Crystal growth in materials scienceV
Effect of natural and forced convection in materials crystallization
The symposium will focus on the role of fluid flow during the solidification or crystallization of materials and on its control from the melt. Its aim is to gather specialists of different material families working on processes where convection plays a role, or where specific material properties depend on convection during the elaboration.
The role of fluid flow during the solidification of materials is usually analyzed in terms of macrosegregation and microstructure. While the heat and solute flows are considered at the scale of the ingot, the scale of the solid-liquid interface is usually pertinent for the study of microstructure and its associated microsegregation. Natural or forced convection can play a role at all scales, for example during dendrite breakup which plays a role on the transition between columnar and equiaxed grain structures in alloys, solute mixing which modifies the macrosegregation in semiconductors or mesosegregation which can lead to channel segregates. Experimental and numerical models are needed to investigate the interplays between the physical mechanisms involved.
In metallurgy, one of the industrial issues is to be able to control the structure as well as the defects formed during the solidification step, since they induce the working properties of the solidified material. The usual microstructure is dendritic and can be either columnar (preferred growth direction), or equiaxed (solid grains without preferential direction), or mixed. The complex phenomena at the origin of equiaxed dendrites include heterogeneous nucleation, controlled by introducing refining particles during the solidification, and fragmentation or other mechanisms bringing germs from the columnar structure. In both cases, the convection plays a very significant role in the origin and transport of the nuclei and equiaxed grains. But also in a completely different materials type, protein crystals, convection affects the final crystal quality and is therefore either experimentally reduced (e.g. using microgravity) or enhanced.
In crystal growth, the fluid flow can modify the effective segregation at the interface, producing macrosegregation with adverse effect (striations in electronic crystals, increase of dopant level during growth) or used for its purification effect when the last solidified part is eliminated (solar silicon). The flow conditions range from very low velocity, when high magnetic fields are used to stop any convection and favour diffusive regimes, to turbulent forced convection that is sometimes used in purification processes to increase the mass transfer and thus the segregation near the growth front.
Hot topics to be covered by the symposium:
- Columnar to Equiaxed Transition under forced convection
- Macrosegregation under forced convection (laminar or turbulent)
- Micro-or mesosegregation with convection effects (freckles, channels, ...)
- Transport-controlled pattern formation
- Solidification and crystal growth without convection (microgravity , gels, microfludics, ...)
- Crystal growth with electromagnetic control of the convection
- Effect of convection on the grain structure of multicrystals
1130 rue de la piscine
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Phone: +33 (0)4 76 82 52 04
Fax: +33 (0)4 76 82 52 11
1130 rue de la piscine
38400 St Martin d'Heres cedex
Phone: +33 (0)4 76 82 52 07
Fax: +33 (0)4 76 82 52 49
Grenoble Institute of Technology
BP 75, PHELMA/SIMAP/EPM
38402 Saint Martin d'Heres cedex
Phone: +33 476 825205
Fax: +33 476 825249
Institute for Molecules and Materials (IMM)
Radboud University Nijmegen
Solid State Chemistry
6525 AJ Nijmegen
Phone: +31-(0)24 3653323
Fax: +31-(0)24 3653067
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Authors : Kader ZAIDAT
Affiliations : Laboratoire SiMaP/Grenoble-inp
Resume : Opening of the symposium V
Authors : S. Kumar1 , E. Liotti1, A. Lui1, R. Vincent1, T. Connolley2, K.A.Q. OReilly1, P.S. Grant1
Affiliations : 1 Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK and The EPSRC Centre for Innovative Manufacturing in Liquid Metal Engineering, 2 Diamond Light Source, Harwell Science and Innovation Campus, Didcot, OX11 0DE, United Kingdom
Resume : Iron is an inevitable element in all Al castings, either added deliberately to improve strength or entering through the recycling chain when re-melting. During subsequent solidification, dissolved Fe in the liquid reacts with other alloying elements to form complex intermetallic phases, which degrade the downstream processability (e.g. in extrusion) and final properties, particularly elongation and toughness. To overcome the detrimental effects of tramp Fe, a cost and energy consuming homogenisation step is used in an attempt to break-up the intermetallic compound (IMC) network and to spheroidize IMCs to a less damaging morphology. In order to minimise homogenisation costs and possibly to eventually overcome the requirement, it is essential to understand and control the initial IMC formation during solidification. In this study, we report the in-situ observation of the Fe intermetallic formation in Al alloys under constrained and un-constrained solidification conditions, and in the presence of natural convection. The constrained (in the presence of extended primary Al columnar dendrites) and un-constrained (in the presence of free floating primary Al equiaxed dendrites) growth solidification conditions were obtained by directionally solidifying the alloy without and with addition of grain refiner. Under constrained primary Al growth conditions upwards and opposite to gravity, the Fe IMCs nucleated and grew in lengthy branched morphologies that passed across primary Al columnar dendrites. As the cooling rate increased, the IMCs then grew progressively only between the primary Al columnar dendrites. Under un-constrained primary Al growth conditions, the Fe IMCs nucleated and grew to much shorter lengths, between the equiaxed grains at all cooling rates studied. Consequently, we show how primary solidification morphologies and growth conditions can be used to manipulate IMC morphology. Finally, the effect of various convection effects in IMC development is also discussed.
