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International audience; Langatate (LGT) crystals of La3Ga5.5Ta0·5O14 composition of diameter 50 mm were grown from the melt by Czochralski technique. Using (1–2 wt %) Ga2O3 excess in the starting charge and growing crystal in mixture argon (0.1–1%O2) gas atmosphere are a good condition to crystallize LGT under stationary stable regime. The LGT crystals grown along Z-axis exhibit strong faceting. The grown crystals were exempt of inclusions, cracks and secondary phases. The presence of oxygen in the growth chamber is necessary to limit gallium oxide evaporation and strongly affect the crystals coloration and the transmission spectra in the range (200–500 nm). The electrical resistivity is sensitive to the oxygen content in the growth environment.

A full three-dimensional (3D) numerical simulation of solidification was carried out for a benchmark experiment on a binary Sn-10 wt. %Pb alloy. The experiment process involves a melting stage, a first holding stage at constant temperature with electromagnetic stirring, setting a mean horizontal temperature difference (second holding stage), and finally solidification stage by decreasing the temperature under a imposed horizontal temperature gradient. The numerical model is applied only to investigate the solidification stage and compared with the measured temperature fields and macrosegregation obtained from the postmortem analysis. A columnar numerical model based on a two-phase volume-averaged approach is used for the numerical simulation, accounting for thermosolutal convection and assuming perfect microscopic mixing (lever rule) in the mushy zone. It demonstrates that such a model is able to predict stratification in the solute from the liquid phase and mushy zone during the solidification. The effect of the sedimentation on macrosegregations and channel segregation or freckles which develop during the solidification stage is also predicted by the model and compared with experimental data. Emphasis is given to the main factors that have a direct effect on the development and morphology of segregated channels, namely, the remelting phenomenon, dendrite fragmentation, and the solidification front instabilities.

Silicon is the most widely used raw material in photovoltaic industry; however, the quality of the silicon photovoltaic solar cells depends on the quality of the raw material (i.e. metallurgical silicon or poly-silicon) and the solidification methods used for the production of the silicon ingot from which the solar cells are produced. This study is related to how improve the quality of the final ingot in the directional solidification process; it is necessary to control the impurity segregation of silicon raw material during the processing. This control can be accomplished by adding an electromagnetic Bitter coil which can generate an external traveling magnetic field (TMF) stirring to control the hydrodynamic flow of silicon melt during the solidification process without contaminating it. To carry out this study, we used a Bridgman vertical directional solidification furnace, equipped with a cylindrical Bitter coil stirrer used in order to have the control of the silicon melt convection on the principal parameters of the solidification process, such as the growth rate, the thermal gradient and the natural convection of silicon melt. For the electromagnetic, heat exchange and silicon melt flow modelling, we used 3D numerical Multiphysics coupled models. Parallel to the numerical results we carried out experimental investigations relating to the characterization of the electromagnetic parameters. This study shows a promising effect of the applied traveling magnetic field on the final ingot quality; indeed, we have the ability to control the silicon melt flow which can affect the thermal configuration, the solidification interface shape and the segregation of impurities by changing the electric current input configuration of the Bitter coil stirrer.

The rapid evolution of photovoltaic silicon cells production induced to use a silicon with a lower purity. This silicon involves silicon carbide and silicon nitride precipitation which lead to new undesirable morphological structure called grits. However physico-chemical parameters which control these phenomenons are still unknown. Thereby, we developed an experimental levitation set-up under ultra high vacuum which allows measuring nucleation undercooling of silicon containing known concentration of carbon and nitrogen. The results showed a rapid decrease of the undercooling whith increasing impurities concentration. In parallel a facetted to equiaxed facetted transition model has been developed. Using analytical theoretical models, it allows predicting the columnar facetted to equiaxed facetted transition all along the ingot solidication and the grain size distribution, L'évolution de la production mondiale de cellule photovoltaïque a entraîné une pénurie en silicium de haute pureté. Le silicium de qualité moindre provoque, lors de la cristallisation, l'apparition de morphologies indésirables (grits) dues à la précipitation de carbure et de nitrure de silicium. Cependant les paramètres physico-chimiques liés à ces phénomènes sont encore inconnus. Dans ce but, nous avons développé une expérience de lévitation électromagn étique sous ultra vide permettant de mesurer la surfusion de germination du silicium en présence de carbone et d'azote dans des conditions contrôlées. Les résultats ont montré une diminution drastique de la surfusion en augmentant la concentration en impureté. En parall èle un modèle de transition facetté-équiaxe a été développé. En s'appuyant sur des modèles analytiques, il permet de prédire les transitions colonnaires-grits au cours de la solidication d'un lingot ainsi que la répartition en taille des grains dans les zones de grits.