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ZrNi is considered a promising candidate for hydrogen storage and nickel-metal hydride rechargeable batteries (Ni-MH). The effect of creating zirconium and nickel vacancy defects on the dehydrogenation properties of ZrNiH3 is investigated by means of first-principles calculations. The results indicate that nickel vacancy is energetically more favorable to form in ZrNiH3 than zirconium vacancy, because of the lesser formation energy of Ni-vacancy. For both Zr and Ni vacancy defects, the formation enthalpy decreases with increasing the concentration of vacancy and, vice versa. In particular, it is found that with ~2.4% of zirconium vacancy defects or with ~4.5% of nickel vacancy defects in ZrNiH3, the formation enthalpy is around - 40 kJ/mol.H2, which is recommended by the U.S. Department of Energy (DOE). It is worth noting also that with slightly higher vacancy defects ~2.8 of Zr-vacancy or ~5.3% of Ni-vacancy in ZrNiH3, it becomes harder to store hydrogen in these systems without cooling. Moreover, the density of states (DOS) analysis indicates that the stability of ZrNiH3 decreases with increasing Zr-vacancy and Ni-vacancy concentrations, through the shrinkage in the size of the total DOS and shifting in the valence bands near to Fermi level.

First-principles calculations based on Plane-Wave Self-Consistent Field (PWSCF) method, implemented in quantum espresso program, have been performed on ZrNiH3 substituted with transition metals (V, Ti, Fe, Mn, and Cr). The study aims to investigate the heat of formation in terms of material stability and desorption temperature. It is found that the substitution by transition metals, results in a significant enhancement in the thermodynamic properties accompanied by an increase of the volumetric and gravimetric hydrogen storage capacities. In addition, the obtained values of heat of formation and desorption temperature corroborate with that required by the U.S. Department of Energy (DOE) for stability and volumetric capacity criteria. Moreover, Mn and Fe elements are found to present the lowest substituting content (34%) to obtain optimum hydrogen storage characteristics (enthalpy of formation of - 40 kJ/mol.H2, decomposition temperature of 300 K and volumetric capacity of 134 g.H2/l), without affecting the electronic structure and the metallic character of ZrNiH3.

We study hydrogen storage properties of Nb doped ZrNiH3 using the first-principles calculations based on density functional theory (DFT). To achieve such calculations, we used full potential linearized augmented plane waves (FP-LAPW) method implanted in WIEN2K code. Total energy and density of states (DOS) are computed for Zr1-xNbxNiH3 (x = 0 and 0.1). It turns out that substituting 10% of zirconium by niobium in ZrNiH3 matrix involves a destabilization of the system. This destabilization is induced by the formation of a new hybridization between niobium and hydrogen atoms. Furthermore, the thermodynamic stability in terms of its energy of formation, as well as the capacity of the material to store hydrogen are discussed. Noteworthy that from FP-LAPW we report that Nb doped ZrNiH3 lower the desorption temperature to 305.77 K without significant reduction of the hydrogen storage capacity. This lowering of the temperature desorption makes the material very useful for hydrogen storage in solids.

International audience; Lead-free ferroelectric Na0.5Bi0.5TiO3 (NBT) and NBT-doped Ln (Ln = Er and Ho) in both bulk and thin films forms were processed via solid-state reaction and solution deposition respectively. SEM investigations showed dense samples with fine-grained and smooth microstructure for thin films. A pure perovskite structure was observed by the X-ray diffraction for all specimens. The frequency dependences of the dielectric constant and loss tangent were investigated at room temperature and revealed a decrease in the dielectric losses with the introduction of the lanthanide element. The energy storage density (Wrec) was calculated using the obtained P–E loops. The stored and delivered energies were found depending on the form and composition of the sample.

Experimental analysis of stratification, electromagnetic stirring (EMS), and solidification were carried out for two solidification experiments for a binary Sn–10 wt % Pb alloy. The objective of this study is to examine the effect of forced convection driven by an intermittent traveling magnetic field on the solidification process. Several aspects were investigated, namely, thermal field, macrostructure, and finally segregation behavior, as well as morphology. The effect of the both thermal and solutal stratification on the intensity of the flow is discussed yet showed that stratification has a stabilizing effect for the flow, which can also slow the convective hydrodynamic movements generated by the buoyancy forces. The consequence of this stratification on macrosegregations and channel segregation, which develop during the solidification period, is experimentally analyzed. EMS by intermittent traveling magnetic promotes the development of the columnar-equiaxed transition mechanism (CET), more particularly the refinement of the grain size. The results illustrate, also, that EMS effectively diminish macrosegregations significantly, while remaining inactive for reducing channels segregation development.

