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Herein, thermodynamic assessment is proposed to screen suitable precursors for the solid-state synthesis of manganese ferrite, by mechanosynthesis at room temperature or by subsequent calcination at relatively low temperatures, and the main findings are validated by experimental results for the representative precursor mixtures MnO + FeO 3 , MnO 2 + Fe 2 O 3 , and MnO 2 +2FeCO 3 . Thermodynamic guidelines are provided for the synthesis of manganese ferrite from (i) oxide and/or metallic precursors; (ii) carbonate + carbonate or carbonate + oxide powder mixtures; (iii) other precursors. It is also shown that synthesis from metallic precursors (Mn + 2Fe) requires a controlled oxygen supply in limited redox conditions, which is hardly achieved by reducing gases H 2 /H 2 O or CO/CO 2 . Oxide mixtures with an overall oxygen balance, such as MnO + Fe 2 O 3 , act as self-redox buffers and offer prospects for mechanosynthesis for a sufficient time (>9 h) at room temperature. On the contrary, the fully oxidised oxide mixture MnO 2 + Fe 2 O 3 requires partial reduction, which prevents synthesis at room temperature and requires subsequent calcination at temperatures above 1100 °C in air or in nominally inert atmospheres above 750 °C. Oxide + carbonate mixtures, such as MnO 2 +2FeCO 3 , also yield suitable oxygen balance by the decomposition of the carbonate precursor and offer prospects for mechanosynthesis at room temperature, and residual fractions of reactants could be converted by firing at relatively low temperatures (≥650 °C).

The alkaline electrolytic production of iron is gaining interest due to the absence of CO 2 emissions and significantly lower electrical energy consumption when compared with traditional steelmaking. The possibility of using an iron-bearing pseudobrookite mineral, Fe 2 TiO 5 , is explored for the first time as an alternative feedstock for the electrochemical reduction process. To assess relevant impacts of the presence of titanium, similar electroreduction processes were also performed for Fe 2 TiO 5 ·Fe 2 O 3 and Fe 2 O 3 . The electroreduction was attempted using dense and porous ceramic cathodes. Potentiostatic studies at the cathodic potentials of -1.15--1.30 V vs. an Hg|HgO|NaOH reference electrode and a galvanostatic approach at 1 A/cm 2 were used together with electroreduction from ceramic suspensions, obtained by grinding the porous ceramics. The complete electroreduction to Fe 0 was only possible at high cathodic polarizations (-1.30 V), compromising the current efficiencies of the electrochemical process due to the hydrogen evolution reaction impact. Microstructural evolution and phase composition studies are discussed, providing trends on the role of titanium and corresponding electrochemical mechanisms. Although the obtained results suggest that pseudobrookite is not a feasible material to be used alone as feedstock for the electrolytic iron production, it can be considered with other iron oxide materials and/or ores to promote electroreduction.

This work further explores the possibilities for designing the high-temperature electrical performance of the thermoelectric Ca 3 Co 4 O 9 phase, by a composite approach involving separate metallic iron and nickel particles additions, and by employing two different sintering schemes, capable to promote the controlled interactions between the components, encouraged by our recent promising results obtained for similar cobalt additions. Iron and nickel were chosen because of their similarities with cobalt. The maximum power factor value of around 200 μWm -1 K -2 at 925 K was achieved for the composite with the nominal nickel content of 3% vol., processed via the two-step sintering cycle, which provides the highest densification from this work. The effectiveness of the proposed approach was shown to be strongly dependent on the processing conditions and added amounts of metallic particles. Although the conventional one-step approach results in Fe- and Ni-containing composites with the major content of the thermoelectric Ca 3 Co 4 O 9 phase, their electrical performance was found to be significantly lower than for the Co-containing analogue, due to the presence of less-conducting phases and excessive porosity. In contrast, the relatively high performance of the composite with a nominal nickel content of 3% vol. processed via a two-step approach is related to the specific microstructural features from this sample, including minimal porosity and the presence of the Ca 2 Co 2 O 5 phase, which partially compensate the complete decomposition of the Ca 3 Co 4 O 9 matrix. The obtained results demonstrate different pathways to tailor the phase composition of Ca 3 Co 4 O 9 -based materials, with a corresponding impact on the thermoelectric performance, and highlight the necessity of more controllable approaches for the phase composition tuning, including lower amounts and different morphologies of the dispersed metallic phases.

