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Progress in Plasma Processing of Materials, 2003

ISBN Imprimer: 978-1-56700-192-1

ISBN En ligne: 978-1-56700-447-2



A new process combining inductive plasma and electromagnetic stirring of molten silicon in a cold crucible has been developed to refine upgraded metallurgical silicon. The addition of reactive gases to the plasma makes it possible to volatilize impurities at the liquid silicon surface. The process has been studied both from a thermodynamic and a kinetic point of view. The concentration of boron impurities decreased from 15 ppmw in the raw material to less than 2 ppmw after plasma treatment. The most volatile form of boron was BOH, which was obtained by treatment with a mixture of oxygen and hydrogen. The volatilization rate depends on the oxygen and hydrogen flow rates. The limitation in boron volatilization is due to the formation, at high oxygen flow rate, of a silica layer at the surface of the molten silicon, which results in a dramatic drop of the volatilization rate. In contrast to boron, the concentration of phosphorus was reduced by only a factor of two, although the remainder seems to be electrically neutral. Thermodynamic study suggests that phosphorus could be trapped as phosphate in combination with metallic impurities or as aluminium phosphide. Cells made of material produced using this technique exhibited a conversion efficiency of 12.4 %.
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