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This work assesses the feasibility of processing CuFe1-xNixO2 (x = 0, 0.02) using the Laser Floating Zone (LFZ) technique to grow fibres with different pulling rates, to tune their thermoelectric performance. Structural analysis showed CuFeO2 as the major phase. Formation of secondary phases is promoted by Ni addition, diminishing with decreasing pulling rate. Grain alignment and crystallite size of the fibres increase with the puling rate and doping. Electrical conductivity is enhanced by decreasing the pulling rate, while Ni-doping decreases the conductivity, and Seebeck coefficient demonstrates quite complex behaviour. Thermal conductivity decreases with temperature and with the pulling rate and Ni-doping. A maximum ZT value of 0.17 was achieved for 10 mm h-1 sample at 1000 K for pure, and 700 K for Ni-doped samples. These ZT values are higher than found in the literature, demonstrating the feasibility of the LFZ method for processing thermoelectric delafossites, indicating a proper optimisation process., This work was developed within the scope of the project i3N (LA/P/0037/2020 & UIDB/50025/2020 & UIDP/50025/2020) and CICECO-Aveiro Institute of Materials (UIDB/50011/2020 & UIDP/50011/2020), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. This work was funded by national funds (OE), through FCT – Fundação para a Ciência e a Tecnologia, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of July 19. M. A. Madre and A. Sotelo acknowledge the Spanish MINECO-FEDER (Project MAT2017–82183-C3–1-R) and Gobierno de Aragón-FEDER (Research Group T 54–20R) for funding. The support of the project POCI-01–0145-FEDER-031875, financed by COMPETE 2020 Program and National Funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement, is greatly acknowledged.

Zinc oxide (ZnO) is a highly versatile and well-known material, widely used for its useful optoelectronic, catalytic, and photochemical properties [1]. In the search for alternative energy sources, ZnO-based ceramics emerge as promising materials for high-temperature thermoelectric (TE) applications [2], capable of direct conversion of (waste) heat to electricity, thanks to the Seebeck effect. Owing to the specific wurtzite crystal structure of this large band-gap semiconductor, donor substitutions are often performed/employed, for tailoring the electrical properties and, in this respect, aluminium is probably the most well-known and often used cation [3] for improving the charge carrier concentration and/or mobility. Additionally, nanostructuring approaches are also considered and implemented for the independent control of the different TE coefficients involved in the charge and heat transport [4], provided by their ability to promote the formation of specific microstructural features, capable of simultaneously enhancing the electrical conductivity and decreasing the lattice thermal conductivity. This work reports on the preliminary results of a combined charge carrier transport improvement / grain boundary tailoring scheme, involving the simultaneous increase in electron concentration/mobility (by doping with Al) and the engineering of defects between the highly packed grains characteristic for this material (by controlled aluminothermic reactions, providing in-situ redox conditions). The solubility of Al and the exothermic effects of aluminothermy are controlled by the careful choice of 3 different aluminium sources (for the targeted nominal composition Zn0.995Al0.005O) and a one stage sintering cycle, performed in air. The samples prepared from metallic micrometric and nanometric Al particles show the highest density values (around 96% of the theoretical value), compared with the conventional, reference Zn0.995Al0.005O samples prepared from Al2O3 (around 92% of the theoretical value). The Wurtzite phase has been found in all cases as the single phase, and the relevant microstructural changes/features, including defects formation at the grain boundaries, have only been observed for the samples prepared from the 2 different metallic aluminium sources, provided by the aluminothermic reactions. The electrical performance results show a dramatic increase in electrical conductivity, for the samples prepared from micro- and nano-sized aluminium particles, leading to a maximum power factor value of around 700 μW/K^2m, at 900 °C for the samples prepared from micrometric particles, being among the best values found in literature. Not Published

