PhD_Thesis_Radu_Andrei_Negrila_EMF_stirring_final

1. The numerical and experimental study of a melt flow under the influence of a special type of electromagnetic field in a model experiment Ph.D. Student Andrei- Radu Negril ă Scientific coordinator Prof. Dr. Daniel Vizman West University of Timişoara Faculty of Physics

2. Many things are linked to a vortex We are here ...

3. Outline Motivation State of the Art in Directional Solidification of Si DS for PV Model Experiment for the study of EMF stirring Numerical results and their experimental validation Flow structure analysis Flow structure engineering Conclusions & perspectives

4. Energy coming from the Sun The solar energy on the earth’s surface is 1000 times it annual energy usage (16 Tw -yr=504.6 Exajoules ) ! Perez et al., 2009, International Energy Agency Update, “A Fundamental Look At Energy Reserves For The Planet”

5. So, why not plug in directly to the Sun ? Human burning of fossil fuels is “the dumbest experiment in history” – Elon Musk, billionaire entrepreneur and physicist

6. However, it all depends if PV is commercially viable …. Levelized Cost of Electricity (LCOE) – here , in EUR per kWh

7. The cost of PV is going down with mass production and innovations We are HERE Very good (perspective) evolution of Levelized Cost of Electricity for PV compared to fossil fuels by 2020 Due to innovations reducing solar cell costs and increasing their efficiency ( our technique is aspiring to be one of them )

8. Bloomberg Financial prediction It is no longer government policy sustaining the advancement of PV, but by the economics of the process, as costs are driven down by innovation and mass production

9. Outline Motivation State of the Art in Directional Solidification of Si DS for PV Model Experiment for the study of EMF stirring Numerical results and their experimental validation Flow structure analysis Flow structure engineering Conclusions & perspectives

10. The dominant technology (> 90% of market) Crystalline Silicon for solar cell manufacture current conversion efficiency for commercial cells 17-21 %; @23% for back contact cells (ITRPV 2015)

11. Market Leader (60% out of 90%): Multi crystalline Silicon Directional Solidification (DS) The DS silicon Ingot is cut into Brics Courtesy of INES-CEA France, LMPS Laboratory Which are sliced into wafer s for Solar Cell fabrication The method has the best cost / efficiency ratio in USD/W !

12. Costs can be reduced by increase of yield (useful % of ingot volume) and of furnace throughput Predicted increase of mc-Si ingot mass will lead to a higher throughput per furnace

13. There are thee main sources of impurities (C,N,O and metals) in the ingots : Impurity content in the feedstock Crucible and crucible coating dissolution Furnace environment (heating elements). When solubility limit is reached precipitates can form SiC and Si 3 N 4 precipitates Catastrophical for electromechanical proprieties Metal oxides precipitates Impede photovoltaic (or dissolute metals) conversion More Impurities in Silicon DS with larger crucibles WITH INCREASING CRUCIBLE SIZE, more impurities can diffuse from the crucible, coating or furnace atmosphere, because of higher melt surface areas and solidification time.

14. Buoyant convection limitations in Si DS V. Pupazan , R. Negrila , O. Bunoiu , I. Nicoara , D. Vizman , Journal of Crystal Growth 401 (2014), 720-726. Impurities and precipitates accumulate in poorly mixed areas, between buoyant convection loops Convective flows may become turbulent for high levels of S-L interface deflection

15. two main convection zones: one due to the S-L interface deflection and one near the melt free surface poorly mixed areas can be observed in between the convection loops, where precipitates may form and be engulfed in the ingot xOz section Buoyant convection is even less effective for larger ingots A. Popescu , D. Vizman ,International Journal of Heat and Mass Transfer 54 (2011), 5540-5544

16. Race towards cost reduction : With the need to maintain or increase Si ingot quality ! Two major trends for cost reduction in DS solidification of mc-Si Increasing crucible size Avoiding detrimental curvature of S-L interface Increase in impurity concentration from crucible, coating or furnace atmosphere, because of higher melt surface areas or crystallization time. Cheaper feedstock of a lesser purity Avoiding precipitate formation or morphological destabilization of growth interface Achievement of a total mixing regime for optimal distribution of impurities through axial segregation CHALLENGES with INCREASING CRUCIBLE SIZE and using a LOWER PURITY FEEDSTOCK can be met by TAYLORING THE MELT CONVECTION !

