Saint-Martin-Vésubie, France
Saint-Martin-Vésubie, France

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Duffar T.,SIMaP EPM
Crystal Research and Technology | Year: 2011

A thermodynamic study is performed for the systems (Ga or In)-Sb-O-Si in order to better understand the difference observed during dewetting experiments of GaSb and InSb in silica ampoules. Results show that the melts can be considered as non reactive toward silica. When the atmosphere is clean (≤ 1 ppm O;bsubesub), no oxide is formed, while, under oxidising atmosphere, oxides exist above the melting point of the antimonide and are known to increase the wetting angle of the melt on the crucible. However the temperature range for oxide stability is smaller in the case of InSb and this may explain why dewetting is easy for GaSb in presence of oxygen, while it is difficult for InSb. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Sylla L.,Cyberstar | Duffar T.,SIMaP EPM
Journal of Crystal Growth | Year: 2011

A series of experiments was performed in order to look at the solidliquid interface and meniscus region in dewetted Bridgman growth of GaSb and InSb in silica crucibles. In agreement with theoretical models, they show that a very stable dewetting was obtained for GaSb only if an oxidising atmosphere is present in the ampoule, and that it increased the wetting and growth angles. Stable dewetting of InSb was never obtained, which can be explained by wetting and thermodynamic considerations. Experiments with an easily dissolved gas (H2) have shown that a model of dissolutionrejection of ampoule gas cannot explain dewetting. Also the formation of hillocks has been observed. Finally, the self-stabilisation of the gas pressure difference is confirmed but remains largely unexplained. © 2011 Elsevier B.V.

Beaudhuin M.,SIMaP EPM | Beaudhuin M.,Lawrence Berkeley National Laboratory | Duffar T.,SIMaP EPM | Lemiti M.,Lyon Institute of Nanotechnologies | Zaidat K.,SIMaP EPM
Journal of Crystal Growth | Year: 2011

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. © 2011 Elsevier B.V.

Duffar T.,SIMAP EPM | Bochu O.,French Atomic Energy Commission | Dusserre P.,French Atomic Energy Commission
Journal of Materials Science | Year: 2010

In order to better understand the BGO-crucible interaction, the wetting of the molten BGO on Ir substrates has been studied by the sessile drop method at 1050 °C. It was found that the oxygen partial pressure in the surrounding atmosphere is an important parameter. For PO2 > 0.1 mbar, full wetting is observed without any visible contact angle. X-ray diffraction analyses have shown that Bi2Ir2O7 is formed at the interface. For PO2 lt; 0.1 mbar, a drop is observed with a contact angle of 70° and no chemical reaction is detected. Attempts to grow BGO samples under low oxygen partial pressure resulted in ingot without sticking with the crucible. © 2009 Springer Science+Business Media, LLC.

Stelian C.,SIMAP EPM | Stelian C.,West University of Timișoara | Duffar T.,SIMAP EPM
Journal of Crystal Growth | Year: 2014

Numerical modeling is used to investigate CdTe crystallization by Traveling Solution Growth (TSG) technique under terrestrial and microgravity conditions. Numerical results are compared to some experimental observations on CdTe growth by a TSG seeded process under ground conditions. It is found that the instability of the growth interface is related to the compositional non-uniformities of the liquid zone near the solidification front, caused by the species convective transport from the dissolution interface to the growth interface. The modeling of the ground experiment shows a large liquid zone (approx. 5 cm) after the dissolution of CdTe feed material into tellurium. The convection is characterized by a two-cell pattern. The vortex located near the dissolution interface mixes the solute in this region, reducing the tellurium transport to the growth interface. The interface is more stable in this case, but the dopant (indium) distribution is not homogeneous in the solidified sample. Numerical simulations performed by reducing the length of the liquid zone (approx. 2.5 cm) show one single vortex which is extended in the whole melt volume. The intense convection accelerates the tellurium transport to the growth interface, which is destabilized. In order to avoid the morphological destabilization of the growth interface, crucible rotation at constant speed can be applied. In this case, the homogeneity of dopant distribution in the solidified ingots is significantly improved. The modeling of CdTe crystallization under microgravity conditions shows favorable conditions for the growth with a stable interface in the absence of the convection. © 2014 Elsevier B.V. All rights reserved.

