Removal of Cu, Mn and Na in multicrystalline silicon by directional solidification under low vacuum condition

A multicrystalline silicon ingot was obtained from metallurgical-grade silicon after directional solidification under low vacuum condition. The concentration distributions of metal impurities such as copper (Cu), manganese (Mn) and sodium (Na) along the growth direction of the ingot were investigated. The result shows that the concentration of Cu and Mn decrease respectively from 28.56 ppmw and 10.53 ppmw to about 0.1 ppmw and 0.01 ppmw in below 80% of the ingot height, which are in good agreement with the value calculated by the Scheil's equation. The concentration of Na decreases from 1096.91 ppmw to about 0.2 ppmw in the whole ingot, due to the evaporation effect. The evaporation model of Na under low vacuum condition is proposed and the mass transfer coefficient of Na is also calculated.