Kinetics of evaporation under vacuum in preparation of solar-grade silicon by electron beam melting

Abstract A kinetics model for silicon evaporation is established in order to investigate the dependence of temperature and chamber pressure on silicon loss. The results show that the rate-limiting step is mass transfer of silicon in gas phase at low temperature; while it transits to free evaporation on melt surface as the temperature rises. As for electron beam melting process, silicon free evaporation is considered as the rate-limiting step due to the extremely high temperature of the melt. The relationship between impurity removal efficiency and silicon loss is obtained according to this calculation model. The silicon loss ratio is less than 10% when the removal efficiency of phosphorus is close to 100%, but that of aluminum is still low, indicating this technology is more effective for the removal of phosphorus. Aluminum can be further removed by increasing temperature or prolonging refining time, but leading to more silicon loss.