Vacuum refining of silicon at ultra-high temperatures
Peer reviewed, Journal article
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In the production of solar grade silicon (SoG-Si), Phosphorus (P) removal from Si is a challenge as it cannot be eliminated effectively by the final key directional solidification step in the value chain of the silicon solar cell technologies. The present research investigates the application of ultra-high temperatures (UHTs) up to 1900 °C for the P removal from Si in the vacuum-induction refining (VIR) process. Kinetic parameters such as mass transfer coefficient and activation energy for P removal from Si melt by vacuum evaporation are determined. It is shown that the P removal kinetics is significantly accelerated with increasing temperature, and about 1800 °C is a critical temperature in which the process rate is doubled. The silicon loss of the process to reach SoG-Si quality is formulated, and it is shown that it is lower at ultra-high temperatures, while it is insignificantly increased with the temperature rise in each temperature regime, and is independent of the melt geometry. The results from UHTs experiments showed complete phosphorus removal from silicon melts with even as high as P concentrations as 92.71 ppmw in short durations. It is shown that the application of UHTs in VIR process reduces the power consumption to reach SoG-Si.