17. Travelling magnetic fields - Ch. Kudla et.al. , Crystallization of 640 kg mc-silicon ingots under traveling magnetic field by using a heater-magnet module, J. Crystal Growth, 365 (2013), 54-58 - K.Dadzis et.al ., U nsteady coupled 3D calculations of melt flow, interface shape, and species transport for directional solidification of silicon in a traveling magnetic, J. Crystal Growth, 367 (2013), 77-87 Other Magnetic fields (Alternating, Rotating, Carousel) - N.Dropka et. al. Comparison of stirring efficiency of various non-steady magnetic fields during unidirectional solidification of large silicon melts, J. Crystal Growth, 365 (2013), 54-58 Mechanical stirring - S. Dumitrica et.al., Numerical studies on a type of mechanical stirring in directional solidification method of multicrystalline silicon for photovoltaic applications, J. of Crystal Growth, 360 (2012) 76-80 - B. Ubbenjans et al. ,, Numerical analysis of the influence of ultrasonic vibration on crystallization processes Crystal Research and Technology 47 (2012) 279-284. Electro magnetic (EMF) stirring - C. Tanasie , D. Vizman , J. Friedrich, Journal of Crystal Growth 318 (2011) 293–297. - D. Vizman , C. Tanasie , Journal of Crystal Growth 372 (2013) 1–8 Convection tailoring in the DS of mc-silicon

18. Lorentz forces TMF - inductors 22x22x22 cm Skin depth: For silicon at 50Hz δ =5cm Limitations of Traveling magnetic field in tailoring DS convection Same limitation for all types of time varying magnetic fields

19. Idea for Electromagnetic field (EMF) stirring Combined application of a vertical magnetic field and electrical current for melt stirring Radial electric current from Electrodes at free melt surface Vertical magnetic field Forced convection(stirring) via the Lorentz Force

20. From EMF Simulation to Model Experiment Numerical simulations in D. Vizman , C. Tanasie , Journal of Crystal Growth 372 (2013) 1–8 of EMF stirring performed using STHAMAS3D software Two study directions for this Thesis Numerical Simulations of Si EMF need to be validated by a Model Experiment Study of additional control parameters’ (I,B, electrode position) influence on Flow Structure (STHAMAS3D)

21. Outline Motivation State of the Art in Directional Solidification of Si DS for PV Model Experiment for the study of EMF stirring Numerical results and their experimental validation Flow structure analysis Flow structure engineering Conclusions & perspectives

22. Velocity measurements: Ultrasound Doppler Velocimeter US-transducers Model Experiment for EMF stirring. UDV Technique US-transducers Electrodes

23. Homogenous Vertical Magnetic field in EM Gap Middle Magnetic field is quasi-homogenous (>90% of maximum value) in crucible region crucible position

24. The “perfect” model melt An easy to handle, non-toxic, room temperature liquid melt with similar proprieties with molten Silicon is needed in order to perform a relevant model exeperiment for Si EMF validation Physical property GaInSn Silcon Unit Density, ρ 6360 2520 Kg/m 3 Dynamic viscosity, μ 2.16 ∙10 -3 7.56∙10 -4 N·s/m 2 Kinematic viscosity (η= μ/ρ) 0.34∙10 -6 0.3∙10 -6 m 2 /s Electrical conductivity, σ 3.2∙10 6 1.29∙10 6 S/m

25. Optimized configuration Uncertainties: - Material properties of Plexiglas compared to glass (sound speed, acoustic impedance) Poor wetting of Plexiglas by GaInSn due to lower density Electrode shape and current distribution Self magnetic field Relative positions error for old system Non magnetic stainless steel hemispherically tipped electrodes : lower self-magnetic field

26. Pulsed ultrasonic echo technique to measure velocity profiles of flowing liquids: Gives velocity projection along the ultrasonic beam direction (depth) from a specified number of volumes Ultrasonic Doppler Velocimetry. DOP3010 Velocimeter V y (mm/s ) Y (um)

27. A t ransducer contains an emitter that sends periodically a short ultrasonic bursts and a receiver that collects continuously echoes from targets present in the path of the ultrasonic beam. Calculation of the autocorrelation function of the signal coming from the same particles in a measurement volume, reflected from different pulses separated in time by T prf , in order to derive the Doppler shift in frequency ! Ultrasonic Doppler Velocimetry. Method principle average Doppler frequency is the Fourier Transform of the autocorrelation function of the echo signal auto-correlation function is estimated using the complex envelope of the echo signal .