International audience; The effect of Zr substitution by alkaline earth metals Mg, Be and post-transition metal Al on the evolution of hydrogen storage properties of ZrNiH 3 has been investigated by ab-initio calculations based on density functional theory. The stability of the quaternary hydrides is studied by the determination of the formation enthalpy and the desorption temperature. The obtained results indicate a reduction of the formation enthalpy as well as the desorption temperature , hence reflecting the enhancement of hydrogen storage properties of ZrNiH 3. Interestingly, each dopant (Mg, Be and Al) achieved its optimum substitution effect at a particular concentration, with Al and Be elements are found to exhibit the lowest substituting content 17% and 23% respectively and Mg with the highest concentration 85%, to achieve an ideal formation enthalpy (ΔH = −40 kJ/mol.H 2) and desorption temperatures (289 to 393 K), as required for practical use of proton exchange membrane fuel cells (PEMFC) without affecting the hydrogen storage capacity as seen in pure ZrNiH 3 .More-over, the electronic structure investigated by partial density of states (PDOS), reveals the metallic nature of Zr 1−x AM x NiH 3 (AM = Mg, Be and Al) hydrides. Highlights • The ZrNiH 3 hydride presents high stability and high decomposition temperature. • The stability decreases significantly when doping ZrNiH 3 with Mg, Be and Al. • The density of states reveals the metallic nature of Zr 1−x AM x NiH 3 hydrides. K E Y W O R D S density functional theory, hydrogen storage, Ni-MH batteries, thermodynamic properties, ZrNiH 3

Novel applications of Halbach array magnets in floating zone processes are proposed. Calculations of the floating zone process with two designs of Halbach array magnets are carried out and compared with the conventional floating zone process. The first part of the calculations including the static and high-frequency electromagnetic fields in the melt and crystal are modelled in COMSOL software. The effect of electromagnetic fields on the melt flow is calculated in three dimensions. From the calculation results, we confirm that Halbach array magnets can provide an adequately strong magnetic field at the surface of silicon to affect the melt flow. From the comparison of calculation results between conventional floating zone and floating zone with Halbach array magnets, the effects of the magnetic field on the melt flow, the temperature distribution, and the crystallization interface are confirmed. Halbach array magnets can locally restrain the melt flow velocity and improve the deflection of the crystallization interface.

The novel bi-molybdate β─Li0.87Na0.13Cr(MoO4)2 was prepared by solid state reaction route. Single crystal X-ray diffraction experiment revealed that the compound crystallizes in the triclinic system, in P −1 space group with a = 6.715 (2), b = 7.160 (3), c = 7.237 (1) A, α = 91.16° (3), β = 110.59° (2), γ = 105.54° (3). Its crystal structure is isotypic to LiCr(MoO4)2 which has interesting magnetic and electrochemical properties [1–3]. Bond valence sum (BVS) and charge distribution (CHARDI) validation tools supported the structural model. The electrical properties were systematically studied by impedance spectroscopy. The ionic conductivity measurements are performed on pellets of 82% and 87% relative density for LiCr(MoO4)2 and β─Li0.87Na0.13Cr(MoO4)2 respectively. AC impedance spectroscopy studies show that the highest overall conductivity is σ326°C = 7.86 × 10−7 S cm−1 Probable diffusion pathways of Li+ ions in the both structures were simulated using the bond valence sum BVS maps method. This analysis shows that the ionic transport in these materials is essentially due to simple hopping of Li+ ions parallel to (101) plane. For β─Li0.87Na0.13Cr(MoO4)2 compound, the in-situ High Temperature X-Ray Diffraction (HTXRD), in the temperature range from 25 to 650 °C, were also performed and Unit-cell thermal expansion has been discussed. The magnetic study show that these compounds present an antiferromagnetic order below the temperatures TN = 16 and 30 K for LiCr(MoO4)2 and β─Li0.87Na0.13Cr(MoO4)2 respectively.

The effect of a submerged heater on the Bridgman crystal growth process under travelling magnetic field is studied. It is known that the magnetic field generates melt flow from crystal center to crucible wall which can suppress morphological instability development, but it also increases the curvature of the crystallization front. The advantage of the submerged heater method is the ability of better control over the temperature distribution in the vicinity of the solid-liquid interface, as well as the constancy of the growth zone geometry, in particular the height of the melt above the phase boundary, which helps reduce the axial heterogeneity of the resulting crystal. Within the steady state approach we perform numerical simulation of Bridgman growth process under simultaneous action of travelling magnetic field and a submerged heater. It is shown that the submerged heater can intensify the flow in the radial direction generated by travelling magnetic field near the growing crystal while the curvature of the crystallization interface remains almost unaffected. On the other hand, it is also possible to decrease significantly the flow close to the crystallization front.

