Using thermoforging process, dense Ca3Co4O9 (Co349) thermoelectric oxides have been successfully textured. The various parameters influencing the formation of the Co349-textured material have been investigated. The electrical transport measurements show an anisotropy of the resistivity in good agreement with scanning electron microscopy observations. Texture is quantified by neutron-diffraction measurements and correlated to anisotropic resistivity measurements and Seebeck coefficients.

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A prototype of oxide thermoelectric module only composed of n-legs Ca0.95Sm0.05MnO3, so-called unileg device, has been fabricated. The electrical and thermal measurements have been investigated at high temperature in air. In this non conventional configuration, the pellets are thermally connected in parallel, electrically in series, and linked by silver strips. The system has been characterized under large temperature difference (ΔT=360K) using a homemade system, allowing to record simultaneously the electrical and thermal parameters of the working device. An open circuit voltage of 260mV was obtained for a four-leg “unileg” module. The module exhibits an internal resistance of 1.09Ω. The maximum power output for this four-leg device reached 16mW in these working conditions. The manufacturing quality is discussed, according to the electric contact resistance values, and the reliability of the thermoelectric device is reported.

Magnetic porous nanostructures consisting of oriented aggregates of iron oxide nanocrystals display very interesting properties such as a lower oxidation state of magnetite, and enhanced saturation magnetization in comparison with individual nanoparticles of similar sizes and porosity. However, the formation mechanism of these promising nanostructures is not well understood, which hampers the fine tuning of their magnetic properties, for instance by doping them with other elements. Therefore the formation mechanism of porous raspberry shaped nanostructures (RSNs) synthesized by a one-pot polyol solvothermal method has been investigated in detail from the early stages by using a wide panel of characterization techniques, and especially by performing original in situ HR-TEM studies in temperature. A time-resolved study showed the intermediate formation of an amorphous iron alkoxide phase with a plate-like lamellar structure (PLS). Then, the fine investigation of PLS transformation upon heating up to 500 °C confirmed that the synthesis of RSNs involves two iron precursors: the starting one (hydrated iron chlorides) and the in situ formed iron alkoxide precursor which decomposes with time and heating and contributes to the growth step of nanostructures. Such an understanding of the formation mechanism of RSNs is necessary to envision efficient and rational enhancement of their magnetic properties.

Nanostructures with controlled size, morphology and composition represent a main challenge in materials science because controlling these parameters is fundamental to optimize the subsequent functional properties. Agregated nanostructures, combining both individual and collective properties of nanocrystals, offer interesting perspectives to design new magnetic nanomaterials. In that context, original porous raspberry shaped nanostructures consisting of oriented aggregates of ferrite nanocrystals have been synthesized by an one-pot polyol solvothermal method. Synthesis conditions have been optimized to obtain nanostructures featured by similar sizes of about 250 nm and nanocrystal sizes modulated from 5 to 60 nm, leading to porous structures with tunable specific surface area. Structural and magnetic studies of nanostructures as a function of the nanocrystal size evidenced their low oxidation state and enhanced magnetic properties. Indeed the oriented aggregation of nanocrystals leads to high interface between nanograins reducing significantly their surface oxidation and enhancing their saturation magnetization in comparison to individual nanoparticles of similar sizes. Magnetic moments of each grains are also consequently strongly coupled by dipolar interactions which led to super spin glass effects.