Boron removal from silicon by hydrogen assistant during the electromagnetic directional solidification of Al Si alloys

Abstract

Abstract Boron (B) removal from silicon (Si) by using an Ar-20% H2 gas mixture during the Al Si solvent refining was investigated. Electromagnetic directional solidification was employed to enrich the primary Si crystals in the melt and simultaneously drive the produced BxHy gas species to migrate out of the Si enrichment zone. Thermodynamic analysis shows that the formation of BxHy gas species by the reaction of dissolved boron [B] and dissolved hydrogen [H] in Al Si melt is feasible. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) analysis confirmed that the impurity carbon (C), oxygen (O), and B adhered to the wall of the gas cavity, indicating that B removal can be attributed to the migration of gas species. As the flow time of Ar-20% H2 gas mixture increased, the B removal fraction significantly increased, and a maximum value of 96.3% for B removal fraction was achieved when the flow time was 150 min. Finally, an overall process for producing solar grade silicon (SoG-Si) from metallurgical grade silicon (MG-Si) with the combination of Ar H2 refining in Al Si solvent and vacuum directional solidification was proposed.