28. UDV parameters US frequency 4.130 Mhz Doppler angle 0° Pulse repetition frequency 310.56 Hz Speed of sound in GaInSn 2730 m/s Measured depth 70 mm Bursts per profile 64 Velocity resolution 0.2 mm/s Time resolution (single profile) 750 ms Number of gates (measurement volumes) 300 Number of profiles 900/1600 Spatial resolution (distance between two measurement volumes) 0.228 mm Axial spatial resolution of measurement volume 2.184 mm Minimum/Maximum US beam diameter 5 mm - 9.5 mm Divergence /spread of the US beam 3.89° Half angle at (-6 dB) / 7.79° spread

29. Outline Motivation State of the Art in Directional Solidification of Si DS for PV Model Experiment for the study of EMF stirring Numerical results and their experimental validation Flow structure analysis Flow structure engineering Conclusions & perspectives

30. Isothermal Melt : GaInSn (melting point 11C ) L =7 cm; h=5 cm; B=10mT, 3mT I=2A,5A,10A US sensors Symmetric configuration Asymmetric configuration U B h L I Numerical Model employed Combined application of current I and magnetic field B causes melt rotation ;

31. Direct solution of Navier -Stokes equations using Finite Volume method Consideration of Lorentz force by solving additional scalar potential equation Higher efficiency for parallel version on a PC cluster Simulations with STHAMAS3D software

32. F b + F s + F L + F V Consideration of Lorentz force by solving additional scalar potential equation (current continuity) at each time step (MHD2 approximation) Navier -Stokes Equations j = σ (- Ñ  + u x B ) Ñ j = => Current injected at electrode positioning Conservation of Momentum : Conservation of mass: External forces:

33. Vortical melt rotation develops in the whole mass of the melt Flow intensity increases with the increase of the electrical current I =5A Bottom view I =2A I =10A General view Melt flow in an Asymmetric electrode configuration B=10mT:

34. Flow structure changes to poloidal with by electrode position Even a magnetic field of 3mT can produce a significant stirring effect Bottom view AC; B=10mT; I =10A General view SC; B=10mT; I =10A SC; B=3mT; I =10A Melt flow in a Symmetric electrode configuration R. A. Negrila , A. Popescu , M. Paulescu , D. Vizman , Procedings of the 9th PAMIR International Conference, Riga, 2014, Volume 2, 205-208

35. Comparison of numerical and experimental results A good agreement between numerical and experimental profiles Better agreement in the AC case due to simpler flow structure than in SC one R. A. Negrila , A. Popescu , M. Paulescu , D. Vizman , Procedings of the 9th PAMIR International Conference, Riga, 2014, Volume 2, 205-208

36. Outline Motivation State of the Art in Directional Solidification of Si DS for PV Model Experiment for the study of EMF stirring Numerical results and their experimental validation Flow structure analysis Flow structure engineering Conclusions & perspectives

37. Phenomenological explanation of the flow structure with the help of a Model Problem There are two Lorentz forces: an acceleration one, F θ , and a damping one F d F d has a component opposing u θ and a component opposing u p The flow around an electrode can be described by an azimuthal u θ and a poloidal u p component F d _ θ F d_p B z

38. AC case – Vortex Formation an azimuthal rotation and a converging meridional flow form a “Vortex” structure The flow is pointed towards the electrode

