La doped SrTiO3 based oxide thermoelectrics

• Main Author: Lu, Zhilun
• Other Authors: Ian, Reaney ; Derek, Sinclair
• Published: University of Sheffield 2016
• Subjects: 620
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.678774

In this project, the thermoelectric properties of La-doped Sr3Ti2O7, Ca3Ti2O7 and SrTiO3 ceramics sintered in air and N2/5%H2 have been investigated. Different defect chemistry models were studied in an attempt to improve the thermoelectric performance of La-doped SrTiO3 and related systems. For La-doped Sr3Ti2O7 Ruddlesden-Popper (RP) ceramics, the three starting nominal compositions, (Sr1-xLax)3Ti2O7 (electronic donor-doping), (Sr1-3y/2Lay)3Ti2O7 (A-site vacancies) and (Sr1-zLaz)3Ti2-z/4O7 (B-site vacancies) were sintered under air and flowing N2/5%H2 at 1773 K. The La-doped air sintered ceramics were all off-white/yellow in appearance and electrical insulators with low bulk conductivity and a high activation energy, Ea, confirming that solid solubility of La was small and that electronic (donor-doping) compensation does not exist for La-doping of ceramics sintered in air. Processing ceramics under reducing atmosphere is sufficient to form dark single-phase samples for the x series (electronic donor-doping series) up to (Sr0.95La0.05)3Ti2O7 (x = 0.05), indicating that reducing conditions and oxygen-loss from the Sr3Ti2O7 lattice are conducive towards electronic La-doping in Sr3Ti2O7-δ ceramics and to extend solid solubility. In all N2/5%H2 sintered samples, an insulating surface layer associated with SrO volatilization and oxygen up-take (during cooling) from the sintering process occurred that, unless removed, masked the conductive nature of the ceramics. In the bulk, significantly higher power factors were obtained for ceramics that were phase mixtures containing highly conductive (Sr, La)TiO3-δ, ST. This highlights the superior power factor properties of reduced perovskite-type ST compared to reduced RP-type Sr3Ti2O7 and serves as a warning for the need to identify low levels of highly conducting perovskite phases when exploring rare-earth doping mechanisms in RP-type phases. For La-doped SrTiO3, the favoured mechanism for doping was through the formation of A-site vacancies independent of P_(O_2 ). Samples with A-site vacancies (Sr1-3y/2LayTiO3) had the highest electrical conductivity for the same La content (i.e. 10 at. %) sintered at 1773 K, independent of P_(O_2 ). In the Sr1-3y/2LayTiO3 system, air sintered ceramics were metrically cubic for 0.1 ≤ y < 0.30, tetragonal with short range strontium vacancy, VSr, ordering for 0.30 ≤ y < 0.50, then orthorhombic with long range ordering of VSr for y ≥ 0.50 by X-ray powder and electron diffraction at room temperature. For samples reduced in N2/5%H2, compositions with 0.1 ≤ y ≤ 0.50 were metrically cubic. Short range VSr ordering and an orthorhombic structure with long range VSr ordering were observed for y = 0.50 and 0.63, respectively. Samples with y = 0.15 sintered in N2/5%H2 revealed the largest dimensionless thermoelectric figure-of-merit (ZT = 0.41 at 973 K) reported for n-type SrTiO3 based materials, suggesting that the accommodation of La through the formation of VSr accompanied by reduction in N2/5%H2 represents a new protocol for the development of oxide-based thermoelectrics.