Stelian C.,SIMAP EPM | Duffar T.,SIMAP EPM
Journal of Crystal Growth | Year: 2015

The influence of rotating magnetic fields (RMF) on species transport and interface stability during the growth of Cd0.96Zn0.04Te:In crystals by using the traveling heater method (THM), under microgravity and terrestrial conditions, is numerically investigated. The numerical results are compared to ground and space experiments. The modeling of THM under ground conditions shows very deleterious effects of the natural convection on the morphological stability of the growth interface. The vertical flow transports the liquid of low Te concentration from the dissolution interface to the growth interface, which is consequently destabilized. The suppression of this flow, in low-gravity conditions, results in higher morphological stability of the growth interface. Application of RMF induces a two flow cell pattern, which has a destabilizing effect on the growth interface. Simulations performed by varying the magnetic field induction in the range of 1-3mT show optimal conditions for the growth with a stable interface at low strength of the magnetic field (B=1mT). Computations of indium distribution show a better homogeneity of crystals grown under purely diffusive conditions. Rotating magnetic fields of B=1mT induce low intensity convection, which generates concentration gradients near the growth interface. These numerical results are in agreement with experiments performed during the FOTON M4 space mission, showing good structural quality of Cd0.96Zn0.04Te crystals grown at very low gravity level. Applying low intensity rotating magnetic fields in ground experiments has no significant influence on the flow pattern and solute distribution. At high intensity of RMF (B=50mT), the buoyancy convection is damped near the growth front, resulting in a more stable advancing interface. However, convection is strengthening in the upper part of the liquid zone, where the flow becomes unsteady. The multi-cellular unsteady flow generates temperature oscillations, having deleterious effects on the growth process. © 2015 Elsevier B.V. All rights reserved.

Cablea M.,SIMAP EPM | Zaidat K.,SIMAP EPM | Gagnoud A.,SIMAP EPM | Nouri A.,Silicor Materials | Delannoy Y.,SIMAP EPM
Journal of Crystal Growth | Year: 2014

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. © 2013 Elsevier B.V.

Cablea M.,SIMAP EPM | Zaidat K.,SIMAP EPM | Gagnoud A.,SIMAP EPM | Nouri A.,Clean Power Innovation | And 2 more authors.
Journal of Crystal Growth | Year: 2014

Abstract 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. © 2014 Published by Elsevier B.V.

Steinbach S.,German Aerospace Center | Ratke L.,German Aerospace Center | Zimmermann G.,ACCESS E.V. | Budenkova O.,SIMAP EPM
IOP Conference Series: Materials Science and Engineering | Year: 2016

Ternary Al-6.5wt.%Si-0.93wt.%Fe alloy samples were directionally solidified on-board of the International Space Station ISS in the ESA payload Materials Science Laboratory (MSL) equipped with Low Gradient Furnace (LGF) under both purely diffusive and stimulated convective conditions induced by a rotating magnetic field. Using different analysis techniques the shape and distribution of the intermetallic phase β-Al5SiFe in the dendritic microstructure was investigated, to study the influence of solidification velocity and fluid flow on the size and spatial arrangement of intermetallics. Deep etching as well as 3-dimensional computer tomography measurements characterized the size and the shape of β-Al5SiFe platelets: Diffusive growth results in a rather homogeneous distribution of intermetallic phases, whereas forced flow promotes an increase in the amount and the size of β-Al5SiFe platelets in the centre region of the samples. The β-Al5SiFe intermetallics can form not only simple platelets, but also be curved, branched, crossed, interacting with dendrites and porosity located. This leads to formation of large and complex groups of Fe-rich intermetallics, which reduce the melt flow between dendrites leading to lower permeability of the mushy zone and might significantly decrease feeding ability in castings.

Stelian C.,SIMAP EPM | Stelian C.,Le Rubis SA | Duffar T.,SIMAP EPM
Journal of Crystal Growth | Year: 2016

Numerical modeling is used to investigate the effect of solute concentration on the melt convection and interface shape in Bridgman growth of Cd1-xZnxTe (CZT). The numerical analysis is compared to experimental growth in cylindrical ampoules having a conical tip performed by Komar et al. (2001) [15]. In these experiments, the solidification process occurs at slow growth rate (V=2·10-7m/s) in a thermal field characterized by a vertical gradient GT=20K/cm at the growth interface. The computations performed by accounting the solutal effect show a progressive damping of the melt convection due to the depleted Zn at the growth interface. The computed shape of the crystallization front is in agreement with the experimental measurement showing a convex-concave shape for the growth through the conical part of the ampoule and a concave shape of the interface in the cylindrical region. The distribution of Zn is nearly uniform over the crystal length except for the end part of the ingots. The anomalous zinc segregation observed in some experiments is explained by introducing the hypothesis of incomplete charge mixing during the homogenization time which precedes the growth process. When the crystallization is started in ampoules having a very sharp conical tip, the heavy CdTe is accumulated at the bottom part of the melt, giving rise to anomalous segregation patterns, featuring very low zinc concentration in the ingots during the first stage of the solidification. © 2016 Elsevier B.V. All rights reserved.

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