39. Radial pressure gradients description of poloidal flow generation The forcing around one electrode is similar to that in these studies Davidson 87 Davidson 92 A radial pressure gradient oppoeses the centrifugal force As long as the angular momentum is an increasing functions from the axis of rotation, the flow is centrifugally stable Centrifugal pumping: axial variation of radial pressure gradients in the core flow generate a poloidal flow Ekman pumping: axial variation of radial pressure gradients (in the boundary layer) generate a poloidal flow P. A. Davidson, J. C. R. Hunt - Journal of Fluid Mechanics 185 (1987), 67-106 P. A. Davidson Journal of Fluid Mechanics 245 (1992), 669-699

40. SC cases considered here present another flow structure The vortex is resupplied by a converging meridional recirculation (as for AC) but a secondary “Poloidal” structure is observed

41. 41 Melt: GaInSn (melting point 10C) L =7 cm; h=5 cm (aspect ratio close to G1 silicon ingot) B=5mT, 10mT , 20mT, 30mT I=0.1A, 0.5 A, 1A, 2A, 5A,10A, 15A, 20A, 25A, 30A, 35A, 40A, 45A Simulations with STHAMAS3D software on symmetrical configuration of electrodes Change of flow structure with electromagnetic parameters – SC parametrical study Parametrical Study for array of currents I and magnetic fields B to understand, quantify and predict flow structure

42. 42 The flow structures: “Vortex” and “Poloidal” dominated Flow structure changed from “Vortex” to “Poloidal” flow domination with the increase of the intensity of electrical current for any magnetic field Particle tracking of flow structure BLUE FROM TOP OF VORTEX ; RED FROM BOTTOM OF VORTEX R. A. Negrila , A. Popescu , D. Vizman , European Journal of Mechanics B/Fluids 52 (2015), 147-159 .

43. The flow structure transition Particle tracking of flow structure for different electrical current I and magnetic field B pairs R. A. Negrila , A. Popescu , D. Vizman , European Journal of Mechanics B/Fluids 52 (2015), 147-159 .

44. Dependence of possible flow structures on an integral energy constraint Flow scaling case Secondary poloidal flow Energy Dissipation around streamlines lengthscale (a) Generated by centrifugal pumping (usually in a deep crucible) Torrodial structure closing inside the core of the flow Long enough so that energy diffuses to the wall where destroyed by shear forces (b) Generated by Ekman pumping (usually in a short crucible) Poloidal structure passing through the boundary layer ( suppressing centrifugal pumping ) Streamline lengthscale short as Ekman pumping flushed them through the wall boundary layer where energy is destroyed by shear forces (c) May be generated by pumping both mechanisms . At wall but also by magnetic damping inside the melt volume

45. Streamline structure Streamlines passing close to the electrode and the bottom are flushed by Ekman pumping through boundary layer for the “Vortex” case

46. Streamline structure For the “Vortex” case streamlines in the bulk flow are centrifugally pumped For the “Poloidal” case streamlines adopt a longer lenghtscale to dissipate the flow

47. Confirmation of “Vortex” and “Poloidal” flow structure for SC case The plane at 6 mm close to the bottom of crucible – area of interest for DS GaInSn poor wetting material - limits of “no-slip” condition imposed in simulation for Ekman flushed streamlines R. A. Negrila , A. Popescu , D. Vizman , European Journal of Mechanics B/Fluids 52 (2015), 147-159 .

48. Outline Motivation State of the Art in Directional Solidification of Si DS for PV Model Experiment for the study of EMF stirring Numerical results and their experimental validation Flow structure analysis Flow structure engineering Conclusions & perspectives

49. Quantification of the flow structure transition. The parameter - L Struct The competition between the vortex driven flow and the poloidal one characterised by L Struct - the distance along the diagonal where the two opposing flows balance at the bottom, marked by a dark arrow I = 0.1A, (vortex flow) I = 1A, (vortex flow) I = 5 A, (transition) I = 25A, (poloidal flow) I = 35 A, (poloidal flow) I = 45A, (poloidal flow) B=20mT

50. 50 The flow structure transition with electromagnetic parameters L struct as a characteristic length defining the “Vortex” to “Poloidal” structure transition for the different electrical currents I and magnetic fields B

51. 51 Scaling analysis of the flow governing equations F 1_Re F 2_Re Flow structure and Re = functions of (Fem#1, Fem#2, boundary conditions, electrode positioning)

52. Reynolds number Re number depends on F em#1 , as the accelerating Lorentz force is balanced in the core by the inertial force R. A. Negrila , A. Popescu , D. Vizman , European Journal of Mechanics B/Fluids 52 (2015), 147-159 .

