Effects of forced convection on the micro and macrostructure of metallurgical grade silicon ingots obtained by unidirectional solidification

Abstract:

The effects of forced convection on the micro-macrostructure and the macro segregation of metallurgical grade silicone ingots obtained by unidirectional solidification were investigated. Solidification was carried out by extracting heat from the melt through the bottom of a crucible in contact with a water-cooled base. In one experiment, a graphite disk rotating at a controlled velocity was immersed into the silicon melt to force convection during the directional solidification process. After solidification, the resulting ingots off 100mm height showed a purified region with: clearly aligned columnar grains, without intermetallic precipitates, and which probably solidified with a planar solid-liquid interface. With forced convection the length of the purified region increases from 8 to 80 mm, indicating an increase in the macrosegregation of impurities and in purification. Chemical analyses along the longitudinal axis of the ingots confirmed the larger macrosegregation of impurities promoted by convection. Convection movement transports impurities within the liquid are well-known to increase macrosegragation, but a heat and mass transfer mathematical model indicates that it might also increase the stability of a planar solid-liquid interface, preventing formation of cells/dendrites and, therefore, facilitating solute transport from the solid-liquid interface into the bulk liquid, interface into the bulk liquid, which increases macrosegregation.


Reference:
NASCIMENTO, Denir Paganini; MARTORANO, Marcelo Aquino; FERRERIRA NETO, João Batista; LIMA, Moysés Leite de; RIBEIRO, Tiago Ramos. Effects of forced convection on the micro and macrostructure of metallurgical grade silicon ingots obtained by unidirectional solidification. In: SILICON FOR THE CHEMICAL AND SOLAR INDUSTRY CONFERENCE, 14., 2018, Noruega.  Proceedings… 12 p


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