The numerical simulation of the horizontal solidification of pure tin under natural convection and under forced convection induced by Electromagnetic Stirring (EMS) is presented and compared with experimental results obtained by the ‘AFRODITE’ benchmark setup, described in several previous publications [6–10]. The experiment consists in solidifying a rectangular ingot (100 × 10 × 60 mm) using two lateral heat exchangers which allow the application of a controlled horizontal temperature difference. The experimental temperature difference between the two lateral sides of the sample DT = 40 K and the cooling rate CR = −0.03K/s. Under these conditions the solidification front is planar throughout the experiment. Enthalpy formulation based on fixed-grid techniques is used for the numerical simulations of the phase-change problems, accounting for buoyancy convection and forced convection created by Lorentz forces generated by an external Traveling Magnetic Field (TMF). The temperature distribution obtained by numerical simulation is demonstrated to effectively reproduce the temperature maps obtained from the experimental measurements. The proposed 3D numerical model has demonstrated its effectiveness in predicting the effect of EM stirring on the solidification process in terms of thermal field, dynamic field and the shape and localization of the solidification front.

During crystal growth, the fluid flow plays an important role on the quantity and distribution of impurities in the crystal. This study examines with modelling and experiments the removal of impurities from the solidification front for the case where a turbulent fluid flow increases the effective diffusivity. The theoretical model is based on the properties of turbulent boundary layers. It predicts a transition between the diffusive regime (no segregation) and the convective regime (efficient segregation) when the stirring velocity is higher than a threshold depending on the solidification rate. Efficient segregation is generally obtained when the stirring velocity is 10 5 times higher than the solidification rate. Silicon solidification experiments in a Taylor-Couette flow showed that concentrations in the crystal are close to the thermodynamic minimum for stirring velocities above the threshold. This result shows that the turbulent fluid flow efficiently removes impurities from the solidification front and validates the theoretical model.

Efficient numerical models are derived for problems of natural convection and material solidification in a horizontal differentially heated slender cavity. These 2D1/2 models are obtained by averaging the equations of momentum, heat, and mass conservation along the transverse direction assuming both a constant temperature and a well defined velocity profile in this direction. Based on our former works, the transverse velocity profile is assumed to be either a Poiseuille profile ( 2 D p 1 / 2 model), or Hartmann-type profiles featuring two boundary layers on the sides of a uniform bulk ( 2 D H 1 / 2 model). For this 2 D H 1 / 2 model, however, a parameter δ (giving the boundary layer thickness) has to be adjusted: optimal values have been found in a large range of the control parameters and expressed as a reliable fitted function of G r . The ability of the model to reproduce 3D results in a 2D framework is investigated in a large range of the control parameters (Prandtl number P r and Grashof number G r ); the validity domain of the model in this parameter space is also clarified and rigorously defined. A good precision is obtained for natural convection problems (intensity of the flow, temperature field) as well as for solid-liquid phase change problems (shape, position, and evolution of the front). A comparison with unpublished experimental data of solidification of pure tin is also conducted. The boundary conditions for the simulation are first defined after a post-treatment of the time-dependent experimental data in order for them to be representative of the experimental process despite a significant and time dependent thermal resistance between the walls of the crucible and the liquid. A very good agreement is observed between the 2 D H 1 / 2 model and the experimental measurements for this pure tin solidification experiment in the AFRODITE setup.

International audience; Within the framework of purification of molten silicon for solar cells, phosphorus is one of few impurities that remains problematic. The present purpose is to develop a new method that leads to a final phosphorus concentration around 0.5 wppm. We have first focused on the main thermodynamic parameters that control the process: relevant thermodynamic parameters are taken from literature, such as activities, partial pressures and interaction coefficients. According to our results, distillation of phosphorus under vacuum occurs at high temperatures. Yet, the residual oxygen present in the vacuum is a hindrance to the distillation, since it evaporates in the form of silicon oxides. (C) 2015 The Electrochemical Society. All rights reserved.