53. Flow structure depends on dimensionless numbers ratios R. A. Negrila , A. Popescu , D. Vizman , European Journal of Mechanics B/Fluids 52 (2015), 147-159 . Vortex” structure depends on ratio of the second Lorentz number “Poloidal” structure tends to depend more on the ratio of first Lorentz number in the “Transition” region both ratios have an important influence on the flow solution

54. Structure “phase” diagram A flow structure “diagram” can be imagined for sets of Lorentz ratios values I increase R. A. Negrila , A. Popescu , D. Vizman , European Journal of Mechanics B/Fluids 52 (2015), 147-159 .

55. MHD damping of poloidal flow for same F em#1 = 9.52E+05 ~ IB product, damping parameter F em#2 ~ uB 2 differs: 2.99E+05 4.78E+06

56. MHD damping of poloidal flow 0.5 A, 20 mT 2 A, 5 mT The flow in the forced region is more azimuthal for the “Vortex” case

57. MHD damping of poloidal flow the MHD interaction parameter F 2_Re =σB 2 /ρ∙L /U = (1/τ)∙(L /U ) ) = t p / τ d Z=45 mm Z=5 mm 2 A, 5 mT; F 2_Re = 0.104 0.5 A, 20 mT; F 2_Re = 1.79

58. Isothermal Silicon melt EMF flow structure conservation with dimensionless numbers “ Vortex” case of I=0.1 A B=30 mT for GaInSn EMF vs. I=0.03 A B=0.0279 mT for Si EMF “ Poloidal” case of I=45 A B=5 mT for GaInSn EMF vs. I=14.91 A B=4.656 mT for Si EMF

59. 59 Melt: GaInSn (melting point 10C) I=45A ; B=30mT same aspect ratio as for L =7 cm; h=5 cm : L =10 cm, h=7 cm; L=15 cm, h=11cm; ; L=25 cm, h=18cm; ; L=35 cm, h=25 cm. Simulations with STHAMAS3D software on symmetrical configuration of electrodes Change of flow structure with up-scaling the crucible size - SC parametrical study for varying (L) at I, B = const Parametrical Study for variation of crucible size to understand, quantify and predict flow structure

60. The flow structure transition Particle tracking of flow structure ( BLUE FROM TOP OF VORTEX ; RED FROM BOTTOM OF VORTEX) for I=45A ; B=30mT and different characteristic lengths with same aspect ratio   R. A. Negrila , A. Popescu , B. Barvinschi , M. Paulescu and D. Vizman , GaInSn melt flow structure variation with crucible size in an isothermal electromagnetic stirring configuration, to be published in AIP Conference Proceedings

61. 61 The flow structure transition quantification Lstruct as a parameter defining the “Vortex” to “Poloidal” structure transition for for I=45A ; B=30mT for the different characteristic lengths with same aspect ratio that have been numerically simulated

62. Reynolds number variation with up-scaling

63. Flow structure depends on MHD interaction parameter F2_Re “Vortex” to “Poloidal” structure transition depends on ratio of the second Lorentz number; this can be generalized for any I and B pair

64. Electrode positioning influence on Poloidal structure formation “ Poloidal” flow structures structure seems to be favored if the intersection of the two vortices around the electrodes is close to the diagonal between them

65. 65 Outline Motivation State of the Art in Directional Solidification of Si DS for PV Model Experiment for the study of EMF stirring Numerical results. Flow structure analysis Experimental results for validation of the numerical model Conclusions & perspectives

66. A dreamer may ask: "Sometimes do you think that crystals grow better if you grow'em with Jazz ?" A laughing man answers: "It depends on the Jazz atmosphere pressure“ with a contribution from Zbignev Kowalski (the laughing man :) ) The link between the advancement of PV and the work of this thesis THE ELECTROMAGNETIC CONTROL OF A MELT FLOW CAN CREATE A “JAZZY” ATMOSPHERE FOR growing a better PV Si ingot, being an innovation that could lead to a reduction in solar cell costs and an increase in their efficiency

