ICT2014 Abstracts

Heavily electron doped SrTiO₃ is a candidate n-type oxide semiconductor for high temperature thermoelectric applications because of its relatively high Seebeck coefficient and large effective carrier mass. Incorporation of boron in the lattice of SrTiO₃ single crystal has shown enhancement for both Seebeck coefficient and electrical conductivity.  Here, we report promising thermoelectric properties obtained by addition of boron to La-doped SrTiO₃ and Nb-doped SrTiO₃ polycrystalline ceramics.

Sr_0.9La_0.1TiO₃ and SrTi_0.8Nb_0.2O₃ ceramics were synthesized by the mixed oxide route. The mixed powders were calcined at 1200^oC for 8 hours. Up to 1 wt% boron oxide was added to the calcined powders. Ceramics were sintered in air at 1400^oC to 1500^oC. Subsequently the sintered pellets were annealed in a reducing atmosphere (argon with 5% H₂) at 1300^oC to 1400^oC for times of 8 to 48 hours. All sintered products were of high density (at least 95% theoretical) with grain sizes of typically 10 μm. X-ray diffraction confirmed that the ceramics stabilised with a cubic structure. High resolution Scanning Electron Microscopy showed a core-shell type structure within the grains for ceramics substituted with both La and Nb. The cores contain nano-size voids and nano-size particles; their sizes depend on the annealing time. Atomic level resolution structural characterization was carried out using an aberration-corrected microscope. HAADF-EELS investigation showed that the distribution of La and Nb in cores and shells of the grains is random being independent of the annealing time. Atomic level EELS analysis showed that the precipitates are rich in Ti and around the voids contain high concentration of Ti³⁺. By controlling the microstructure and oxidation state of the Ti, ZT values of greater than 0.35 at 700^°C were obtained.