Kinetics and Mechanism of Phosphorus Removal from Silicon in Vacuum Induction Refining

Abstract

Vacuum induction refining is a process that can be applied to remove phosphorus from molten silicon for the production of solar grade silicon. Pure silicon was doped by phosphorus to make molten silicon containing around 17 ppmw phosphorus. The kinetics of phosphorus removal from this silicon was studied at 0.5 Pa through the application of vacuum induction refining. It was observed that vacuum removal of phosphorus occurs through a first-order reaction. The rate constants of phosphorus evaporation were determined as 2.28 × 10-6 m/s and 4.93 × 10-6 m/s at 1500 °C and 1600 °C, respectively. Moreover, an apparent activation energy 213.1 kJ/mol for phosphorus evaporation from molten silicon was calculated. It was found that mass transfer of phosphorus in the melt is not rate limiting in the inductively stirred silicon melt. The vacuum removal of phosphorus is mix-controlled by chemical reaction and gas phase mass transfer. Under medium vacuum conditions, the mass transfer in the gas phase is more rate-limiting than the chemical reaction at higher refining temperatures.

Keywords

• silicon
• phosphorus
• kinetics
• mechanism
• vacuum induction refining