Macrosegregation of impurities in directionally solidified silicon

Abstract:

Directional solidification of molten metallurgical-grade Si was carried out in a vertical Bridgman
furnace. The effects of changing the mold velocity from 5 to 110 lm seconds–1 on the macro-
segregation of impurities during solidification were investigated. The macrostructures of the
cylindrical Si ingots obtained in the experiments consist mostly of columnar grains parallel to the
ingot axis. Because neither cells nor dendrites can be observed on ingot samples, the absence of
precipitated particles and the fulfillment of the constitutional supercooling criterion suggest a
planar solid–liquid interface for mold velocities £10 lm seconds–1. Concentration profiles of
several impurities were measured along the ingots, showing that their bottom and middle are
purer than the metallurgical Si from which they solidified. At the ingot top, however, impurities
accumulated, indicating the typical normal macrosegregation. When the mold velocity decreases,
the macrosegregation and ingot purity increase, changing abruptly for a velocity variation from
20 to 10 lm seconds–1. A mathematical model of solute transport during solidification shows
that, for mold velocities ‡20 lm seconds–1, macrosegregation is caused mainly by diffusion in a
stagnant liquid layer assumed at the solid–liquid interface, whereas for lower velocities, mac-
rosegregation increases as a result of more intense convective solute transport.


Reference:
MARTORANO, Marcelo A.; FERREIRA NETO, João Batista ; OLIVEIRA, Theógenes S. ; TSUBAKI, Tomoe Oide . Macrosegregation of impurities in directionally solidified silicon. Metallurgical and Materials Transactions A, v. 4A, p. 1870-1886, jul., 2011.

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