Macrosegregation of Impurities during Solidification of Metallurgical Grade Silicon in a Vertical Bridgman Furnace

Abstract

Directional solidification of metallurgical-grade Si was carried out in a vertical Bridgman furnace. The effects of changing the mold velocity on the macrosegregation of impurities were investigated. Macrostructures of cylindrical Si ingots consist mostly of columnar grains parallel to the ingot axis. Neither dendrites nor cells were observed, although there is some indication of their presence in the microstructures obtained at the largest velocities. Measured concentration profiles of impurities showed that ingot bottom and middle are purer than the metallurgical Si. Impurities accumulated at the ingot top, creating a typical normal macrosegregation. When mold velocity decreases, macrosegregation and ingot purity increase, changing abruptly below 20 μm s. A mathematical model of solute transport shows that, for mold velocities ≥ 20 μm s, macrosegregation is caused mainly by diffusion in a stagnant liquid layer assumed at the solidliquid interface, while for lower velocities, macrosegregation increases owing to convective solute transport.