International audience; A solidification benchmark experiment was designed and developed at SIMAP Laboratory in Grenoble in order to investigate the structure formation as well as solute macro-mesosegregation by means of a well-controlled solidification experiment. The experiment consists in solidifying a rectangular ingot of Sn-10 wt.%Pb alloy by using two lateral heat exchangers, allowing extraction of the heat flux from one or two vertical sides of the sample. The domain is a quasi-two dimensional rectangular ingot (100 x 60 x 10 mm). The temperature difference AT between the two lateral sides is 40K, and the cooling rate CR = 0.03 K/s. The instrumentation firstly consists in recording the instantaneous temperature maps by means of a lattice of 50 thermocouples to provide the time evolution of the isotherms. Measurements of the velocity field by ultrasonic Doppler velocimetry (UDV) method in a Ga-In-Sn pool were taken and transposed to the tin-lead alloy case before solidification. After each experiment, the segregation patterns were obtained by X-ray radiograph, and confirmed by quantitative analysis of the solutal distribution obtained by chemical method coupled with the technical ICP (Inductively Coupled Plasma). The originality of the present study is to examine the effect of various types of forced convection driven by a traveling magnetic field (TMF). Three cases of TMF were investigated: in the same direction as natural convection, reversed with respect to natural convection, and periodically reversed with a modulation frequency equal to 0.125 Hz. This study allows us to evaluate the evolution due to the forced convection induced by a TMF field, as well as its influence on the initial conditions, the solidification macrostructure and the segregation behavior. While forced convection promotes equiaxed structures, the stirring pattern influences grain size according to its direction with respect to natural convection. The results show that all electromagnetic stirring modes effectively reduce macrosegregations significantly, while remaining inactive for reducing development of segregated channels.

Kyropoulos crystal growth seeding process for silicon was studied with 3D modeling of the full furnace. The process had three heating zones (top, bottom and side) to improve the control of the horizontal and vertical thermal gradients in the melt; and, a square crucible to reshape the final crystal into a square horizontal section. Modeling of the starting configuration, where crystal growth from the seed was not possible experimentally, explained that the ascendant melt flow in the center is bringing hot melt to the seed. The addition of a ceramic tube as a radiation shield around the seeding zone and the increase of side heating were modeled numerically as solutions to reverse the melt flow direction. The applied solutions on the experimental furnace allowed the growth from the seed. The process was sensitive to symmetry loss. The asymmetric growth of the crystal towards one preferential direction was explained by a coupling between the sudden increase of emissivity due to solidification of silicon and the local decrease of convection on the side of the preferred growth direction. A symmetric growth was obtained in the model by lowering radiative cooling at the top of the melt surface during the first stage of the growth, and by homogenizing the melt using crystal rotation. As a result, symmetrical circular crystals were obtained experimentally using the same growth conditions without contact with the crucible and with no upward pulling.

International audience; This paper presents a quasi-two-dimensional solidification benchmark experiment with controlled thermal boundary conditions. The experiment consists in solidifying a rectangular ingot of Sn-3wt.%Pb using two lateral heat exchangers to extract the heat flux from one or both vertical sides of the sample. The aim is to assess the reproducibility of the solidification experiments performed under rigorously identical conditions. The temperature difference (Delta T) between the two vertical sides is 40 K and the cooling rate (CR) is 0.03 K/s. This slow-cooling condition favours segregation formation. A lattice of fifty thermocouples placed on the front of the sample between the two vertical sides is used to determine the instantaneous temperature distribution. During solidification, the time evolution of the temperature field is recorded, and analyzed. This allows us to estimate the time evolution of the temperatures due to natural convection and assess the velocity field, the solidification macrostructure and the segregation behaviour. Post-mortem segregation patterns are obtained both by X-ray radiography and induction coupled plasma analysis. The recorded data is analyzed statistically. An ensemble average procedure is applied to both instantaneous temperature and post-mortem composition measurements. This makes it possible to determine the mean values of the solidification parameters as well as their standard deviations. Temperature fluctuation standard deviations are deterministic and characterize the solidification dynamics. Among other results, it is shown that mesoscale segregation locations are not fully predictable. (C) 2016 Elsevier Masson SAS. All rights reserved.

Numerical simulations of stationary thermal convection in a differentially heated enclosure corresponding to the AFRODITE solidification benchmark experiment [1e5] are presented. The cavity of relative dimensions 10:6:1 (length:height:width) is characterized by a small transverse width. The Prandtl number Pr is varied within the range [0.0045, 0.03], typical of liquid metals, whereas the Grashof number, defined as Gr ¼ gb(Dq/L)H 4 /n 2 , is varied within the range [1.3 � 10 6 ,1 .6� 10 7 ]. As shown by the reference 3D simulations, the temperature field in these situations is 2D (independent of the transverse direction); 2D simulations are, however, not able to catch the physics of the flow and the resulting temperature results are also erroneous. To improve these 2D simulation results while keeping reasonable computational times, a 2D½ model is developed, which will take into account the no-slip condition at the side walls. This model is obtained by averaging the governing equations over the width of the domain, with a transverse profile for the velocity featuring a uniform central part and two boundary layers of size d (d is fixed for the whole domain). The relative deviation of the temperature field between the 2D½ and 3D computations is investigated as a function of the Prandtl number, the Grashof number and the chosen boundary layer thickness. It is shown that an optimum value exists for d, which gives a mean deviation in the middle plane of less than 2%, whereas the choice of a more usual parabolic profile would lead to a twice larger deviation. Good comparisons are also obtained with the original experimental results reported at the end of the paper. The 2D½ model is thus able to give results which compare well with fully 3D results. It can then be used for extensive parametric studies at a reasonable cost.