67. Conclusions The combination between an vertical magnetic field and an electrical current is an interesting option in the control of melt convection in the DS growth of silicon ingots Good qualitative agreement between experimental and numerical results The results prove the potential of numerical simulation to support the process development. Numerical experiments are much less more expensive than real ones. It was shown that even small values of the magnetic field and electrical current can improve the level of mixing in the melt The flow structure can be changed through electrode AC or SC positioning, electrical current or magnetic field variation

68. Conclusions The resulting flow structure was analyzed and described in terms of a "Vortex“, respectively a “Poloidal” flow domination on the flow structure Quantification of flow structures through a dedicated parameter A scaling analysis of the flow governing equation, was done indentifying a direct correlation between ratios containing dimensionless forcing parameters and the Reynolds number or the flow structure parameter By increasing the crucible size and conserving the electromagnetic parameters I and B, the transition a “Poloidal” to a “Vortex” flow structure is observed The apparition of a “Poloidal” flow structure seems to be related to the distance between the intersection of the vortices surrounding each electrode and the diagonal lying between them. The same flow structures will be obtained for a Si melt as for GaInSn by conserving the Reynolds and forcing parameters

69. Perspectives The forcing parameters must be adapted to be able to predict the influence of changing the aspect ratio of the crucible on the flow structure A full description of the application of EMF to DS flow must take into account the Grashof number for buoyant convection as an independent variable A correlation must be made between the flow structure and impurity concentration and growth interface deflection through the employment of some additional numerical simulations and experimental characterization work, for Si DS

70. Conferences 4th European Conference on Crystal Growth, 7-12 Iunie 2012, Glasgow, Marea Britanie: D. Vizman , C.Tanasie , R. Negrila - Novel method for melt flow control in unidirectional solidification of multi-crystalline silicon (Poster) The 7th International Conference On Advanced Materials (ROCAM), 28-31 August 2012: R. A. Negrila , D. Vizman - Numerical and Experimental Studies on Melt Flow in a Model Experiment for Electromagnetic Stirring, Brașov , România (Poster) 6th International Workshop on Crystalline Silicon Solar Cells (CSSC6), 8-11 Octombrie 2012, Aix-les- Bains , Franța : D. Vizman , C. Tanasie , R. Negrila - A new type of electromagnetic stirring in directional solidification of mc-Si (Poster) 7th International Workshop On Modeling In Crystal Growth, 28-31 Octombrie 2012, Taipei, Taiwan: D. Vizman , C. Tanasie , R. Negrila - Numerical studies on a type of electromagnetical stirring in directional solidification method of multicrystalline silicon ,, (Oral) Physics Conference TIM-12, 27-30 Noiembrie 2012, Timișoara , România : R. A. Negrilă , A. Popescu , M. Paulescu , D. Vizman - Melt stirring based on a combination of electrical current and magnetic field (Oral); V. Pupăzan , R. A. Negrila , M. Bunoiu , M. Stef , I. Nicoara , D. Vizman - Impurity distribution study in multi-crystalline silicon grown by Bridgman method (Oral) A Treia Sesiune Națională De Comunicări Științifice A Doctoranzilor , 10-12 Iunie 2013, Timișoara , România: R .-A. Negrila - Experiment model al unei noi configurații de stirring electromagnetic într -o topitură de GaInSn (Oral) The 19th American Conference on Crystal Growth and Epitaxy , 21-26 Iulie 2013, Keystone, Colorado, Statele Unite ale Americii : D. Vizman , R. Negrila - Numerical and experimental studies on a special type of electromagnetic stirring, (Oral); V. Pupazan , R. Negrila , O. M. Bunoiu , I. Nicoara , D. Vizman - Growth and characterization of multi-crystalline silicon obtained by Bridgman technique (Poster), 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, 11-16 August 2013, Varșovia , Polonia : R.-A. Negrila , M. Paulescu, A. Popescu, D. Vizman - Model experiment on a special type of electromagnetical stirring in a GaInSn melt (Oral); V. Pupazan , R.-A. Negrila , M. Stef, M. O. Bunoiu, I. Nicoara, D. Vizman - Effects of crucible coating on the quality of multi-crystalline silicon grown by a Bridgman technique (Poster); R.-A. Negrila , V. Pupazan, M. O. Bunoiu, D. Vizman - Study of resistivity and lifetime profiles in highly polluted multi-crystalline silicon grown in a graphite crucible by a Bridgman technique (Poster); 7th International Workshop on Crystalline Silicon Solar Cells, 22-25 Octombrie 2013, Fukuoka, Japonia R. Negrila , S. Dumitrica , A. Popescu , D. Vizman Model experiments on some melt stirring techniques in a rectangular crucible (Oral):