Oxide materials possess notable high-temperature and chemical stability, thus representing certain advantages over traditional thermoelectrics. The thermoelectric performance of oxides is strongly affected by the microstructure, often even dominating over the chemical composition effects. Thus, engineering the microstructure in these materials represents a powerful tool towards improving their thermoelectric performance, both in terms of the maximum output and stable operation at elevated temperatures. This work reviews some representative cases observed for the main thermoelectric oxide families, such as ZnO, SrTiO3, Ca3Co4O9 and CaMnO3-based systems. The selected examples highlight the impacts of the relevant microstructural features, including the presence of porosity, phase morphology and interfaces, on the thermoelectric properties and their evolution with time. published

We present a study of magnetic and structural properties of La1−xGdxCoO3 compounds with x = 0, 0.2, 0.5 and 1. The Gd term in magnetic susceptibility obeys the Curie-Weiss law with antiferromagnetic interaction between the magnetic moments of Gd3+ ions. The value of the paramagnetic Curie temperature Θ is proportional to Gd content and for GdCoO3 it reaches Θ ' −5.5 K. The Co contribution, χCo(T), derived by subtracting the Gd term from the total susceptibility, shows a non-monotonous behavior which is originated by the temperature induced transition of Co3+ ions from the ground low spin state to a higher spin state separated by an energy ∆. The revealed strong increase in ∆ with increasing Gd content is presumably due to the manifestation of a chemical pressure upon replacing La with Gd having a smaller ionic size. published

Oxide materials represent certain advantages as compared to traditional thermoelectric materials, including stability at high temperatures and versatility regarding the gas atmosphere composition. Microstructural aspects play a crucial role in the performance of oxides, often dominating even over chemical composition effects. Thus, engineering the microstructure in these materials represents a powerful tool towards improving their thermoelectric performance, both in terms of the maximum output and stable operation at elevated temperatures. This work reviews some representative cases observed for the main thermoelectric oxide families, such as ZnO, SrTiO3, Ca3Co4O9 and CaMnO3-based systems. The selected examples highlight the impacts of the relevant microstructural features, including the presence of porosity, phase morphology and interfaces, on the thermoelectric properties and their evolution with time. Not Published

For La1–xPrxCoO3cobaltites (x¼0, 0.1, 0.2, and 0.3), the dependence ofmagnetic susceptibilityχðTÞis studied in the temperature range 5–400 K. Also,the crystal structure of these cobaltites is investigated, and the effect ofpressure up to 2 kbar on their susceptibility is measured atT¼78, 150, and300 K. The specificdependenciesχðTÞandthelargenegativepressureeffectare assumed to arise from Co3þions contribution to the total susceptibilityevaluated using La1 xPrxAlO3as a reference system. The obtained experi-mental data on temperature and pressure effects in magnetism are analyzedwithin a two-level model with energy gapΔbetween the ground state of thesystem with zero spin of Co3þions and the excited higher-spin state. In thismodel, magnetism of Co3þions is determined by the temperature-inducedpopulation of the excited state, and magnitude of the pressure effect is gov-erned by the volume dependence ofΔ. The results of the analysis, supple-mented by the theoretical calculations of the electronic structures of LaCoO3and PrCoO3, indicate significant increase inΔwith decrease in the unit cellvolume both under hydrostatic pressure and by substituting La with Pr having asmaller ionic radius. published

Broad societal needs have focused increased attention to providing a sustainable energy supply to the population, based on technologies with minimal environmental impact and reduced fossil fuels usage. One solution is to improve energy conversion efficiency in key consuming sectors. Since most of the energy (60-70%) used worldwide is discharged as waste heat, ”green” thermoelectric (TE) conversion has received considerable attention due to its intrinsic simplicity, employing no moving parts, silent operation, excellent scalability and reliability, and self-sufficiency to enable mobile or remote applications. In some energy-conversion scenarios, the cost and thermal stability requirements may dominate over efficiency issues, making abundant, high-temperature-stable and low-toxic oxides an interesting alternative TE material. This talk will feature some oxide-specific approaches towards tuning the thermoelectric performance in strontium titanate and zincoxide-based materials, including defects engineering and in-situ induced nanostructuring. published