Study of the growth of Al-based alloys and of silicon for photovoltaic applications using X-ray synchrotron radiography and topography

Transcription

1 Etude de la croissance d alliages à base aluminium et du silicium pour les applications photovoltaïques par radiographie et topographie X synchrotron Study of the growth of Al-based alloys and of silicon for photovoltaic applications using X-ray synchrotron radiography and topography Nathalie Mangelinck-Noël, Guillaume Reinhart, Henri Nguyen-Thi, Amina Tandjaoui, Bernard Billia, Tamzin Lafford, Jurgen Härtwig, José Baruchel

2 Columnar to Equiaxed Transition (CET) in casting processes V6 Engine block (Cast aluminium) [Hydro Aluminium ] Equiaxed grains Directional solidification (columnar grains) Turbine blade Al-7.0wt%Si

3 Multi-crystalline Silicon Photowatt International S.A. 10 cm Small size grains: Wafer cutting difficulties during cell fabrication step Important decrease of the PV efficiency (Wafer rejection)

4 In collaboration with ESRF: Device for X-ray imaging (radiography + topography) during solidification (<900 C) Bridgman type furnace: Two heating zones (resistive) Temperature gradient or Isothermal Pulling system G L S X-Rays Sample V p Diaphragm Graphite Foils Diaphragm Sample Size : e = µm L = 40 mm l = 6 mm

5 Device for X-ray imaging (radiography + topography) during solidification (1800 C) Sample holder Rigid Boron Nitride crucibles with molybdenum clips Different grades of Si: Pure (6N), MG, UMG, SoG Dimensions: 40 mm x 6mm x 0.3 mm

6 Aluminum alloy solidification Cu (wt%) X = 0.58 mm X = 2.35 mm X = 4.7 mm Z (mm)

7 Dynamical evolution of a columnar front Al 3.5 wt% Ni, V P = 1 µm/s 1) Interface destabilisation 1 mm 304 s X-ray Radiography 2) Development of perturbations ( = 140 µm) 1453 s 4320 s 3) - Formation of dendrites - Localization ( eutectic front ) 2605 s 4) Coupled growth of a single columnar dendrite / eutectic Eutectic Front

8 Fluid flow: Dendrite / Eutectic competition Clustering Liquid X-ray Radiography Dendrites Fluid flow Ni - rich Eutectic front Al-3.5 wt% Ni V P = 1 µm/s G = 30 K/cm t = 4320 s after birth of morphological instability Eutectic 1 mm Solid

9 Dynamical formation of solidification microstructure X-ray Radiography Al 3.5 wt% Ni V P = 1 µm/s G = 30 C/cm Exposure time : 3 s Video: 1 h 20 min 1 mm

10 CET in refined alloys Refined Al - 3.5wt%Ni G 20K/cm, At t 0 : V = 1.5 µm/s V = 15 µm/s Al - dendrites Equiaxed grains 1 mm t s t s t s t s 1 mm 1) Dendritic columnar growth 2) Nucleation of equiaxed grains 3) Sedimentation of equiaxed grains 4) Blocking of columnar structures Ful equiaxed growth X-ray Radiography

11 CET in refined alloys 1 mm X-ray Radiography Refined Al 3.5wt%Ni G = 30 K/cm V = 1,5 15 µm/s

12 Misorientation inside a dendrite 1 mm Radiography Al-3,5wt%Ni G 30K/cm, t min t min t min V = 1 µm/s (022) Topography

13 Misorientation inside a dendrite 1 mm Radiography Al-3,5wt%Ni G 30K/cm, t min t min t min V = 1 µm/s 0.12 (022) Topography 0.3

14 Silicon solidification

15 Radiography/Image processing Silicon: ρ s = 2.31 g.cm -3 ; ρ l = 2.56 g.cm -3 Interface hardly distinguishable Pixel by pixel division of the current image by the first image before solidification L S Front view L S+L S : Interface : Crucible (BN) L S Side view 6 mm Pixel by pixel division of two successive images L L Front view S+L Side view S : Interface : Crucible (BN) S 6 mm

16 Twinning and grain competition UMG-Si, R = 0.4 K/min, G app = 16 K/cm 6 mm 1,3 mm X-ray Radiography t 0 t s t s t s Groove at the level of a grain boundary Shape of the groove: facetted/facetted Groove is marked several times during solidification.

17 Twinning and grain competition 6 mm Si UMG#1 R = 0.4K/min G app ~ 16K/cm Zone 2 Interface velocity: V 1 = 3.3 µm/s V 2 =2.7 µm/s Zone 1 X-ray Radiography

18 Twinning and grain competition Si UMG#1 R= 0.4 C/min G app ~ 16 C/cm X-ray Topography Growth Direction 2 cm t 0 +8 min t min t min t 0 g 1 mm

19 Twinning and grain competition Si UMG#1 R= 0.4 C/min G app ~ 16 C/cm X-ray Topography Growth Direction 2 cm t min t min g t 0 +8 min t 0 1 mm 1,6 mm

20 Twinning and grain competition Si UMG#1 R= 0.4 C/min G app ~ 16 C/cm 2 cm X-ray Topography Growth Direction t min t min t 0 t 0 +8 min g 2 mm

21 Twinning and grain competition X-ray Topography Movies from topographs Diffraction diagram = + Twinning during Si crystal development t min t min t min

22 Successive Twinning MG Si, R= 1K/min, G ~ 15K/cm, Vg~11 µm/s X-ray Radiography t s t s t s t s 5 mm Twins Birth of the twins at S/L interface at the level of facets

23 Successive Twinning 6 mm X-ray Topography Si MG R= 1K/min, G ~ 15K/cm Vg~11 µm/s

24 Successive Twinning X-ray Topography Si (6N) Cooling rate = 0.2K/min G ~ 16K/cm Growth t min g 1 cm t min t+ 18min t min t 0 1 mm

25 Successive Twinning Successive twins revealed by chemical etching after solidification. Grain orientation identified by Laue diffraction: (100). Twin plane orientation : (111) Stereographic projection Laue diagram Si (6N), G=30K/cm, R=0.5K/cm

26 Conclusion Two X-ray imaging techniques in situ and in real time during solidification: Synchrotron X-ray radiography: information on the dynamics of the solid-liquid interface Synchrotron white beam X-ray topography: grain structure, crystallographic orientation, strains, defects Investigation of solidification microstructure and grain structure formation in thin samples (Al-based alloys and Silicon) CET Solute field ahead of the front Solutal interactions between equiaxed grains Dendrite deformation Twinning in silicon Grain competition Cu (wt%) 8 X = 0.58 mm 7 X = 2.35 mm 6 X = 4.7 mm Z (mm)

27 Outlook Other materials with a high melting point Microgravity experiments: Ni-based superalloys, Steel ESA XRMON project : X-ray radiography in microgravity conditions Sounding rocket MASER 12, February 2012 Comparison with quantitative simulations of grain structure G. Salloum Abou Jaoudé Ph. D. Thesis XR2Sol CAFE Coll. with Ch.-A. Gandin (CEMEF, MinesParisTech, Sophia Antipolis)

28 Acknowledgments

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