REMOVAL OF C/SIC FROM LIQUID SILICON BY DIRECTIONAL SOLIDIFICATION

Silicon, produced by a carbothermic reduction of high-purity silica with high-purity carbon in an arc furnace, contains a high amount of carbon and silicon carbide. For this reason, a subsequent purification step is required to obtain solar grade silicon for the production of solar cells with high efficiencies. The investigation of a directional solidification process for that purpose is described. The content of dissolved impurities is reduced by segregation, whereas a sufficient separation of silicon carbide particles can only be obtained with a convenient convectional flow in the melt, which depends on the thermal conditions during the crystallization procedure. The motion of the particles in front of the freezing interface can prevent the incorporation into the growing crystallites. This correlation is derived from experimental results and the numerical simulation of the thermal free convection in the silicon melt.