Abstract.We have studied YMnO 3 by high-temperature synchrotron X-ray powder diffraction, and have carried out differential thermal analysis and dilatometry on a single crystal sample. These experiments show two phase transitions at about 1100K and 1350K, respectively. This demonstrates the existence of an intermediate phase between the room temperature ferroelectric and the high temperature centrosymmetric phase. This study identifies for the first time the different high-temperature phase transitions in YMnO 3 .

The top-seeding melt texture growth (TSMTG) process is investigated in a small-vertical-thermal-gradient furnace in order to better understand the conditions for growing large single-grain pellets. An crystal was used as seed in a Y123 - Y211 composite with additions. It was shown that the role of the seed is to initiate the sympathetic nucleation growth mechanism. Indeed, the additional driving force induced by the seed is clearly demonstrated by a decrease in the undercooling of the system. The seeding crystal introduces a heterogeneous nucleation centre, leading to a very well controlled nucleation rate. Furthermore, we discuss the relation between intrinsic anisotropic growth rate and the processing rate, showing the importance of the thermal conditions of the texturing process to stabilize a sympathetic growth. By careful control of the texturing parameters, we succeeded in producing single-grain pellets as confirmed by a pole figure experiment and neutron diffraction measurements. The levitation force for pellets of 2 cm in diameter reaches 16 N when an NdFeB magnet of the same diameter is used.

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Thermoelectric performances of misfit cobaltites can be controlled by grain orientation and/or by cation substitution. Both processes have been simultaneously performed by directional solidification at 0.03 m/h, using the laser floating zone technique, of Bi 2-x Pb x Sr 2 Co 1.8 O y (with x = 0.0, 0.2, 0.4 and 0.6). The microstructure has shown two different main phases as a function of Pb content, a Co-poor phase for low Pb content (0.0 and 0.2) and a Co-rich one for higher Pb substitution. These microstructural changes are reflected on the thermoelectric properties leading to an important decrease on the resistivity and increase of thermopower for samples with 0.4Pb substitution. Both improvements lead to power factor values higher than usual in textured misfit cobaltites.

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The synthesis of Bi 2 Ca 2 Co 1.7 O x misfit compounds by different processing routes, i.e. classical solid-state, and two solution methods namely sol-gel via nitrates and a polymer route, has been investigated. A comparison among the final products has been performed using DTA, TGA, X-ray diffraction, scanning electron microscopy, and thermoelectric characterizations. All the samples obtained by solution synthesis show a very important increase in ZT, as compared to the solid state prepared samples, due to a significant decrease in the electrical resistivity values.

The ultra fast synthesis of nanocrystalline Mg(2)Si was carried out using microwave radiation. The elemental precursors were first milled together under dry conditions to get fine particles. The resulting mixture of powders of Mg and Si was cold pressed before being heated by microwave irradiation. Precursors and products were analyzed by X-ray diffraction and scanning electron microscopy. The high energy ball milling parameters utilized to prepare the reactive powders have quite an influence on the behavior of the mixture under irradiation. Moreover, SEM imaging demonstrates that the power and time of irradiation are crucial for the grain growth of the Mg(2)Si and must be adequately controlled in order to avoid the decomposition of the phase. Our results show that we successfully managed to easily and quickly synthesize homogeneous nanocrystalline Mg(2)Si with particle size smaller than 100 nm using a microwave power of only 175 W for two minutes on powders ball milled for two hours.