Authors : Faiza Mokhtari 1,3, Abdelkrim Merah 2,3 & Ahcene Bouabdallah 3
Affiliations : 1Faculté des Sciences, Université Mouloud Mammeri de Tizi Ouzou, Algeria 2Faculté des Sciences de lIngénieur, Université MHamed Bougara de Boumerdes, Algeria 3LTSE Laboratory, University of Science and Technology. Algiers, Algeria
Resume : The time-dependent VOF (Volume Of Fluid) formulation is used to track the shape of the free surface and the flow field inside the silicon crucible. The VOF formulation relies on the fact that two fluids (liquid silicon and argon gase) are not interpenetrating. The volume fraction is introduced as a new variable of the phase in the computational cell. In each control volume the volume fractions of all phases sum to unity. The fields for all variables and properties are shared by the phases and represent volume-averaged values, as long as the volume fraction of each of the phases is known at each location. In order to understand the influence of the crucible rotation on melt flow pattern and free surface in silicon melt, a set of numerical simulations are conducted using the finite volume method. The analysis of the obtained results led to conclude that the forced convection affects strongly the free surface shape and the melt flow.
Authors : Zhongyun FAN
Affiliations : The EPSRC Centre-LiME, BCAST, Brunel University, Uxbridge, UB8 3PH, UK
Resume : Liquid metal engineering refers to the treatment of liquid metals by either chemical or physical means to enhance heterogeneous nucleation through manipulation of the chemical and physical nature of both endogenous (naturally occurred) and exogenous (externally added) nucleating particles prior to solidification processing. A prime aim of liquid metal engineering is to produce solidified metallic materials/components with fine and uniform microstructure, uniform chemical composition and reduced/eliminated cast defects. In this paper I offer an overview on our current understanding of enhancing heterogeneous nucleation through liquid metal engineering by intensive melt shearing and our current progress on development of solidification technologies by implementation of liquid metal engineering principles.
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Authors : M. Gonik^1, A. Croel ^ 2 , A. Le Donne^3 , M.Acciarri^3 and S. Binetti^3
Affiliations : 1 Centre for Material Researches (PHOTON), Alexandrov (Russian); 2 Institute for Geosciences, Albert-Ludwigs-University of Freiburg , Albertstr. 23b D- 79104 Freiburg (Germany) ; 3 Milano-Bicocca Solar Energy Research Center (MIB-SOLAR), Dept. of Materials Science, University of Milano-Bicocca, via Cozzi 55, Milano (Italy)
Resume : In any crystallization processes convection governs the formation and distribution of impurities and foreign phases. Therefore, experimentation under microgravity can contribute to a better understanding of the processes during solidification as the chemical segregation, the interaction of particles and their effect on defects can be studied under purely diffusive transport conditions. Silicon is actually a well studied material; however, the above mention phenomena are still not investigated from the point of view of diffusion and segregation phenomena. This is why it is important to study heat and mass process in Si melts also in view of using upgraded metallurgical grade (UMG) Si in solar cells. It contains high amount of deleterious impurities and precipitates, but can substantially contribute to reduce the solar energy cost. In ground experiments, the effect of closed to diffuse regime could be studied during solidification from a thin melt layer by the Axial Heating Process (AHP). In this work, the modified FZ technique using an additional heater submerged into the melt was applied to grow silicon. The samples were then characterized by electrical (resistivity, hall mobility) and spectroscopic techniques (PL, Raman, IR). The concentration and spatial distribution of impurities and defects were determined and compared with standard silicon for solar cells. These samples will be compared with those to be grown in microgravity conditions (ESA project A0-2009-1051).
Authors : Andrey Sadovskiy, Ekaterina Sukhanova, Stanislav Belov, Igor Avetissov
Affiliations : D. Mendeleyev University of Chemical Technology of Russia, Armoled Ltd.
