Refining of metallurgical silicon by directional solidification


The directional solidification of a typical and a previously refined metallurgical silicon was carried out in a vertical Bridgman furnace. The mold velocity out of the hot zone of the furnace changed from one experiment to another in the range between 5 and 110 m s−1 . Samples were extracted from the cylin- drical ingots obtained in the experiments to investigate the effects of the mold velocity on the micro and macrostructures and on the concentration profiles of impurities along the ingots. At the lowest mold velocity, the macrostructures consist of columnar grains oriented approximately parallel to the ingot axis. As velocity increases, grains become thinner and more inclined in the radial direction. Precipitated par- ticles containing Si, Fe, Al, and Ti are observed at the top of all ingots and, as the mold velocity increases, they are also seen at the ingot bottom and middle. The concentration profiles of several impurities have been measured along the ingots by inductively coupled plasma atomic emission spectrometry (ICP), indi- cating an accumulation of impurities at the ingot top. Consequently, the bottom and middle of the ingots are purer than the corresponding metallurgical silicon from which they solidified. Slices from the ingot bottom have also been analyzed by the glow discharge mass spectrometry technique (GDMS), allowing measurement of impurity concentrations that were below the quantification limit of the ICP. The purifica- tion effect and the accumulation of impurities at the ingot top are more pronounced as the mold velocity decreases. In the ingots obtained from the typical metallurgical silicon at the lowest mold velocities (5 and 10 m s−1 ), except for Al, all impurities are in concentrations below an important maximum limit for the feedstock of solar grade silicon. At the same mold velocities, the concentrations of Fe, Ti, Cu, Mn, and Ni measured at the bottom of the ingots obtained from both types of metallurgical silicon (typical and previously refined) are even below some limits suggested directly for solar grade silicon.

MARTORANO, Marcelo A.; FERREIRA NETO, João Batista ; OLIVEIRA, Theógenes S. ; TSUBAKI, Tomoe Oide . Refining of metallurgical silicon by directional solidification. Materials Science and Engineering B, v.176, n.3, febr., p.217-226, 2011.

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