International audience; The numerical simulation of macrosegregation formation during the horizontal solidification of a Sn-3 wt %Pb alloy in a rectangular cavity is presented and compared to experimental results. The benchmark experiment consists in solidifying a rectangular ingot of Sn-3 wt%Pb alloy using a solidification setup with a precise control of the thermal boundary conditions. The 3D simulation is based on a model formerly derived with a two-phase volume averaging technique. It is focused on the solidification stage of the experiment, during which lead segregation occurs. Both the temperature field measured in situ during the solidification and the post-mortem observations of macrostructures can be compared with the numerical simulation results. These comparisons are rather good: the evolution of the temperature field during the solidification process is well reproduced, and the main zones of lead enrichment are recovered. A particular focus is made on the segregated channels development mechanism, as well as their size and geometric shape. They are observed to grow in the vicinity of the side walls and to feature a curved tubular shape in the simulations. (C) 2016 Elsevier Ltd. All rights reserved.

International audience; A quasi-two-dimensional solidification benchmark experiment with controlled thermal boundary conditions is proposed. The experiment consists in solidifying a rectangular ingot of Sn-3 wt.%Pb alloy using two lateral heat exchangers to extract the heat flux from one or two sides of the sample. The temperature difference between the two sides of the heat exchangers may vary from 0 to 40 K and the cooling rate from 0.02 to 0.04 K/s. This slow-cooling condition has been used to promote segregation formation. An array of fifty thermocouples placed on the corresponding sample walls is used to determine the instantaneous temperature distribution. During the solidification process, the temperature field is recorded versus time and analyzed. This makes it possible to estimate the change in temperature due to natural convection, the velocity field and the solidification macrostructure and segregation behavior. After each experiment, the segregation patterns are obtained by X-ray analysis and confirmed by eutectic fraction measurements. The local solute distribution is determined by means of induction coupled plasma analysis. (C) 2011 Elsevier Ltd. All rights reserved

International audience

We analyzed the columnar solidification of a binary alloy under the influence of an electromagnetic forced convection of various types and investigated the influence of a rotating magnetic field on segregation during directional solidification of Al-Si alloy as well as the influence of a travelling magnetic field on segregation during solidification of Al-Ni alloy through directional solidification experiments and numerical modeling of macrosegregation. The numerical model is capable of predicting fluid flow, heat transfer, solute concentration field, and columnar solidification and takes into account the existence of a mushy zone. Fluid flows are created by both natural convection as well as electromagnetic body forces. Both the experiments and the numerical modeling, which were achieved in axisymmetric geometry, show that the forced-flow configuration changes the segregation pattern. The change is a result of the coupling between the liquid flow and the top of the mushy zone via the pressure distribution along the solidification front. In a forced flow, the pressure difference along the front drives a mush flow that transports the solute within the mushy region. The channel forms at the junction of two meridional vortices in the liquid zone where the fluid leaves the front. The latter phenomenon is observed for both the rotating magnetic field (RMF) and traveling magnetic field (TMF) cases. The liquid enrichment in the segregated channel is strong enough that the local solute concentration may reach the eutectic composition.

International audience; : Multi crystalline silicon wafers have a lower production cost compared to mono crystalline wafers. This comes at the price of reduced quality in terms of electrical properties and as a result the solar cells made from such materials have a reduced efficiency. The presence of different impurities in the bulk material plays an important role during the solidification process. The impurities are related to different defects (dislocations, grain boundaries) encountered in multi-crystalline wafers. Applying an alternative magnetic field during the solidification process has various benefits, impurities concentration in the final ingot could be reduced, especially metallic species, due to a convective term added in the liquid that reduces the concentration of impurities in the solute boundary layer. Another aspect is the solidification interface shape that is influenced by the electromagnetic stirring. A vertical Bridgman type furnace was used in order to study the solidification process of Si under the influence of a travelling magnetic field able to induce a convective flow in the liquid. The furnace was equipped with a Bitter type three-phase electromagnet that provides the required magnetic field. A numerical model of the furnace was developed in ANSYS Fluent commercial software. This paper presents experimental and numerical results of this approach, where interface markings were performed.

