ICT2014 Abstracts

Ceramics based on SrTiO₃ are of growing interest as thermoelectric materials because of their high temperature stability and non-toxicity. Substitution of La and Nb into the perovskite structure provides control on both the microstructure and properties. Here, we report promising thermoelectric properties of ceramic solid solutions of  (1-x)SrTiO₃ – (x)La_1/3NbO₃.

The conventional mixed oxide route of was used to prepare (1-x)SrTiO₃ – (x)La_1/3NbO₃, with steps of x=0.1. Pressed pellets were sintered at temperatures of 1300⁰C to 1450⁰C in air and Ar/H₂(5%) atmospheres, for time of 8 to 48 hours. All sintered products were of high density (at least 90% theoretical) with grain sizes of typically 10 μm.  X-ray diffraction confirmed that the ceramics stabilised with a cubic structure. Scanning electron microscopy showed that samples sintered in air were single phase, whilst those sintered under reducing conditions contained La-rich and TiO₂-rich second phases.

Addition of aliovalent ions (La³⁺, Nb⁵⁺) on the A/B sites (Sr²⁺, Ti⁴⁺)  led to A-Site cation deficiency in the stoichiometric compositions; direct sintering in reducing conditions resulted in oxygen vacancies and other defect structures which increased carrier concentration.

Thermoelectric properties were determined for all the samples at temperatures up to 700°C. It was found that samples of 0.8SrTiO₃ – 0.2La_1/3NbO₃, (i.e. x – 0.2), exhibited a maximum power factor of 1.0 x 10^-3 W/mK² at 450⁰C and a ZT of 0.30 at 700⁰C.

It is proposed that A-site cation deficiency, the absence of La ordering, the Ti⁴⁺ reduction to Ti³⁺ and microstructure control improved electrical conductivity. The substitution of (La³⁺, Nb⁵⁺) ions coupled with grain boundary control enhanced phonon scattering to reduce thermal conductivity and systematically improve ZT.