Resume : Crystal growth from the melt is non-equilibrium process affected by the time and spatial behavior of temperature and concentration gradients near the interface. Many methods of melt activation during crystal growth optimize and stabilize these gradients through the hydrodynamical impact on liquid . Axial vibration control (AVC) technique is one of the most energy efficient for these purposes . However besides thermal and chemical diffusion under non-isothermal conditions of a growing setup, the phenomenon of structure evolution of the melt should be taken into consideration. Melt structural changings have been declared for metals, oxides, chalcogenides, silicon, germanium, and complex semiconductors. In present work we analyzed temperature changing of melt structures for modeling materials NaNO3 and LiNO3 by XRD and Raman spectroscopy. To analyze the AVC impact on melt structure Raman spectrum have been analyzed for different temperature and for different vibrating regimes. Physical experiment has been conducted together with numerical simulation, from which flows parameters and viscous dissipation rate in boundary layer have been withdrawn. Single crystals grown by AVC-CZ technique for the melt structure control demonstrated high perfection comparing to the conventional CZ-grown crystals. 1. Semiconductors and Dielectrics Eds. P. Capper, P. Rudolph, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2010 2. Avetissov, I. Ch. et al. Doklady physics, 54 (2009) 410-412
Authors : V.I. Shymanski1, N.N. Cherenda1, V.V. Uglov1, V.M. Astashynski2
Affiliations : 1Belarusian State University, Minsk, Belarus; 2Heat and mass transfer Institute of National Academy of Science of Belarus, Minsk, Belarus
Resume : Compression plasma flows generated by a magnetoplasma compressor are a unique technique for surface materials modification. Specific configuration of electric and magnetic fields allows to reach the period of stable existence of a directed plasma flow of up to 100 microseconds. The influence of such plasma flows with the absorbed by the surface energy density up to 40 J/cm2 on the coating/substrate system results in surface layer melting and penetration of the coating element into the metal substrate. The atoms of the coating (Mo, Cr, Zr, Ni, Ti) are distributed homogeneously along the depth of 10 30 micrometers in the metal (Ti, Fe, Al) matrix. The main mechanism of mass transfer providing deeper penetration than the diffusion depth can be attributed to convective fluxes that appear in the liquid phase because of the Kelvin-Helmholtz hydrodynamic instability development. The plasma flow interacting with the melt surface results in the liquid motion with both tangential and normal components of the velocity, which obeys to the Navier-Stokes equation. Due to viscous friction between the layers in the melt the motion is spread to deeper layers causing mixing of coating and substrate elements in the liquid state. The model describing the experimental data is proposed.
Authors : Lakhdar HACHANI1,2 , Kader ZAIDAT1, Yves FAUTRELLE1
Affiliations : 1 CNRS/SIMAP/EPM, ENSHMG BP75 38402 St Martin dHères Cedex, France 2Laboratoire de mécanique, université de Laghouat, Algérie
Resume : The introduction of magnetic field in the solidification process is one of the effective methods to improve the microstructure and mechanical performance of alloys (by controlling the defects as freckles or segregated channels). At SIMaP-EPM laboratory, we have proposed to control the fluid flow with a travelling magnetic field (TMF). With this kind of electromagnetic field the control of the intensity and the direction of Lorentz force are easy (by changing the order of electric phases). Since ten years, EPM team developed some experiments and models around the TMF. The present work deals with a experimental and numerical studies of solidification process under forced convection induced by a travelling magnetic field (TMF). The impact of TMF and gravity on segregations of an Tin-Lead alloy has been examined.
Authors : A.Kharicha, M.Stefan-Kharicha, M.Wu, A. Ludwig
Affiliations : Lehrstuhl für Simulation und Modellierung metallurgischer Prozesse Department of Metallurgy University of Leoben
Resume : In this paper we present experimental results showing the influence of the thermal Raleigh number on the CET transition during the solidification of a transparent alloy. In a cast cell of 10*3*40cm3, different solidification experiments of a 29,52 wt% hypereutectic alloy NH4Cl-H2O were performed. Wall?s temperature was measured during the solidification experiment. The starting temperature was 42?C and the ending temperature 5?C. The cooling rate was 1.2K/min. The liquid solution was filled in the cell at different heights: 8 cm, 12.5 cm, 20cm and 35cm. The influence of the Rayleigh number on the thermosolutal convection was studied in the cast cell. A PIV system (laser light and CCD camera) was used to record pictures during the experiments. The thermal Rayleight numbers varies from 107 for the 8 cm height to 109 for 35 cm height. The experiments were very different in terms of solidification type (Fig.1). Almost purely columnar and no apparent equiaxed crystals were observed for the height of 8 cm (Ra=107). For the 12.5 cm height (Ra=107) large number of equiaxed crystals settling was observed. The mush at the end of the experiment showed a large area of equiaxed in the centre of the cell (Figure 1 b). For the experiment with the 35 cm filling height (Ra~109 ) the solidification took place almost in a purely equiaxed and the mush had a equiaxed structure. The influence of the Rayleigh number on the CET will be discussed in term of undercooling temperatures and hydrodynamics.
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