International audience; A comparative study among solidification experiments for three selected alloys (Sn-3 wt.%Pb, Sn-6.5 wt.%Pb and Sn-10 wt.%Pb) was conducted on a benchmark experiment model under the same experimental conditions. The goal of this paper is to analyze the effect of variation in concentration on the solidification process with respect to different aspects: thermal, dynamic, structure, and morphology of segregation. Experimental results consist of instantaneous temperature maps provided by a lattice of 50 thermocouples welded on the large crucible side and post-mortem characterizations of the samples, such as X-ray imaging, solute local composition and metallography. Measurement of the instantaneous temperature field and numerical computation of liquid solid interface evolution allows us to evaluate the effect of variation in concentration on thermosolutal convection behavior. Experimental results show that an increase in concentration greatly enhances the mechanism of the columnar-to-equiaxed transition (CET) and leads to refinement of the equiaxed structure. However, a significant effect of solutal element (lead) stratification is observed, which can slow down thermosolutal convection, in particular for large concentrations. Especially, without any stirring lead segregation which likely occurs during the melting phase may suppress natural convection. Furthermore, lead stratification is significantly reduced when electromagnetic stirring is opposed to natural convection before the solidification phase begins.

The specific objective of the paper is to investigate the effect of fluid flow on the development of a dendritic microstructure and more particularly on the columnar and equiaxed solidification regimes. A gradient furnace (BATMAF) with a possibility of applying a travelling magnetic field was designed for the solidification of aluminium-based alloys. The travelling magnetic field (TMF) drives a forced flow in the liquid above the solidifying interface. Some experiments are achieved in this furnace on an Al-Ni3.5wt% refined alloy. As a result, a significant change of the microstructure due to the transversal melt flow is observed. This work is performed in the framework of the CETSOL/MICAST Microgravity Application Projects of the European Space Agency (ESA).

International audience; Silicon is the most widely used semiconductor for photovoltaic solar cells, and also the most extensively studied. The need for cost reduction lead solar cell producers to look for cheaper raw silicon, allowing a higher impurity content in the charge used as input for crystallization and making it very different from the extensively studied electronic-grade silicon. In order to keep a good efficiency of the solar cells, the impurities that have a huge impact over the electrical properties of the material should be removed from the final ingot. Controlling the segregation of impurities during the solidification is therefore mandatory.A study of the solidification of metallurgical grade silicon under controlled solidification parameters such as growth rate, thermal gradient and forced convection flow was conducted in a Bridgman set-up. Forced convection was induced by a travelling magnetic field (TMF) during the solidification, and resulted in a mixing of excess impurities in the liquid, removing them from the vicinity of the solidification front. As a result the purity of the final ingot is increased. The impact of the forced convection on the segregation of metallic impurities and on the orientation of grain boundaries during the silicon solidification is presented in this paper.

International audience; In solidification processes the fluid flow occurs almost at every scale from the bulk, near the interfaces and deeply in the mushy zone. Numerical modeling is a valuable tool for understanding and master the solidification processes, however, macro-scale models are not always able to predict in detail the random behavior of the solidification process whereas models for micro scales are not capable to take into account a complex structure of flows which enter into the mushy zone. In the present paper the variety of the flows and imprints they left on solidification structure are discussed and illustrated with experimental data which naturally comprise every flow occurring in the process.

The present paper is aimed at showing how solidification of metallic alloys can be influenced by AC or DC magnetic fields through various types of effects. The application of AC magnetic fields leads to the generation of either fluid flows or vibration. It has been shown both numerically and experimentally that the electromagnetically-driven flows created by travelling or rotating magnetic fields promoted segregations and influenced their distribution. The flow may also promote the CET thanks to its effects on both the temperature and solute fields as well as the possible fragmentation mechanism. As far as DC magnetic fields are concerned, it was known that they usually exert a damping of the bulk fluid flows. However, it has been shown recently that for some alloys high intensity magnetic field interacts with the small thermoelectric current to create significant electromagnetic forces which are responsible for strong liquid metal flows both in the bulk and in the mushy zone. Orientation changes as well as possible modifications of thermodynamic properties were also observed.

International audience; A solidification benchmark experiment is simulated using a three-dimensional cellular automaton-finite element solidification model. The experiment consists of a rectangular cavity containing a Sn-3 wt pct Pb alloy. The alloy is first melted and then solidified in the cavity. A dense array of thermocouples permits monitoring of temperatures in the cavity and in the heat exchangers surrounding the cavity. After solidification, the grain structure is revealed by metallography. X-ray radiography and inductively coupled plasma spectrometry are also conducted to access a distribution map of Pb, or macrosegregation map. The solidification model consists of solutions for heat, solute mass, and momentum conservations using the finite element method. It is coupled with a description of the development of grain structure using the cellular automaton method. A careful and direct comparison with experimental results is possible thanks to boundary conditions deduced from the temperature measurements, as well as a careful choice of the values of the material properties for simulation. Results show that the temperature maps and the macrosegregation map can only be approached with a three-dimensional simulation that includes the description of the grain structure.

