the thermal conductivity in comparison with Sr(Ti0.8Nb0.2)O-3 due to the enhanced phonon scattering observed at the planar faults, which proves that introducing such defects is a promising method for lowering the thermal conductivity of the Sr(Ti0.8Nb0.2)O-3 thermoelectric material. The highest figure of merit (ZT = 0.08) was achieved with CaO doping, since the significantly reduced thermal conductivity was accompanied by an increased power factor.