71. Conferences Al XV-lea Simpozion „ Tinerii și cercetarea multidisciplinară ”, 14-15 Noiembrie 2013, Timișoara , România : M. O. Bunoiu , R. A. Negrila , D. Vizman - Materiale fotovoltaice și dezvoltarea de procese industriale pentru promovarea energiei solare prin scăderea costurilor de producție (Oral) TIM-13 Physics Conference, 21-24 Noiembrie 2013, Timișoara , România : R. A. Negrilă , A. Popescu , M. Paulescu , D. Vizman , B. Barvinschi - Comparaison between numerical and experimental results in the case of a special type of electromagnetical stirring for conducting melts (Oral); R. A. Negrilă , V. Pupăzan , O. M. Bunoiu , D. Vizman - Influence of process time on electrical proprieties of highly polluted multi-crystalline silicon grown by bridgman technique (Poster) E-MRS 2014 Spring Meeting, 26-30 Mai 2014, Lille, Franţa : R. Negrila , A. Popescu , D. Vizman – Novel electromagnetic stirring technique in a direct-solidification configuration (Oral) 9th PAMIR International Conference on "Fundamental and Applied MHD, Thermo acoustic and Space technologies", Riga, 16-20 Iunie , 2014: R. A. Negrila , A. Popescu , M. Paulescu , D. Vizman - Control of convective flows in a rectangular crucible by a special type of electromagnetical stirring (Poster), Physics Conference TIM14, 20 - 22 Noiembrie 2014, Timişoara , România : R. A. Negrila , A. Popescu , B. Barvinschi , M. Paulescu and D. Vizman - GaInSn melt flow structure variation with crucible size in an isothermal electromagnetic stirring configuration (Oral) CSSC-8 / Crystalline Silicon for Solar Cells / May 5-8 2015, Bamberg, Germany: R. A. Negrilă , V. Pupăzan , A. Popescu , D. Vizman - Influence of process time on electrical proprieties of highly polluted multi-crystalline silicon grown by the Bridgman technique (Poster); R. A. Negrilă , A. Popescu , D. Vizman - Melt flow structure variation with crucible size in an isothermal electromagnetic stirring configuration for Silicon Directional Solidification (Poster) 8th International Conference on Advanced Materials, ROCAM 2015, 7-10 July 2015, Bucharest, Romania: R. A. Negrilă , V. Pupăzan , A. Popescu , D. Vizman - The influence of varying growth parameters on the quality of multi-crystalline silicon grown by a Bridgman technique (Oral), R. A. Negrilă , A. Popescu , D. Vizman - Up-scaling of an electromagnetic stirring configuration for silicon directional solidification (Oral) 5th European Conference on Crystal Growth, 9-11 September 2015, Bologna, Italy: R. A. Negrilă , A. Popescu , D. Vizman - Numerical modeling and experimental validation of electromagnetical stirring in unidirectional solidification of multicrystalline silicon (Oral); R. A. Negrilă , V. Pupăzan , A. Popescu , D. Vizman - Influence of growth parameters on electrical proprieties of highly polluted mc-Si grown by the Bridgman technique with various crucible-coating combinations (Poster)