International audience; Numerical simulations of macrosegregation formation during horizontal solidification of a hypoeutectic Sn-Pb alloy in a rectangular cavity is presented and compared to experimental results. The experiment involves a melting phase, a holding stage at constant temperature with or without application of electromagnetic stirring, and finally solidification. We simulate all stages of the experiment and study the segregation of Pb throughout the experiment. A two-phase volume-averaged model is used for the numerical simulations, accounting for thermo-solutal convection and assuming perfect microscopic mixing (lever rule). The influence of the melting phase and of the electromagnetic stirring is studied with different cases to illustrate the consequences on the flow pattern and on Pb segregation during melting and during solidification. The numerical simulations are compared to the experimental data and the impact of chemical inhomogeneities prior to solidification on macrosegregation formation is discussed.

International audience; Metallurgical grade silicon (MG-Si) has become a new source of raw material for the photovoltaic industry. The use of this material as an alternative feed stock has however introduced phenomena that are detrimental to both the yield of the manufacturing process and the performance of the photovoltaic cells produced. This is mainly related to the presence of carbon, which precipitates to silicon carbide (SiC) in the ingot. This article focuses on the effect of carbon on silicon nucleation. Statistical experimental results of silicon nucleation are obtained as a function of carbon concentration and are presented and compared to thermodynamic calculations.

International audience; During solidification of low purity silicon for photovoltaic (PV) cells, solute rejection at the growth interface leads to an increase of the carbon concentration in the liquid phase and then to the precipitation of silicon carbide (SiC). When the precipitate radius becomes higher than the silicon critical nucleus radius, SiC can act as a refining agent for the Si and Si equiaxed grains appear in the liquid. The grain structure of the ingot changes from columnar to small grains, also known as grits. We developed a one-dimensional analytical model of this series of phenomena, including C segregation, SiC nucleation and growth. Si nucleation on the SiC precipitates and subsequent growth of the Si equiaxed grains. The equations are implemented under Matlab software in order to predict the columnar to equiaxed transition (CET) during the directional solidification of PV Si. We carried out calculations of the position and thickness of the equiaxed areas and of the number and size of Si grits as a function of the main process parameters: thermal gradient and growth velocity. Recommendations in order to adapt the growth process parameters to the initial carbon content are given. It is expected that coupling this model to global 3D numerical simulation codes could help improving the yield of ingot solidification. (C) 2011 Elsevier B.V. All rights reserved.

International audience; Effect of a high magnetic field on the growth of the alpha-Al dendrite in three Al-based alloys during directional solidification at a higher growth speed has been investigated experimentally. It has been found that for different alloy, the effect of the magnetic field on the growth of the alpha-Al dendrite is different. Indeed, a high magnetic field has caused the occurrence of the columnar-to-equiaxed transitions (CET) of the alpha-Al dendrite in the Al-Si alloy. However, for the Al-Ni alloy, the effect of the magnetic field on the alpha-Al dendrite is negligible. Further, the thermoelectric magnetic force (TEMF) imposed on the dendrite under a high magnetic field has been investigated numerically and the results show that the value of the TEMF imposed on the dendrite during directional solidification under a 10 T high magnetic field is about 10(5) N/cm(3) and this force should be responsible for the occurrence of the CET in the Al-Si alloy. This may act as an experimental proof that the force imposed on the dendrite will induce the occurrence of the CET. Above results are discussed from the TEMF imposed on the dendrite and the difference of the stacking fault energy of alpha-Al dendrites in the different alloy.

International audience; The rapid evolution of photovoltaic Si production induced a shortage of high purity silicon raw material. The use of lowest purity silicon has a strong effect on the casting conditions and ingot structure and properties. During solidification, solute rejection at the growth interface leads to an increase of the impurities concentration in the liquid phase and then to the precipitation of silicon nitride and silicon carbide. As a consequence, the grain structure of the ingot changes from columnar to small grains, also known as grits. A new electromagnetic levitation setup which has been developed in order to measure the undercooling versus impurity concentration is presented. The impurity concentration in the levitated Si drop is controlled by the partial pressure of nitrogen or hydrocarbon gas. As nucleation is a random phenomenon, statistical measurements are presented, from samples which showed numerous heating/melting and cooling/solidification phases. The effect of carbon impurities on the undercooling of silicon droplet is discussed.

Colloque avec actes et comité de lecture. Internationale.; International audience; La solidification des alliages de Titane-Aluminium est problématique dans la mesure où de nombreuses hétérogénéités apparaissent. Un modèle numérique de solidification basé sur le moyennage statistique a été établi en prenant en compte la spécificité de ces alliages. Les calculs montrent que la convection a une influence certaine sur la qualité des lingots obtenus.

