Thermo-Calc Determination of Phase Diagram of Si-B Binary System

Abstract

Deep understanding of the Si-B binary system is mainly essential for producing solar-grade silicon from metallurgical-grade silicon (MG-Si) as raw material. The phase diagram and thermodynamic properties are the main pillars of this understanding, where the phase diagram is useful for determining the Si terminal solubility, which is necessary for the success of the solvent refining process, by careful examination of the solvus line. Since Si-B phase diagrams found in literature are old and based on old experimental data, Thermo-Calc software 2019b is used in this work to reassess the temperature–composition phase diagram of the Si-B binary system at pressure of 1 bar. The resulting phase diagram is compared with experimental and computed diagrams found in literature and used to deduce the different reactions, melting temperatures, and solubilities of Si in B and B in Si, and all the identified discrepancies are discussed. There are six single phases other than elemental boron and elemental silicon: liquid, diamond-Si solid solution, β-B solid solution, SiB3 solid solution, SiB6 considered as a stoichiometric compound, and SiBn solid solution. In addition, the phase diagram includes nine regions of two mixed phases, all of which are thoroughly discussed. The melting points of pure β-B and pure diamond-Si are determined. Two peritectic reactions are found, a eutectic reaction, and a peritectoid reaction. The maximum solubility of Si in B is found to be 5.37 Si mass percent at 2310.04 ± 0.025 K, while that of B in Si is found to be 0.41 B mass percent at 1657.56 ± 0.025 K.