n-Type Thermoelectric Oxide Ceramics for High Temperature Power Generation

Conventional non-oxide materials utilised in thermoelectric devices have unfavourable properties such as instability at high temperatures with scarce and toxic raw materials that constitute an environmental hazard. Transition metal oxide thermoelectric materials are stable at high temperatures, abundant, less toxic and have been suggested as potential alternatives given improvements in thermoelectric figures of merit (ZT). La-Sm co-doped SrTiO3, Sm-doped Sr5LaTi3Nb7O30 and Nb-doped La2Ti2O7 n-type oxide ceramics synthesised using solid state reaction technique, sintered in air and 5% H2/N2 have thus been investigated.
For La-Sm co-doped SrTiO3 ceramics, the following aliovalent doping mechanisms were adopted; Sr1-xLax/2Smx/2TiO3 (electron donor-doping), Sr1-3x/2Lax/2Smx/2TiO3 (A-site vacancies or ionic donor-doping) which were calcined in air or 5% H2/N2 and sintered in air or 5% H2/N2 at 1773 K. La-Sm co-doped SrTiO3 pellets sintered in air were white/pale yellow in colour, indicating they were stoichiometric with respect to oxygen concentration hence electrical insulators. All La-Sm co-doped 5% H2/N2 sintered ceramics were black single-phase pellets up to 15 mol% (x = 0.15) doping concentration for electron donor-doped compositions. For the A-site vacancy, La-Sm doped SrTiO3, the single-phase materials spanned across all the compositions (x ≥ 0.30). This indicates that processing in reducing atmosphere improves the electronic conduction by generating oxygen vacancies (VO) in the lattice with a lower La-Sm solid solubility limit observed in electron donor-doped samples. Vacancy doping with a double calcination in 5% H2/N2 at elevated temperatures optimised the ZT values. x = 0.20 (1400H) doubled calcined (at 1573 and 1673 K) and sintered in 5% H2/N2 for 8 hours showed the highest ZT (0.35 at 973 K) reported for RE co-doped n-type SrTiO3 ceramics. The result shows that La-Sm co-doping of SrTiO3 through creation of VSr with processing in 5% H2/N2 opens a new window for the synthesis, fabrication and characterisation of oxide thermoelectrics.
For Sm-doped Sr5LaTi3Nb7O30 ceramics, Sr5La1-xSmxTi3Nb7O30 and Sr5-3x/2SmxLaTi3Nb7O30 were sintered in air and 5% H2/N2 at 1673 K for 6 hours. The Sm-doped Sr5LaTi3Nb7O30 air sintered ceramics showed single-phase, homogenous ceramics with dense microstructures and a white/pale yellow appearance. The white or pale yellow colour shows the samples are electrical insulators, hence were not characterized for TE performance. In contrast, Sm-doped Sr5LaTi3Nb7O30 sintered in 5% H2/N2 were identified with secondary phases consisting of SrTiO3 (Sm doped) and an Nb-rich oxide (most likely Nb2O5). The A-site vacancy samples (Sr5-3x/2SmxLaTi3Nb7O30) exhibited lower thermal conductivity when compared to its electron doped counterpart (Sr5La1-xSmxTi3Nb7O30), indicating that secondary phase mixtures present in the microstructure and the vacancy defects (VSr and VO) created in the lattice contributed in shortening the mean free path (MFP) of phonons, resulting in a maximum ZT (0.21) at 973 K for x = 0.30.
Nb-doped La2Ti2O7 ceramics with an electron compensation mechanism were also investigated. All compositions were single-phase with porous microstructures consistent with their low experimental densities. The thermoelectric results showed improved properties in comparison to pure La2Ti2O7 but PF and ZT values were too low and not useful for thermoelectric applications. However, the high Seebeck coefficient and glass-like (low) thermal conductivity values achieved have established La2Ti2O7 as a potential thermoelectric material.