Development of Thermoelectric n-type Oxides in the La-Sr-Ti-O-System
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Development of donor-doped thermoelectric (TE) perovskite oxides based on strontium titanate (STO) was investigated with respect to synthesis, phase stability, and TE properties. La was employed as an A-site donor-dopant and compositions belonging to the LaxSr1-xTiO3 and LaxSrxTiO3 series were produced. Furthermore, effects of deliberately introduced A-site deficiency on the TE properties for both series have been addressed. The purpose of the A-site deficiency was to provoke an exsolution mechanism in the La-doped STO systems, which potentially could result in a uniformly distributed exsolution of secondary-phase particles capable of effectively reducing phonon propagation, and hence, reducing the thermal conductivity.The STO system demonstrated strong tolerance towards accommodation of La-donors and A-site vacancies, and compositions of La0.12Sr0.88TiO3, (La0.12Sr0.88)0.95TiO3, and La0.40Sr0.40TiO3 were successfully synthesized as single-phase cubic perovskite structures (Pm3 ̅m) by the conventional solid-state reaction method. All samples were sintered in reducing atmosphere to achieve electrical conductivity, while the two A-site deficient compositions were subjected to an additional prolonged heat treatment (also in reducing atmosphere) to provoke the exsolution mechanism. Exsolutions of a titanium oxide phase were found in (La0.12Sr0.88)0.95TiO3 after heat treatment and in La0.40Sr0.40TiO3 after both sintering and heat treatment. The sizes of the exsolutions were generally much larger in La0.40Sr0.40TiO3 compared to the ones in (La0.12Sr0.88)0.95TiO3. The exsolutions nucleated on grain boundaries in both compositions, and it appears that the formation was associated with the presence of pores; probably due to the available oxygen trapped inside. Small pores in the grains indicate that an exsolution mechanism might have occurred inside the grains as well. Despite the large exsolution sizes, a reducing effect on the thermal conductivity was observed. This is assumed to be due to the large volume fraction of titanium oxide phase, i.e. that the phonons were disrupted by a frequent change in phase structure. Generally, the electrical conductivity results showed that conductivity is strongly dependent on the extent of reduction during processing. The large volume fraction of insulating titanium oxide exsolutions was assumed to have a reducing effect on the electrical conductivity. The measured Seebeck coefficients were found to be reflected by the electrical conductivity results and showed no signs of obvious influence from the exsolutions. Moreover, an exsolution-free (La0.12Sr0.88)0.95TiO3-δ sample achieved the largest ZT over the whole measurement temperature range, reaching a maximum value of 0.21 at 1073 K.