72. Published articles V. Pupazan , R. Negrila , O. Bunoiu , I. Nicoara , D. Vizman - Effects of crucible coating on the quality of multicrystalline silicon grown by a Bridgman technique, Journal of Crystal Growth 401 (2014), 720-726 . ( IF: 1.698, AIS: 0.401 ) R. A. Negrila , A. Popescu , D. Vizman - Numerical and experimental modeling of melt flow in a directional solidification configuration under the combined influence of electrical current and magnetic field, European Journal of Mechanics B/Fluids 52 (2015), 147-159 . ( IF: 1.656, AIS: 0.701 ) R. A. Negrila , A. Popescu , B. Barvinschi , M. Paulescu and D. Vizman , GaInSn melt flow structure variation with crucible size in an isothermal electromagnetic stirring configuration, în curs de publicare în AIP Conference Proceedings R. A. Negrila , A. Popescu , M. Paulescu , D. Vizman - Control of convective flows in a rectangular crucible by a special type of electromagnetical stirring , Procedings of the 9th PAMIR International Conference on "Fundamental and Applied MHD, Thermo acoustic and Space technologies", Riga, 2014, Volume 2, 205-208

73. Acknowledgments All the mc-Si team at Physics Faculty, West University of Timisoara for continuous help and crucial discussions I would like to thank Prof. Dr. Daniel Vizman , who set me up with a very challenging PhD position and for his practical advice in professional and personal matters I would like to thank all the members of the PhD advisory and defense commission for support and for accepting to review my thesis. For the financial support : the work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project : “Control of melt flow in a directional solidification configuration using an electromagnetic field” number PN-II-ID-PCE-2011-3-0789 This work was supported by the strategic grant POSDRU/159/1.5/S/137750, ”Project Doctoral and Postdoctoral programs support for increased competitiveness in Exact Sciences research” cofinanced by the European Social Found within the Sectorial Operational Program Human Resources Development 2007 – 2013 K.Dadzis andV.Bojarevics for the fruitful discussions concerning scaling and EVF flow analysis

74. Published articles THANK YOU FOR YOUR ATTENTION ! V. Pupazan , R. Negrila , O. Bunoiu , I. Nicoara , D. Vizman - Effects of crucible coating on the quality of multicrystalline silicon grown by a Bridgman technique, Journal of Crystal Growth 401 (2014), 720-726 . ( IF: 1.698, AIS: 0.401 ) R. A. Negrila , A. Popescu , D. Vizman - Numerical and experimental modeling of melt flow in a directional solidification configuration under the combined influence of electrical current and magnetic field, European Journal of Mechanics B/Fluids 52 (2015), 147-159 . ( IF: 1.656, AIS: 0.701 ) R. A. Negrila , A. Popescu , B. Barvinschi , M. Paulescu and D. Vizman , GaInSn melt flow structure variation with crucible size in an isothermal electromagnetic stirring configuration, în curs de publicare în AIP Conference Proceedings R. A. Negrila , A. Popescu , M. Paulescu , D. Vizman - Control of convective flows in a rectangular crucible by a special type of electromagnetical stirring , Procedings of the 9th PAMIR International Conference on "Fundamental and Applied MHD, Thermo acoustic and Space technologies", Riga, 2014, Volume 2, 205-208

75. Role of damping force “Vortex”: and Transition: and “Poloidal”: and

76. MHD damping of vortical flow -the convergent poloidal part 0.5 A, 20 mT 2 A, 5 mT 0.1 A, 20 mT

77. Vorticity Azimuthal and Poloidal Governing Equations Governing equation for Poloidal vorticity (azimuthal velocity) Governing equation for Azimuthal vorticity (poloidal velocity) Axial gradients in azimuthal velocity generate a poloidal flow ! Inertial forces balance the accelerating Lorentz force in the core flow P. A. Davidson, J. C. R. Hunt - Journal of Fluid Mechanics 185 (1987), 67-106

78. Centrifugal pumping for vortex case for streamlines in the middle

79. Keeping impurity concentrations below solubility limit through melt mixing Diffusion boundary layer thickness reduced by forced mixing prevents precipitation formation and morphological destabilization of growth interface. Controlling the convection structure in DS is an important issue for Si crystal quality improvement

80. Purification through segregation – the UMG route cheaper than Electronic Grade silicon Controlling the convection structure in DS is important for a better purification through axial segregation and Si crystal quality improvement