This paper reports a novel composite-based processing route for improving the electrical performance of Ca 3 Co 4 O 9 thermoelectric (TE) ceramics. The approach involves the addition of metallic Co, acting as a pore filler on oxidation, and considers two simple sintering schemes. The (1-x)Ca 3 Co 4 O 9 /xCo composites (x = 0%, 3%, 6% and 9% vol.) have been prepared through a modified Pechini method, followed by one- and two-stage sintering, to produce low-density (one-stage, 1ST) and high-density (two-stage, 2ST) ceramic samples. Their high-temperature TE properties, namely the electrical conductivity (σ), Seebeck coefficient (α) and power factor (PF), were investigated between 475 and 975 K, in air flow, and related to their respective phase composition, morphology and microstructure. For the 1ST case, the porous samples (56%-61% of ρ th ) reached maximum PF values of around 210 and 140 μWm -1 ·K -2 for the 3% and 6% vol. Co-added samples, respectively, being around two and 1.3 times higher than those of the pure Ca 3 Co 4 O 9 matrix. Although 2ST sintering resulted in rather dense samples (80% of ρ th ), the efficiency of the proposed approach, in this case, was limited by the complex phase composition of the corresponding ceramics, impeding the electronic transport and resulting in an electrical performance below that measured for the Ca 3 Co 4 O 9 matrix (224 μWm -1 ·K -2 at 975K).

This work aims at the preparation of multifunctional titania-based photocatalysts with inherent capabilities for thermal co-activation and stabilisation of anatase polymorph, by designing the phase composition and microstructure of rutile-silicon carbide mixture. The processing involved a conventional solid state route, including partial pre-reduction of rutile by SiC in inert Ar atmosphere, followed by post-oxidation in air. The impacts of processing conditions on the phase composition and photocatalytic activity were evaluated using Taguchi planning. The XRD studies confirmed the presence of rutile/anatase mixtures in the post-oxidised samples. The results emphasise that pre-reduction and post-oxidation temperatures are critical in defining the phase composition, while post-oxidation time is relevant for the photocatalytic performance. Microstructural studies revealed the formation of core-shell particles, which can suppress the photocatalytic activity. The highest apparent reaction rate of the photodegradation of methylene blue was observed for the sample pre-reduced in Ar at 1300 °C for 5 h and then calcined in air at 400 °C for 25 h. Though its performance was ~1.6-times lower than that for the same amount of nanostructured industrial P25 photocatalyst, it was achieved in the material possessing 2-3 times lower surface area and containing ~50 mol% of SiO 2 and SiC, thus demonstrating excellent prospects for further improvements.

Zinc oxide (ZnO) has being recognised as a potentially interesting thermoelectric material, allowing flexible tuning of the electrical properties by donor doping. This work focuses on the assessment of tantalum doping effects on the relevant structural, microstructural, optical and thermoelectric properties of ZnO. Processing of the samples with a nominal composition Zn 1-x Ta x O by conventional solid-state route results in limited solubility of Ta in the wurtzite structure. Electronic doping is accompanied by the formation of other defects and dislocations as a compensation mechanism and simultaneous segregation of ZnTa 2 O 6 at the grain boundaries. Highly defective structure and partial blocking of the grain boundaries suppress the electrical transport, while the evolution of Seebeck coefficient and band gap suggest that the charge carrier concentration continuously increases from x = 0 to 0.008. Thermal conductivity is almost not affected by the tantalum content. The highest ZT ~0.07 at 1175 K observed for Zn 0.998 Ta 0.002 O is mainly provided by high Seebeck coefficient (-464 V/K) along with a moderate electrical conductivity of ~13 S/cm. The results suggest that tantalum may represent a suitable dopant for thermoelectric zinc oxide, but this requires the application of specific processing methods and compositional design to enhance the solubility of Ta in wurtzite lattice., Competing Interests: The authors declare no conflicts of interest.