Inspired by recent research results that have demonstrated appealing thermoelectric performance of A-site cation-deficient titanates, this work focuses on detailed analysis of the changes in performance promoted by altering the defect chemistry mechanisms. The series of cation-stoichiometric SrTi 1-x Ta x O 3±δ and A-site deficient Sr 1-x/2 Ti 1-x Ta x O 3-δ compositions (0.05≤x≤ 0.30) with cubic perovskite-like structure were selected to demonstrate the defect chemistry engineering approaches, which result in promising electric and thermal properties. High power factors were observed in compositions where appropriate concentration of the charge carriers and their mobility were attained by presence of strontium-and oxygen vacancies and suppressed formation of the oxygen-rich layers. Noticeable deviations from stoichiometric oxygen content were found to decrease the lattice thermal conductivity, suggesting good phonon scattering ability for oxygen vacancies, vacant A-sites and oxygen-excessive defects, while the effect from donor substitution on the thermal transport was less pronounced. The obtained guidelines for the defect chemistry engineering in donor-substituted strontium titanates open new possibilities for boosting the thermoelectric performance, especially if followed by complementary microstructural design to further promote electrical and thermal transport.
The applicability of perovskite‐type SrVO3−δ in high‐temperature electrochemical energy conversion technology is hampered by the limited stability domain of the perovskite phase. The aim of the present work was to find a compromise between the phase stability and electrical performance by designing solid solutions in the SrVO3–SrTiO3 system. Increasing titanium content in SrV1−yTiyO3−δ (y=0–0.9) perovskites is demonstrated to result in a gradual shift of the upper‐p(O2) phase stability boundary toward oxidizing conditions: from ≈10−15 bar at 900 °C for undoped SrVO3−δ to ≈10−11–10−5 bar for y=0.3–0.5. Although the improvement in the phase stability is accompanied by a decrease in electrical conductivity, the conductivities of SrV0.7Ti0.3O3−δ and SrV0.5Ti0.5O3−δ at 900 °C remain as high as 80 and 20 S cm−1, respectively, and is essentially independent of p(O2) within the phase‐stability domain. Combined XRD, thermogravimetric analysis, and electrical studies revealed very sluggish kinetics of oxidation of SrV0.5Ti0.5O3−δ ceramics under inert gas conditions and a nearly reversible behavior after exposure to an inert atmosphere at elevated temperatures. Substitution by titanium in the SrV1−yTiyO3−δ system results also in a decrease of oxygen deficiency in perovskite lattice and a favorable suppression of thermochemical expansion. Variations of oxygen nonstoichiometry and electrical properties in the SrV1−yTiyO3−δ series are discussed in combination with the simulated defect chemistry of solid solutions.
Phase stability domain of SrVO3-based SOFC anode materials is expanded towards higher oxygen partial pressures by donor-type substitutions into strontium and/or vanadium sublattices.
Structural and defect chemistry guidelines were used for Nb-substituted SrVO3-δ materials, designed to meet SOFC anode requirements, with emphasis on redox tolerance, thermochemical compatibility with other SOFC materials, electrical conductivity and adjustable changes in oxygen stoichiometry for their prospective impact on electrocatalytic performance. SrV1-xNbxO3-δ (x = 0-0.30) ceramics were prepared by solid-state synthesis and sintered at 1773 K in a reducing atmosphere. XRD and SEM/EDS showed that under these conditions a single-phase cubic perovskite structure appears up to x ≈ 0.25. Electrical conductivity is metallic-like and nearly p(O2)-independent. Although substitution by niobium decreases the conductivity, which still exceeds 100 S cm(-1) for x ≤ 0.20 at temperatures below 1273 K, it also expands the stability domain of the cubic perovskite phase and suppresses partly high thermochemical expansion characteristic of parent SrVO3-δ. The upper p(O2) limit of phase stability was found to shift from ∼2 × 10(-15) atm for the undoped material to ∼2 × 10(-12) atm for x = 0.30, whereas the average thermal expansion coefficient at 773-1223 K decreased from 22.7 × 10(-6) to 13.3 × 10(-6) K(-1). SrV1-xNbxO3-δ perovskites undergo oxidative decomposition in air, which causes dimensional and microstructural changes. However, sluggish kinetics of oxidation under inert gas conditions results in nearly reversible behavior in relatively short-term redox cycles between reducing and inert atmospheres. Subtle structural changes and a close correlation with point defect chemistry clarify these sluggish changes and provide guidelines to retain the metastability.
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