La solidification des alliages binaires est problematique dans la mesure ou de nombreuses heterogeneites susceptibles de fragiliser le produit apparaissent. Ces heterogeneites peuvent etre a la fois de composition (appelees segregations) et de structures. Les segregations presentent, en outre, l'inconvenient d'apparaitre aussi bien a l'echelle mesoscopique (freckles) qu'a l'echelle du lingot (macrosegregation). Il apparait que la gravite joue un role predominant dans la formation et le devenir des heterogeneites observees. S'affranchir de cette composante peut etre un des enjeux des nouveaux procedes de solidification. Par ailleurs, les alliages de titane-aluminium presentent une solidification peritectique : au cours de la solidification peritectique deux phases solides sont presentes dans le lingot. Un modele numerique de solidification base sur le moyennage statistique a ete etabli qui prend en compte la specificite de ces alliages. Les calculs menes en condition de microgravite et de convection forcee montrent que la convection au voisinage du front de solidification influence la localisation des heterogeneites.

In the field of the metallic alloy development, the main industrial goals are the control of the metallurgical structure and defects. Hydrodynamic movements in the liquid phase have a significant influence on properties of the solidified product. We focus our attention on two major effects influenced by forced convection induced by a travelling magnetic field: the macrosegregation and the grain structure for Al-3.5 wt% Ni alloys. We show that this configuration can control macrosegregations and that, moreover, the dendritic primary spacing maybe modified by varying the applied field. With regards to the grain structure, we exhibit the effect of the forced convection on the microstructure. For the equiaxed regime growth, the effect of the travelling magnetic field on the constitutional undercooling and on the refiner distribution induces an equiaxed to an elongated transition. This mechanism enhances the nucleation and the growth of equiaxed grains. In the case of non-refined alloys, a mode of elongated free grains is obtained most likely because of fragmentation. To cite this article: K. Zaidat et al., C. R. Mecanique 335 (2007).

International audience; A new Bridgman-type furnace has been designed and built.This furnace has two specific items: a device conceived to minimise any residual radial thermal gradient near the solid–liquid interface (induced by the difference in thermal conductivities between liquid and solid), and a travelling magnetic field to drive a forced flow in the liquid above the solidifying interface. We have already performed comparative experiments of directional solidification on refined and non-refined Al-based alloys, under natural convection and under forced flow, controlled by the travelling magnetic field. We will present some characteristics of this new furnace, and the preliminary results obtained by solidifying refined Al–3.5 wt% Ni, with and without the travelling magnetic field.

International audience; The main objective of the research project of the European Space Agency (ESA) Microgravity Application Promotion (MAP) programme entitled Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL) is the investigation of the formation of the transition from columnar to equiaxed macrostructure that takes place in casting. Indeed, grain structures observed in most casting processes of metallic alloys are the result of a competition between the growth of several arrays of dendrites that develop under constrained and unconstrained conditions, leading to the CET. A dramatic effect of buoyancy-driven flow on the transport of equiaxed crystals on earth is acknowledged. This leads to difficulties in conducting precise investigations of the origin of the formation of the equiaxed crystals and their interaction with the development of the columnar grain structure. Consequently, critical benchmark data to test fundamental theories of grain structure formation are required, that would benefit from microgravity investigations. Accordingly, the ESA-MAP CETSOL project has gathered together European groups with complementary skills to carry out experiments and to model the processes, in particular with a view to utilization of the reduced-gravity environment that will be afforded by the International Space Station (ISS) to get benchmark data. The ultimate objective of the research program is to significantly contribute to the improvement of integrated modelling of grain structure in industrially important castings. To reach this goal, the approach is devised to deepen the quantitative understanding of the basic physical principles that, from the microscopic to the macroscopic scales, govern microstructure formation in solidification processing under diffusive conditions and with fluid flow in the melt. Pertinent questions are attacked by well-defined model experiments on technical alloys and/or on model transparent systems, physical modelling at microstructure and mesoscopic scales (e.g. large columnar front or equiaxed crystals) and numerical simulation at all scales, up to the macroscopic scales of casting with integrated numerical models.

The formation of the grain structure in multicrystalline photovoltaic Silicon relies on basic phenomena which are not well understood and mastered in industrial processes. Nucleation, growth modes (facetted or not), grain competition and kinetics, coarsening are at the origin of the mean grain size and of the morphology of the grain structure which impacts drastically on the photovoltaic properties. During solidification, solute rejection (especially carbon and nitrogen) at the growth interface leads to an increase of the impurity concentration in the liquid phase and then to the precipitation of silicon nitride (Si3N4) and silicon carbide (SiC). As a consequence, the grain structure of the ingot changes from columnar to small grains, also known as grits.

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 solidification 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 électromagnétique 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 parallèle 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 solidification d'un lingot ainsi que la répartition en taille des grains dans les zones de grits.