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.

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.