• PDF / 862,655 Bytes
• 17 Pages / 593.972 x 792 pts Page_size
• 67 Downloads / 200 Views

DOWNLOAD

REPORT

---

ing demand for Si in photovoltaic applications has triggered the investigation of alternative more economical routes to produce feedstock for the solar grade silicon.[1–4] In the metallurgical routes, the metallurgical grade silicon (MG-Si), with typical purity of 98.5 pct Si, is refined to fulfill the more rigid composition constraint needed for the solar grade. As a possible step in these routes, directional solidification of Si is adopted to eliminate a large number of important impurities,[4–7] which segregate to the last portion of the ingot to solidify, leaving a purer solid Si. The intensity of this segregation is related strongly to the solute partition coefficient, which is also known as the equilibrium segregation coefficient, defined as k0 ¼

Cs Cl

½1

where Cs and Cl are the equilibrium impurity concentrations in the solid and liquid, respectively, at a given temperature. Except for B, P, and O, k0 > 1, the liquid convection does not affect the solute redistribution and the concentration profile in the solid follows the initial transient behavior. For D 5 lm seconds–1), however, there were fewer precipitates at the top, appearing isolated or forming strings. The ingots grown at 5 and 10 lm seconds–1 show precipitates only at the top, whereas for 110 and 20 lm seconds–1, precipitates were viewed throughout the ingot (bottom, middle, and top). For a mold velocity of 110 lm seconds–1, for example, Figure 2(c) shows a string of precipitates in a slice cut from the middle of the ingot. A microprobe analysis by energy dispersive spectroscopy of the precipitates indicated different compositions, which consisted of a combination of the following elements: Si, Fe, Al, Ti, Cr, Ni, Cu, Zr, and P.

V.

STABILITY OF THE SOLID–LIQUID INTERFACE

Dendrites or cells were not viewed in the samples observed at the optical or scanning electron (SEM) microscopes. Nevertheless, it is not possible to conclude that the morphology of the solid–liquid interface during solidification was planar. In high-purity metals, the microsegregation and the amount of precipitates formed in interdendritic regions might be insufficient to reveal a dendritic or cellular microstructure by chemical etching or by the backscattered electron contrast of a SEM. For example, Liu et al.[41] observed faceted dendrites in high-purity Si droplets solidified in contact with a Cu chill plate because the dendritic structure became visible by the surface relief formed on the solidified droplet. C¸iftja[42] concluded that the presence of precipitates rich in Al, Fe, or made of SiC is an indication that the planar solid–liquid interface broke down during solidification, giving rise to a cellular structure. This conclusion was based on the assumption that during solidification, some precipitates that were present already in the molten Si were pushed in between the cells, becoming an evidence of the cell existence. Even when precipitates do not form in the liquid ahead of the solid–liquid interface, these might form during 1874—VOLUME 42A, JULY 2011

solidification in between the