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A3: High temperature materials
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)La1/3NbO3.
The conventional mixed oxide route of was used to prepare (1-x)SrTiO3 – (x)La1/3NbO3, 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 – 0.2La1/3NbO3, (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.
