Structural and defect chemistry guidelines for Sr(V,Nb)O3-based SOFC anode materials.

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.

Prospects and challenges of iron pyroelectrolysis in magnesium aluminosilicate melts near minimum liquidus temperature.

Although steel production by molten oxide electrolysis offers potential economic and environmental advantages over classic extractive metallurgy, its feasibility is far from being convincingly demonstrated, mainly due to inherent experimental difficulties exerted by harsh conditions and lack of knowledge regarding relevant mechanisms and physico-chemical processes in the melts. The present work was intended to demonstrate the concept of pyroelectrolysis at very high temperature near the minimum liquidus point of magnesium aluminosilicate, being conducted under electron-blocking conditions using yttria-stabilized zirconia cells, and to provide a new insight into electrochemistry behind this process. Significant current yields are possible for pyroelectrolysis performed in electron-blocking mode using a solid electrolyte membrane to separate the anode and the molten electrolyte. Parasitic electrochemical processes rise gradually as the concentration of iron oxide dissolved in the molten electrolytes is depleted, impairing faradaic efficiency. Reduction of silica to metallic silicon was identified as a significant contribution to those parasitic currents, among other plausible processes. Direct pyroelectrolysis without electron blocking was found much less plausible, due to major limitations on faradaic efficiency imposed by electronic leakage and insufficient ionic conductivity of the aluminosilicate melt. Ohmic losses may consume an excessive fraction of the applied voltage, thus failing to sustain the Nernst potential required for reduction to metallic iron. The results suggest the need for further optimization of the molten electrolyte composition to promote ionic conductivity and to suppress electronic transport contribution, possibly, by tuning the Al/Si ratio and altering the network-forming/modifying behaviour of the iron cations.

Towards a high thermoelectric performance in rare-earth substituted SrTiO3: effects provided by strongly-reducing sintering conditions.

Donor-substituted strontium titanate ceramics demonstrate one of the most promising performances among n-type oxide thermoelectrics. Here we report a marked improvement of the thermoelectric properties in rare-earth substituted titanates Sr0.9R0.1TiO3±Î´ (R = La, Ce, Pr, Nd, Sm, Gd, Dy, Y) to achieve maximal ZT values of as high as 0.42 at 1190 K < T < 1225 K, prepared via a conventional solid state route followed by sintering under strongly reducing conditions (10%H2-90%N2, 1773 K). As a result of complex defect chemistry, both electrical and thermal properties were found to be dependent on the nature of the rare-earth cation and exhibit an apparent correlation with the unit cell size. High power factors of 1350-1550 µW m(-1) K(-2) at 400-550 K were observed for R = Nd, Sm, Pr and Y, being among the largest reported so far for n-type conducting bulk-ceramic SrTiO3-based materials. Attractive ZT values at high temperatures arise primarily from low thermal conductivity, which, in turn, stem from effective phonon scattering in oxygen-deficient perovskite layers formed upon reduction. The results suggest that highly-reducing conditions are essential and should be employed, whenever possible, in other related micro/nanostructural engineering approaches to suppress the thermal conductivity in target titanate-based ceramics.

Unusual redox behaviour of the magnetite/hematite core-shell structures processed by the laser floating zone method.

Magnetite (Fe3O4) offers unique physical and chemical properties, being an important material for many industrial applications. Certain limitations on the application conditions are, however, imposed by the redox stability issue. Fine control of the iron oxidation states represents a challenge for materials engineering. The present work explores relevant redox processes in iron oxides, processed under highly non-equilibrium laser floating zone (LFZ) conditions under atmospheres with different oxygen activities. The as-grown fibres showed a structure composed of the Fe3O4 core and the Fe2O3 shell. This study uncovers unexpectedly lower hematite content and shell thickness for the fibres processed under more oxidizing conditions. Combined structural and microstructural studies, supported by the analysis of the existing literature data, strongly suggest that the redox processes during the LFZ process can be rather determined by kinetics of melt crystallization, nuclei formation and heat transfer than by the oxygen content in the gas phase. The proposed mechanisms are further confirmed by electrical and magnetic studies of the composite fibres.

Enhanced BaZrO3 mechanosynthesis by the use of metastable ZrO2 precursors.

The current work assesses the impact of structural differences between stable and metastable ZrO2 precursors on the mechanochemical preparation of BaZrO3. Monoclinic (m-ZrO2) and tetragonal (t-ZrO2) zirconia polymorphs were prepared without stabilizing additives by slow alkaline precipitation. High-energy milling of the individual ZrO2 precursors induced different partial transformations in each case. The as-synthesized m-ZrO2 powders showed partial conversion to the tetragonal polymorph on mechanical activation, reaching about 10% t-ZrO2 after 420 min accompanied by increases in strain. In contrast, the as synthesized t-ZrO2 powders underwent the inverse transformation to the monoclinic phase, producing about 50% m-ZrO2 after 120 min with the liberation of strain. The t-ZrO2 precursor was shown to exhibit the higher reactivity with barium peroxide, yielding significantly earlier formation of barium zirconate under room-temperature mechanosynthesis. The progress of the mechanochemical formation of BaZrO3 has been discussed with respect to the differing behaviour of the ZrO2 precursors upon mechanical activation and associated thermodynamic perspectives.